Clarify v0.9.0 import and collection migration guidance

README.md

@@ -17,7 +17,8 @@ then report domain errors and warnings through read-only semantic validation. Se
 tags and a path predicate drive deterministic redacted effective YAML. Typed secret
 references resolve environment names before validation, and explicit
 secrets-directory bootstrap is available as a separate startup operation. Explicit
-CLI override records apply above overlays. Value explanation is available through
+CLI override records apply after inheritance and typed decoding, before application
+resolution. Value explanation is available through
 detached structured records and deterministic redacted output. Built-in
 `extends` relationships let a mapping inherit from a named sibling during the
 same load, retaining explicit presence and source/derivation history. The root
@@ -26,12 +27,14 @@ the [configuration contract](docs/config.md#effective-configuration-diffs) and
 [consumer guide](docs/consumers/README.md#effective-configuration-diffs) describe
 its safe changed-region records and maintained workflows.
 
-Sources can also be explicitly placed as sequence items, and definitions can
-register selected sequences to append records with distinct string IDs during
-strict composition. Higher-priority sequences still replace lower-priority
-sequences as whole values. See the [sequence contract](docs/config.md#strict-document-composition),
-[maintained prompt example](docs/consumers/README.md#identified-prompt-sequences),
-and [ADR 0014](docs/adr/0014-sequence-placement-and-composition.md).
+Sources can also be explicitly placed as sequence items. Configuration
+declarations and application registrations can give selected sequences scalar
+identities and choose how they combine across precedence and inheritance.
+Strict peers combine distinct records, while higher-priority collections can
+replace, append, or upsert whole records. See the
+[sequence contract](docs/config.md#strict-document-composition),
+[maintained catalog examples](docs/consumers/README.md#identified-prompt-sequences),
+and [ADR 0017](docs/adr/0017-collection-sequence-policy.md).
 
 Imports are also explicit: a source may name files, search declared directories,
 or select files with path globs. Applications can set load-wide import defaults
@@ -76,12 +79,13 @@ mappings combine, while overlapping non-mapping definitions are errors. Between
 precedence tiers, mappings combine recursively, retaining omitted lower-priority
 children; other values replace lower-priority values. The main document and its
 imports form one strictly composed tier. Explicit underlay groups sit below it
-and above typed defaults; a selected overlay and explicit CLI overrides sit
-above it. The [configuration contract](docs/config.md#strict-document-composition)
-owns the detailed combination rules and input-specific constraints. At
-definition-registered paths only, strict composition can append records with
-distinct explicit IDs; ordinary and unregistered sequences keep their conflict
-or whole-value replacement behavior.
+and above typed defaults; a selected overlay applies before inheritance, while
+explicit CLI overrides apply after decoding and before application resolution.
+The [configuration contract](docs/config.md#strict-document-composition) owns
+the detailed combination rules and input-specific constraints. Declared or
+registered collections combine distinct identified records within strict tiers
+and use their selected operation at precedence boundaries; other sequences keep
+their strict-conflict and whole-value-replacement behavior.
 
 The [architecture goals](docs/policy/architecture.md#success-criteria) explain
 these priorities and how genuine new needs can justify extending the shared

docs/adr/0017-collection-sequence-policy.md

@@ -138,6 +138,6 @@ Record inheritance also requires parent lookup by identity and checking for
 missing parents and cycles. Copied settings retain the same source, history,
 and sensitivity information as mapping-sibling inheritance.
 
-The configuration contract will define declaration syntax, identity comparison,
-omission and empty-value behavior, and diagnostics when the feature is
-implemented.
+The [configuration contract](../config.md#strict-document-composition) defines
+declaration syntax, identity comparison, omission and empty-value behavior, and
+diagnostics for the implemented feature.

docs/adr/0018-cli-after-structural-resolution.md

@@ -80,5 +80,5 @@ must remain consistent throughout loading.
 CLI input can no longer add or cancel inheritance relationships or bypass an
 earlier error in source assembly or inheritance. It also cannot fix a value that
 fails the earlier decoding step. Semantic validation still runs after overrides
-and application resolution. The configuration contract and consumer guidance
-must explain these behavior changes when they are implemented.
+and application resolution. The [configuration contract](../config.md#explicit-cli-overrides)
+and [consumer guidance](../consumers/README.md) describe this behavior.

docs/config.md

@@ -52,10 +52,11 @@ preserves already validated secret state without rereading the environment.
 The current lifecycle is:
 
 ```text
-validate options → parse the selected source → traverse/compose enabled imports
+validate options → parse the selected source → extract source controls and
+  traverse/strictly compose enabled imports
   → build schema → check the optional application version → apply a selected overlay
-  → apply explicit CLI overrides → expand sibling inheritance
-  → create and clone defaults → strictly decode
+  → expand sibling inheritance
+  → create and clone defaults → strictly decode → apply explicit CLI overrides
   → recorded application resolution
   → resolve typed secret references → semantic validation → classify secrets
   → snapshot configuration and metadata
@@ -70,15 +71,16 @@ missing file remains an error; it is never interpreted as no input.
 `Options` accepts the application definition, ordinary-source `Destination` and
 `PlacementMode`, explicit imports, one selected overlay, filesystem
 context, explicit CLI overrides, and shared source-version controls. The definition
-may register strict sequence composition paths through
-`SequenceCompositions`. Its zero-valued placement mode places the source mapping
+may register sequence identities through `Definition.SequenceIdentities`. Its
+zero-valued placement mode places the source mapping
 at the destination; `sequence-item` explicitly wraps the mapping as one item at a
 non-root destination. Unknown modes, invalid destinations, and invalid or
 conflicting sequence registrations fail before source acquisition. Overlay and
-CLI coordinates remain rooted at the application root. `NoSource` rejects
+CLI coordinates remain rooted at the application root. Ordinary `Load` recognizes
+source-root collection declarations whether or not imports are enabled; an
+`imports` declaration still requires imports to be enabled. `NoSource` rejects
 imports, active overlays, and filesystem context as invalid combinations but
-accepts overrides. Input containing a reserved `configkit` mapping key is rejected
-while imports are disabled.
+accepts overrides and remains an absent source rather than an empty document.
 
 Every source, options, schema, version, defaults, decode, resolution, or validation
 failure returns a nil result and a `*LoadError`. Successful results contain caller-owned `Config`, detached
@@ -128,16 +130,20 @@ nothing. Underlay groups are listed from lowest to highest priority. Repeating
 a file within one group remains an import/repetition error; the same reusable
 source may be selected again in another tier.
 
-ConfigKit acquires and composes the main input first, constructs the schema,
-and checks the optional application version from the main tier only. Underlay
-roots cannot claim that version path (`underlay.version`); imported fragments
-retain `import.version` protection. It then acquires and composes underlay
-groups in order, recursively combines the groups, and applies the main tier,
-one selected overlay, and explicit CLI overrides. Scalar, sequence, and null
-definitions replace lower values between tiers; mappings combine recursively.
-Syntax and strict-composition errors inside a tier cannot be masked by a
-higher-priority tier. Defaults, typed decoding, resolution, secret lookup,
-validation, and publication run once on the final assembled input.
+ConfigKit acquires the main input first and constructs the schema. It then
+acquires and strictly composes underlay groups from lowest to highest priority,
+passing each group's accumulated lower collection policy into the next group.
+Each group is applied once as a precedence tier. The main tier is then composed
+with the accumulated underlay policy, and the optional application version is
+checked against that composed main tier only. Underlay roots cannot claim that
+version path (`underlay.version`); imported fragments retain `import.version`
+protection. ConfigKit then applies the selected overlay and resolves inheritance.
+Scalar, sequence, and null definitions replace lower values between tiers unless
+a collection policy defines the sequence operation; mappings combine recursively.
+After successful defaults and typed decoding, CLI overrides are applied to that
+typed configuration before application resolution. Syntax, strict-composition,
+inheritance, and decoding errors cannot be masked by a CLI override. Resolution,
+secret lookup, validation, and publication run once on the resulting values.
 
 An active layered overlay requires `LayeredOptions.OverlayFilesystem` explicitly,
 independent of the main source's namespace. Its name and root-relative lookup
@@ -147,7 +153,10 @@ Ordinary `Load` continues to infer its selected-overlay context from its source.
 
 Source IDs follow acquisition order—main and its depth-first imports, then each
 underlay tier's inputs and depth-first imports, then overlay and CLI—even though
-history follows precedence order. Source `Layer` describes precedence while
+history follows precedence order. Default main-version checking happens after
+underlay composition because the main tier inherits the accumulated collection
+policy. Enabled per-source version checks retain acquisition order and source
+local headers. Source `Layer` describes precedence while
 `Kind` describes acquisition. Underlay records use `LayerUnderlay` and a
 one-based `UnderlayIndex` giving their original tier position; all other records
 have index zero. Explicit empty groups and `NoSource` consume no source ID.
@@ -213,35 +222,55 @@ is recorded in [ADR 0011](adr/0011-input-destinations.md) and
 
 Any input mapping may declare `extends: name` to inherit from the mapping named
 `name` among its immediate siblings. Names are exact keys, including the empty
-string; lookup does not search ancestor or unrelated scopes. The document root
-and a sequence-element mapping have no sibling namespace for their own string
-relationship, though mappings nested inside them can have one. An `extends`
+string; lookup does not search ancestor or unrelated scopes. A mapping inside a
+sequence can also participate in record inheritance when that sequence has an
+effective identity declaration from `configkit.sequences` or an application
+registration. In that case, `extends` is a mapping containing exactly the
+sequence's identity fields, with scalar values; lookup is limited to that final
+sequence. The document root and a sequence-element mapping without an effective
+identity declaration have no sibling namespace for non-null string
+relationships, though mappings nested inside them can have one. An `extends`
 value of null cancels a relationship without looking up a parent. Other value
-types, missing or non-mapping parents, and cycles fail the load with value-free
-diagnostics.
+types, incomplete or extra reference fields, missing parents, and cycles fail
+the load with value-free diagnostics.
 
-Within each sibling scope, direct mapping definitions resolve by dependency
-readiness. A parent is resolved before a child; among currently eligible
-definitions, the lexically smallest canonical semantic pointer is selected,
-including newly eligible definitions as soon as their parent completes. This
-ordering governs inheritance operations and their history, controls, and
-transfers. It does not reorder the mappings in the resulting configuration.
+Within each mapping sibling scope or identified sequence, direct definitions
+resolve by dependency readiness. A parent is resolved before a child; among
+currently eligible definitions, the lexically smallest canonical semantic
+pointer is selected, including newly eligible definitions as soon as their
+parent completes. This ordering governs inheritance operations and their
+history, controls, and transfers. It does not reorder mappings or sequence
+records in the resulting configuration.
 
 `extends` is reserved structural input across ordinary and layered loads, every
 source kind, imports, selected overlays, and explicit CLI overrides. It is
 consumed before typed decoding and need not appear in the application's Go
 types. Applications previously using `extends` as an ordinary YAML field must
 rename that key and its field. No collection registration or keyword setup is
-required.
+required for mapping-sibling relationships; identified-sequence relationships
+require effective identity fields for that collection.
 
 ConfigKit completes strict composition within tiers and precedence assembly
-between tiers before resolving relationships. An omitted `extends` retains a
-lower-priority relationship; `extends: null` cancels it while preserving the
-child's own settings. Superseded references are historical evidence, not active
-relationships to validate. Each child recursively overlays its own settings
-over its resolved parent: explicit zero, false, empty string, sequence, and
-null values replace inherited values, while mappings combine recursively.
-The child retains its key. All effective relationships resolve, including
+between tiers before resolving relationships. Only records that survive that
+assembly participate in identified-sequence lookup: an upserted parent supplies
+its final record, a replaced parent is unavailable, and a higher replacement
+without `extends` supersedes a lower relationship. An omitted `extends` retains
+a lower-priority mapping-sibling relationship; an identified record's effective
+directive depends on the record that survives assembly. `extends: null` cancels
+a relationship while preserving the child's own settings. Superseded references
+are historical evidence, not active relationships to validate. Each child
+recursively overlays its own settings over its resolved parent: explicit zero,
+false, empty string, and
+null values replace inherited values, while mappings combine recursively. For
+identified records, the child's identity fields and their origins remain its
+own; parent identity fields do not contribute to the payload.
+Sequences at collection destinations use the shared collection operation with
+the parent as lower input and the child as higher input. The destination's
+explicit mode applies; otherwise a copied identity selects upsert and an
+unkeyed destination replaces. A parent's explicit mode does not move to a
+different destination. Sequences outside collection destinations remain atomic
+replacements.
+All effective relationships resolve, including
 entries the application may never select for execution. Partial parent
 definitions undergo normal typed decoding and application-owned validation,
 not a special structural exemption.
@@ -423,10 +452,11 @@ the descendant can therefore retain history after `Lookup` stops reporting it as
 effective. Equal-valued input is still an explicit base definition, while a derived
 value for absent input remains non-explicit.
 
-Appending registered records during strict composition assigns each contribution
-its assembled sequence index in history. A later precedence tier that supplies a
-sequence records replacement at the sequence path; history does not imply
-identity-based merging across tiers.
+Strict composition assigns contributed records their assembled sequence positions
+in history. Across tiers, history follows the selected
+[collection operation](#strict-document-composition), including append and
+whole-record upsert. Incoming contributions use their assigned positions, and
+replaced payload remains available as historical evidence.
 
 A mapping assembled from the selected file and imports has one composite base origin
 whose locations follow [scoped assembly order](#explicit-imports), not source-record
@@ -501,30 +531,33 @@ historical path. Its effective value and each chronological before/after value a
 parser values. `Present=false` with empty YAML means absence or deletion; explicit
 null is present and renders as `null`. The history starts with the canonical typed
 application-default or Go-zero baseline, followed by relevant underlay and main-tier
-contributions in precedence order, the selected overlay, supplied CLI records,
-and audited net resolution operations. Within each strict tier, replay follows
+contributions in precedence order, the selected overlay, and structural inheritance
+events. Typed defaulting and decoding reconcile that source history before supplied
+CLI records and audited net resolution operations. Within each strict tier, replay follows
 [scoped assembly order](#explicit-imports), not source-record acquisition order.
-Structural inheritance events appear after source replay
-and before typed reconciliation, including consumed or canceled `extends` input
-and equal-valued child replacements. Overlapping parent/child resolution records
+Inheritance events include consumed or canceled `extends` input and equal-valued
+child replacements. Overlapping parent/child resolution records
 are rendered
 as one net subtree event with combined reasons and dependencies, rather than
 pretending that the recorded assignments ran in lexical order. Explicit equal
-writes remain visible. When input replay
-differs from the final typed input, a `ChangeDecoding` event records the transition
-before domain resolution. Its origin retains the decoded path's source/default
-attribution; its reason identifies static-default retention and canonical decoding.
-This built-in inspection operation is not accepted by `Resolution.Record`.
+writes remain visible. When structural replay differs from the pre-CLI typed input,
+a `ChangeDecoding` event records retained defaults and canonical decoding. CLI
+contributions then retain their raw input representation while effective and
+resolution values use the typed projection. This built-in inspection operation is
+not accepted by `Resolution.Record`.
 
 Base, overlay, and CLI history preserves the policy-validated input representation,
 including a superseded value that could not have decoded into the final Go field.
 Baseline, effective, and resolution values use canonical typed projection. A
 decoding event bridges replayed input and canonical typed values, so intermediate
 null/scalar replacements cannot incorrectly imply permanent default removal. Mapping
-queries reconstruct each source or override contribution, while sequences remain
-atomic: a descendant history identifies its presence before and after the containing
-sequence replacement without implying element-wise merging. Removed paths remain
-explainable; unknown and malformed paths return false.
+queries reconstruct each source or override contribution. A sequence snapshot
+represents the collection at that point in replay, and sequence diffs remain
+atomic. Descendant queries describe their presence and values before and after
+containing sequence operations. Combination follows the
+[collection operation tables](#strict-document-composition); upsert replaces
+matching records in full. Removed paths remain explainable; unknown and malformed
+paths return false.
 
 Every snapshot applies the same historical tag, predicate, and recorded-dependency
 sensitivity closure as effective rendering before YAML is exposed. A redacted scalar
@@ -611,13 +644,15 @@ must resolve to a nonempty value. Secret references work in typed structs, maps,
 pointers, slices, and arrays. A dynamic `any` value does not establish secret
 semantics and cannot contain `Secret`; applications must use a typed slot.
 
-ConfigKit resolves only the effective names after recorded application resolution
-and before semantic validation. Each unique name is read with `os.LookupEnv` once
-per load. Validation can inspect the credential through the deliberately named
-`Value()` accessor and distinguish success with `IsResolved()`. Later loads read
-the environment again, and cloned defaults are rebuilt from names so a previously
-resolved result cannot retain a stale credential. `Load` never writes to the
-environment.
+ConfigKit resolves only the effective names after CLI assignment and recorded
+application resolution, and before semantic validation. CLI overrides may supply
+or replace a typed secret reference; lookup uses that effective name and retains
+its effective source location for diagnostics. Each unique name is read with
+`os.LookupEnv` once per load. Validation can inspect the credential through the
+deliberately named `Value()` accessor and distinguish success with `IsResolved()`.
+Later loads read the environment again, and cloned defaults are rebuilt from names
+so a previously resolved result cannot retain a stale credential. `Load` never
+writes to the environment.
 
 Credential bytes are held only in the per-load lookup state and the trusted returned
 configuration, and are returned only by `Value`. Secret formatting uses a safe
@@ -681,18 +716,19 @@ files when appropriate; ConfigKit stores no implicit global report.
 
 ### Recorded application resolution
 
-An optional `Definition.Resolve` callback runs after successful typed decoding. It
-receives the working configuration and a per-load `Resolution` context. The context's
+An optional `Definition.Resolve` callback runs after successful defaults, typed
+decoding, and explicit CLI assignment. It receives the working configuration and a
+per-load `Resolution` context. The context's
 `IsExplicit` query uses surviving input presence, so explicit false, zero, empty, and
 null are never confused with absence or static defaults.
 
 `Resolution.InputInfo(path)` returns detached effective metadata from the fixed
-decoded entry state, before application resolution runs. It includes retained static
-defaults, preserves explicit null, and uses the same exact semantic path matching as
-`Metadata.Lookup`; missing, malformed, and key/index-shape-mismatched paths return
-false without diagnostics. Its `Explicit` value agrees with `IsExplicit` for every
-found entry path. Resolution changes, additions, removals, and moves do not alter
-this entry view.
+post-decode entry state, including explicit CLI overrides, before application
+resolution runs. It includes retained static defaults, preserves explicit null, and
+uses the same exact semantic path matching as `Metadata.Lookup`; missing,
+malformed, and key/index-shape-mismatched paths return false without diagnostics.
+Its `Explicit` value agrees with `IsExplicit` for every found entry path. Resolution
+changes, additions, removals, and moves do not alter this entry view.
 
 `Resolution.Source(id)` returns a detached record for an already acquired source in
 the same load: the main source, imports, selected overlay, and supplied CLI records
@@ -767,8 +803,11 @@ live services as a substitute for configuration validation.
 
 ## Strict document composition
 
-ConfigKit strictly composes parsed base documents when `Options.Imports` is true.
-Application defaults do not participate in this operation.
+Ordinary `Load` applies strict document composition to its parsed source scope;
+`Options.Imports` adds imported documents as peers in that scope. Application
+defaults do not participate. With imports disabled, the single source still has
+its controls extracted and any declared or registered collection identity is
+validated, but there are no imported peers to combine.
 
 | Existing node | Incoming node | Result |
 | --- | --- | --- |
@@ -779,46 +818,139 @@ Application defaults do not participate in this operation.
 | Any other pair of present nodes | Any other pair of present nodes | `compose.conflict` |
 
 Every non-mapping overlap is a conflict, even when values are equal, except for
-sequence/sequence contributions at a registered sequence-composition path. Nulls
-remain atomic conflicts. Unregistered sequences conflict when repeated and are
-never joined by index. Disjoint entries under shared mappings compose, making
-stable keyed mappings the natural representation for independently contributed
-entries.
+sequence/sequence contributions at a destination with effective identity from an
+application registration, a current YAML `key` declaration, or retained lower
+policy. Nulls remain atomic conflicts.
+Sequences without effective identity conflict when repeated and are never joined
+by index. Disjoint entries under shared mappings compose, making stable keyed
+mappings the natural representation for independently contributed entries.
 
-`Definition.SequenceCompositions` registers literal mapping paths and one literal
-`IdentityField` for sequences that should compose independently authored records.
-Registration paths follow input-destination key rules: parsed numeric tokens are
-literal mapping keys, explicit index segments and reserved `configkit` or
-mapping-valued `extends` segments are invalid, and the root registration is
-harmless and unused. The identity field is one literal string key; neither the
-field name nor identity strings are normalized.
-At each registered path, every actual sequence contribution in a strict tier is
-validated, including a singleton contribution: each record must be a mapping with
-a present string identity field. Distinct exact identity strings append in source
-and list order; duplicate identities conflict even if the records are otherwise
-equal or disjoint. Empty identity strings are valid. Malformed records produce
-`compose.invalid`; duplicate identities produce `compose.conflict`, with the
-sequence container and source locations identifying a duplicate without exposing
-identity values. Missing paths and non-sequence values do not fail by themselves.
-Identical registrations coalesce; conflicting registrations at one path and
-invalid registration paths produce `options.invalid`. Unused and root registrations
-are allowed and have no effect.
+`Definition.SequenceIdentities` contains registrations created by
+`SequenceIdentityAt` or `SequenceIdentityMatching`. The exact form accepts a
+mapping-only `Path`; the matching form accepts literal-key and any-mapping-key
+segments created by `SequenceLiteralKey` and `SequenceAnyMappingKey`. A literal
+`"*"` key is distinct from the wildcard constructor. Parsed numeric path tokens
+remain literal mapping keys; explicit index segments and reserved `configkit` or
+mapping-valued `extends` segments are invalid. The root selector is harmless under
+the mapping-root schema. Selector segments consume one mapping level and match a
+complete destination; an any-mapping-key segment matches exactly one key, not an
+arbitrary prefix or suffix. Each registration requires one or more literal,
+direct field names; compound field sets are normalized as sets, and field names
+are not otherwise normalized. The same field-set and scalar-equality rules apply
+to YAML `key` declarations.
 
-Registration changes strict composition only. Each underlay group and the main
-tier has independent duplicate-identity state. Precedence assembly is unchanged:
-a present higher-priority sequence replaces the lower sequence as one value,
-including at a registered path. Overlays, CLI overrides, defaults, inheritance,
-and application resolution do not run the registered identity check. The accepted
-rationale is recorded in [ADR 0014](adr/0014-sequence-placement-and-composition.md).
-The [maintained prompt catalog](consumers/README.md#identified-prompt-sequences)
-shows item path and glob imports, helper-supplied identity, and main-tier
-replacement of a lower-priority sequence.
+At each selected path, records must provide every effective identity field as a
+scalar string, number, boolean, or explicit null. Field names form a set, so
+order and repetition do not affect compound identity. The type-tagged tuple
+preserves category distinctions: strings compare exactly, including empty
+strings; numbers, booleans, and null remain distinct categories. Finite numbers
+compare by exact mathematical value, so `1`, `1.0`, and `1e0` are equal, as are
+`0` and `-0`, without floating-point rounding. Distinct tuples append in source
+and list order; duplicate tuples conflict even when records are otherwise equal
+or disjoint. Malformed records produce `compose.invalid`; duplicate identities
+produce `compose.conflict`, with paths and source locations identifying a
+duplicate without exposing identity values. Missing paths and non-sequence values
+do not fail by themselves. Equivalent overlapping registrations coalesce.
+Incompatible overlaps and invalid registrations produce `options.invalid` before
+source acquisition. Unused and root registrations are allowed and have no effect.
+
+Registrations provide identity for strict composition, sequence operations,
+inheritance lookup, and final validation. Dynamic selectors apply at concrete
+strict-composition, inheritance, and final-result destinations without requiring
+the application to enumerate mapping keys. Each strict tier has independent
+duplicate-identity state among its peers; layered precedence and inheritance
+then apply the collection operation described below.
+
+Final registration checks run in both ordinary and layered loads after effective
+secret lookup and before application semantic validation. They examine actual
+final typed sequences, including values introduced by defaults, CLI, or recorded
+resolution; they do not require path existence or sequence shape. Diagnostics
+use concrete collection and item paths and available source locations, without
+exposing identity or credential values. The guarantee ends when loading publishes
+successfully and does not cover later caller mutations.
+YAML-only identity is enforced during structural composition, precedence, and
+identified inheritance; it carries no final-result guarantee after typed
+defaults, CLI, or application resolution. Applications that need that guarantee
+register the identity constraint.
+The [maintained catalog examples](consumers/README.md#identified-prompt-sequences)
+show strict peers, precedence operations, inheritance, dynamic destinations, and
+the distinction between YAML-only declarations and application registrations.
 When an enabled source-version mode is selected, declarations are checked in each
 source's own coordinates before placement and composition. Accepted non-main
 headers are consumed before strict composition and cannot supply a registered
 record identity. Final typed decoding and application validation still run on the
 completed configuration.
 
+Ordinary sources, layered inputs, and selected overlays may declare collection
+behavior under `configkit.sequences` at the document root. Each entry has a
+required literal `destination` and may specify `key` as one string or a nonempty
+list of strings, plus a `mode` of `replace`, `append`, or `upsert`. These
+controls are removed before application decoding. A declaration destination is
+relative to that source's placement, so a mounted fragment's declaration is
+mounted with its values. Declarations from strict peers must agree on identity
+fields and on any explicitly supplied mode; omission expresses no mode
+preference. They apply even when the declaring source has no value at that
+destination: a key validates an existing retained sequence or remains pending
+until records arrive, while a mode-only declaration affects later combinations
+without recombining earlier tiers. A key supplies the identity used by `append`
+and `upsert` and is checked against any application registration.
+
+Strict peers at an identified destination first combine distinct records in
+source and list order. Duplicate identities among peers or within one
+contribution fail under every mode. After strict composition, precedence and
+inheritance use one operation for the assembled incoming collection:
+
+| Mode | Behavior at source-precedence and inheritance boundaries |
+| --- | --- |
+| `replace` | Replace the complete lower sequence with the assembled higher sequence. |
+| `append` | Retain lower records and append higher records; duplicate identities fail. |
+| `upsert` | Replace matching records in full at their lower positions, retain unmatched lower records, and append new identities. |
+
+Upsert replaces the complete matched record; it does not recursively merge
+record fields. Append and upsert require identity. With no explicit mode in the
+current tier, an explicit lower mode persists; if none exists, effective identity
+selects upsert and its absence selects replace. Inferred modes do not persist.
+Explicit modes persist at their destination until changed by a higher explicit
+mode, independently of YAML identity lifetime. A declaration alone does not
+clear or recombine a collection.
+
+The presence and value kind at a precedence or inheritance boundary determine
+whether a collection changes:
+
+| Higher contribution | `replace` | `append` / `upsert` |
+| --- | --- | --- |
+| Absent | Leave the lower value unchanged. | Leave the lower value unchanged. |
+| Explicit `[]` | Clear the sequence. | Contribute no records. |
+| Explicit null or another non-sequence value | Replace the lower value, subject to application schema. | Error; the operation requires a sequence. |
+
+An absent lower value can be initialized by an incoming sequence under any mode.
+A present lower null or other non-sequence value is not absence: append and
+upsert cannot combine with it. An empty sequence still requires a valid mode and
+identity where applicable. Null used as an identity component inside a record is
+distinct from null used as the collection value.
+
+Declarations govern ordinary strict source composition, including loads with
+imports disabled, and each strict scope in layered loading. Before composing a
+higher tier's peers, ConfigKit makes applicable lower identity and explicit mode
+available; after peer composition, the tier applies its resolved operation
+against the lower collection. Selected overlays use the same collection policy
+after the base source graph is composed. During mapping-sibling inheritance, a
+parent's YAML identity can follow its collection to the child destination, while
+the parent's explicit operation stays at the parent destination. The child
+destination selects its operation; absent one, effective identity implies
+upsert and its absence implies replace. Whole-sequence replacement discards
+lower YAML identity and uses the replacing scope's key declaration, if any;
+without one, the collection is unkeyed unless an application registration still
+supplies identity. Replacement discards policies owned by removed records, while
+an independently declared explicit mode and application registration remain
+applicable. Item-local
+policies follow their owning records: retaining a record retains its rules, while
+whole-record upsert replacement discards the displaced record's rules. Matching
+identity or reusing an index does not transfer them. These policies do not change
+CLI behavior: CLI sequence assignments replace whole sequences after decoding.
+A root `configkit.imports` declaration still requires `Options.Imports`;
+`NoSource` has no declaration scope.
+
 An empty mapping contributes no child definitions. Two empty mappings compose, and
 an empty mapping neither clears nor conflicts with another mapping's children.
 Shared mapping containers retain all contributing value locations as their composite
@@ -843,7 +975,9 @@ and typed decoding. Overlay bytes, nodes, and source records count toward the sa
 aggregate limits as base inputs. Its canonical path must remain within the configured
 root and must not duplicate a file already present in the base graph.
 
-Overlay is intentionally distinct from strict composition:
+Collection destinations follow the
+[collection operation and presence rules](#strict-document-composition) above.
+The following table describes ordinary overlay behavior elsewhere:
 
 | Existing lower node | Incoming overlay node | Result |
 | --- | --- | --- |
@@ -852,18 +986,24 @@ Overlay is intentionally distinct from strict composition:
 | Mapping | Mapping | Recurse by mapping key |
 | Any other pair of present nodes | Any other pair of present nodes | Replace the entire lower node |
 
-Scalars, sequences, and null are atomic. Thus an overlay sequence replaces rather
-than extends a base sequence, `[]` clears it, null is an explicit replacement, and
-cross-kind replacement is allowed when the final application schema accepts it.
+Under these ordinary rules, scalars, sequences, and null are atomic. An overlay
+sequence replaces rather than extends a base sequence, `[]` clears it, null is an
+explicit replacement, and cross-kind replacement is allowed when the final
+application schema accepts it.
 Replacing a subtree removes all displaced descendants from effective presence and
 origin data; history retains the displaced definitions and identifies the parent
 replacement. Surviving overlay paths have overlay origins, while recursively shared
 mapping containers retain their contributing locations.
 
-An overlay may not contain `configkit` anywhere, contribute the configured schema
-version, or replace an ancestor of that version with a scalar or null. These rules
-apply even if the supplied version text would be unchanged. Overlays cannot mask a
-base parse, composition, or version failure because those checks run first.
+An overlay may contain only root `configkit.sequences` declarations; overlay
+imports and nested directives are rejected. Sequence declarations are removed
+before application decoding. At declared or registered sequence destinations,
+`replace`, `append`, and `upsert` use the same identity and policy rules as source
+composition. Other overlay sequences remain atomic replacements. An overlay may
+not contribute the configured schema version or replace an ancestor of that
+version with a scalar or null. These rules apply even if the supplied version
+text would be unchanged. Overlays cannot mask a base parse, composition, or
+version failure because those checks run first.
 
 ## Explicit CLI overrides
 
@@ -875,10 +1015,16 @@ must never contain the raw argument or configured value. General environment
 variable overlays are an explicit non-goal: environment access is reserved for
 typed secret references.
 
-Overrides apply after the selected overlay and before typed defaults/decoding, so
-the input precedence is application defaults < composed base < selected overlay <
-CLI. Recorded resolution, typed secret lookup, and semantic validation follow the
-effective override tree. Overrides also work with `NoSource`.
+Overrides are prepared after source assembly and inheritance, then assigned after
+typed defaults and decoding and before application resolution. The order is:
+defaults and decoded sources < CLI < application resolution. A CLI mapping
+recursively overlays the decoded mapping; a CLI sequence replaces the complete
+decoded sequence regardless of source collection operations. Inheritance and
+typed defaulting do not run again after CLI assignment. Earlier parsing,
+composition, inheritance, and decoding failures remain errors even if an override
+would replace the failing value. Recorded resolution, typed secret lookup, and
+semantic validation use the effective post-override values. Overrides also work
+with `NoSource`.
 
 Values are converted directly into ConfigKit's owned tree without YAML parsing or
 string coercion. Supported values are `nil`, built-in strings and booleans, all
@@ -889,7 +1035,8 @@ false, and zero remain intentional; empty sequences replace whole lists; and map
 values recursively overlay existing mappings. Map keys are sorted canonically and
 sequence order is preserved.
 
-All records are detached and validated before any is applied. Root and explicit
+All records are detached and validated before any is applied. They cannot create
+`configkit` or `extends` controls. Root and explicit
 sequence-index paths are invalid; parsed numeric tokens remain mapping keys. A path
 may create missing mapping ancestors but cannot traverse an existing scalar, null,
 or sequence. Duplicate and ancestor/descendant-overlapping records are rejected.
@@ -906,8 +1053,12 @@ unsupported types, nonfinite floats, and limit excesses fail with safe `override
 diagnostics. Diagnostics and provenance identify the safe `Flag` source, never raw
 arguments or values.
 
-Override origins are explicit CLI origins. History retains replaced base/overlay
-definitions and records equal-value replacements as well as structural changes.
+Override origins are explicit CLI origins. Resolution receives CLI values with
+their explicit-presence and source information. Absence-based defaults preserve
+those values, while recorded normalization and derivation may change them. History
+retains replaced source definitions and records equal-value replacements as well
+as structural changes. Secret lookup and validation use post-resolution values
+with their effective source locations.
 Caller-owned maps and slices are snapshotted during loading and must not be mutated
 concurrently with the call. The executable `ExampleOverride` shows translating a
 parser's explicit-set state into records while preserving an explicitly supplied
@@ -915,9 +1066,11 @@ zero.
 
 ## Explicit imports
 
-Imports are opt-in. With `Options.Imports` enabled, a document may reserve its root
-`configkit` mapping for an `imports` sequence. ConfigKit removes that mapping before
-application decoding. Each sequence item is a mapping with exactly one `path`,
+Imports are opt-in. With `Options.Imports` enabled, a document may use its root
+`configkit` mapping for `imports` and `sequences` controls. Ordinary `Load` may use
+`sequences` without enabling imports, but an `imports` field still requires
+`Options.Imports`. ConfigKit removes recognized root controls before application
+decoding. Each imports sequence item is a mapping with exactly one `path`,
 `dir`, or `glob` selector. `path` and `dir` accept a string or a list of strings;
 `glob` accepts one path pattern string. The optional `required` boolean controls
 whether an empty selection or a missing named path is an error. Its default is
@@ -927,8 +1080,8 @@ strings. Every destination string is one literal mapping key and follows the
 input-destination validation rules above; quote numeric-looking keys so YAML
 reads them as strings. Null, scalar destinations, and non-string members are
 `import.invalid`, as are all other unsupported shapes and fields. A `configkit`
-key below the root is invalid. With imports disabled, any occurrence remains
-`import.disabled`.
+key below the root is invalid. With imports disabled, root imports and nested
+`configkit` keys remain `import.disabled`.
 
 The directive key is `configkit`, not the `_configkit` spelling used in v0.1.0.
 There is no legacy alias: `_configkit` is ordinary application data, subject to
@@ -976,6 +1129,12 @@ filenames unrestricted; a non-nil empty slice selects none. Defaults do not make
 a field applicable to selectors that do not support it, and load calls snapshot
 the supplied pointers and slice before source acquisition.
 
+A source's `configkit.sequences` declarations apply independently of import
+enablement; a `configkit.imports` declaration still requires `Options.Imports`.
+Sequence identities can be supplied either by these YAML declarations or by
+application registrations. The [strict composition contract](#strict-document-composition)
+defines their agreement, operation, and lifetime rules.
+
 Acquisition visits declarations in authored order and each declaration's selected
 paths in namespace-path lexical order, preserving source-record and version-callback
 order. Ordinary mapping contributions in a main tier are assembled dependency-first:
@@ -987,9 +1146,10 @@ mapping scope, with helpers before the bodies that import them. Those local scop
 do not use application collection policies and are not flattened into the outer
 schedule. Item positions and diagnostic paths follow the resulting assembly order.
 Each underlay group likewise acquires its inputs before dependency-ready assembly
-in one strict scope. Independent groups remain separate precedence tiers; a higher
-tier still replaces a lower tier's sequence values wholesale. Assembly order never
-permits replacement.
+in one strict scope. Independent groups remain separate precedence tiers; at a
+declared or registered destination, the assembled higher sequence uses the
+resolved collection operation against the lower sequence. Other sequences retain
+whole-value replacement between tiers. Assembly order never permits replacement.
 
 Acquisition order and resolution order serve different purposes: acquisition
 assigns source records and callback order, while the shared scheduler determines
@@ -1020,13 +1180,17 @@ Each declaration may include `placement: mapping` or
 including every match of a glob; placement is not inherited. Sequence-item
 placement requires a nonempty effective destination and wraps the imported
 mapping as one element of a sequence there. It does not split source sequences
-or infer identity fields. Repeated sequences still conflict unless their path is
-registered through `Definition.SequenceCompositions`; registration does not
-change placement or precedence behavior. Sequence records retain source
-locations through placement and history; indexes describe their assembled
-positions while source coordinates continue to identify the declaring files.
-Header/version policy and final application validation retain their existing
-owners and timing; sequence registration is a strict-composition rule only.
+or infer identity fields. Repeated sequences combine only when the destination
+has an applicable YAML identity declaration or application registration; source
+placement remains independent of collection identity and operation. Sequence
+records retain source locations through placement and history; indexes describe
+their assembled positions while source coordinates continue to identify the
+declaring files.
+Header/version policy retains its existing owner and timing. Application
+registrations also enforce identity on the final effective typed configuration;
+YAML-only declarations are enforced at structural combination and inheritance
+boundaries. The [configuration contract](#strict-document-composition) defines
+these validation boundaries.
 
 An item import first opens a local mapping scope. Its body and ordinary helper
 imports compose within that record. An omitted helper destination or

docs/consumers/README.md

@@ -21,7 +21,7 @@ sanitized shapes using only ConfigKit and the Go standard library:
 | Named presets | One-load sibling inheritance, explicit false, nested destination resolution, and source/derivation inspection | Choosing and executing a preset |
 | Effective configuration diff | Same-type frozen effective comparisons for two selected files or base-versus-overlay impact | Source/options selection and consumer-owned safe presentation |
 | Shared source-version checks | One `Definition.Version` policy for accepted declarations across participating source documents, with optional or required declarations and sanitized evidence | Choosing which declarations the application accepts; final assembled schema and domain validation |
-| Identified prompt sequences | Explicit item path/glob imports, helper-supplied IDs, reusable single-field registration, and main-tier replacement of a lower-priority collection | Prompt meaning and final domain validation |
+| Sequence catalogs | Declared collection operations, record inheritance, typed CLI and resolution, inspection, exact and wildcard application registrations | Catalog meaning and final domain validation |
 
 The YAML under [`testdata`](../../examples/consumers/testdata) is loaded by the
 executable examples and ordinary tests. It contains no private application data,
@@ -184,36 +184,35 @@ import, source-location, and ownership behavior.
 
 ## Identified prompt sequences
 
-[`LoadPromptCatalog`](../../examples/consumers/prompt_catalog.go) uses one
-definition registration for the literal `prompts` path and `id` field. Its
-load-wide filename defaults accept two patterns, an explicit directory
-declaration overrides those patterns, and an exclusion omits one matching
-candidate. A nested ordinary helper import supplies an item's ID. A separate
-empty optional directory search is read from `SourceInfo.ImportSelections`.
-The executable example prints both the scheduled prompt order and the single
-empty-search outcome; the behavior test also follows a mounted resource through
-its effective metadata origin to the declaring source context. The completed
-main sequence replaces the lower-priority default collection as one value.
-The maintained [executable example and behavior tests](../../examples/consumers/prompt_catalog_test.go)
-show the scheduled result, empty-search inspection, excluded candidate, and
-resource lookup without a consumer-side discovery pass. The corresponding
-[complex fixture](../../examples/consumers/testdata/prompt-catalog/config.yml)
-and [simple required-file fixture](../../examples/consumers/testdata/prompt-catalog/simple.yml)
-are complete local examples.
+[`LoadPromptCatalog`](../../examples/consumers/prompt_catalog.go) shows the
+smaller import-focused adoption path: a registered `prompts.id` identity,
+directory and glob selection, a helper import that supplies an item's ID, and
+resource lookup through the effective source context. Its executable examples
+and tests are in
+[`prompt_catalog_test.go`](../../examples/consumers/prompt_catalog_test.go), with
+complete [catalog](../../examples/consumers/testdata/prompt-catalog/config.yml)
+and [required-file](../../examples/consumers/testdata/prompt-catalog/simple.yml)
+fixtures.
 
-[`LoadRequiredPromptCatalog`](../../examples/consumers/prompt_catalog.go) and
-[`Example_requiredPromptCatalog`](../../examples/consumers/prompt_catalog_test.go)
-keep the smaller adoption path visible: one explicitly required file import,
-the same identity registration, and the same application validation.
+[`LoadSequenceCatalog`](../../examples/consumers/sequence_catalog.go) integrates
+an underlay declaration, imported main fragments, identified-record inheritance,
+selected overlay `append`, `upsert`, and `replace`, a typed CLI override, and
+recorded application resolution. Its tests and runnable example demonstrate
+stable output order, whole-record upsert, provenance/history inspection,
+resource lookup for inherited records, and secret-safe explanation in
+[`sequence_catalog_test.go`](../../examples/consumers/sequence_catalog_test.go).
+[`LoadNamespacedMapCatalog`](../../examples/consumers/sequence_catalog.go) adds
+an exact root registration and a wildcard namespace registration, showing that
+each concrete sequence has an independent identity scope. The same tests show
+that a YAML identity declaration without an application registration does not
+create final-result identity enforcement.
 
-The example's `validatePromptCatalog` remains application-owned. It checks that
-prompt names and message lists satisfy this consumer's requirements after
-ConfigKit has composed, projected, and decoded the configuration. It does not
-add recursive record merging, identity-based overlay, or final uniqueness checks
-after defaults and resolution. See the [strict composition contract](../config.md#strict-document-composition),
-[import syntax](../config.md#explicit-imports), and
-[ADR 0014](../adr/0014-sequence-placement-and-composition.md) for the exact
-boundaries.
+The examples keep prompt meaning and domain checks in the consuming application.
+The [configuration contract](../config.md#strict-document-composition) owns
+declaration, identity, operation, ownership, and final-validation semantics;
+[sibling inheritance](../config.md#sibling-inheritance) defines record
+relationships, and the [CLI contract](../config.md#explicit-cli-overrides) owns
+override ordering.
 
 Known module catalogs should use canonical object entries with a module identifier
 and typed options. A small union struct is sufficient when the available option

docs/development.md

@@ -135,12 +135,13 @@ Current behavior belongs to the configuration contract's
 [input destinations](config.md#input-destinations),
 [imports](config.md#explicit-imports), and
 [shared source-version checks](config.md#shared-source-version-checks) sections.
-Explicit sequence-item placement and registered strict composition are owned by
-the [strict composition](config.md#strict-document-composition) and
-[imports](config.md#explicit-imports) sections. The
-[maintained prompt catalog](consumers/README.md#identified-prompt-sequences)
-shows the current consumer boundary, and [ADR 0014](adr/0014-sequence-placement-and-composition.md)
-records the decision.
+Explicit sequence-item placement, collection operations, identity registration,
+and final registered-identity validation are owned by the
+[strict composition](config.md#strict-document-composition) section;
+[imports](config.md#explicit-imports) owns acquisition and scheduling. The
+[maintained sequence catalog](consumers/README.md#identified-prompt-sequences)
+shows the consumer boundary, and [ADR 0017](adr/0017-collection-sequence-policy.md)
+records the collection decision.
 
 The completed sequence-composition roadmap, implementation plan, and resolved
 remediation record are preserved in Git history at `008d2ba` under
@@ -166,6 +167,21 @@ The subsequent import-integration audit and completed remediation record are
 preserved at `6903c9e:docs/roadmap/remediation.md`; retrieve them with
 `git show 6903c9e:docs/roadmap/remediation.md`.
 
+The completed sequence-identity/collection-policy roadmap and implementation
+plan are preserved in Git history at `d0a605a`:
+
+```sh
+git show d0a605a:docs/roadmap/sequences.md
+git show d0a605a:docs/roadmap/implementation.md
+```
+
+Current behavior belongs to the configuration contract's
+[strict composition](config.md#strict-document-composition),
+[sibling inheritance](config.md#sibling-inheritance), and
+[CLI overrides](config.md#explicit-cli-overrides) sections.
+The completed remediation record, including the algorithm-audit findings, is
+preserved at `ea46001:docs/roadmap/remediation.md`.
+
 Potential enhancements are tracked in [future features](roadmap/future.md).
 
 Install the Go version required by `go.mod` or a compatible newer toolchain.
@@ -218,6 +234,7 @@ code changes. Use the component overview to find the corresponding implementatio
 | Import defaults, empty-selection evidence, and dependency-ready ordering | [Defaults and facade evidence](../facade_import_defaults_test.go), [engine import evidence](../internal/engine/import_evidence_test.go), [scheduler](../internal/engine/dependency_order_test.go), [scheduling integrations](../internal/engine/imports_test.go) | `go test ./internal/engine -run 'Import|ScheduleDependencies|Inheritance'`, `go test . -run 'ImportDefaults|FacadeImport'`, and `go test ./examples/consumers` |
 | Native filesystem and mapping sources | [Native acquisition](../internal/engine/native_filesystem_test.go), [filesystem facade](../facade_filesystem_test.go), [mapping facade](../facade_mapping_test.go) | `go test ./internal/engine -run 'NativeFilesystem'` and `go test . -run 'TestFacade'` |
 | Composition, selected overlays, CLI precedence | [Composition](../internal/engine/compose_test.go), [overlay](../internal/engine/overlay_test.go), [selected overlays](../internal/engine/selected_overlay_test.go), [overrides](../internal/engine/override_test.go) | `go test ./internal/engine -run 'Compose\|Overlay\|Override'` |
+| Sequence identity, declarations, and operations | [Identity](../internal/engine/sequence_identity_test.go), [registrations](../internal/engine/sequence_registration_test.go), [declarations](../internal/engine/sequence_declaration_test.go), [combination](../internal/engine/sequence_combine_test.go), [final facade validation](../facade_final_sequence_identity_test.go), and [consumer catalog](../examples/consumers/sequence_catalog_test.go) | `go test ./internal/engine -run 'Sequence\|Collection'`, `go test . -run 'SequenceIdentity\|SequenceRegistration'`, and `go test ./examples/consumers` |
 | Sibling inheritance, provenance, history, sensitivity | [Expansion](../internal/engine/inheritance_test.go), [destination scopes](../internal/engine/inheritance_walk_test.go), [origins](../internal/engine/inheritance_origin_test.go), [history](../internal/engine/inheritance_history_test.go), [secrets](../internal/engine/inheritance_secret_test.go), [public loading](../facade_inheritance_test.go) | `go test ./internal/engine -run 'Inheritance\|Inherited'` and `go test . -run '^TestFacadeInheritance\|^TestFacadeInheritedSecrets'` |
 | Layered loading and source equivalence | [Grouped lifecycle](../internal/engine/load_request_test.go), [layered facade](../facade_layered_test.go), [equivalence](../facade_equivalence_test.go) | `go test ./internal/engine -run '^TestGrouped'`, `go test . -run '^TestFacadeLayered'`, and `go test . -run '^TestEquivalentSourceForms'` |
 | Input and relative-import destinations, mounted sources, and source ownership | [Destination engine tests](../internal/engine/destination_test.go), [tier tests](../internal/engine/destination_tiers_test.go), [import traversal and version tests](../internal/engine/destination_import_test.go), [accounting tests](../internal/engine/destination_accounting_test.go), [lifecycle inspection tests](../internal/engine/destination_import_lifecycle_test.go), [public source/error tests](../facade_destination_test.go), [resource contexts](../facade_resource_context_test.go), and [consumer examples](../examples/consumers/destinations_test.go) | `go test ./internal/engine -run 'Destination|Import|Glob|Mounted'`, `go test . -run 'Destination|Import|SourceContext|FacadeMounted'`, and `go test ./examples/consumers` |
@@ -230,6 +247,7 @@ code changes. Use the component overview to find the corresponding implementatio
 | Effective configuration diff | [Engine comparison and masking tests](../internal/engine/diff_test.go), [engine operation tests](../internal/engine/diff_operation_test.go), [root facade tests](../facade_diff_test.go), and [consumer examples](../examples/consumers/diff_test.go) | `go test ./internal/engine -run 'Diff|Explain|History|Secret|Independent'`, `go test . -run 'Diff|FacadeInspection|PublicFacadeBoundary|DependencyImportBoundary'`, `go test ./examples/consumers`, and `go test . -run '^ExampleDiff$'` |
 | Consumer examples and migration fixtures | [Consumer tests](../examples/consumers/consumers_test.go), [native, layered, and ordinary relative-import examples](../examples/consumers/layered_test.go), [mounted collection examples](../examples/consumers/destinations_test.go), [API examples](../example_test.go) | `go test ./examples/consumers` and `go test . -run '^Example'` |
 | Assembly and layered scaling evidence | [Assembly benchmarks](../internal/engine/assembly_bench_test.go), [layered benchmarks](../internal/engine/underlay_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkUnderlay' -benchmem`; repeat with `^BenchmarkCompose` and `^BenchmarkOverlay` |
+| Identified-sequence scaling evidence | [Mixed-upsert benchmark](../internal/engine/sequence_combine_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkSequenceMixedUpsert$' -benchmem` |
 | Inheritance scaling evidence | [Expansion benchmarks](../internal/engine/inheritance_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkInheritance' -benchmem` |
 | Documentation links and dependency isolation | [Link checks](../documentation_test.go), [import boundary](../import_boundary_test.go), [public boundary](../public_boundary_test.go) | `go test . -run 'TestLocalDocumentationLinks\|TestDependencyImportBoundary\|TestPublicFacadeBoundary'` |
 | Development check behavior | [Development checker test](../development_check_test.go) | `go test . -run '^TestDevelopmentCheckHandlesFilenamesAndStopsOnFailure$'` |

docs/internal/imports.md

@@ -65,11 +65,18 @@ returns a closed cycle rotated to its smallest key; blocked dependents are not
 reported as cycle members.
 
 Import composition schedules a main tier or one underlay group as a single
-strict-scope graph. Item contributions own nested local strict-mapping scopes;
-their helpers complete before the item is placed in its outer scope. Inheritance
-uses the same ready-node rule inside each sibling mapping scope, while preserving
-the authored mapping order in its output. These scopes do not flatten into one
-global graph, and independent precedence tiers remain separate.
+strict-scope graph. Applicable collection declarations are gathered for the
+scope before peer composition, including identity inherited from lower tiers.
+Identified peer sequences then combine distinct records in assembled source and
+list order; after the strict scope completes, layered assembly applies the
+resolved collection operation against the lower value, as defined by the
+[configuration contract](../config.md#strict-document-composition). Item
+contributions own nested local strict-mapping scopes; their helpers complete
+before the item is placed in its outer scope. Inheritance uses the same
+ready-node rule inside each sibling mapping scope, while preserving the authored
+mapping order in its output.
+These scopes do not flatten into one global graph, and independent precedence
+tiers remain separate.
 
 The main owners are [`import_selection.go`](../../internal/engine/import_selection.go),
 [`directory_walk.go`](../../internal/engine/directory_walk.go),

docs/internal/overview.md

@@ -9,12 +9,13 @@ Implemented component owners:
 
 - `internal/engine` owns host and native-filesystem source acquisition,
   grouped strict composition, recursive precedence assembly, per-input mapping
-  and sequence-item placement, definition-registered distinct-ID sequence
-  composition within strict tiers, and immediate-parent-relative import
-  placement within that assembly. Higher-priority sequences replace lower
-  sequences as whole values. The engine also owns dependency-ready,
-  destination-local sibling inheritance before typed decoding, plus shared
-  source-version checking with sanitized declaration evidence.
+  and sequence-item placement, YAML collection declarations and operations for
+  identified sequences, and application identity registrations for strict
+  composition, inheritance, and final validation. It applies the selected
+  collection operation at precedence boundaries and owns dependency-ready,
+  destination-local inheritance before typed decoding. CLI assignment follows
+  inheritance and decoding, before application resolution. The engine also owns
+  shared source-version checking with sanitized declaration evidence.
   Explicit import discovery supports declared paths, directory searches, and
   globs; load-wide defaults and source-local empty-selection evidence cross the
   root facade without changing acquisition order. Import and inheritance
@@ -56,9 +57,10 @@ semantics, including [input destinations](../config.md#input-destinations),
 [shared source-version checks](../config.md#shared-source-version-checks), and
 [effective configuration diffs](../config.md#effective-configuration-diffs).
 The [strict composition contract](../config.md#strict-document-composition)
-owns sequence placement, registration, replacement, and history behavior; the
-[consumer guide](../consumers/README.md#identified-prompt-sequences) shows the
-maintained example.
+owns sequence declarations, identity, operations, ownership, and final
+validation; [CLI overrides](../config.md#explicit-cli-overrides) own override
+ordering. The [consumer guide](../consumers/README.md#identified-prompt-sequences)
+links maintained workflow examples.
 The [consumer adoption guide](../consumers/README.md) owns application guidance.
 The [architecture policy](../policy/architecture.md) owns contributor
 boundaries; [ADR 0006](../adr/0006-public-facade.md) records the facade decision
@@ -74,5 +76,7 @@ records the source-version decision.
 sequence placement and composition. [ADR 0015](../adr/0015-shared-dependency-ready-ordering.md)
 records the shared dependency-ready ordering decision; [ADR
 0016](../adr/0016-independent-import-controls.md) records the independent
-selection controls. The [import internals](imports.md) document candidate and
+selection controls. [ADR 0017](../adr/0017-collection-sequence-policy.md) and
+[ADR 0018](../adr/0018-cli-after-structural-resolution.md) record collection
+operations and CLI ordering. The [import internals](imports.md) document candidate and
 namespace identities, discovery and scheduling, and the bounded algorithms.

docs/policy/architecture.md

@@ -32,11 +32,11 @@ structure as implemented when its changes land.
 Constraints must reflect deliberately selected requirements and decisions.
 Do not promote incidental implementation choices into architectural invariants.
 
-The collection-policy and CLI-ordering requirements below reflect accepted
-[ADR 0017](../adr/0017-collection-sequence-policy.md) and
-[ADR 0018](../adr/0018-cli-after-structural-resolution.md). The
-[sequence roadmap](../roadmap/sequences.md) tracks the work to implement them;
-the configuration contract continues to describe implemented behavior.
+The implemented collection-policy and CLI-ordering constraints below reflect
+accepted [ADR 0017](../adr/0017-collection-sequence-policy.md) and
+[ADR 0018](../adr/0018-cli-after-structural-resolution.md). The configuration
+contract owns their current behavior; these architectural constraints describe
+the system boundaries and guarantees that guide changes to that behavior.
 
 ## Success Criteria
 
@@ -373,8 +373,8 @@ as records survive, move, or are replaced. Identity diagnostics use paths and
 available origin information without exposing identity values. The guarantee
 requires no new mutation-history system or exact reconstruction of causes for
 records supplied by defaults or callbacks. Exact identity equality, declaration
-syntax, presence cases, and operation tables belong in the configuration contract
-when implemented; the roadmap owns those details until then.
+syntax, presence cases, and operation tables belong in the
+[configuration contract](../config.md#strict-document-composition).
 
 ## Ownership And Confidentiality
 

docs/releases/v0.9.0.md

@@ -0,0 +1,111 @@
+# ConfigKit v0.9.0
+
+This minor release expands import selection, adds author-declared collection
+identity and operations, and supports inheritance between identified records.
+It also moves CLI assignments after inheritance and typed decoding. These changes
+include breaking API and configuration behavior changes from v0.8.0; review the
+migration guidance below before upgrading.
+
+## Changes
+
+- Extend imports with named-file and directory lists, directory filename patterns,
+  exclusions, recursion and depth controls, and explicit hidden-file and symlink
+  discovery controls. Add `ImportDefaults` to ordinary and layered options, with
+  authored declarations taking precedence over application defaults. Imports are
+  optional by default. `SourceInfo.ImportSelections` exposes completed optional
+  declarations that selected no sources. See the
+  [import contract](../config.md#explicit-imports) and
+  [consumer guidance](../consumers/README.md#import-defaults-and-selection-inspection).
+- Use shared dependency-ready ordering for import composition and sibling
+  inheritance. Dependencies complete before dependents, with deterministic
+  selection among currently eligible contributions. Source acquisition and
+  callback order remain separate from assembly order. See the
+  [ordering decision](../adr/0015-shared-dependency-ready-ordering.md).
+- Add root `configkit.sequences` declarations for collection identity and
+  `replace`, `append`, or `upsert` operations, including in selected overlays.
+  Strict peers combine distinct identities and reject duplicates; precedence
+  and inheritance apply the destination's collection operation. Upsert replaces
+  a whole matching record at its retained position. Identity supports compound
+  field sets and scalar strings, numbers, booleans, and null. See the
+  [collection contract](../config.md#strict-document-composition).
+- Add `Definition.SequenceIdentities`, exact `SequenceIdentityAt` registrations,
+  and dynamic `SequenceIdentityMatching` selectors. Application registrations
+  govern structural collection operations and check existing matching final
+  typed sequences, including values from defaults, CLI assignments, and
+  application resolution. YAML-only declarations govern structural operations
+  without adding that final-result check.
+- Support record inheritance within an identified sequence using an `extends`
+  mapping containing the parent's complete identity. The child retains its own
+  identity, relationships use surviving assembled records, and output sequence
+  order is preserved. See [sibling inheritance](../config.md#sibling-inheritance)
+  and the maintained [sequence catalog](../consumers/README.md#identified-prompt-sequences).
+- Apply explicit CLI overrides to the decoded configuration before application
+  resolution. Mappings retain omitted fields and their presence metadata;
+  sequences replace whole lists independently of collection operations. See the
+  [CLI contract](../config.md#explicit-cli-overrides).
+- Correct native import alias and contained-symlink handling, source-version
+  path projection, and provenance/history through collection movement,
+  replacement, inheritance, and CLI assignment. Reduce repeated scans,
+  accumulated policy copying, and sparse-projection storage during construction.
+
+## Compatibility and upgrade
+
+This pre-v1 minor release intentionally changes the v0.8.0 contracts:
+
+1. **Migrate import requiredness.** The YAML `optional` field is removed and is
+   rejected. Replace `optional: true` with `required: false`; omission is equivalent
+   when the application does not default imports to required. Replace
+   `optional: false` with `required: true`. Imports that previously relied on
+   implicit requiredness now need `required: true`, or an application
+   `ImportDefaults.Required` default set to true. Review discovery filters:
+   `follow` and `hidden` default to true, and directory searches select all
+   filenames unless a pattern is supplied.
+2. **Replace sequence registrations.** `SequenceComposition` and
+   `Definition.SequenceCompositions` are removed without compatibility aliases.
+   For a previous registration at `prompts` using `id`, use:
+
+   ```go
+   rules.SequenceIdentities = []configkit.SequenceIdentity{
+       configkit.SequenceIdentityAt(configkit.Keys("prompts"), "id"),
+   }
+   ```
+
+   These registrations now also enforce identity in final typed sequences. Fix
+   missing or duplicate identities introduced by defaults, CLI assignments, or
+   application resolution. Registration does not require a collection to exist
+   or determine its allowed shape; schema and application validation retain
+   those responsibilities.
+3. **Review collection precedence and ordering.** An identified collection now
+   defaults to upsert across layers and inheritance when no explicit mode
+   applies, rather than unconditional whole-sequence replacement. Declare
+   `mode: replace` at the relevant destination where replacement is intended.
+   With append or upsert, `[]` contributes no records and does not clear the
+   collection; null is invalid. Use `mode: replace` with `[]` to clear it.
+   An explicit mode persists at its destination until changed; a parent's mode
+   does not transfer to a different inheriting destination. Unidentified
+   sequences retain strict peer conflicts and whole-value precedence
+   replacement. Dependency-ready assembly can change sequence positions and
+   diagnostic or history order compared with the former traversal order.
+4. **Review CLI recovery and inheritance assumptions.** CLI overrides can no
+   longer repair values that fail earlier decoding, bypass source or inheritance
+   errors, or add or cancel inheritance relationships. Supply structural
+   directives in source configuration. Application resolution still receives
+   explicit CLI values and may record subsequent normalization or derivation.
+5. **Update affected Go literals and diagnostic handling.** Use keyed literals
+   for expanded option and source-information structs. Collection validation adds
+   `CodeSequenceInvalid` (`sequence.invalid`) and `CodeSequenceConflict`
+   (`sequence.conflict`); update diagnostic handling where applicable.
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.9.0
+```
+
+The consumer minimum remains Go 1.26.0. The YAML dependency remains
+`go.yaml.in/yaml/v4 v4.0.0-rc.6`, and the maintainer tool dependency remains
+ReleaseKit v0.4.1. Linux amd64 and arm64 remain release-validated targets;
+macOS remains best-effort. Publication follows the existing
+[release procedure](../release.md).
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.9.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.9.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.9.0/docs/oss/attribution.md)

docs/roadmap/complexity.md

@@ -0,0 +1,222 @@
+# Internal Complexity Refactoring Roadmap
+
+Status: proposed target state. This roadmap defines a behavior-preserving
+refactor; a later implementation plan will define stages and exact private
+signatures and file names.
+
+## Purpose
+
+Make ConfigKit's import, inheritance, and projection implementation easier to
+understand, review, and change while preserving its public APIs and configuration
+behavior. Concentrate on functions that coordinate several responsibilities and
+carry substantial shared state.
+
+The target is clearer responsibility and ownership. Adding a few well-named
+helpers, private records, or focused files can serve that target even when total
+line count increases. Complexity measurements help locate candidates; they do
+not establish function-size limits, score thresholds, or release gates.
+
+The [architecture policy](../policy/architecture.md) owns package and ownership
+constraints. The [configuration contract](../config.md) and exported Go comments
+remain authoritative for consumer behavior.
+
+## Current Baseline
+
+Reviewed baseline: `7a83704f43ef9870db33661231cdff5d6ee502f7`.
+
+The root facade delegates to a cohesive private engine, supported by the ordered
+document representation and YAML adapter. Most functions are small. The
+complexity audit identified these concentrated maintenance costs:
+
+| Area | Current implementation | Required improvement |
+| --- | --- | --- |
+| Imports | `imports.go` spans acquisition, item-local composition, and evidence projection. Private loading variants pass long positional argument lists. Source admission and directory selection contain substantial mixed control flow. | Give import entry points explicit, cohesive inputs and separate the responsibilities of acquisition, scope assembly, and evidence projection. |
+| Inheritance | `applyInheritanceScopeTemplates` coordinates payload preparation, parent combination, policy ownership, origins, history, and accounting in one executor. | Keep one readable executor with focused operations and explicit state handoffs. |
+| Projection | `valueOperation.project` combines type dispatch with struct flattening, map and sequence traversal, scalar conversion, sensitivity, and accounting. | Retain clear dispatch while moving substantial value-kind handling into focused helpers. |
+
+The implemented inventory belongs to the
+[component overview](../internal/overview.md). Existing import flow and scheduling
+are described in [import internals](../internal/imports.md).
+
+## Target Architecture
+
+The refactor stays within `internal/engine`. The root facade, `internal/document`,
+and `internal/yaml` retain their existing responsibilities and dependency
+direction. Focused files and ordinary private functions express the new internal
+organization. Package extraction is outside this roadmap's selected design.
+
+`executeLoadRequest` remains the visible coordinator of the shared load lifecycle.
+Its phase order should remain easy to follow. Helpers expose meaningful work and
+results rather than distributing that order across a generic execution framework.
+
+Existing shared mechanisms remain shared: recursive mapping combination,
+identified-sequence operations, dependency-ready scheduling, schema descriptors,
+and projection/accounting state. New helper boundaries make their ownership and
+use clearer without creating parallel implementations.
+
+### Imports: Cohesive Inputs And Scope Ownership
+
+Import loading has a clear private entry surface using named request/context
+fields for related inputs. Reuse the existing load-request, acquisition-scope,
+and traversal models where they fit. Consolidate forwarding variants whose only
+purpose is passing another optional positional argument through the chain.
+Retain convenience helpers when they express a useful, distinct operation.
+
+Keep these state lifetimes identifiable:
+
+- Load-wide state: accounting, shared inclusion/repetition information,
+  namespaces, and source-version processing.
+- Acquisition requests: source and filesystem context, placement, declarations,
+  applicable policy, and the relevant assembly handoff.
+- Composition scopes: peer contributions, item-local helpers, record ownership,
+  and pending declarations.
+- Evidence: physical source records, effective destinations, origins, and retained
+  history projections.
+
+Group acquisition, pending-item assembly, and evidence projection in focused
+files. A caller should be able to identify which operation acquires resources,
+which composes already acquired content, and which projects retained evidence.
+Shared mutable state has a visible owner; a request record need not duplicate
+state already held by that owner.
+
+Within dense admission, directory-selection, and declaration-parsing routines,
+extract coherent operations where that reduces nested, mixed responsibilities.
+Preserve the existing selection algorithms and host/native filesystem behavior.
+Resource ownership and closure remain explicit across success and failure paths.
+
+The resulting flow preserves the distinction among acquisition order, strict
+scope scheduling, and precedence. Completing a helper scope before placing its
+item remains part of the same import flow. Ordinary and layered loading continue
+to share the applicable mechanisms.
+
+### Inheritance: One Executor With Explicit Phases
+
+Keep the mapping and identified-sequence adapters and their shared template
+executor. The executor coordinates focused operations for:
+
+- Preparing child payload, directives, and destination-local state.
+- Preparing the resolved parent payload and applicable inherited policy.
+- Combining values through the existing mapping and sequence mechanisms.
+- Producing policy, origin, replacement, and replay evidence from that combination.
+- Accounting for expansion and retained history under the existing budgets.
+
+These are responsibility boundaries, not a required helper count or a new
+scheduler. Small private result records are appropriate when related outputs
+otherwise travel through long argument lists or implicit mutations.
+
+Authored templates, resolved values, scoped policy, and inspection evidence remain
+distinguishable. Immutable borrowed inputs and owned mutable results have clear
+lifetimes. Nested instructions remain available for destination-local evaluation.
+For identified-record inheritance, the child's own identity fields and origins
+remain intact; the parent's identity fields are excluded from inherited payload
+and its history, as the current contract requires.
+
+One sequence-operation result continues to drive value construction, record
+ownership, origins, and historical position projection. Helpers consume that
+evidence directly. Scheduling remains dependency-ready, enclosing relationships
+resolve before nested ones, and assembled sequence order remains unchanged.
+
+### Projection: Explicit Value-Kind Handlers
+
+Keep `projectConfiguration` as the schema-checked entry and `valueOperation` as
+the owner of per-operation traversal and accounting state. The central dispatch
+clearly selects the appropriate handling for pointers, dynamic values, structs,
+maps, sequences, and scalars.
+
+Substantial struct, map, and sequence work lives in focused handlers. Struct
+projection retains declaration order, inline flattening, collision diagnostics,
+and sensitivity propagation. Map projection retains deterministic key ordering.
+Scalar conversion retains the existing number, duration, text-codec, and secret
+reference representations.
+
+Depth and node accounting retain their existing meaning, including flattened
+containers. Cycle tracking stays active through recursive child traversal and
+is balanced on error paths as well as success. Passing state through a helper
+does not introduce another full-tree projection or an unnecessary ownership copy.
+
+Cloning and source-value construction remain distinct operations with their
+existing semantics. Reuse small helpers when the semantics coincide; keep the
+projection refactor centered on making the existing traversal readable.
+
+## Behavior And Scope Preservation
+
+The implementation preserves:
+
+- Root-owned public types, signatures, supported import paths, and facade boundaries.
+- Supported source forms, opt-ins, import controls, placement, acquisition and
+  composition ordering, and source/version evidence.
+- Strict peer composition, precedence operations, identity agreement and lifetime,
+  record ownership, and destination-local inheritance.
+- The load lifecycle, including typed defaults, post-decode CLI assignment,
+  recorded application resolution, and final validation.
+- Explicit presence, provenance, history, sensitivity, resource contexts, and
+  detached publication/query results.
+- Established diagnostic codes, semantic paths, source coordinates, ordering,
+  resource lifetime, and existing limit behavior.
+
+The owning contracts define the extent of these obligations. Incidental private
+call sequences, exact helper names, and allocation counts do not acquire new
+guarantees through this roadmap. The work adds no consumer options, configuration
+syntax, dependencies, or complexity-based gates.
+
+## Relationship To Remediation And Other Work
+
+The completed remediation record at `ea46001:docs/roadmap/remediation.md`
+documents the identified correctness and algorithmic efficiency findings.
+Several overlap these files, particularly item-scope application matching and
+inheritance policy localization.
+
+Use the documented contract and completed remediation as the behavioral baseline.
+Keep corrective changes reviewable under their findings, and retain their
+regressions through this refactor. The later implementation plan should coordinate
+overlapping work and select its starting revision rather than duplicating the
+remediation plan here. These structural changes do not close a finding merely
+because its function moved or became smaller.
+
+The pending [discovery roadmap](discovery.md) remains separate feature work.
+Import cleanup should leave clear private boundaries for future use without
+implementing discovery behavior as part of this refactor.
+
+## Work Needed And Validation
+
+Implementation needs to consolidate import inputs and adapters, separate import
+responsibilities, extract inheritance operations with explicit outputs, and
+decompose projection handling. Remove superseded private plumbing as its callers
+migrate. Review the resulting call flow for repeated traversals, policy scans,
+copies, or hidden state transitions introduced by extraction.
+
+Reuse the existing behavioral suites under the
+[development guide](../development.md) and [testing policy](../policy/testing.md):
+
+- Imports: selection, scheduling, source/version evidence, placement, helper scopes,
+  ordinary/layered equivalence, filesystem failures, and resource lifetime.
+- Inheritance: mapping and record relationships, nested destination resolution,
+  cancellation and replacement, policy ownership, history, sensitivity, and
+  independent loads.
+- Projection: schema/decoding agreement, inline fields, text codecs, nil/empty
+  values, cycles and budgets, secrets, and detached results.
+
+Add focused regressions only where a consequential handoff risk lacks coverage.
+Tests should continue to describe behavior as private functions and files move.
+Use the existing everyday check and applicable ownership race checks during
+implementation. Complexity measurements can inform review; acceptance does not
+depend on numerical scores or a new benchmark campaign.
+
+Update the component overview and import internals to describe the implemented
+ownership and file responsibilities when the refactor lands. Update contributor
+navigation and test-owner references where moves affect them. Public behavior
+continues to be defined by its current canonical owners.
+
+## Completion Criteria
+
+- Import callers pass cohesive named inputs, with clearly owned load-wide and
+  scope-local state and fewer redundant forwarding variants.
+- Import acquisition, item assembly, and evidence projection are easy to locate
+  and follow across focused private files.
+- Inheritance orchestration exposes value, policy, evidence, and accounting work
+  without requiring readers to reconstruct their interaction from one large body.
+- Projection dispatch and substantial value-kind handlers have clear boundaries
+  and share the original operation state correctly.
+- Existing APIs and contracted behavior, including completed remediation, remain
+  covered by the relevant behavioral and ownership checks.
+- Internal documentation matches the resulting implementation.

docs/roadmap/discovery.md

@@ -0,0 +1,381 @@
+# Composable Configuration Discovery Roadmap
+
+Status: proposal with OD-1 through OD-5 and the final review clarifications
+settled and integrated below.
+This document describes future work, not an implemented contract. A staged
+implementation plan will follow review of the complete roadmap.
+
+## Purpose And Ownership
+
+Make ordinary file-based loading concise for applications that want conventional
+configuration locations. ConfigKit supplies useful built-in candidate lists and
+an ordered selection operation. Consumers continue to own main-file search
+conventions: they choose which lists apply, their order, custom candidates, and
+whether a configuration file is required.
+
+A consumer can select one preset for a simple application or compose several
+presets and custom path lists. The application supplies an explicit slug where
+needed to construct conventional paths. ConfigKit does not infer application
+identity from the executable name or enable discovery for existing explicit
+sources.
+
+Discovery selects one source for the existing loading lifecycle. It does not
+create additional configuration tiers or merge every matching file. Existing
+layered loading remains available when consumers intentionally combine sources.
+
+## Current Baseline
+
+The [source contract](../config.md#sources) provides explicit host files, native
+filesystem files, bytes, readers, mappings, and `NoSource()`. A missing explicit
+file fails. The zero source is invalid. There is no conventional main-file search.
+
+[Ordinary loading](../config.md#single-source-loading) infers a selected overlay's
+filesystem context from the main source. `NoSource()` currently rejects enabled
+imports, active overlays, and filesystem context. In
+[layered loading](../config.md#layered-loading), an absent main input can coexist
+with underlays; a selected overlay requires an explicit `OverlayFilesystem`.
+These distinctions matter when an optional search finds nothing.
+
+The [architecture policy](../policy/architecture.md#responsibility-boundaries)
+assigns search conventions to consumers and currently excludes implicit main-file
+discovery. The addition should clarify that consumers may explicitly select
+library-provided search conventions. Consumer ownership remains unchanged.
+
+## Agreed Direction
+
+### Filename Groups, Directory Searches, And Search Presets
+
+Organize discovery around three composable descriptions. These are ordered lists:
+their order determines precedence.
+
+**Filename groups** provide reusable, ordered filename templates. The built-in
+groups are:
+
+| Group | Filenames, in order |
+| --- | --- |
+| Named | `<appname>.yml`, `<appname>.yaml` |
+| Generic | `config.yml`, `config.yaml`, `main.yml`, `main.yaml` |
+
+`<appname>` is the application slug supplied by the consumer.
+
+**Directory searches** pair a directory with an ordered list of filename groups.
+The default selection is `[Named, Generic]`, producing the six filenames above
+in that order. A directory search may select different groups or reverse their
+precedence, such as `[Generic, Named]`, while retaining each group's internal
+order. Consumers can also supply custom filenames, subject to the literal
+single-component rules under OD-5 below.
+
+**Search presets** bundle ordered directory searches. Consumers can select a
+complete preset for the simple case or compose presets, custom directory searches,
+and explicit file candidates in their desired order. Explicit file candidates
+are literal paths and receive no filename expansion.
+
+### Ordered Expansion And Concatenation
+
+The selection model is **ordered expansion and concatenation**, followed by
+first-match selection:
+
+1. Expand the selected presets into directory searches in their supplied order.
+2. For each directory search, concatenate its selected filename groups in order,
+   substitute the application slug where needed, and join each filename to that
+   directory.
+3. Concatenate the resulting candidate lists, preserving the positions of custom
+   directory searches and explicit file candidates.
+4. Try candidates in order, stopping at the first selected file or an error as
+   described below.
+
+Directory priority comes before filename priority: exhaust one directory's
+filenames before trying the next directory. When several directories share a
+filename list, their expansion is a Cartesian product traversed directory first.
+Different directories may select different lists, so ordered expansion and
+concatenation describes the complete model. Set union does not express its
+precedence semantics.
+
+The eventual documentation should explain this model using the applicable
+mathematical nomenclature: ordered lists, ordered concatenation, and Cartesian
+product where a shared filename list applies. Retain these names in relevant
+implementation comments where they help readers understand the operations. The
+model describes ordering without requiring eager construction of every candidate.
+
+### Unix System Preset
+
+The agreed Unix system preset is:
+
+| Directory, in search order | Filename groups, in order |
+| --- | --- |
+| `/usr/local/etc/<appname>` | `[Named, Generic]` |
+| `/etc/<appname>` | `[Named, Generic]` |
+| `/etc` | `[Named]` |
+
+This produces 14 candidates: six in each application-specific directory, then
+`/etc/<appname>.yml` and `/etc/<appname>.yaml`. For example,
+`/usr/local/etc/<appname>/main.yaml` precedes
+`/etc/<appname>/<appname>.yml`. Selecting only `Named` for `/etc` is an ordinary
+directory-search description; the selection algorithm needs no special exception.
+
+### Additional Presets (OD-1 — Settled)
+
+Provide the following independently selectable presets alongside Unix system.
+Within each preset, search directories in the order shown:
+
+| Preset | Directory | Filename groups, in order |
+| --- | --- | --- |
+| User configuration | `<user-config-dir>/<appname>` | `[Named, Generic]` |
+| User configuration | `<user-config-dir>` | `[Named]` |
+| Home directory | `<home>/.<appname>` | `[Named, Generic]` |
+| Home directory | `<home>` | `[Named]` |
+| Working directory | Current working directory | `[Named]` |
+
+Resolve the user-configuration base with
+[`os.UserConfigDir`](https://pkg.go.dev/os#UserConfigDir), the home base with
+[`os.UserHomeDir`](https://pkg.go.dev/os#UserHomeDir), and the working-directory
+base with [`os.Getwd`](https://pkg.go.dev/os#Getwd). Adopt the Go helpers'
+platform-specific directory and environment behavior, including their errors;
+the settled OD-2 rules below govern how discovery handles resolution errors.
+
+The goal is to support familiar conventions through idiomatic Go. This feature
+does not add a separate XDG compatibility layer or XDG system preset. A selected
+user-configuration base has no additional hard-coded `<home>/.config` fallback.
+The Home directory preset is a separate, explicit choice.
+
+The working-directory preset searches only that directory and uses application-named
+files; it does not walk ancestor directories. The consumer chooses precedence
+between all presets. Selecting one does not automatically select any others.
+
+### Illustrative Consumer API
+
+Names below are illustrative and are not implemented Go API:
+
+```go
+// A simple system-wide application.
+source := configkit.SearchConfig("myapp", configkit.UnixSystemDirectories())
+```
+
+```go
+// A consumer-selected working-directory, user, then system search.
+source := configkit.SearchConfig(
+    "myapp",
+    configkit.WorkingDirectory(),
+    configkit.UserConfigDirectory(),
+    configkit.UnixSystemDirectories(),
+)
+```
+
+```go
+// Explicit custom candidates followed by a selected conventional fallback.
+source := configkit.SearchConfig(
+    "myapp",
+    configkit.Paths("/run/myapp/config.yml", "/opt/myapp/config.yaml"),
+    configkit.UnixSystemDirectories(),
+)
+```
+
+Use the resulting source with ordinary loading or as a layered main input. Custom
+searches and file lists participate at their specified position; they do not
+silently append unselected presets. Reordering presets or their directory
+searches changes search priority.
+
+### Selection And Failure Behavior
+
+| Situation | Selected behavior |
+| --- | --- |
+| Candidate is absent | Continue to the next candidate. |
+| A candidate file is selected | Stop searching and load that file. |
+| Candidate is inaccessible, unreadable, or not an accepted file type | Fail using normal source diagnostics. |
+| Selected file fails parsing, version checks, composition, decoding, or validation | Fail without trying another candidate. |
+| Entire search has no match | Absence is permitted by default; follow the shared absent-main lifecycle settled under OD-3 below. |
+| Entire search has no match and is required | Fail with a diagnostic explaining the unsuccessful search. |
+| Caller explicitly uses `File(path)` | Retain existing exclusive-file behavior, including errors for a missing file. |
+
+Requiredness applies to the complete search, not to each preset or directory.
+Permitting an absent file does not waive application version, schema, or domain
+requirements; the remaining lifecycle may still reject an incomplete configuration.
+
+Perform discovery before parsing the chosen main file. Do not implement fallback
+by repeatedly running complete loads. Defaults, application resolution, secret
+lookup, and final validation retain their existing lifecycle execution rules.
+Once a file has been selected, later failure does not restart discovery.
+
+### Construction And Filesystem Behavior
+
+Constructors describe the search without filesystem access. Resolve dynamic base
+directories and access candidate files during loading, so reusable descriptions
+can observe changes on subsequent loads. Do not change the process working
+directory or environment.
+
+Reuse the host-file acquisition rules for selected files, including regular-file
+checks, symlink handling, resource ownership, and source limits. The feature does
+not require a new filesystem traversal engine, recursive directory search, or
+new file-size limits. Candidate filenames and groups are bounded by their
+explicit descriptions, rather than by scanning unrelated directories.
+
+The exact Go names, constructors, and representation of search-wide options
+belong in the implementation plan. Keep the public surface root-owned and keep
+the single-preset case concise.
+
+### Unavailable Bases And Platform Applicability (OD-2 — Settled)
+
+Resolve directory bases only as the ordered search reaches them. Once an earlier
+candidate is selected, later searches do not resolve their bases. Static invalid
+search arguments still fail ordinary option validation before source acquisition.
+An explicitly empty filename selection skips directory resolution as specified
+under OD-4; it has no candidates to locate.
+
+If a selected directory's base cannot be resolved, stop and report the error.
+A missing directory or candidate file beneath a resolved base allows the search
+to continue; a directory-resolution error does not. This distinction also applies
+to optional searches: permitting no match does not suppress errors encountered
+while searching. Preserve the agreed errors for inaccessible or unreadable
+candidates.
+
+Treat using the Unix system preset on an unsupported platform as an explicit
+configuration error when that preset is reached. Specify its supported platforms
+in the implementation plan. Do not translate the Unix preset into a different
+platform's convention. User, home, and working-directory presets use their native
+Go directory helpers.
+
+This keeps search priority predictable when directory resolution fails. Consumers
+select the presets appropriate to their application and environment; this proposal
+does not add an option to skip unavailable bases or unsupported presets.
+
+### No Match, Imports, And Selected Overlays (OD-3 — Settled)
+
+Use a shared absent-main lifecycle for optional no-match and explicit `NoSource()`.
+Allow imports to be enabled when the main file is absent: there are no main-file
+import declarations to process, and enablement does not introduce automatic
+directory scanning. Other actual sources retain their normal import behavior.
+In layered loads, process underlays normally and retain the existing explicit
+`OverlayFilesystem` requirement for an active overlay.
+
+For ordinary loads with an active overlay and no main file, require an explicit
+filesystem context to locate the overlay. Do not infer an overlay root from the
+first or last searched directory. A missing selected overlay remains an error;
+absence of a main file does not silently disable a requested overlay.
+
+For an absent main input, a host filesystem context specifying only `Root` is
+sufficient to locate the overlay; no main-file `BaseDir` is needed. If discovery
+selects a host file, retain the normal file-context rules: the selected file
+determines its base directory, an explicit `BaseDir` is rejected, and the file
+must be contained within any supplied root. This allows one root-only context
+to work with both selected and absent main inputs, subject to those existing
+root and containment checks. An invalid context is an error, not a reason to
+resume discovery.
+
+Defaults, CLI overrides, application resolution, and validation continue through
+the normal lifecycle. Main-version checking receives an absent main input and
+may reject it under the application's existing policy. Create no synthetic empty
+document or source record for the absent main file.
+
+This deliberately relaxes the current `NoSource()` restrictions on imports and
+on overlays with explicit context. Both ways of obtaining an absent main input
+have the same behavior. Implementation must cover this lifecycle change as well
+as candidate selection.
+
+### Empty Composition And Implicit Preset Selection (OD-4 — Settled)
+
+Require explicit preset or search selection to activate a convention. Supplying
+no presets or custom searches produces no candidates and activates no implicit
+fallback. Empty custom path lists and explicitly empty filename-group selections
+also contribute no candidates. An empty component does not prevent later
+components from contributing candidates.
+
+A directory search with an explicitly empty filename selection does not resolve
+its directory base. Static argument validation still applies. This gives empty
+selections their no-candidate meaning without introducing directory-resolution
+errors for a search that cannot select a file.
+
+If the complete search is empty, an optional search follows the absent-main
+lifecycle settled under OD-3; a required search fails as exhausted. Empty lists
+are valid, supporting consumers that construct their searches programmatically.
+
+Distinguish omission from an explicitly empty filename selection for a directory
+search. Omitting the selection uses the agreed `[Named, Generic]` default;
+explicitly selecting an empty list means no filenames. The eventual API must
+represent both intentions clearly. Presets retain their specified filename groups.
+
+Selecting a single preset keeps the simple case concise. Search-wide requiredness
+provides the check for consumers that must find a configuration file.
+
+### Slugs And Custom Path Interpretation (OD-5 — Settled)
+
+Require a nonempty single-component application slug whenever a selected search
+uses it to construct a directory or filename. Reject path separators, `.` and
+`..`, and forms unusable as that component on the target platform. Preserve
+spelling and case; do not trim, lowercase, or impose a kebab-case naming scheme.
+Validation keeps the slug from changing the intended directory structure without
+imposing a separate naming style.
+
+Searches that do not use the slug do not require one. Examples include explicit
+file candidates and a custom directory searched with only Generic filenames.
+
+Treat custom paths as literal host paths. Resolve relative paths against the
+working directory at load time. Do not expand shell variables, `~`, or wildcard
+syntax. Consumers can construct expanded paths themselves; User and Home presets
+provide the common directory-resolution conveniences. These rules keep custom
+paths consistent with explicit file loading.
+
+Custom filenames within directory searches are literal single filename
+components. They cannot be absolute paths or contain subdirectory components;
+use custom directory searches or explicit file candidates for those layouts.
+They receive no application-name placeholder expansion. The Named group supplies
+application-name expansion, while consumers may construct custom filenames
+themselves before supplying them.
+
+### Placement, Provenance, And Existing Features
+
+The selected file participates as the actual file source. Preserve its physical
+identity, effective placement, source origin, and passive resource context.
+Relative imports and relative application resources use the selected file's
+normal context. Discovery does not relocate overlay or CLI coordinates.
+
+Missing candidates are not acquired sources and do not create source IDs,
+configuration history, or explicit input presence. An entirely absent search
+contributes no synthetic empty configuration file. Diagnostics should distinguish
+an exhausted required search from a failed candidate. Existing selected-source
+inspection remains the foundation; a new public per-attempt trace is not needed
+for this proposal.
+
+Preserve collection policies, inheritance, source-version rules, CLI ordering,
+final validation, and frozen inspection after selection. Search-list order does
+not change precedence among underlays, the main input, an overlay, and CLI values.
+An explicitly selected overlay is not another main-file fallback candidate.
+
+## Work Needed From The Baseline
+
+The subsequent implementation plan should cover:
+
+- Define filename-group, directory-search, preset, and search-source representations,
+  slug validation, and a search-wide requiredness option. Keep construction
+  separate from acquisition and the simple preset selection concise. Distinguish
+  omitted and explicitly empty directory-search filename selections.
+- Implement ordered expansion and concatenation for presets, filename groups,
+  custom searches, and explicit file candidates, followed by first-match selection
+  through existing file access. Resolve each source once per consumption before
+  entering its parse and composition lifecycle.
+- Integrate selected and absent outcomes with ordinary and layered main inputs,
+  placement, imports, application version checking, and overlay contexts according
+  to OD-3, including root-only host contexts for absent main inputs and normal
+  file-context checks when a file is selected. Preserve ownership and close
+  acquired handles on success and failure.
+- Preserve real selected-source metadata and safe diagnostics without treating
+  absent candidates as configuration contributions.
+- Exercise preset, directory, filename-group, and filename ordering, reversed
+  group precedence, the Unix system preset's 14 candidates and named-only `/etc`
+  search, custom-list composition, both filename extensions, no-match behavior,
+  empty components and searches, omitted versus empty filename selections,
+  skipped directory resolution for empty filename selections, slug-dependent and
+  slug-independent searches, literal custom paths and filename components,
+  required searches, unavailable preset bases, errors that must stop fallback,
+  repeated loads, and untouched explicit `File`
+  behavior. Cover imports, overlays, underlays, defaults, CLI, source versions,
+  source contexts, and source-list/inspection evidence at their boundaries.
+- Clarify the architecture policy's consumer-selected discovery boundary. Record
+  any warranted durable ownership decision in an ADR; keep preset filenames and
+  API mechanics in their canonical contract when implemented. Update consumer
+  examples and the configuration contract when the feature lands. Explain the
+  three-part discovery model and its mathematical terminology in the eventual
+  documentation, with simple preset selection and custom composition examples.
+
+This roadmap does not change implementation, policy, or ADRs by itself. Coordinate
+implementation with the ongoing sequence integration audit rather than assuming
+its in-progress changes are part of a fixed implementation baseline.

docs/roadmap/future.md

@@ -6,112 +6,19 @@ commitments. Adopting a candidate requires an explicit design decision and a
 separate plan. Significant architectural exclusions and their rationale belong
 in ADRs; this document does not maintain a list of rejected proposals.
 
-## 1. Identified record sequences
+## 1. Recursive body merging for identified records
 
-Status: deferred. PromptKit proposed this capability from its retired proposal 2,
-**Namespaced Collections As Identified Records**. The original proposal is in the
-PromptKit repository at
-`b5866dd:docs/roadmap/configuration-collections-identified-records.md`.
-PromptKit selected mounted mappings instead; this feature is not a dependency
-of that design or a request to restore multiple installed prompt versions.
+Status: deferred. The implemented
+[collection policy](../config.md#strict-document-composition) uses `upsert` to
+replace a complete matched record. Recursive merging of fields inside matching
+records remains a separate candidate and would revisit the whole-record update
+boundary selected by [ADR 0017](../adr/0017-collection-sequence-policy.md).
 
-A narrower [sequence placement and composition feature](../config.md#strict-document-composition)
-is implemented: consumers can place imported mappings as sequence items and
-register a single literal string identity field for duplicate-rejecting strict
-composition. Precedence still replaces sequences as whole values. This entry
-tracks broader identified-record behavior that remains deferred and is not part
-of that implemented contract.
-
-The active [sequence roadmap](sequences.md) now selects a narrower design for
-collection policy, compound scalar identities, final application-registered
-identity validation, and inheritance within identified sequences. It owns those
-selected behaviors and their remaining decisions. The original broader proposal
-below remains background for further consideration, particularly recursive
-combination of matching record bodies; it does not override the active roadmap.
-
-### Motivation and proposed behavior
-
-Allow applications to declare that selected sequences contain records identified
-by explicit fields, while retaining sequence-shaped output for slice decoding
-and effective YAML. This could make composite identities and independently
-authored catalogs more convenient than encoding identity in nested mapping keys.
-Current [composition](../config.md#strict-document-composition) appends distinct
-records only at explicitly registered paths; other sequences remain atomic.
-Selected [overlays](../config.md#selected-overlays) always treat sequences
-atomically.
-
-The proposed broader opt-in registration would extend the current reusable
-application definition from one identity field to a composite identity made of
-an ordered, nonempty set of distinct direct string-field names. For example,
-records at `profiles` could be identified by `(namespace, id)`. Records
-`(provider, brief)`
-and `(application, brief)` would coexist, and a later layer could update the
-provider record's temperature while retaining its backend and model.
-
-The requested semantics, all still subject to design review, are:
-
-- Match exact string tuples, preserving component boundaries and spelling,
-  including empty strings. Every contribution supplies its complete identity;
-  defaults, inheritance, filenames, and normalization do not supply it.
-- Retain distinct identities. Matching records from strict peers recursively
-  compose disjoint body fields and conflict on overlapping non-mapping body
-  values, even equal values. Equal identity fields act as structural keys rather
-  than conflicts and retain their contributing source evidence.
-- Higher layers recursively overlay matching bodies. A changed identity denotes
-  a different record, not a rename. Proposed ordering is first contribution in
-  precedence-assembly order; matching records keep position and new ones append.
-- Reject malformed identities, non-mapping records, non-null non-sequence values
-  at registered paths, and duplicate tuples within one source sequence. Later
-  layers cannot repair malformed structural identity in an acquired contribution.
-- Treat an empty registered sequence as contributing no records; whole-path null
-  retains ordinary conflict/replacement semantics. Unregistered sequences,
-  including nested message lists, retain existing atomic behavior.
-- Support record inheritance within the same registered sequence using an
-  `extends` mapping containing the complete parent tuple. Resolve only final
-  active relationships after precedence, preserve the child's identity, overlay
-  its body on the parent's body, and consume the directive before decoding.
-  Preserve cancellation, missing-parent/cycle errors, and declaring resource
-  contexts. Existing string-based mapping-sibling inheritance stays unchanged.
-
-Integrate ordinary and layered loads, imports, mounted paths, selected overlays,
-and whole-sequence CLI overrides. Keep origins, presence, history, sensitivity,
-and resource contexts attached to the correct record despite different source
-and final indexes. Diagnostics must retain source locations without exposing
-sensitive identity values. Final inspection may use snapshot-specific sequence
-indexes; existing atomic sequence diffs need not become identity-aware.
-
-Successful results would have valid, unique identities, including after defaults
-and application resolution, without a second merge or inheritance pass. Existing
-source-version checks remain independent; consumed headers cannot supply missing
-record identities. No identity-selector path syntax, arbitrary merge callbacks,
-wildcard registration, per-record deletion/rename operator, multiple inheritance,
-or cross-path parent references are requested.
-
-### Questions to resolve before implementation
-
-- **Need and delivery scope:** does an active consumer benefit enough to justify
-  the added semantics over keyed mappings? Are composite identities required?
-  Staging composition before inheritance is possible but would leave part of
-  the request incomplete.
-- **Defaults and callbacks:** do default records merge by identity or retain
-  existing sequence-replacement behavior? How are callback-created duplicates,
-  identity edits, and reordering validated and reflected in provenance?
-- **Null, empty, and order:** what happens when null clears a collection and a
-  later layer reintroduces records? How do absent and explicitly empty sequences
-  interact with defaults and first-contribution order?
-- **Inheritance references:** how do peers compose relationship declarations,
-  and how are complete references enforced without assembling partial tuples
-  accidentally? What happens if identity fields overlap structural controls?
-- **Registration paths:** how do overlapping registrations and paths beneath
-  sequence indexes behave when assembly changes those indexes?
-- **Evidence and lifecycle:** how are record identities mapped between source
-  trees, history, inheritance, callback snapshots, and final indexes without
-  mixing records' provenance or sensitivity? Where do identity checks run relative
-  to header consumption and other structural processing?
-
-Keep the feature deferred until its consumer value and full lifecycle design
-justify reconsidering the sequence-merging exclusion. A correct final sequence
-alone would not establish correct inspection or disclosure behavior.
+Consider this only in response to a concrete consumer need. A proposal would need
+to define conflicts for overlapping body fields, removal of omitted fields,
+presence and ordering, and how provenance, history, sensitivity, and resource
+context follow partially retained records. Any change requires a new decision and
+implementation plan; this entry does not alter current upsert behavior.
 
 ## 2. Required version declarations for selected sources
 

docs/roadmap/implementation.md

@@ -1,894 +0,0 @@
-# Sequence Identity And Combination Implementation Plan
-
-Status: ready for implementation; no stages completed.
-
-Audience: a Luna coding agent implementing one numbered stage per prompt, in
-order. This plan starts at commit
-`4f941f8ba4c9a9ed0afa87b07fe6ba29f131d1c6`. It implements the settled
-[sequence roadmap](sequences.md), [ADR 0017](../adr/0017-collection-sequence-policy.md),
-and [ADR 0018](../adr/0018-cli-after-structural-resolution.md).
-
-## Working Rules
-
-- Read [development guidance](../development.md) and the applicable policies,
-  especially [agents](../policy/agents.md), [architecture](../policy/architecture.md),
-  [testing](../policy/testing.md), and [documentation](../policy/documentation.md).
-  Prefer the repository knowledge graph for code discovery.
-- The roadmap owns the selected feature semantics. This plan supplies execution
-  order and implementation choices, not additional product requirements. Preserve
-  unchanged source, parser, version, schema, ownership, and inspection contracts.
-- Complete each stage's implementation and behavioral tests together. Record its
-  completion and validation here, then commit the nonempty unit with a short,
-  plain-English message. Preserve unrelated changes. Do not publish intermediate
-  stages or a release, update consumers' dependencies, or wait for CI.
-- Run focused tests while iterating and `./scripts/check.sh` before handing off
-  each code stage. Use the installed compatible Go toolchain with dependencies
-  already acquired, as the development guide describes. Add race coverage at the
-  ownership milestones below. These are implementation checks, not new release
-  gates. Reuse successful checks for unchanged content rather than repeating them
-  at a documentation-only milestone. Do not require human platform validation.
-- Update affected declaration comments and canonical current-behavior prose when
-  behavior lands. Keep incomplete target behavior in this roadmap/plan until it
-  works. The final documentation stage reconciles the cross-feature narrative;
-  it is not permission to leave misleading current contracts in earlier stages.
-- Transitional internal adapters are acceptable where specified. Remove them by
-  completion. Do not add public compatibility aliases, permanent feature flags,
-  alternate semantics, new dependencies, new resource caps, or an extension/plugin
-  framework for this work.
-- Preparation stages keep existing public behavior working and test new machinery
-  privately. Activation stages integrate values, policy, metadata, and relevant
-  failure handling together; do not defer correct inspection to a later stage.
-
-## Implementation Decisions
-
-### Public Registration Surface
-
-Replace `SequenceComposition` and `Definition.SequenceCompositions` with:
-
-```go
-type SequenceIdentity struct { /* private, root-owned registration state */ }
-type SequenceSelector []SequenceSelectorSegment
-type SequenceSelectorSegment struct { /* private literal-or-any-mapping-key state */ }
-
-func SequenceLiteralKey(key string) SequenceSelectorSegment
-func SequenceAnyMappingKey() SequenceSelectorSegment
-func SequenceIdentityAt(path Path, fields ...string) SequenceIdentity
-func SequenceIdentityMatching(selector SequenceSelector, fields ...string) SequenceIdentity
-
-// Field on Definition[T]:
-// SequenceIdentities []SequenceIdentity
-```
-
-For example, the reusable definition can contain both registrations:
-
-```go
-SequenceIdentities: []configkit.SequenceIdentity{
-    configkit.SequenceIdentityAt(configkit.Keys("prompts"), "id"),
-    configkit.SequenceIdentityMatching(configkit.SequenceSelector{
-        configkit.SequenceLiteralKey("collections"),
-        configkit.SequenceAnyMappingKey(),
-        configkit.SequenceLiteralKey("prompts"),
-    }, "id"),
-},
-```
-
-The exact constructor converts mapping-only `Path` segments into literal selector
-segments. Preserve invalid index segments long enough to report an ordinary
-definition error; do not panic, silently drop them, or change `Path` semantics.
-The zero selector is the exact root selector, harmless under the existing
-mapping-root schema. A zero selector segment denotes the literal empty key,
-not a wildcard. Empty field names are valid literal field names; an empty field
-set is invalid. Constructors snapshot slices; load-entry conversion also takes
-an owned snapshot. Definition validation, not construction, reports errors.
-Root types and functions must not expose internal types or bridge methods.
-
-### Identity Representation And Matching
-
-- Normalize identity field sets once: deduplicate and sort exact field names.
-  Use the same normalizer for YAML and application registration.
-- Encode a record's identity as a hashable, type-tagged, length-delimited tuple in
-  normalized field order. Mapping lookup indexes avoid scanning record fields
-  repeatedly. Distinguish structural-payload lookup from final typed-payload and
-  parent-reference lookup: only fields acting as directives are excluded, not
-  every occurrence of the same literal spelling. Never include tuple encodings
-  in diagnostics.
-- Normalize finite JSON numbers to sign, significant decimal digits, and a signed
-  base-10 exponent. Strip insignificant zeroes and give all zero spellings one
-  representation. Use `math/big.Int` for exponent arithmetic, not a power-of-ten
-  expansion or a float conversion. Thus large exponent spellings do not allocate
-  storage proportional to their mathematical magnitude. This comparison-only
-  representation does not rewrite document nodes or typed projection.
-- Compile selectors into a mapping-segment trie with literal and wildcard edges.
-  Carry applicable trie states through traversal; stop at sequence-index edges.
-  Index concrete matches within one assembly/inheritance scope where useful, but
-  do not save an assembly-only destination list for final validation.
-- Check registration overlap pairwise once during definition validation using
-  equal length and compatible corresponding segments. This straightforward
-  `O(R² × D)` check, for R registrations and maximum selector depth D, is
-  proportionate for registration definitions. Equivalent constraints coalesce;
-  conflicting overlaps fail even with no matching input.
-  Do not impose a registration-count limit to simplify this operation.
-
-### Policy And Evidence Storage
-
-Keep application constraints separate from YAML policy. Add private engine
-collection-policy records containing normalized identity, explicit mode, and
-declaration locations. Track whether identity is pending or belongs to a retained
-collection. Retain declaration presence separately from inferred defaults.
-
-Use the existing acquisition/item scopes as ownership anchors, with an opaque
-per-load owner token when a retained record needs one. Store item-local policies
-relative to their owning record, and project concrete paths when that record is
-placed. Do not key ownership solely by numeric index, identity tuple, or source
-filename: all three can coincide for a replacement that must lose old policies.
-Inheritance clones the relevant scoped state; it does not create registrations.
-
-Resolve policies through shared engine helpers used by strict composition,
-precedence, and inheritance. Do not attach engine policy to public values or add
-policy fields to the parser-independent `document.Node`. The sidecar can be
-extended with concrete-path indexes; it need not become a general event store.
-
-Sequence combination returns the resulting list plus explicit input-to-output
-item projections and retained/replaced contribution information. Use that same
-result for values, policy transfer, origins, source destinations, sensitivity,
-and historical replay. Extend the existing sequence-history descriptors for
-noncontiguous upsert positions rather than inferring them from equal values.
-Sharing immutable internal evidence is acceptable under existing ownership rules;
-independently returned mutable records must remain detached.
-
-### Diagnostics
-
-Use existing `options.invalid` for invalid application definitions and existing
-`compose.invalid` / `compose.conflict` for malformed or duplicate strict-peer
-contributions. Add root-owned and engine diagnostic constants
-`CodeSequenceInvalid` (`sequence.invalid`) and `CodeSequenceConflict`
-(`sequence.conflict`) for malformed collection declarations/operations or final
-identities, and conflicting policy / precedence duplicates respectively.
-Final duplicate identities use `sequence.conflict`. Keep existing inheritance
-invalid/missing/cycle codes for record relationships. Use existing override and
-schema/decode codes at the CLI boundary; do not invent a new error taxonomy there.
-
-Report collection/item paths and available locations, not identity values,
-normalized tuple strings, raw parent references, or raw hook errors. Declaration
-conflicts include both sites. Definition overlap errors identify registration
-positions without fabricating a concrete matched configuration path. Exact
-English diagnostic wording is not a new guarantee.
-
-## Stage 1 — Implement Shared Scalar Identity Primitives
-
-Scope: new private identity helpers and their focused tests; no load-path change.
-
-1. Add `internal/engine/sequence_identity.go`. Implement field-set normalization,
-   scalar tuple construction, and exact decimal normalization as chosen above.
-   Accept string, number, boolean, and explicit null nodes; distinguish categories
-   and absence. Exclude fields acting as structural directives from payload lookup.
-2. Validate mapping records, complete direct-field sets, and uniqueness using one
-   tuple-to-first-record index. Return paths and source locations, not values.
-   Support caller-selected diagnostic families for strict and other boundaries.
-3. Cover equivalent field sets; empty strings and null; missing/non-scalar fields;
-   literal punctuation; compound tuple boundaries; `1`/`1.0`/`1e0`; signed zero;
-   close unequal decimals; and large exponents without expansion. Verify that
-   node text remains unchanged. Include `extends: null` as a structural field
-   that cannot supply source identity.
-
-Validation: `go test ./internal/engine -run 'SequenceIdentity|SequenceNumber'`
-and the everyday check. Retain the old registration path until its adapters are
-replaced; do not generalize unrelated YAML scalar decoding.
-
-## Stage 2 — Compile Exact And Dynamic Application Constraints
-
-Scope: private registration model, selector matching, overlap validation.
-
-1. Extend `sequence_registration.go` with the common registration representation
-   and compiled selector trie. Represent literal and arbitrary-mapping-key segments
-   structurally, not with a magic string. Snapshot registration inputs.
-2. Validate field sets and mapping-only selectors before source acquisition.
-   Preserve existing destination validation without adding restrictions on identity
-   field spelling. Root and unused selectors remain harmless.
-3. Implement full-destination matching, equivalent overlap coalescing, and
-   incompatible-overlap errors. Exact rules have no priority over dynamic rules.
-   Keep a temporary conversion from the current single-field registration so
-   existing callers still use their current behavior.
-4. Test exact and wildcard matches, multiple wildcard levels, empty/punctuation
-   keys, sequence-index nonmatches, disjoint lengths/literals, equivalent sets,
-   incompatible overlaps with absent input, and independent uniqueness scopes.
-
-Validation: focused registration/selector tests and the everyday check. This
-stage does not export the replacement API or add final-result validation yet.
-
-## Stage 3 — Parse Collection Declarations Without Filesystem Coupling
-
-Scope: private parser and directive extraction helpers, initially unit-tested
-without activating collection combination in public loads. Keep source-form
-activation out of this stage; ordinary sources and selected overlays are wired
-in Stages 13 and 14 respectively.
-
-1. Add `sequence_declaration.go` for source-root `configkit.sequences`. Each entry
-   has a required existing literal `destination` form and optional `key` and
-   `mode`. `key` is a string or nonempty string list; `mode` is exactly replace,
-   append, or upsert. Use shared identity normalization. A destination-only entry
-   can be a harmless no-op; do not add an unrequested ban on empty declarations.
-2. Split common root-directive extraction from import-specific parsing in
-   `import_spec.go` / `parse.go`. Extract imports and sequences together, retaining
-   source-local locations and explicit presence. Reject unrecognized directive
-   fields and wrong supplied field types using existing strict parsing style.
-3. Keep declaration destinations relative to source placement, with item-local
-   coordinates retained until their scope is assembled. Do not project them
-   prematurely to a guessed numeric index.
-4. Give extraction an explicit overlay context: sequences are allowed, imports
-   are not. Import enablement remains separately enforced; do not turn on
-   filesystem access because a source declares sequences. Keep enabled
-   source-version checks in raw source coordinates before consuming controls;
-   extend existing actual header/control-overlap handling to sequence directives
-   without treating an absent header beneath a control as an actual overlap.
-5. Test key-only, mode-only, combined, repeated-field-set equivalence, no-op,
-   malformed types/modes/destinations, mixed imports/sequences, and overlay rules.
-   Do not add import-level key/mode shorthand or sequence wrappers.
-
-Validation: focused declaration/parser tests, existing import-spec tests, and the
-everyday check. Existing import-only behavior remains intact during preparation.
-
-## Stage 4 — Reconcile Declarations And Select Collection Operations
-
-Scope: pure agreement and operation-selection helpers, independent of loading
-and of the ownership transformations introduced next.
-
-1. Add `collection_policy.go`. Gather the current scope's declarations before
-   combining peers. Reconcile equivalent keys and agreeing explicit modes;
-   conflicting peers fail. Omitted fields have no preference. Apply authoritative
-   application constraints even for key-only declarations without a value.
-2. Implement shared operation selection: current explicit mode, retained explicit
-   mode, inferred upsert with effective identity, otherwise replace. Persist only
-   explicit modes. Policy-only declarations must not replay earlier combinations.
-3. Accept eligible lower policy as an input; do not decide record survival or
-   ancestor removal inside declaration reconciliation. Retain declaration
-   locations/presence separately from inferred mode and application constraints.
-4. Test equivalent/different field sets, agreeing/conflicting peer modes,
-   omission, explicit default modes, and operation inference. Include key-only
-   declarations agreeing or conflicting with application constraints before a
-   sequence exists.
-
-Validation: focused policy-agreement/selection tests and the everyday check.
-Do not activate collection combination or implement the ownership layer here.
-
-## Stage 5 — Track Policy Lifetime And Record Ownership
-
-Scope: private scoped-state transformations built on Stage 4's resolution helpers.
-
-1. Implement key agreement across retained values, replacement reset, pending
-   key declarations, and immediate validation when a key-only declaration
-   identifies retained records.
-   Changing mode to replace without a replacement value does not reset identity.
-2. Implement scope/record ownership transfer and retirement hooks. Actual ancestor
-   replacement removes descendant collection identity and removed-record policy;
-   independently owned modes and application constraints survive as specified in
-   the roadmap. Do not repeatedly reapply old declarations and resurrect reset
-   identity. Pending declarations retain their owning-scope rules.
-3. Use item/acquisition scope anchors and record-owned relative policy as selected
-   above. Provide transformations for retention, whole-record replacement,
-   whole-sequence replacement, and inheritance copies without wiring them into
-   public loading yet. Application registrations remain destination constraints.
-4. Use table-driven tests derived from the roadmap's lifetime, persistence, and
-   replacement tables, including absent/non-sequence destinations, equal outer
-   identities with distinct record owners, and independent root versus item policy.
-
-Validation: focused collection-policy tests and the everyday check. This stage
-must not add path-existence, shape, nullability, or global YAML final-check rules.
-
-## Stage 6 — Implement The Shared Sequence Operations
-
-Scope: a pure document-level combination helper, not separate implementations
-for overlays and inheritance.
-
-1. Add `sequence_combine.go`, accepting lower/higher sequence nodes, resolved
-   policy, and combination context. Reuse Stage 1 identity extraction and indexes.
-2. Implement replace, duplicate-rejecting append, and whole-record upsert. Upsert
-   retains matched lower positions and appends new higher identities in order.
-   Return item projections and replacement/retention evidence with the new value.
-3. Keep peer validation distinct: duplicate identities within any contribution or
-   among peers fail under every mode. Do not merge record payloads or sort by ID.
-4. Cover absent versus null/non-sequence lower values; higher omission, `[]`, null,
-   and non-sequence values; missing identity under append/upsert; empty inputs;
-   stable ordering; equivalent/different keys; and explicit replacement identity
-   reset. An empty input does not bypass kind checks or identity requirements.
-5. Verify clones do not alias mutable input nodes or policy slices. Build indexes
-   once per operation rather than scanning the accumulated list for every record.
-
-Validation: focused combination tests and the everyday check. Do not change the
-ordinary recursive mapping rule or add unkeyed append.
-
-## Stage 7 — Extend Sequence History Representation And Replay
-
-Scope: represent and replay sequence operations without yet changing all metadata
-projection consumers or public precedence.
-
-1. Extend `sequenceHistoryContribution`, sequence replay in `history.go`, and
-   its contribution indexes to consume Stage 6's explicit item mappings. Existing
-   append ranges may remain a compact case; upsert cannot be represented by one
-   offset.
-2. Preserve authored raw contribution snapshots and displaced values. A replaced
-   record's omitted fields must appear as removed, not as retained lower payload.
-   Preserve equal-valued replacement events. Keep existing index-based query and
-   atomic sequence-diff APIs; add no public identity-selector path syntax.
-3. Exercise synthetic append/upsert/replace histories, noncontiguous matches,
-   nested paths, and discarded descendants. Compare replayed final values with
-   the actual combination result, including first/last and unchanged neighboring
-   records. Reuse existing escaped-path and detached-query tests rather than
-   duplicating them.
-
-Validation: sequence/history/replay tests and the everyday check. Keep current
-append/replace callers working through the extended descriptors.
-
-## Stage 8 — Project Sequence Origins And Inspection Evidence
-
-Scope: use the prepared item mappings consistently in metadata consumers.
-
-1. Project composition origins, nested source destinations, historical paths, and
-   sensitivity through Stage 6's item mapping and Stage 7's history descriptors.
-   Physical source IDs/locations and resource contexts continue to identify the
-   declaring source. Reuse one projection mechanism rather than searching again
-   by identity or equal payload.
-2. Test synthetic noncontiguous upserts, replaced descendants, nested source
-   destinations, unchanged neighbors, and secret-marked replaced records. Verify
-   projected queries agree with replayed and actual final values, retaining
-   displaced evidence and detached results.
-3. Keep the new precedence operations unactivated until their value, policy, and
-   projection machinery are integrated in the later load-path stages.
-
-Validation: metadata/history/sequence/provenance/sensitivity tests,
-`go test -race ./internal/engine`, and the everyday check.
-
-## Stage 9 — Separate Per-Input Acquisition From Composition
-
-Scope: a behavior-preserving acquired-input handoff; grouped underlay coordination
-is the next stage.
-
-1. In `load_input.go`, `imports.go`, and related graph helpers, retain acquisition
-   scopes, raw contributions, inclusion edges, source records,
-   contexts, and extracted declarations without irrevocably composing them.
-   Reuse existing `deferAssembly` and acquired-scope mechanisms instead of adding
-   a second loader. Keep already acquired document handles closed as before.
-2. Separate acquisition from the per-input composition entry. Retain pending
-   item/helper scope boundaries, namespace identities, inclusion/repetition checks,
-   source records, accounting, and scheduler inputs. Do not reacquire sources to
-   compose them later or change grouped coordination yet.
-3. Preserve existing version checks for this refactor. Retain a small adapter
-   invoking current composition semantics at the existing lifecycle point.
-   Preserve source-version callbacks at acquisition, including header consumption.
-4. Run existing acquisition, dependency scheduling, item placement, source-version,
-   ordinary/layered equivalence, resource-lifetime, and source-context tests. Add
-   only targeted regressions for any newly split handoff not already covered.
-
-Validation: the focused suites above and the everyday check. This refactor must
-not change source IDs or turn acquisition order into precedence order.
-
-## Stage 10 — Separate Underlay-Group Acquisition From Composition
-
-Scope: behavior-preserving group coordination in `load_request.go`, using the
-per-input handoff already prepared in Stage 9.
-
-1. Separate acquiring an underlay group from scheduling/composing its acquired
-   scopes. Preserve namespace reanchoring, shared inclusion/repetition state,
-   item-local helper scopes, source/context aggregation, and group accounting.
-2. Keep authored acquisition order: main and depth-first imports, each underlay
-   and input, overlay, then synthetic CLI records. Preserve underlay indexes and
-   current version-check timing in this refactor; policy-dependent layered
-   composition is activated only in Stage 15.
-3. Expose the common scope-composition handoff with inputs for accumulated lower
-   policy and compiled application constraints. Initially adapt it to the current
-   semantics. Do not add another loader or repeat acquisition/version callbacks.
-4. Test the split with existing grouped/dependency/version/destination coverage,
-   including empty inputs/groups and repeated sources within versus across tiers.
-   Confirm unchanged values, source ordering, contexts, and failure evidence.
-
-Validation: grouped/layered, import scheduling, version, destination, and
-resource-context tests plus the everyday check.
-
-## Stage 11 — Compose Strict Collections With Resolved Identity
-
-Scope: the policy-aware strict composition core. Nested pending-item scope
-integration is deferred to Stage 12, not absorbed into this prompt.
-
-1. Extend `composeDocumentsWithPolicy` and `validateRegisteredSequences` to use
-   normalized field sets and scalar tuples. Resolve current declarations,
-   applicable application constraints, and eligible lower identity before peer
-   composition, using the Stage 4 agreement and Stage 5 lifetime helpers.
-2. Use the existing dependency-ready order for contributions and authored list
-   order. Validate every identified sequence, including singleton contributions;
-   strictly append distinct peers and reject duplicates under all modes.
-3. For explicit replacement, do not use obsolete lower YAML identity to authorize
-   otherwise unkeyed peer composition. Still apply independently registered
-   application constraints and genuinely applicable declarations.
-4. Attach Stages 7–8 evidence and Stage 5 scoped policy to composed results. Test
-   completed-document contributions with lower-established identity, declaration
-   conflicts, compound/scalar IDs, singleton validation, and replacement resets.
-
-Validation: composition and sequence identity tests plus the everyday check.
-Keep unregistered non-mapping conflicts unchanged. Public load activation remains
-in the later integration stages.
-
-## Stage 12 — Integrate Policy-Aware Item And Helper Scopes
-
-Scope: adapt pending-item/helper composition to Stage 11's core using the acquired
-scope boundaries from Stages 9–10.
-
-1. Gather declarations before composing each relevant scope, complete item-local
-   helper imports before item placement, and consume applicable non-main version
-   headers before checking record payload identity. Keep relative policy with its
-   owner until concrete placement is known.
-2. Apply eligible lower identity at each scope, but prevent a replacement outer
-   record from borrowing nested identity owned by the discarded lower record.
-   Preserve independently applicable policy and application constraints.
-3. Retained structural directives cannot serve as identity fields. A helper may
-   supply payload identity components. Do not require a helper-built compound
-   parent reference to be complete at this point; validate the completed surviving
-   reference during inheritance.
-4. Project item evidence and owned policies through Stages 7–8 machinery. Test
-   nested items, helper-supplied identity, header consumption, declaration scope,
-   discarded owner identity, and exact diagnostic paths with no value disclosure.
-
-Validation: item/destination/import/composition/source-version tests and the
-everyday check. Keep current public behavior until load-path activation.
-
-## Stage 13 — Activate Ordinary Source Collection Declarations
-
-Scope: ordinary `Load` base-source assembly, excluding activation of the new
-selected-overlay policy surface until Stage 14.
-
-1. Wire common directive extraction into every ordinary source form: file,
-   native filesystem, bytes, reader, mapping, imported mapping, and sequence item.
-   Declarations work without `Imports`; disabled imports retain their existing
-   rejection semantics. `NoSource` remains absence rather than an empty document.
-2. Use Stages 11–12 composition and Stage 5 scoped policy. Strip collection
-   controls from application decoding, but retain their locations/evidence for
-   their structural lifetime and diagnostics.
-3. Integrate policy/projections together with the values. Cover inline/imported
-   equivalence, placement-relative declarations, singleton and peer validation,
-   and declarations without accompanying values. Maintain ordinary loading's
-   existing non-collection overlay behavior while preparing its policy handoff.
-4. Update the implemented declaration/composition contract sections. Collection
-   precedence, inheritance, and later CLI behavior remain incomplete until their
-   integration stages; do not claim them implemented here.
-
-Validation: ordinary-load, import, composition, and metadata tests plus
-the everyday check. Include one public ordinary-load declaration example test.
-
-## Stage 14 — Activate Selected Overlay Collection Policies
-
-Scope: ordinary loading's selected overlay and shared precedence integration,
-using the source/declaration handoff already active in Stage 13.
-
-1. Permit `configkit.sequences` in selected overlays while continuing to reject
-   overlay imports. Preserve source-version header/control handling and ordinary
-   overlay namespace selection. Consume controls before application decoding.
-2. Use Stage 6's sequence operations and Stages 4–5 policy helpers in the assembly
-   accumulator. Mappings still recurse and unrelated sequences still replace.
-   Wire value, policy retirement/transfer, and Stages 7–8 inspection projections
-   together, not as a later best-effort repair pass.
-3. Cover append/upsert/replace, empty/null/kind cases, key-only/mode-only changes,
-   retained explicit modes, overlay identity agreement, and independently owned
-   policies. Check values, history, origins, and discarded descendants together.
-4. Update the selected-overlay and collection-operation contract sections. Make
-   the shared precedence entry reusable by layered loading without adding another
-   operation implementation or turning overlay imports on.
-
-Validation: ordinary-load, overlay, composition, policy, and metadata tests plus
-the everyday check.
-
-## Stage 15 — Integrate Lower Policy Across Layered Assembly
-
-Scope: source-backed underlay coordination and the layered selected-overlay path,
-using the prepared acquisition, ownership, strict-scope, and precedence machinery.
-Do not absorb unfinished work from Stages 5–14 into this stage.
-
-1. Acquire main first, but assemble underlays from lowest to highest, strictly
-   composing each group with applicable accumulated lower policy before applying
-   its one precedence operation. Then compose/apply main and the selected overlay.
-   Do not sequentially upsert peer sources as if they were different tiers.
-2. Pass lower identity into higher strict scopes before peer composition. Cover an
-   otherwise empty underlay establishing identity, inherited explicit mode, and
-   replacement that deliberately discards the old YAML key. Invalid lower tiers
-   still fail even if a higher replacement would hide their values.
-3. Keep main-only version ownership and every enabled per-source version rule.
-   In default source-version mode, run the main checker once on the composed
-   main tier before overlay/inheritance/defaults, never on underlay-augmented
-   values. Its former main-before-underlay composition timing need not persist:
-   main composition now depends on lower policy. Enabled-mode source checks keep
-   acquisition order and exact source-local header handling. No extra callbacks
-   or second acquisition pass are permitted.
-4. Keep histories in precedence order, sources in acquisition order, underlay
-   indexes stable across empty groups, and shared budgets intact. Update the
-   layered contract's acquisition-versus-composition description accordingly.
-   Remove the acquisition/composition adapters superseded by this integration;
-   the old public-registration adapter remains until Stage 22.
-5. Cover multiple inputs/groups, mounted main inputs, item scopes, empty main,
-   explicit empty groups, layered overlays, version protection and diagnostics,
-   and equivalence with ordinary loading where the inputs express the same model.
-
-Validation: layered/grouped, source-version, dependency scheduling, destination,
-and metadata tests plus the everyday check.
-
-## Stage 16 — Verify Replacement And Nested Policy Ownership End To End
-
-Scope: the consequential lifetime cases across the now-active source assembly.
-
-1. Exercise an item-local nested collection through retaining the outer record,
-   upserting the same outer ID, and replacing the entire outer sequence. Retained
-   items keep their local key/mode; replacements do not inherit discarded policy
-   merely because identity, source label, or final index is equal.
-2. Exercise ancestor replacement and later reintroduction. Removed collection
-   identity does not reappear; independently declared append mode can survive
-   and require renewed identity. Cover renewed YAML key, application constraint,
-   explicit replace, and the intended failure with no available identity.
-3. Cover key-only validation of retained records, pending declarations, a no-value
-   replace declaration that cannot authorize key changes, and independent policy
-   whose owning scope survives a record replacement.
-4. Correct any integration defects in the Stage 5 ownership transformations;
-   consolidate tests with their mechanism owners. Do not introduce special-case
-   ownership by numeric path or a historical identity cache to make cases pass.
-5. Verify frozen metadata, origin/resource context, sensitivity of discarded
-   records, and mutable independence across repeated loads for these scenarios.
-
-Validation: focused policy/sequence/layered/ownership tests,
-`go test -race ./internal/engine`, and the everyday check.
-
-## Stage 17 — Prepare Inheritance Policy And Template Transport
-
-Scope: behavior-preserving inheritance state transport, before activating
-collection-aware inheritance operations in Stage 18.
-
-1. Extend inheritance templates/expansion sidecars to carry collection identity
-   and record-local policy alongside nodes and origins. Keep existing scheduler
-   keys, cancellation, version protection, missing-parent/cycle diagnostics, and
-   destination-local evaluation of nested templates.
-2. Provide owned-state copy/transfer helpers from Stage 5 and path projections from
-   Stage 8. Preserve original nested instructions for each destination's later
-   evaluation rather than retaining only a parent's expanded output. Keep
-   application registration matching separate from copied YAML policy.
-3. Test template reuse at multiple destinations, independent scoped state, and
-   retained instruction locations/resource context. Preserve current inheritance
-   values/history while these new helpers remain preparatory; do not activate a
-   value change without its corresponding policy/evidence handling.
-
-Validation: existing inheritance, ownership, history, and resource-context tests,
-focused transport tests, and the everyday check.
-
-## Stage 18 — Apply Collection Policy During Mapping-Sibling Inheritance
-
-Scope: activate shared collection operations using the prepared template/state
-handoff; existing mapping-sibling relationship selection remains unchanged.
-
-1. Use the shared sequence combiner at the inheriting destination: parent lower,
-   child higher, destination explicit mode or identity-based default. Copy YAML
-   identity where applicable, but do not automatically copy a parent collection's
-   explicit mode to a different destination. Match application constraints at the
-   destination, including newly created matching paths.
-2. Transfer record-owned state only with surviving records using Stage 17's
-   helpers. Integrate inheritance evidence with Stage 8's projections in the same
-   activation, preserving origins, historical sensitivity, resource context,
-   cancellation, and existing expansion/history accounting.
-3. Cover nested append/upsert/replace, explicit empty/null/kind cases, copied key
-   with inferred upsert, destination explicit mode, incompatible keys, application
-   constraints, cancellation, and policy at an inherited new namespace.
-4. Update current mapping-sibling inheritance documentation for collection
-   operations. Identified-record parent lookup remains incomplete until Stage 21.
-
-Validation: inheritance, history, provenance, sensitivity, and resource-context
-tests plus the everyday check. No identified-record parent lookup is active yet.
-
-## Stage 19 — Build Identified-Record Inheritance Graphs
-
-Scope: relationship extraction/validation and shared scheduling for sequence
-scopes; test these helpers before activating payload expansion.
-
-1. Extend sibling-scope discovery to identified sequences. Build the tuple-to-item
-   lookup once using effective identity. Every record supplies its own complete
-   identity before inheritance; neither parent lookup nor typed defaults supplies
-   a missing child field.
-2. Parse a surviving record's `extends` as null cancellation or a mapping with
-   exactly the normalized identity fields and supported scalar components.
-   Mapping-sibling references keep their existing string form. Do not accept a
-   scalar shorthand, multiple parents, positional or cross-collection lookup.
-3. Parent-reference mappings have already used ordinary strict mapping
-   composition: disjoint helper fields can complete one reference, while equal
-   overlapping fields conflict. Validate completeness/extra fields only now on
-   the surviving reference. Discarded references remain historical evidence.
-4. Build one-parent dependency edges and reuse `scheduleDependencies` with stable
-   semantic-pointer keys. Missing parents, self-reference, and longer cycles
-   use existing inheritance diagnostics without identity values. Keep authored
-   output positions independent from graph schedule.
-5. Cover compound and null references, numeric equality, forward edges, chains,
-   unrelated equal identities in different lists, malformed refs, missing parents,
-   cycles, helper-built references, and overwritten/cancelled relationships.
-
-Validation: focused inheritance graph/reference tests and the everyday check.
-
-## Stage 20 — Prepare Reusable Inheritance Scope Execution
-
-Scope: remove mapping-container assumptions from the private template executor
-without activating identified-record inheritance in public loads.
-
-1. Refactor template execution to separate relationship scheduling/payload
-   expansion from reading and rebuilding its scope container. Preserve current
-   mapping-sibling output, locations, cancellation, evidence, and budgets.
-2. Provide private mapping- and identified-sequence-scope adapters using Stage 19's
-   relationship graphs. The sequence adapter preserves output positions and
-   exposes child identity fields to the payload-expansion step. Keep this private;
-   do not add a public generic inheritance or merge callback API.
-3. Reuse Stage 17's template/policy transport and Stage 18's collection-aware
-   payload operations. Existing origin/history/retained-node handling must remain
-   available to both adapters rather than duplicating an entire executor.
-4. Run existing mapping-inheritance regressions and privately exercise a small
-   sequence graph through the adapter. Check output order, independent templates,
-   and operation/evidence paths before public activation in Stage 21.
-
-Validation: inheritance execution, scheduling, history, and budget tests plus
-the everyday check. This refactor does not introduce new parent-reference forms
-or change public loading semantics yet.
-
-## Stage 21 — Resolve Record Inheritance With Ownership And Evidence
-
-Scope: integrate Stage 19's graphs and Stage 20's executor into the structural
-expansion phase, with values and inspection working together.
-
-1. Expand parent payload into the child while preserving the child's own identity
-   fields and their origin/presence. Exclude parent identity fields from payload
-   inheritance. Recursively combine mappings and use destination collection policy
-   for nested sequences; retain whole-record upsert at the earlier boundary.
-2. Consume the child's directive before decoding. Keep parents as ordinary records
-   subject to the same schema/semantic validation; create no abstract templates or
-   acquired source records.
-3. Resolve only records surviving tier assembly. Cover parent upsert/removal,
-   replacement of a child without `extends`, self-reference not exposing a lower
-   predecessor, and nested destination-local relationships. Enclosing scopes
-   finish before nested scopes; assembled list order stays unchanged.
-4. Carry policy, origin/history, sensitivity, resource context, and existing
-   expansion/retained-history accounting through copies. Do not widen budgets or
-   require a new inspection-path API.
-5. Add public-load regressions for complete inheritance chains and nested
-   collections, including secret canaries on success and failure. Update the
-   implemented inheritance contract and consumer guidance for this scope.
-
-Validation: all inheritance and cross-feature inspection tests,
-`go test -race ./internal/engine`, and the everyday check.
-
-## Stage 22 — Expose The Root-Owned Registration API
-
-Scope: the settled facade surface and migration of in-repository callers.
-
-1. Implement the exact public names/signatures above in a focused root file, and
-   add `Definition.SequenceIdentities`. Convert root records/selectors explicitly
-   into the engine model. Snapshot fields, selectors, and paths on construction
-   and loading, preserving immutable definition reuse.
-2. Remove `SequenceComposition`, `Definition.SequenceCompositions`, and the private
-   old-shape adapters. Migrate all maintained code/tests/examples; do not edit
-   historical release notes solely to erase the old name or add deprecated aliases.
-3. Cover mixed exact/dynamic constructors, compound scalar fields, literal `*`
-   versus wildcard segments, root/unused selectors, invalid index paths and empty
-   field sets, overlap failure before source access, and independent loads.
-4. Prove dynamic constraints apply before strict composition and at inherited
-   destinations, without consumer namespace enumeration. Keep final-result
-   enforcement described as pending until Stage 27 actually adds it.
-5. Verify the public boundary and declaration documentation. Do not expose engine
-   enums, policy stores, parser nodes, or internal methods.
-
-Validation: root sequence/facade, registration, public/dependency-boundary, and
-consumer example tests plus the everyday check.
-
-## Stage 23 — Separate Reusable CLI Override Preparation
-
-Scope: mechanical preparation refactor and a privately tested post-structural
-validation entry, without changing current CLI processing order.
-
-1. Refactor override preparation so labels, paths, snapshots, type support,
-   disjointness, version protection, and existing budgets remain reusable outside
-   the source assembly accumulator. Allocate synthetic sources once in the same
-   order; do not reopen or reinterpret CLI values as YAML.
-2. Prepare post-structural validation that rejects `extends` as well as
-   `configkit` in supplied paths and nested values, using their established
-   structural roles. Test this entry privately; retain the existing entry's
-   behavior until the lifecycle switches in Stage 26. Do not reject previously
-   valid early-CLI instructions before that switch or add unrelated restrictions.
-3. Test detached prepared batches, synthetic source ordering, and the new
-   directive validation. Reuse existing label/path/cycle/type-support/budget tests
-   rather than adding another inventory of the same preparation cases.
-
-Validation: override preparation/source tests and the everyday check. Keep
-ordinary/layered behavior unchanged; typed assignment is the next stage.
-
-## Stage 24 — Implement Schema-Aware Post-Decode CLI Assignment
-
-Scope: a private typed assignment helper using Stage 23's prepared overrides,
-without activating the later lifecycle order yet.
-
-1. Add `applyDecodedOverrides`, using existing schema/decode mechanisms to bind
-   only supplied override subtrees against a detached copy of the decoded state.
-   Mappings recursively overlay, sequences replace, and explicit zero/empty/null
-   retain normal schema rules. Current typed values supply retained children,
-   not new defaults for replaced list items. Do not call `Definition.Defaults`
-   again or re-decode the whole projected configuration as newly authored input.
-2. Return the updated typed value, canonical post-CLI projection, presence/origin
-   changes, and existing raw CLI history contributions. Preserve a pre-CLI typed
-   projection for decoding reconciliation/history. No resolver runs on a failed
-   override; no partially applied result is returned.
-3. Test unknown/type-invalid fields, mapping retention, sequence replacement,
-   nested maps/lists, named/text/duration/secret schema fields, explicit null/false/
-   zero, discarded children, structural controls, and mutable ownership.
-
-Validation: override, decode, schema, projection, and ownership tests plus the
-everyday check. Only the helper is added here; the shared lifecycle does not
-switch until its inspection/context support is prepared in Stage 25.
-
-## Stage 25 — Prepare Post-Decode CLI Inspection And Effective Context
-
-Scope: prepare history, resolution-input, and secret-lookup context handling;
-retain the existing public CLI lifecycle until Stage 26.
-
-1. Extend existing replay/reconciliation to represent the target order:
-   source tiers, inheritance, typed decoding/defaults, CLI, application resolution.
-   Keep the structural input, pre-CLI typed snapshot, raw CLI contributions, and
-   post-CLI projection distinct where needed. Use the existing history machinery,
-   not a second replay framework, and retain legacy input compatibility until the
-   lifecycle switch. Do not mislabel CLI changes as defaults or inheritance.
-2. Prepare resolution-entry metadata from Stage 24's result so explicit presence,
-   origins, source records, and resource contexts describe the overridden state.
-3. Prepare the effective reference/location handoff for secret lookup and final
-   validation using the post-override/post-resolution values, not stale structural
-   input. Preserve existing no-credential-byte and ownership behavior.
-4. Test the target pipeline with privately assembled snapshots: equal-value CLI
-   replacement, replaced descendants, decoding reconciliation before CLI, later
-   recorded resolver transformations, explicit-presence defaults, and an overridden
-   secret reference. Verify current public-load regressions still pass unchanged.
-
-Validation: history/metadata/resolution/secret tests and the everyday check.
-Public activation must not require a later stage to repair these mechanisms.
-
-## Stage 26 — Activate CLI After Decoding In The Shared Lifecycle
-
-Scope: activate ADR 0018 through both loading paths using Stages 23–25's complete
-assignment, inspection, and effective-value handoffs.
-
-1. Remove CLI assignment from ordinary/layered structural assembly. Invoke
-   Stage 24 after inheritance and successful typed-default creation/decoding,
-   immediately before application resolution. Activate Stage 23's post-structural
-   validation. Keep the single shared lifecycle, not separate CLI implementations.
-2. Wire Stage 25's history and context handoffs in the same change. Resolution
-   receives explicit-presence/origin metadata and effective CLI values;
-   `Resolution.Default` preserves them while recorded transformations can change
-   them normally. Secret lookup uses effective references and locations. Preserve
-   exactly-once default-factory behavior and synthetic source ordering.
-3. Cover `NoSource`, ordinary/layered equivalence, CLI clearing/replacing an
-   identified list despite append/upsert mode, derived values seeing CLI input,
-   equal-value provenance, and resolver changes after CLI. Add explicit regressions
-   that CLI cannot fix earlier decode/inheritance errors or introduce/cancel a
-   relationship. Update obsolete early-CLI tests deliberately.
-4. Update the lifecycle, override, history, and secret-reference contracts and
-   affected examples to describe the now-implemented order.
-5. Remove the superseded early-CLI application paths, old preparation entry, and
-   temporary replay compatibility paths. Keep their still-relevant behavioral
-   tests under the new lifecycle rather than postponing code cleanup to the
-   documentation stage.
-
-Validation: override/lifecycle/resolution/inheritance/history/secret tests,
-`go test -race ./...`, and the everyday check.
-
-## Stage 27 — Enforce Final Application Identity Constraints
-
-Scope: the final-result guarantee, not another combination or inheritance phase.
-
-1. Add shared final identity validation after effective secret lookup and before
-   application semantic validation/publication in the common lifecycle. Use the
-   canonical effective typed projection, which retains secret references rather
-   than credential bytes. Preserve the existing validated publication snapshot
-   and detachment behavior.
-2. Traverse the actual final tree with the compiled selector trie, validating each
-   matching sequence independently with Stage 1 primitives. This must discover
-   paths introduced by typed defaults, CLI, and callbacks, not only assembly paths.
-   Support root/namespaced rules and unrelated unselected sequences.
-3. Validate actual sequences only. Missing paths, null, and other shapes remain
-   the schema/application validator's responsibility. Identity extraction here
-   examines effective typed payload, not whether a field was authored. Do not
-   recheck YAML-only policies or every intermediate callback state.
-4. Cover duplicate/missing/non-scalar identities produced at each late phase,
-   valid null/scalar identities, numeric equality after typed conversion, dynamic
-   namespaces created by each late phase, harmless unmatched rules, concrete-path
-   diagnostics with available origins, and secret-value non-disclosure.
-5. Prove both `Load` and `LoadLayered` use this guarantee and return no usable
-   result on failure. Update registration comments and canonical contract with
-   this final boundary, excluding later caller mutations from the guarantee.
-
-Validation: final identity, selector, facade, defaults/resolution/override, and
-secret tests plus the everyday check.
-
-## Stage 28 — Exercise Representative Consumer Workflows
-
-Scope: a small number of meaningful integrations, not a Cartesian test matrix.
-
-1. Extend `examples/consumers` with an executable source-backed catalog: underlay
-   records, main fragments, selected overlay append/upsert/replace, and explicit
-   record inheritance. Demonstrate complete replacement of a matched record,
-   stable order, and final inspection without application merge code.
-2. Add a namespaced typed-map example using one wildcard registration plus an
-   exact root registration, with independent uniqueness. Show YAML-declared
-   identity without registration separately so the final guarantee is not implied.
-3. Include one integrated CLI/resolution case and one representative provenance/
-   resource-context/secret-history case where those interactions add protection
-   beyond mechanism tests. Keep application domain validation recognizable.
-4. Audit coverage against the roadmap's work inventory, especially helper-built
-   parent references, version-header consumption, pending declarations, lower
-   identity before higher peers, replacement ownership, and dynamic destinations
-   introduced through inheritance and late typed changes. Add only missing,
-   consequential regressions at their narrowest stable owners.
-5. Consolidate obsolete/brittle tests. Do not restore old replacement semantics
-   merely to keep fixtures unchanged, and do not preserve duplicates of the same
-   semantic table across every layer.
-
-Validation: `go test ./examples/consumers`, focused root integration tests,
-`go test -race ./...`, and the everyday check.
-
-## Stage 29 — Reconcile Documentation
-
-Scope: finish the current-contract narrative. Code cleanup belongs with the
-integrations that supersede each adapter, not in this documentation prompt.
-
-1. Reconcile `docs/config.md` for declarations, identity equality, selectors,
-   operations/persistence, ownership, inheritance, final validation, and CLI order.
-   Keep its operation and presence tables authoritative; Go comments own exported
-   symbols and consumer guidance links rather than copying full contracts.
-2. Update `docs/consumers/README.md`, relevant root examples, `README.md`,
-   `docs/internal/overview.md`, `docs/internal/imports.md`, and
-   `docs/development.md` where the implemented structure or reading/test map has
-   changed. Remove stale claims about string-only registration, universal sequence
-   replacement, early CLI, and overlay prohibition of all directives.
-3. Remove implemented portions from the deferred identified-record discussion in
-   `future.md`, retaining genuinely deferred recursive body merging and other
-   still-relevant ideas. Preserve accepted ADR decision content; update only status
-   or implementation references where appropriate. Architecture already adopts
-   the decisions: remove pending-implementation wording when true, without adding
-   new policy. Leave release version selection/publication to the release task.
-4. Search maintained prose/examples for obsolete registration names and lifecycle
-   claims; retain intentional historical references. Follow canonical ownership
-   rather than copying the complete contract into each guide.
-
-Validation: local documentation-link checks and whitespace review; run relevant
-example tests if executable examples changed. Reuse Stage 28's successful
-code/race validation when its covered content is unchanged.
-
-## Stage 30 — Audit Scaling And Verify The Completed Target
-
-Scope: a bounded implementation review and final validation, not another broad
-refactor or expansion of the feature/performance scope.
-
-1. Inspect the completed code for duplicate combination/identity algorithms,
-   per-record scans of whole collections, repeated declaration gathering, numeric
-   magnitude expansion, stale ownership/path caches, and unnecessary copies.
-   Prefer shared indexed mechanisms without pooling/global caches. Reuse existing
-   composition/underlay/inheritance benchmarks and add a focused mixed-upsert or
-   wildcard-collection benchmark only where existing fixtures cannot exercise the
-   new scaling risk. Record commands and qualitative findings here; timing or
-   allocation thresholds and a quiet-host benchmark campaign are not gates.
-2. Confirm transitional adapters/dead early-CLI paths have been removed. Search
-   current code and maintained examples for `SequenceCompositions` and
-   `SequenceComposition`; only intentional historical references may remain.
-   Confirm no new dependencies, public internal-type exposure, background work,
-   release machinery, or additional limits were introduced. Review the full diff
-   against the roadmap, not merely passing tests. Record bounded review findings
-   here; significant corrective
-   work requires its own focused unit rather than being hidden inside this stage.
-   Do not declare completion while a target behavior remains incorrect or missing.
-
-Validation: `./scripts/check.sh`, `go test -race ./...`, and the development
-guide's local documentation/public-boundary checks, reusing successful checks
-from Stages 28–29 if their covered content is unchanged. Record completed scope,
-validation, benchmark commands/qualitative findings, and any real remaining
-issues accurately. No new timing thresholds or release gates are introduced.
-
-Acceptance: all roadmap operations and lifecycle boundaries work through ordinary
-and layered loads; values and inspection agree; application constraints hold on
-successful final results; deferred behavior and YAML-only guarantees have not
-expanded. All thirty stages are complete and their nonempty units committed.
-
-## Open Questions
-
-None. Feature choices are settled in the roadmap and accepted ADRs; this plan
-selects the remaining API, representation, validation, and execution decisions.

docs/roadmap/sequences.md

@@ -1,999 +0,0 @@
-# Sequence Identity And Combination Roadmap
-
-Status: settled feature roadmap; implementation is specified in the
-[staged implementation plan](implementation.md). This document defines the
-targeted end state, not an implemented contract. The implementation plan owns
-execution order, exact Go names and signatures, bookkeeping, and validation.
-
-## Purpose And Scope
-
-Allow configuration authors to assemble collections from independently authored
-records, declare identity in configuration, and progressively extend a baseline
-collection through higher layers. Authors can replace the entire collection,
-append distinct records, or replace complete records by identity while retaining
-unmatched records. Sibling inheritance uses the same sequence operations and
-identity-based defaults. Within an identified sequence, a record can explicitly
-inherit settings from another record in that sequence while keeping its own
-identity.
-
-The motivating use case is a prompt catalog whose source-backed baseline supplies
-records and whose application or installation layers add or supersede records.
-Applications can optionally impose identity constraints that configuration must
-respect and that hold in the final loaded configuration. These constraints can
-cover exact destinations or collections beneath dynamically named mapping keys,
-without requiring the application to discover those names before loading.
-
-The feature covers ordinary and layered loading, imported items, inline
-sequences, selected overlays, and sibling inheritance. Typed defaults, CLI
-assignments, and application resolution retain their own combination semantics;
-CLI application moves later in the loading lifecycle.
-
-This scope is narrower than the deferred
-[identified-record sequences proposal](future.md#1-identified-record-sequences).
-Automatic recursive merging of matching records remains outside this scope;
-explicit inheritance between distinct records is included. The initial
-declaration surface is `configkit.sequences`; `key` and `mode` shorthand on
-imports is deferred. Application lists remain
-ordinary YAML sequences without control wrappers. Unkeyed sequence concatenation
-is outside this feature.
-
-## Current Baseline And Required Changes
-
-The [import contract](../config.md#explicit-imports) owns existing selectors,
-destinations, placement, and ordering. `placement: sequence-item` already places
-each selected source mapping as one item at its effective destination. Item-local
-helper imports can contribute fields before placement.
-
-The [composition contract](../config.md#strict-document-composition),
-[layered-loading contract](../config.md#layered-loading), and
-[inheritance contract](../config.md#sibling-inheritance) describe the implemented
-baseline:
-
-| Area | Current behavior | Target behavior |
-| --- | --- | --- |
-| Identity declaration | `Definition.SequenceCompositions` registers one literal string field at each registered mapping path. | YAML collection declarations and optional authoritative application registrations support field sets and scalar identity components. |
-| Application registration destinations | Registrations select exact literal mapping paths. | Exact and structural selectors can impose identity beneath arbitrary mapping keys, independently at each matched destination. |
-| Strict composition | Registered sequences append distinct string identities and reject duplicates; repeated unregistered sequences conflict. | Collections with effective identity append distinct records and reject duplicates. Applicable lower-tier identity is available before composing higher-tier peers. Unidentified sequences retain strict-conflict behavior. |
-| Source precedence and selected overlays | A present higher sequence replaces the lower sequence wholesale. | Collection policy selects `replace`, `append`, or `upsert`; overlays can declare policy for inline sequences. |
-| Sibling inheritance | An inheriting mapping recursively overlays its resolved parent; supplied sequences replace wholesale. | Mappings retain recursive combination, while sequences use destination policy and the same three operations. |
-| Sequence-member inheritance | A sequence element has no sibling scope for its own inheritance relationship. | An identified sequence provides a sibling scope: a record can inherit from one record in the same sequence, selected by complete identity. |
-| CLI overrides | Apply before inheritance and typed defaults/decoding. | Apply after inheritance and typed defaults/decoding, before application resolution. |
-| Identity enforcement | Registration governs strict sequence composition. | Structural identity checks cover identified combinations; application registrations additionally constrain final effective sequences. |
-
-Source-backed underlays participate in source assembly. `Definition.Defaults`
-provides typed defaults later; extending source combination does not turn typed
-default lists into source underlays.
-
-## Collection Declarations
-
-Use a source-root `configkit.sequences` block containing collection declarations.
-Each entry identifies a `destination` and may declare `key`, `mode`, or both.
-Identity and mode can be supplied independently. This form serves imported items,
-inline records, ordinary mapping imports, and inline sequences in selected
-overlays.
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      key: [namespace, id]
-      mode: append
-  imports:
-    - dir: prompts
-      pattern: '*.yaml'
-      destination: [prompts]
-      placement: sequence-item
-```
-
-Each selected file contributes a record such as:
-
-```yaml
-namespace: support
-id: welcome
-text: Welcome
-```
-
-Here append retains a lower-layer collection and adds distinct higher-layer
-records. Identity and combination controls belong to the collection declaration,
-so imported and inline contributions share one policy surface.
-
-### Destinations And Scope
-
-`destination` uses the existing sequence of literal mapping keys. Resolve it
-relative to the declaring source's placement: a fragment mounted under `catalog`
-can declare `[prompts]` locally. A sequence-item source can declare a nested
-collection within its own item without referring to an eventual numeric index.
-Preserve source and item-local scope boundaries.
-
-A collection's scope is its resolved destination within its composition scope.
-Resolve relative destinations before reconciling declarations. Different imported
-items retain separate scopes for their nested collections. A declaration governs
-all contributions to its collection in the tier, including imported and inline
-records; it does not govern only records in the declaring file.
-
-Gather declarations before composing the relevant scope so their effect does not
-depend on which contribution is assembled first. Retain applicable controls
-through source precedence and inheritance, then keep them out of application
-decoding.
-
-Sequence declarations are independent of filesystem import enablement. Selected
-overlays may contain `configkit.sequences`, extending the current blanket
-prohibition on overlay `configkit` declarations. This does not enable imports
-in overlays.
-
-### Overlay Authoring
-
-For an existing sequence `[foo, bar]`, an overlay can replace `foo` in full:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      key: id
-      mode: upsert
-prompts:
-  - id: foo
-    text: Revised definition
-```
-
-The result is `[foo*, bar]`. The explicit upsert mode can be omitted when no
-different explicit mode applies from a lower tier: effective identity then
-selects upsert. To add a distinct record instead:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      key: id
-      mode: append
-prompts:
-  - id: baz
-    text: Another prompt
-```
-
-The result is `[foo, bar, baz]`. To replace the whole sequence:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      mode: replace
-prompts:
-  - id: bar
-    text: Bar definition
-  - id: baz
-    text: Baz definition
-```
-
-The result is `[bar, baz]`. Explicitly selecting the default operation is valid.
-With no applicable explicit mode, omitting this declaration selects replace for
-an unidentified collection and upsert for an identified one. A retained explicit
-mode continues to govern.
-
-## Identity Definitions And Constraints
-
-### Field Sets
-
-`key` accepts a string or a nonempty list of strings naming direct, literal
-fields in each record. A name such as `metadata.id` denotes that literal field,
-not a nested lookup. Treat field names as a set: remove repeated names and
-ignore authored order when comparing definitions.
-
-Thus `id`, `[id]`, and `[id, id]` are equivalent; `[namespace, id]`,
-`[id, namespace]`, and `[namespace, id, id]` are also equivalent. An empty list
-is invalid because it supplies no identity fields. These rules apply to both
-configuration declarations and application registrations.
-
-Use a consistent field order internally to construct identity tuples. Preserve
-component types and tuple boundaries so unstructured concatenation cannot cause
-otherwise distinct identities to collide.
-
-### Scalar Components And Equality
-
-Each identified record must be a mapping with every designated identity field
-present. Components may be strings, numbers, booleans, or explicit null. Missing
-fields and mapping or sequence components are invalid. Present null differs
-from absence; two single-field null identities are equal. Compound identities
-are equal when all corresponding components are equal.
-
-Strings compare exactly, including empty strings, without trimming, case folding,
-or normalization. Scalar categories remain distinct: `"1"`, `1`, `true`, and
-null are different identities.
-
-Numbers compare by mathematical value across integer, fractional, and exponent
-spellings: `1`, `1.0`, and `1e0` are equal, as are `0` and `-0`. Use exact numeric
-normalization for identity comparison without rounding through a floating-point
-conversion. This does not rewrite authored values or change existing typed
-decoding and projection semantics. The parser's rejection of non-finite numeric
-forms remains unchanged.
-
-### Application-Owned Constraints
-
-Retain an optional application interface for registering collections and imposing
-their identity definitions. Configuration may define identity where no application
-constraint exists. When a declaration omits `key`, an applicable registration
-supplies identity. An explicit equivalent definition is accepted, and a different
-definition fails under every mode, including replace.
-
-For example, if the application requires `id`, `key: [id, name]` is invalid:
-that compound identity would allow repeated `id` values with different names.
-A registration is an authoritative constraint, not an omission-filling default.
-Copying a collection through inheritance does not create another application
-registration; registrations constrain their registered destinations.
-
-Identity constraints validate actual sequences. They do not independently require
-a path to exist, be non-null, or have sequence shape. The application schema and
-validation own those requirements. Final-result enforcement for application
-registrations is defined under [Validation And Diagnostics](#validation-and-diagnostics).
-
-### Application Registration API
-
-Use one public sequence-identity registration type, collected in one field on
-`Definition`. Each registration contains an explicit destination selector and
-its identity fields. Exact and dynamic registrations share matching, overlap
-validation, identity comparison, and lifecycle behavior.
-
-Provide two constructors returning that same registration type:
-
-- An exact-path convenience constructor accepting a literal `Path` and identity
-  field names. It converts the path into a selector containing literal segments.
-- An explicit-selector constructor accepting a structural selector and identity
-  field names, for destinations that include arbitrary mapping keys.
-
-Both constructors accept identity fields as variadic strings, so one-field and
-compound identities use the same construction form. Apply the settled field-set
-rules, including deduplication. Invalid registrations, including an empty identity
-field set, fail during application-definition validation.
-
-Existing `Path` values retain their literal meaning throughout ConfigKit. The
-exact-path constructor supplies convenience at the registration boundary; dynamic
-matching uses the explicit selector type. Both forms can appear together in the
-same definition field.
-
-Replace the current registration shape with this model. Preserving the existing
-`SequenceComposition` fields and `Definition.SequenceCompositions` spelling is not
-a requirement, and no compatibility facade is needed solely to retain that syntax.
-Choose names centered on sequence identity, reflecting its role in composition,
-inheritance, and final validation. Exact Go type, field, and constructor names
-and signatures belong in the implementation plan.
-
-### Dynamic Application Registration Destinations
-
-Allow applications to register identity for collections whose containing mapping
-keys become known only during loading. For example:
-
-```yaml
-collections:
-  weatherreporter:
-    prompts:
-      - id: foo.bar
-        text: Report the weather.
-  homewatch:
-    prompts:
-      - id: foo.bar
-        text: Report the household status.
-```
-
-One application registration can require `id` for every namespace's `prompts`
-list. The equal IDs above are valid because each list has its own uniqueness
-scope. An optional root `prompts` list can have a separate exact registration.
-
-Use an explicit application-side structural selector composed of literal mapping
-keys and an arbitrary-mapping-key segment. Conceptually, the namespace rule is:
-
-```text
-Literal("collections") -> AnyMappingKey -> Literal("prompts")
-```
-
-This notation explains the selector; it is not a Go signature or YAML syntax.
-Each segment consumes exactly one mapping level. Match the complete destination,
-not an arbitrary prefix or suffix. Literal keys remain exact, including empty
-strings and keys containing punctuation such as `*`, `/`, or `.`. An arbitrary-key
-segment is distinct from any literal spelling and does not match a sequence
-index. Existing `Path` values retain their literal meaning.
-
-This capability belongs to the reusable application definition and applies to
-ordinary and layered loads. YAML collection destinations and import destinations
-retain their existing literal syntax. Recursive search, regular expressions,
-filesystem glob rules, sequence-index selectors, and custom matching callbacks
-are outside this addition.
-
-At each concrete matched destination, apply the same authoritative identity
-constraint as an exact registration. Use the settled field-set and scalar-equality
-rules. Do not pool identities across destinations or require records to repeat
-the containing mapping key as an identity component. Applications retain ownership
-of namespace interpretation, qualified runtime IDs, and domain validation.
-
-Matching supplies effective identity before relevant strict composition and
-precedence operations. Without an applicable explicit mode, this identity selects
-upsert under the existing default rule. It also supplies identity for inheritance
-within the matched sequence. YAML declarations must agree with it under every
-mode, including replace. These are ordinary application constraints with a broader
-destination selector, not a separate final-validation facility.
-
-### Registration Overlap
-
-Equivalent application constraints coalesce at any shared destination. Equivalence
-uses the identity field-set rules: for example, `id` and `[id, id]` agree.
-Incompatible constraints fail; exact registrations do not override broader
-selectors, and registration order grants no precedence.
-
-Detect incompatible overlapping registrations while validating the application
-definition, before source acquisition. For these fixed-depth mapping selectors,
-overlap exists when the selectors have the same number of segments and every
-corresponding pair can match a common key. Unequal literal keys at any position
-make the selectors disjoint; an arbitrary-key segment can overlap a literal or
-another arbitrary-key segment. Compare segment structure rather than rendered
-path strings. No configuration pre-read is needed.
-
-Thus an `id` rule for all namespace prompt lists conflicts with a `[id, name]`
-rule for one named namespace, even if that namespace is absent from a particular
-load. This is an invalid application definition. A valid selector matching no
-collections is harmless: registration does not require a path to exist, create
-collections, or impose shape and nullability requirements.
-
-### Matching Across The Loading Lifecycle
-
-Match application selectors against effective configuration destinations after
-accounting for input placement. Physical filenames and source-local paths do not
-determine applicability. Make constraints available before composing contributions
-at those destinations, including namespaces first introduced by imports, mounted
-inputs, or higher tiers. Apply matching constraints to key-only YAML declarations
-as well, even before their sequence exists. Consumers need neither a preliminary
-load nor source inspection to enumerate namespaces.
-
-During inheritance, apply registrations at the inheriting destination, including
-newly created matching destinations. Copying a collection does not transfer its
-application registrations to a different path. The existing YAML-policy ownership
-and transfer rules remain separate from application selector matching.
-
-During final validation, find matching actual sequences in the final effective
-configuration. This includes namespaces introduced or replaced by typed defaults,
-CLI overrides, and application resolution. Reusing only destinations found during
-source assembly would miss these values. Use the same selector meaning and
-identity validator in both phases; later assignments do not trigger composition
-or inheritance again. YAML-only identities retain their existing enforcement
-boundary.
-
-## Strict Composition And Precedence Operations
-
-### Compose Peers Before Applying Precedence
-
-Resolve applicable identity and mode policy, strictly compose the tier's peer
-contributions, then apply one selected operation per collection against the
-accumulated lower value. The tier operation does not sequentially replace peer
-source contributions. Selected overlay contributions use the same combination
-semantics.
-
-Within a strict tier, equivalent identity definitions coalesce and different
-field sets fail. Agreeing explicit modes coalesce; conflicting explicit modes
-fail. Omission expresses no preference. A root declaration or a later declaration
-has no precedence over conflicting peers. Explicitly selecting a default mode
-is still a real instruction and can conflict with another explicit mode.
-
-Identified peer sequences append distinct records. Duplicate identities among
-peers or within a contributed list fail under all three modes, including replace
-and upsert. Equal or disjoint record payloads do not excuse a duplicate identity.
-Unidentified sequences retain their current strict-composition conflict behavior.
-
-For example, higher-tier fragments `[C]` and `[D]` with an agreed identity assemble
-as `[C, D]`. Against lower `[A, B]`, append or upsert produces `[A, B, C, D]`,
-while replace produces `[C, D]`. Peers `[B]` and `[B*]` conflict even under
-upsert because neither has higher precedence.
-
-### Available Operations
-
-Use one spelling for each mode: `replace`, `append`, and `upsert`.
-
-| Mode | Behavior at source-precedence and inheritance boundaries |
-| --- | --- |
-| `replace` | Replace the complete lower sequence with the assembled higher sequence. |
-| `append` | Retain lower records and append higher records; duplicate identities fail. |
-| `upsert` | Replace matching records in full, retain unmatched lower records, and append new identities. |
-
-Append and upsert require effective identity. ConfigKit does not infer identity
-from an `id` field or record position. Whole-sequence replacement does not require
-identity agreement with lower YAML policy, but application constraints still apply.
-
-Letters denote record identities; `B*` has the same identity as `B` with a different
-payload:
-
-| Lower | Higher | Mode | Result |
-| --- | --- | --- | --- |
-| `[A, B]` | `[C]` | `append` | `[A, B, C]` |
-| `[A, B]` | `[C]` | `replace` | `[C]` |
-| `[A, B]` | `[B*]` | `append` | Duplicate-identity error |
-| `[A, B]` | `[B*]` | `replace` | `[B*]` |
-| `[A, B]` | `[B*]` | `upsert` | `[A, B*]` |
-| `[A, B]` | `[B*, C]` | `upsert` | `[A, B*, C]` |
-
-Upsert replaces the whole matched record. If lower `B` has `id`, `text`, and
-`temperature`, while higher `B` has only `id` and `text`, the result at this
-boundary has only `id` and `text`. Subsequent defaulting or other lifecycle
-operations retain their own semantics. Mappings continue to combine recursively;
-matching sequence records do not recursively merge fields.
-
-### Mode Selection And Persistence
-
-Select the operation in this order:
-
-1. Reconcile explicit modes within the current tier; conflicting peers fail.
-2. Use the current tier's explicit mode if supplied.
-3. Otherwise retain an applicable explicit mode from a lower tier.
-4. With no explicit mode, infer upsert when effective identity is available.
-5. Otherwise infer replace.
-
-Effective identity can come from an application registration, current declarations,
-or applicable lower policy, subject to the identity agreement and lifetime rules
-below. Determine the inferred mode from available identity before applying the
-operation. A plain higher-tier sequence can therefore use upsert without repeating
-a key declaration.
-
-An explicit mode persists at its applicable destination until a higher-priority
-explicit declaration changes it. New sequence values alone do not reset it, and
-an unrelated overlay leaves it intact. A mode-only declaration can change policy
-without supplying records; it neither clears the collection nor retroactively
-recombines earlier tiers. Later source contributions and inheritance use the
-resulting policy.
-
-| Lower explicit mode | Higher declaration | Effective mode |
-| --- | --- | --- |
-| `append` | Mode omitted | `append` |
-| `append` | `upsert` | `upsert` |
-| `upsert` | `replace` | `replace` |
-| None | Mode omitted, effective identity available | Inferred `upsert` |
-| None | Mode omitted, no effective identity | Inferred `replace` |
-
-Only explicit modes persist. Inferred defaults are recomputed from effective
-identity when needed. Explicit replace continues to apply in subsequent tiers
-until another mode is declared; introducing identity alone does not override it.
-Mode persistence does not preserve YAML identity discarded by replacement, nor
-policies owned by discarded records. Source precedence and inheritance use the
-same destination policy, without separate per-phase settings. CLI assignments
-remain outside mode selection.
-
-### Presence, Empty Sequences, Null, And Kind Mismatches
-
-At a collection's precedence boundary:
-
-| Higher contribution | `replace` | `append` / `upsert` |
-| --- | --- | --- |
-| Absent | Leave the lower value unchanged. | Leave the lower value unchanged. |
-| Explicit `[]` | Clear the sequence. | Contribute no records. |
-| Explicit null or another non-sequence value | Ordinary replacement, subject to application schema. | Error: the operation requires a sequence. |
-
-A declaration alone does not clear a collection. An optional import selecting
-nothing contributes nothing and does not erase existing records. An absent lower
-collection can be initialized by an incoming sequence under any mode, subject to
-identity requirements. Explicit lower null or another non-sequence value is not
-absence: append and upsert fail against it, while replacement remains available.
-An empty incoming sequence bypasses neither kind checks nor identity requirements.
-
-With effective identity and no explicit mode, `prompts: []` uses upsert and does
-not clear the collection. Clearing requires replace, selected in the current tier
-or retained from lower policy. Replacing an identified collection with null also
-requires replace and a schema permitting null. Collection null is distinct from
-a present null identity component inside a record.
-
-### Ordering
-
-Retain existing dependency-ready source scheduling within each tier and authored
-item order within lists. Gathering declarations does not change assembly order;
-identity values do not sort records.
-
-- Append keeps lower records in order, followed by higher records in assembled order.
-- Replace keeps only higher records in their assembled order.
-- Upsert keeps a matched record's lower position and appends new identities in
-  higher-tier order.
-
-For example, lower `[A, B, C]` and higher `[C*, A*, D]` produce `[A*, B, C*, D]`
-under upsert. Whole-sequence replacement gives the higher tier control of the
-complete sequence order.
-
-## Policy Agreement, Lifetime, And Ownership
-
-### Identity Agreement And Replacement
-
-Append and upsert use one agreed identity for all participating records:
-
-| Lower identity | Higher declaration | Result when combining records |
-| --- | --- | --- |
-| `id` | Omitted | Use `id`. |
-| `id` | `id`, `[id]`, or `[id, id]` | Accept the equivalent definition. |
-| `id` | `[namespace, id]` | Error: different definitions. |
-| None | `id` | Establish `id` and validate all participating records. |
-
-Introducing identity for an unkeyed lower sequence requires checking both lower
-and incoming records, including uniqueness within each contribution. With no
-identity from the application or either participating collection, append and
-upsert fail.
-
-YAML identity follows the collection that survives an operation:
-
-| Operation | Effective YAML identity |
-| --- | --- |
-| Append or upsert | Retain or establish the agreed identity. |
-| Whole-sequence replacement | Discard lower YAML identity and use the replacing tier's identity declaration, if any. |
-| Replacement by null or another non-sequence value | Discard lower YAML identity and retain any identity explicitly declared by the replacing tier. |
-
-Application constraints remain authoritative under every operation. Without an
-application constraint, replacement can introduce a different identity, such as
-changing from `id` to `name`. Replacing a sequence without a new key declaration
-or application registration produces an unkeyed sequence. No identity-removal
-directive is needed.
-
-The reset occurs only when the collection value is actually replaced. Declaring
-replace without supplying a replacement does not authorize changing the identity
-of retained records. Explicit mode lifetime is separate from YAML identity
-lifetime.
-
-### Ancestor Replacement
-
-Replacing an enclosing mapping can remove a collection without replacing that
-collection directly. Apply the same identity and ownership rules to this removal:
-discard the removed collection's lower YAML identity, and discard policies owned
-by any removed records. Reintroducing the same path later does not restore those
-discarded policies.
-
-Independently declared explicit modes retain their existing persistence rules.
-Removing a descendant collection does not remove a mode whose declaring scope
-survives. Application registrations remain applicable at matching destinations,
-including destinations reintroduced later. Identity newly declared by the
-replacing tier follows the existing declaration and ownership rules.
-
-For example, a root-level declaration establishes `key: id` and `mode: append`
-for `[catalog, prompts]`. A higher tier replaces `catalog` with null, removing
-the prompts collection and its YAML identity. The independently declared append
-mode survives. If a later tier recreates `catalog.prompts`, append requires
-identity supplied by a new declaration or an application registration. Without
-that identity, loading fails; the later tier can instead explicitly choose
-replace. The replacement of `catalog` itself follows ordinary ancestor-value
-precedence and does not attempt to append null to the removed prompts list.
-
-### Declarations Without Sequence Values
-
-A key declaration without accompanying records applies immediately to an existing
-retained sequence: check agreement and validate its records, without recombining
-earlier tiers. For example, a higher tier that only declares
-`destination: [prompts], key: id` identifies and validates the retained lower
-`prompts` sequence.
-
-If no collection exists, retain the declaration in its owning scope for later
-contributions. An otherwise empty underlay can establish identity for main-tier
-fragments. An explicit identity declaration can also remain pending at a
-non-sequence destination; it does not independently impose a shape requirement.
-The presence and kind rules still determine whether an attempted operation fails.
-Item-local declarations retain their ownership boundaries.
-
-### Lower-Tier Identity During Strict Composition
-
-Resolve applicable identity before composing a higher tier's peers, using
-application registrations, available lower-tier policy, and declarations gathered
-within the higher tier's scope. Authors need not repeat a baseline key solely
-because the higher tier has multiple fragments.
-
-For example, an underlay establishes `key: id` for `prompts`, and two main-tier
-fragments contribute `[foo]` and `[bar]` without a key declaration. They strictly
-compose as `[foo, bar]` before the main tier combines with the accumulated lower
-collection. If both contribute `foo`, composition fails; upsert does not give
-precedence to peers.
-
-Lower YAML identity remains subject to agreement, replacement, and ownership.
-A replacing collection must not be constrained by obsolete lower identity just
-because policy resolution precedes composition. Likewise, a replacement record
-cannot borrow nested identity owned by the discarded lower record, even when
-outer identity or numeric position matches. Application constraints still apply.
-Higher-tier declarations do not retroactively repair invalid lower-tier composition.
-
-### Nested Record Ownership
-
-Attach item-local policies to their owning contributions so they follow surviving
-records independently of numeric indexes. Whole-record upsert discards the old
-record's local policies along with its fields. Whole-sequence replacement discards
-policies owned by removed records. A replacement record supplies its own local
-declarations; equal outer identity or final index does not transfer discarded
-policies.
-
-For example, a source imported as one record into `prompts` can declare:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [steps]
-      key: id
-      mode: append
-id: foo
-steps:
-  - id: prepare
-```
-
-The declaration governs `steps` inside that `foo` record. Retaining `foo` retains
-its local policy, including the explicit mode. Upserting a new `foo` discards the
-old `steps` and its declaration even though the outer record keeps its position.
-
-Policies independently applicable at a destination are not discarded merely
-because an individual record is replaced. Ownership determines which declarations
-are removed. Application registrations remain constraints at their registered
-destinations. Inheritance copies transfer identity and choose operations as
-specified next.
-
-## Sibling Inheritance
-
-Complete source-tier assembly before resolving inheritance. Retain existing
-mapping-sibling scopes and add identified-sequence scopes as described below.
-Both use the existing dependency-ready algorithm. At each inheritance combination,
-the resolved parent's collection is the lower value and the inheriting mapping's
-local collection is the higher value. Mappings combine recursively; upsert within
-an identified sequence remains whole-record replacement. Explicit record
-inheritance is a separate construction step after those replacements.
-
-Use the inheriting destination collection's policy: an applicable explicit mode,
-including one retained through source tiers, wins. Otherwise infer upsert with
-effective identity and replace without it. There is no separate default-append
-rule for inheritance. The same destination policy governs source-precedence and
-inheritance combinations.
-
-YAML identity follows a collection copied through inheritance, subject to agreement
-and replacement rules. A source collection's mode does not automatically become an
-instruction at a different inheriting destination. For example, source `steps`
-has `key: id` and `mode: append`; an inheriting mapping supplies local `steps`
-without an applicable explicit mode. The copied identity selects upsert there.
-An explicit destination append instead selects append. A matching destination key
-agrees; a different key conflicts when combining records, while explicit replace
-can establish new YAML identity subject to application constraints.
-
-For example:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [parent, items]
-      key: id
-child:
-  items:
-    - id: foo
-      value: A
-    - id: bar
-      value: B
-parent:
-  extends: child
-  items:
-    - id: baz
-      value: C
-```
-
-Despite the names, `parent` inherits from `child`. Identity at `[parent, items]`
-selects upsert, producing `[foo, bar, baz]`. If the local sequence instead supplies
-a revised `foo`, the result is `[foo*, bar]`. Append rejects that repeated identity;
-replace retains only the local sequence.
-
-The same ordering, presence, and kind rules apply at inheritance boundaries.
-An omitted local sequence retains the inherited sequence. Explicit `[]` clears it
-under replace and contributes no records under append or upsert. Retain controls
-until inheritance uses them, then exclude them from application decoding.
-
-### Inheritance Between Identified Sequence Records
-
-An identified sequence supplies a sibling scope for its records. Identity can
-come from an application registration or an applicable YAML declaration. A record
-may inherit from one other record in that same sequence using an `extends`
-mapping that selects the parent's complete identity:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      key: id
-prompts:
-  - id: base
-    model: standard
-    temperature: 0.5
-  - id: concise
-    extends: {id: base}
-    temperature: 0.2
-```
-
-The final `concise` record is:
-
-```yaml
-id: concise
-model: standard
-temperature: 0.2
-```
-
-The `base` record remains in the collection. Parents are ordinary records subject
-to the same decoding and validation as other records; there is no abstract
-template category.
-
-Use mapping-form references for both single and compound keys. With
-`key: [namespace, id]`, a reference can be
-`extends: {namespace: shared, id: base}`. Match components using the collection's
-field-set and scalar-equality rules, including mathematical numeric equality.
-`extends: {id: null}` selects a null-valued identity; `extends: null` cancels a
-relationship.
-
-Parent-reference mappings use ordinary strict mapping composition. Item-local
-helper imports may supply disjoint fields of a compound reference; overlapping
-fields conflict, even when equal. For example, a helper's
-`extends: {namespace: shared}` and the importing record's `extends: {id: base}`
-compose into `extends: {namespace: shared, id: base}`. This assembles one record;
-it does not permit strict peers to contribute duplicate record identities.
-
-Before parent lookup, require the surviving effective reference mapping to contain
-exactly the collection's identity fields, with supported scalar values. Missing
-or extra fields fail. Validate the completed reference during inheritance, rather
-than requiring each helper contribution to be independently complete. Superseded
-references remain historical evidence; earlier strict-composition conflicts
-remain errors.
-
-Every record must supply its complete own identity before inheritance. Preserve
-the child's identity fields and inherit only the parent's nonidentity payload;
-inheritance cannot supply a missing child identity. Child payload fields take
-precedence, mappings combine recursively, and nested sequences use the inheriting
-destination's collection policy. Consume the record's directive before decoding.
-
-Support one parent within the same identified sequence, including forward
-references and chains. Missing parents and cycles, including self-reference,
-fail the load. Cross-collection references, position-based references, multiple
-parents, and inheritance between unidentified sequence members are outside this
-scope. Existing string-form references between mapping siblings retain their
-meaning; they do not become shorthand for sequence identity references.
-
-Resolve relationships against records surviving source-tier assembly and selected
-overlays. A higher layer can replace a parent before inheritance runs; removing
-that parent makes a surviving reference fail. Whole-record upsert discards the
-old record's directive along with its payload. Omitting `extends` in the
-replacement record does not retain the displaced record's relationship. An
-explicit reference to the record's own identity is a cycle, not a way to access
-its lower-tier predecessor.
-
-Reuse Kahn's algorithm with the existing stable canonical semantic-pointer key
-for dependency-ready ordering. Build parent lookup from the sequence's effective
-identity, and preserve the assembled list order independently of resolution
-order. Enclosing relationships resolve before nested relationships. Parent
-templates retain nested instructions for evaluation at each inheriting
-destination, following the existing destination-local inheritance rule. Copied
-collection identity, destination operation selection, and record-owned policy
-transfer follow the rules above.
-
-Record inheritance runs in the existing structural phase, before typed defaults,
-decoding, CLI overrides, and application resolution. Those later stages do not
-trigger another inheritance pass. Preserve inherited presence, origin and
-history evidence, sensitivity, declaring resource contexts, and existing
-expansion budgets just as for mapping-sibling inheritance. Identity lookup must
-not expose identity values in diagnostics. Existing index-based inspection paths
-remain sufficient; no new identity-selector path API is needed.
-
-## Loading Lifecycle
-
-Use the following order:
-
-1. Assemble source tiers and selected overlays, resolving policy before each
-   tier's strict composition and applying collection operations at precedence
-   boundaries.
-2. Resolve sibling inheritance in mappings and identified sequences, then apply
-   typed defaults and decode the assembled configuration.
-3. Apply explicit CLI overrides to that configuration and check supplied values
-   against the application schema.
-4. Run application resolution against the overridden configuration.
-5. Resolve effective secret references, validate the final configuration including
-   application-registered identities, and publish it.
-
-### Typed Defaults And Application Resolution
-
-Source-backed underlays, including application-supplied mapping sources,
-participate in sequence combination. Typed `Definition.Defaults` remains the
-initial decoded state: supplied sequences replace default sequences. Source
-append does not append to a typed default list.
-
-Application resolution retains assignment and recorded-transformation semantics.
-ConfigKit does not automatically combine a callback's assigned sequence with its
-previous value. Application-registered identities receive final validation
-independently of how defaults, CLI overrides, or callbacks produce their values.
-
-### CLI Precedence
-
-CLI overrides are the highest-precedence input to application resolution.
-Supplied sequences replace whole sequences, including sequences inside supplied
-mappings. Mappings retain recursive override behavior; empty sequences and
-permitted nulls retain explicit replacement meaning. Configuration append or
-upsert does not reinterpret CLI assignments. Inheritance and defaulting do not
-run again afterward: overriding assembled `[A, B]` with `[C]` presents `[C]` to
-application resolution without inheritance reintroducing prior records.
-
-Application resolution sees CLI values and their explicit-presence and source
-metadata. `Resolution.Default` preserves explicit CLI values; normalization and
-recorded application transformations may still change them. For example, a derived
-URL can use a CLI hostname, while an explicit CLI URL can be preserved by an
-absence-based default. Retain recording and provenance requirements for those
-transformations.
-
-Reject structural directives such as `extends` and `configkit` in CLI override
-paths or supplied values. CLI input supplies application values after structural
-processing; it cannot supply or cancel inheritance directives, change completed
-relationships, or prevent earlier inheritance failures.
-
-Typed decoding also precedes CLI application, so a CLI replacement cannot repair
-an earlier decoding failure. A source string in an integer field fails before a
-valid CLI integer can apply. This changes current override-before-decoding
-behavior. Semantic validation remains after overrides and application resolution.
-
-Secret lookup uses effective references after overrides and resolution. Neither
-stage bypasses final application identity constraints or semantic validation.
-
-## Validation And Diagnostics
-
-### Structural Identity Checks
-
-Allow item-local helper imports to complete and consume applicable non-main
-source-version headers before checking payload identity for collection assembly.
-A helper can supply an identity component; a consumed header cannot supply one
-after removal.
-
-Structural controls are likewise unavailable as source identity components.
-Check identity against application payload fields, excluding fields acting as
-structural directives even when the engine retains them for later processing.
-For example, `key: extends` with an item containing `extends: null` does not
-supply a null identity: that field cancels inheritance and is consumed. Report
-the ordinary missing-identity-component error at the collection/item path using
-available origins, without exposing values. Do not defer this failure until
-directive consumption or final application validation.
-
-This rule follows a field's structural role; it does not introduce a broader
-restriction on unrelated literal field names. Final application identity checks
-continue to examine the effective typed payload under their existing rules.
-
-Validate each identified contribution before combination and check uniqueness in
-the resulting identified collection, including inheritance operations. Upsert
-matches across precedence are replacements; duplicates within a contribution
-remain errors. Later inheritance, typed defaults, or callbacks cannot repair an
-identity missing at an earlier assembly boundary.
-
-YAML-only identities are enforced during structural composition, precedence
-assembly, and identified inheritance operations. They have no final-result
-guarantee after typed defaulting, CLI overrides, or application resolution. Those
-stages can produce duplicate identities or otherwise violate a YAML-only identity.
-Applications needing a final guarantee register the constraint through the
-application-owned interface.
-
-Adding record inheritance does not expand this guarantee: a YAML declaration
-can enable parent lookup without imposing final validation on values later
-produced by defaults, CLI overrides, or application resolution.
-
-### Final Application Constraints
-
-On successful loading, final effective collections covered by application
-registrations satisfy their declared identity constraints. Retain assembly-time
-checks and validate final effective sequences before publication, after defaults,
-inheritance, overrides, application resolution, and secret lookup.
-
-This covers both exact and dynamic application registrations. Evaluate dynamic
-selectors against the final effective configuration, with independent validation
-for each concrete matched sequence as described under
-[Matching Across The Loading Lifecycle](#matching-across-the-loading-lifecycle).
-
-ConfigKit performs these identity checks; consumers register the constraint
-without needing to repeat duplicate detection in `Definition.Validate`.
-Application-owned semantic validation continues to cover domain-specific rules.
-
-This guarantee covers actual sequences, not path existence or allowed shape.
-It does not require every intermediate callback state to satisfy the constraint
-and does not cover caller mutations after loading returns. Final checks examine
-effective typed values; assembly checks validate authored contributions. A final
-scalar alone does not prove that a source explicitly supplied that field.
-
-Use the existing canonical final-value representation and shared publication
-path as the foundation. The guarantee requires neither a new mutation-history
-system nor exact causal reconstruction for records supplied by defaults or
-callbacks.
-
-### Diagnostics And Inspection
-
-Identity diagnostics identify collection and item paths using available origin
-information without exposing identity values. Conflicting declarations identify
-the collection destination and declaration locations.
-
-For dynamic registrations, use concrete matched collection and item paths for
-value diagnostics, with available origins and existing sensitivity rules.
-Definition-time overlap errors identify the conflicting registrations; they do
-not require or invent a concrete destination from a loaded document.
-
-Correctness includes final values, projected item paths, source destinations,
-provenance, history, sensitivity, and passive resource contexts. Appending can
-change final indexes; replacing changes which contributions survive. Preserve
-the existing loading and inspection contracts through both operations, including
-policy ownership and inheritance transfer.
-
-## Work Needed From The Baseline
-
-The baseline needs the following connected capabilities to reach the target
-state. Execution order and test sequencing belong to the
-[implementation plan](implementation.md):
-
-- **Declarations and public API:** extend application registration to the settled
-  identity model with one public registration type and definition field, an
-  exact-path convenience constructor, and an explicit-selector constructor.
-  Preserve literal `Path` semantics. Parse and consume `configkit.sequences` across
-  supported source forms and selected overlays; preserve placement-relative and
-  item-local scopes. Keep sequence declarations independent of filesystem import
-  enablement.
-- **Identity and policy representation:** implement field-set comparison, typed
-  scalar identity tuples with exact numeric equality, declaration agreement,
-  pending declarations, explicit-mode persistence, replacement lifetime, and
-  record-owned policy transfer. Choose bookkeeping proportional to these semantics.
-  Handle descendant collection removal through ancestor replacement, preserving
-  independently declared modes and application constraints while discarding
-  removed collection identity and record-owned policies.
-- **Registration matching:** support literal and arbitrary-mapping-key segments,
-  detect incompatible selector overlap during definition validation, and resolve
-  applicable constraints at concrete placed destinations. Reuse the identity
-  rules and validator for exact and dynamic registrations. Keep matching separate
-  from source discovery and from YAML declaration ownership.
-- **Assembly handoffs:** make accumulated lower policy available before higher-tier
-  strict composition. Separate source acquisition from policy-aware composition
-  or otherwise revise assembly sequencing; current main-before-underlay composition
-  is not a design constraint. Preserve distinctions among acquisition order,
-  dependency-ready scheduling within tiers, and precedence order. Account for
-  source records, callbacks, main-version checks, and failure diagnostics. Retain
-  version ownership and validation semantics; higher replacement cannot hide
-  lower-tier structural failures.
-- **Combination and inheritance:** implement the three operations with strict peer
-  handling, whole-record replacement, presence rules, stable ordering, and the
-  destination policy rules for inheritance. Extend sibling-scope discovery and
-  parent lookup to identified sequences, validate mapping-form references, and
-  preserve child identities while copying parent payloads. Reuse the shared
-  dependency-ready scheduler and destination-local nested resolution. Preserve
-  provenance, history, sensitivity, paths, and resource contexts as records survive,
-  move, or disappear.
-- **Lifecycle and validation:** move CLI application after inheritance and typed
-  defaults/decoding, preserve explicit-presence and source metadata for application
-  resolution, reject CLI structural directives, and add final application identity
-  validation through the shared publication path. Keep YAML-only enforcement
-  within structural operations. Match dynamic registrations during assembly and
-  inheritance, and against the final effective configuration rather than a saved
-  list of source-backed destinations.
-  Exclude structural controls from source identity checks even while retaining
-  those controls for later inheritance processing.
-- **Documentation and verification:** exercise ordinary and layered loading,
-  imported and inline collections, item-local scopes, version-header consumption,
-  selected overlays, inheritance, typed defaults, CLI, callbacks, and inspection.
-  Cover single and compound parent references, scalar equality, forward references
-  and chains, missing parents and cycles, parent replacement/removal, child identity
-  preservation, nested destination policy, cancellation, and stable output order.
-  Verify helper-composed references, overlapping reference fields, missing or extra
-  fields, and replacement of references together with their containing records.
-  Verify independent namespace uniqueness, root and namespaced registrations,
-  literal punctuation keys, equivalent and incompatible overlaps, and conflicts
-  detected even without a matching input destination. Cover namespaces introduced
-  by imports, placement, higher tiers, inheritance, defaults, CLI, and callbacks;
-  preserve ordinary strict-peer and upsert behavior. Check YAML disagreement under
-  replace, harmless absent or empty collections, schema-owned shape requirements,
-  unrelated sequences remaining unselected, and concrete-path diagnostics.
-  Verify both constructors feed the same registration model, including mixed
-  exact and dynamic registrations, compound identities, and definition-time
-  rejection of empty identity field sets.
-  Cover ancestor replacement followed by collection reintroduction, including a
-  surviving append mode with and without renewed identity, explicit replacement,
-  and application-registered identity. Verify that `extends: null` cannot supply
-  an identity component and that structural-control exclusion uses ordinary
-  value-free missing-identity diagnostics.
-  Update the public contract and consumer examples when behavior lands.
-
-The collection-policy and CLI-ordering decisions are accepted in
-[ADR 0017](../adr/0017-collection-sequence-policy.md) for collection policy and
-[ADR 0018](../adr/0018-cli-after-structural-resolution.md) for CLI ordering.
-Their scoped supersession is recorded in the affected earlier ADRs, and the
-[architecture policy](../policy/architecture.md) reflects the accepted decisions.
-These govern the target architecture; the configuration contract continues to
-describe implemented behavior until the feature lands. This roadmap does not
-select migration machinery.
-
-ADR 0017 now includes inheritance within identified sequences;
-[ADR 0009](../adr/0009-sibling-inheritance.md) records that scope extension in its
-status while preserving its decision content. The architecture policy is aligned.
-Dynamic selectors extend the destinations covered by application registration;
-they retain the accepted application authority and final-validation boundary.
-
-No open feature decisions remain. The implementation plan translates this end
-state into reviewable stages without adding guarantees, restrictions, or
-combination semantics.

example_test.go

@@ -19,6 +19,28 @@ func ExampleLoad() {
 	// Output: {true 3} <nil>
 }
 
+func ExampleLoad_sequenceDeclaration() {
+	type Item struct {
+		ID string `yaml:"id"`
+	}
+	type Config struct {
+		Items []Item `yaml:"items"`
+	}
+	result, err := configkit.Load[Config](configkit.Bytes("catalog.yml", []byte(`configkit:
+  sequences:
+    - destination: [items]
+      key: id
+items:
+  - id: prompt
+`)))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(result.Config.Items[0].ID)
+	// Output: prompt
+}
+
 func ExampleNoSource() {
 	type Config struct {
 		Workers int `yaml:"workers"`
@@ -154,8 +176,15 @@ func ExampleOverride() {
 		})
 	}
 	result, err := configkit.Load(configkit.NoSource(), configkit.Options[Config]{
-		Definition: configkit.Definition[Config]{Defaults: func() Config { return Config{Workers: 4} }},
-		Overrides:  overrides,
+		Definition: configkit.Definition[Config]{
+			Defaults: func() Config { return Config{Workers: 4} },
+			Resolve: func(configuration *Config, resolution *configkit.Resolution) error {
+				configkit.Default(resolution, configkit.Keys("workers"), &configuration.Workers, 100,
+					"preserve an explicitly supplied worker count")
+				return nil
+			},
+		},
+		Overrides: overrides,
 	})
 	fmt.Println(result.Config.Workers, result.Sources()[0].Label, err)
 	// Output: 0 --workers <nil>

examples/consumers/prompt_catalog.go

@@ -45,10 +45,9 @@ func LoadRequiredPromptCatalog(mainPath string) (*configkit.Result[PromptCatalog
 
 func promptCatalogDefinition() configkit.Definition[PromptCatalogConfig] {
 	return configkit.Definition[PromptCatalogConfig]{
-		SequenceCompositions: []configkit.SequenceComposition{{
-			Path:          configkit.Keys("prompts"),
-			IdentityField: "id",
-		}},
+		SequenceIdentities: []configkit.SequenceIdentity{
+			configkit.SequenceIdentityAt(configkit.Keys("prompts"), "id"),
+		},
 		Validate: validatePromptCatalog,
 	}
 }

examples/consumers/sequence_catalog.go

@@ -0,0 +1,134 @@
+package consumers
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// SequenceCatalogConfig is a source-backed prompt catalog with an application
+// supplied run label and a value derived from that label during resolution.
+type SequenceCatalogConfig struct {
+	RunLabel      string                 `yaml:"run_label"`
+	ResolvedLabel string                 `yaml:"resolved_label"`
+	Prompts       []SequenceCatalogEntry `yaml:"prompts"`
+}
+
+// SequenceCatalogEntry keeps prompt meaning, resources, and credentials in the
+// consuming application while ConfigKit handles collection assembly.
+type SequenceCatalogEntry struct {
+	ID          string           `yaml:"id"`
+	Name        string           `yaml:"name"`
+	Description string           `yaml:"description"`
+	Messages    []string         `yaml:"messages"`
+	Resource    string           `yaml:"resource"`
+	Credential  configkit.Secret `yaml:"credential"`
+}
+
+// LoadSequenceCatalog assembles imported main fragments over a file-backed
+// underlay, applies one selected collection overlay, then resolves and validates
+// the resulting application configuration.
+func LoadSequenceCatalog(mainPath, overlayName, runLabel string) (*configkit.Result[SequenceCatalogConfig], error) {
+	root := filepath.Dir(mainPath)
+	filesystem := configkit.NewFilesystem("sequence catalog", os.DirFS(root))
+	filesystemContext := &configkit.FilesystemContext{BaseDir: ".", Root: ".", FS: &filesystem}
+	definition := configkit.Definition[SequenceCatalogConfig]{
+		SequenceIdentities: []configkit.SequenceIdentity{
+			configkit.SequenceIdentityAt(configkit.Keys("prompts"), "id"),
+		},
+		Resolve: func(configuration *SequenceCatalogConfig, resolution *configkit.Resolution) error {
+			configuration.ResolvedLabel = "for:" + configuration.RunLabel
+			resolution.Record(configkit.Keys("resolved_label"), configkit.ChangeNormalization,
+				"derive the resolved label from the selected run label", configkit.Keys("run_label"))
+			return nil
+		},
+		Validate: validateSequenceCatalog,
+	}
+	return configkit.LoadLayered(configkit.Input{
+		Source:  filesystem.File(filepath.Base(mainPath)),
+		Imports: true,
+	}, configkit.LayeredOptions[SequenceCatalogConfig]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+			Source: filesystem.File("underlay.yml"),
+		}}}},
+		Overlay:           overlayName,
+		OverlayFilesystem: filesystemContext,
+		Overrides: []configkit.Override{{
+			Path: configkit.Keys("run_label"), Value: runLabel, Flag: "--run-label",
+		}},
+		Definition: definition,
+	})
+}
+
+// ReadSequenceCatalogResource follows the final prompt field's source context,
+// including when record inheritance retained a resource from an underlay.
+func ReadSequenceCatalogResource(result *configkit.Result[SequenceCatalogConfig], promptID string) (string, error) {
+	if result == nil {
+		return "", errors.New("sequence catalog result is nil")
+	}
+	for index, prompt := range result.Config.Prompts {
+		if prompt.ID != promptID {
+			continue
+		}
+		resourcePath := configkit.Keys("prompts").Index(index).Key("resource")
+		info, found := result.Metadata().Lookup(resourcePath)
+		if !found || len(info.Origin.Locations) != 1 {
+			return "", errors.New("prompt resource has no single declaring source")
+		}
+		context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+		if !found {
+			return "", errors.New("prompt resource source context is unavailable")
+		}
+		data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, prompt.Resource))
+		if err != nil {
+			return "", fmt.Errorf("read prompt resource: %w", err)
+		}
+		return string(data), nil
+	}
+	return "", fmt.Errorf("prompt %q is unavailable", promptID)
+}
+
+func validateSequenceCatalog(configuration *SequenceCatalogConfig, validation *configkit.Validation) {
+	for index, prompt := range configuration.Prompts {
+		if strings.TrimSpace(prompt.Name) == "" {
+			validation.Error(configkit.Keys("prompts").Index(index).Key("name"), "must not be empty")
+		}
+		if len(prompt.Messages) == 0 {
+			validation.Error(configkit.Keys("prompts").Index(index).Key("messages"), "must contain at least one message")
+		}
+	}
+	if len(configuration.Prompts) == 0 {
+		validation.Error(configkit.Keys("prompts"), "must contain at least one prompt")
+	}
+}
+
+// NamespacedMapCatalogConfig demonstrates typed application maps whose
+// independent prompt sequences use root and dynamic namespace registrations.
+type NamespacedMapCatalogConfig struct {
+	Prompts     []map[string]any          `yaml:"prompts"`
+	Collections map[string]map[string]any `yaml:"collections"`
+}
+
+// LoadNamespacedMapCatalog applies the same scalar identity constraint to one
+// root collection and each namespace's independently scoped collection.
+func LoadNamespacedMapCatalog(source configkit.Source) (*configkit.Result[NamespacedMapCatalogConfig], error) {
+	selector := configkit.SequenceSelector{
+		configkit.SequenceLiteralKey("collections"),
+		configkit.SequenceAnyMappingKey(),
+		configkit.SequenceLiteralKey("prompts"),
+	}
+	return configkit.Load(source, configkit.Options[NamespacedMapCatalogConfig]{
+		Definition: configkit.Definition[NamespacedMapCatalogConfig]{
+			SequenceIdentities: []configkit.SequenceIdentity{
+				configkit.SequenceIdentityAt(configkit.Keys("prompts"), "id"),
+				configkit.SequenceIdentityMatching(selector, "id"),
+			},
+		},
+	})
+}

examples/consumers/sequence_catalog_test.go

@@ -0,0 +1,239 @@
+package consumers_test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func Example_sequenceCatalog() {
+	const oldSecret = "CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET"
+	const newSecret = "CONFIGKIT_SEQUENCE_CATALOG_NEW_SECRET"
+	restoreOld, restoreNew := preserveEnvironment(oldSecret), preserveEnvironment(newSecret)
+	defer restoreOld()
+	defer restoreNew()
+	_ = os.Setenv(oldSecret, "disposable-old-example-credential")
+	_ = os.Setenv(newSecret, "disposable-new-example-credential")
+
+	result, err := consumers.LoadSequenceCatalog(fixture("sequence-catalog", "config.yml"), "upsert", "preview")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	ids := make([]string, 0, len(result.Config.Prompts))
+	for _, prompt := range result.Config.Prompts {
+		ids = append(ids, prompt.ID)
+	}
+	shared := configkit.Keys("prompts").Index(1).Key("name")
+	info, _ := result.Metadata().Lookup(shared)
+	resource, resourceErr := consumers.ReadSequenceCatalogResource(result, "concise")
+	fmt.Println(ids, result.Config.ResolvedLabel, info.Explicit, len(result.Metadata().History(shared)), resource, resourceErr == nil, err)
+	// Output: [foundation shared welcome concise extra] for:preview true 2 Foundation resource text.
+	//  true <nil>
+}
+
+func TestSequenceCatalogSelectedOverlayOperations(t *testing.T) {
+	const oldSecret = "CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET"
+	const newSecret = "CONFIGKIT_SEQUENCE_CATALOG_NEW_SECRET"
+	t.Setenv(oldSecret, "sequence-catalog-old-canary")
+	t.Setenv(newSecret, "sequence-catalog-new-canary")
+
+	tests := []struct {
+		name    string
+		wantIDs []string
+		shared  *consumers.SequenceCatalogEntry
+	}{
+		{name: "append", wantIDs: []string{"foundation", "shared", "welcome", "concise", "extra"}},
+		{name: "upsert", wantIDs: []string{"foundation", "shared", "welcome", "concise", "extra"}, shared: &consumers.SequenceCatalogEntry{ID: "shared", Name: "Shared replacement", Messages: []string{"Replacement message"}, Credential: configkit.SecretEnv(newSecret)}},
+		{name: "replace", wantIDs: []string{"replacement", "last"}},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := consumers.LoadSequenceCatalog(fixture("sequence-catalog", "config.yml"), test.name, "test")
+			if err != nil {
+				var loadError *configkit.LoadError
+				if errors.As(err, &loadError) {
+					t.Fatalf("load selected overlay: %v (%#v)", err, loadError.Diagnostics())
+				}
+				t.Fatal(err)
+			}
+			gotIDs := make([]string, 0, len(result.Config.Prompts))
+			for _, prompt := range result.Config.Prompts {
+				gotIDs = append(gotIDs, prompt.ID)
+			}
+			if strings.Join(gotIDs, ",") != strings.Join(test.wantIDs, ",") {
+				t.Fatalf("prompt order = %v, want %v", gotIDs, test.wantIDs)
+			}
+			if test.shared == nil {
+				return
+			}
+			shared := result.Config.Prompts[1]
+			if shared.ID != test.shared.ID || shared.Name != test.shared.Name || len(shared.Messages) != 1 ||
+				shared.Messages[0] != test.shared.Messages[0] || shared.Description != "" || shared.Resource != "" || shared.Credential.EnvName() != newSecret {
+				t.Fatalf("upsert did not replace the complete matched record: %#v", shared)
+			}
+		})
+	}
+}
+
+func TestSequenceCatalogInspectionTracksResourcesAndSecretHistory(t *testing.T) {
+	const oldSecret = "CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET"
+	const newSecret = "CONFIGKIT_SEQUENCE_CATALOG_NEW_SECRET"
+	const oldCanary = "sequence-catalog-old-canary"
+	const newCanary = "sequence-catalog-new-canary"
+	t.Setenv(oldSecret, oldCanary)
+	t.Setenv(newSecret, newCanary)
+
+	result, err := consumers.LoadSequenceCatalog(fixture("sequence-catalog", "config.yml"), "upsert", "history")
+	if err != nil {
+		var loadError *configkit.LoadError
+		if errors.As(err, &loadError) {
+			t.Fatalf("load sequence catalog: %v (%#v)", err, loadError.Diagnostics())
+		}
+		t.Fatal(err)
+	}
+	resourcePath := configkit.Keys("prompts").Index(3).Key("resource")
+	resourceInfo, found := result.Metadata().Lookup(resourcePath)
+	if !found || len(resourceInfo.Origin.Locations) != 1 || len(resourceInfo.Origin.Inheritance) != 1 {
+		t.Fatalf("inherited resource inspection = %#v, %v", resourceInfo, found)
+	}
+	resource, err := consumers.ReadSequenceCatalogResource(result, "concise")
+	if err != nil || resource != "Foundation resource text.\n" {
+		t.Fatalf("resource from inherited source context = %q, %v", resource, err)
+	}
+	if len(result.Metadata().History(configkit.Keys("resolved_label"))) == 0 {
+		t.Fatal("recorded CLI-driven resolution was absent from final inspection")
+	}
+	credentialPath := configkit.Keys("prompts").Index(1).Key("credential")
+	credentialHistory := result.Metadata().History(credentialPath)
+	if len(credentialHistory) == 0 {
+		t.Fatal("upsert did not retain the replaced secret reference in history")
+	}
+	var explanation bytes.Buffer
+	if err := result.WriteExplanation(&explanation, credentialPath); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(explanation.String(), oldCanary) || strings.Contains(explanation.String(), newCanary) {
+		t.Fatalf("secret explanation exposed a credential value: %s", explanation.String())
+	}
+}
+
+func TestNamespacedTypedMapIdentityScopesAreIndependent(t *testing.T) {
+	source := configkit.Bytes("namespaced maps", []byte(`
+prompts:
+  - id: shared
+    text: root prompt
+collections:
+  team:
+    prompts:
+      - id: shared
+        text: team prompt
+  operations:
+    prompts:
+      - id: shared
+        text: operations prompt
+`))
+	result, err := consumers.LoadNamespacedMapCatalog(source)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Config.Prompts[0]["text"] != "root prompt" ||
+		result.Config.Collections["team"]["prompts"].([]any)[0].(map[string]any)["text"] != "team prompt" ||
+		result.Config.Collections["operations"]["prompts"].([]any)[0].(map[string]any)["text"] != "operations prompt" {
+		t.Fatalf("typed map collections = %#v", result.Config)
+	}
+}
+
+func TestNamespacedTypedMapIdentityFindsCollectionsIntroducedByInheritance(t *testing.T) {
+	result, err := consumers.LoadNamespacedMapCatalog(configkit.Bytes("inherited namespace", []byte(`
+collections:
+  source:
+    prompts:
+      - id: shared
+        text: inherited prompt
+  copied:
+    extends: source
+`)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	copied := result.Config.Collections["copied"]["prompts"].([]any)
+	if len(copied) != 1 || copied[0].(map[string]any)["text"] != "inherited prompt" {
+		t.Fatalf("inherited typed-map collection = %#v", result.Config.Collections["copied"])
+	}
+}
+
+func Example_yamlDeclaredIdentityOnly() {
+	result, err := configkit.Load(configkit.Bytes("YAML identity only", []byte(`
+configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+prompts:
+  - id: initial
+`)), configkit.Options[map[string]any]{
+		Overrides: []configkit.Override{{
+			Path: configkit.Keys("prompts"),
+			Value: []any{
+				map[string]any{"id": "same"},
+				map[string]any{"id": "same"},
+			},
+			Flag: "--prompts",
+		}},
+	})
+	fmt.Println(len(result.Config["prompts"].([]any)), err)
+	// Output: 2 <nil>
+}
+
+func TestYAMLDeclaredIdentityDoesNotRegisterFinalTypedConstraint(t *testing.T) {
+	result, err := configkit.Load(configkit.Bytes("YAML identity only", []byte(`
+configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+prompts:
+  - id: initial
+`)), configkit.Options[map[string]any]{
+		Overrides: []configkit.Override{{
+			Path: configkit.Keys("prompts"),
+			Value: []any{
+				map[string]any{"id": "same"},
+				map[string]any{"id": "same"},
+			},
+			Flag: "--prompts",
+		}},
+	})
+	if err != nil {
+		t.Fatalf("YAML-only identity incorrectly became an application constraint: %v", err)
+	}
+	prompts := result.Config["prompts"].([]any)
+	if len(prompts) != 2 || prompts[0].(map[string]any)["id"] != prompts[1].(map[string]any)["id"] {
+		t.Fatalf("CLI result = %#v", result.Config)
+	}
+}
+
+func TestNamespacedTypedMapRejectsDuplicatesWithinOneCollection(t *testing.T) {
+	_, err := consumers.LoadNamespacedMapCatalog(configkit.Bytes("duplicate namespace", []byte(`
+collections:
+  team:
+    prompts:
+      - id: duplicate
+      - id: duplicate
+`)))
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 ||
+		loadError.Diagnostics()[0].Code != configkit.CodeComposeConflict ||
+		loadError.Diagnostics()[0].Path.String() != "/collections/team/prompts" {
+		diagnostics := []configkit.Diagnostic(nil)
+		if loadError != nil {
+			diagnostics = loadError.Diagnostics()
+		}
+		t.Fatalf("duplicate namespaced identity error = %v (%#v)", err, diagnostics)
+	}
+}

examples/consumers/testdata/prompt-catalog/config.yml

@@ -1,4 +1,8 @@
 configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+      mode: replace
   imports:
     - dir: prompts
       exclude: [generated-ignored.yml]

examples/consumers/testdata/prompt-catalog/invalid.yml

@@ -1,3 +1,8 @@
+configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+      mode: replace
 prompts:
   - id: invalid
     name: Invalid prompt

examples/consumers/testdata/sequence-catalog/config.yml

@@ -0,0 +1,7 @@
+run_label: source-default
+configkit:
+  imports:
+    - dir: fragments
+      pattern: '*.yml'
+      destination: [prompts]
+      placement: sequence-item

examples/consumers/testdata/sequence-catalog/foundation.txt

@@ -0,0 +1 @@
+Foundation resource text.

examples/consumers/testdata/sequence-catalog/fragments/01-welcome.yml

@@ -0,0 +1,4 @@
+id: welcome
+name: Welcome
+messages: [Welcome message]
+credential: CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET

examples/consumers/testdata/sequence-catalog/fragments/02-concise.yml

@@ -0,0 +1,3 @@
+id: concise
+extends: {id: foundation}
+name: Concise

examples/consumers/testdata/sequence-catalog/overlays/append.yml

@@ -0,0 +1,10 @@
+configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+      mode: append
+prompts:
+  - id: extra
+    name: Extra
+    messages: [Extra message]
+    credential: CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET

examples/consumers/testdata/sequence-catalog/overlays/replace.yml

@@ -0,0 +1,14 @@
+configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+      mode: replace
+prompts:
+  - id: replacement
+    name: Replacement
+    messages: [Replacement message]
+    credential: CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET
+  - id: last
+    name: Last
+    messages: [Last message]
+    credential: CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET

examples/consumers/testdata/sequence-catalog/overlays/upsert.yml

@@ -0,0 +1,14 @@
+configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+      mode: upsert
+prompts:
+  - id: shared
+    name: Shared replacement
+    messages: [Replacement message]
+    credential: CONFIGKIT_SEQUENCE_CATALOG_NEW_SECRET
+  - id: extra
+    name: Extra
+    messages: [Extra message]
+    credential: CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET

examples/consumers/testdata/sequence-catalog/underlay.yml

@@ -0,0 +1,18 @@
+configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+      mode: append
+prompts:
+  - id: foundation
+    name: Foundation
+    description: Shared foundation instruction
+    messages: [Foundation message]
+    resource: foundation.txt
+    credential: CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET
+  - id: shared
+    name: Shared draft
+    description: This field must not survive a whole-record replacement
+    messages: [Draft message]
+    resource: shared.txt
+    credential: CONFIGKIT_SEQUENCE_CATALOG_OLD_SECRET

facade_final_sequence_identity_test.go

@@ -0,0 +1,249 @@
+package configkit_test
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestFinalSequenceIdentityTreatsLateExtendsFieldsAsPayload(t *testing.T) {
+	for _, phase := range []string{"defaults", "resolution"} {
+		for _, component := range []any{"application-data", nil} {
+			for _, duplicate := range []bool{false, true} {
+				t.Run(fmt.Sprintf("%s/%v/duplicate=%t", phase, component, duplicate), func(t *testing.T) {
+					items := []any{map[string]any{"extends": component}}
+					if duplicate {
+						items = append(items, map[string]any{"extends": component})
+					}
+					definition := configkit.Definition[map[string]any]{
+						SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("items"), "extends")},
+						Defaults: func() map[string]any {
+							if phase == "defaults" {
+								return map[string]any{"items": items}
+							}
+							return map[string]any{}
+						},
+					}
+					if phase == "resolution" {
+						definition.Resolve = func(value *map[string]any, resolution *configkit.Resolution) error {
+							(*value)["items"] = items
+							resolution.Record(configkit.Keys("items"), configkit.ChangeNormalization, "supply application payload")
+							return nil
+						}
+					}
+					loaded, err := configkit.Load(configkit.NoSource(), configkit.Options[map[string]any]{Definition: definition})
+					if duplicate {
+						loadErr := requireFacadeLoadError(t, err)
+						diagnostics := loadErr.Diagnostics()
+						if loaded != nil || len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeSequenceConflict || diagnostics[0].Path.String() != "/items" {
+							t.Fatalf("duplicate effective payload diagnostics = %#v", diagnostics)
+						}
+						return
+					}
+					if err != nil || loaded == nil {
+						t.Fatalf("ordinary effective identity was interpreted as a directive: %v", err)
+					}
+					value := loaded.Config["items"].([]any)[0].(map[string]any)
+					if actual, present := value["extends"]; !present || actual != component {
+						t.Fatalf("effective payload = %#v", value)
+					}
+				})
+			}
+		}
+	}
+}
+
+func TestFinalSequenceIdentityValidatesLateValues(t *testing.T) {
+	identity := configkit.SequenceIdentityAt(configkit.Keys("items"), "id")
+	for _, source := range []string{"defaults", "CLI", "resolution"} {
+		for _, shape := range []string{"duplicate", "missing", "non-scalar"} {
+			t.Run(source+"/"+shape, func(t *testing.T) {
+				items := finalIdentityRecords(shape)
+				definition := configkit.Definition[map[string]any]{
+					SequenceIdentities: []configkit.SequenceIdentity{identity},
+					Defaults: func() map[string]any {
+						if source == "defaults" {
+							return map[string]any{"items": items}
+						}
+						return map[string]any{}
+					},
+				}
+				options := configkit.Options[map[string]any]{Definition: definition}
+				switch source {
+				case "CLI":
+					options.Overrides = []configkit.Override{{Path: configkit.Keys("items"), Value: items, Flag: "--items"}}
+				case "resolution":
+					options.Definition.Resolve = func(value *map[string]any, resolution *configkit.Resolution) error {
+						(*value)["items"] = items
+						resolution.Record(configkit.Keys("items"), configkit.ChangeNormalization, "create effective records")
+						return nil
+					}
+				}
+				result, err := configkit.Load(configkit.NoSource(), options)
+				if result != nil {
+					t.Fatalf("invalid final identities returned a result: %#v", result.Config)
+				}
+				loadErr := requireFacadeLoadError(t, err)
+				if len(loadErr.Diagnostics()) != 1 {
+					t.Fatalf("diagnostics = %#v", loadErr.Diagnostics())
+				}
+				diagnostic := loadErr.Diagnostics()[0]
+				wantCode, wantPath := configkit.CodeSequenceInvalid, "/items/0/id"
+				if shape == "duplicate" {
+					wantCode, wantPath = configkit.CodeSequenceConflict, "/items"
+				}
+				if diagnostic.Code != wantCode || diagnostic.Path.String() != wantPath {
+					t.Fatalf("diagnostic = %#v, want %s at %s", diagnostic, wantCode, wantPath)
+				}
+				if source == "CLI" && (diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1") {
+					t.Fatalf("CLI identity diagnostic lost its source: %#v", diagnostic)
+				}
+			})
+		}
+	}
+}
+
+func TestFinalSequenceIdentityMatchesLateDynamicNamespaces(t *testing.T) {
+	selector := configkit.SequenceSelector{
+		configkit.SequenceLiteralKey("collections"),
+		configkit.SequenceAnyMappingKey(),
+		configkit.SequenceLiteralKey("items"),
+	}
+	for _, source := range []string{"defaults", "CLI", "resolution"} {
+		t.Run(source, func(t *testing.T) {
+			name := map[string]string{"defaults": "team", "CLI": "cloud", "resolution": "orbit"}[source]
+			path := configkit.Keys("collections", name, "items")
+			items := finalIdentityRecords("duplicate")
+			definition := configkit.Definition[map[string]any]{
+				SequenceIdentities: []configkit.SequenceIdentity{
+					configkit.SequenceIdentityMatching(selector, "id"),
+				},
+				Defaults: func() map[string]any {
+					if source == "defaults" {
+						return map[string]any{"collections": map[string]any{name: map[string]any{"items": items}}}
+					}
+					return map[string]any{}
+				},
+			}
+			options := configkit.Options[map[string]any]{Definition: definition}
+			switch source {
+			case "CLI":
+				options.Overrides = []configkit.Override{{Path: path, Value: items, Flag: "--items"}}
+			case "resolution":
+				options.Definition.Resolve = func(value *map[string]any, resolution *configkit.Resolution) error {
+					(*value)["collections"] = map[string]any{name: map[string]any{"items": items}}
+					resolution.Record(configkit.Keys("collections"), configkit.ChangeNormalization, "create namespaced records")
+					return nil
+				}
+			}
+			result, err := configkit.Load(configkit.NoSource(), options)
+			if result != nil {
+				t.Fatalf("invalid dynamic identity returned a result: %#v", result.Config)
+			}
+			loadErr := requireFacadeLoadError(t, err)
+			if diagnostics := loadErr.Diagnostics(); len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeSequenceConflict || diagnostics[0].Path.String() != path.String() {
+				t.Fatalf("dynamic identity diagnostics = %#v", diagnostics)
+			}
+		})
+	}
+}
+
+func TestFinalSequenceIdentityAppliesToLayeredLoads(t *testing.T) {
+	definition := configkit.Definition[map[string]any]{
+		SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("items"), "id")},
+		Defaults: func() map[string]any {
+			return map[string]any{"items": finalIdentityRecords("duplicate")}
+		},
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{Definition: definition})
+	if result != nil {
+		t.Fatalf("invalid layered final identities returned a result: %#v", result.Config)
+	}
+	loadErr := requireFacadeLoadError(t, err)
+	if diagnostics := loadErr.Diagnostics(); len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeSequenceConflict {
+		t.Fatalf("layered identity diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestFinalSequenceIdentityUsesTypedNumbersAndIgnoresUnmatchedShapes(t *testing.T) {
+	definition := configkit.Definition[map[string]any]{
+		SequenceIdentities: []configkit.SequenceIdentity{
+			configkit.SequenceIdentityAt(configkit.Keys("items"), "id"),
+			configkit.SequenceIdentityAt(configkit.Keys("unmatched"), "id"),
+		},
+	}
+	_, err := configkit.Load(configkit.NoSource(), configkit.Options[map[string]any]{
+		Definition: configkit.Definition[map[string]any]{
+			SequenceIdentities: definition.SequenceIdentities,
+			Defaults: func() map[string]any {
+				return map[string]any{"items": []any{
+					map[string]any{"id": int64(1)}, map[string]any{"id": float64(1)},
+				}}
+			},
+		},
+	})
+	loadErr := requireFacadeLoadError(t, err)
+	if diagnostics := loadErr.Diagnostics(); len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeSequenceConflict {
+		t.Fatalf("typed numeric identity diagnostics = %#v", diagnostics)
+	}
+
+	valid, err := configkit.Load(configkit.Mapping("valid", map[string]any{
+		"items": []any{
+			map[string]any{"id": nil}, map[string]any{"id": true},
+			map[string]any{"id": int64(1)}, map[string]any{"id": "one"},
+		},
+		"other": []any{map[string]any{"id": "same"}, map[string]any{"id": "same"}},
+	}), configkit.Options[map[string]any]{Definition: definition})
+	if err != nil || valid == nil {
+		t.Fatalf("valid scalar identities and unmatched duplicate sequence = %#v, %v", valid, err)
+	}
+	if _, err := configkit.Load(configkit.Mapping("other-shapes", map[string]any{"items": nil}), configkit.Options[map[string]any]{Definition: definition}); err != nil {
+		t.Fatalf("null at a registered path was validated as an identity sequence: %v", err)
+	}
+}
+
+func TestFinalSequenceIdentityDiagnosticDoesNotRevealResolvedSecrets(t *testing.T) {
+	const canary = "FINAL-IDENTITY-CREDENTIAL-CANARY"
+	const secretName = "CONFIGKIT_FINAL_IDENTITY_SECRET"
+	t.Setenv(secretName, canary)
+	type item struct {
+		ID    string           `yaml:"id"`
+		Token configkit.Secret `yaml:"token"`
+	}
+	type config struct {
+		Items []item `yaml:"items"`
+	}
+	definition := configkit.Definition[config]{
+		SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("items"), "id")},
+		Defaults: func() config {
+			return config{Items: []item{
+				{ID: "duplicate", Token: configkit.SecretEnv(secretName)},
+				{ID: "duplicate", Token: configkit.SecretEnv(secretName)},
+			}}
+		},
+	}
+	result, err := configkit.Load(configkit.NoSource(), configkit.Options[config]{Definition: definition})
+	if result != nil {
+		t.Fatalf("duplicate identity returned a result: %#v", result.Config)
+	}
+	if err == nil || strings.Contains(err.Error(), canary) {
+		t.Fatalf("identity diagnostic leaked a resolved secret: %v", err)
+	}
+	loadErr := requireFacadeLoadError(t, err)
+	if diagnostics := loadErr.Diagnostics(); len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeSequenceConflict {
+		t.Fatalf("secret-bearing identity diagnostics = %#v", diagnostics)
+	}
+}
+
+func finalIdentityRecords(shape string) []any {
+	switch shape {
+	case "duplicate":
+		return []any{map[string]any{"id": "same"}, map[string]any{"id": "same"}}
+	case "missing":
+		return []any{map[string]any{"other": "value"}}
+	default:
+		return []any{map[string]any{"id": map[string]any{"nested": "value"}}}
+	}
+}

facade_import_lifecycle_test.go

@@ -40,8 +40,8 @@ entries:
 		Imports:    true,
 		Filesystem: &FilesystemContext{Root: "bundle", FS: &filesystem},
 		Definition: Definition[map[string]any]{
-			Version:              &VersionRule{Path: Keys("version"), Check: func(Version, *Validation) error { return nil }},
-			SequenceCompositions: []SequenceComposition{{Path: Keys("services", "records"), IdentityField: "id"}},
+			Version:            &VersionRule{Path: Keys("version"), Check: func(Version, *Validation) error { return nil }},
+			SequenceIdentities: []SequenceIdentity{SequenceIdentityAt(Keys("services", "records"), "id")},
 			Secret: func(candidate Path) bool {
 				return candidate.String() == "/services/records/0/version"
 			},
@@ -105,8 +105,8 @@ entries:
 		Imports:    true,
 		Filesystem: &FilesystemContext{Root: "bundle", FS: &filesystem},
 		Definition: Definition[map[string]any]{
-			Version:              &VersionRule{Path: Keys("version"), Check: func(Version, *Validation) error { return nil }},
-			SequenceCompositions: []SequenceComposition{{Path: Keys("services", "records"), IdentityField: "version"}},
+			Version:            &VersionRule{Path: Keys("version"), Check: func(Version, *Validation) error { return nil }},
+			SequenceIdentities: []SequenceIdentity{SequenceIdentityAt(Keys("services", "records"), "version")},
 			Secret: func(candidate Path) bool {
 				return candidate.String() == "/services/records/0/version"
 			},

facade_inheritance_test.go

@@ -91,6 +91,214 @@ func TestFacadeInheritancePreservesPresenceOriginsAndHistory(t *testing.T) {
 	}
 }
 
+func TestFacadeIdentifiedInheritanceResolvesChainsAndNestedCollections(t *testing.T) {
+	const secretName = "CONFIGKIT_IDENTIFIED_INHERITANCE_SECRET"
+	const canary = "IDENTIFIED-INHERITANCE-SECRET-CANARY"
+	t.Setenv(secretName, canary)
+	type step struct {
+		ID    string `yaml:"id"`
+		Value string `yaml:"value"`
+	}
+	type job struct {
+		ID         string                       `yaml:"id"`
+		Name       string                       `yaml:"name"`
+		Credential configkit.Secret             `yaml:"credential"`
+		Resource   string                       `yaml:"resource"`
+		Steps      []step                       `yaml:"steps"`
+		Settings   map[string]map[string]string `yaml:"settings"`
+	}
+	type configuration struct {
+		Jobs []job `yaml:"jobs"`
+	}
+	input := []byte(`jobs:
+- id: child
+  extends: {id: parent}
+  name: child-name
+- id: parent
+  extends: {id: base}
+- id: base
+  name: base-name
+  credential: CONFIGKIT_IDENTIFIED_INHERITANCE_SECRET
+  resource: icon.txt
+  steps:
+    - id: base-step
+      value: base-step-value
+    - id: second-step
+      value: second-step-value
+  settings:
+    primary: {value: base-setting}
+    secondary: {extends: primary}
+`)
+	filesystem := configkit.NewFilesystem("identified-bundle", fstest.MapFS{
+		"config/identified.yml": &fstest.MapFile{Data: input},
+		"config/icon.txt":       &fstest.MapFile{Data: []byte("identified-parent-resource")},
+	})
+	registrations := []configkit.SequenceIdentity{
+		configkit.SequenceIdentityAt(configkit.Keys("jobs"), "id"),
+	}
+	result, err := configkit.Load(filesystem.File("config/identified.yml"), configkit.Options[configuration]{
+		Definition: configkit.Definition[configuration]{
+			SequenceIdentities: registrations,
+			Secret: func(path configkit.Path) bool {
+				return path.String() == "/jobs/0/credential" || path.String() == "/jobs/1/credential" || path.String() == "/jobs/2/credential"
+			},
+		},
+	})
+	if err != nil {
+		if loadError, ok := err.(*configkit.LoadError); ok {
+			t.Fatalf("load identified inheritance: %v (%#v)", err, loadError.Diagnostics())
+		}
+		t.Fatal(err)
+	}
+	if len(result.Config.Jobs) != 3 || result.Config.Jobs[0].ID != "child" || result.Config.Jobs[1].ID != "parent" || result.Config.Jobs[2].ID != "base" {
+		t.Fatalf("inherited job positions and identities = %#v", result.Config.Jobs)
+	}
+	child := result.Config.Jobs[0]
+	if child.Name != "child-name" || child.Credential.Value() != canary || child.Resource != "icon.txt" || len(child.Steps) != 2 {
+		t.Fatalf("inherited child = %#v", child)
+	}
+	if got := []string{child.Steps[0].ID, child.Steps[1].ID}; strings.Join(got, ",") != "base-step,second-step" {
+		t.Fatalf("inherited nested steps = %v", got)
+	}
+	if child.Steps[0].Value != "base-step-value" || child.Steps[1].Value != "second-step-value" || child.Settings["secondary"]["value"] != "base-setting" {
+		t.Fatalf("inherited nested collection and mapping = %#v", child)
+	}
+	credentialPath := configkit.Keys("jobs").Index(0).Key("credential")
+	credentialInfo, found := result.Metadata().Lookup(credentialPath)
+	if !found || len(credentialInfo.Origin.Inheritance) != 2 {
+		t.Fatalf("inherited secret provenance = %#v, %v", credentialInfo, found)
+	}
+	resourceInfo, found := result.Metadata().Lookup(configkit.Keys("jobs").Index(0).Key("resource"))
+	if !found || len(resourceInfo.Origin.Inheritance) != 2 || len(resourceInfo.Origin.Locations) != 1 {
+		t.Fatalf("inherited resource provenance = %#v, %v", resourceInfo, found)
+	}
+	context, found := result.SourceContext(resourceInfo.Origin.Locations[0].SourceID)
+	if !found || context.BaseDir != "config" || context.FS == nil {
+		t.Fatalf("inherited resource context = %#v, %v", context, found)
+	}
+	resource, readErr := fs.ReadFile(context.FS, context.BaseDir+"/"+child.Resource)
+	if readErr != nil || string(resource) != "identified-parent-resource" {
+		t.Fatalf("inherited resource = %q, %v", resource, readErr)
+	}
+	resourceHistory, found := result.Explain(configkit.Keys("jobs").Index(0).Key("resource"))
+	if !found || len(resourceHistory.History) == 0 {
+		t.Fatalf("inherited resource history = %#v, %v", resourceHistory, found)
+	}
+	credentialHistory, found := result.Explain(credentialPath)
+	if !found || !credentialHistory.Effective.Redacted {
+		t.Fatalf("inherited secret explanation = %#v, %v", credentialHistory, found)
+	}
+	identityInfo, found := result.Metadata().Lookup(configkit.Keys("jobs").Index(0).Key("id"))
+	if !found || !identityInfo.Explicit || len(identityInfo.Origin.Inheritance) != 0 ||
+		len(identityInfo.Origin.Locations) != 1 {
+		t.Fatalf("child identity origin and presence = %#v, %v", identityInfo, found)
+	}
+	if _, found := result.Metadata().Lookup(configkit.Keys("jobs").Index(0).Key("extends")); found {
+		t.Fatal("identified record relationship remained in effective metadata")
+	}
+	var rendered bytes.Buffer
+	if err := result.WriteEffective(&rendered); err != nil || strings.Contains(rendered.String(), canary) || strings.Contains(rendered.String(), "extends:") {
+		t.Fatalf("effective inherited configuration = %q, %v", rendered.String(), err)
+	}
+}
+
+func TestFacadeIdentifiedInheritanceFailureDoesNotRevealSecretCanary(t *testing.T) {
+	const secretName = "CONFIGKIT_IDENTIFIED_INHERITANCE_FAILURE_SECRET"
+	const canary = "IDENTIFIED-INHERITANCE-FAILURE-CANARY"
+	t.Setenv(secretName, canary)
+	type job struct {
+		ID         string           `yaml:"id"`
+		Credential configkit.Secret `yaml:"credential"`
+	}
+	type configuration struct {
+		Jobs []job `yaml:"jobs"`
+	}
+	input := []byte(`jobs:
+- id: child
+  extends: {id: missing}
+  credential: CONFIGKIT_IDENTIFIED_INHERITANCE_FAILURE_SECRET
+`)
+	_, err := configkit.Load(configkit.Bytes("identified-failure.yml", input), configkit.Options[configuration]{
+		Definition: configkit.Definition[configuration]{
+			SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("jobs"), "id")},
+			Secret:             func(path configkit.Path) bool { return path.String() == "/jobs/0/credential" },
+		},
+	})
+	if err == nil || strings.Contains(err.Error(), canary) {
+		t.Fatalf("identified inheritance failure = %v; secret canary must remain undisclosed", err)
+	}
+}
+
+func TestFacadeLayeredIdentifiedInheritanceUsesSurvivingRecords(t *testing.T) {
+	policy := func(mode string) string {
+		return "configkit:\n  sequences:\n    - destination: [jobs]\n      key: id\n      mode: " + mode + "\n"
+	}
+	tests := []struct {
+		name        string
+		lower       string
+		higher      string
+		wantCode    configkit.Code
+		wantValue   string
+		wantSuccess bool
+	}{
+		{
+			name:      "upserted parent supplies its final payload",
+			lower:     policy("upsert") + "jobs: [{id: parent, value: lower}]\n",
+			higher:    policy("upsert") + "jobs: [{id: parent, value: higher}, {id: child, extends: {id: parent}}]\n",
+			wantValue: "higher", wantSuccess: true,
+		},
+		{
+			name:     "replacement removes a lower parent before lookup",
+			lower:    policy("upsert") + "jobs: [{id: parent, value: lower}]\n",
+			higher:   policy("replace") + "jobs: [{id: child, extends: {id: parent}}]\n",
+			wantCode: configkit.CodeInheritanceMissing,
+		},
+		{
+			name:      "upserted child without a directive replaces the invalid lower child",
+			lower:     policy("upsert") + "jobs: [{id: child, extends: {id: missing}, value: lower}]\n",
+			higher:    policy("upsert") + "jobs: [{id: child, value: higher}]\n",
+			wantValue: "higher", wantSuccess: true,
+		},
+		{
+			name:     "self-reference cannot resolve through a lower predecessor",
+			lower:    policy("upsert") + "jobs: [{id: parent, value: lower}]\n",
+			higher:   policy("upsert") + "jobs: [{id: parent, extends: {id: parent}, value: higher}]\n",
+			wantCode: configkit.CodeInheritanceCycle,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main.yml", []byte(test.higher))}, configkit.LayeredOptions[map[string]any]{
+				Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Bytes("lower.yml", []byte(test.lower))}}}},
+			})
+			if test.wantSuccess {
+				if err != nil {
+					t.Fatal(err)
+				}
+				jobs, ok := result.Config["jobs"].([]any)
+				if !ok || len(jobs) == 0 {
+					t.Fatalf("assembled jobs = %#v", result.Config["jobs"])
+				}
+				child, ok := jobs[len(jobs)-1].(map[string]any)
+				if !ok || child["value"] != test.wantValue {
+					t.Fatalf("surviving record = %#v, want inherited value %q", jobs[len(jobs)-1], test.wantValue)
+				}
+				return
+			}
+			loadError, ok := err.(*configkit.LoadError)
+			if !ok {
+				t.Fatalf("load error = %v, want code %s", err, test.wantCode)
+			}
+			for _, diagnostic := range loadError.Diagnostics() {
+				if diagnostic.Code == test.wantCode {
+					return
+				}
+			}
+			t.Fatalf("load diagnostics = %#v, want code %s", loadError.Diagnostics(), test.wantCode)
+		})
+	}
+}
+
 func TestFacadeInheritanceLayeredPrecedenceAndSourceContext(t *testing.T) {
 	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
 		"config/main.yml":          &fstest.MapFile{Data: []byte("entries: {child: {extends: parent}}\n")},
@@ -206,27 +414,24 @@ func TestFacadeInheritanceFailureStopsLaterCallbacks(t *testing.T) {
 	}
 }
 
-func TestFacadeInheritanceResolvesAfterImportsAndCLIReferenceChange(t *testing.T) {
+func TestFacadeCLIOverridesCannotChangeInheritanceRelationship(t *testing.T) {
 	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
 		"main.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: parent.yml}]}\nentries: {child: {extends: first}}\n")},
 		"parent.yml": &fstest.MapFile{Data: []byte("entries: {first: {count: 1}, second: {count: 2}}\n")},
 	})
-	result, err := configkit.Load(filesystem.File("main.yml"), configkit.Options[inheritedConfiguration]{
+	_, err := configkit.Load(filesystem.File("main.yml"), configkit.Options[inheritedConfiguration]{
 		Imports: true, Overrides: []configkit.Override{{Path: configkit.Keys("entries", "child", "extends"), Value: "second", Flag: "--extends"}},
 	})
-	if err != nil || result.Config.Entries["child"].Count != 2 {
-		t.Fatalf("import and CLI reference = %#v, %v", result, err)
-	}
-	history := result.Metadata().History(configkit.Keys("entries", "child", "extends"))
-	if len(history) < 3 || history[len(history)-1].Origin.Operation != configkit.ChangeInheritance {
-		t.Fatalf("superseded reference history = %#v", history)
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeOverrideDirective {
+		t.Fatalf("CLI inheritance relationship change = %v", err)
 	}
 	conflicting := configkit.NewFilesystem("bundle", fstest.MapFS{
 		"main.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: parent.yml}]}\nentries: {child: {extends: first}, first: {count: 3}}\n")},
 		"parent.yml": &fstest.MapFile{Data: []byte("entries: {first: {count: 1}}\n")},
 	})
 	_, err = configkit.Load[inheritedConfiguration](conflicting.File("main.yml"), configkit.Options[inheritedConfiguration]{Imports: true})
-	loadError, ok := err.(*configkit.LoadError)
+	loadError, ok = err.(*configkit.LoadError)
 	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeComposeConflict {
 		t.Fatalf("strict conflict before expansion = %v", err)
 	}

facade_item_registration_scope_test.go

@@ -0,0 +1,69 @@
+package configkit_test
+
+import (
+	"fmt"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestFacadeItemScopesDoNotReinterpretGlobalRegistrations(t *testing.T) {
+	t.Parallel()
+	for _, layered := range []bool{false, true} {
+		for _, dynamic := range []bool{false, true} {
+			for _, missingOuterID := range []bool{false, true} {
+				t.Run(fmt.Sprintf("layered=%t/dynamic=%t/missing-outer-id=%t", layered, dynamic, missingOuterID), func(t *testing.T) {
+					payload := "prompts: [{text: helper}]\n"
+					unused := configkit.SequenceIdentityAt(configkit.Keys("prompts"), "id")
+					if dynamic {
+						payload = "catalog: {local: {prompts: [{text: helper}]}}\n"
+						unused = configkit.SequenceIdentityMatching(configkit.SequenceSelector{
+							configkit.SequenceLiteralKey("catalog"), configkit.SequenceAnyMappingKey(), configkit.SequenceLiteralKey("prompts"),
+						}, "id")
+					}
+					item := "configkit: {imports: [{path: helper.yml}, {path: child.yml, destination: [children], placement: sequence-item}]}\n"
+					if !missingOuterID {
+						item += "id: outer\n"
+					}
+					files := configkit.NewFilesystem("items", fstest.MapFS{
+						"main.yml": {Data: []byte("configkit: {imports: [{path: item.yml, destination: [items], placement: sequence-item}]}\n")},
+						"item.yml": {Data: []byte(item)}, "helper.yml": {Data: []byte(payload)},
+						"child.yml":        {Data: []byte("id: child\nconfigkit: {imports: [{path: child-helper.yml}]}\n")},
+						"child-helper.yml": {Data: []byte(payload)},
+					})
+					definition := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{
+						unused, configkit.SequenceIdentityAt(configkit.Keys("items"), "id"),
+					}}
+					var result *configkit.Result[map[string]any]
+					var err error
+					if layered {
+						result, err = configkit.LoadLayered(configkit.Input{Source: files.File("main.yml"), Imports: true}, configkit.LayeredOptions[map[string]any]{Definition: definition})
+					} else {
+						result, err = configkit.Load(files.File("main.yml"), configkit.Options[map[string]any]{Imports: true, Definition: definition})
+					}
+					if missingOuterID {
+						failure := requireFacadeLoadError(t, err)
+						if result != nil || len(failure.Diagnostics()) != 1 || failure.Diagnostics()[0].Code != configkit.CodeComposeInvalid || failure.Diagnostics()[0].Path.String() != "/items/0/id" {
+							t.Fatalf("outer constraint diagnostics = %#v", failure.Diagnostics())
+						}
+						return
+					}
+					if err != nil {
+						t.Fatal(err)
+					}
+					for _, record := range []configkit.Path{configkit.Keys("items").Index(0), configkit.Keys("items").Index(0).Key("children").Index(0)} {
+						path := record
+						if dynamic {
+							path = path.Key("catalog").Key("local")
+						}
+						info, found := result.Metadata().Lookup(path.Key("prompts").Index(0).Key("text"))
+						if !found || !info.Explicit {
+							t.Fatalf("nested helper payload lost at %s", path)
+						}
+					}
+				})
+			}
+		}
+	}
+}

facade_sequence_identity_test.go

@@ -0,0 +1,240 @@
+package configkit_test
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestFacadeSequenceIdentitySnapshotsSelectorsAndFields(t *testing.T) {
+	fields := []string{"namespace", "id", "id"}
+	selector := configkit.SequenceSelector{
+		configkit.SequenceLiteralKey("collections"),
+		configkit.SequenceAnyMappingKey(),
+		configkit.SequenceLiteralKey("prompts"),
+	}
+	identity := configkit.SequenceIdentityMatching(selector, fields...)
+	selector[1] = configkit.SequenceLiteralKey("other")
+	fields[0] = "changed"
+	definition := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{identity}}
+
+	underlay := configkit.Mapping("underlay", map[string]any{"collections": map[string]any{
+		"team": map[string]any{"prompts": []any{map[string]any{"namespace": "team", "id": "kept"}}},
+	}})
+	main := configkit.Mapping("main", map[string]any{"collections": map[string]any{
+		"team": map[string]any{"prompts": []any{map[string]any{"namespace": "team", "id": "added"}}},
+	}})
+	loaded, err := configkit.LoadLayered(configkit.Input{Source: main}, configkit.LayeredOptions[map[string]any]{
+		Definition: definition,
+		Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: underlay}}}},
+	})
+	if err != nil {
+		t.Fatalf("load after mutating constructor inputs: %v", err)
+	}
+	prompts := loaded.Config["collections"].(map[string]any)["team"].(map[string]any)["prompts"].([]any)
+	if len(prompts) != 2 {
+		t.Fatalf("registered compound identity did not append records: %#v", prompts)
+	}
+}
+
+func TestFacadeSequenceIdentityUsesExactAndDynamicSelectors(t *testing.T) {
+	dynamic := configkit.SequenceIdentityMatching(configkit.SequenceSelector{
+		configkit.SequenceLiteralKey("collections"),
+		configkit.SequenceAnyMappingKey(),
+		configkit.SequenceLiteralKey("prompts"),
+	}, "id")
+	definition := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{
+		configkit.SequenceIdentityAt(configkit.Keys("prompts"), "namespace", "id"),
+		dynamic,
+	}}
+	underlay := configkit.Mapping("underlay", map[string]any{
+		"prompts": []any{map[string]any{"namespace": "alpha", "id": "same"}},
+		"collections": map[string]any{
+			"team":  map[string]any{"prompts": []any{map[string]any{"id": "team-one"}}},
+			"cloud": map[string]any{"prompts": []any{map[string]any{"id": "cloud-one"}}},
+		},
+	})
+	main := configkit.Mapping("main", map[string]any{
+		"prompts": []any{map[string]any{"namespace": "beta", "id": "same"}},
+		"collections": map[string]any{
+			"team":  map[string]any{"prompts": []any{map[string]any{"id": "team-two"}}},
+			"cloud": map[string]any{"prompts": []any{map[string]any{"id": "cloud-two"}}},
+		},
+	})
+	loaded, err := configkit.LoadLayered(configkit.Input{Source: main}, configkit.LayeredOptions[map[string]any]{
+		Definition: definition,
+		Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: underlay}}}},
+	})
+	if err != nil {
+		t.Fatalf("load mixed exact and dynamic registrations: %v", err)
+	}
+	rootRecords := loaded.Config["prompts"].([]any)
+	if len(rootRecords) != 2 {
+		t.Fatalf("compound root identities = %#v", rootRecords)
+	}
+	collections := loaded.Config["collections"].(map[string]any)
+	for _, name := range []string{"team", "cloud"} {
+		prompts := collections[name].(map[string]any)["prompts"].([]any)
+		if len(prompts) != 2 {
+			t.Errorf("dynamic prompts for %q = %#v", name, prompts)
+		}
+	}
+}
+
+func TestFacadeSequenceIdentityLiteralStarIsNotWildcard(t *testing.T) {
+	definition := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{
+		configkit.SequenceIdentityMatching(configkit.SequenceSelector{
+			configkit.SequenceLiteralKey("collections"),
+			configkit.SequenceLiteralKey("*"),
+			configkit.SequenceLiteralKey("prompts"),
+		}, "id"),
+	}}
+	underlay := configkit.Mapping("underlay", map[string]any{"collections": map[string]any{
+		"*":    map[string]any{"prompts": []any{map[string]any{"id": "one"}}},
+		"team": map[string]any{"prompts": []any{map[string]any{"id": "team-one"}}},
+	}})
+	main := configkit.Mapping("main", map[string]any{"collections": map[string]any{
+		"*":    map[string]any{"prompts": []any{map[string]any{"id": "two"}}},
+		"team": map[string]any{"prompts": []any{map[string]any{"id": "team-two"}}},
+	}})
+	_, err := configkit.LoadLayered(configkit.Input{Source: configkit.Mapping("main", map[string]any{})}, configkit.LayeredOptions[map[string]any]{
+		Definition: definition,
+		Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: underlay}, {Source: main}}}},
+	})
+	loadErr := requireFacadeLoadError(t, err)
+	if len(loadErr.Diagnostics()) == 0 || loadErr.Diagnostics()[0].Path.String() != "/collections/team/prompts" {
+		t.Fatalf("literal star registration also matched another key: %#v", loadErr.Diagnostics())
+	}
+
+	wildcard := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{
+		configkit.SequenceIdentityMatching(configkit.SequenceSelector{
+			configkit.SequenceLiteralKey("collections"),
+			configkit.SequenceAnyMappingKey(),
+			configkit.SequenceLiteralKey("prompts"),
+		}, "id"),
+	}}
+	loaded, err := configkit.LoadLayered(configkit.Input{Source: configkit.Mapping("main", map[string]any{})}, configkit.LayeredOptions[map[string]any]{
+		Definition: wildcard,
+		Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: underlay}, {Source: main}}}},
+	})
+	if err != nil {
+		t.Fatalf("wildcard registration did not match mapping keys: %v", err)
+	}
+	for name, raw := range loaded.Config["collections"].(map[string]any) {
+		if got := len(raw.(map[string]any)["prompts"].([]any)); got != 2 {
+			t.Errorf("wildcard result for %q has %d records", name, got)
+		}
+	}
+}
+
+func TestFacadeSequenceIdentityRejectsInvalidDefinitionsBeforeReadingSource(t *testing.T) {
+	tests := []struct {
+		name     string
+		identity configkit.SequenceIdentity
+		wantPath string
+	}{
+		{
+			name:     "index path",
+			identity: configkit.SequenceIdentityAt(configkit.Keys("records").Index(0), "id"),
+			wantPath: "/records/0",
+		},
+		{
+			name:     "empty fields",
+			identity: configkit.SequenceIdentityAt(configkit.Keys("records")),
+			wantPath: "/records",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			reader := strings.NewReader("records: []\n")
+			_, err := configkit.Load(configkit.Reader("input.yml", reader), configkit.Options[map[string]any]{
+				Definition: configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{test.identity}},
+			})
+			loadErr := requireFacadeLoadError(t, err)
+			if len(loadErr.Diagnostics()) != 1 || loadErr.Diagnostics()[0].Code != configkit.CodeOptionsInvalid || loadErr.Diagnostics()[0].Path.String() != test.wantPath {
+				t.Fatalf("definition diagnostics = %#v", loadErr.Diagnostics())
+			}
+			if reader.Len() == 0 {
+				t.Fatal("source was read before registration validation")
+			}
+		})
+	}
+}
+
+func TestFacadeSequenceIdentityOverlapFailsBeforeSourceAccess(t *testing.T) {
+	selector := configkit.SequenceSelector{configkit.SequenceLiteralKey("collections"), configkit.SequenceAnyMappingKey()}
+	definition := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{
+		configkit.SequenceIdentityMatching(selector, "id"),
+		configkit.SequenceIdentityMatching(configkit.SequenceSelector{
+			configkit.SequenceLiteralKey("collections"), configkit.SequenceLiteralKey("team"),
+		}, "slug"),
+	}}
+	reader := strings.NewReader("collections: {}\n")
+	_, err := configkit.Load(configkit.Reader("input.yml", reader), configkit.Options[map[string]any]{Definition: definition})
+	loadErr := requireFacadeLoadError(t, err)
+	if len(loadErr.Diagnostics()) == 0 || loadErr.Diagnostics()[0].Code != configkit.CodeOptionsInvalid {
+		t.Fatalf("overlap diagnostics = %#v", loadErr.Diagnostics())
+	}
+	if reader.Len() == 0 {
+		t.Fatal("source was read before overlapping registrations were rejected")
+	}
+}
+
+func TestFacadeSequenceIdentityConstraintsApplyAtInheritedDestinations(t *testing.T) {
+	definition := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{
+		configkit.SequenceIdentityMatching(configkit.SequenceSelector{
+			configkit.SequenceLiteralKey("entries"),
+			configkit.SequenceLiteralKey("child"),
+			configkit.SequenceLiteralKey("collections"),
+			configkit.SequenceAnyMappingKey(),
+			configkit.SequenceLiteralKey("prompts"),
+		}, "id"),
+	}}
+	loaded, err := configkit.Load(configkit.Bytes("main", []byte(`jobs:
+entries:
+  parent:
+    collections:
+      team:
+        prompts: [{id: inherited, text: base}]
+  child:
+    extends: parent
+    collections:
+      team:
+        prompts: [{id: added, text: child}]
+`)), configkit.Options[map[string]any]{Definition: definition})
+	if err != nil {
+		t.Fatalf("load with dynamic inherited destination: %v", err)
+	}
+	child := loaded.Config["entries"].(map[string]any)["child"].(map[string]any)
+	prompts := child["collections"].(map[string]any)["team"].(map[string]any)["prompts"].([]any)
+	if len(prompts) != 2 {
+		t.Fatalf("inherited dynamic collection = %#v, want parent and child records", prompts)
+	}
+}
+
+func TestFacadeSequenceIdentityLoadsUseIndependentScopes(t *testing.T) {
+	definition := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{
+		configkit.SequenceIdentityMatching(configkit.SequenceSelector{
+			configkit.SequenceLiteralKey("collections"), configkit.SequenceAnyMappingKey(),
+		}, "id"),
+	}}
+	for _, source := range []string{"first", "second"} {
+		loaded, err := configkit.Load(configkit.Mapping(source, map[string]any{"collections": map[string]any{
+			source: []any{map[string]any{"id": "reused"}},
+		}}), configkit.Options[map[string]any]{Definition: definition})
+		if err != nil || len(loaded.Config["collections"].(map[string]any)[source].([]any)) != 1 {
+			t.Fatalf("independent load %q = %#v, %v", source, loaded, err)
+		}
+	}
+}
+
+func requireFacadeLoadError(t *testing.T, err error) *configkit.LoadError {
+	t.Helper()
+	var loadErr *configkit.LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("error = %v, want *LoadError", err)
+	}
+	return loadErr
+}

facade_sequence_test.go

@@ -21,7 +21,7 @@ func TestFacadeNoSourcePreservesAbsenceWithSequencePolicy(t *testing.T) {
 				definition.Defaults = func() map[string]any { return map[string]any{"fallback": true} }
 			}
 			if registered {
-				definition.SequenceCompositions = []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}}
+				definition.SequenceIdentities = []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("records"), "id")}
 			}
 			ordinary, err := configkit.Load(configkit.NoSource(), configkit.Options[map[string]any]{Definition: definition})
 			if err != nil {
@@ -63,7 +63,7 @@ func TestFacadeItemHelperHistoryPreservesScheduledPlacement(t *testing.T) {
 		"child-helper.yml": {Data: []byte("text: nested helper\n")},
 	})
 	result, err := configkit.Load(filesystem.File("main.yml"), configkit.Options[map[string]any]{
-		Imports: true, Definition: configkit.Definition[map[string]any]{SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}}},
+		Imports: true, Definition: configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("records"), "id")}},
 	})
 	if err != nil {
 		t.Fatal(err)
@@ -107,8 +107,14 @@ func TestFacadeRegisteredReplacementExplainsRemovedDescendants(t *testing.T) {
 		{"removed field", []any{map[string]any{"id": "main"}}, configkit.Keys("records").Index(0).Key("text")},
 	} {
 		t.Run(test.name, func(t *testing.T) {
-			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Mapping("main", map[string]any{"records": test.upper})}, configkit.LayeredOptions[map[string]any]{
-				Definition: configkit.Definition[map[string]any]{SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}}},
+			mainYAML := "configkit:\n  sequences:\n    - destination: [records]\n      key: id\n      mode: replace\n"
+			if len(test.upper) == 0 {
+				mainYAML += "records: []\n"
+			} else {
+				mainYAML += "records: [{id: main}]\n"
+			}
+			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Bytes("main", []byte(mainYAML))}, configkit.LayeredOptions[map[string]any]{
+				Definition: configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("records"), "id")}},
 				Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"records": []any{map[string]any{"id": "one", "text": "old"}, map[string]any{"id": "two"}}})}}}},
 			})
 			if err != nil {
@@ -137,7 +143,7 @@ func TestFacadeItemConflictDiagnosticsFollowEnclosingIndexes(t *testing.T) {
 		"bad.yml":    {Data: []byte("id: bad\nconfigkit: {imports: [{path: helper.yml}]}\n")},
 		"helper.yml": {Data: []byte("id: duplicate-field\n")},
 	})
-	definition := configkit.Definition[map[string]any]{SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}}}
+	definition := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("records"), "id")}}
 	for _, test := range []struct {
 		name, file, path  string
 		grouped, itemMain bool
@@ -194,7 +200,7 @@ func TestFacadeItemConflictDiagnosticsFollowEnclosingIndexes(t *testing.T) {
 
 func TestFacadeSequencePlacementControlsAcrossLoadEntryPoints(t *testing.T) {
 	definition := configkit.Definition[map[string]any]{
-		SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("entries"), IdentityField: "id"}},
+		SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("entries"), "id")},
 	}
 	ordinary, err := configkit.Load(configkit.Mapping("ordinary", map[string]any{"id": "one"}), configkit.Options[map[string]any]{
 		Definition: definition, Destination: configkit.Keys("entries"), PlacementMode: configkit.PlacementModeSequenceItem,
@@ -240,8 +246,8 @@ func TestFacadeSequencePlacementControlsAcrossLoadEntryPoints(t *testing.T) {
 	if err != nil {
 		t.Fatalf("registered sequence precedence load: %v", err)
 	}
-	if got := precedence.Config["entries"].([]any); len(got) != 1 || got[0].(map[string]any)["id"] != "main" {
-		t.Fatalf("higher-priority list did not replace the lower sequence: %#v", precedence.Config)
+	if got := precedence.Config["entries"].([]any); len(got) != 2 || got[0].(map[string]any)["id"] != "lower" || got[1].(map[string]any)["id"] != "main" {
+		t.Fatalf("registered upsert did not retain the lower sequence: %#v", precedence.Config)
 	}
 
 	for _, test := range []struct {
@@ -295,7 +301,7 @@ items:
 	result, err := configkit.Load(configkit.File(mainPath), configkit.Options[map[string]any]{
 		Imports: true,
 		Definition: configkit.Definition[map[string]any]{
-			SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("items"), IdentityField: "id"}},
+			SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("items"), "id")},
 			Resolve: func(_ *map[string]any, resolution *configkit.Resolution) error {
 				for _, id := range []string{"source-2", "source-3"} {
 					source, ok := resolution.Source(id)
@@ -455,15 +461,17 @@ func TestFacadeNestedItemFallbackUsesNearestResolvedContainer(t *testing.T) {
 	}
 }
 
-func TestFacadeSequenceCompositionRegistrationPathIsDetached(t *testing.T) {
-	registrations := []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}}
+func TestFacadeSequenceIdentityRegistrationIsDetached(t *testing.T) {
+	fields := []string{"id"}
+	path := configkit.Keys("records")
+	registration := configkit.SequenceIdentityAt(path, fields...)
+	fields[0] = "changed"
 	definition := configkit.Definition[map[string]any]{
-		SequenceCompositions: registrations,
+		SequenceIdentities: []configkit.SequenceIdentity{registration},
 		Version: &configkit.VersionRule{
 			Path: configkit.Keys("version"),
 			Check: func(configkit.Version, *configkit.Validation) error {
-				registrations[0].Path = configkit.Keys("other")
-				registrations[0].IdentityField = "changed"
+				path = configkit.Keys("other")
 				return nil
 			},
 		},
@@ -506,6 +514,31 @@ func TestFacadeSourceInfoPlacementCopiesAndZeroValues(t *testing.T) {
 	}
 }
 
+func TestLayeredSequenceUpsertHistoryRetainsLowerTierRecords(t *testing.T) {
+	definition := configkit.Definition[map[string]any]{
+		SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("entries"), "id")},
+	}
+	result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Bytes("main", []byte("entries: [{id: main}]\n"))}, configkit.LayeredOptions[map[string]any]{
+		Definition: definition,
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+			Source: configkit.Bytes("lower", []byte("entries: [{id: lower}]\n")),
+		}}}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for index, id := range []string{"lower", "main"} {
+		path := configkit.Keys("entries").Index(index).Key("id")
+		explanation, ok := result.Explain(path)
+		if !ok || !explanation.Effective.Present || explanation.Effective.YAML != id+"\n" || len(explanation.History) == 0 {
+			t.Fatalf("tier history at %s = %#v, %v", path.String(), explanation, ok)
+		}
+		if explanation.History[len(explanation.History)-1].After.YAML != id+"\n" {
+			t.Fatalf("tier history at %s does not retain final record: %#v", path.String(), explanation.History)
+		}
+	}
+}
+
 func TestFacadeSequenceReplacementKeepsTypedAndHistoricalEvidenceAligned(t *testing.T) {
 	type record struct {
 		ID       string  `yaml:"id"`
@@ -523,7 +556,12 @@ func TestFacadeSequenceReplacementKeepsTypedAndHistoricalEvidenceAligned(t *test
 	directory := t.TempDir()
 	mainPath := filepath.Join(directory, "main.yml")
 	underlayPath := filepath.Join(directory, "underlay.yml")
-	if err := os.WriteFile(mainPath, []byte(`entries:
+	if err := os.WriteFile(mainPath, []byte(`configkit:
+  sequences:
+    - destination: [entries]
+      key: id
+      mode: replace
+entries:
   - id: current
     count: 0
     enabled: false
@@ -559,7 +597,7 @@ func TestFacadeSequenceReplacementKeepsTypedAndHistoricalEvidenceAligned(t *test
 	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(mainPath)}, configkit.LayeredOptions[configuration]{
 		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.File(underlayPath), Imports: true}}}},
 		Definition: configkit.Definition[configuration]{
-			SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("entries"), IdentityField: "id"}},
+			SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("entries"), "id")},
 			Secret: func(path configkit.Path) bool {
 				return path.String() == "/entries/0/token" || path.String() == "/entries/1/token"
 			},

facade_sequence_version_test.go

@@ -0,0 +1,197 @@
+package configkit_test
+
+import (
+	"fmt"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type movedVersionItem struct {
+	ID      string `yaml:"id"`
+	Version string `yaml:"version"`
+}
+
+type movedVersionCatalog struct {
+	Items []movedVersionItem `yaml:"items"`
+}
+
+func TestMainVersionDecodingFollowsSequenceMovement(t *testing.T) {
+	t.Parallel()
+	for _, mounted := range []bool{false, true} {
+		for _, matching := range []bool{false, true} {
+			for _, version := range []string{"4", "'4'"} {
+				t.Run(fmt.Sprintf("mounted=%t/matched=%t/%s", mounted, matching, version), func(t *testing.T) {
+					body := "items: [{id: main, version: " + version + "}]\n"
+					path := configkit.Keys("items").Index(0).Key("version")
+					main := configkit.Input{}
+					if mounted {
+						body, path = "id: main\nversion: "+version+"\n", configkit.Keys("version")
+						main.Destination, main.PlacementMode = configkit.Keys("items"), configkit.PlacementModeSequenceItem
+					}
+					main.Source = configkit.Bytes("main", []byte(body))
+					keep := "{id: keep}"
+					if mounted {
+						keep = "{id: keep, version: ordinary}"
+					}
+					lower := "items: [" + keep + "]\n"
+					if matching {
+						lower = "items: [" + keep + ", {id: main}]\n"
+					}
+					calls := 0
+					loaded, err := configkit.LoadLayered(main, configkit.LayeredOptions[movedVersionCatalog]{
+						SourceVersionMode: configkit.SourceVersionModeIfPresent,
+						Underlays:         []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Bytes("lower", []byte(lower))}}}},
+						Definition: configkit.Definition[movedVersionCatalog]{
+							SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("items"), "id")},
+							Version: &configkit.VersionRule{Path: path, Check: func(version configkit.Version, _ *configkit.Validation) error {
+								calls++
+								if !version.Present || version.Text != "4" {
+									t.Errorf("checked version = %#v", version)
+								}
+								return nil
+							}},
+						},
+					})
+					if err != nil {
+						t.Fatal(err)
+					}
+					if calls != 1 || len(loaded.Config.Items) != 2 || loaded.Config.Items[1].ID != "main" || loaded.Config.Items[1].Version != "4" {
+						t.Fatalf("moved main = %#v, checker calls = %d", loaded.Config, calls)
+					}
+					if mounted && loaded.Config.Items[0].Version != "ordinary" {
+						t.Fatal("unrelated underlay payload was treated as the main declaration")
+					}
+					for _, source := range loaded.Sources() {
+						if source.Layer == configkit.LayerBase && source.VersionCheck.Path.String() != path.String() {
+							t.Fatal("effective movement changed source-local version evidence")
+						}
+					}
+				})
+			}
+		}
+	}
+}
+
+func TestOverlayVersionProtectionUsesAssignedRecordPositions(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name, payload string
+		code          configkit.Code
+	}{
+		{name: "change main header", payload: "[{id: main, version: changed}]", code: configkit.CodeOverlayVersion},
+		{name: "remove main header", payload: "[{id: main}]", code: configkit.CodeOverlayVersion},
+		{name: "other record version", payload: "[{id: keep, version: ordinary}]"},
+		{name: "new record version", payload: "[{id: new, version: ordinary}]"},
+		{name: "only main header gets integer conversion", payload: "[{id: new, version: 5}]", code: configkit.CodeDecodeType},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			files := configkit.NewFilesystem("catalog", fstest.MapFS{
+				"overlays/test.yml": {Data: []byte("items: " + test.payload + "\n")},
+			})
+			calls := 0
+			loaded, err := configkit.LoadLayered(configkit.Input{
+				Source: configkit.Bytes("main", []byte("id: main\nversion: 4\n")), Destination: configkit.Keys("items"), PlacementMode: configkit.PlacementModeSequenceItem,
+			}, configkit.LayeredOptions[movedVersionCatalog]{
+				SourceVersionMode: configkit.SourceVersionModeIfPresent,
+				Overlay:           "test", OverlayFilesystem: &configkit.FilesystemContext{FS: &files, BaseDir: ".", Root: "."},
+				Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Bytes("lower", []byte("items: [{id: keep}]\n"))}}}},
+				Definition: configkit.Definition[movedVersionCatalog]{
+					SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("items"), "id")},
+					Version:            &configkit.VersionRule{Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { calls++; return nil }},
+				},
+			})
+			if calls != 1 {
+				t.Fatalf("checker calls = %d", calls)
+			}
+			if test.code != "" {
+				loadErr := requireFacadeLoadError(t, err)
+				if loaded != nil || len(loadErr.Diagnostics()) != 1 || loadErr.Diagnostics()[0].Code != test.code {
+					t.Fatalf("version diagnostics = %#v", loadErr.Diagnostics())
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			if loaded.Config.Items[1].Version != "4" {
+				t.Fatal("unrelated overlay changed main header")
+			}
+		})
+	}
+}
+
+func TestUnderlayVersionProtectionUsesMainRecordPosition(t *testing.T) {
+	t.Parallel()
+	for _, imported := range []bool{false, true} {
+		t.Run(fmt.Sprintf("imported=%t", imported), func(t *testing.T) {
+			payload := "items: [{id: keep}, {id: main, version: forbidden}]\n"
+			lower := configkit.Input{Source: configkit.Bytes("lower", []byte(payload))}
+			want := configkit.CodeUnderlayVersion
+			if imported {
+				files := configkit.NewFilesystem("lower", fstest.MapFS{
+					"main.yml":     {Data: []byte("configkit: {imports: [{path: fragment.yml}]}\n")},
+					"fragment.yml": {Data: []byte(payload)},
+				})
+				lower.Source, lower.Imports = files.File("main.yml"), true
+				want = configkit.CodeImportVersion
+			}
+			calls := 0
+			result, err := configkit.LoadLayered(configkit.Input{
+				Source:      configkit.Bytes("main", []byte("id: main\nversion: 4\n")),
+				Destination: configkit.Keys("items"), PlacementMode: configkit.PlacementModeSequenceItem,
+			}, configkit.LayeredOptions[movedVersionCatalog]{
+				SourceVersionMode: configkit.SourceVersionModeIfPresent,
+				Underlays:         []configkit.Underlay{{Inputs: []configkit.Input{lower}}},
+				Definition: configkit.Definition[movedVersionCatalog]{
+					SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("items"), "id")},
+					Version:            &configkit.VersionRule{Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { calls++; return nil }},
+				},
+			})
+			failure := requireFacadeLoadError(t, err)
+			if result != nil || calls != 1 || len(failure.Diagnostics()) != 1 || failure.Diagnostics()[0].Code != want || failure.Diagnostics()[0].Path.String() != "/items/1/version" {
+				t.Fatalf("result=%#v calls=%d diagnostics=%#v", result, calls, failure.Diagnostics())
+			}
+		})
+	}
+}
+
+func TestMainVersionFollowsInheritedSequenceMovement(t *testing.T) {
+	t.Parallel()
+	for _, parentID := range []string{"keep", "main"} {
+		t.Run(parentID, func(t *testing.T) {
+			body := "base: {items: [{id: " + parentID + ", version: ordinary}]}\nchild:\n  extends: base\n  items: [{id: main, version: 4}]\n"
+			type config struct {
+				Base, Child movedVersionCatalog
+			}
+			calls := 0
+			result, err := configkit.Load(configkit.Bytes("main", []byte(body)), configkit.Options[config]{
+				SourceVersionMode: configkit.SourceVersionModeIfPresent,
+				Definition: configkit.Definition[config]{
+					SequenceIdentities: []configkit.SequenceIdentity{
+						configkit.SequenceIdentityAt(configkit.Keys("base", "items"), "id"),
+						configkit.SequenceIdentityAt(configkit.Keys("child", "items"), "id"),
+					},
+					Version: &configkit.VersionRule{Path: configkit.Keys("child", "items").Index(0).Key("version"), Check: func(configkit.Version, *configkit.Validation) error { calls++; return nil }},
+				},
+			})
+			if calls != 1 {
+				t.Fatalf("checker calls = %d", calls)
+			}
+			if parentID == "main" {
+				failure := requireFacadeLoadError(t, err)
+				if result != nil || len(failure.Diagnostics()) != 1 || failure.Diagnostics()[0].Code != configkit.CodeInheritanceVersion {
+					t.Fatalf("diagnostics = %#v", failure.Diagnostics())
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			if len(result.Config.Child.Items) != 2 || result.Config.Child.Items[1].Version != "4" || result.Config.Child.Items[0].Version != "ordinary" {
+				t.Fatalf("inherited items = %#v", result.Config.Child.Items)
+			}
+		})
+	}
+}

facade_source_version_mode_test.go

@@ -23,9 +23,9 @@ func TestFacadeSourceVersionEvidenceFollowsAssembledSequenceRanges(t *testing.T)
 		for _, sensitivePath := range []string{"", fmt.Sprintf("/records/%d/version", index), fmt.Sprintf("/records/%d", index)} {
 			t.Run(fmt.Sprintf("grouped=%t/secret=%s", grouped, sensitivePath), func(t *testing.T) {
 				definition := configkit.Definition[map[string]any]{
-					SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}},
-					Version:              &configkit.VersionRule{Path: configkit.Keys("records").Index(0).Key("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil }},
-					Secret:               func(path configkit.Path) bool { return sensitivePath != "" && path.String() == sensitivePath },
+					SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("records"), "id")},
+					Version:            &configkit.VersionRule{Path: configkit.Keys("records").Index(0).Key("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil }},
+					Secret:             func(path configkit.Path) bool { return sensitivePath != "" && path.String() == sensitivePath },
 				}
 				first := "records: [{id: first, version: first-version}]\n"
 				child := "records: [{id: child, version: PRIVATE-VERSION-CANARY}]\n"
@@ -86,8 +86,8 @@ func TestFacadeSourceVersionOwnershipAcceptsContextualIndexes(t *testing.T) {
 	for _, grouped := range []bool{false, true} {
 		for _, path := range []configkit.Path{configkit.Keys("records").Index(0).Key("version"), parsed} {
 			definition := configkit.Definition[map[string]any]{
-				SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}},
-				Version:              &configkit.VersionRule{Path: path, Check: func(configkit.Version, *configkit.Validation) error { return nil }},
+				SequenceIdentities: []configkit.SequenceIdentity{configkit.SequenceIdentityAt(configkit.Keys("records"), "id")},
+				Version:            &configkit.VersionRule{Path: path, Check: func(configkit.Version, *configkit.Validation) error { return nil }},
 			}
 			first := "records: [{id: first, version: first}]\n"
 			child := "records: [{id: child, version: child}]\n"
@@ -104,11 +104,7 @@ func TestFacadeSourceVersionOwnershipAcceptsContextualIndexes(t *testing.T) {
 			if err != nil || result == nil {
 				t.Fatalf("grouped=%t path=%#v: %v", grouped, path, err)
 			}
-			want := 2
-			if grouped {
-				want = 1
-			}
-			if len(result.Config["records"].([]any)) != want {
+			if len(result.Config["records"].([]any)) != 2 {
 				t.Fatal("version path changed composition")
 			}
 		}

internal/engine/collection_ancestor_test.go

@@ -0,0 +1,106 @@
+package engine
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+	"testing/fstest"
+)
+
+func TestNestedCollectionInitializationIsIndependentOfAncestorPresence(t *testing.T) {
+	t.Parallel()
+	for _, ancestor := range []string{"{}", "group: {}", "group: null"} {
+		for _, test := range []struct {
+			mode, key, value string
+			valid            bool
+		}{
+			{mode: "append", value: "[{id: a}]"},
+			{mode: "upsert", value: "[{id: a}]"},
+			{mode: "append", value: "[]"},
+			{mode: "upsert", value: "[]"},
+			{mode: "append", key: ", key: id", value: "{}"},
+			{mode: "upsert", key: ", key: id", value: "null"},
+			{mode: "append", key: ", key: id", value: "[{id: a}]", valid: true},
+			{mode: "upsert", key: ", key: id", value: "[]", valid: true},
+		} {
+			t.Run(fmt.Sprintf("%s/%s/%s/%s", ancestor, test.mode, test.key, test.value), func(t *testing.T) {
+				main := "configkit:\n  sequences: [{destination: [group, items], mode: " + test.mode + test.key + "}]\ngroup: {items: " + test.value + "}\n"
+				loaded, err := testGroupedLoad(loadRequest[map[string]any]{
+					main:      loadInput{source: Bytes("main", []byte(main))},
+					underlays: []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte(ancestor))}}}},
+				})
+				if test.valid {
+					if err != nil || loaded == nil {
+						t.Fatalf("valid initialization failed: %v", err)
+					}
+					return
+				}
+				var loadErr *LoadError
+				if !errors.As(err, &loadErr) || loaded != nil {
+					t.Fatalf("invalid collection operation succeeded: %#v, %v", loaded, err)
+				}
+				diagnostics := loadErr.Diagnostics()
+				if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid || diagnostics[0].Path.String() != "/group/items" {
+					t.Fatalf("operation diagnostics = %#v", diagnostics)
+				}
+			})
+		}
+	}
+}
+
+func TestAddedAncestorTransfersNestedRecordPolicies(t *testing.T) {
+	t.Parallel()
+	type record struct {
+		ID    string           `yaml:"id"`
+		Steps []map[string]any `yaml:"steps"`
+	}
+	type config struct {
+		Group struct {
+			Items []record `yaml:"items"`
+		} `yaml:"group"`
+	}
+	files := fstest.MapFS{
+		"main.yml": {Data: []byte(`configkit:
+  sequences: [{destination: [group, items], key: id}]
+  imports: [{path: base.yml, destination: [group, items], placement: sequence-item}]
+group:
+  items: [{id: child, extends: {id: base}, steps: [{name: child}]}]
+`)},
+		"base.yml": {Data: []byte("configkit:\n  sequences: [{destination: [steps], key: name}]\nid: base\nsteps: [{name: base}]\n")},
+	}
+	loaded, err := testGroupedLoad(loadRequest[config]{
+		main:      loadInput{source: NewFilesystem("catalog", files).File("main.yml"), imports: true},
+		underlays: []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte("{}"))}}}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, item := range loaded.Config.Group.Items {
+		if item.ID == "child" {
+			if len(item.Steps) != 2 || item.Steps[0]["name"] != "base" || item.Steps[1]["name"] != "child" {
+				t.Fatalf("record-local identity was lost when adding its ancestor: %#v", item.Steps)
+			}
+			return
+		}
+	}
+	t.Fatal("missing child record")
+}
+
+func TestInheritanceChecksCollectionOperationUnderAddedAncestor(t *testing.T) {
+	t.Parallel()
+	_, err := testLoad[map[string]any](Bytes("inheritance", []byte(`configkit:
+  sequences: [{destination: [child, group, items], mode: append}]
+base: {}
+child:
+  extends: base
+  group: {items: [{id: a}]}
+`)))
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("missing nested operation error: %v", err)
+	}
+	diagnostics := loadErr.Diagnostics()
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid || diagnostics[0].Path.String() != "/child/group/items" {
+		t.Fatalf("nested inheritance operation diagnostics = %#v", diagnostics)
+	}
+}

internal/engine/collection_lookup_test.go

@@ -0,0 +1,98 @@
+package engine
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestOverlayBroadCollectionPoliciesAndRetainedDeclarations(t *testing.T) {
+	var baseBody, overlayBody strings.Builder
+	basePolicies := collectionPolicyScope{}
+	var declarations []sequenceDeclarationSpec
+	for index := range 64 {
+		name := fmt.Sprintf("group/%03d~", index)
+		fmt.Fprintf(&baseBody, "%s: {items: [{id: base}], retained: [{id: retained}], scalar: pending}\n", name)
+		fmt.Fprintf(&overlayBody, "%s: {items: [{id: overlay}], added: true}\n", name)
+		basePolicies.set(Keys(name, "items"), collectionPolicy{
+			identityFields: []string{"id"}, identityPresent: true,
+			explicitMode: sequenceModeAppend, explicitModePresent: true,
+		})
+		for _, key := range []string{"retained", "missing", "scalar"} {
+			declarations = append(declarations, sequenceDeclarationSpec{
+				destination: Keys(name, key), identityFields: []string{"id"}, identityPresent: true,
+				mode: sequenceModeAppend, modePresent: true,
+			})
+		}
+	}
+	base, diagnostics := composeDocuments(parseCompositionDocument(t, "base", baseBody.String()))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	base.collectionPolicies = basePolicies
+	assembly := newAssemblyAccumulator(base)
+	overlay := parseCompositionDocument(t, "overlay", overlayBody.String())
+	diagnostics = assembly.applyOverlayWithCollectionPolicy(overlay, nil, collectionPolicyScope{}, nil, LayerOverlay, sequencePolicy{}, declarations)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	lookup := buildDocumentLookup(assembly.current.root)
+	for index := range 64 {
+		name := fmt.Sprintf("group/%03d~", index)
+		items, found := documentAtPathWithLookup(assembly.current.root, Keys(name, "items"), lookup)
+		if !found || items.Kind != document.Sequence || len(items.Sequence) != 2 || items.Sequence[0].Mapping[0].Value.Text != "base" || items.Sequence[1].Mapping[0].Value.Text != "overlay" {
+			t.Fatalf("collection %s did not append in order", name)
+		}
+		for _, key := range []string{"retained", "missing", "scalar"} {
+			policy, found := assembly.current.collectionPolicies.at(Keys(name, key))
+			if !found || !policy.identityPresent || !policy.explicitModePresent || policy.explicitMode != sequenceModeAppend {
+				t.Fatalf("retained/pending declaration lost at %s/%s", name, key)
+			}
+		}
+		if _, found := documentAtPathWithLookup(assembly.current.root, Keys(name, "missing"), lookup); found {
+			t.Fatal("pending declaration created a value")
+		}
+		if origin := assembly.current.origins[Keys(name, "items").Index(1).Key("id").String()]; origin.Layer != LayerOverlay {
+			t.Fatal("appended item lost overlay attribution")
+		}
+	}
+}
+
+func TestInheritanceBroadCollectionsInOneDefinition(t *testing.T) {
+	var body strings.Builder
+	policies := collectionPolicyScope{}
+	for _, definition := range []string{"base", "child"} {
+		fmt.Fprintf(&body, "%s:\n", definition)
+		if definition == "child" {
+			body.WriteString("  extends: base\n")
+		}
+		for index := range 64 {
+			name := fmt.Sprintf("group/%03d~", index)
+			fmt.Fprintf(&body, "  %s: {items: [{id: %s}]}\n", name, definition)
+			policy := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+			if definition == "child" {
+				policy.explicitMode, policy.explicitModePresent = sequenceModeAppend, true
+			}
+			policies.set(Keys(definition, name, "items"), policy)
+			policies.set(Keys(definition, name, "pending"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+		}
+	}
+	root := parseCompositionDocument(t, "wide-collections", body.String())
+	result, diagnostics := expandInheritanceWithPolicies(root, nil, policies, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	lookup := buildDocumentLookup(result.root)
+	for index := range 64 {
+		name := fmt.Sprintf("group/%03d~", index)
+		items, found := documentAtPathWithLookup(result.root, Keys("child", name, "items"), lookup)
+		if !found || len(items.Sequence) != 2 || items.Sequence[0].Mapping[0].Value.Text != "base" || items.Sequence[1].Mapping[0].Value.Text != "child" {
+			t.Fatalf("inherited collection %s did not append in order", name)
+		}
+		if policy, found := result.collectionPolicies.at(Keys("child", name, "pending")); !found || !policy.identityPresent {
+			t.Fatalf("pending inherited policy lost at %s", name)
+		}
+	}
+}

internal/engine/collection_policy.go

@@ -0,0 +1,209 @@
+package engine
+
+type collectionPolicy struct {
+	identityFields      []string
+	identityPresent     bool
+	identityLocation    *Location
+	explicitMode        sequenceMode
+	explicitModePresent bool
+	modeLocation        *Location
+	recordOrder         []*collectionRecordOwner
+	recordPolicies      map[*collectionRecordOwner]collectionPolicyScope
+}
+
+type collectionPolicyResolution struct {
+	policy                     collectionPolicy
+	operation                  sequenceMode
+	declarations               []sequenceDeclarationSpec
+	declarationsPresent        bool
+	identityDeclarationPresent bool
+	modeDeclarationPresent     bool
+	applicationIdentityFields  []string
+	applicationIdentityPresent bool
+	effectiveIdentityFields    []string
+}
+
+// resolveCollectionPolicy reconciles declarations for one destination before
+// peers are composed. The caller supplies only lower policy that is applicable
+// to this destination; record survival and ownership are resolved elsewhere.
+func resolveCollectionPolicy(
+	destination Path,
+	declarations []sequenceDeclarationSpec,
+	applicationIdentityFields []string,
+	lower *collectionPolicy,
+) (collectionPolicyResolution, []Diagnostic) {
+	normalizedDeclarations := cloneSequenceDeclarations(declarations)
+	for index := range normalizedDeclarations {
+		declaration := &normalizedDeclarations[index]
+		if !declaration.identityPresent {
+			continue
+		}
+		fields, err := normalizeSequenceIdentityFields(declaration.identityFields)
+		if err != nil {
+			return collectionPolicyResolution{}, []Diagnostic{{
+				Code: CodeSequenceInvalid, Severity: SeverityError, Path: destination.clone(),
+				Message: "collection identity declaration requires at least one field",
+				Primary: collectionDeclarationLocation(declaration.identityLocation, declaration.location),
+			}}
+		}
+		declaration.identityFields = fields
+	}
+	normalizedApplicationIdentityFields := cloneStrings(applicationIdentityFields)
+	if len(applicationIdentityFields) != 0 {
+		var err error
+		normalizedApplicationIdentityFields, err = normalizeSequenceIdentityFields(applicationIdentityFields)
+		if err != nil {
+			return collectionPolicyResolution{}, []Diagnostic{{
+				Code: CodeOptionsInvalid, Severity: SeverityError, Path: destination.clone(),
+				Message: "application collection identity fields are invalid",
+			}}
+		}
+	}
+	result := collectionPolicyResolution{
+		declarations:               normalizedDeclarations,
+		declarationsPresent:        len(normalizedDeclarations) != 0,
+		applicationIdentityFields:  normalizedApplicationIdentityFields,
+		applicationIdentityPresent: len(normalizedApplicationIdentityFields) != 0,
+	}
+
+	var diagnostics []Diagnostic
+	var declaredIdentityFields []string
+	var declaredIdentityLocation *Location
+	var declaredMode sequenceMode
+	var declaredModeLocation *Location
+	for _, declaration := range normalizedDeclarations {
+		if declaration.identityPresent {
+			result.identityDeclarationPresent = true
+			if declaredIdentityFields == nil {
+				declaredIdentityFields = cloneStrings(declaration.identityFields)
+				declaredIdentityLocation = collectionDeclarationLocation(declaration.identityLocation, declaration.location)
+			} else if !equalSequenceIdentityFields(declaredIdentityFields, declaration.identityFields) {
+				diagnostics = append(diagnostics, collectionPolicyConflictDiagnostic(
+					destination, declaredIdentityLocation,
+					collectionDeclarationLocation(declaration.identityLocation, declaration.location),
+					"collection identity declarations conflict",
+				))
+			}
+		}
+		if declaration.modePresent {
+			result.modeDeclarationPresent = true
+			if declaredMode == 0 {
+				declaredMode = declaration.mode
+				declaredModeLocation = collectionDeclarationLocation(declaration.modeLocation, declaration.location)
+			} else if declaredMode != declaration.mode {
+				diagnostics = append(diagnostics, collectionPolicyConflictDiagnostic(
+					destination, declaredModeLocation,
+					collectionDeclarationLocation(declaration.modeLocation, declaration.location),
+					"collection operation declarations conflict",
+				))
+			}
+		}
+	}
+	if len(diagnostics) != 0 {
+		return collectionPolicyResolution{}, canonicalDiagnostics(diagnostics)
+	}
+
+	if lower != nil && lower.identityPresent && declaredIdentityFields != nil && !equalSequenceIdentityFields(lower.identityFields, declaredIdentityFields) {
+		diagnostics = append(diagnostics, collectionPolicyConflictDiagnostic(
+			destination, declaredIdentityLocation,
+			collectionDeclarationLocation(lower.identityLocation, Location{}),
+			"collection identity declaration conflicts with retained identity",
+		))
+	}
+	if result.applicationIdentityPresent {
+		if declaredIdentityFields != nil && !equalSequenceIdentityFields(normalizedApplicationIdentityFields, declaredIdentityFields) {
+			diagnostics = append(diagnostics, collectionPolicyConflictDiagnostic(
+				destination, declaredIdentityLocation, nil,
+				"collection identity declaration conflicts with application constraint",
+			))
+		}
+		if lower != nil && lower.identityPresent && !equalSequenceIdentityFields(normalizedApplicationIdentityFields, lower.identityFields) {
+			diagnostics = append(diagnostics, collectionPolicyConflictDiagnostic(
+				destination, declaredIdentityLocation,
+				collectionDeclarationLocation(lower.identityLocation, Location{}),
+				"retained collection identity conflicts with application constraint",
+			))
+		}
+	}
+	if len(diagnostics) != 0 {
+		return collectionPolicyResolution{}, canonicalDiagnostics(diagnostics)
+	}
+
+	if declaredIdentityFields != nil {
+		result.policy.identityFields = declaredIdentityFields
+		result.policy.identityPresent = true
+		result.policy.identityLocation = cloneLocation(declaredIdentityLocation)
+	} else if lower != nil && lower.identityPresent {
+		result.policy.identityFields = cloneStrings(lower.identityFields)
+		result.policy.identityPresent = true
+		result.policy.identityLocation = cloneLocation(lower.identityLocation)
+	}
+	if result.applicationIdentityPresent {
+		result.effectiveIdentityFields = cloneStrings(normalizedApplicationIdentityFields)
+	} else if result.policy.identityPresent {
+		result.effectiveIdentityFields = cloneStrings(result.policy.identityFields)
+	}
+
+	if result.modeDeclarationPresent {
+		result.policy.explicitMode = declaredMode
+		result.policy.explicitModePresent = true
+		result.policy.modeLocation = cloneLocation(declaredModeLocation)
+	} else if lower != nil && lower.explicitModePresent {
+		result.policy.explicitMode = lower.explicitMode
+		result.policy.explicitModePresent = true
+		result.policy.modeLocation = cloneLocation(lower.modeLocation)
+	}
+	result.operation = collectionOperationMode(declaredMode, result.policy.explicitMode, len(result.effectiveIdentityFields) != 0)
+
+	return result, nil
+}
+
+func collectionOperationMode(current, retained sequenceMode, identified bool) sequenceMode {
+	if current != 0 {
+		return current
+	}
+	if retained != 0 {
+		return retained
+	}
+	if identified {
+		return sequenceModeUpsert
+	}
+	return sequenceModeReplace
+}
+
+func cloneSequenceDeclarations(declarations []sequenceDeclarationSpec) []sequenceDeclarationSpec {
+	if declarations == nil {
+		return nil
+	}
+	cloned := make([]sequenceDeclarationSpec, len(declarations))
+	for index, declaration := range declarations {
+		cloned[index] = declaration
+		cloned[index].path = declaration.path.clone()
+		cloned[index].destination = declaration.destination.clone()
+		cloned[index].identityFields = cloneStrings(declaration.identityFields)
+		cloned[index].identityLocation = cloneLocation(declaration.identityLocation)
+		cloned[index].modeLocation = cloneLocation(declaration.modeLocation)
+	}
+	return cloned
+}
+
+func collectionDeclarationLocation(specific *Location, fallback Location) *Location {
+	if specific != nil {
+		return cloneLocation(specific)
+	}
+	if fallback != (Location{}) {
+		return cloneLocation(&fallback)
+	}
+	return nil
+}
+
+func collectionPolicyConflictDiagnostic(path Path, primary, related *Location, message string) Diagnostic {
+	diagnostic := Diagnostic{
+		Code: CodeSequenceConflict, Severity: SeverityError,
+		Path: path.clone(), Message: message, Primary: cloneLocation(primary),
+	}
+	if related != nil {
+		diagnostic.Related = []Location{*cloneLocation(related)}
+	}
+	return diagnostic
+}

internal/engine/collection_policy_state.go

@@ -0,0 +1,258 @@
+package engine
+
+import (
+	"sort"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type collectionPolicyScope struct {
+	collections map[string]collectionPolicyEntry
+}
+
+type collectionPolicyEntry struct {
+	destination Path
+	policy      collectionPolicy
+}
+
+type collectionRecordPolicy struct {
+	owner *collectionRecordOwner
+	scope collectionPolicyScope
+}
+
+// The nonzero field ensures simultaneously live record owners have distinct
+// addresses. The token is per-record ownership, not a source or identity key.
+type collectionRecordOwner struct {
+	marker byte
+}
+
+type projectedCollectionPolicy struct {
+	path   Path
+	policy collectionPolicy
+}
+
+func newCollectionRecordPolicy() collectionRecordPolicy {
+	return collectionRecordPolicy{owner: &collectionRecordOwner{marker: 1}}
+}
+
+func retainedCollectionRecordPolicy(record collectionRecordPolicy) collectionRecordPolicy {
+	return collectionRecordPolicy{
+		owner: record.owner,
+		scope: cloneCollectionPolicyScope(record.scope),
+	}
+}
+
+func replacedCollectionRecordPolicy() collectionRecordPolicy {
+	return newCollectionRecordPolicy()
+}
+
+func (policy *collectionPolicy) setRecordPolicy(record collectionRecordPolicy) {
+	if record.owner == nil {
+		record.owner = newCollectionRecordPolicy().owner
+	}
+	if policy.recordPolicies == nil {
+		policy.recordPolicies = make(map[*collectionRecordOwner]collectionPolicyScope)
+	}
+	if _, exists := policy.recordPolicies[record.owner]; !exists {
+		policy.recordOrder = append(policy.recordOrder, record.owner)
+	}
+	policy.recordPolicies[record.owner] = cloneCollectionPolicyScope(record.scope)
+}
+
+func (policy *collectionPolicy) setRecordPolicyAt(index int, record collectionRecordPolicy) {
+	if index < 0 {
+		return
+	}
+	if record.owner == nil {
+		record.owner = newCollectionRecordPolicy().owner
+	}
+	if policy.recordPolicies == nil {
+		policy.recordPolicies = make(map[*collectionRecordOwner]collectionPolicyScope)
+	}
+	for len(policy.recordOrder) <= index {
+		owner := newCollectionRecordPolicy().owner
+		policy.recordOrder = append(policy.recordOrder, owner)
+		policy.recordPolicies[owner] = collectionPolicyScope{}
+	}
+	delete(policy.recordPolicies, policy.recordOrder[index])
+	policy.recordOrder[index] = record.owner
+	policy.recordPolicies[record.owner] = cloneCollectionPolicyScope(record.scope)
+}
+
+func (policy collectionPolicy) recordPolicyAt(index int) (collectionRecordPolicy, bool) {
+	if index < 0 || index >= len(policy.recordOrder) {
+		return collectionRecordPolicy{}, false
+	}
+	owner := policy.recordOrder[index]
+	scope, ok := policy.recordPolicies[owner]
+	if !ok {
+		return collectionRecordPolicy{}, false
+	}
+	return collectionRecordPolicy{owner: owner, scope: cloneCollectionPolicyScope(scope)}, true
+}
+
+func collectionPolicyForReplacement(lower *collectionPolicy, current collectionPolicy) collectionPolicy {
+	result := cloneCollectionPolicy(current)
+	if !current.explicitModePresent && lower != nil && lower.explicitModePresent {
+		result.explicitMode = lower.explicitMode
+		result.explicitModePresent = true
+		result.modeLocation = cloneLocation(lower.modeLocation)
+	}
+	return result
+}
+
+// collectionPolicyWithoutValue keeps identity and record ownership because no
+// collection value was replaced. A key-only declaration can remain pending
+// when no sequence is present, or validate the retained sequence immediately.
+func collectionPolicyWithoutValue(
+	lower *collectionPolicy,
+	current collectionPolicy,
+	retainedValue *document.Node,
+	path Path,
+) (collectionPolicy, []Diagnostic) {
+	result := collectionPolicy{}
+	if lower != nil {
+		result = cloneCollectionPolicy(*lower)
+	}
+	if lower != nil && lower.identityPresent && current.identityPresent && !equalSequenceIdentityFields(lower.identityFields, current.identityFields) {
+		return collectionPolicy{}, []Diagnostic{collectionPolicyConflictDiagnostic(
+			path, current.identityLocation, lower.identityLocation,
+			"collection identity declaration conflicts with retained identity",
+		)}
+	}
+	if current.identityPresent {
+		result.identityFields = cloneStrings(current.identityFields)
+		result.identityPresent = true
+		result.identityLocation = cloneLocation(current.identityLocation)
+	}
+	if current.explicitModePresent {
+		result.explicitMode = current.explicitMode
+		result.explicitModePresent = true
+		result.modeLocation = cloneLocation(current.modeLocation)
+	}
+	if current.identityPresent && retainedValue != nil && retainedValue.Kind == document.Sequence {
+		_, diagnostics := validateSequenceIdentity(retainedValue, path, result.identityFields, sequenceIdentityDiagnosticCodes{
+			invalid: CodeSequenceInvalid, conflict: CodeSequenceConflict,
+		})
+		if len(diagnostics) != 0 {
+			return collectionPolicy{}, diagnostics
+		}
+	}
+	return result, nil
+}
+
+func (scope *collectionPolicyScope) set(destination Path, policy collectionPolicy) {
+	if scope.collections == nil {
+		scope.collections = make(map[string]collectionPolicyEntry)
+	}
+	scope.collections[destination.String()] = collectionPolicyEntry{
+		destination: destination.clone(), policy: cloneCollectionPolicy(policy),
+	}
+}
+
+func (scope collectionPolicyScope) at(destination Path) (collectionPolicy, bool) {
+	entry, ok := scope.collections[destination.String()]
+	if !ok {
+		return collectionPolicy{}, false
+	}
+	return cloneCollectionPolicy(entry.policy), true
+}
+
+// retireCollectionPoliciesAtOrBelow clears YAML identity and record-owned state
+// below a replaced ancestor while retaining independently scoped explicit modes.
+func (scope *collectionPolicyScope) retireCollectionPoliciesAtOrBelow(ancestor Path) {
+	if scope == nil {
+		return
+	}
+	for key, entry := range scope.collections {
+		if !pathHasPrefix(entry.destination, ancestor) {
+			continue
+		}
+		clearCollectionIdentity(&entry.policy)
+		scope.collections[key] = entry
+	}
+}
+
+func projectCollectionPolicyScope(scope collectionPolicyScope, prefix Path) []projectedCollectionPolicy {
+	keys := make([]string, 0, len(scope.collections))
+	for key := range scope.collections {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	var projected []projectedCollectionPolicy
+	for _, key := range keys {
+		entry := scope.collections[key]
+		concrete := appendDestination(prefix, entry.destination)
+		projected = append(projected, projectedCollectionPolicy{path: concrete, policy: cloneCollectionPolicy(entry.policy)})
+		for index, owner := range entry.policy.recordOrder {
+			if recordScope, ok := entry.policy.recordPolicies[owner]; ok {
+				projected = append(projected, projectCollectionPolicyScope(recordScope, concrete.Index(index))...)
+			}
+		}
+	}
+	return projected
+}
+
+func cloneCollectionPolicyScope(scope collectionPolicyScope) collectionPolicyScope {
+	if scope.collections == nil {
+		return collectionPolicyScope{}
+	}
+	cloned := collectionPolicyScope{collections: make(map[string]collectionPolicyEntry, len(scope.collections))}
+	for key, entry := range scope.collections {
+		cloned.collections[key] = collectionPolicyEntry{
+			destination: entry.destination.clone(), policy: cloneCollectionPolicy(entry.policy),
+		}
+	}
+	return cloned
+}
+
+func cloneCollectionPolicyScopeForInheritance(scope collectionPolicyScope) collectionPolicyScope {
+	if scope.collections == nil {
+		return collectionPolicyScope{}
+	}
+	cloned := collectionPolicyScope{collections: make(map[string]collectionPolicyEntry, len(scope.collections))}
+	for key, entry := range scope.collections {
+		cloned.collections[key] = collectionPolicyEntry{
+			destination: entry.destination.clone(), policy: cloneCollectionPolicyForInheritance(entry.policy),
+		}
+	}
+	return cloned
+}
+
+func cloneCollectionPolicy(policy collectionPolicy) collectionPolicy {
+	cloned := policy
+	cloned.identityFields = cloneStrings(policy.identityFields)
+	cloned.identityLocation = cloneLocation(policy.identityLocation)
+	cloned.modeLocation = cloneLocation(policy.modeLocation)
+	if policy.recordOrder != nil {
+		cloned.recordOrder = append([]*collectionRecordOwner(nil), policy.recordOrder...)
+	}
+	if policy.recordPolicies != nil {
+		cloned.recordPolicies = make(map[*collectionRecordOwner]collectionPolicyScope, len(policy.recordPolicies))
+		for owner, scope := range policy.recordPolicies {
+			cloned.recordPolicies[owner] = cloneCollectionPolicyScope(scope)
+		}
+	}
+	return cloned
+}
+
+func cloneCollectionPolicyForInheritance(policy collectionPolicy) collectionPolicy {
+	cloned := policy
+	cloned.identityFields = cloneStrings(policy.identityFields)
+	cloned.identityLocation = cloneLocation(policy.identityLocation)
+	cloned.modeLocation = cloneLocation(policy.modeLocation)
+	if policy.recordOrder != nil {
+		cloned.recordOrder = make([]*collectionRecordOwner, 0, len(policy.recordOrder))
+	}
+	if policy.recordPolicies != nil {
+		cloned.recordPolicies = make(map[*collectionRecordOwner]collectionPolicyScope, len(policy.recordPolicies))
+		for _, sourceOwner := range policy.recordOrder {
+			owner := newCollectionRecordPolicy().owner
+			cloned.recordOrder = append(cloned.recordOrder, owner)
+			if sourceScope, ok := policy.recordPolicies[sourceOwner]; ok {
+				cloned.recordPolicies[owner] = cloneCollectionPolicyScopeForInheritance(sourceScope)
+			}
+		}
+	}
+	return cloned
+}

internal/engine/collection_policy_state_test.go

@@ -0,0 +1,292 @@
+package engine
+
+import (
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestCollectionPolicyWithoutValueRetainsIdentityAndMode(t *testing.T) {
+	lower := collectionPolicy{
+		identityFields:      []string{"id"},
+		identityPresent:     true,
+		explicitMode:        sequenceModeAppend,
+		explicitModePresent: true,
+	}
+	lower.setRecordPolicy(newCollectionRecordPolicy())
+	current := collectionPolicy{explicitMode: sequenceModeReplace, explicitModePresent: true}
+	retained, diagnostics := collectionPolicyWithoutValue(&lower, current, nil, Keys("prompts"))
+	if len(diagnostics) != 0 || !retained.identityPresent || !reflect.DeepEqual(retained.identityFields, []string{"id"}) {
+		t.Fatalf("no-value replace reset identity: %#v, %#v", retained, diagnostics)
+	}
+	if retained.explicitMode != sequenceModeReplace || !retained.explicitModePresent || len(retained.recordOrder) != 1 {
+		t.Fatalf("no-value policy update lost mode or record ownership: %#v", retained)
+	}
+	retainedRecord, _ := retained.recordPolicyAt(0)
+	lowerRecord, _ := lower.recordPolicyAt(0)
+	if retainedRecord.owner != lowerRecord.owner {
+		t.Fatal("no-value policy update changed record ownership")
+	}
+}
+
+func TestCollectionPolicyWithoutValueLeavesAbsentAndNonSequenceDestinationsUnconstrained(t *testing.T) {
+	current := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+	for _, value := range []*document.Node{nil, parsePolicyNode(t, "value: scalar\n").Mapping[0].Value} {
+		retained, diagnostics := collectionPolicyWithoutValue(nil, current, value, Keys("prompts"))
+		if len(diagnostics) != 0 || !retained.identityPresent || !reflect.DeepEqual(retained.identityFields, []string{"id"}) {
+			t.Fatalf("pending identity at absent/non-sequence destination = %#v, %#v", retained, diagnostics)
+		}
+	}
+}
+
+func TestCollectionPolicyWithoutValueValidatesRetainedSequenceImmediately(t *testing.T) {
+	current := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+	valid := parsePolicyNode(t, "items: [{id: one}, {id: two}]\n").Mapping[0].Value
+	retained, diagnostics := collectionPolicyWithoutValue(nil, current, valid, Keys("prompts"))
+	if len(diagnostics) != 0 || !retained.identityPresent {
+		t.Fatalf("valid retained records were rejected: %#v, %#v", retained, diagnostics)
+	}
+
+	invalid := parsePolicyNode(t, "items: [{name: missing-id}]\n").Mapping[0].Value
+	retained, diagnostics = collectionPolicyWithoutValue(nil, current, invalid, Keys("prompts"))
+	if !reflect.DeepEqual(retained, collectionPolicy{}) || len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid || diagnostics[0].Path.String() != "/prompts/0/id" {
+		t.Fatalf("invalid retained records = %#v, %#v", retained, diagnostics)
+	}
+}
+
+func TestCollectionPolicyWithoutValueRejectsIdentityChange(t *testing.T) {
+	lower := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+	current := collectionPolicy{identityFields: []string{"name"}, identityPresent: true}
+	retained, diagnostics := collectionPolicyWithoutValue(&lower, current, nil, Keys("prompts"))
+	if !reflect.DeepEqual(retained, collectionPolicy{}) || len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict {
+		t.Fatalf("no-value identity change = %#v, %#v", retained, diagnostics)
+	}
+}
+
+func TestCollectionPolicyLifetimePersistenceAndReplacementTable(t *testing.T) {
+	tests := []struct {
+		name            string
+		current         collectionPolicy
+		replaceValue    bool
+		wantIdentity    []string
+		wantIdentitySet bool
+		wantMode        sequenceMode
+		wantRecords     int
+	}{
+		{
+			name: "mode-only replace declaration does not reset retained identity",
+			current: collectionPolicy{
+				explicitMode: sequenceModeReplace, explicitModePresent: true,
+			},
+			wantIdentity: []string{"id"}, wantIdentitySet: true,
+			wantMode: sequenceModeReplace, wantRecords: 1,
+		},
+		{
+			name:            "whole-sequence replacement resets lower identity",
+			replaceValue:    true,
+			wantIdentitySet: false,
+			wantMode:        sequenceModeAppend,
+			wantRecords:     0,
+		},
+		{
+			name:            "replacement can establish a new identity",
+			current:         collectionPolicy{identityFields: []string{"name"}, identityPresent: true},
+			replaceValue:    true,
+			wantIdentity:    []string{"name"},
+			wantIdentitySet: true,
+			wantMode:        sequenceModeAppend,
+			wantRecords:     0,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			lower := collectionPolicy{
+				identityFields: []string{"id"}, identityPresent: true,
+				explicitMode: sequenceModeAppend, explicitModePresent: true,
+			}
+			lower.setRecordPolicy(newCollectionRecordPolicy())
+			var got collectionPolicy
+			if test.replaceValue {
+				got = collectionPolicyForReplacement(&lower, test.current)
+			} else {
+				var diagnostics []Diagnostic
+				got, diagnostics = collectionPolicyWithoutValue(&lower, test.current, nil, Keys("prompts"))
+				if len(diagnostics) != 0 {
+					t.Fatalf("policy diagnostics = %#v", diagnostics)
+				}
+			}
+			if got.identityPresent != test.wantIdentitySet || !reflect.DeepEqual(got.identityFields, test.wantIdentity) || got.explicitMode != test.wantMode || len(got.recordOrder) != test.wantRecords {
+				t.Fatalf("transition result = %#v", got)
+			}
+		})
+	}
+}
+
+func TestCollectionPolicyValueReplacementResetsIdentityButRetainsExplicitMode(t *testing.T) {
+	lower := collectionPolicy{
+		identityFields:      []string{"id"},
+		identityPresent:     true,
+		explicitMode:        sequenceModeAppend,
+		explicitModePresent: true,
+	}
+	lower.setRecordPolicy(newCollectionRecordPolicy())
+	current := collectionPolicy{identityFields: []string{"name"}, identityPresent: true}
+	replaced := collectionPolicyForReplacement(&lower, current)
+	if !replaced.identityPresent || !reflect.DeepEqual(replaced.identityFields, []string{"name"}) {
+		t.Fatalf("replacement identity was not reset: %#v", replaced)
+	}
+	if replaced.explicitMode != sequenceModeAppend || !replaced.explicitModePresent || len(replaced.recordOrder) != 0 {
+		t.Fatalf("replacement failed to retain explicit mode or retire lower record policy: %#v", replaced)
+	}
+
+	cleared := collectionPolicyForReplacement(&lower, collectionPolicy{})
+	if cleared.identityPresent || len(cleared.identityFields) != 0 || cleared.explicitMode != sequenceModeAppend || !cleared.explicitModePresent {
+		t.Fatalf("replacement without a new key did not clear YAML identity while retaining mode: %#v", cleared)
+	}
+}
+
+func TestCollectionRecordRetentionAndReplacementUseOpaqueOwners(t *testing.T) {
+	oldRecord := newCollectionRecordPolicy()
+	oldRecord.scope.set(Keys("steps"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+	retained := retainedCollectionRecordPolicy(oldRecord)
+	if retained.owner != oldRecord.owner {
+		t.Fatal("retained record did not keep its owner")
+	}
+	if got, ok := retained.scope.at(Keys("steps")); !ok || !got.identityPresent {
+		t.Fatalf("retained record lost its local policy: %#v, %t", got, ok)
+	}
+	if retained.scope.collections["/steps"].policy.identityFields[0] = "changed"; oldRecord.scope.collections["/steps"].policy.identityFields[0] != "id" {
+		t.Fatal("retained record policy aliases its source")
+	}
+
+	// A whole-record replacement at the same list position, even with equal
+	// authored identity/source metadata, receives new ownership and no old rules.
+	replacement := replacedCollectionRecordPolicy()
+	if replacement.owner == oldRecord.owner || len(replacement.scope.collections) != 0 {
+		t.Fatal("replacement record inherited discarded owner policy")
+	}
+}
+
+func TestCollectionPolicyAncestorRetirementPreservesIndependentMode(t *testing.T) {
+	var scope collectionPolicyScope
+	owner := newCollectionRecordPolicy()
+	owner.scope.set(Keys("steps"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+	scope.set(Keys("catalog", "prompts"), collectionPolicyWithRecords(collectionPolicy{
+		identityFields:      []string{"id"},
+		identityPresent:     true,
+		explicitMode:        sequenceModeAppend,
+		explicitModePresent: true,
+	}, owner))
+	scope.retireCollectionPoliciesAtOrBelow(Keys("catalog"))
+	policy, ok := scope.at(Keys("catalog", "prompts"))
+	if !ok || policy.identityPresent || len(policy.recordOrder) != 0 || policy.explicitMode != sequenceModeAppend || !policy.explicitModePresent {
+		t.Fatalf("ancestor retirement = %#v, %t", policy, ok)
+	}
+	reintroduced, diagnostics := collectionPolicyWithoutValue(&policy, collectionPolicy{}, nil, Keys("catalog", "prompts"))
+	if len(diagnostics) != 0 || reintroduced.identityPresent || reintroduced.explicitMode != sequenceModeAppend {
+		t.Fatalf("retired identity was resurrected while mode persisted: %#v, %#v", reintroduced, diagnostics)
+	}
+}
+
+func TestEqualOuterIdentitiesHaveDistinctRecordOwners(t *testing.T) {
+	retained := newCollectionRecordPolicy()
+	replacement := replacedCollectionRecordPolicy()
+	if retained.owner == replacement.owner {
+		t.Fatal("replacement with the same outer identity reused the old owner")
+	}
+	if retained.owner == nil || replacement.owner == nil {
+		t.Fatal("record owner token is missing")
+	}
+}
+
+func TestCollectionPolicyScopesKeepRootAndItemRulesIndependent(t *testing.T) {
+	var root collectionPolicyScope
+	root.set(Keys("steps"), collectionPolicy{identityFields: []string{"root-id"}, identityPresent: true})
+	owner := newCollectionRecordPolicy()
+	owner.scope.set(Keys("steps"), collectionPolicy{identityFields: []string{"item-id"}, identityPresent: true})
+	root.set(Keys("items"), collectionPolicyWithRecords(collectionPolicy{}, owner))
+
+	root.retireCollectionPoliciesAtOrBelow(Keys("steps"))
+	rootPolicy, ok := root.at(Keys("steps"))
+	if !ok || rootPolicy.identityPresent {
+		t.Fatalf("root-local policy was not retired: %#v", rootPolicy)
+	}
+	itemPolicy, ok := root.at(Keys("items"))
+	if !ok || len(itemPolicy.recordOrder) != 1 {
+		t.Fatalf("unrelated item owner disappeared: %#v", itemPolicy)
+	}
+	itemRecord, _ := itemPolicy.recordPolicyAt(0)
+	nested, ok := itemRecord.scope.at(Keys("steps"))
+	if !ok || !nested.identityPresent || !reflect.DeepEqual(nested.identityFields, []string{"item-id"}) {
+		t.Fatalf("item-local policy was coupled to root destination: %#v", nested)
+	}
+}
+
+func TestCollectionPolicyProjectionUsesConcretePlacement(t *testing.T) {
+	first := newCollectionRecordPolicy()
+	first.scope.set(Keys("steps"), collectionPolicy{identityFields: []string{"step"}, identityPresent: true})
+	second := newCollectionRecordPolicy()
+	second.scope.set(Keys("steps"), collectionPolicy{explicitMode: sequenceModeAppend, explicitModePresent: true})
+	var scope collectionPolicyScope
+	scope.set(Keys("items"), collectionPolicyWithRecords(collectionPolicy{}, first, second))
+
+	projected := projectCollectionPolicyScope(scope, Keys("mounted"))
+	if len(projected) != 3 {
+		t.Fatalf("projected policies = %#v", projected)
+	}
+	want := []string{"/mounted/items", "/mounted/items/0/steps", "/mounted/items/1/steps"}
+	for index, policy := range projected {
+		if policy.path.String() != want[index] {
+			t.Fatalf("projected path[%d] = %q, want %q", index, policy.path.String(), want[index])
+		}
+	}
+}
+
+func TestCollectionPolicyInheritanceCopiesOwnersAndState(t *testing.T) {
+	owner := newCollectionRecordPolicy()
+	owner.scope.set(Keys("steps"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+	original := collectionPolicyWithRecords(collectionPolicy{}, owner)
+	copy := cloneCollectionPolicyForInheritance(original)
+	copyOwner := copy.recordOrder[0]
+	if copyOwner == owner.owner {
+		t.Fatal("inheritance copy retained the source record owner")
+	}
+	copy.recordPolicies[copyOwner].collections["/steps"].policy.identityFields[0] = "changed"
+	if owner.scope.collections["/steps"].policy.identityFields[0] != "id" {
+		t.Fatal("inheritance copy aliases source policy")
+	}
+}
+
+func TestAcquisitionScopeCloneOwnsCollectionPolicyCopy(t *testing.T) {
+	owner := newCollectionRecordPolicy()
+	owner.scope.set(Keys("steps"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+	var scope acquisitionScope
+	scope.collectionPolicies.set(Keys("items"), collectionPolicyWithRecords(collectionPolicy{}, owner))
+	cloned := cloneAcquisitionScope(scope)
+	clonedOwner := cloned.collectionPolicies.collections["/items"].policy.recordOrder[0]
+	cloned.collectionPolicies.collections["/items"].policy.recordPolicies[clonedOwner].collections["/steps"].policy.identityFields[0] = "changed"
+	sourceOwner := scope.collectionPolicies.collections["/items"].policy.recordOrder[0]
+	if scope.collectionPolicies.collections["/items"].policy.recordPolicies[sourceOwner].collections["/steps"].policy.identityFields[0] != "id" {
+		t.Fatal("acquisition scope clone aliases collection policy")
+	}
+	if clonedOwner != owner.owner {
+		t.Fatal("acquisition scope clone changed record ownership")
+	}
+}
+
+func collectionPolicyWithRecords(policy collectionPolicy, records ...collectionRecordPolicy) collectionPolicy {
+	for _, record := range records {
+		policy.setRecordPolicy(record)
+	}
+	return policy
+}
+
+func parsePolicyNode(t *testing.T, input string) *document.Node {
+	t.Helper()
+	parsed, diagnostics := parseSource(Bytes("collection-policy-state.yml", []byte(input)), "collection-policy-state", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("parse policy value: %#v", diagnostics)
+	}
+	return parsed.document
+}

internal/engine/collection_policy_test.go

@@ -0,0 +1,192 @@
+package engine
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestResolveCollectionPolicyReconcilesDeclarationsBeforeComposition(t *testing.T) {
+	declarations := extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [prompts]
+      key: [namespace, id]
+      mode: append
+    - destination: [prompts]
+      key: [id, namespace, id]
+    - destination: [prompts]
+      mode: append
+`, rootDirectiveContextSource).sequences
+
+	resolved, diagnostics := resolveCollectionPolicy(Keys("prompts"), declarations, nil, nil)
+	if len(diagnostics) != 0 {
+		t.Fatalf("policy diagnostics = %#v", diagnostics)
+	}
+	if !resolved.declarationsPresent || !resolved.identityDeclarationPresent || !resolved.modeDeclarationPresent {
+		t.Fatalf("declaration presence was lost: %#v", resolved)
+	}
+	wantFields := []string{"id", "namespace"}
+	if !reflect.DeepEqual(resolved.policy.identityFields, wantFields) || !reflect.DeepEqual(resolved.effectiveIdentityFields, wantFields) {
+		t.Fatalf("effective identity = %#v / %#v, want %#v", resolved.policy.identityFields, resolved.effectiveIdentityFields, wantFields)
+	}
+	if !resolved.policy.identityPresent || !resolved.policy.explicitModePresent || resolved.policy.explicitMode != sequenceModeAppend || resolved.operation != sequenceModeAppend {
+		t.Fatalf("resolved explicit policy = %#v, operation %d", resolved.policy, resolved.operation)
+	}
+	if len(resolved.declarations) != 3 || resolved.declarations[0].identityLocation == nil || resolved.declarations[2].location.Line == 0 {
+		t.Fatalf("declaration locations were not retained: %#v", resolved.declarations)
+	}
+}
+
+func TestResolveCollectionPolicyRejectsConflictingPeersAtBothSites(t *testing.T) {
+	tests := []struct {
+		name string
+		body string
+	}{
+		{
+			name: "identity fields",
+			body: `configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+    - destination: [prompts]
+      key: name
+`,
+		},
+		{
+			name: "explicit modes",
+			body: `configkit:
+  sequences:
+    - destination: [prompts]
+      mode: append
+    - destination: [prompts]
+      mode: replace
+`,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			declarations := extractSequenceDeclarationsYAML(t, test.body, rootDirectiveContextSource).sequences
+			resolved, diagnostics := resolveCollectionPolicy(Keys("prompts"), declarations, nil, nil)
+			if !reflect.DeepEqual(resolved, collectionPolicyResolution{}) || len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict {
+				t.Fatalf("resolution/diagnostics = %#v / %#v", resolved, diagnostics)
+			}
+			diagnostic := diagnostics[0]
+			if diagnostic.Path.String() != "/prompts" || diagnostic.Primary == nil || len(diagnostic.Related) != 1 {
+				t.Fatalf("conflict does not identify both declaration sites: %#v", diagnostic)
+			}
+			if diagnostic.Primary.SourceID != "sequence-declarations" || diagnostic.Related[0].SourceID != "sequence-declarations" || diagnostic.Primary.Line == diagnostic.Related[0].Line {
+				t.Fatalf("conflict locations = %#v and %#v", diagnostic.Primary, diagnostic.Related)
+			}
+		})
+	}
+}
+
+func TestResolveCollectionPolicySelectsCurrentThenRetainedExplicitMode(t *testing.T) {
+	lower := &collectionPolicy{
+		identityFields:      []string{"id"},
+		identityPresent:     true,
+		explicitMode:        sequenceModeAppend,
+		explicitModePresent: true,
+		modeLocation:        &Location{SourceID: "lower", Line: 2},
+	}
+	current := extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [prompts]
+      mode: upsert
+`, rootDirectiveContextSource).sequences
+	resolved, diagnostics := resolveCollectionPolicy(Keys("prompts"), current, nil, lower)
+	if len(diagnostics) != 0 || resolved.operation != sequenceModeUpsert || resolved.policy.explicitMode != sequenceModeUpsert || !resolved.policy.explicitModePresent {
+		t.Fatalf("current mode did not override retained mode: %#v, %#v", resolved, diagnostics)
+	}
+
+	resolved, diagnostics = resolveCollectionPolicy(Keys("prompts"), nil, nil, lower)
+	if len(diagnostics) != 0 || resolved.operation != sequenceModeAppend || resolved.policy.explicitMode != sequenceModeAppend || !resolved.policy.explicitModePresent {
+		t.Fatalf("omitted current mode did not retain lower explicit mode: %#v, %#v", resolved, diagnostics)
+	}
+	if !reflect.DeepEqual(resolved.effectiveIdentityFields, []string{"id"}) {
+		t.Fatalf("retained identity = %#v", resolved.effectiveIdentityFields)
+	}
+}
+
+func TestResolveCollectionPolicyInfersWithoutPersistingMode(t *testing.T) {
+	keyOnly := extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+`, rootDirectiveContextSource).sequences
+	resolved, diagnostics := resolveCollectionPolicy(Keys("prompts"), keyOnly, nil, nil)
+	if len(diagnostics) != 0 || resolved.operation != sequenceModeUpsert || resolved.policy.explicitModePresent {
+		t.Fatalf("identity-based mode inference = %#v, %#v", resolved, diagnostics)
+	}
+
+	resolved, diagnostics = resolveCollectionPolicy(Keys("prompts"), nil, []string{"id"}, nil)
+	if len(diagnostics) != 0 || resolved.operation != sequenceModeUpsert || resolved.policy.identityPresent || !resolved.applicationIdentityPresent {
+		t.Fatalf("application identity was not kept distinct from YAML policy: %#v, %#v", resolved, diagnostics)
+	}
+
+	noIdentity := extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [prompts]
+`, rootDirectiveContextSource).sequences
+	resolved, diagnostics = resolveCollectionPolicy(Keys("prompts"), noIdentity, nil, nil)
+	if len(diagnostics) != 0 || resolved.operation != sequenceModeReplace || resolved.policy.explicitModePresent {
+		t.Fatalf("unkeyed mode inference = %#v, %#v", resolved, diagnostics)
+	}
+
+	explicitDefault := extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [prompts]
+      mode: replace
+`, rootDirectiveContextSource).sequences
+	resolved, diagnostics = resolveCollectionPolicy(Keys("prompts"), explicitDefault, nil, nil)
+	if len(diagnostics) != 0 || resolved.operation != sequenceModeReplace || !resolved.policy.explicitModePresent {
+		t.Fatalf("explicit default mode lost its presence: %#v, %#v", resolved, diagnostics)
+	}
+}
+
+func TestResolveCollectionPolicyChecksKeyOnlyAgainstApplicationConstraint(t *testing.T) {
+	declarations := extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [prompts]
+      key: [namespace, id]
+`, rootDirectiveContextSource).sequences
+	resolved, diagnostics := resolveCollectionPolicy(Keys("prompts"), declarations, []string{"id", "namespace", "id"}, nil)
+	if len(diagnostics) != 0 || resolved.operation != sequenceModeUpsert || !reflect.DeepEqual(resolved.effectiveIdentityFields, []string{"id", "namespace"}) {
+		t.Fatalf("equivalent key-only declaration was rejected: %#v, %#v", resolved, diagnostics)
+	}
+
+	resolved, diagnostics = resolveCollectionPolicy(Keys("prompts"), declarations, []string{"slug"}, nil)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict {
+		t.Fatalf("conflicting key-only declaration = %#v, %#v", resolved, diagnostics)
+	}
+	if diagnostics[0].Primary == nil || diagnostics[0].Path.String() != "/prompts" || len(diagnostics[0].Related) != 0 {
+		t.Fatalf("application conflict diagnostic = %#v", diagnostics[0])
+	}
+}
+
+func TestResolveCollectionPolicyChecksRetainedIdentityAgainstApplicationConstraint(t *testing.T) {
+	lower := &collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+	resolved, diagnostics := resolveCollectionPolicy(Keys("prompts"), nil, []string{"name"}, lower)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict || diagnostics[0].Path.String() != "/prompts" {
+		t.Fatalf("retained/application conflict = %#v, %#v", resolved, diagnostics)
+	}
+}
+
+func TestResolveCollectionPolicyRequiresCurrentIdentityToAgreeWithRetainedIdentity(t *testing.T) {
+	declarations := extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [prompts]
+      key: name
+`, rootDirectiveContextSource).sequences
+	lower := &collectionPolicy{
+		identityFields:   []string{"id"},
+		identityPresent:  true,
+		identityLocation: &Location{SourceID: "lower", Line: 4},
+	}
+	resolved, diagnostics := resolveCollectionPolicy(Keys("prompts"), declarations, nil, lower)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict || diagnostics[0].Path.String() != "/prompts" {
+		t.Fatalf("current/retained identity conflict = %#v, %#v", resolved, diagnostics)
+	}
+	if diagnostics[0].Primary == nil || len(diagnostics[0].Related) != 1 || diagnostics[0].Related[0].SourceID != "lower" {
+		t.Fatalf("current/retained conflict locations = %#v", diagnostics[0])
+	}
+}

internal/engine/collection_retirement.go

@@ -0,0 +1,81 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// A true entry replaces the collection itself as well as its descendants;
+// false records ordinary ancestor replacement without selecting a collection
+// operation at that exact path.
+type collectionReplacementPaths map[string]bool
+
+func (paths collectionReplacementPaths) contains(path Path, includeExact bool) bool {
+	for length := 0; length <= len(path.segments); length++ {
+		prefix := Path{segments: path.segments[:length]}
+		inclusive, found := paths[prefix.String()]
+		if found && (length < len(path.segments) || includeExact && inclusive) {
+			return true
+		}
+	}
+	return false
+}
+
+// lowerCollectionPoliciesForClaims makes replacement eligibility independent of
+// declaration order. It removes only old YAML identity/record state; new pending
+// declarations and application constraints are reconciled afterward.
+func lowerCollectionPoliciesForClaims(
+	lower collectionPolicyScope,
+	claims []compositionClaim,
+	declarations map[string][]sequenceDeclarationSpec,
+	application sequencePolicy,
+) (collectionPolicyScope, collectionReplacementPaths) {
+	byPath := make(map[string]compositionClaim, len(claims))
+	rootPath := ""
+	if len(claims) != 0 {
+		rootPath = claims[0].path.String()
+	}
+	for _, claim := range claims {
+		byPath[claim.path.String()] = claim
+	}
+	replaced := make(collectionReplacementPaths)
+	for key, claim := range byPath {
+		old, hasOld := lower.collections[key]
+		current := declarations[key]
+		if claim.kind == document.Mapping && (key == rootPath || !hasOld && len(current) == 0) {
+			continue // Ordinary mapping combination preserves descendants.
+		}
+		mode := sequenceMode(0)
+		identity := old.policy.identityPresent
+		for _, declaration := range current {
+			identity = identity || declaration.identityPresent
+			if mode == 0 && declaration.modePresent {
+				mode = declaration.mode
+			}
+		}
+		retainedMode := sequenceMode(0)
+		if old.policy.explicitModePresent {
+			retainedMode = old.policy.explicitMode
+		}
+		_, registered := application.identityFieldsAt(claim.path)
+		mode = collectionOperationMode(mode, retainedMode, identity || registered)
+		if mode == sequenceModeReplace {
+			replaced[key] = true
+		} else if claim.kind != document.Mapping && claim.kind != document.Sequence {
+			replaced[key] = false
+		}
+	}
+	result := cloneCollectionPolicyScope(lower)
+	for key, entry := range result.collections {
+		if replaced.contains(entry.destination, true) {
+			clearCollectionIdentity(&entry.policy)
+			result.collections[key] = entry
+		}
+	}
+	return result, replaced
+}
+
+func clearCollectionIdentity(policy *collectionPolicy) {
+	policy.identityFields = nil
+	policy.identityPresent = false
+	policy.identityLocation = nil
+	policy.recordOrder = nil
+	policy.recordPolicies = nil
+}

internal/engine/collection_retirement_test.go

@@ -0,0 +1,111 @@
+package engine
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestSourceReplacementRetiresIdentityForEveryValueKind(t *testing.T) {
+	t.Parallel()
+	for _, value := range []string{"null", "42", "{}", "[{name: new}]"} {
+		t.Run(value, func(t *testing.T) {
+			lower := "configkit:\n  sequences: [{destination: [items], key: id}]\nitems: [{id: old}]\n"
+			main := "configkit:\n  sequences: [{destination: [items], key: name, mode: replace}]\nitems: " + value + "\n"
+			loaded, err := testGroupedLoad(loadRequest[map[string]any]{
+				main:      loadInput{source: Bytes("main", []byte(main))},
+				underlays: []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte(lower))}}}},
+			})
+			if err != nil || loaded == nil {
+				t.Fatalf("replacement retained obsolete identity: %v", err)
+			}
+		})
+	}
+}
+
+func TestAncestorRemovalCanEstablishPendingReplacementIdentity(t *testing.T) {
+	t.Parallel()
+	for _, inheritance := range []bool{false, true} {
+		name := "tiers"
+		if inheritance {
+			name = "inheritance"
+		}
+		t.Run(name, func(t *testing.T) {
+			var loaded *testResult[map[string]any]
+			var err error
+			if inheritance {
+				loaded, err = testLoad[map[string]any](Bytes("source", []byte(`configkit:
+  sequences:
+    - {destination: [base, group, items], key: id}
+    - {destination: [child, group, items], key: name}
+base: {group: {items: [{id: old}]}}
+child:
+  extends: base
+  group: null
+grandchild:
+  extends: child
+  group: {items: [{name: new}]}
+`)))
+			} else {
+				loaded, err = testGroupedLoad(loadRequest[map[string]any]{
+					main: loadInput{source: Bytes("main", []byte("group: {items: [{name: new}]}\n"))},
+					underlays: []loadGroup{
+						{inputs: []loadInput{{source: Bytes("base", []byte("configkit:\n  sequences: [{destination: [group, items], key: id}]\ngroup: {items: [{id: old}]}\n"))}}},
+						{inputs: []loadInput{{source: Bytes("remove", []byte("configkit:\n  sequences: [{destination: [group, items], key: name}]\ngroup: null\n"))}}},
+					},
+				})
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			value := loaded.Config
+			if inheritance {
+				value = value["grandchild"].(map[string]any)
+			}
+			items := value["group"].(map[string]any)["items"].([]any)
+			if len(items) != 1 || items[0].(map[string]any)["name"] != "new" {
+				t.Fatalf("reintroduced collection = %#v", items)
+			}
+		})
+	}
+}
+
+func TestInheritanceAncestorRetirementDoesNotResurrectOldIdentity(t *testing.T) {
+	t.Parallel()
+	_, err := testLoad[map[string]any](Bytes("source", []byte(`configkit:
+  sequences: [{destination: [base, group, items], key: id}]
+base: {group: {items: [{id: old}]}}
+child:
+  extends: base
+  group: null
+grandchild:
+  extends: child
+  group: {items: [{name: new}]}
+`)))
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestReplacementRetirementPreservesApplicationConstraintsAndRetainedKeys(t *testing.T) {
+	t.Parallel()
+	for _, withValue := range []bool{false, true} {
+		main := "configkit:\n  sequences: [{destination: [items], key: name, mode: replace}]\n"
+		if withValue {
+			main += "items: null\n"
+		}
+		definition := Definition[map[string]any]{}
+		if withValue {
+			definition.sequenceIdentityRegistrations = []sequenceIdentityRegistration{{
+				selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"},
+			}}
+		}
+		_, err := testGroupedLoad(loadRequest[map[string]any]{
+			main: loadInput{source: Bytes("main", []byte(main))}, definition: definition,
+			underlays: []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte("configkit:\n  sequences: [{destination: [items], key: id}]\nitems: [{id: old}]\n"))}}}},
+		})
+		var loadErr *LoadError
+		if !errors.As(err, &loadErr) || len(loadErr.Diagnostics()) == 0 || loadErr.Diagnostics()[0].Code != CodeSequenceConflict {
+			t.Fatalf("retirement bypassed identity agreement: %v", err)
+		}
+	}
+}

internal/engine/compose.go

@@ -1,21 +1,26 @@
 package engine
 
-import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+import (
+	"maps"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
 
 // composedDocument is the private handoff between source assembly and later
 // precedence layers. Claims remain ordered by source and declaration traversal;
 // origins retain every source that contributes to a composite mapping.
 type composedDocument struct {
-	root           *document.Node
-	acquisition    []acquisitionScope
-	importGraph    importInclusionGraph
-	claims         []compositionClaim
-	sequenceRanges []sequenceRange
-	origins        map[string]Origin
-	overlays       []overlayRecord
-	applications   []historyApplication
-	inputBytes     int64
-	inputNodes     int
+	root               *document.Node
+	acquisition        []acquisitionScope
+	importGraph        importInclusionGraph
+	collectionPolicies collectionPolicyScope
+	claims             []compositionClaim
+	sequenceRanges     []sequenceRange
+	origins            map[string]Origin
+	overlays           []overlayRecord
+	applications       []historyApplication
+	inputBytes         int64
+	inputNodes         int
 }
 
 type sequenceHistoryMode uint8
@@ -23,13 +28,16 @@ type sequenceHistoryMode uint8
 const (
 	sequenceHistoryReplace sequenceHistoryMode = iota + 1
 	sequenceHistoryAppend
+	sequenceHistoryMapped
 )
 
 type sequenceHistoryContribution struct {
-	path   Path
-	mode   sequenceHistoryMode
-	offset int
-	length int
+	path         Path
+	mode         sequenceHistoryMode
+	offset       int
+	length       int
+	items        []sequenceItemProjection
+	replacements []sequenceItemReplacement
 }
 
 // assemblyAccumulator owns one mutable working document. Acquired trees held by
@@ -51,10 +59,11 @@ func cloneComposedDocument(base *composedDocument) *composedDocument {
 	}
 	return &composedDocument{
 		root: document.Clone(base.root), claims: cloneCompositionClaims(base.claims),
-		acquisition:    cloneAcquisitionScopes(base.acquisition),
-		importGraph:    base.importGraph.clone(),
-		sequenceRanges: cloneSequenceRanges(base.sequenceRanges),
-		origins:        cloneOriginIndex(base.origins), overlays: cloneOverlayRecords(base.overlays),
+		acquisition:        cloneAcquisitionScopes(base.acquisition),
+		importGraph:        base.importGraph.clone(),
+		collectionPolicies: cloneCollectionPolicyScope(base.collectionPolicies),
+		sequenceRanges:     cloneSequenceRanges(base.sequenceRanges),
+		origins:            cloneOriginIndex(base.origins), overlays: cloneOverlayRecords(base.overlays),
 		applications: cloneHistoryApplications(base.applications),
 		inputBytes:   base.inputBytes, inputNodes: base.inputNodes,
 	}
@@ -80,6 +89,53 @@ type sequenceRange struct {
 	path        Path
 	start       int
 	end         int
+	items       []sequenceItemProjection
+	projection  sequenceItemProjectionIndex
+}
+
+func (assigned sequenceRange) outputForInput(inputIndex int) (int, bool) {
+	if assigned.projection.outputByInput != nil {
+		return assigned.projection.outputForInput(inputIndex)
+	}
+	if assigned.items != nil {
+		output, found := 0, false
+		for _, item := range assigned.items {
+			if item.inputIndex == inputIndex && inputIndex >= 0 && item.outputIndex >= 0 {
+				output, found = item.outputIndex, true
+			}
+		}
+		return output, found
+	}
+	if inputIndex < 0 || inputIndex >= assigned.end-assigned.start {
+		return 0, false
+	}
+	return assigned.start + inputIndex, true
+}
+
+func (assigned sequenceRange) inputForOutput(outputIndex int) (int, bool) {
+	if assigned.projection.inputByOutput != nil {
+		return assigned.projection.inputForOutput(outputIndex)
+	}
+	if assigned.items != nil {
+		input, found := 0, false
+		for _, item := range assigned.items {
+			if item.outputIndex == outputIndex && outputIndex >= 0 && item.inputIndex >= 0 {
+				input, found = item.inputIndex, true
+			}
+		}
+		return input, found
+	}
+	if outputIndex < assigned.start || outputIndex >= assigned.end {
+		return 0, false
+	}
+	return outputIndex - assigned.start, true
+}
+
+func indexSequenceRange(assigned sequenceRange) sequenceRange {
+	if assigned.items != nil && assigned.projection.outputByInput == nil && assigned.projection.inputByOutput == nil {
+		assigned.projection = newSequenceItemProjectionIndex(assigned.items)
+	}
+	return assigned
 }
 
 // composeDocuments recursively joins disjoint mapping entries. Source order
@@ -93,6 +149,17 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 // sequence rules when policy contains registrations. The ordinary adapter above
 // deliberately preserves the existing unregistered behavior for current callers.
 func composeDocumentsWithPolicy(policy sequencePolicy, inputs ...*document.Node) (*composedDocument, []Diagnostic) {
+	return composeDocumentsWithCollectionPolicy(policy, collectionPolicyScope{}, nil, inputs...)
+}
+
+// composeDocumentsWithCollectionPolicy resolves collection declarations and
+// eligible lower policy before strictly composing completed peer documents.
+func composeDocumentsWithCollectionPolicy(
+	policy sequencePolicy,
+	lower collectionPolicyScope,
+	declarations []sequenceDeclarationSpec,
+	inputs ...*document.Node,
+) (*composedDocument, []Diagnostic) {
 	if len(inputs) == 0 {
 		return nil, []Diagnostic{compositionInvalidDiagnostic("strict composition requires at least one document", nil)}
 	}
@@ -110,6 +177,13 @@ func composeDocumentsWithPolicy(policy sequencePolicy, inputs ...*document.Node)
 		collected.append(input, Path{}, sourceIndex)
 	}
 
+	identityFieldsByPath, collectionPolicies, diagnostics := resolveStrictCollectionPolicies(
+		collected.ordered, policy, lower, declarations,
+	)
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+
 	checked := make(map[string]struct{}, len(collected.byPath))
 	for _, claim := range collected.ordered {
 		pointer := claim.path.String()
@@ -118,13 +192,13 @@ func composeDocumentsWithPolicy(policy sequencePolicy, inputs ...*document.Node)
 		}
 		checked[pointer] = struct{}{}
 		claims := collected.byPath[pointer]
-		if len(claims) < 2 || allMappingClaims(claims) || (allSequenceClaims(claims) && policy.registered(claim.path)) {
+		if len(claims) < 2 || allMappingClaims(claims) || (allSequenceClaims(claims) && len(identityFieldsByPath[pointer]) != 0) {
 			continue
 		}
 		return nil, []Diagnostic{compositionConflictDiagnostic(claim.path, claims)}
 	}
 
-	validated, ranges, diagnostics := validateRegisteredSequences(collected.ordered, policy)
+	validated, ranges, diagnostics := validateRegisteredSequences(collected.ordered, identityFieldsByPath)
 	if len(diagnostics) != 0 {
 		return nil, diagnostics
 	}
@@ -132,7 +206,7 @@ func composeDocumentsWithPolicy(policy sequencePolicy, inputs ...*document.Node)
 	root := document.Clone(inputs[0])
 	index := new(mappingIndex)
 	for sourceIndex, input := range inputs[1:] {
-		mergeComposedMappings(root, input, Path{}, index, sourceIndex+1, policy, validated)
+		mergeComposedMappings(root, input, Path{}, index, sourceIndex+1, identityFieldsByPath, validated)
 	}
 	applications := make([]historyApplication, 0, len(inputs))
 	seenSequences := make(map[string]bool)
@@ -159,11 +233,12 @@ func composeDocumentsWithPolicy(policy sequencePolicy, inputs ...*document.Node)
 	origins := projectedCompositionOrigins(inputs, ranges)
 	completeCompositionOrigins(root, Path{}, origins)
 	return &composedDocument{
-		root:           root,
-		claims:         cloneCompositionClaims(collected.ordered),
-		sequenceRanges: cloneSequenceRanges(ranges),
-		origins:        origins,
-		applications:   applications,
+		root:               root,
+		collectionPolicies: collectionPolicies,
+		claims:             cloneCompositionClaims(collected.ordered),
+		sequenceRanges:     cloneSequenceRanges(ranges),
+		origins:            origins,
+		applications:       applications,
 	}, nil
 }
 
@@ -219,71 +294,128 @@ type validatedSequence struct {
 	items []*document.Node
 }
 
-func validateRegisteredSequences(claims []compositionClaim, policy sequencePolicy) (map[string]map[int]validatedSequence, []sequenceRange, []Diagnostic) {
+func resolveStrictCollectionPolicies(
+	claims []compositionClaim,
+	applicationPolicy sequencePolicy,
+	lower collectionPolicyScope,
+	declarations []sequenceDeclarationSpec,
+) (map[string][]string, collectionPolicyScope, []Diagnostic) {
+	paths := make([]Path, 0, len(claims)+len(declarations))
+	seen := make(map[string]bool, len(claims)+len(declarations))
+	valuePresent := make(map[string]bool)
+	declarationsByPath := make(map[string][]sequenceDeclarationSpec, len(declarations))
+	for _, claim := range claims {
+		if len(lower.collections) != 0 {
+			valuePresent[claim.path.String()] = true
+		}
+		if claim.kind != document.Sequence {
+			continue
+		}
+		key := claim.path.String()
+		if !seen[key] {
+			seen[key] = true
+			paths = append(paths, claim.path.clone())
+		}
+	}
+	for _, declaration := range declarations {
+		key := declaration.destination.String()
+		declarationsByPath[key] = append(declarationsByPath[key], declaration)
+		if !seen[key] {
+			seen[key] = true
+			paths = append(paths, declaration.destination.clone())
+		}
+	}
+
+	identityFieldsByPath := make(map[string][]string)
+	effectiveLower := lower
+	if len(lower.collections) != 0 {
+		effectiveLower, _ = lowerCollectionPoliciesForClaims(lower, claims, declarationsByPath, applicationPolicy)
+	}
+	collectionPolicies := cloneCollectionPolicyScope(effectiveLower)
+	var diagnostics []Diagnostic
+	for _, path := range paths {
+		key := path.String()
+		currentDeclarations := declarationsByPath[key]
+
+		lowerEntry, hasLower := effectiveLower.collections[key]
+		var lowerPolicy *collectionPolicy
+		if hasLower {
+			copy := cloneCollectionPolicy(lowerEntry.policy)
+			lowerPolicy = &copy
+		}
+		applicationIdentity, _ := applicationPolicy.identityFieldsAt(path)
+
+		resolution, found := resolveCollectionPolicy(path, currentDeclarations, applicationIdentity, lowerPolicy)
+		if len(found) != 0 {
+			diagnostics = append(diagnostics, found...)
+			continue
+		}
+		if len(resolution.effectiveIdentityFields) != 0 {
+			identityFieldsByPath[key] = cloneStrings(resolution.effectiveIdentityFields)
+		}
+
+		policyForPath := resolution.policy
+		if hasLower && !valuePresent[key] {
+			// A declaration without a new value changes policy metadata but
+			// leaves retained record ownership untouched.
+			policyForPath = cloneCollectionPolicy(lowerEntry.policy)
+			policyForPath.identityFields = cloneStrings(resolution.policy.identityFields)
+			policyForPath.identityPresent = resolution.policy.identityPresent
+			policyForPath.identityLocation = cloneLocation(resolution.policy.identityLocation)
+			policyForPath.explicitMode = resolution.policy.explicitMode
+			policyForPath.explicitModePresent = resolution.policy.explicitModePresent
+			policyForPath.modeLocation = cloneLocation(resolution.policy.modeLocation)
+		}
+		if collectionPolicies.collections == nil {
+			collectionPolicies.collections = make(map[string]collectionPolicyEntry)
+		}
+		collectionPolicies.collections[key] = collectionPolicyEntry{destination: path.clone(), policy: policyForPath}
+	}
+	return identityFieldsByPath, collectionPolicies, canonicalDiagnostics(diagnostics)
+}
+
+func validateRegisteredSequences(claims []compositionClaim, identityFieldsByPath map[string][]string) (map[string]map[int]validatedSequence, []sequenceRange, []Diagnostic) {
 	itemsByPath := make(map[string]map[int]validatedSequence)
-	identitiesByPath := make(map[string]map[string]Location)
+	identitiesByPath := make(map[string]map[string]document.Location)
 	lengthsByPath := make(map[string]int)
 	var ranges []sequenceRange
 	for _, claim := range claims {
 		if claim.kind != document.Sequence {
 			continue
 		}
-		identityField, registered := policy.identityFields[claim.path.String()]
-		if !registered {
+		identityFields := identityFieldsByPath[claim.path.String()]
+		if len(identityFields) == 0 {
 			continue
 		}
 		node := claim.node
 		pathKey := claim.path.String()
 		identities := identitiesByPath[pathKey]
 		if identities == nil {
-			identities = make(map[string]Location, len(node.Sequence))
+			identities = make(map[string]document.Location, len(node.Sequence))
 			identitiesByPath[pathKey] = identities
 		}
 		start := lengthsByPath[pathKey]
 		contribution := validatedSequence{items: node.Sequence}
-		for itemIndex, item := range node.Sequence {
-			itemPath := claim.path.Index(start + itemIndex)
-			if item == nil || item.Kind != document.Mapping {
-				location := (*Location)(nil)
-				if item != nil {
-					converted := publicLocation(item.Location)
-					location = &converted
-				}
-				return nil, nil, []Diagnostic{{
-					Code: CodeComposeInvalid, Severity: SeverityError,
-					Path: itemPath, Message: "registered sequence items must be mappings", Primary: location,
-				}}
-			}
-			fieldIndex := documentEntryIndex(item.Mapping, identityField)
-			if fieldIndex < 0 {
-				location := publicLocation(item.Location)
-				return nil, nil, []Diagnostic{{
-					Code: CodeComposeInvalid, Severity: SeverityError,
-					Path: itemPath, Message: "registered sequence item is missing its identity field",
-					Primary: &location,
-				}}
-			}
-			field := item.Mapping[fieldIndex].Value
-			if field == nil || field.Kind != document.String {
-				location := publicLocation(item.Mapping[fieldIndex].ValueLocation)
-				if field != nil {
-					location = publicLocation(field.Location)
-				}
-				return nil, nil, []Diagnostic{{
-					Code: CodeComposeInvalid, Severity: SeverityError,
-					Path: itemPath.Key(identityField), Message: "registered sequence identity must be a string",
-					Primary: &location,
-				}}
-			}
-			if first, duplicate := identities[field.Text]; duplicate {
-				current := publicLocation(field.Location)
+		records, diagnostics := validateNormalizedSequenceIdentityFrom(node, claim.path, identityFields, sequenceIdentityDiagnosticCodes{
+			invalid: CodeComposeInvalid, conflict: CodeComposeConflict,
+		}, start, sequenceStructuralRecord)
+		if len(diagnostics) != 0 {
+			return nil, nil, diagnostics
+		}
+		byIndex := make([]sequenceIdentityRecord, len(node.Sequence))
+		for _, record := range records {
+			byIndex[record.index] = record
+		}
+		for _, record := range byIndex {
+			if first, duplicate := identities[record.tuple]; duplicate {
+				current := publicLocation(record.location)
 				return nil, nil, []Diagnostic{{
 					Code: CodeComposeConflict, Severity: SeverityError,
 					Path: claim.path.clone(), Message: "registered sequence contains duplicate identities",
-					Primary: &first, Related: []Location{current},
+					Primary: ptrLocation(publicLocation(first)), Related: []Location{current},
 				}}
 			}
-			identities[field.Text] = publicLocation(field.Location)
+			identities[record.tuple] = record.location
 		}
 		if itemsByPath[pathKey] == nil {
 			itemsByPath[pathKey] = make(map[int]validatedSequence)
@@ -297,7 +429,7 @@ func validateRegisteredSequences(claims []compositionClaim, policy sequencePolic
 	return itemsByPath, ranges, nil
 }
 
-func mergeComposedMappings(target, incoming *document.Node, path Path, index *mappingIndex, sourceIndex int, policy sequencePolicy, validated map[string]map[int]validatedSequence) {
+func mergeComposedMappings(target, incoming *document.Node, path Path, index *mappingIndex, sourceIndex int, identityFieldsByPath map[string][]string, validated map[string]map[int]validatedSequence) {
 	for _, incomingEntry := range incoming.Mapping {
 		entryPath := path.Key(incomingEntry.Key)
 		position := index.find(target, incomingEntry.Key)
@@ -308,7 +440,7 @@ func mergeComposedMappings(target, incoming *document.Node, path Path, index *ma
 			continue
 		}
 		current := target.Mapping[position].Value
-		if current.Kind == document.Sequence && incomingEntry.Value.Kind == document.Sequence && policy.registered(entryPath) {
+		if current.Kind == document.Sequence && incomingEntry.Value.Kind == document.Sequence && len(identityFieldsByPath[entryPath.String()]) != 0 {
 			contribution := validated[entryPath.String()][sourceIndex]
 			if contribution.items != nil {
 				for _, item := range contribution.items {
@@ -317,20 +449,15 @@ func mergeComposedMappings(target, incoming *document.Node, path Path, index *ma
 			}
 			continue
 		}
-		mergeDisjointMappingsAtPath(current, incomingEntry.Value, entryPath, index, sourceIndex, policy, validated)
+		mergeDisjointMappingsAtPath(current, incomingEntry.Value, entryPath, index, sourceIndex, identityFieldsByPath, validated)
 	}
 }
 
-func mergeDisjointMappingsAtPath(target, incoming *document.Node, path Path, index *mappingIndex, sourceIndex int, policy sequencePolicy, validated map[string]map[int]validatedSequence) {
+func mergeDisjointMappingsAtPath(target, incoming *document.Node, path Path, index *mappingIndex, sourceIndex int, identityFieldsByPath map[string][]string, validated map[string]map[int]validatedSequence) {
 	if target.Kind != document.Mapping || incoming.Kind != document.Mapping {
 		return
 	}
-	mergeComposedMappings(target, incoming, path, index, sourceIndex, policy, validated)
-}
-
-func (p sequencePolicy) registered(path Path) bool {
-	_, ok := p.identityFields[path.String()]
-	return ok
+	mergeComposedMappings(target, incoming, path, index, sourceIndex, identityFieldsByPath, validated)
 }
 
 func cloneSequenceRanges(ranges []sequenceRange) []sequenceRange {
@@ -338,6 +465,11 @@ func cloneSequenceRanges(ranges []sequenceRange) []sequenceRange {
 	for i, assigned := range ranges {
 		cloned[i] = assigned
 		cloned[i].path = assigned.path.clone()
+		if assigned.items != nil {
+			cloned[i].items = append([]sequenceItemProjection{}, assigned.items...)
+		}
+		cloned[i].projection.outputByInput = maps.Clone(assigned.projection.outputByInput)
+		cloned[i].projection.inputByOutput = maps.Clone(assigned.projection.inputByOutput)
 	}
 	return cloned
 }
@@ -377,12 +509,28 @@ func documentEntryIndex(entries []document.Entry, key string) int {
 
 func projectedCompositionOrigins(inputs []*document.Node, ranges []sequenceRange) map[string]Origin {
 	origins := make(map[string]Origin)
-	rangeBySourcePath := make(map[sequenceRangeKey]int, len(ranges))
-	for index, assigned := range ranges {
-		rangeBySourcePath[sequenceRangeKey{sourceIndex: assigned.sourceIndex, path: assigned.path.String()}] = index
+	sequencesBySource := make([][]sequenceHistoryContribution, len(inputs))
+	for _, assigned := range ranges {
+		contribution := sequenceHistoryContribution{
+			path: assigned.path.clone(), mode: sequenceHistoryAppend,
+			offset: assigned.start, length: assigned.end - assigned.start,
+		}
+		if assigned.items != nil {
+			contribution.mode = sequenceHistoryMapped
+			contribution.items = assigned.items
+		}
+		sequencesBySource[assigned.sourceIndex] = append(sequencesBySource[assigned.sourceIndex], contribution)
 	}
 	for sourceIndex, input := range inputs {
-		collectProjectedCompositionOrigins(input, Path{}, Path{}, sourceIndex, ranges, rangeBySourcePath, origins)
+		application := historyApplication{value: input, sequences: sequencesBySource[sourceIndex]}
+		application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
+		walkHistoryApplication(application, func(_, effective Path, node *document.Node) {
+			pointer := effective.String()
+			origin := origins[pointer]
+			origin.Layer = LayerBase
+			origin.Locations = append(origin.Locations, publicLocation(node.Location))
+			origins[pointer] = origin
+		})
 	}
 	return origins
 }
@@ -400,39 +548,12 @@ func indexSequenceRanges(ranges []sequenceRange) sequenceRangeIndex {
 	}
 	index := make(sequenceRangeIndex, len(ranges))
 	for _, assigned := range ranges {
+		assigned = indexSequenceRange(assigned)
 		index[sequenceRangeKey{sourceIndex: assigned.sourceIndex, path: assigned.path.String()}] = assigned
 	}
 	return index
 }
 
-// Keep local paths for matching Stage 2 ranges and effective paths for emitted
-// evidence. The paths are transformed segment by segment; pointer strings are
-// only used as map keys after projection is complete.
-func collectProjectedCompositionOrigins(node *document.Node, localPath, effectivePath Path, sourceIndex int, ranges []sequenceRange, rangeBySourcePath map[sequenceRangeKey]int, origins map[string]Origin) {
-	pointer := effectivePath.String()
-	origin := origins[pointer]
-	origin.Layer = LayerBase
-	origin.Locations = append(origin.Locations, publicLocation(node.Location))
-	origins[pointer] = origin
-
-	switch node.Kind {
-	case document.Mapping:
-		for _, entry := range node.Mapping {
-			collectProjectedCompositionOrigins(entry.Value, localPath.Key(entry.Key), effectivePath.Key(entry.Key), sourceIndex, ranges, rangeBySourcePath, origins)
-		}
-	case document.Sequence:
-		start := 0
-		if rangeIndex, ok := rangeBySourcePath[sequenceRangeKey{sourceIndex: sourceIndex, path: localPath.String()}]; ok {
-			start = ranges[rangeIndex].start
-		}
-		for index, child := range node.Sequence {
-			localChild := localPath.Index(index)
-			effectiveChild := effectivePath.Index(start + index)
-			collectProjectedCompositionOrigins(child, localChild, effectiveChild, sourceIndex, ranges, rangeBySourcePath, origins)
-		}
-	}
-}
-
 func applyCompositionOrigins(decoded *decodedConfiguration, origins map[string]Origin) {
 	for pointer, origin := range origins {
 		if _, present := decoded.presence[pointer]; present {

internal/engine/compose_test.go

@@ -10,17 +10,20 @@ import (
 )
 
 func TestComposeWideRegisteredCollectionsPreservesEvidenceAndReplay(t *testing.T) {
-	policy := sequencePolicy{identityFields: make(map[string]string)}
+	registrations := make([]sequenceIdentityRegistration, 0, 128)
 	var inputs []*document.Node
 	for source := 0; source < 3; source++ {
 		var text strings.Builder
 		for collection := 0; collection < 128; collection++ {
 			key := fmt.Sprintf("records%d", collection)
-			policy.identityFields[Keys(key).String()] = "id"
+			registrations = append(registrations, sequenceIdentityRegistration{
+				selector: sequenceIdentitySelector{literalSequenceSelector(key)}, identityFields: []string{"id"},
+			})
 			fmt.Fprintf(&text, "%s: [{id: source%d}]\n", key, source)
 		}
 		inputs = append(inputs, parseCompositionDocument(t, fmt.Sprintf("source-%d", source), text.String()))
 	}
+	policy := mustCompileSequenceIdentityPolicy(t, registrations)
 	composed, diagnostics := composeDocumentsWithPolicy(policy, inputs...)
 	if len(diagnostics) != 0 {
 		t.Fatal(diagnostics)
@@ -242,7 +245,7 @@ func TestComposeDocumentsAppendsRegisteredSequencesAndReturnsRanges(t *testing.T
 	first := parseCompositionDocument(t, "source-1", "records: [{id: '', left: 1}, {id: ' spaced ', right: 1}]\nother: [one]\n")
 	second := parseCompositionDocument(t, "source-2", "records: [{id: ' ', text: second}]\n")
 	firstBefore, secondBefore := document.Clone(first), document.Clone(second)
-	policy := sequencePolicy{identityFields: map[string]string{"/records": "id"}}
+	policy := mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}})
 
 	composed, diagnostics := composeDocumentsWithPolicy(policy, first, second)
 	if len(diagnostics) != 0 {
@@ -287,10 +290,10 @@ escaped/~:
       name: escaped-second-name
 `)
 	firstBefore, secondBefore := document.Clone(first), document.Clone(second)
-	policy := sequencePolicy{identityFields: map[string]string{
-		Keys("mounted", "records").String(): "id",
-		Keys("escaped/~", "7").String():     "id",
-	}}
+	policy := mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{
+		{selector: sequenceIdentitySelector{literalSequenceSelector("mounted"), literalSequenceSelector("records")}, identityFields: []string{"id"}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("escaped/~"), literalSequenceSelector("7")}, identityFields: []string{"id"}},
+	})
 
 	composed, diagnostics := composeDocumentsWithPolicy(policy, first, second)
 	if len(diagnostics) != 0 {
@@ -357,6 +360,78 @@ escaped/~:
 	}
 }
 
+func TestProjectedCompositionOriginsUseNoncontiguousItemMappings(t *testing.T) {
+	lowerRoot := parseCompositionDocument(t, "lower", `records:
+  - id: first
+    label: equal
+  - id: middle
+  - id: last
+    nested: {old: lower}
+`)
+	higherRoot := parseCompositionDocument(t, "higher", `records:
+  - id: last
+    nested: {new: higher}
+  - id: first
+    label: equal
+  - id: added
+`)
+	lower, _ := documentAtPath(lowerRoot, Keys("records"))
+	higher, _ := documentAtPath(higherRoot, Keys("records"))
+	combined, diagnostics := combineSequence(
+		lower, higher, sequenceCombinationPolicy(sequenceModeUpsert, "id"),
+		sequenceCombinationContext{destination: Keys("records")},
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("combine diagnostics = %#v", diagnostics)
+	}
+	inputs := []*document.Node{lowerRoot, higherRoot}
+	ranges := []sequenceRange{
+		{
+			sourceIndex: 0, path: Keys("records"), start: 0, end: 3,
+			items: combined.lowerItems,
+		}, {
+			sourceIndex: 1, path: Keys("records"), start: 0, end: 3,
+			items: combined.higherItems,
+		},
+	}
+	origins := projectedCompositionOrigins(inputs, ranges)
+	if got, want := sequenceIdentityValues(combined.value, "id"), []string{"first", "middle", "last", "added"}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("combined IDs = %#v, want %#v", got, want)
+	}
+	for path, want := range map[string][]string{
+		"/records/0/label":      {"higher"},
+		"/records/1/id":         {"lower"},
+		"/records/2/nested/new": {"higher"},
+		"/records/3/id":         {"higher"},
+		"/records/0/nested/new": nil,
+		"/records/2/nested/old": nil,
+	} {
+		origin := origins[path]
+		var got []string
+		for _, location := range origin.Locations {
+			got = append(got, location.SourceID)
+		}
+		if !reflect.DeepEqual(got, want) {
+			t.Errorf("origin sources at %s = %#v, want %#v", path, got, want)
+		}
+	}
+}
+
+func TestCloneSequenceRangesPreservesPreparedProjection(t *testing.T) {
+	ranges := []sequenceRange{{
+		start: 0, end: 1,
+		projection: newSequenceItemProjectionIndex([]sequenceItemProjection{{inputIndex: 0, outputIndex: 4}}),
+	}}
+	cloned := cloneSequenceRanges(ranges)
+	if output, ok := cloned[0].outputForInput(0); !ok || output != 4 {
+		t.Fatalf("cloned projection = %d, %v; want output index 4", output, ok)
+	}
+	cloned[0].projection.outputByInput[0] = 9
+	if output, ok := ranges[0].outputForInput(0); !ok || output != 4 {
+		t.Fatalf("cloned projection aliases original: %d, %v", output, ok)
+	}
+}
+
 func TestComposeDocumentsValidatesRegisteredSequenceSingletons(t *testing.T) {
 	tests := []struct {
 		name     string
@@ -365,15 +440,12 @@ func TestComposeDocumentsValidatesRegisteredSequenceSingletons(t *testing.T) {
 		wantCode Code
 	}{
 		{name: "nonmapping item", body: "records: [plain]\n", path: "/records/0", wantCode: CodeComposeInvalid},
-		{name: "missing identity", body: "records: [{name: plain}]\n", path: "/records/0", wantCode: CodeComposeInvalid},
-		{name: "null identity", body: "records: [{id: null}]\n", path: "/records/0/id", wantCode: CodeComposeInvalid},
-		{name: "numeric identity", body: "records: [{id: 12}]\n", path: "/records/0/id", wantCode: CodeComposeInvalid},
-		{name: "boolean identity", body: "records: [{id: false}]\n", path: "/records/0/id", wantCode: CodeComposeInvalid},
+		{name: "missing identity", body: "records: [{name: plain}]\n", path: "/records/0/id", wantCode: CodeComposeInvalid},
 	}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
 			_, diagnostics := composeDocumentsWithPolicy(
-				sequencePolicy{identityFields: map[string]string{"/records": "id"}},
+				mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}}),
 				parseCompositionDocument(t, "source-1", test.body),
 			)
 			if len(diagnostics) != 1 || diagnostics[0].Code != test.wantCode || diagnostics[0].Path.String() != test.path {
@@ -386,14 +458,138 @@ func TestComposeDocumentsValidatesRegisteredSequenceSingletons(t *testing.T) {
 	}
 }
 
+func TestComposeDocumentsUsesLowerIdentityForStrictPeers(t *testing.T) {
+	path := Keys("records")
+	lower := collectionPolicyScope{collections: map[string]collectionPolicyEntry{
+		path.String(): {
+			destination: path,
+			policy:      collectionPolicy{identityFields: []string{"id"}, identityPresent: true},
+		},
+	}}
+	composed, diagnostics := composeDocumentsWithCollectionPolicy(
+		sequencePolicy{}, lower, nil,
+		parseCompositionDocument(t, "source-1", "records: [{id: first}]\n"),
+		parseCompositionDocument(t, "source-2", "records: [{id: second}]\n"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose strict peers with lower identity: %#v", diagnostics)
+	}
+	value, present := documentAtPath(composed.root, path)
+	if !present || value.Kind != document.Sequence || len(value.Sequence) != 2 {
+		t.Fatalf("composed records = %#v, %v; want two strict peer records", value, present)
+	}
+	resolved, present := composed.collectionPolicies.collections[path.String()]
+	if !present || !resolved.policy.identityPresent || !reflect.DeepEqual(resolved.policy.identityFields, []string{"id"}) {
+		t.Fatalf("composed collection policy = %#v, %v; want retained lower identity", resolved, present)
+	}
+	clone := cloneComposedDocument(composed)
+	resolved.policy.identityFields[0] = "mutated"
+	if !reflect.DeepEqual(clone.collectionPolicies.collections[path.String()].policy.identityFields, []string{"id"}) {
+		t.Fatal("cloned composition shares mutable collection identity fields")
+	}
+}
+
+func TestComposeDocumentsResolvesDeclarationsBeforePeerComposition(t *testing.T) {
+	path := Keys("records")
+	declaration := func(source string, line int, fields ...string) sequenceDeclarationSpec {
+		location := Location{SourceID: source, Line: line}
+		return sequenceDeclarationSpec{
+			destination: path, identityFields: fields, identityPresent: true,
+			identityLocation: &location,
+		}
+	}
+
+	_, diagnostics := composeDocumentsWithCollectionPolicy(
+		sequencePolicy{}, collectionPolicyScope{},
+		[]sequenceDeclarationSpec{declaration("source-1", 2, "id"), declaration("source-2", 4, "name")},
+		parseCompositionDocument(t, "source-1", "left: true\n"),
+		parseCompositionDocument(t, "source-2", "right: true\n"),
+	)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict || diagnostics[0].Path.String() != "/records" {
+		t.Fatalf("conflicting policy-only declarations = %#v", diagnostics)
+	}
+	if diagnostics[0].Primary == nil || len(diagnostics[0].Related) != 1 {
+		t.Fatalf("declaration conflict lacks both locations: %#v", diagnostics[0])
+	}
+
+	composed, diagnostics := composeDocumentsWithCollectionPolicy(
+		sequencePolicy{}, collectionPolicyScope{},
+		[]sequenceDeclarationSpec{declaration("source-1", 2, "tenant", "id"), declaration("source-2", 4, "id", "tenant")},
+		parseCompositionDocument(t, "source-1", "records: [{tenant: true, id: null}, {tenant: \"alpha\", id: 1}]\n"),
+		parseCompositionDocument(t, "source-2", "records: [{id: 1.0, tenant: \"beta\"}, {id: \"1\", tenant: \"alpha\"}]\n"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose normalized compound identities: %#v", diagnostics)
+	}
+	value, present := documentAtPath(composed.root, path)
+	if !present || len(value.Sequence) != 4 {
+		t.Fatalf("compound identity records = %#v, %v; want four distinct records", value, present)
+	}
+
+	_, diagnostics = composeDocumentsWithCollectionPolicy(
+		sequencePolicy{}, collectionPolicyScope{},
+		[]sequenceDeclarationSpec{declaration("source-1", 2, "id")},
+		parseCompositionDocument(t, "source-1", "records: [{id: 1}]\n"),
+		parseCompositionDocument(t, "source-2", "records: [{id: 1.0}]\n"),
+	)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeConflict || diagnostics[0].Path.String() != "/records" {
+		t.Fatalf("equivalent numeric peer identities = %#v", diagnostics)
+	}
+}
+
+func TestComposeDocumentsReplacementDoesNotBorrowLowerYAMLIdentity(t *testing.T) {
+	path := Keys("records")
+	lower := collectionPolicyScope{collections: map[string]collectionPolicyEntry{
+		path.String(): {
+			destination: path,
+			policy:      collectionPolicy{identityFields: []string{"id"}, identityPresent: true},
+		},
+	}}
+	declaration := sequenceDeclarationSpec{destination: path, mode: sequenceModeReplace, modePresent: true}
+	_, diagnostics := composeDocumentsWithCollectionPolicy(
+		sequencePolicy{}, lower, []sequenceDeclarationSpec{declaration},
+		parseCompositionDocument(t, "source-1", "records: [{name: first}]\n"),
+		parseCompositionDocument(t, "source-2", "records: [{name: second}]\n"),
+	)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeConflict || diagnostics[0].Path.String() != "/records" {
+		t.Fatalf("replacement peers borrowing lower identity = %#v", diagnostics)
+	}
+
+	identityReplacement := sequenceDeclarationSpec{
+		destination: path, identityFields: []string{"name"}, identityPresent: true,
+		mode: sequenceModeReplace, modePresent: true,
+	}
+	composed, diagnostics := composeDocumentsWithCollectionPolicy(
+		sequencePolicy{}, lower, []sequenceDeclarationSpec{identityReplacement},
+		parseCompositionDocument(t, "source-1", "records: [{name: first}]\n"),
+		parseCompositionDocument(t, "source-2", "records: [{name: second}]\n"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("replacement with a new identity declaration: %#v", diagnostics)
+	}
+	resolved := composed.collectionPolicies.collections[path.String()].policy
+	if !reflect.DeepEqual(resolved.identityFields, []string{"name"}) || !resolved.identityPresent {
+		t.Fatalf("replacement collection identity = %#v; want name", resolved)
+	}
+
+	_, diagnostics = composeDocumentsWithCollectionPolicy(
+		mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}}), lower,
+		[]sequenceDeclarationSpec{declaration},
+		parseCompositionDocument(t, "source-1", "records: [{name: replacement}]\n"),
+	)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeInvalid || diagnostics[0].Path.String() != "/records/0/id" {
+		t.Fatalf("replacement ignored application identity constraint = %#v", diagnostics)
+	}
+}
+
 func TestComposeDocumentsReportsAssignedPathForMalformedRegisteredSequenceContribution(t *testing.T) {
 	_, diagnostics := composeDocumentsWithPolicy(
-		sequencePolicy{identityFields: map[string]string{"/records": "id"}},
+		mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}}),
 		parseCompositionDocument(t, "source-1", "records: [{id: first}]\n"),
 		parseCompositionDocument(t, "source-2", "records: [{name: malformed}]\n"),
 	)
-	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeInvalid || diagnostics[0].Path.String() != "/records/1" {
-		t.Fatalf("diagnostics = %#v, want invalid item at /records/1", diagnostics)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeInvalid || diagnostics[0].Path.String() != "/records/1/id" {
+		t.Fatalf("diagnostics = %#v, want invalid identity field at /records/1/id", diagnostics)
 	}
 	if diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-2" {
 		t.Fatalf("malformed item location = %#v, want source-2", diagnostics[0].Primary)
@@ -420,7 +616,7 @@ func TestComposeDocumentsRejectsRegisteredDuplicateIdentities(t *testing.T) {
 				inputs = append(inputs, parseCompositionDocument(t, "source-2", test.second))
 			}
 			_, diagnostics := composeDocumentsWithPolicy(
-				sequencePolicy{identityFields: map[string]string{"/records": "id"}}, inputs...,
+				mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}}), inputs...,
 			)
 			if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeConflict || diagnostics[0].Path.String() != "/records" {
 				t.Fatalf("diagnostics = %#v, want duplicate conflict at /records", diagnostics)
@@ -438,9 +634,12 @@ func TestComposeDocumentsRejectsRegisteredDuplicateIdentities(t *testing.T) {
 }
 
 func TestComposeDocumentsSkipsMissingAndNonSequenceRegistrations(t *testing.T) {
-	policy := sequencePolicy{identityFields: map[string]string{
-		"/absent": "id", "/not-a-sequence": "id", "/catalog": "id", "/catalog/records": "id",
-	}}
+	policy := mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{
+		{selector: sequenceIdentitySelector{literalSequenceSelector("absent")}, identityFields: []string{"id"}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("not-a-sequence")}, identityFields: []string{"id"}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("catalog")}, identityFields: []string{"id"}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("catalog"), literalSequenceSelector("records")}, identityFields: []string{"id"}},
+	})
 	composed, diagnostics := composeDocumentsWithPolicy(policy, parseCompositionDocument(t, "source-1", `
 not-a-sequence: null
 catalog:
@@ -454,7 +653,7 @@ catalog:
 	}
 
 	composed, diagnostics = composeDocumentsWithPolicy(
-		sequencePolicy{identityFields: map[string]string{"/records": "id"}},
+		mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}}),
 		parseCompositionDocument(t, "source-1", "records: []\n"),
 		parseCompositionDocument(t, "source-2", "records: []\n"),
 	)
@@ -471,7 +670,7 @@ catalog:
 
 func TestComposeDocumentsKeepsUnregisteredSequenceConflicts(t *testing.T) {
 	_, diagnostics := composeDocumentsWithPolicy(
-		sequencePolicy{identityFields: map[string]string{"/registered": "id"}},
+		mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("registered")}, identityFields: []string{"id"}}}),
 		parseCompositionDocument(t, "source-1", "unregistered: [one]\n"),
 		parseCompositionDocument(t, "source-2", "unregistered: [two]\n"),
 	)
@@ -491,7 +690,7 @@ func TestComposeDocumentsKeepsNonSequenceRegisteredOverlapsConflicting(t *testin
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
 			_, diagnostics := composeDocumentsWithPolicy(
-				sequencePolicy{identityFields: map[string]string{"/records": "id"}},
+				mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}}),
 				parseCompositionDocument(t, "source-1", test.first),
 				parseCompositionDocument(t, "source-2", test.second),
 			)

internal/engine/destination_accounting_test.go

@@ -34,7 +34,7 @@ func TestDestinationPlacementCountsAgainstSharedNodeBudget(t *testing.T) {
 		t.Run(test.name, func(t *testing.T) {
 			t.Parallel()
 			accounting := &importAccounting{nodes: limits.nodes - perInput + test.delta}
-			composition, _, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{
+			composition, _, _, diagnostics := testAcquireUnderlayGroup(loadGroup{inputs: []loadInput{{
 				source: empty, destination: destination,
 			}}}, 1, nil, accounting)
 			if test.wantError {
@@ -62,7 +62,7 @@ func TestMultipleMountedInputsSharePlacementCharges(t *testing.T) {
 	perInput := countInputNodes(&document.Node{Kind: document.Mapping}) + destinationWrapperNodeCount(Keys("collection", "one"))
 	charge := len(inputs) * perInput
 	accounting := &importAccounting{nodes: limits.nodes - charge}
-	composition, _, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: inputs}, 1, nil, accounting)
+	composition, _, _, diagnostics := testAcquireUnderlayGroup(loadGroup{inputs: inputs}, 1, nil, accounting)
 	if len(diagnostics) != 0 || composition == nil || accounting.nodes != limits.nodes || composition.inputNodes != charge {
 		t.Fatalf("shared placement accounting = %#v, %#v, accounting %#v; want charge %d", composition, diagnostics, accounting, charge)
 	}
@@ -107,7 +107,7 @@ id: parent
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			accounting := &importAccounting{nodes: limits.nodes - wantNodes + test.delta}
-			composition, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{
+			composition, sources, _, diagnostics := testAcquireUnderlayGroup(loadGroup{inputs: []loadInput{{
 				source: File(mainPath), imports: true, destination: destination, placementMode: placementModeSequenceItem,
 			}, {
 				source: Mapping("other", map[string]any{"id": "other"}), destination: otherDestination, placementMode: placementModeSequenceItem,
@@ -160,7 +160,7 @@ func TestImportedEmptyBodiesChargeEachDestinationWrapper(t *testing.T) {
 		t.Run(test.name, func(t *testing.T) {
 			t.Parallel()
 			accounting := &importAccounting{nodes: limits.nodes - rawNodes - placementNodes + test.delta}
-			composition, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{
+			composition, sources, _, diagnostics := testAcquireUnderlayGroup(loadGroup{inputs: []loadInput{{
 				source: mainSource, imports: true, destination: destination,
 			}}}, 1, nil, accounting)
 			if test.wantError {
@@ -387,7 +387,7 @@ func TestNoSourceAndRootPlacementAddNoWrapperCharges(t *testing.T) {
 	t.Parallel()
 
 	accounting := &importAccounting{}
-	composition, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{
+	composition, sources, _, diagnostics := testAcquireUnderlayGroup(loadGroup{inputs: []loadInput{
 		{source: NoSource(), destination: Keys("absent")},
 		{source: Mapping("root", map[string]any{})},
 	}}, 1, nil, accounting)
@@ -431,7 +431,13 @@ func TestSelectedOverlayAndCLIUsePlacementNodeCharges(t *testing.T) {
 			overrides: []Override{override},
 		}
 		accounting := &importAccounting{nodes: limits.nodes - baseNodes - overrideNodes + 1}
-		_, diagnostics := assembleLayeredInput(request, assembledInput{}, accounting)
+		input, diagnostics := assembleLayeredInput(request, assembledInput{}, accounting)
+		if len(diagnostics) != 0 {
+			t.Fatalf("assembly diagnostics = %#v", diagnostics)
+		}
+		_, diagnostics = preparePostStructuralOverrides(overridePreparationInput{
+			root: input.document, inputBytes: accounting.bytes, inputNodes: accounting.nodes,
+		}, input.sources, request.overrides, nil, limits)
 		if len(diagnostics) != 1 || diagnostics[0].Code != CodeOverrideLimitExceeded {
 			t.Fatalf("override diagnostics = %#v", diagnostics)
 		}

internal/engine/destination_integration_test.go

@@ -197,8 +197,8 @@ func TestLayeredItemVersionProtectionTracksAssignedPeerIndexes(t *testing.T) {
 	t.Parallel()
 
 	definition := Definition[map[string]any]{
-		Version:               &VersionRule{Path: Keys("schema", "version"), Check: func(Version, *Validation) error { return nil }},
-		sequenceRegistrations: []sequenceRegistration{{path: Keys("catalog", "entries"), identityField: "id"}},
+		Version:                       &VersionRule{Path: Keys("schema", "version"), Check: func(Version, *Validation) error { return nil }},
+		sequenceIdentityRegistrations: []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("catalog"), literalSequenceSelector("entries")}, identityFields: []string{"id"}}},
 	}
 	item := func(label string, value map[string]any) Input {
 		return Input{Source: Mapping(label, value), Destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem}
@@ -209,7 +209,7 @@ func TestLayeredItemVersionProtectionTracksAssignedPeerIndexes(t *testing.T) {
 	main := item("main", map[string]any{"id": "main", "schema": map[string]any{"version": "v1"}})
 
 	t.Run("same assigned main path is protected", func(t *testing.T) {
-		_, err := LoadLayered(main, testSecretBinding(), options(item("underlay", map[string]any{"id": "underlay", "schema": nil})))
+		_, err := LoadLayered(main, testSecretBinding(), options(item("underlay", map[string]any{"id": "main", "schema": nil})))
 		if got := groupedErrorCode(t, err); got != CodeUnderlayVersion {
 			t.Fatalf("protected item claim code = %s, want %s", got, CodeUnderlayVersion)
 		}
@@ -224,7 +224,7 @@ func TestLayeredItemVersionProtectionTracksAssignedPeerIndexes(t *testing.T) {
 			t.Fatalf("unrelated appended peer rejected: %v", err)
 		}
 		entries := layeredMapValue(t, testLoaded(loaded).Config, "catalog", "entries").([]any)
-		if len(entries) != 1 || entries[0].(map[string]any)["id"] != "main" {
+		if len(entries) != 3 || entries[0].(map[string]any)["id"] != "first" || entries[1].(map[string]any)["id"] != "peer" || entries[2].(map[string]any)["id"] != "main" {
 			t.Fatalf("main item projection = %#v", entries)
 		}
 	})

internal/engine/directives.go

@@ -9,29 +9,104 @@ import (
 
 const directiveKey = "configkit"
 
+type rootDirectiveContext uint8
+
+const (
+	rootDirectiveContextImportsOnly rootDirectiveContext = iota + 1
+	rootDirectiveContextSource
+	rootDirectiveContextOverlay
+	rootDirectiveContextLayered
+)
+
+type rootDirectiveExtraction struct {
+	body             *document.Node
+	imports          []importDeclarationSpec
+	sequences        []sequenceDeclarationSpec
+	importsPresent   bool
+	sequencesPresent bool
+}
+
 func splitImportDirectives(root *document.Node) (*document.Node, []importDeclarationSpec, []Diagnostic) {
+	extracted, diagnostics := extractRootDirectives(root, rootDirectiveContextImportsOnly)
+	if len(diagnostics) != 0 {
+		return nil, nil, diagnostics
+	}
+	return extracted.body, extracted.imports, nil
+}
+
+func extractRootDirectives(root *document.Node, context rootDirectiveContext) (rootDirectiveExtraction, []Diagnostic) {
+	if root == nil {
+		return rootDirectiveExtraction{}, nil
+	}
 	body := document.Clone(root)
-	var declarations []importDeclarationSpec
 	directiveIndex := -1
 	for index, entry := range body.Mapping {
 		if entry.Key != directiveKey {
 			continue
 		}
 		directiveIndex = index
-		parsed, diagnostics := parseImportSpecificationDirective(entry.Value, Keys(directiveKey))
+		extracted, diagnostics := parseRootDirectiveBlock(entry.Value, Keys(directiveKey), context)
 		if len(diagnostics) != 0 {
-			return nil, nil, diagnostics
+			return rootDirectiveExtraction{}, diagnostics
 		}
-		declarations = parsed
-		break
-	}
-	if directiveIndex >= 0 {
-		body.Mapping = append(body.Mapping[:directiveIndex], body.Mapping[directiveIndex+1:]...)
+		extracted.body = body
+		if directiveIndex >= 0 {
+			extracted.body.Mapping = append(extracted.body.Mapping[:directiveIndex], extracted.body.Mapping[directiveIndex+1:]...)
+		}
+		if diagnostics := nestedDirectiveDiagnostics(extracted.body, Path{}); len(diagnostics) != 0 {
+			return rootDirectiveExtraction{}, diagnostics
+		}
+		return extracted, nil
 	}
 	if diagnostics := nestedDirectiveDiagnostics(body, Path{}); len(diagnostics) != 0 {
-		return nil, nil, diagnostics
+		return rootDirectiveExtraction{}, diagnostics
 	}
-	return body, declarations, nil
+	return rootDirectiveExtraction{body: body}, nil
+}
+
+func parseRootDirectiveBlock(node *document.Node, directivePath Path, context rootDirectiveContext) (rootDirectiveExtraction, []Diagnostic) {
+	if node == nil || node.Kind != document.Mapping {
+		return rootDirectiveExtraction{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, directivePath, "configuration directive must be a mapping", node)}
+	}
+	var extracted rootDirectiveExtraction
+	var diagnostics []Diagnostic
+	for _, entry := range node.Mapping {
+		fieldPath := directivePath.Key(entry.Key)
+		switch entry.Key {
+		case "imports":
+			extracted.importsPresent = true
+			if context == rootDirectiveContextOverlay {
+				diagnostics = append(diagnostics, nodeImportDiagnostic(CodeOverlayDirective, fieldPath, "overlays cannot contain imports", entry.Value))
+				continue
+			}
+			if context != rootDirectiveContextImportsOnly && context != rootDirectiveContextSource && context != rootDirectiveContextLayered {
+				diagnostics = append(diagnostics, nodeImportDiagnostic(CodeImportInvalid, fieldPath, "directive extraction context is invalid", entry.Value))
+				continue
+			}
+			parsed, issues := parseImportDeclarations(entry.Value, fieldPath)
+			extracted.imports = append(extracted.imports, parsed...)
+			diagnostics = append(diagnostics, issues...)
+		case "sequences":
+			extracted.sequencesPresent = true
+			if context == rootDirectiveContextImportsOnly {
+				diagnostics = append(diagnostics, sequenceDeclarationDiagnostic(fieldPath, "sequence declarations are not enabled in this extraction context", entry.Value))
+				continue
+			}
+			if context != rootDirectiveContextSource && context != rootDirectiveContextOverlay && context != rootDirectiveContextLayered {
+				diagnostics = append(diagnostics, sequenceDeclarationDiagnostic(fieldPath, "directive extraction context is invalid", entry.Value))
+				continue
+			}
+			parsed, issues := parseSequenceDeclarationDirective(entry.Value, fieldPath)
+			extracted.sequences = append(extracted.sequences, parsed...)
+			diagnostics = append(diagnostics, issues...)
+		default:
+			diagnostics = append(diagnostics, nodeImportDiagnostic(CodeImportInvalid, fieldPath, "configuration directive field is not recognized", entry.Value))
+		}
+	}
+	if len(diagnostics) != 0 {
+		return rootDirectiveExtraction{}, canonicalDiagnostics(diagnostics)
+	}
+	return extracted, nil
 }
 
 func nestedDirectiveDiagnostics(node *document.Node, current Path) []Diagnostic {
@@ -91,10 +166,11 @@ func fragmentVersionClaimWithIndex(root *document.Node, rule *VersionRule, sourc
 				return current
 			}
 			if assigned, found := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: currentPath.String()}]; found {
-				if index < assigned.start || index >= assigned.end {
+				projected, ok := assigned.inputForOutput(index)
+				if !ok {
 					return nil
 				}
-				index -= assigned.start
+				index = projected
 			}
 			if index < 0 || index >= len(current.Sequence) {
 				return nil
@@ -143,24 +219,28 @@ func nodeImportDiagnostic(code Code, path Path, message string, node *document.N
 	return Diagnostic{Code: code, Severity: SeverityError, Path: path, Message: message, Primary: &location}
 }
 
-func overlayDirectiveDiagnostics(node *document.Node, current Path) []Diagnostic {
+func overlayDirectiveDiagnostics(node *document.Node, current Path, allowSequences ...bool) []Diagnostic {
+	sequencesAllowed := len(allowSequences) != 0 && allowSequences[0]
 	if node.Kind == document.Mapping {
 		for _, entry := range node.Mapping {
 			childPath := current.Key(entry.Key)
 			if entry.Key == directiveKey {
+				if sequencesAllowed && len(current.segments) == 0 && entry.Value != nil && entry.Value.Kind == document.Mapping && len(entry.Value.Mapping) == 1 && entry.Value.Mapping[0].Key == "sequences" {
+					continue
+				}
 				location := publicLocation(entry.KeyLocation)
 				return []Diagnostic{{
 					Code: CodeOverlayDirective, Severity: SeverityError, Path: childPath,
-					Message: "overlays cannot contain configuration directives", Primary: &location,
+					Message: "overlays may contain only root collection sequence declarations", Primary: &location,
 				}}
 			}
-			if diagnostics := overlayDirectiveDiagnostics(entry.Value, childPath); len(diagnostics) != 0 {
+			if diagnostics := overlayDirectiveDiagnostics(entry.Value, childPath, sequencesAllowed); len(diagnostics) != 0 {
 				return diagnostics
 			}
 		}
 	} else if node.Kind == document.Sequence {
 		for index, child := range node.Sequence {
-			if diagnostics := overlayDirectiveDiagnostics(child, current.Index(index)); len(diagnostics) != 0 {
+			if diagnostics := overlayDirectiveDiagnostics(child, current.Index(index), sequencesAllowed); len(diagnostics) != 0 {
 				return diagnostics
 			}
 		}
@@ -189,3 +269,72 @@ func disabledDirectiveDiagnostics(node *document.Node, path Path) []Diagnostic {
 	}
 	return diagnostics
 }
+
+func disabledRootImportDiagnostics(node *document.Node) []Diagnostic {
+	if node == nil || node.Kind != document.Mapping {
+		return nil
+	}
+	var rootDirective *document.Node
+	var rootLocation document.Location
+	var foundRootDirective bool
+	for index := range node.Mapping {
+		if node.Mapping[index].Key == directiveKey {
+			rootDirective = node.Mapping[index].Value
+			rootLocation = node.Mapping[index].KeyLocation
+			foundRootDirective = true
+			break
+		}
+	}
+	if foundRootDirective {
+		if rootDirective == nil || rootDirective.Kind != document.Mapping {
+			return disabledDirectiveDiagnostic(rootLocation)
+		}
+		for _, directive := range rootDirective.Mapping {
+			if directive.Key == "imports" {
+				return disabledDirectiveDiagnostic(rootLocation)
+			}
+		}
+	}
+	for _, entry := range node.Mapping {
+		if entry.Key == directiveKey {
+			continue
+		}
+		if diagnostic := disabledNestedDirectiveDiagnostics(entry.Value, Path{}.Key(entry.Key)); len(diagnostic) != 0 {
+			return diagnostic
+		}
+	}
+	return nil
+}
+
+func disabledNestedDirectiveDiagnostics(node *document.Node, path Path) []Diagnostic {
+	if node.Kind == document.Mapping {
+		for _, entry := range node.Mapping {
+			childPath := path.Key(entry.Key)
+			if entry.Key == directiveKey {
+				location := Location{SourceID: entry.KeyLocation.SourceID, Line: entry.KeyLocation.Line, Column: entry.KeyLocation.Column}
+				return []Diagnostic{{
+					Code: CodeImportDisabled, Severity: SeverityError, Path: childPath,
+					Message: "configuration directives require imports to be enabled", Primary: &location,
+				}}
+			}
+			if diagnostics := disabledNestedDirectiveDiagnostics(entry.Value, childPath); len(diagnostics) != 0 {
+				return diagnostics
+			}
+		}
+	} else if node.Kind == document.Sequence {
+		for index, child := range node.Sequence {
+			if diagnostics := disabledNestedDirectiveDiagnostics(child, path.Index(index)); len(diagnostics) != 0 {
+				return diagnostics
+			}
+		}
+	}
+	return nil
+}
+
+func disabledDirectiveDiagnostic(location document.Location) []Diagnostic {
+	public := Location{SourceID: location.SourceID, Line: location.Line, Column: location.Column}
+	return []Diagnostic{{
+		Code: CodeImportDisabled, Severity: SeverityError, Path: Keys(directiveKey),
+		Message: "configuration directives require imports to be enabled", Primary: &public,
+	}}
+}

internal/engine/explain.go

@@ -76,20 +76,55 @@ func (r *Inspection) Explain(path Path) (Explanation, bool) {
 
 	// Intermediate input replacements do not permanently suppress defaults:
 	// decoding applies them to the final input tree. Show that reconciliation
-	// explicitly, including canonical scalar conversions, before domain changes.
+	// explicitly before typed CLI contributions and domain resolution.
+	decodedInput := r.preCLI
+	decodedOrigins := r.preCLIOrigins
 	replayed, _ := documentAtPath(working, path)
-	decoded, _ := documentAtPathWithLookup(r.input, path, r.immutableLookup)
+	decoded, _ := documentAtPathWithLookup(decodedInput, path, r.immutableLookup)
 	if !configurationValuesEqual(replayed, decoded) {
+		origin := cloneOrigin(decodedOrigins[path.String()])
+		origin.Operation = ChangeDecoding
+		explanation.History = append(explanation.History, ValueChange{
+			Before:  valueSnapshot(working, path, r.secrets),
+			After:   valueSnapshotWithLookup(decodedInput, path, r.secrets, r.immutableLookup),
+			Origin:  origin,
+			Reasons: []string{"applied retained defaults and canonical typed decoding"},
+		})
+	}
+
+	working = document.Clone(decodedInput)
+	for _, application := range r.postDecodeApps {
+		operationPath, relevant := applicationOperationPath(application, path, r.immutableLookup)
+		var before ValueSnapshot
+		if relevant {
+			before = valueSnapshot(working, path, r.secrets)
+		}
+		applyHistoryApplication(&working, application, replayIndex)
+		if !relevant {
+			continue
+		}
+		after := valueSnapshot(working, path, r.secrets)
+		if !before.Present && !after.Present {
+			continue
+		}
+		explanation.History = append(explanation.History, ValueChange{
+			Before: before,
+			After:  after,
+			Origin: historyApplicationOrigin(application, operationPath, r.immutableLookup),
+		})
+	}
+	replayed, _ = documentAtPath(working, path)
+	effectiveInput, _ := documentAtPathWithLookup(r.input, path, r.immutableLookup)
+	if !configurationValuesEqual(replayed, effectiveInput) {
 		origin := cloneOrigin(r.inputOrigins[path.String()])
 		origin.Operation = ChangeDecoding
 		explanation.History = append(explanation.History, ValueChange{
 			Before:  valueSnapshot(working, path, r.secrets),
 			After:   valueSnapshotWithLookup(r.input, path, r.secrets, r.immutableLookup),
 			Origin:  origin,
-			Reasons: []string{"applied retained defaults and canonical typed decoding"},
+			Reasons: []string{"applied the typed CLI assignment"},
 		})
 	}
-
 	working = document.Clone(r.input)
 	for _, snapshot := range r.resolutionReplay {
 		relevant := pathsIntersect(snapshot.change.Path, path)

internal/engine/explain_test.go

@@ -351,6 +351,10 @@ func TestExplanationReconcilesDefaultsAfterInputReplacement(t *testing.T) {
 					opts.Overlay = "p"
 				}
 				result, err := testLoad(File(main), opts)
+				if cli && base == "replaced-text" {
+					requireSecretLoadCode(t, err, CodeDecodeType)
+					return
+				}
 				if err != nil {
 					t.Fatal(err)
 				}
@@ -360,6 +364,18 @@ func TestExplanationReconcilesDefaultsAfterInputReplacement(t *testing.T) {
 						t.Fatal("missing explanation")
 					}
 					last := explanation.History[len(explanation.History)-1]
+					if cli && path.String() == "/m" {
+						if last.After != explanation.Effective || last.Origin.Layer != LayerCLI || last.Origin.Operation != "" {
+							t.Fatalf("CLI assignment was not the final history event: %#v", explanation)
+						}
+						continue
+					}
+					if cli && path.String() == "/m/a" {
+						if explanation.Effective.Present {
+							t.Fatalf("CLI map override retained descendants discarded by source null: %#v", explanation)
+						}
+						continue
+					}
 					if last.After != explanation.Effective || last.Origin.Operation != ChangeDecoding {
 						t.Fatalf("history does not reconcile final value: %#v", explanation)
 					}

internal/engine/final_identity.go

@@ -0,0 +1,51 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+func validateFinalSequenceIdentities(root *document.Node, policy sequencePolicy) []Diagnostic {
+	if root == nil || policy.selectorTrie == nil {
+		return nil
+	}
+	var diagnostics []Diagnostic
+	var visit func(*document.Node, Path, []*sequenceSelectorTrie)
+	visit = func(node *document.Node, path Path, states []*sequenceSelectorTrie) {
+		if node == nil {
+			return
+		}
+		if node.Kind == document.Sequence {
+			var fields []string
+			for _, state := range states {
+				if state.terminal {
+					fields = state.fields
+					break
+				}
+			}
+			if fields != nil {
+				_, issues := validateNormalizedSequenceIdentityFrom(node, path, fields, sequenceIdentityDiagnosticCodes{
+					invalid: CodeSequenceInvalid, conflict: CodeSequenceConflict,
+				}, 0, sequencePayloadRecord)
+				diagnostics = append(diagnostics, issues...)
+			}
+			return
+		}
+		if node.Kind != document.Mapping {
+			return
+		}
+		for _, entry := range node.Mapping {
+			next := make([]*sequenceSelectorTrie, 0, len(states)*2)
+			for _, state := range states {
+				if literal := state.literals[entry.Key]; literal != nil {
+					next = append(next, literal)
+				}
+				if state.wildcard != nil {
+					next = append(next, state.wildcard)
+				}
+			}
+			if len(next) != 0 {
+				visit(entry.Value, path.Key(entry.Key), next)
+			}
+		}
+	}
+	visit(root, Path{}, []*sequenceSelectorTrie{policy.selectorTrie})
+	return canonicalDiagnostics(diagnostics)
+}

internal/engine/history.go

@@ -42,15 +42,15 @@ func (c *retainedNodeCounter) add(root *document.Node) bool {
 
 // History applications borrow immutable policy-validated input trees. Metadata
 // records answer presence and origin queries; replay reconstructs value changes.
-// Replay reconciles with canonical decoding before domain resolution so both
-// contracts agree on the effective result without retaining a tree per field.
+// Replay reconciles with canonical decoding, applies supplied CLI contributions,
+// then records domain resolution without retaining a tree per field.
 type historyApplication struct {
 	layer           Layer
 	path            Path
 	value           *document.Node
 	origin          Origin
 	sequences       []sequenceHistoryContribution
-	sequencesByPath map[string]sequenceHistoryContribution
+	sequencesByPath map[string]sequenceHistoryContributionIndex
 }
 
 func cloneHistoryApplications(values []historyApplication) []historyApplication {
@@ -92,7 +92,14 @@ func walkHistoryApplication(application historyApplication, visit func(local, ef
 			for index, child := range node.Sequence {
 				effectiveIndex := index
 				if projected {
-					effectiveIndex += contribution.offset
+					if contribution.mode == sequenceHistoryMapped {
+						effectiveIndex = contribution.outputForInput(index)
+						if effectiveIndex < 0 {
+							continue
+						}
+					} else {
+						effectiveIndex += contribution.offset
+					}
 				}
 				walk(child, local.Index(index), effective.Index(effectiveIndex))
 			}
@@ -112,7 +119,18 @@ func historyApplicationLocalPath(application historyApplication, effective Path)
 	for _, segment := range relative.segments {
 		if contribution, found := sequenceHistoryAt(application.sequencesByPath, local); found {
 			index, valid := schemaIndex(segment)
-			if !valid || segment.kind == pathSegmentKey || index < contribution.offset || index >= contribution.offset+contribution.length {
+			if !valid || segment.kind == pathSegmentKey {
+				return Path{}, false
+			}
+			if contribution.mode == sequenceHistoryMapped {
+				inputIndex := contribution.inputForOutput(index)
+				if inputIndex < 0 {
+					return Path{}, false
+				}
+				local = local.Index(inputIndex)
+				continue
+			}
+			if index < contribution.offset || index >= contribution.offset+contribution.length {
 				return Path{}, false
 			}
 			local = local.Index(index - contribution.offset)
@@ -149,6 +167,16 @@ func applicationOperationPath(application historyApplication, query Path, lookup
 					return container, true
 				}
 			}
+			indexed, _ := sequenceHistoryAt(application.sequencesByPath, contribution.path)
+			if indexed.mode == sequenceHistoryMapped && len(query.segments) > len(container.segments) {
+				outputIndex, valid := schemaIndex(query.segments[len(container.segments)])
+				_, replaced := indexed.replacedOutputs[outputIndex]
+				if valid && replaced {
+					if _, present := documentAtPathWithLookup(application.value, local, lookup); !ok || !present {
+						return container.Index(outputIndex), true
+					}
+				}
+			}
 			if !ok {
 				continue
 			}
@@ -206,14 +234,14 @@ func applyHistoryApplication(root **document.Node, application historyApplicatio
 		mergeHistoryDocumentAtPath(root, application.value, Path{}, index, application.sequencesByPath)
 		return
 	}
-	mergeHistoryAtPath(root, application.path, application.value, index)
+	mergeHistoryAtPath(root, application.path, application.value, index, application.sequencesByPath)
 }
 
 func mergeHistoryDocument(target **document.Node, incoming *document.Node, index *mappingIndex) {
 	mergeHistoryDocumentAtPath(target, incoming, Path{}, index, nil)
 }
 
-func mergeHistoryDocumentAtPath(target **document.Node, incoming *document.Node, path Path, index *mappingIndex, sequences map[string]sequenceHistoryContribution) {
+func mergeHistoryDocumentAtPath(target **document.Node, incoming *document.Node, path Path, index *mappingIndex, sequences map[string]sequenceHistoryContributionIndex) {
 	if incoming == nil {
 		return
 	}
@@ -238,23 +266,53 @@ func mergeHistoryDocumentAtPath(target **document.Node, incoming *document.Node,
 	}
 }
 
-func sequenceHistoryAt(contributions map[string]sequenceHistoryContribution, path Path) (sequenceHistoryContribution, bool) {
+func sequenceHistoryAt(contributions map[string]sequenceHistoryContributionIndex, path Path) (sequenceHistoryContributionIndex, bool) {
 	contribution, found := contributions[path.String()]
 	return contribution, found
 }
 
-func indexSequenceHistoryContributions(values []sequenceHistoryContribution) map[string]sequenceHistoryContribution {
+type sequenceHistoryContributionIndex struct {
+	sequenceHistoryContribution
+	projection      sequenceItemProjectionIndex
+	replacedOutputs map[int]struct{}
+}
+
+func indexSequenceHistoryContributions(values []sequenceHistoryContribution) map[string]sequenceHistoryContributionIndex {
 	if len(values) == 0 {
 		return nil
 	}
-	index := make(map[string]sequenceHistoryContribution, len(values))
+	index := make(map[string]sequenceHistoryContributionIndex, len(values))
 	for _, value := range values {
-		index[value.path.String()] = value
+		contribution := sequenceHistoryContributionIndex{sequenceHistoryContribution: value}
+		if value.mode == sequenceHistoryMapped {
+			contribution.projection = newSequenceItemProjectionIndex(value.items)
+			contribution.replacedOutputs = make(map[int]struct{}, len(value.replacements))
+			for _, replacement := range value.replacements {
+				contribution.replacedOutputs[replacement.outputIndex] = struct{}{}
+			}
+		}
+		index[value.path.String()] = contribution
 	}
 	return index
 }
 
-func applySequenceHistory(target **document.Node, incoming *document.Node, contribution sequenceHistoryContribution) {
+func (contribution sequenceHistoryContributionIndex) outputForInput(inputIndex int) int {
+	outputIndex, ok := contribution.projection.outputForInput(inputIndex)
+	if !ok {
+		return -1
+	}
+	return outputIndex
+}
+
+func (contribution sequenceHistoryContributionIndex) inputForOutput(outputIndex int) int {
+	inputIndex, ok := contribution.projection.inputForOutput(outputIndex)
+	if !ok {
+		return -1
+	}
+	return inputIndex
+}
+
+func applySequenceHistory(target **document.Node, incoming *document.Node, contribution sequenceHistoryContributionIndex) {
 	if incoming == nil || incoming.Kind != document.Sequence {
 		return
 	}
@@ -262,6 +320,18 @@ func applySequenceHistory(target **document.Node, incoming *document.Node, contr
 		*target = document.Clone(incoming)
 		return
 	}
+	if contribution.mode == sequenceHistoryMapped {
+		for _, projection := range contribution.items {
+			if projection.inputIndex < 0 || projection.inputIndex >= len(incoming.Sequence) || projection.outputIndex < 0 {
+				continue
+			}
+			for len((*target).Sequence) <= projection.outputIndex {
+				(*target).Sequence = append((*target).Sequence, nil)
+			}
+			(*target).Sequence[projection.outputIndex] = document.Clone(incoming.Sequence[projection.inputIndex])
+		}
+		return
+	}
 	for _, item := range incoming.Sequence {
 		(*target).Sequence = append((*target).Sequence, document.Clone(item))
 	}
@@ -275,13 +345,15 @@ func cloneSequenceHistoryContributions(values []sequenceHistoryContribution) []s
 	for index, value := range values {
 		cloned[index] = value
 		cloned[index].path = value.path.clone()
+		cloned[index].items = append([]sequenceItemProjection(nil), value.items...)
+		cloned[index].replacements = append([]sequenceItemReplacement(nil), value.replacements...)
 	}
 	return cloned
 }
 
-func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node, index *mappingIndex) {
+func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node, index *mappingIndex, sequences map[string]sequenceHistoryContributionIndex) {
 	if root == nil || *root == nil || len(path.segments) == 0 {
-		mergeHistoryDocument(root, incoming, index)
+		mergeHistoryDocumentAtPath(root, incoming, Path{}, index, sequences)
 		return
 	}
 	current := *root
@@ -292,7 +364,7 @@ func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node
 				return
 			}
 			if segmentIndex == len(path.segments)-1 {
-				mergeHistoryDocument(&current.Sequence[position], incoming, index)
+				mergeHistoryDocumentAtPath(&current.Sequence[position], incoming, Path{}, index, sequences)
 				return
 			}
 			current = current.Sequence[position]
@@ -306,7 +378,7 @@ func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node
 			if position < 0 {
 				index.append(current, document.Entry{Key: segment.key, Value: document.Clone(incoming)})
 			} else {
-				mergeHistoryDocument(&current.Mapping[position].Value, incoming, index)
+				mergeHistoryDocumentAtPath(&current.Mapping[position].Value, incoming, Path{}, index, sequences)
 			}
 			return
 		}

internal/engine/history_sequence_test.go

@@ -10,7 +10,10 @@ import (
 )
 
 func TestSequenceHistoryReplaysAppendAtAssignedIndexes(t *testing.T) {
-	policy := sequencePolicy{identityFields: map[string]string{"/records": "id", "/other": "id"}}
+	policy := mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{
+		{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("other")}, identityFields: []string{"id"}},
+	})
 	first := parseCompositionDocument(t, "source-1", "records: [{id: a, value: first}]\nother: [{id: x}]\n")
 	second := parseCompositionDocument(t, "source-2", "records: [{id: b, value: second}]\nother: [{id: y}]\n")
 	composed, diagnostics := composeDocumentsWithPolicy(policy, first, second)
@@ -75,8 +78,76 @@ func TestSequenceHistoryReplaysAppendAtAssignedIndexes(t *testing.T) {
 	}
 }
 
+func TestTierSequenceHistoryReplaysSelectedOperation(t *testing.T) {
+	t.Parallel()
+	policy := mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{
+		selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"},
+	}})
+	for _, test := range []struct {
+		name string
+		mode sequenceMode
+	}{{"append", sequenceModeAppend}, {"upsert", sequenceModeUpsert}, {"replace", sequenceModeReplace}} {
+		t.Run(test.name, func(t *testing.T) {
+			mode := test.mode
+			lower, diagnostics := composeDocumentsWithPolicy(policy, parseCompositionDocument(t, "lower", "records: [{id: a, removed: old}, {id: b, value: old, removed: old}]\n"))
+			if len(diagnostics) != 0 {
+				t.Fatal(diagnostics)
+			}
+			secondID := "d"
+			if mode == sequenceModeUpsert {
+				secondID = "b"
+			}
+			declarations := []sequenceDeclarationSpec{{destination: Keys("records"), modePresent: true, mode: mode}}
+			higher, diagnostics := composeDocumentsWithCollectionPolicy(policy, lower.collectionPolicies, declarations,
+				parseCompositionDocument(t, "first-peer", "records: [{id: c, value: first}]\n"),
+				parseCompositionDocument(t, "second-peer", "records: [{id: "+secondID+", value: second}]\n"),
+			)
+			if len(diagnostics) != 0 {
+				t.Fatal(diagnostics)
+			}
+			assembly := newAssemblyAccumulator(lower)
+			if diagnostics := assembly.applyOverlayWithCollectionPolicy(higher.root, higher.origins, higher.collectionPolicies, higher.applications, LayerBase, policy, declarations); len(diagnostics) != 0 {
+				t.Fatal(diagnostics)
+			}
+			if len(assembly.current.applications) != 3 {
+				t.Fatalf("source applications = %d, want all three sources", len(assembly.current.applications))
+			}
+			if mode == sequenceModeUpsert {
+				query := Keys("records").Index(1).Key("value")
+				if _, relevant := applicationOperationPath(assembly.current.applications[1], query, nil); relevant {
+					t.Fatal("first peer was attributed to another peer's upserted record")
+				}
+				origin := historyApplicationOrigin(assembly.current.applications[2], query, nil)
+				if len(origin.Locations) != 1 || origin.Locations[0].SourceID != "second-peer" {
+					t.Fatalf("upsert history source = %#v", origin)
+				}
+			}
+			working := parseCompositionDocument(t, "initial", "{}\n")
+			index := new(mappingIndex)
+			for _, application := range assembly.current.applications {
+				applyHistoryApplication(&working, application, index)
+			}
+			if !configurationValuesEqual(working, assembly.current.root) {
+				t.Fatal("tier history differs from structural assembly before decoding")
+			}
+			overlay := parseCompositionDocument(t, "overlay", "records: [{id: c, value: overlay}]\n")
+			declarations[0].mode = sequenceModeUpsert
+			if diagnostics := assembly.applyOverlayWithCollectionPolicy(overlay, nil, collectionPolicyScope{}, nil, LayerOverlay, policy, declarations); len(diagnostics) != 0 {
+				t.Fatal(diagnostics)
+			}
+			applyHistoryApplication(&working, assembly.current.applications[3], index)
+			if !configurationValuesEqual(working, assembly.current.root) {
+				t.Fatal("subsequent overlay history differs from structural assembly")
+			}
+		})
+	}
+}
+
 func TestSequenceHistoryPreservesTierReplacementAndEmptyLists(t *testing.T) {
-	policy := sequencePolicy{identityFields: map[string]string{"/records": "id", "/independent": "id"}}
+	policy := mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{
+		{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("independent")}, identityFields: []string{"id"}},
+	})
 	first, diagnostics := composeDocumentsWithPolicy(policy,
 		parseCompositionDocument(t, "underlay-1a", "records: [{id: shared}]\nindependent: [{id: x}]\n"),
 		parseCompositionDocument(t, "underlay-1b", "records: [{id: lower}]\n"),
@@ -145,10 +216,53 @@ func TestHistoryMergeSupportsSemanticIndexedPrefixes(t *testing.T) {
 	if !present || value.Text != "new" {
 		t.Fatalf("indexed helper replay = %#v, %v", value, present)
 	}
+
+	root = parseCompositionDocument(t, "base", "records: [{id: one, helper: {items: [{id: a}]}}]\n")
+	application.value = parseCompositionDocument(t, "helper", "items: [{id: b}]\n")
+	application.sequences = []sequenceHistoryContribution{{path: Keys("items"), mode: sequenceHistoryAppend, offset: 1, length: 1}}
+	application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
+	applyHistoryApplication(&root, application, new(mappingIndex))
+	items, present := documentAtPath(root, Keys("records").Index(0).Key("helper").Key("items"))
+	if !present || len(items.Sequence) != 2 {
+		t.Fatalf("indexed helper lost its local sequence operation: %#v", items)
+	}
+}
+
+func TestLayeredUpsertExplanationFollowsStructuralOrder(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		Records []map[string]any `yaml:"records"`
+	}
+	loaded, err := testGroupedLoad(loadRequest[config]{
+		main:      loadInput{source: Bytes("main", []byte("configkit:\n  sequences: [{destination: [records], key: id}]\nrecords: [{id: b, value: new-b}, {id: c, value: new-c}, {id: a, value: new-a}]\n"))},
+		underlays: []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte("records: [{id: a, value: old-a}, {id: b, value: old-b, removed: old}]\n"))}}}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	explanation, ok := loaded.Explain(Keys("records"))
+	if !ok || len(explanation.History) == 0 {
+		t.Fatal("missing collection history")
+	}
+	for _, change := range explanation.History {
+		if change.Origin.Operation == ChangeDecoding {
+			t.Fatal("structural upsert was misreported as a decoding reconciliation")
+		}
+	}
+	if last := explanation.History[len(explanation.History)-1]; last.After.YAML != explanation.Effective.YAML || last.Origin.Layer != LayerBase {
+		t.Fatalf("main contribution did not produce the effective collection: %#v", last)
+	}
+	removed, ok := loaded.Explain(Keys("records").Index(1).Key("removed"))
+	if !ok || len(removed.History) == 0 || removed.Effective.Present {
+		t.Fatalf("missing displaced field history: %#v, %v", removed, ok)
+	}
+	if last := removed.History[len(removed.History)-1]; last.After.Present || last.Origin.Layer != LayerBase || last.Origin.Operation == ChangeDecoding {
+		t.Fatalf("upsert removal attribution = %#v", last)
+	}
 }
 
 func TestProjectedSequenceHistoryAgreesAcrossMetadataAndExplanation(t *testing.T) {
-	policy := sequencePolicy{identityFields: map[string]string{"/records": "id"}}
+	policy := mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}})
 	composed, diagnostics := composeDocumentsWithPolicy(policy,
 		parseCompositionDocument(t, "first", "records: [{id: first, token: public}]"),
 		parseCompositionDocument(t, "second", "records: [{id: second, token: private}]"),
@@ -165,7 +279,7 @@ func TestProjectedSequenceHistoryAgreesAcrossMetadataAndExplanation(t *testing.T
 
 	inspection := &Inspection{
 		metadata: metadata, baseline: &document.Node{Kind: document.Mapping},
-		input: composed.root, final: composed.root, applications: composed.applications,
+		input: composed.root, preCLI: composed.root, final: composed.root, applications: composed.applications,
 	}
 	inspection.secrets = classifySecretPaths(nil, func(candidate Path) bool {
 		return reflect.DeepEqual(candidate, path)
@@ -193,7 +307,7 @@ func TestProjectedSequenceHistoryAgreesAcrossMetadataAndExplanation(t *testing.T
 }
 
 func TestReplacedProjectedSensitiveRecordRemainsRedacted(t *testing.T) {
-	policy := sequencePolicy{identityFields: map[string]string{"/records": "id"}}
+	policy := mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}})
 	composed, diagnostics := composeDocumentsWithPolicy(policy,
 		parseCompositionDocument(t, "first", "records: [{id: first, token: public}]"),
 		parseCompositionDocument(t, "second", "records: [{id: second, token: PRIVATE-RECORD-CANARY}]"),
@@ -212,7 +326,7 @@ func TestReplacedProjectedSensitiveRecordRemainsRedacted(t *testing.T) {
 	finalizeMetadataPaths(&metadata, final, applications)
 	inspection := &Inspection{
 		metadata: metadata, baseline: &document.Node{Kind: document.Mapping},
-		input: final, final: final, applications: applications,
+		input: final, preCLI: final, final: final, applications: applications,
 	}
 	inspection.secrets = classifySecretPaths(nil, func(candidate Path) bool {
 		return reflect.DeepEqual(candidate, path)
@@ -230,6 +344,241 @@ func TestReplacedProjectedSensitiveRecordRemainsRedacted(t *testing.T) {
 	}
 }
 
+func TestMappedUpsertProjectsSensitiveReplacementEvidence(t *testing.T) {
+	lowerRoot := parseCompositionDocument(t, "lower", `records:
+  - id: first
+  - id: middle
+  - id: last
+    token: MAPPED-SECRET-CANARY
+`)
+	higherRoot := parseCompositionDocument(t, "higher", `records:
+  - id: last
+    token: visible
+  - id: first
+  - id: added
+`)
+	lower, _ := documentAtPath(lowerRoot, Keys("records"))
+	higher, _ := documentAtPath(higherRoot, Keys("records"))
+	combined, diagnostics := combineSequence(
+		lower, higher,
+		sequenceCombinationPolicy(sequenceModeUpsert, "id"),
+		sequenceCombinationContext{destination: Keys("records")},
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("combine diagnostics = %#v", diagnostics)
+	}
+	secretPath := Keys("records").Index(2).Key("token")
+	secretNode, present := documentAtPath(lowerRoot, Keys("records").Index(2).Key("token"))
+	if !present {
+		t.Fatal("sensitive source token is missing")
+	}
+	secretNode.Sensitive = true
+	application := historyApplication{
+		layer: LayerOverlay, value: higherRoot,
+		sequences: []sequenceHistoryContribution{{
+			path: Keys("records"), mode: sequenceHistoryMapped,
+			items: combined.higherItems, replacements: combined.replacements,
+		}},
+	}
+	application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
+	applications := []historyApplication{{value: lowerRoot}, application}
+	replayed := &document.Node{Kind: document.Mapping}
+	for _, current := range applications {
+		applyHistoryApplication(&replayed, current, new(mappingIndex))
+	}
+	replayedRecords, _ := documentAtPath(replayed, Keys("records"))
+	if !reflect.DeepEqual(replayedRecords.Sequence, combined.value.Sequence) {
+		t.Fatalf("replayed value = %#v, want combination = %#v", replayedRecords.Sequence, combined.value.Sequence)
+	}
+	finalRoot := document.Clone(lowerRoot)
+	finalRecords, _ := documentAtPath(finalRoot, Keys("records"))
+	finalRecords.Sequence = document.Clone(combined.value).Sequence
+
+	var metadata Metadata
+	finalizeMetadataPaths(&metadata, finalRoot, applications)
+	inspection := &Inspection{
+		metadata: metadata, baseline: &document.Node{Kind: document.Mapping},
+		input: finalRoot, preCLI: finalRoot, final: finalRoot, applications: applications,
+	}
+	secretRule := func(candidate Path) bool { return reflect.DeepEqual(candidate, secretPath) }
+	inspection.secrets = classifySecretPaths(nil, secretRule, metadata, finalRoot, applications)
+	explanation, ok := inspection.Explain(secretPath)
+	if !ok || !explanation.Effective.Redacted {
+		t.Fatalf("mapped replacement explanation = %#v, %v", explanation, ok)
+	}
+	var rendered bytes.Buffer
+	if err := inspection.WriteExplanation(&rendered, secretPath); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(rendered.String(), "MAPPED-SECRET-CANARY") || !strings.Contains(rendered.String(), "[REDACTED]") {
+		t.Fatalf("mapped replacement history leaked or was not redacted: %s", rendered.String())
+	}
+}
+
+func TestSequenceHistoryReplaysCombinationEvidence(t *testing.T) {
+	tests := []struct {
+		name      string
+		operation sequenceMode
+		lower     string
+		higher    string
+	}{
+		{
+			name:      "append",
+			operation: sequenceModeAppend,
+			lower:     "records: [{id: first}, {id: last}]\n",
+			higher:    "records: [{id: added}]\n",
+		},
+		{
+			name:      "upsert",
+			operation: sequenceModeUpsert,
+			lower: `records:
+  - id: first
+    label: same
+    nested: {removed: lower}
+  - id: middle
+  - id: last
+    nested: {removed: lower}
+`,
+			higher: `records:
+  - id: last
+    nested: {kept: higher}
+  - id: first
+    label: same
+  - id: added
+`,
+		},
+		{
+			name:      "replace",
+			operation: sequenceModeReplace,
+			lower:     "records: [{id: discarded}]\n",
+			higher:    "records: [{id: replacement}]\n",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			lowerRoot := parseCompositionDocument(t, "lower", test.lower)
+			higherRoot := parseCompositionDocument(t, "higher", test.higher)
+			lower, _ := documentAtPath(lowerRoot, Keys("records"))
+			higher, _ := documentAtPath(higherRoot, Keys("records"))
+			fields := []string(nil)
+			if test.operation != sequenceModeReplace {
+				fields = []string{"id"}
+			}
+			combined, diagnostics := combineSequence(lower, higher, sequenceCombinationPolicy(test.operation, fields...), sequenceCombinationContext{destination: Keys("records")})
+			if len(diagnostics) != 0 {
+				t.Fatalf("combine diagnostics = %#v", diagnostics)
+			}
+
+			mode := sequenceHistoryMapped
+			if test.operation == sequenceModeReplace {
+				mode = sequenceHistoryReplace
+			}
+			length := 0
+			if mode == sequenceHistoryReplace {
+				length = len(combined.higherItems)
+			}
+			application := historyApplication{
+				layer: LayerOverlay,
+				value: higherRoot,
+				sequences: []sequenceHistoryContribution{{
+					path: Keys("records"), mode: mode, length: length,
+					items: combined.higherItems, replacements: combined.replacements,
+				}},
+			}
+			application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
+			replayed := document.Clone(lowerRoot)
+			applyHistoryApplication(&replayed, application, new(mappingIndex))
+			got, present := documentAtPath(replayed, Keys("records"))
+			if !present || !reflect.DeepEqual(got.Sequence, combined.value.Sequence) {
+				t.Fatalf("replayed sequence = %#v, want combination = %#v", got, combined.value.Sequence)
+			}
+		})
+	}
+}
+
+func TestMappedSequenceHistoryReplacesWholeRecordsAndKeepsDisplacedEvidence(t *testing.T) {
+	lowerRoot := parseCompositionDocument(t, "lower", `records:
+  - id: first
+    label: same
+  - id: middle
+  - id: last
+    nested: {removed: LOWER-CANARY}
+`)
+	higherRoot := parseCompositionDocument(t, "higher", `records:
+  - id: last
+    nested: {kept: higher}
+  - id: first
+    label: same
+  - id: added
+`)
+	lower, _ := documentAtPath(lowerRoot, Keys("records"))
+	higher, _ := documentAtPath(higherRoot, Keys("records"))
+	combined, diagnostics := combineSequence(
+		lower, higher,
+		sequenceCombinationPolicy(sequenceModeUpsert, "id"),
+		sequenceCombinationContext{destination: Keys("records")},
+	)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	application := historyApplication{
+		layer: LayerOverlay, value: higherRoot,
+		sequences: []sequenceHistoryContribution{{
+			path: Keys("records"), mode: sequenceHistoryMapped,
+			items: combined.higherItems, replacements: combined.replacements,
+		}},
+	}
+	application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
+	clonedApplications := cloneHistoryApplications([]historyApplication{application})
+	clonedApplications[0].sequences[0].items[0].outputIndex = 99
+	clonedApplications[0].sequences[0].replacements[0].outputIndex = 99
+	if application.sequences[0].items[0].outputIndex == 99 || application.sequences[0].replacements[0].outputIndex == 99 {
+		t.Fatal("cloned mapped history aliases its source projections")
+	}
+
+	query := Keys("records").Index(2).Key("nested").Key("removed")
+	operationPath, relevant := applicationOperationPath(application, query, nil)
+	if !relevant || operationPath.String() != "/records/2" {
+		t.Fatalf("removed descendant operation = %s, %v", operationPath.String(), relevant)
+	}
+	localPath, ok := historyApplicationLocalPath(application, Keys("records").Index(2).Key("nested").Key("kept"))
+	if !ok || localPath.String() != "/records/0/nested/kept" {
+		t.Fatalf("mapped effective path localizes to %s, %v", localPath.String(), ok)
+	}
+	origin := historyApplicationOrigin(application, operationPath, nil)
+	if len(origin.Locations) != 1 || origin.Locations[0].SourceID != "higher" {
+		t.Fatalf("replacement operation origin = %#v", origin)
+	}
+	var walkedMappedPath bool
+	walkHistoryApplication(application, func(local, effective Path, _ *document.Node) {
+		if local.String() == "/records/0/nested/kept" && effective.String() == "/records/2/nested/kept" {
+			walkedMappedPath = true
+		}
+	})
+	if !walkedMappedPath {
+		t.Fatal("history walk did not project a noncontiguous incoming item")
+	}
+
+	replayed := document.Clone(lowerRoot)
+	applyHistoryApplication(&replayed, application, new(mappingIndex))
+	sequence, _ := documentAtPath(replayed, Keys("records"))
+	if _, present := documentAtPath(sequence.Sequence[2], Keys("nested", "removed")); present {
+		t.Fatal("whole-record replacement retained an omitted descendant")
+	}
+	if value, present := documentAtPath(sequence.Sequence[2], Keys("nested", "kept")); !present || value.Text != "higher" {
+		t.Fatalf("replacement descendant = %#v, %v", value, present)
+	}
+	if sequence.Sequence[1].Mapping[0].Value.Text != "middle" {
+		t.Fatal("unchanged neighboring record moved or changed")
+	}
+	if value, present := documentAtPath(lowerRoot, Keys("records").Index(2).Key("nested").Key("removed")); !present || value.Text != "LOWER-CANARY" {
+		t.Fatalf("displaced lower contribution was not retained as raw history: %#v, %v", value, present)
+	}
+	if got := application.sequences[0].replacements; !reflect.DeepEqual(got, combined.replacements) || len(got) != 2 {
+		t.Fatalf("replacement evidence lost equal-valued or noncontiguous events: %#v", got)
+	}
+}
+
 func mustParsePath(t *testing.T, value string) Path {
 	t.Helper()
 	path, err := ParsePath(value)

internal/engine/import_spec.go

@@ -193,18 +193,26 @@ func parseImportSpecificationDirective(node *document.Node, directivePath Path)
 			diagnostics = append(diagnostics, nodeImportDiagnostic(CodeImportInvalid, fieldPath, "configuration directive field is not recognized", entry.Value))
 			continue
 		}
-		if entry.Value.Kind != document.Sequence {
-			diagnostics = append(diagnostics, nodeImportDiagnostic(CodeImportInvalid, fieldPath, "imports must be a sequence", entry.Value))
+		parsed, issues := parseImportDeclarations(entry.Value, fieldPath)
+		specs = append(specs, parsed...)
+		diagnostics = append(diagnostics, issues...)
+	}
+	return specs, canonicalDiagnostics(diagnostics)
+}
+
+func parseImportDeclarations(node *document.Node, declarationPath Path) ([]importDeclarationSpec, []Diagnostic) {
+	if node == nil || node.Kind != document.Sequence {
+		return nil, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, declarationPath, "imports must be a sequence", node)}
+	}
+	var specs []importDeclarationSpec
+	var diagnostics []Diagnostic
+	for index, declarationNode := range node.Sequence {
+		spec, issues := parseImportDeclarationSpec(declarationNode, declarationPath.Index(index))
+		if len(issues) != 0 {
+			diagnostics = append(diagnostics, issues...)
 			continue
 		}
-		for index, declarationNode := range entry.Value.Sequence {
-			spec, issues := parseImportDeclarationSpec(declarationNode, fieldPath.Index(index))
-			if len(issues) != 0 {
-				diagnostics = append(diagnostics, issues...)
-				continue
-			}
-			specs = append(specs, spec)
-		}
+		specs = append(specs, spec)
 	}
 	return specs, canonicalDiagnostics(diagnostics)
 }

internal/engine/imports.go

@@ -46,12 +46,13 @@ type importAccounting struct {
 }
 
 type importTraversal struct {
-	access       *filesystemAccess
-	accounting   *importAccounting
-	limits       importLimits
-	rule         *VersionRule
-	versionState *sourceVersionState
-	defaults     importDefaults
+	access           *filesystemAccess
+	accounting       *importAccounting
+	limits           importLimits
+	rule             *VersionRule
+	versionState     *sourceVersionState
+	directiveContext rootDirectiveContext
+	defaults         importDefaults
 
 	scope        acquisitionScope
 	itemScope    bool
@@ -166,12 +167,17 @@ func loadImportedWithAccessTrackedAndProcessingPolicyDeferred(access *filesystem
 	if versionState != nil {
 		defaults = snapshotImportDefaults(versionState.importDefaults)
 	}
+	directiveContext := rootDirectiveContextImportsOnly
+	if processing != nil && processing.directiveContext != 0 {
+		directiveContext = processing.directiveContext
+	}
 	state := importTraversal{
 		access: access, accounting: accounting, limits: limits, rule: rule, versionState: versionState,
-		defaults: defaults,
-		scope:    acquisitionScope{owner: mainNodeKey, destination: destination.clone()},
-		sources:  []SourceInfo{mainInfo},
-		active:   make(map[resourceIdentity]int), visited: visited,
+		directiveContext: directiveContext,
+		defaults:         defaults,
+		scope:            acquisitionScope{owner: mainNodeKey, destination: destination.clone()},
+		sources:          []SourceInfo{mainInfo},
+		active:           make(map[resourceIdentity]int), visited: visited,
 		importGraph: newImportInclusionGraph(),
 	}
 	if mainIsItem {
@@ -201,10 +207,12 @@ func loadImportedWithAccessTrackedAndProcessingPolicyDeferred(access *filesystem
 	state.stack = append(state.stack, mainRoute)
 	var body *document.Node
 	var declarations []importDeclarationSpec
+	var sequenceDeclarations []sequenceDeclarationSpec
 	var diagnostics []Diagnostic
 	if processing != nil {
 		body = processing.payload
 		declarations = processing.imports
+		sequenceDeclarations = cloneSequenceDeclarations(processing.sequences)
 	} else {
 		body, declarations, diagnostics = splitImportDirectives(main.document)
 	}
@@ -213,6 +221,7 @@ func loadImportedWithAccessTrackedAndProcessingPolicyDeferred(access *filesystem
 			state.scope.contributions = append(state.scope.contributions, acquiredContribution{
 				source: cloneSourceInfo(mainInfo), identity: mainNodeKey, role: importRoleMain,
 				scopeOwner: mainNodeKey, payload: body, document: body, processing: processing,
+				sequenceDeclarations: cloneSequenceDeclarations(sequenceDeclarations),
 			})
 			diagnostics = state.walkDeclarations(basePath, declarations, 0, Path{})
 		} else {
@@ -228,6 +237,7 @@ func loadImportedWithAccessTrackedAndProcessingPolicyDeferred(access *filesystem
 					source: cloneSourceInfo(mainInfo), payload: body, document: placed,
 					identity: mainNodeKey, role: importRoleMain, scopeOwner: mainNodeKey,
 					destination: destination.clone(), processing: processing,
+					sequenceDeclarations: cloneSequenceDeclarations(sequenceDeclarations),
 				})
 				diagnostics = state.walkDeclarations(basePath, declarations, 0, destination)
 			}
@@ -235,9 +245,6 @@ func loadImportedWithAccessTrackedAndProcessingPolicyDeferred(access *filesystem
 	}
 	if len(diagnostics) == 0 {
 		state.scheduleAcquisitionScopes(&state.scope)
-		if !deferAssembly {
-			diagnostics = state.composePendingItems(&state.scope)
-		}
 	}
 
 	closeErr := access.close()
@@ -250,19 +257,19 @@ func loadImportedWithAccessTrackedAndProcessingPolicyDeferred(access *filesystem
 	if closeErr != nil {
 		return nil, cloneSourceInfos(state.sources), []Diagnostic{importDiagnostic(CodeImportRead, "configuration root could not be closed", nil)}
 	}
-	if deferAssembly {
-		acquisition := state.scope
-		if mainIsItem {
-			localScope := state.scope
-			acquisition = acquisitionScope{
-				owner: mainNodeKey, destination: destination.clone(),
-				contributions: []acquiredContribution{{
-					source: cloneSourceInfo(mainInfo), payload: body, identity: mainNodeKey,
-					role: importRoleMain, scopeOwner: mainNodeKey, destination: destination.clone(),
-					processing: processing, pendingItem: &localScope,
-				}},
-			}
+	acquisition := state.scope
+	if mainIsItem {
+		localScope := state.scope
+		acquisition = acquisitionScope{
+			owner: mainNodeKey, destination: destination.clone(),
+			contributions: []acquiredContribution{{
+				source: cloneSourceInfo(mainInfo), payload: body, identity: mainNodeKey,
+				role: importRoleMain, scopeOwner: mainNodeKey, destination: destination.clone(),
+				processing: processing, sequenceDeclarations: cloneSequenceDeclarations(sequenceDeclarations), pendingItem: &localScope,
+			}},
 		}
+	}
+	if deferAssembly {
 		if diagnostic := reservePendingItemPlacements(&acquisition, limits, accounting); diagnostic != nil {
 			return nil, cloneSourceInfos(state.sources), []Diagnostic{*diagnostic}
 		}
@@ -271,20 +278,65 @@ func loadImportedWithAccessTrackedAndProcessingPolicyDeferred(access *filesystem
 			importGraph: state.importGraph.clone(), inputBytes: accounting.bytes, inputNodes: accounting.nodes,
 		}, cloneSourceInfos(state.sources), nil
 	}
-	if mainIsItem {
-		localScope := state.scope
+	acquired := &composedDocument{
+		acquisition: []acquisitionScope{cloneAcquisitionScope(acquisition)},
+		importGraph: state.importGraph.clone(), inputBytes: accounting.bytes, inputNodes: accounting.nodes,
+	}
+	input := assembledInput{
+		parsed: main, composition: acquired, acquisition: cloneAcquisitionScopes(acquired.acquisition),
+		sources: state.sources, versionState: versionState, accounting: accounting,
+	}
+	composed, diagnostics := composeAcquiredInputWithPolicies(input, collectionPolicyScope{}, policy)
+	return composed, cloneSourceInfos(state.sources), diagnostics
+}
+
+// composeAcquiredInputWithPolicy is the per-input adapter from retained
+// acquisition scopes to the established strict-composition behavior. It uses
+// the already acquired documents and graph; source handles are closed before
+// this handoff.
+func composeAcquiredInputWithPolicies(input assembledInput, lower collectionPolicyScope, policy sequencePolicy) (*composedDocument, []Diagnostic) {
+	if input.composition == nil || input.composition.root != nil || len(input.composition.acquisition) == 0 {
+		return input.composition, nil
+	}
+	acquired := input.composition
+	if len(acquired.acquisition) != 1 {
+		return nil, []Diagnostic{importDiagnostic(CodeImportInvalid, "acquired input has an invalid composition scope", nil)}
+	}
+	scope := cloneAcquisitionScope(acquired.acquisition[0])
+	var rule *VersionRule
+	if input.versionState != nil {
+		rule = input.versionState.effectiveRule
+	}
+	accounting := input.accounting
+	if accounting == nil {
+		accounting = &importAccounting{sources: len(input.sources), bytes: acquired.inputBytes, nodes: acquired.inputNodes}
+	}
+	traversal := importTraversal{
+		accounting: accounting,
+		limits:     defaultImportLimits(), rule: rule, versionState: input.versionState,
+		importGraph: acquired.importGraph.clone(),
+	}
+	if len(scope.contributions) == 1 && scope.contributions[0].role == importRoleMain && scope.contributions[0].pendingItem != nil {
+		main := scope.contributions[0]
+		localScope := cloneAcquisitionScope(*main.pendingItem)
+		if diagnostics := traversal.composePendingItems(&localScope); len(diagnostics) != 0 {
+			return nil, prependCompositionDiagnosticPaths(diagnostics, main.destination.Index(0))
+		}
 		documents := make([]*document.Node, 0, len(localScope.contributions))
 		for _, contribution := range localScope.contributions {
 			documents = append(documents, contribution.document)
 		}
-		completed, failures := composeDocuments(documents...)
-		if len(failures) != 0 {
-			return nil, cloneSourceInfos(state.sources), prependCompositionDiagnosticPaths(failures, destination.Index(0))
+		completed, diagnostics := composeDocumentsWithCollectionPolicy(
+			policy, localScope.collectionPolicies,
+			acquiredSequenceDeclarations(localScope), documents...,
+		)
+		if len(diagnostics) != 0 {
+			return nil, prependCompositionDiagnosticPaths(diagnostics, main.destination.Index(0))
 		}
-		if versionState != nil {
-			protectionRule := versionState.localRule
-			if versionState.mode == sourceVersionModeDefault && protectionRule != nil {
-				prefix := destination.Index(0)
+		if input.versionState != nil {
+			protectionRule := input.versionState.localRule
+			if input.versionState.mode == sourceVersionModeDefault && protectionRule != nil {
+				prefix := main.destination.Index(0)
 				relative, inside := relativeHistoryPath(prefix, protectionRule.Path)
 				if !inside || !matchesPathShape(Path{segments: protectionRule.Path.segments[:len(prefix.segments)]}, prefix) {
 					protectionRule = nil
@@ -298,56 +350,84 @@ func loadImportedWithAccessTrackedAndProcessingPolicyDeferred(access *filesystem
 				if rule != nil {
 					diagnostic.Path = rule.Path.clone()
 				}
-				return nil, cloneSourceInfos(state.sources), []Diagnostic{*diagnostic}
+				return nil, []Diagnostic{*diagnostic}
 			}
 		}
 		completed.acquisition = []acquisitionScope{cloneAcquisitionScope(localScope)}
 		projectNestedEvidence(completed, localScope)
-		_, placed, err := placeDocumentWithMode(completed.root, destination, placementModeSequenceItem, body.Location, maxDocumentDepth, maxDocumentNodes)
+		_, placed, err := placeDocumentWithMode(completed.root, main.destination, placementModeSequenceItem, main.payload.Location, maxDocumentDepth, maxDocumentNodes)
 		if err != nil {
-			return nil, cloneSourceInfos(state.sources), []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, main.source.ID, "configuration exceeds the source depth or node limit after destination placement")}
+			return nil, []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, main.source.ID, "configuration exceeds the source depth or node limit after destination placement")}
 		}
-		placementNodes := destinationWrapperNodeCountForMode(destination, placementModeSequenceItem)
-		if accounting.nodes+placementNodes > limits.nodes {
-			return nil, cloneSourceInfos(state.sources), []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", nil)}
+		if !main.placementCharged {
+			placementNodes := destinationWrapperNodeCountForMode(main.destination, placementModeSequenceItem)
+			if accounting.nodes+placementNodes > traversal.limits.nodes {
+				return nil, []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", main.source.ImportedFrom)}
+			}
+			accounting.nodes += placementNodes
 		}
-		accounting.nodes += placementNodes
-		composed, failures := composeDocumentsWithPolicy(policy, placed)
-		if composed != nil {
-			synthetic := acquisitionScope{contributions: []acquiredContribution{{
-				source: cloneSourceInfo(mainInfo), payload: completed.root, document: placed,
-				identity: mainNodeKey, role: importRoleMain, scopeOwner: mainNodeKey,
-				destination: destination.clone(), processing: processing, nested: completed,
-			}}}
-			synthetic.owner = mainNodeKey
-			projectNestedEvidence(composed, synthetic)
-			projectAcquiredDestinations(composed, synthetic, state.sources)
-			composed.acquisition = []acquisitionScope{cloneAcquisitionScope(synthetic)}
-			composed.inputBytes = accounting.bytes
-			composed.inputNodes = accounting.nodes
-			composed.importGraph = state.importGraph.clone()
+		composition, diagnostics := composeDocumentsWithPolicy(policy, placed)
+		if len(diagnostics) != 0 {
+			return nil, diagnostics
 		}
-		return composed, cloneSourceInfos(state.sources), failures
+		synthetic := acquisitionScope{owner: main.scopeOwner, contributions: []acquiredContribution{{
+			source: cloneSourceInfo(main.source), payload: completed.root, document: placed,
+			identity: main.identity, role: importRoleMain, scopeOwner: main.scopeOwner,
+			destination: main.destination.clone(), processing: main.processing, nested: completed,
+		}}}
+		projectNestedEvidence(composition, synthetic)
+		projectAcquiredDestinations(composition, synthetic, input.sources)
+		composition.acquisition = []acquisitionScope{cloneAcquisitionScope(synthetic)}
+		composition.inputBytes = accounting.bytes
+		composition.inputNodes = accounting.nodes
+		composition.importGraph = acquired.importGraph.clone()
+		return composition, nil
 	}
-	documents := make([]*document.Node, 0, len(state.scope.contributions))
-	for _, contribution := range state.scope.contributions {
+	return composeAcquiredScopeWithPolicy(
+		&scope, lower, policy, acquired.importGraph, input.sources, rule, input.versionState, accounting,
+	)
+}
+
+// composeAcquiredScopeWithPolicy is shared by per-input and underlay-group
+// handoffs. Lower policy is carried separately from registered application
+// constraints; current strict composition continues to use its existing rules.
+func composeAcquiredScopeWithPolicy(
+	scope *acquisitionScope,
+	lower collectionPolicyScope,
+	policy sequencePolicy,
+	graph importInclusionGraph,
+	sources []SourceInfo,
+	rule *VersionRule,
+	versionState *sourceVersionState,
+	accounting *importAccounting,
+) (*composedDocument, []Diagnostic) {
+	traversal := importTraversal{
+		accounting: accounting, limits: defaultImportLimits(), rule: rule,
+		versionState: versionState, importGraph: graph.clone(),
+	}
+	if diagnostics := traversal.composePendingItems(scope); len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+	documents := make([]*document.Node, 0, len(scope.contributions))
+	for _, contribution := range scope.contributions {
 		documents = append(documents, contribution.document)
 	}
-	composed, diagnostics := composeDocumentsWithPolicy(policy, documents...)
-	if composed != nil {
-		if diagnostic := importedVersionClaim(documents, state.scope, composed, rule); diagnostic != nil {
-			return nil, cloneSourceInfos(state.sources), []Diagnostic{*diagnostic}
-		}
+	composition, diagnostics := composeDocumentsWithCollectionPolicy(
+		policy, lower, acquiredSequenceDeclarations(*scope), documents...,
+	)
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
 	}
-	if composed != nil {
-		projectNestedEvidence(composed, state.scope)
-		projectAcquiredDestinations(composed, state.scope, state.sources)
-		composed.acquisition = []acquisitionScope{cloneAcquisitionScope(state.scope)}
-		composed.inputBytes = accounting.bytes
-		composed.inputNodes = accounting.nodes
-		composed.importGraph = state.importGraph.clone()
+	if diagnostic := importedVersionClaim(documents, *scope, composition, rule); diagnostic != nil {
+		return nil, []Diagnostic{*diagnostic}
 	}
-	return composed, cloneSourceInfos(state.sources), diagnostics
+	projectNestedEvidence(composition, *scope)
+	projectAcquiredDestinations(composition, *scope, sources)
+	composition.acquisition = []acquisitionScope{cloneAcquisitionScope(*scope)}
+	composition.inputBytes = accounting.bytes
+	composition.inputNodes = accounting.nodes
+	composition.importGraph = graph.clone()
+	return composition, nil
 }
 
 func reservePendingItemPlacements(scope *acquisitionScope, limits importLimits, accounting *importAccounting) *Diagnostic {
@@ -547,25 +627,37 @@ func (s *importTraversal) visitPath(candidate selectedImportCandidate, spec impo
 
 	var body *document.Node
 	var declarations []importDeclarationSpec
+	var sequenceDeclarations []sequenceDeclarationSpec
 	var diagnostics []Diagnostic
 	var processing *sourceVersionProcessingResult
+	directiveContext := s.directiveContext
+	if directiveContext == 0 {
+		directiveContext = rootDirectiveContextImportsOnly
+	}
 	if s.versionState != nil && s.versionState.mode != sourceVersionModeDefault {
 		versionDestination := destination
 		if s.itemScope {
 			versionDestination = appendDestination(s.itemPrefix.Index(0), destination)
 		}
-		processed, failures := s.versionState.process(root, sourceVersionContribution, versionDestination, sourceID, policy.placement)
+		processed, failures := s.versionState.processWithDirectiveContext(root, sourceVersionContribution, versionDestination, sourceID, policy.placement, directiveContext)
 		if len(failures) != 0 {
 			return failures
 		}
 		processing = &processed
 		body, declarations = processed.payload, processed.imports
 	} else {
-		body, declarations, diagnostics = splitImportDirectives(root)
-		if len(diagnostics) != 0 {
-			return diagnostics
+		versionDestination := destination
+		if s.itemScope {
+			versionDestination = appendDestination(s.itemPrefix.Index(0), destination)
 		}
+		processed := processSourceVersionWithDirectiveContext(root, s.rule, sourceVersionContribution, versionDestination, sourceVersionModeDefault, directiveContext)
+		if len(processed.diagnostics) != 0 {
+			return processed.diagnostics
+		}
+		processing = &processed
+		body, declarations = processed.payload, processed.imports
 	}
+	sequenceDeclarations = cloneSequenceDeclarations(processing.sequences)
 	placementDestination := destination
 	mode := placementModeMapping
 	if policy.placement == placementModeSequenceItem {
@@ -582,7 +674,7 @@ func (s *importTraversal) visitPath(candidate selectedImportCandidate, spec impo
 		s.active[identity] = len(s.stack)
 		s.visited[identity] = route
 		s.stack = append(s.stack, route)
-		diagnostics := s.visitItemSource(canonical, nodeKey, destination, relativeDestination, depth, info, body, declarations, processing)
+		diagnostics := s.visitItemSource(canonical, nodeKey, destination, relativeDestination, depth, info, body, declarations, sequenceDeclarations, processing)
 		delete(s.active, identity)
 		s.stack = s.stack[:len(s.stack)-1]
 		return diagnostics
@@ -601,6 +693,7 @@ func (s *importTraversal) visitPath(candidate selectedImportCandidate, spec impo
 		source: cloneSourceInfo(info), payload: body, document: placed,
 		identity: nodeKey, role: role, scopeOwner: s.scope.owner,
 		destination: placementDestination.clone(), relativeDestination: relativeDestination.clone(), processing: processing,
+		sequenceDeclarations: cloneSequenceDeclarations(sequenceDeclarations),
 	})
 	s.active[identity] = len(s.stack)
 	s.visited[identity] = route
@@ -631,7 +724,7 @@ func (s *importTraversal) sourceDestination(destination Path) Path {
 	return s.itemFallback.clone()
 }
 
-func (s *importTraversal) visitItemSource(canonical string, identity importNodeKey, destination, relativeDestination Path, depth int, info SourceInfo, body *document.Node, declarations []importDeclarationSpec, processing *sourceVersionProcessingResult) []Diagnostic {
+func (s *importTraversal) visitItemSource(canonical string, identity importNodeKey, destination, relativeDestination Path, depth int, info SourceInfo, body *document.Node, declarations []importDeclarationSpec, sequenceDeclarations []sequenceDeclarationSpec, processing *sourceVersionProcessingResult) []Diagnostic {
 	parentScope := s.scope
 	parentItemScope, parentPrefix, parentFallback := s.itemScope, s.itemPrefix, s.itemFallback
 	s.scope = acquisitionScope{owner: identity}
@@ -645,7 +738,7 @@ func (s *importTraversal) visitItemSource(canonical string, identity importNodeK
 	s.scope.contributions = append(s.scope.contributions, acquiredContribution{
 		source: cloneSourceInfo(info), payload: body, document: body,
 		identity: identity, role: importRoleItem, scopeOwner: identity,
-		processing: processing,
+		processing: processing, sequenceDeclarations: cloneSequenceDeclarations(sequenceDeclarations),
 	})
 	diagnostics := s.walkDeclarations(s.access.dir(canonical), declarations, depth, Path{})
 	itemScope := s.scope
@@ -686,7 +779,13 @@ func (s *importTraversal) composePendingItems(scope *acquisitionScope) []Diagnos
 		for _, nested := range itemScope.contributions {
 			documents = append(documents, nested.document)
 		}
-		completed, diagnostics := composeDocuments(documents...)
+		// An item scope is below a sequence index. Application selectors match
+		// mapping-only effective paths, so none can apply inside this scope.
+		// Its YAML declarations remain relative to the owning record.
+		completed, diagnostics := composeDocumentsWithCollectionPolicy(
+			sequencePolicy{}, itemScope.collectionPolicies,
+			acquiredSequenceDeclarations(*itemScope), documents...,
+		)
 		if len(diagnostics) != 0 {
 			return projectItemDiagnosticPaths(diagnostics, *scope, contribution.destination, index)
 		}
@@ -713,6 +812,18 @@ func (s *importTraversal) composePendingItems(scope *acquisitionScope) []Diagnos
 	return nil
 }
 
+func acquiredSequenceDeclarations(scope acquisitionScope) []sequenceDeclarationSpec {
+	var declarations []sequenceDeclarationSpec
+	for _, contribution := range scope.contributions {
+		projected := cloneSequenceDeclarations(contribution.sequenceDeclarations)
+		for index := range projected {
+			projected[index].destination = appendDestination(contribution.relativeDestination, projected[index].destination)
+		}
+		declarations = append(declarations, projected...)
+	}
+	return declarations
+}
+
 func importedVersionClaim(documents []*document.Node, scope acquisitionScope, composed *composedDocument, rule *VersionRule) *Diagnostic {
 	if composed == nil || rule == nil {
 		return nil
@@ -817,8 +928,13 @@ func projectNestedEvidence(composed *composedDocument, scope acquisitionScope) {
 			projectedApplications = append(projectedApplications, applications[sourceIndex])
 			continue
 		}
-		start := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}].start
-		prefix := contribution.destination.Index(start)
+		itemIndex := 0
+		if assigned, found := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}]; found {
+			if projected, ok := assigned.outputForInput(0); ok {
+				itemIndex = projected
+			}
+		}
+		prefix := contribution.destination.Index(itemIndex)
 		if sourceIndex < len(applications) {
 			outer := applications[sourceIndex]
 			structural := document.Clone(outer.value)
@@ -840,6 +956,71 @@ func projectNestedEvidence(composed *composedDocument, scope acquisitionScope) {
 		projectNestedOrigins(composed.origins, contribution.nested.root, contribution.nested.origins, prefix)
 	}
 	composed.applications = projectedApplications
+	projectNestedCollectionPolicies(composed, scope, ranges)
+}
+
+func projectNestedCollectionPolicies(composed *composedDocument, scope acquisitionScope, ranges map[sequenceRangeKey]sequenceRange) {
+	if composed == nil {
+		return
+	}
+	type itemPolicyProjection struct {
+		destination Path
+		policy      collectionPolicy
+		base        int
+	}
+	projected := make(map[string]*itemPolicyProjection)
+	var prefixLengths map[string]int
+	for sourceIndex, contribution := range scope.contributions {
+		if prefixLengths != nil && sourceIndex > 0 {
+			addSequencePrefixLengths(scope.contributions[sourceIndex-1].document, Path{}, prefixLengths)
+		}
+		if contribution.nested == nil || len(contribution.nested.collectionPolicies.collections) == 0 {
+			continue
+		}
+		key := contribution.destination.String()
+		assigned := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: key}]
+		itemIndex, found := assigned.outputForInput(0)
+		if !found {
+			// Normal composition supplies the assignment directly. For a
+			// handoff without ranges, scan each preceding tree only once.
+			if prefixLengths == nil {
+				prefixLengths = make(map[string]int)
+				for _, previous := range scope.contributions[:sourceIndex] {
+					addSequencePrefixLengths(previous.document, Path{}, prefixLengths)
+				}
+			}
+			itemIndex = prefixLengths[key]
+		}
+		working, initialized := projected[key]
+		if !initialized {
+			// Read one owned copy per destination, then add each incoming
+			// record without recopying earlier records and their scopes.
+			policy, _ := composed.collectionPolicies.at(contribution.destination)
+			working = &itemPolicyProjection{destination: contribution.destination, policy: policy, base: len(policy.recordOrder)}
+			projected[key] = working
+		}
+		working.policy.setRecordPolicyAt(working.base+itemIndex, collectionRecordPolicy{
+			owner: newCollectionRecordPolicy().owner,
+			scope: contribution.nested.collectionPolicies,
+		})
+	}
+	for _, working := range projected {
+		composed.collectionPolicies.set(working.destination, working.policy)
+	}
+}
+
+func addSequencePrefixLengths(node *document.Node, path Path, lengths map[string]int) {
+	if node == nil {
+		return
+	}
+	if node.Kind == document.Sequence {
+		lengths[path.String()] += len(node.Sequence)
+		return
+	}
+	// Contribution destinations select mapping paths, not sequence members.
+	for _, entry := range node.Mapping {
+		addSequencePrefixLengths(entry.Value, path.Key(entry.Key), lengths)
+	}
 }
 
 func projectNestedOrigins(target map[string]Origin, root *document.Node, origins map[string]Origin, prefix Path) {
@@ -875,7 +1056,12 @@ func projectAcquiredDestinations(composed *composedDocument, scope acquisitionSc
 			}
 			contributionPrefix := prefix
 			contributionInsideItem := insideItem
-			start := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}].start
+			itemIndex := 0
+			if assigned, found := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}]; found {
+				if projected, ok := assigned.outputForInput(0); ok {
+					itemIndex = projected
+				}
+			}
 			if contribution.source.placementMode == placementModeSequenceItem {
 				container := appendDestination(contributionPrefix, contribution.destination)
 				contribution.source.Destination = container.clone()
@@ -885,7 +1071,7 @@ func projectAcquiredDestinations(composed *composedDocument, scope acquisitionSc
 					sources[position].Destination = container
 					sources[position].destinationUnresolved = false
 				}
-				contributionPrefix = appendDestination(container, Path{}.Index(start))
+				contributionPrefix = appendDestination(container, Path{}.Index(itemIndex))
 				contributionInsideItem = true
 				projectSourceVersionEvidence(contribution.processing, contributionPrefix, nil, sourceIndex)
 			} else if contributionInsideItem {
@@ -928,7 +1114,9 @@ func projectSourceVersionEvidence(processing *sourceVersionProcessingResult, des
 				}
 				item, ok := schemaIndex(path.segments[position])
 				if ok && path.segments[position].kind != pathSegmentKey {
-					path.segments[position] = pathSegment{kind: pathSegmentIndex, index: assigned.start + item}
+					if outputIndex, projected := assigned.outputForInput(item); projected {
+						path.segments[position] = pathSegment{kind: pathSegmentIndex, index: outputIndex}
+					}
 				}
 			}
 			paths[index] = path

internal/engine/imports_test.go

@@ -115,7 +115,7 @@ tasks: [{id: z-parent}]
 
 	input, diagnostics := assembleBaseInput[config](File(mainPath), Options[config]{
 		Imports:        true,
-		sequencePolicy: sequencePolicy{identityFields: map[string]string{"/tasks": "id"}},
+		sequencePolicy: mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("tasks")}, identityFields: []string{"id"}}}),
 	})
 	if len(diagnostics) != 0 {
 		t.Fatalf("assemble imports: %#v", diagnostics)
@@ -525,6 +525,141 @@ func TestLoadMainItemPlacementKeepsItsHelpersInTheItem(t *testing.T) {
 	}
 }
 
+func TestProjectAcquiredDestinationsUsesMappedSequenceItemIndex(t *testing.T) {
+	child := &composedDocument{}
+	childScope := acquisitionScope{contributions: []acquiredContribution{{
+		source: SourceInfo{ID: "nested-source"}, destination: Keys("settings"),
+	}}}
+	child.acquisition = []acquisitionScope{childScope}
+	composed := &composedDocument{sequenceRanges: []sequenceRange{{
+		sourceIndex: 0, path: Keys("records"), start: 0, end: 1,
+		items: []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}},
+	}}}
+	scope := acquisitionScope{contributions: []acquiredContribution{{
+		source:      SourceInfo{ID: "outer-source", placementMode: placementModeSequenceItem},
+		destination: Keys("records"), nested: child,
+	}}}
+	sources := []SourceInfo{{ID: "outer-source"}, {ID: "nested-source"}}
+
+	projectAcquiredDestinations(composed, scope, sources)
+	if got, want := sources[0].Destination.String(), "/records"; got != want {
+		t.Errorf("outer source destination = %q, want %q", got, want)
+	}
+	if got, want := sources[1].Destination.String(), "/records/2/settings"; got != want {
+		t.Errorf("nested source destination = %q, want %q", got, want)
+	}
+}
+
+func TestProjectNestedEvidenceUsesMappedSequenceItemIndex(t *testing.T) {
+	outer := historyApplication{
+		value: parseCompositionDocument(t, "outer-source", "records: [{id: outer}]\n"),
+		sequences: []sequenceHistoryContribution{{
+			path: Keys("records"), mode: sequenceHistoryMapped,
+			items: []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}},
+		}},
+	}
+	outer.sequencesByPath = indexSequenceHistoryContributions(outer.sequences)
+	nestedRoot := parseCompositionDocument(t, "nested-source", "settings: {value: nested}\n")
+	nestedOrigin := Origin{Layer: LayerBase, Locations: []Location{{SourceID: "nested-source"}}}
+	nested := &composedDocument{
+		root: nestedRoot, applications: []historyApplication{{value: nestedRoot}},
+		origins: map[string]Origin{"/settings/value": nestedOrigin},
+	}
+	scope := acquisitionScope{contributions: []acquiredContribution{{
+		source: SourceInfo{ID: "outer-source"}, destination: Keys("records"), nested: nested,
+	}}}
+	composed := &composedDocument{
+		applications: []historyApplication{outer},
+		sequenceRanges: []sequenceRange{{
+			sourceIndex: 0, path: Keys("records"), start: 0, end: 1,
+			items: []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}},
+		}},
+		origins: make(map[string]Origin),
+	}
+
+	projectNestedEvidence(composed, scope)
+	if got, want := composed.applications[1].path.String(), "/records/2"; got != want {
+		t.Fatalf("nested history application path = %q, want %q", got, want)
+	}
+	if origin := composed.origins["/records/2/settings/value"]; len(origin.Locations) != 1 || origin.Locations[0].SourceID != "nested-source" {
+		t.Fatalf("nested origin = %#v", origin)
+	}
+	query := Keys("records").Index(2).Key("settings").Key("value")
+	if operation, relevant := applicationOperationPath(composed.applications[1], query, nil); !relevant || operation.String() != query.String() {
+		t.Fatalf("nested historical query = %s, %v", operation.String(), relevant)
+	}
+	replayed := parseCompositionDocument(t, "lower", "records: [{id: first}, {id: middle}, {id: last}]\n")
+	for _, application := range composed.applications {
+		applyHistoryApplication(&replayed, application, new(mappingIndex))
+	}
+	if value, present := documentAtPath(replayed, query); !present || value.Text != "nested" {
+		t.Fatalf("replayed nested value = %#v, %v", value, present)
+	}
+	composed.applications[1].path.segments[0].key = "detached"
+	if got := nested.applications[0].path.String(); got != "" {
+		t.Fatalf("projected nested history path aliases its source: %q", got)
+	}
+}
+
+func TestDeferredInputCompositionUsesRetainedAcquisitionScopes(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: item.yml
+      destination: [records]
+      placement: sequence-item
+name: main
+`)
+	itemPath := writeImportFile(t, directory, "item.yml", `configkit:
+  imports:
+    - path: helper.yml
+id: item
+`)
+	helperPath := writeImportFile(t, directory, "helper.yml", "value: acquired\n")
+	accounting := &importAccounting{}
+	input, diagnostics := assembleBaseInputTracked(File(mainPath), Options[struct{}]{
+		Imports: true, Filesystem: &FilesystemContext{Root: directory}, deferAssembly: true,
+	}, Path{}, accounting, nil, nil)
+	if len(diagnostics) != 0 {
+		t.Fatalf("acquire input: %#v", diagnostics)
+	}
+	if input.composition == nil || input.composition.root != nil || len(input.composition.acquisition) != 1 {
+		t.Fatalf("acquisition handoff was composed early: %#v", input.composition)
+	}
+	if len(input.sources) != 3 || input.sources[0].ID != "source-1" || input.sources[1].ID != "source-2" || input.sources[2].ID != "source-3" {
+		t.Fatalf("acquired source order/identity = %#v", input.sources)
+	}
+	if _, err := os.Stat(itemPath); err != nil {
+		t.Fatalf("item source disappeared before composition: %v", err)
+	}
+	if _, err := os.Stat(helperPath); err != nil {
+		t.Fatalf("helper source disappeared before composition: %v", err)
+	}
+
+	composition, diagnostics := compositionForInputWithPolicy(input, sequencePolicy{})
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose acquired input: %#v", diagnostics)
+	}
+	if value, present := documentAtPath(composition.root, Keys("name")); !present || value.Text != "main" {
+		t.Fatalf("main value = %#v, %v", value, present)
+	}
+	for path, want := range map[string]string{
+		"/records/0/id":    "item",
+		"/records/0/value": "acquired",
+	} {
+		value, present := documentAtPath(composition.root, mustParsePath(t, path))
+		if !present || value.Text != want {
+			t.Errorf("composed %s = %#v, %v; want %q", path, value, present, want)
+		}
+	}
+	if got, want := input.sources[1].Destination.String(), "/records"; got != want {
+		t.Errorf("item destination = %q, want %q", got, want)
+	}
+	if got, want := input.sources[2].Destination.String(), "/records/0"; got != want {
+		t.Errorf("helper destination = %q, want %q", got, want)
+	}
+}
+
 func TestItemImportCanJoinAnAuthoredListUnderRegisteredComposition(t *testing.T) {
 	t.Parallel()
 	type config struct {
@@ -544,7 +679,7 @@ func TestItemImportCanJoinAnAuthoredListUnderRegisteredComposition(t *testing.T)
 		t.Fatalf("assemble item import: %#v", diagnostics)
 	}
 	authored := parseDocumentForDecode(t, "items: [{id: authored-id, name: authored}]\n")
-	composed, diagnostics := composeDocumentsWithPolicy(sequencePolicy{identityFields: map[string]string{"/items": "id"}}, authored, input.document)
+	composed, diagnostics := composeDocumentsWithPolicy(mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"}}}), authored, input.document)
 	if len(diagnostics) != 0 {
 		t.Fatalf("compose authored and imported lists: %#v", diagnostics)
 	}
@@ -874,7 +1009,7 @@ func TestItemContributionsUseDependencyReadySchedules(t *testing.T) {
 		type config struct {
 			Items []item `yaml:"items"`
 		}
-		definition := Definition[config]{sequenceRegistrations: []sequenceRegistration{{path: Keys("items"), identityField: "id"}}}
+		definition := Definition[config]{sequenceIdentityRegistrations: []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"}}}}
 		directory := t.TempDir()
 		mainPath := writeImportFile(t, directory, "main.yml", `configkit:
   imports:

internal/engine/inheritance.go

@@ -7,10 +7,13 @@ const inheritanceKey = "extends"
 // These private records keep structural evidence separate from acquired source
 // records. Later inspection can project a transfer onto individual descendants.
 type inheritanceTransfer struct {
-	from      Path
-	to        Path
-	reference Path
-	locations []Location
+	from              Path
+	to                Path
+	reference         Path
+	locations         []Location
+	policyTemplates   []collectionPolicyScope
+	templatePolicies  collectionPolicyScope
+	projectedPolicies []projectedCollectionPolicy
 }
 
 type inheritanceControl struct {
@@ -20,22 +23,110 @@ type inheritanceControl struct {
 }
 
 type inheritanceScope struct {
-	root         *document.Node
-	origins      map[string]Origin
-	transfers    []inheritanceTransfer
-	controls     []inheritanceControl
-	replacements []inheritanceReplacement
-	operations   []inheritanceOperation
+	root               *document.Node
+	origins            map[string]Origin
+	collectionPolicies collectionPolicyScope
+	transfers          []inheritanceTransfer
+	controls           []inheritanceControl
+	replacements       []inheritanceReplacement
+	operations         []inheritanceOperation
 }
 
 type inheritanceOperation struct {
-	control          *inheritanceControl
-	transfer         *inheritanceTransfer
-	controlOrigin    Origin
-	parent           *document.Node
-	effective        *document.Node
-	parentOrigins    map[string]Origin
-	effectiveOrigins map[string]Origin
+	control           *inheritanceControl
+	transfer          *inheritanceTransfer
+	controlOrigin     Origin
+	parent            *document.Node
+	effective         *document.Node
+	parentOrigins     map[string]Origin
+	effectiveOrigins  map[string]Origin
+	parentPolicies    []collectionPolicyScope
+	templatePolicies  collectionPolicyScope
+	effectivePolicies []collectionPolicyScope
+	sequences         []sequenceHistoryContribution
+	identityFields    []string
+}
+
+func cloneInheritancePolicyTemplates(templates []collectionPolicyScope) []collectionPolicyScope {
+	if templates == nil {
+		return nil
+	}
+	cloned := make([]collectionPolicyScope, len(templates))
+	for index, template := range templates {
+		cloned[index] = cloneCollectionPolicyScopeForInheritance(template)
+	}
+	return cloned
+}
+
+func cloneProjectedInheritancePolicies(policies []projectedCollectionPolicy) []projectedCollectionPolicy {
+	if policies == nil {
+		return nil
+	}
+	cloned := make([]projectedCollectionPolicy, len(policies))
+	for index, policy := range policies {
+		cloned[index] = projectedCollectionPolicy{
+			path: policy.path.clone(), policy: cloneCollectionPolicyForInheritance(policy.policy),
+		}
+	}
+	return cloned
+}
+
+func cloneInheritanceTransfer(transfer inheritanceTransfer) inheritanceTransfer {
+	cloned := transfer
+	cloned.from = transfer.from.clone()
+	cloned.to = transfer.to.clone()
+	cloned.reference = transfer.reference.clone()
+	cloned.locations = append([]Location(nil), transfer.locations...)
+	cloned.policyTemplates = cloneInheritancePolicyTemplates(transfer.policyTemplates)
+	cloned.templatePolicies = cloneCollectionPolicyScopeForInheritance(transfer.templatePolicies)
+	cloned.projectedPolicies = cloneProjectedInheritancePolicies(transfer.projectedPolicies)
+	return cloned
+}
+
+func cloneInheritanceOperations(operations []inheritanceOperation) []inheritanceOperation {
+	if operations == nil {
+		return nil
+	}
+	cloned := make([]inheritanceOperation, len(operations))
+	for index, operation := range operations {
+		cloned[index] = operation
+		if operation.control != nil {
+			control := *operation.control
+			control.path = operation.control.path.clone()
+			control.locations = append([]Location(nil), operation.control.locations...)
+			cloned[index].control = &control
+		}
+		if operation.transfer != nil {
+			transfer := cloneInheritanceTransfer(*operation.transfer)
+			cloned[index].transfer = &transfer
+		}
+		cloned[index].parent = document.Clone(operation.parent)
+		cloned[index].effective = document.Clone(operation.effective)
+		cloned[index].parentOrigins = cloneOriginIndex(operation.parentOrigins)
+		cloned[index].effectiveOrigins = cloneOriginIndex(operation.effectiveOrigins)
+		cloned[index].parentPolicies = cloneInheritancePolicyTemplates(operation.parentPolicies)
+		cloned[index].templatePolicies = cloneCollectionPolicyScopeForInheritance(operation.templatePolicies)
+		cloned[index].effectivePolicies = cloneInheritancePolicyTemplates(operation.effectivePolicies)
+		cloned[index].sequences = cloneSequenceHistoryContributions(operation.sequences)
+		cloned[index].identityFields = cloneStrings(operation.identityFields)
+	}
+	return cloned
+}
+
+func inheritancePolicyScopeAtPath(scope collectionPolicyScope, prefix Path) collectionPolicyScope {
+	return newInheritancePolicyIndex(scope).localize(prefix)
+}
+
+func projectInheritancePolicyTemplates(templates []collectionPolicyScope, prefix Path) []projectedCollectionPolicy {
+	var projected []projectedCollectionPolicy
+	for _, template := range templates {
+		for _, policy := range projectCollectionPolicyScope(template, prefix) {
+			projected = append(projected, projectedCollectionPolicy{
+				path: policy.path.clone(), policy: cloneCollectionPolicyForInheritance(policy.policy),
+			})
+		}
+	}
+	return projected
 }
 
 // A replacement retains the displaced source/derivation record while the

internal/engine/inheritance_collection.go

@@ -0,0 +1,309 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+func projectInheritanceCollectionPolicies(scope collectionPolicyScope, prefix Path) collectionPolicyScope {
+	projected := collectionPolicyScope{}
+	for _, entry := range scope.collections {
+		policy := cloneCollectionPolicyForInheritance(entry.policy)
+		stripInheritanceModes(&policy)
+		projected.set(appendDestination(prefix, entry.destination), policy)
+	}
+	return projected
+}
+
+func projectCurrentInheritanceCollectionPolicies(scope collectionPolicyScope, prefix Path) collectionPolicyScope {
+	projected := collectionPolicyScope{}
+	for _, entry := range scope.collections {
+		projected.set(appendDestination(prefix, entry.destination), cloneCollectionPolicyForInheritance(entry.policy))
+	}
+	return projected
+}
+
+func projectInheritancePolicyRoots(target *collectionPolicyScope, scope collectionPolicyScope, prefix Path) {
+	for _, entry := range scope.collections {
+		target.set(appendDestination(prefix, entry.destination), cloneCollectionPolicyForInheritance(entry.policy))
+	}
+}
+
+func stripInheritanceModes(policy *collectionPolicy) {
+	if policy == nil {
+		return
+	}
+	policy.explicitMode = 0
+	policy.explicitModePresent = false
+	policy.modeLocation = nil
+	for owner, nested := range policy.recordPolicies {
+		for key, entry := range nested.collections {
+			stripInheritanceModes(&entry.policy)
+			nested.collections[key] = entry
+		}
+		policy.recordPolicies[owner] = nested
+	}
+}
+
+func inheritanceDeclarationsAt(path Path, policy collectionPolicy) []sequenceDeclarationSpec {
+	if !policy.identityPresent && !policy.explicitModePresent {
+		return nil
+	}
+	return []sequenceDeclarationSpec{{
+		destination:    path.clone(),
+		identityFields: cloneStrings(policy.identityFields), identityPresent: policy.identityPresent,
+		identityLocation: cloneLocation(policy.identityLocation),
+		mode:             policy.explicitMode, modePresent: policy.explicitModePresent,
+		modeLocation: cloneLocation(policy.modeLocation),
+	}}
+}
+
+func inheritanceCollectionResolution(
+	path Path,
+	lowerScope, higherScope collectionPolicyScope,
+	applicationPolicy sequencePolicy,
+	incoming *document.Node,
+) (collectionPolicyResolution, collectionPolicy, collectionPolicy, bool, []Diagnostic) {
+	lower, hasLower := lowerScope.at(path)
+	higher, hasHigher := higherScope.at(path)
+	applicationIdentity, registered := applicationPolicy.identityFieldsAt(path)
+	if !hasLower && !hasHigher && !registered {
+		return collectionPolicyResolution{}, collectionPolicy{}, collectionPolicy{}, false, nil
+	}
+	lowerForResolution := cloneCollectionPolicy(lower)
+	// The parent policy is copied to a different sibling destination. Its YAML
+	// identity follows the inherited collection, but its explicit mode does not.
+	lowerForResolution.explicitMode = 0
+	lowerForResolution.explicitModePresent = false
+	lowerForResolution.modeLocation = nil
+	if incoming != nil && higher.explicitModePresent && higher.explicitMode == sequenceModeReplace {
+		// Actual replacement discards the parent's YAML identity before
+		// agreement. A mode-only declaration still governs retained records.
+		lowerForResolution = collectionPolicy{}
+	}
+	var lowerPointer *collectionPolicy
+	if hasLower {
+		lowerPointer = &lowerForResolution
+	}
+	resolution, diagnostics := resolveCollectionPolicy(
+		path, inheritanceDeclarationsAt(path, higher), applicationIdentity, lowerPointer,
+	)
+	return resolution, lower, higher, true, diagnostics
+}
+
+func mergeInheritanceCollectionPolicies(
+	parent, child collectionPolicyScope,
+	parentRootPath Path,
+	parentValue, childValue *document.Node,
+	applicationPolicy sequencePolicy,
+	replaced collectionReplacementPaths,
+) (collectionPolicyScope, []Diagnostic) {
+	result := cloneCollectionPolicyScopeForInheritance(parent)
+	paths := make(map[string]Path, len(parent.collections)+len(child.collections))
+	for key, entry := range parent.collections {
+		paths[key] = entry.destination.clone()
+	}
+	for key, entry := range child.collections {
+		paths[key] = entry.destination.clone()
+	}
+	var lookup *documentLookup
+	if len(paths) != 0 {
+		lookup = buildDocumentLookup(parentValue, childValue)
+	}
+	var diagnostics []Diagnostic
+	for _, path := range paths {
+		relative, inside := relativeHistoryPath(parentRootPath, path)
+		var retained, incoming *document.Node
+		if inside {
+			if !replaced.contains(path, false) {
+				retained, _ = documentAtPathWithLookup(parentValue, relative, lookup)
+			}
+			incoming, _ = documentAtPathWithLookup(childValue, relative, lookup)
+		}
+		resolution, lower, _, active, issues := inheritanceCollectionResolution(path, parent, child, applicationPolicy, incoming)
+		if len(issues) != 0 {
+			diagnostics = append(diagnostics, issues...)
+			continue
+		}
+		if !active {
+			continue
+		}
+		var lowerPointer *collectionPolicy
+		if _, found := parent.at(path); found {
+			lower.explicitMode = 0
+			lower.explicitModePresent = false
+			lower.modeLocation = nil
+			lowerPointer = &lower
+		}
+		policy := resolution.policy
+		if incoming == nil || resolution.operation != sequenceModeReplace {
+			var foundDiagnostics []Diagnostic
+			policy, foundDiagnostics = collectionPolicyWithoutValue(lowerPointer, resolution.policy, retained, path)
+			if len(foundDiagnostics) != 0 {
+				diagnostics = append(diagnostics, foundDiagnostics...)
+				continue
+			}
+		}
+		if higher, ok := child.at(path); ok {
+			policy = inheritanceRecordPoliciesFromHigher(policy, higher)
+		}
+		result.set(path, policy)
+	}
+	return result, canonicalDiagnostics(diagnostics)
+}
+
+func inheritanceRecordPoliciesFromHigher(policy, higher collectionPolicy) collectionPolicy {
+	if len(higher.recordOrder) == 0 {
+		return policy
+	}
+	policy.recordOrder = nil
+	policy.recordPolicies = nil
+	for index := range higher.recordOrder {
+		if record, found := higher.recordPolicyAt(index); found {
+			policy.setRecordPolicyAt(index, currentCollectionRecordPolicy(record))
+		}
+	}
+	return policy
+}
+
+func inheritedCollectionRecordPolicy(record collectionRecordPolicy) collectionRecordPolicy {
+	cloned := cloneCollectionPolicyForInheritance(collectionPolicy{
+		recordOrder:    []*collectionRecordOwner{record.owner},
+		recordPolicies: map[*collectionRecordOwner]collectionPolicyScope{record.owner: record.scope},
+	})
+	if len(cloned.recordOrder) == 0 {
+		return newCollectionRecordPolicy()
+	}
+	owner := cloned.recordOrder[0]
+	scope := cloned.recordPolicies[owner]
+	for key, entry := range scope.collections {
+		stripInheritanceModes(&entry.policy)
+		scope.collections[key] = entry
+	}
+	return collectionRecordPolicy{owner: owner, scope: scope}
+}
+
+func currentCollectionRecordPolicy(record collectionRecordPolicy) collectionRecordPolicy {
+	cloned := cloneCollectionPolicyForInheritance(collectionPolicy{
+		recordOrder:    []*collectionRecordOwner{record.owner},
+		recordPolicies: map[*collectionRecordOwner]collectionPolicyScope{record.owner: record.scope},
+	})
+	if len(cloned.recordOrder) == 0 {
+		return newCollectionRecordPolicy()
+	}
+	owner := cloned.recordOrder[0]
+	return collectionRecordPolicy{owner: owner, scope: cloned.recordPolicies[owner]}
+}
+
+func applyInheritanceSequencePolicy(
+	path Path,
+	combined sequenceCombinationResult,
+	resolution collectionPolicyResolution,
+	lowerScope, higherScope collectionPolicyScope,
+) collectionPolicy {
+	policy := cloneCollectionPolicyForInheritance(resolution.policy)
+	policy.recordOrder = nil
+	policy.recordPolicies = nil
+	if combined.value == nil || combined.value.Kind != document.Sequence {
+		return policy
+	}
+
+	lowerPolicy, hasLower := lowerScope.at(path)
+	higherPolicy, hasHigher := higherScope.at(path)
+	lowerProjection := newSequenceItemProjectionIndex(combined.lowerItems)
+	higherProjection := newSequenceItemProjectionIndex(combined.higherItems)
+	for outputIndex := range combined.value.Sequence {
+		if higherIndex, exists := higherProjection.inputForOutput(outputIndex); exists {
+			if record, found := higherPolicy.recordPolicyAt(higherIndex); hasHigher && found {
+				policy.setRecordPolicyAt(outputIndex, currentCollectionRecordPolicy(record))
+			} else {
+				policy.setRecordPolicyAt(outputIndex, replacedCollectionRecordPolicy())
+			}
+			continue
+		}
+		if lowerIndex, exists := lowerProjection.inputForOutput(outputIndex); exists && hasLower {
+			if record, found := lowerPolicy.recordPolicyAt(lowerIndex); found {
+				policy.setRecordPolicyAt(outputIndex, inheritedCollectionRecordPolicy(record))
+			}
+		}
+	}
+	return policy
+}
+
+func applyInheritanceSequenceOrigins(
+	result *inheritanceScope,
+	origins, incomingOrigins map[string]Origin,
+	path Path,
+	lower, higher *document.Node,
+	combined sequenceCombinationResult,
+	operation sequenceMode,
+) {
+	if operation == sequenceModeReplace {
+		if lower != nil {
+			displaced := displacedOrigins(lower, origins, path)
+			result.replacements = append(result.replacements, inheritanceReplacement{
+				path: path.clone(), before: lower, after: higher,
+				previous: displaced, effective: cloneOrigin(incomingOrigins[path.String()]),
+			})
+			deleteOriginSubtree(origins, path, lower)
+		}
+		mergeInheritanceOrigin(origins, path, incomingOrigins[path.String()])
+		if higher != nil && higher.Kind != document.Sequence {
+			copyInheritanceSubtree(origins, incomingOrigins, higher, path)
+		}
+		for _, projection := range combined.higherItems {
+			if higher != nil && higher.Kind == document.Sequence && projection.inputIndex >= 0 && projection.inputIndex < len(higher.Sequence) {
+				copyInheritanceSubtreeAt(origins, incomingOrigins, higher.Sequence[projection.inputIndex], path.Index(projection.inputIndex), path.Index(projection.outputIndex))
+			}
+		}
+		return
+	}
+
+	mergeInheritanceOrigin(origins, path, incomingOrigins[path.String()])
+	for _, replacement := range combined.replacements {
+		if lower == nil || lower.Kind != document.Sequence || replacement.lowerIndex < 0 || replacement.lowerIndex >= len(lower.Sequence) ||
+			higher == nil || higher.Kind != document.Sequence || replacement.higherIndex < 0 || replacement.higherIndex >= len(higher.Sequence) {
+			continue
+		}
+		lowerPath := path.Index(replacement.lowerIndex)
+		higherPath := path.Index(replacement.higherIndex)
+		outputPath := path.Index(replacement.outputIndex)
+		previous := displacedOrigins(lower.Sequence[replacement.lowerIndex], origins, lowerPath)
+		deleteOriginSubtree(origins, lowerPath, lower.Sequence[replacement.lowerIndex])
+		copyInheritanceSubtreeAt(origins, incomingOrigins, higher.Sequence[replacement.higherIndex], higherPath, outputPath)
+		result.replacements = append(result.replacements, inheritanceReplacement{
+			path: outputPath, before: lower.Sequence[replacement.lowerIndex], after: higher.Sequence[replacement.higherIndex],
+			previous: previous, effective: cloneOrigin(incomingOrigins[higherPath.String()]),
+		})
+	}
+	for _, projection := range combined.higherItems {
+		if higher != nil && higher.Kind == document.Sequence && projection.inputIndex >= 0 && projection.inputIndex < len(higher.Sequence) {
+			copyInheritanceSubtreeAt(origins, incomingOrigins, higher.Sequence[projection.inputIndex], path.Index(projection.inputIndex), path.Index(projection.outputIndex))
+		}
+	}
+}
+
+func validateRegisteredInheritanceSequences(root *document.Node, rootPath Path, applicationPolicy sequencePolicy) []Diagnostic {
+	var diagnostics []Diagnostic
+	var visit func(*document.Node, Path)
+	visit = func(node *document.Node, path Path) {
+		if node == nil {
+			return
+		}
+		if node.Kind == document.Sequence {
+			if fields, registered := applicationPolicy.identityFieldsAt(path); registered {
+				if _, issues := validateSequenceIdentity(node, path, fields, sequenceCombinationIdentityCodes); len(issues) != 0 {
+					diagnostics = append(diagnostics, issues...)
+				}
+			}
+			for index, item := range node.Sequence {
+				visit(item, path.Index(index))
+			}
+			return
+		}
+		if node.Kind == document.Mapping {
+			for _, entry := range node.Mapping {
+				visit(entry.Value, path.Key(entry.Key))
+			}
+		}
+	}
+	visit(root, rootPath)
+	return canonicalDiagnostics(diagnostics)
+}

internal/engine/inheritance_collection_test.go

@@ -0,0 +1,369 @@
+package engine
+
+import (
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestInheritanceReplacementResetsOnlyReplacedYamlIdentity(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name, key, value, applicationKey string
+		mode                             sequenceMode
+		wantFields                       []string
+		conflict                         bool
+	}{
+		{name: "changed key", key: "name", value: "[{name: child}]", mode: sequenceModeReplace, wantFields: []string{"name"}},
+		{name: "unkeyed replacement", value: "[{name: child}]", mode: sequenceModeReplace},
+		{name: "non-sequence replacement", value: "42", mode: sequenceModeReplace},
+		{name: "agreeing application", key: "name", value: "[{name: child}]", applicationKey: "name", mode: sequenceModeReplace, wantFields: []string{"name"}},
+		{name: "conflicting application", key: "name", value: "[{name: child}]", applicationKey: "id", mode: sequenceModeReplace, conflict: true},
+		{name: "application still validates replacement", value: "[{name: child}]", applicationKey: "id", mode: sequenceModeReplace, conflict: true},
+		{name: "mode-only key change", key: "name", mode: sequenceModeReplace, conflict: true},
+		{name: "mode-only keeps identity", mode: sequenceModeReplace, wantFields: []string{"id"}},
+		{name: "append key change", key: "name", value: "[{name: child}]", mode: sequenceModeAppend, conflict: true},
+		{name: "upsert key change", key: "name", value: "[{name: child}]", mode: sequenceModeUpsert, conflict: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			input := "base: {items: [{id: parent}]}\nchild:\n  extends: base\n"
+			if test.value != "" {
+				input += "  items: " + test.value + "\n"
+			}
+			root := parseItemPolicyDocument(t, "replacement", input)
+			policies := collectionPolicyScope{}
+			policies.set(Keys("base", "items"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+			higher := collectionPolicy{explicitMode: test.mode, explicitModePresent: true}
+			if test.key != "" {
+				higher.identityFields, higher.identityPresent = []string{test.key}, true
+			}
+			policies.set(Keys("child", "items"), higher)
+			application := sequencePolicy{}
+			if test.applicationKey != "" {
+				application = mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{
+					selector: sequenceIdentitySelector{literalSequenceSelector("child"), literalSequenceSelector("items")}, identityFields: []string{test.applicationKey},
+				}})
+			}
+			expanded, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, application, defaultProjectionLimits())
+			if test.conflict {
+				wantCode := CodeSequenceConflict
+				if test.applicationKey != "" && test.key == "" {
+					wantCode = CodeSequenceInvalid
+				}
+				if len(diagnostics) == 0 || diagnostics[0].Code != wantCode || diagnostics[0].Path.String() != "/child/items" && diagnostics[0].Path.String() != "/child/items/0/id" {
+					t.Fatalf("missing identity/constraint failure: %#v", diagnostics)
+				}
+				return
+			}
+			if len(diagnostics) != 0 {
+				t.Fatal(diagnostics)
+			}
+			policy, found := expanded.collectionPolicies.at(Keys("child", "items"))
+			if !found || !reflect.DeepEqual(policy.identityFields, test.wantFields) || policy.identityPresent != (test.wantFields != nil) {
+				t.Fatalf("effective YAML identity = %#v, want %v", policy, test.wantFields)
+			}
+			if test.value != "" {
+				actual, _ := documentAtPath(expanded.root, Keys("child", "items"))
+				want, _ := documentAtPath(root, Keys("child", "items"))
+				if !configurationValuesEqual(actual, want) {
+					t.Fatal("explicit replacement retained parent payload")
+				}
+			}
+		})
+	}
+}
+
+func TestLoadInheritanceReplacementUsesNewYamlKey(t *testing.T) {
+	t.Parallel()
+	loaded, err := testLoad[map[string]any](Bytes("replacement", []byte(`configkit:
+  sequences:
+    - {destination: [base, items], key: id}
+    - {destination: [child, items], key: name, mode: replace}
+base: {items: [{id: parent}]}
+child:
+  extends: base
+  items: [{name: child}]
+`)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	value, found := loaded.Explain(Keys("child", "items"))
+	if !found || value.Effective.YAML != "- name: child\n" {
+		t.Fatalf("replacement payload = %#v", value)
+	}
+}
+
+func TestInheritanceCombinesIdentifiedSequencesAtDestination(t *testing.T) {
+	root := parseItemPolicyDocument(t, "inheritance-values", `base:
+  items:
+    - id: shared
+      value: parent
+child:
+  extends: base
+  items:
+    - id: shared
+      value: child
+`)
+	modeLocation := Location{SourceID: "base-policy", Line: 4, Column: 5}
+	policy := collectionPolicy{
+		identityFields: []string{"id"}, identityPresent: true,
+		explicitMode: sequenceModeAppend, explicitModePresent: true, modeLocation: &modeLocation,
+	}
+	policies := collectionPolicyScope{}
+	policies.set(Keys("base", "items"), policy)
+
+	expanded, diagnostics := expandInheritanceWithCollectionRules(
+		root, nil, policies, sequencePolicy{}, defaultProjectionLimits(),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("expand identified inheritance: %#v", diagnostics)
+	}
+	items, found := documentAtPath(expanded.root, Keys("child", "items"))
+	if !found || items.Kind != document.Sequence || len(items.Sequence) != 1 {
+		t.Fatalf("child items = %#v, found %t; parent mode should not force an append", items, found)
+	}
+	if value, exists := documentAtPath(items.Sequence[0], Keys("value")); !exists || value.Text != "child" {
+		t.Fatalf("matched record = %#v, want complete child replacement", items.Sequence[0])
+	}
+	childPolicy, found := expanded.collectionPolicies.at(Keys("child", "items"))
+	if !found || !childPolicy.identityPresent || childPolicy.identityFields[0] != "id" {
+		t.Fatalf("inherited destination identity = %#v, found %t; scope=%#v", childPolicy, found, expanded.collectionPolicies.collections)
+	}
+	if childPolicy.explicitModePresent {
+		t.Fatalf("parent explicit mode leaked to a different destination: %#v", childPolicy)
+	}
+
+	var transfer *inheritanceOperation
+	for index := range expanded.operations {
+		if expanded.operations[index].transfer != nil && expanded.operations[index].transfer.to.String() == "/child" {
+			transfer = &expanded.operations[index]
+			break
+		}
+	}
+	if transfer == nil || len(transfer.sequences) != 1 || transfer.sequences[0].path.String() != "/child/items" || len(transfer.sequences[0].replacements) != 1 {
+		t.Fatalf("inheritance sequence projection = %#v, want one mapped replacement", transfer)
+	}
+	if origin, exists := expanded.origins["/child/items/0/value"]; !exists || len(origin.Locations) == 0 || origin.Locations[0].SourceID != "inheritance-values" {
+		t.Fatalf("replacement origin lost the child's source context: %#v", origin)
+	}
+}
+
+func TestLoadAppliesYamlCollectionIdentityDuringSiblingInheritance(t *testing.T) {
+	input := `configkit:
+  sequences:
+    - destination: [base, items]
+      key: id
+      mode: append
+base:
+  items:
+    - id: shared
+      value: parent
+child:
+  extends: base
+  items:
+    - id: shared
+      value: child
+`
+	loaded, err := Load[map[string]any](Bytes("inheritance-policy", []byte(input)), testSecretBinding())
+	if err != nil {
+		if loadError, ok := err.(*LoadError); ok {
+			t.Fatalf("load inherited collection policy: %v; diagnostics=%#v", err, loadError.Diagnostics())
+		}
+		t.Fatal(err)
+	}
+	value, found := loaded.Inspection.Explain(Keys("child", "items").Index(0).Key("value"))
+	if !found || value.Effective.YAML != "child\n" {
+		t.Fatalf("inherited sequence value = %#v, found %t", value, found)
+	}
+	if len(value.History) == 0 {
+		t.Fatal("inherited sequence replacement has no inspection history")
+	}
+	parentValueVisible, childValueVisible := false, false
+	for _, change := range value.History {
+		parentValueVisible = parentValueVisible || change.Before.YAML == "parent\n" || change.After.YAML == "parent\n"
+		childValueVisible = childValueVisible || change.After.YAML == "child\n"
+	}
+	if !parentValueVisible || !childValueVisible {
+		t.Fatalf("sequence replacement history omitted its parent or child value: %#v", value.History)
+	}
+}
+
+func TestInheritanceUsesDestinationExplicitModeAndApplicationConstraint(t *testing.T) {
+	t.Run("destination append", func(t *testing.T) {
+		root := parseItemPolicyDocument(t, "append", `base:
+  items:
+    - id: base
+child:
+  extends: base
+  items:
+    - id: child
+`)
+		policies := collectionPolicyScope{}
+		policies.set(Keys("base", "items"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+		childLocation := Location{SourceID: "child-policy", Line: 4, Column: 5}
+		policies.set(Keys("child", "items"), collectionPolicy{
+			identityFields: []string{"id"}, identityPresent: true,
+			explicitMode: sequenceModeAppend, explicitModePresent: true, modeLocation: &childLocation,
+		})
+		expanded, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, defaultProjectionLimits())
+		if len(diagnostics) != 0 {
+			t.Fatalf("expand with destination append: %#v", diagnostics)
+		}
+		items, _ := documentAtPath(expanded.root, Keys("child", "items"))
+		if len(items.Sequence) != 2 || items.Sequence[0].Mapping[0].Value.Text != "base" || items.Sequence[1].Mapping[0].Value.Text != "child" {
+			t.Fatalf("appended items = %#v", items)
+		}
+	})
+
+	t.Run("new matching namespace", func(t *testing.T) {
+		root := parseItemPolicyDocument(t, "dynamic", `base:
+  enabled: true
+child:
+  extends: base
+  extensions:
+    - id: duplicate
+    - id: duplicate
+`)
+		applicationPolicy, diagnostics := compileSequenceIdentityPolicy([]sequenceIdentityRegistration{{
+			selector: sequenceIdentitySelector{
+				{kind: sequenceSelectorLiteral, key: "child"},
+				{kind: sequenceSelectorLiteral, key: "extensions"},
+			},
+			identityFields: []string{"id"},
+		}})
+		if len(diagnostics) != 0 {
+			t.Fatalf("compile identity constraint: %#v", diagnostics)
+		}
+		_, diagnostics = expandInheritanceWithCollectionRules(root, nil, collectionPolicyScope{}, applicationPolicy, defaultProjectionLimits())
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict || diagnostics[0].Path.String() != "/child/extensions" {
+			t.Fatalf("dynamic application constraint diagnostics = %#v", diagnostics)
+		}
+	})
+}
+
+func TestInheritanceReplacementModesAndCancellation(t *testing.T) {
+	for _, test := range []struct {
+		name  string
+		value string
+		kind  document.Kind
+	}{
+		{name: "empty sequence", value: "[]", kind: document.Sequence},
+		{name: "null", value: "null", kind: document.Null},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			root := parseItemPolicyDocument(t, "replacement", "base:\n  items:\n    - id: prior\nchild:\n  extends: base\n  items: "+test.value+"\n")
+			policies := collectionPolicyScope{}
+			policies.set(Keys("base", "items"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+			policies.set(Keys("child", "items"), collectionPolicy{
+				identityFields: []string{"id"}, identityPresent: true,
+				explicitMode: sequenceModeReplace, explicitModePresent: true,
+			})
+			expanded, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, defaultProjectionLimits())
+			if len(diagnostics) != 0 {
+				t.Fatalf("expand replacement: %#v", diagnostics)
+			}
+			items, found := documentAtPath(expanded.root, Keys("child", "items"))
+			if !found || items.Kind != test.kind || (items.Kind == document.Sequence && len(items.Sequence) != 0) {
+				t.Fatalf("replacement value = %#v, found %t", items, found)
+			}
+		})
+	}
+
+	for _, value := range []string{"null", "{}"} {
+		t.Run("append rejects "+value, func(t *testing.T) {
+			root := parseItemPolicyDocument(t, "append-kind", "base:\n  items: [{id: prior}]\nchild:\n  extends: base\n  items: "+value+"\n")
+			policies := collectionPolicyScope{}
+			policies.set(Keys("base", "items"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+			policies.set(Keys("child", "items"), collectionPolicy{
+				identityFields: []string{"id"}, identityPresent: true,
+				explicitMode: sequenceModeAppend, explicitModePresent: true,
+			})
+			_, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, defaultProjectionLimits())
+			if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid || diagnostics[0].Path.String() != "/child/items" {
+				t.Fatalf("append kind diagnostics = %#v", diagnostics)
+			}
+		})
+	}
+
+	t.Run("incompatible inherited identity", func(t *testing.T) {
+		root := parseItemPolicyDocument(t, "identity", `base:
+  items: [{id: one}]
+child:
+  extends: base
+  items: [{id: two}]
+`)
+		policies := collectionPolicyScope{}
+		policies.set(Keys("base", "items"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+		policies.set(Keys("child", "items"), collectionPolicy{identityFields: []string{"slug"}, identityPresent: true})
+		_, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, defaultProjectionLimits())
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict || diagnostics[0].Path.String() != "/child/items" {
+			t.Fatalf("identity conflict diagnostics = %#v", diagnostics)
+		}
+	})
+
+	t.Run("cancelled relationship", func(t *testing.T) {
+		root := parseItemPolicyDocument(t, "cancel", `base:
+  items: [{id: parent}]
+child:
+  extends: null
+  items: [{id: local}]
+`)
+		policies := collectionPolicyScope{}
+		policies.set(Keys("base", "items"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+		expanded, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, defaultProjectionLimits())
+		if len(diagnostics) != 0 {
+			t.Fatalf("expand cancelled relationship: %#v", diagnostics)
+		}
+		if _, found := expanded.collectionPolicies.at(Keys("child", "items")); found {
+			t.Fatal("cancelled relationship transferred the parent's collection policy")
+		}
+	})
+}
+
+func TestInheritanceReplacementRetiresRecordLocalCollectionPolicy(t *testing.T) {
+	root := parseItemPolicyDocument(t, "record-scope", `base:
+  items:
+    - id: shared
+      children:
+        - key: old
+child:
+  extends: base
+  items:
+    - id: shared
+      children:
+        - key: new
+`)
+	nested := collectionPolicy{identityFields: []string{"key"}, identityPresent: true, explicitMode: sequenceModeAppend, explicitModePresent: true}
+	recordScope := collectionPolicyScope{}
+	recordScope.set(Keys("children"), nested)
+	outer := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+	outer.setRecordPolicyAt(0, newCollectionRecordPolicy())
+	owner := outer.recordOrder[0]
+	outer.recordPolicies[owner] = recordScope
+	policies := collectionPolicyScope{}
+	policies.set(Keys("base", "items"), outer)
+
+	expanded, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("expand replacement record: %#v", diagnostics)
+	}
+	children, found := documentAtPath(expanded.root, Keys("child", "items").Index(0).Key("children"))
+	if !found || children.Kind != document.Sequence || len(children.Sequence) != 1 {
+		t.Fatalf("replacement children = %#v, found %t", children, found)
+	}
+	if key, exists := documentAtPath(children.Sequence[0], Keys("key")); !exists || key.Text != "new" {
+		t.Fatalf("replacement child = %#v, want only new record", children.Sequence[0])
+	}
+	childPolicy, exists := expanded.collectionPolicies.at(Keys("child", "items"))
+	if !exists {
+		t.Fatal("inherited outer collection policy is missing")
+	}
+	record, found := childPolicy.recordPolicyAt(0)
+	if !found {
+		t.Fatal("replacement item has no independent record owner")
+	}
+	if nestedPolicy, exists := record.scope.at(Keys("children")); exists && nestedPolicy.explicitModePresent {
+		t.Fatalf("replaced record retained its former nested operation: %#v", nestedPolicy)
+	}
+}

internal/engine/inheritance_explain.go

@@ -25,6 +25,12 @@ func appendInheritanceExplanation(explanation *Explanation, working **document.N
 		}
 		before := valueSnapshot(*working, query, secrets)
 		replaceHistoryPath(working, operation.transfer.to, operation.parent)
+		// A record keeps its own identity throughout inheritance. Restore it
+		// before taking enclosing snapshots as well as direct field snapshots.
+		for _, field := range operation.identityFields {
+			identity, _ := documentAtPath(operation.effective, Keys(field))
+			replaceHistoryPath(working, operation.transfer.to.Key(field), identity)
+		}
 		afterParent := valueSnapshot(*working, query, secrets)
 		parent, parentPresent := transferredOperationOrigin(operation, operationPathForQuery(operation.transfer.to, query))
 		if pathsIntersect(operation.transfer.to, query) && parentPresent {

internal/engine/inheritance_history.go

@@ -1,6 +1,7 @@
 package engine
 
 import (
+	"slices"
 	"sort"
 
 	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
@@ -73,13 +74,13 @@ func applyInheritanceMetadata(metadata *Metadata, raw *document.Node, rawOrigins
 	})
 	applyOverlayMetadata(&sourceView, overlays)
 	metadata.history = sourceView.history
-	metadata.inheritance = append([]inheritanceOperation(nil), expansion.operations...)
+	metadata.inheritance = cloneInheritanceOperations(expansion.operations)
 }
 
 func historyWithInheritance(path Path, stored []Change, operations []inheritanceOperation) []Change {
 	split := len(stored)
 	for index, change := range stored {
-		if change.Origin.Layer == LayerResolution {
+		if change.Origin.Layer == LayerCLI || change.Origin.Layer == LayerResolution {
 			split = index
 			break
 		}
@@ -114,7 +115,7 @@ func projectedInheritanceChanges(query Path, operation inheritanceOperation) []C
 		return []Change{{Path: operation.control.path.clone(), Origin: origin,
 			Dependencies: []Path{operation.control.path.clone()}}}
 	}
-	if operation.transfer == nil || !pathsIntersect(operation.transfer.to, query) {
+	if operation.transfer == nil || !pathsIntersect(operation.transfer.to, query) || inheritanceOperationIdentity(operation, query) {
 		return nil
 	}
 	path := query
@@ -155,6 +156,9 @@ func projectedInheritanceChanges(query Path, operation inheritanceOperation) []C
 }
 
 func transferredOperationOrigin(operation inheritanceOperation, destination Path) (Origin, bool) {
+	if inheritanceOperationIdentity(operation, destination) {
+		return Origin{}, false
+	}
 	relative, ok := relativeHistoryPath(operation.transfer.to, destination)
 	if !ok {
 		return Origin{}, false
@@ -168,3 +172,12 @@ func transferredOperationOrigin(operation inheritanceOperation, destination Path
 		operation.transfer.from, operation.transfer.to, *operation.transfer)
 	return transferred[destination.String()], true
 }
+
+func inheritanceOperationIdentity(operation inheritanceOperation, path Path) bool {
+	if operation.transfer == nil || len(operation.identityFields) == 0 {
+		return false
+	}
+	relative, within := relativeHistoryPath(operation.transfer.to, path)
+	return within && len(relative.segments) != 0 && relative.segments[0].kind != pathSegmentIndex &&
+		slices.Contains(operation.identityFields, relative.segments[0].key)
+}

internal/engine/inheritance_history_test.go

@@ -126,7 +126,7 @@ func inheritanceInspectionFromComposition(t *testing.T, composed *composedDocume
 	applyInheritanceMetadata(&metadata, composed.root, composed.origins, composed.overlays, expanded)
 	finalizeMetadataPaths(&metadata, expanded.root, composed.applications, baseline, composed.root)
 	return &Inspection{
-		metadata: metadata, baseline: baseline, input: expanded.root,
+		metadata: metadata, baseline: baseline, input: expanded.root, preCLI: expanded.root,
 		inputOrigins: expanded.origins, final: expanded.root,
 		applications: composed.applications, inheritance: expanded.operations,
 	}, expanded

internal/engine/inheritance_identified.go

@@ -0,0 +1,172 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+type identifiedInheritanceTemplate struct {
+	item          *document.Node
+	path          Path
+	tuple         string
+	parent        int
+	hasParent     bool
+	cancelled     bool
+	directive     *document.Node
+	directivePath Path
+}
+
+type identifiedInheritanceGraph struct {
+	scope      *document.Node
+	scopePath  Path
+	fields     []string
+	nodes      []identifiedInheritanceTemplate
+	edges      []dependencyEdge
+	byIdentity map[string]int
+}
+
+func sequenceHasIdentifiedInheritanceReferences(sequence *document.Node) bool {
+	if sequence == nil || sequence.Kind != document.Sequence {
+		return false
+	}
+	for _, item := range sequence.Sequence {
+		if directive, found := inheritanceDirective(item); found && directive.Value.Kind != document.Null {
+			return true
+		}
+	}
+	return false
+}
+
+// analyzeIdentifiedInheritanceGraph validates relationships inside one
+// identified sequence. It prepares a graph only; mapping-sibling expansion
+// remains the active inheritance behavior until the reusable scope executor is
+// activated.
+func analyzeIdentifiedInheritanceGraph(sequence *document.Node, scopePath Path, identityFields []string) (*identifiedInheritanceGraph, []Diagnostic) {
+	if sequence == nil || sequence.Kind != document.Sequence {
+		var primary *Location
+		if sequence != nil {
+			location := publicLocation(sequence.Location)
+			primary = &location
+		}
+		return nil, []Diagnostic{{
+			Code: CodeSequenceInvalid, Severity: SeverityError, Path: scopePath.clone(),
+			Message: "identified inheritance scope must be a sequence", Primary: primary,
+		}}
+	}
+	fields, err := normalizeSequenceIdentityFields(identityFields)
+	if err != nil {
+		location := publicLocation(sequence.Location)
+		return nil, []Diagnostic{{
+			Code: CodeSequenceInvalid, Severity: SeverityError, Path: scopePath.clone(),
+			Message: "identified inheritance scope requires identity fields", Primary: &location,
+		}}
+	}
+	lookup, diagnostics := validateNormalizedSequenceIdentityFrom(
+		sequence, scopePath, fields, sequenceCombinationIdentityCodes, 0, sequenceStructuralRecord,
+	)
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+	graph := &identifiedInheritanceGraph{
+		scope: sequence, scopePath: scopePath.clone(), fields: cloneStrings(fields),
+		nodes:      make([]identifiedInheritanceTemplate, len(sequence.Sequence)),
+		byIdentity: make(map[string]int, len(lookup)),
+	}
+	for tuple, record := range lookup {
+		graph.byIdentity[tuple] = record.index
+		graph.nodes[record.index] = identifiedInheritanceTemplate{
+			item: sequence.Sequence[record.index], path: scopePath.Index(record.index), tuple: tuple,
+		}
+	}
+
+	for index := range graph.nodes {
+		node := &graph.nodes[index]
+		directive, found := inheritanceDirective(node.item)
+		if !found {
+			continue
+		}
+		node.directive = directive.Value
+		node.directivePath = node.path.Key(inheritanceKey)
+		referenceLocation := publicLocation(directive.Value.Location)
+		switch directive.Value.Kind {
+		case document.Null:
+			node.cancelled = true
+		case document.Mapping:
+			reference, issues := identifiedInheritanceReference(directive.Value, node.directivePath, fields)
+			if len(issues) != 0 {
+				return nil, issues
+			}
+			parent, exists := graph.byIdentity[reference]
+			if !exists {
+				return nil, []Diagnostic{inheritanceDiagnostic(
+					CodeInheritanceMissing, node.directivePath, referenceLocation, nil,
+					"referenced record does not exist in this collection",
+				)}
+			}
+			node.parent = parent
+			node.hasParent = true
+			graph.edges = append(graph.edges, dependencyEdge{prerequisite: parent, dependent: index})
+		default:
+			return nil, []Diagnostic{inheritanceDiagnostic(
+				CodeInheritanceInvalid, node.directivePath, referenceLocation, nil,
+				"identified record inheritance reference must be a mapping or null",
+			)}
+		}
+	}
+	return graph, nil
+}
+
+func identifiedInheritanceReference(reference *document.Node, path Path, identityFields []string) (string, []Diagnostic) {
+	allowed := make(map[string]struct{}, len(identityFields))
+	for _, field := range identityFields {
+		allowed[field] = struct{}{}
+	}
+	seen := make(map[string]struct{}, len(reference.Mapping))
+	for _, entry := range reference.Mapping {
+		if _, ok := allowed[entry.Key]; !ok {
+			return "", []Diagnostic{inheritanceDiagnostic(
+				CodeInheritanceInvalid, path.Key(entry.Key), publicLocation(entry.Value.Location), nil,
+				"identified record inheritance reference has an unsupported field",
+			)}
+		}
+		seen[entry.Key] = struct{}{}
+	}
+	for _, field := range identityFields {
+		if _, ok := seen[field]; !ok {
+			return "", []Diagnostic{inheritanceDiagnostic(
+				CodeInheritanceInvalid, path.Key(field), publicLocation(reference.Location), nil,
+				"identified record inheritance reference is missing an identity field",
+			)}
+		}
+	}
+	index := newSequenceMappingIndex(reference, sequencePayloadRecord)
+	tuple, _, missing, invalid := index.identity(identityFields)
+	if missing != "" || invalid >= 0 || invalid == -2 {
+		value := reference
+		fieldPath := path.Key(missing)
+		if invalid >= 0 && invalid < len(reference.Mapping) {
+			value = reference.Mapping[invalid].Value
+			fieldPath = path.Key(reference.Mapping[invalid].Key)
+		}
+		location := publicLocation(value.Location)
+		return "", []Diagnostic{inheritanceDiagnostic(
+			CodeInheritanceInvalid, fieldPath, location, nil,
+			"identified record inheritance identity fields must be scalar values",
+		)}
+	}
+	return tuple, nil
+}
+
+func (graph *identifiedInheritanceGraph) schedule() ([]int, []Diagnostic) {
+	keys := make([]dependencyKey, len(graph.nodes))
+	for index, node := range graph.nodes {
+		keys[index] = dependencyKey{"record", graph.scopePath.String(), node.path.String()}
+	}
+	order, cycle := scheduleDependencies(keys, graph.edges)
+	if len(cycle) == 0 {
+		return order, nil
+	}
+	parent, child := graph.nodes[cycle[0]], graph.nodes[cycle[1]]
+	return nil, []Diagnostic{inheritanceDiagnostic(
+		CodeInheritanceCycle, child.directivePath, publicLocation(child.directive.Location),
+		[]Location{publicLocation(parent.item.Location)},
+		"identified record inheritance contains a cycle",
+	)}
+}

internal/engine/inheritance_identified_test.go

@@ -0,0 +1,373 @@
+package engine
+
+import (
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func parseIdentifiedInheritanceSequence(t *testing.T, sourceID, input string) *document.Node {
+	t.Helper()
+	var indented strings.Builder
+	indented.WriteString("jobs:\n")
+	for _, line := range strings.Split(strings.TrimSuffix(input, "\n"), "\n") {
+		indented.WriteString("  ")
+		indented.WriteString(line)
+		indented.WriteByte('\n')
+	}
+	root := parseItemPolicyDocument(t, sourceID, indented.String())
+	sequence, found := documentAtPath(root, Keys("jobs"))
+	if !found || sequence.Kind != document.Sequence {
+		t.Fatalf("identified sequence missing from test document: %#v", root)
+	}
+	return sequence
+}
+
+func TestIdentifiedInheritanceGraphSchedulesForwardReferencesInPlace(t *testing.T) {
+	sequence := parseIdentifiedInheritanceSequence(t, "identified-graph", `- id: 2
+  tenant: acme
+  extends: {id: 1.0, tenant: acme}
+- id: 1e0
+  tenant: acme
+  extends: {id: 0, tenant: acme}
+- id: 0
+  tenant: acme
+- id: 3
+  tenant: acme
+  extends: null
+`)
+	path := Keys("jobs")
+	graph, diagnostics := analyzeIdentifiedInheritanceGraph(sequence, path, []string{"tenant", "id"})
+	if len(diagnostics) != 0 {
+		t.Fatalf("analyze identified graph: %#v", diagnostics)
+	}
+	if len(graph.edges) != 2 {
+		t.Fatalf("edges = %#v, want the two links in the inheritance chain", graph.edges)
+	}
+	order, diagnostics := graph.schedule()
+	if len(diagnostics) != 0 {
+		t.Fatalf("schedule identified graph: %#v", diagnostics)
+	}
+	if len(order) != 4 || order[0] != 2 || order[1] != 1 || order[2] != 0 {
+		t.Fatalf("schedule = %v, want the forward parent chain before its children", order)
+	}
+	if graph.nodes[0].path.String() != "/jobs/0" || graph.nodes[0].item != sequence.Sequence[0] ||
+		!graph.nodes[3].cancelled || len(graph.nodes[3].directivePath.segments) == 0 {
+		t.Fatalf("graph nodes changed authored positions or cancellation: %#v", graph.nodes)
+	}
+	if _, found := graph.byIdentity[graph.nodes[1].tuple]; !found {
+		t.Fatal("identity index does not contain the parent record")
+	}
+}
+
+func TestIdentifiedInheritanceScopeAdapterReusesTemplateExecution(t *testing.T) {
+	sequence := parseIdentifiedInheritanceSequence(t, "identified-scope", `- id: child-a
+  tenant: team-a
+  extends: {id: parent-a, tenant: team-a}
+  settings: {child: a}
+- id: parent-a
+  tenant: team-a
+  settings: {parent: a}
+- id: child-b
+  tenant: team-b
+  extends: {id: parent-b, tenant: team-b}
+  settings: {child: b}
+- id: parent-b
+  tenant: team-b
+  settings: {parent: b}
+`)
+	path := Keys("jobs")
+	graph, diagnostics := analyzeIdentifiedInheritanceGraph(sequence, path, []string{"tenant", "id"})
+	if len(diagnostics) != 0 {
+		t.Fatalf("analyze identified graph: %#v", diagnostics)
+	}
+	policies := collectionPolicyScope{}
+	policies.set(path.Index(1).Key("steps"), collectionPolicy{
+		identityFields: []string{"step-id"}, identityPresent: true,
+	})
+	result, diagnostics := applyIdentifiedInheritanceTemplates(graph, defaultProjectionLimits(), nil, policies, sequencePolicy{})
+	if len(diagnostics) != 0 {
+		t.Fatalf("apply identified templates: %#v", diagnostics)
+	}
+	if got := len(result.root.Sequence); got != 4 {
+		t.Fatalf("result sequence length = %d, want 4", got)
+	}
+	for index, want := range []string{"child-a", "parent-a", "child-b", "parent-b"} {
+		id, found := documentAtPath(result.root.Sequence[index], Path{}.Key("id"))
+		if !found || id.Text != want {
+			t.Fatalf("result identity at index %d = %#v, found %t; want %q", index, id, found, want)
+		}
+	}
+	for index, want := range []string{"a", "b"} {
+		value, found := documentAtPath(result.root.Sequence[index*2], Path{}.Key("settings").Key("parent"))
+		if !found || value.Text != want {
+			t.Fatalf("inherited value at record %d = %#v, found %t; want %q", index*2, value, found, want)
+		}
+	}
+	if len(result.transfers) != 2 || result.transfers[0].from.String() != "/jobs/1" || result.transfers[0].to.String() != "/jobs/0" ||
+		result.transfers[1].from.String() != "/jobs/3" || result.transfers[1].to.String() != "/jobs/2" {
+		t.Fatalf("record transfers = %#v, want independent transfers at authored positions", result.transfers)
+	}
+	if len(result.controls) != 2 || result.controls[0].path.String() != "/jobs/0/extends" || result.controls[1].path.String() != "/jobs/2/extends" {
+		t.Fatalf("relationship controls = %#v, want paths from child positions", result.controls)
+	}
+	if len(result.operations) != 4 || result.operations[1].transfer == nil || result.operations[1].transfer.to.String() != "/jobs/0" ||
+		result.operations[3].transfer == nil || result.operations[3].transfer.to.String() != "/jobs/2" {
+		t.Fatalf("relationship history operations = %#v, want transfer evidence at child positions", result.operations)
+	}
+	if _, found := result.origins[path.Index(0).Key("id").String()]; !found {
+		t.Fatal("resolved sequence output did not retain the child identity origin path")
+	}
+	projectedPolicy, found := result.collectionPolicies.at(path.Index(0).Key("steps"))
+	if !found || !projectedPolicy.identityPresent || len(projectedPolicy.identityFields) != 1 || projectedPolicy.identityFields[0] != "step-id" {
+		t.Fatalf("inherited nested collection policy = %#v, found %t", projectedPolicy, found)
+	}
+	adapter := identifiedSequenceInheritanceScope{graph: graph, policies: policies}
+	fields := adapter.templates()[0].identityFields
+	if len(fields) != 2 || fields[0] != "id" || fields[1] != "tenant" {
+		t.Fatalf("payload identity fields = %v, want [id tenant]", fields)
+	}
+	templates := adapter.templates()
+	parentPolicy, found := templates[1].collectionPolicies.at(Keys("steps"))
+	if !found || len(parentPolicy.identityFields) != 1 || parentPolicy.identityFields[0] != "step-id" {
+		t.Fatalf("parent nested policy = %#v, found %t", parentPolicy, found)
+	}
+	if _, found := templates[0].collectionPolicies.at(Keys("steps")); found {
+		t.Fatal("nested policy from one record leaked into a sibling template")
+	}
+	parentPolicy.identityFields[0] = "changed"
+	templates[1].collectionPolicies.set(Keys("steps"), parentPolicy)
+	reused, found := adapter.templates()[1].collectionPolicies.at(Keys("steps"))
+	if !found || len(reused.identityFields) != 1 || reused.identityFields[0] != "step-id" {
+		t.Fatal("reusing the adapter observed a mutation to a prior template snapshot")
+	}
+	if sequence.Sequence[0].Mapping[0].Value.Text != "child-a" {
+		t.Fatal("executing the sequence adapter mutated its source scope")
+	}
+}
+
+func TestIdentifiedInheritanceResolvesNestedCollectionsAtEachDestination(t *testing.T) {
+	root := parseItemPolicyDocument(t, "identified-nested", `jobs:
+  - id: child
+    extends: {id: parent}
+  - id: parent
+    extends: {id: base}
+    steps:
+      - id: child-step
+        extends: {id: parent-step}
+        value: child-value
+      - id: parent-step
+        value: parent-value
+  - id: base
+    steps:
+      - id: base-step
+        value: base-value
+`)
+	identity := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+	policies := collectionPolicyScope{}
+	policies.set(Keys("jobs"), identity)
+	for index := 0; index < 3; index++ {
+		policies.set(Keys("jobs").Index(index).Key("steps"), identity)
+	}
+	expanded, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("expand nested identified inheritance: %#v", diagnostics)
+	}
+	steps, found := documentAtPath(expanded.root, Keys("jobs").Index(0).Key("steps"))
+	if !found || steps.Kind != document.Sequence || len(steps.Sequence) != 3 {
+		t.Fatalf("child nested steps = %#v, found %t", steps, found)
+	}
+	for index, want := range []string{"base-step", "child-step", "parent-step"} {
+		id, exists := documentAtPath(steps.Sequence[index], Keys("id"))
+		if !exists || id.Text != want {
+			t.Fatalf("child step %d identity = %#v, found %t; want %q", index, id, exists, want)
+		}
+	}
+	value, found := documentAtPath(steps.Sequence[1], Keys("value"))
+	if !found || value.Text != "child-value" {
+		t.Fatalf("nested record inheritance = %#v, found %t", value, found)
+	}
+	if containsInheritanceDirective(expanded.root) {
+		t.Fatal("a nested record relationship remained after destination-local expansion")
+	}
+	childIdentity, found := expanded.origins["/jobs/0/id"]
+	if !found || len(childIdentity.Locations) == 0 || childIdentity.Locations[0].SourceID != "identified-nested" {
+		t.Fatalf("child identity origin = %#v, found %t", childIdentity, found)
+	}
+	if _, found := expanded.origins["/jobs/0/steps/1/value"]; !found {
+		t.Fatal("nested inherited value did not retain its destination origin")
+	}
+	for _, replacement := range expanded.replacements {
+		if replacement.path.String() == "/jobs/0/id" {
+			t.Fatal("parent identity was treated as payload replaced by the child's identity")
+		}
+	}
+}
+
+func TestIdentifiedInheritanceRespectsOutputNodeBudget(t *testing.T) {
+	root := parseItemPolicyDocument(t, "identified-budget", `jobs:
+  - id: child
+    extends: {id: parent}
+  - id: parent
+    value: inherited
+`)
+	policies := collectionPolicyScope{}
+	policies.set(Keys("jobs"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+	baseline, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("unbounded identified inheritance: %#v", diagnostics)
+	}
+	outputNodes, within := inheritanceLowerBoundCount(baseline.root, -1)
+	if !within || outputNodes < 2 {
+		t.Fatalf("identified output node count = %d, within %t", outputNodes, within)
+	}
+	limits := defaultProjectionLimits()
+	limits.nodes = outputNodes
+	if result, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, limits); result == nil || len(diagnostics) != 0 {
+		t.Fatalf("at output-node boundary: result = %#v, diagnostics = %#v", result, diagnostics)
+	}
+	limits.nodes = outputNodes - 1
+	if result, diagnostics := expandInheritanceWithCollectionRules(root, nil, policies, sequencePolicy{}, limits); result != nil ||
+		len(diagnostics) != 1 || diagnostics[0].Code != CodeSchemaProjectionLimit {
+		t.Fatalf("below output-node boundary: result = %#v, diagnostics = %#v", result, diagnostics)
+	}
+}
+
+func TestIdentifiedInheritanceRequiresChildIdentityBeforeParentLookup(t *testing.T) {
+	sequence := parseIdentifiedInheritanceSequence(t, "missing-child-field", `- id: child
+  extends: {id: parent, tenant: acme}
+- id: parent
+  tenant: acme
+`)
+	_, diagnostics := analyzeIdentifiedInheritanceGraph(sequence, Keys("jobs"), []string{"id", "tenant"})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid || diagnostics[0].Path.String() != "/jobs/0/tenant" {
+		t.Fatalf("missing child identity diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestIdentifiedInheritanceRejectsMalformedAndMissingReferences(t *testing.T) {
+	tests := []struct {
+		name     string
+		value    string
+		wantCode Code
+	}{
+		{name: "string shorthand", value: "extends: parent\n", wantCode: CodeInheritanceInvalid},
+		{name: "sequence shorthand", value: "extends: [{id: parent, tenant: acme}]\n", wantCode: CodeInheritanceInvalid},
+		{name: "missing component", value: "extends: {id: parent}\n", wantCode: CodeInheritanceInvalid},
+		{name: "extra component", value: "extends: {id: parent, tenant: acme, extra: value}\n", wantCode: CodeInheritanceInvalid},
+		{name: "mapping component", value: "extends: {id: {nested: value}, tenant: acme}\n", wantCode: CodeInheritanceInvalid},
+		{name: "missing parent", value: "extends: {id: missing, tenant: acme}\n", wantCode: CodeInheritanceMissing},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			sequence := parseIdentifiedInheritanceSequence(t, "bad-reference", "- id: child\n  tenant: acme\n  "+test.value)
+			_, diagnostics := analyzeIdentifiedInheritanceGraph(sequence, Keys("jobs"), []string{"id", "tenant"})
+			if len(diagnostics) != 1 || diagnostics[0].Code != test.wantCode || diagnostics[0].Path.String() != "/jobs/0/extends" && diagnostics[0].Code != CodeInheritanceInvalid {
+				t.Fatalf("diagnostics = %#v", diagnostics)
+			}
+			if diagnostics[0].Code == CodeInheritanceMissing && diagnostics[0].Path.String() != "/jobs/0/extends" {
+				t.Fatalf("missing-parent path = %s", diagnostics[0].Path)
+			}
+		})
+	}
+}
+
+func TestIdentifiedInheritanceGraphFindsSelfAndLongerCycles(t *testing.T) {
+	for _, input := range []string{
+		`- id: self
+  tenant: acme
+  extends: {id: self, tenant: acme}
+`,
+		`- id: alpha
+  tenant: acme
+  extends: {id: beta, tenant: acme}
+- id: beta
+  tenant: acme
+  extends: {id: gamma, tenant: acme}
+- id: gamma
+  tenant: acme
+  extends: {id: alpha, tenant: acme}
+`,
+	} {
+		graph, diagnostics := analyzeIdentifiedInheritanceGraph(parseIdentifiedInheritanceSequence(t, "cycle", input), Keys("jobs"), []string{"id", "tenant"})
+		if len(diagnostics) != 0 {
+			t.Fatalf("analyze cycle graph: %#v", diagnostics)
+		}
+		_, diagnostics = graph.schedule()
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeInheritanceCycle || diagnostics[0].Path.String() == "" {
+			t.Fatalf("cycle diagnostics = %#v", diagnostics)
+		}
+		if diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "cycle" {
+			t.Fatalf("cycle primary location = %#v", diagnostics[0].Primary)
+		}
+	}
+}
+
+func TestIdentifiedInheritanceScopesCannotReferenceOtherCollections(t *testing.T) {
+	other := parseIdentifiedInheritanceSequence(t, "other-collection", `- id: parent
+  tenant: acme
+`)
+	_, diagnostics := analyzeIdentifiedInheritanceGraph(other, Keys("other"), []string{"id", "tenant"})
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	first := parseIdentifiedInheritanceSequence(t, "first-collection", `- id: child
+  tenant: acme
+  extends: {id: parent, tenant: acme}
+`)
+	_, diagnostics = analyzeIdentifiedInheritanceGraph(first, Keys("first"), []string{"id", "tenant"})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeInheritanceMissing {
+		t.Fatalf("cross-collection reference diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestIdentifiedInheritanceAcceptsStrictlyComposedParentReference(t *testing.T) {
+	left := parseCompositionDocument(t, "reference-left", "extends: {id: parent}\n")
+	right := parseCompositionDocument(t, "reference-right", "extends: {tenant: acme}\n")
+	reference, diagnostics := composeDocuments(left, right)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose disjoint reference helpers: %#v", diagnostics)
+	}
+	child, diagnostics := composeDocuments(parseCompositionDocument(t, "child-fields", "id: child\ntenant: acme\n"), reference.root)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose child fields and reference: %#v", diagnostics)
+	}
+	parent := parseCompositionDocument(t, "parent", "id: parent\ntenant: acme\n")
+	sequence := &document.Node{Kind: document.Sequence, Sequence: []*document.Node{child.root, parent}}
+	graph, diagnostics := analyzeIdentifiedInheritanceGraph(sequence, Keys("jobs"), []string{"id", "tenant"})
+	if len(diagnostics) != 0 {
+		t.Fatalf("analyze composed reference: %#v", diagnostics)
+	}
+	if len(graph.edges) != 1 || graph.edges[0] != (dependencyEdge{prerequisite: 1, dependent: 0}) {
+		t.Fatalf("composed reference edge = %#v", graph.edges)
+	}
+
+	_, diagnostics = composeDocuments(
+		parseCompositionDocument(t, "overlap-a", "extends: {id: first}\n"),
+		parseCompositionDocument(t, "overlap-b", "extends: {id: second}\n"),
+	)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeConflict {
+		t.Fatalf("overlapping reference fields should conflict during strict composition: %#v", diagnostics)
+	}
+}
+
+func TestIdentifiedInheritanceValidatesOnlySurvivingRelationships(t *testing.T) {
+	lower := parseIdentifiedInheritanceSequence(t, "superseded", `- id: shared
+  extends: {id: absent, tenant: acme}
+`)
+	higher := parseIdentifiedInheritanceSequence(t, "cancelled", `- id: shared
+  extends: null
+`)
+	resolution := collectionPolicyResolution{
+		operation: sequenceModeUpsert, effectiveIdentityFields: []string{"id"},
+	}
+	combined, diagnostics := combineSequence(lower, higher, resolution, sequenceCombinationContext{destination: Keys("jobs")})
+	if len(diagnostics) != 0 {
+		t.Fatalf("combine surviving reference: %#v", diagnostics)
+	}
+	graph, diagnostics := analyzeIdentifiedInheritanceGraph(combined.value, Keys("jobs"), []string{"id"})
+	if len(diagnostics) != 0 || len(graph.edges) != 0 || !graph.nodes[0].cancelled {
+		t.Fatalf("superseded missing reference was validated: graph=%#v diagnostics=%#v", graph, diagnostics)
+	}
+}

internal/engine/inheritance_identity_history_test.go

@@ -0,0 +1,87 @@
+package engine
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestRecordInheritanceHistoryPreservesChildIdentity(t *testing.T) {
+	t.Parallel()
+	for _, compound := range []bool{false, true} {
+		t.Run(fmt.Sprintf("compound=%t", compound), func(t *testing.T) {
+			key, reference := "id", "{id: base}"
+			identities := map[string]string{"id": "child"}
+			if compound {
+				key, reference = "[id, tenant]", "{id: base, tenant: parent-team}"
+				identities["tenant"] = "child-team"
+			}
+			body := "configkit:\n  sequences: [{destination: [items], key: " + key + "}]\nitems:\n  - id: base\n    tenant: parent-team\n    value: inherited\n  - id: child\n    tenant: child-team\n    extends: " + reference + "\n"
+			loaded, err := testLoad(Bytes("records", []byte(body)), Options[map[string]any]{})
+			if err != nil {
+				t.Fatal(err)
+			}
+			child := Keys("items").Index(1)
+			for field, want := range identities {
+				path := child.Key(field)
+				info, found := loaded.Metadata().Lookup(path)
+				if !found || !info.Explicit || len(info.Origin.Inheritance) != 0 {
+					t.Fatalf("child identity provenance = %#v, found %t", info, found)
+				}
+				for _, change := range loaded.Metadata().History(path) {
+					if change.Origin.Operation == ChangeInheritance {
+						t.Fatalf("child identity received inheritance history: %#v", change)
+					}
+				}
+				explanation, found := loaded.Explain(path)
+				if !found {
+					t.Fatalf("missing identity explanation at %s", path)
+				}
+				for _, change := range explanation.History {
+					for _, snapshot := range []ValueSnapshot{change.Before, change.After} {
+						if snapshot.Present && snapshot.YAML != want+"\n" {
+							t.Fatalf("child identity snapshot = %q, want %q", snapshot.YAML, want)
+						}
+					}
+				}
+			}
+			for _, query := range []Path{child, Keys("items"), {}} {
+				explanation, found := loaded.Explain(query)
+				if !found {
+					t.Fatalf("missing ancestor explanation at %s", query)
+				}
+				relative, _ := relativeHistoryPath(query, child)
+				for _, change := range explanation.History {
+					for _, snapshot := range []ValueSnapshot{change.Before, change.After} {
+						if !snapshot.Present {
+							continue
+						}
+						wrapped := "snapshot:\n  " + strings.ReplaceAll(snapshot.YAML, "\n", "\n  ") + "\n"
+						root := parseItemPolicyDocument(t, "history", wrapped)
+						for field, want := range identities {
+							value, present := documentAtPath(root, appendDestination(Keys("snapshot"), relative.Key(field)))
+							if present && value.Text != want {
+								t.Fatalf("ancestor %s snapshot has child %s = %q", query, field, value.Text)
+							}
+						}
+					}
+				}
+			}
+			payload, found := loaded.Explain(child.Key("value"))
+			if !found || payload.Effective.YAML != "inherited\n" {
+				t.Fatalf("inherited payload = %#v, found %t", payload, found)
+			}
+			inherited := false
+			for _, change := range payload.History {
+				inherited = inherited || change.Origin.Operation == ChangeInheritance
+			}
+			if !inherited {
+				t.Fatal("ordinary inherited payload lost its history")
+			}
+			parent, found := loaded.Explain(Keys("items").Index(0).Key("id"))
+			if !found || parent.Effective.YAML != "base\n" || len(parent.History) == 0 {
+				t.Fatalf("parent identity history = %#v, found %t", parent, found)
+			}
+		})
+	}
+}

internal/engine/inheritance_load.go

@@ -27,11 +27,12 @@ func containsInheritanceDirective(root *document.Node) bool {
 	return visit(root)
 }
 
-func inheritanceVersionDiagnostic(rule *VersionRule, expanded *inheritanceScope) *Diagnostic {
+func projectInheritanceVersion(rule *VersionRule, expanded *inheritanceScope) (*VersionRule, *Diagnostic) {
 	if rule == nil || expanded == nil {
-		return nil
+		return rule, nil
 	}
 	for _, operation := range expanded.operations {
+		rule = projectVersionRule(rule, operation.sequences)
 		var location Location
 		var affected bool
 		if operation.control != nil {
@@ -50,18 +51,18 @@ func inheritanceVersionDiagnostic(rule *VersionRule, expanded *inheritanceScope)
 			}
 		}
 		if affected {
-			return &Diagnostic{
+			return rule, &Diagnostic{
 				Code: CodeInheritanceVersion, Severity: SeverityError, Path: rule.Path.clone(),
 				Message: "inheritance cannot change the schema-version path", Primary: &location,
 			}
 		}
 	}
-	return nil
+	return rule, nil
 }
 
 func inheritanceOperations(expanded *inheritanceScope) []inheritanceOperation {
 	if expanded == nil {
 		return nil
 	}
-	return expanded.operations
+	return cloneInheritanceOperations(expanded.operations)
 }

internal/engine/inheritance_origin.go

@@ -70,8 +70,14 @@ func mergeInheritanceOrigin(origins map[string]Origin, path Path, incoming Origi
 }
 
 func copyInheritanceSubtree(target, incoming map[string]Origin, node *document.Node, root Path) {
-	walkDocument(node, root, func(path Path, _ *document.Node) {
-		target[path.String()] = cloneOrigin(incoming[path.String()])
+	copyInheritanceSubtreeAt(target, incoming, node, root, root)
+}
+
+func copyInheritanceSubtreeAt(target, incoming map[string]Origin, node *document.Node, from, to Path) {
+	walkDocument(node, Path{}, func(relative Path, _ *document.Node) {
+		source := appendDestination(from, relative)
+		destination := appendDestination(to, relative)
+		target[destination.String()] = cloneOrigin(incoming[source.String()])
 	})
 }
 

internal/engine/inheritance_policy_index.go

@@ -0,0 +1,123 @@
+package engine
+
+// inheritancePolicyIndex borrows immutable policy entries. Semantic path
+// segments keep mapping keys distinct from sequence indexes. Record-owned
+// scopes have their own indexes, just as they have their own policy tables.
+type inheritancePolicyIndex struct {
+	entry    *collectionPolicyEntry
+	children map[pathSegment]*inheritancePolicyIndex
+	records  map[int]*inheritancePolicyIndex
+}
+
+func newInheritancePolicyIndex(scope collectionPolicyScope) *inheritancePolicyIndex {
+	index := &inheritancePolicyIndex{}
+	for _, entry := range scope.collections {
+		index.set(entry)
+	}
+	return index
+}
+
+func (index *inheritancePolicyIndex) set(entry collectionPolicyEntry) {
+	node := index
+	for _, segment := range entry.destination.segments {
+		if node.children == nil {
+			node.children = make(map[pathSegment]*inheritancePolicyIndex)
+		}
+		if node.children[segment] == nil {
+			node.children[segment] = &inheritancePolicyIndex{}
+		}
+		node = node.children[segment]
+	}
+	node.entry = &entry
+	node.records = nil
+	for position, owner := range entry.policy.recordOrder {
+		scope := entry.policy.recordPolicies[owner]
+		if len(scope.collections) == 0 {
+			continue
+		}
+		if node.records == nil {
+			node.records = make(map[int]*inheritancePolicyIndex)
+		}
+		node.records[position] = newInheritancePolicyIndex(scope)
+	}
+}
+
+// localize copies only policies in the requested scope. Earlier record-owned
+// policies are considered before direct entries added by destination expansion.
+func (index *inheritancePolicyIndex) localize(prefix Path) collectionPolicyScope {
+	localized := collectionPolicyScope{}
+	node := index
+	for offset, segment := range prefix.segments {
+		if node == nil {
+			return localized
+		}
+		if segment.kind == pathSegmentIndex {
+			if record := node.records[segment.index]; record != nil {
+				remainder := Path{segments: prefix.segments[offset+1:]}
+				for _, entry := range record.localize(remainder).collections {
+					localized.set(entry.destination, entry.policy)
+				}
+			}
+		}
+		node = node.children[segment]
+	}
+	var collect func(*inheritancePolicyIndex)
+	collect = func(node *inheritancePolicyIndex) {
+		if node == nil {
+			return
+		}
+		if node.entry != nil {
+			relative, _ := relativeHistoryPath(prefix, node.entry.destination)
+			localized.set(relative, cloneCollectionPolicyForInheritance(node.entry.policy))
+		}
+		for _, child := range node.children {
+			collect(child)
+		}
+	}
+	collect(node)
+	return localized
+}
+
+func (index *inheritancePolicyIndex) absoluteScope(prefix Path) collectionPolicyScope {
+	scope := collectionPolicyScope{}
+	for _, entry := range index.localize(prefix).collections {
+		scope.set(appendDestination(prefix, entry.destination), entry.policy)
+	}
+	return scope
+}
+
+// replace updates the walker's owned table and its borrowed index together.
+// Retirement visits only direct entries below the changed destination.
+func (index *inheritancePolicyIndex) replace(scope *collectionPolicyScope, prefix Path, replacement collectionPolicyScope) {
+	node := index
+	var parent *inheritancePolicyIndex
+	var last pathSegment
+	for _, segment := range prefix.segments {
+		if node == nil {
+			break
+		}
+		parent, last, node = node, segment, node.children[segment]
+	}
+	var remove func(*inheritancePolicyIndex)
+	remove = func(node *inheritancePolicyIndex) {
+		if node == nil {
+			return
+		}
+		if node.entry != nil {
+			delete(scope.collections, node.entry.destination.String())
+		}
+		for _, child := range node.children {
+			remove(child)
+		}
+	}
+	remove(node)
+	if parent == nil {
+		*index = inheritancePolicyIndex{}
+	} else {
+		delete(parent.children, last)
+	}
+	for _, entry := range replacement.collections {
+		scope.set(entry.destination, entry.policy)
+		index.set(scope.collections[entry.destination.String()])
+	}
+}

internal/engine/inheritance_policy_index_test.go

@@ -0,0 +1,111 @@
+package engine
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestInheritancePolicyIndexKeepsSemanticScopesAndOwnedTemplates(t *testing.T) {
+	policy := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+	nested := collectionPolicyScope{}
+	nested.set(Keys("group", "base", "items"), policy)
+	outer := policy
+	record := newCollectionRecordPolicy()
+	record.scope = nested
+	outer.setRecordPolicyAt(0, record)
+	scope := collectionPolicyScope{}
+	scope.set(Keys("records"), outer)
+	scope.set(Keys("numeric", "0", "items"), policy)
+	index := newInheritancePolicyIndex(scope)
+
+	first := index.localize(Keys("records").Index(0).Key("group"))
+	second := index.localize(Keys("records").Index(0).Key("group"))
+	if len(first.collections) != 1 || len(second.collections) != 1 {
+		t.Fatal("record-owned scope was not localized")
+	}
+	entry := first.collections["/base/items"]
+	entry.policy.identityFields[0] = "changed"
+	if second.collections["/base/items"].policy.identityFields[0] != "id" || nested.collections["/group/base/items"].policy.identityFields[0] != "id" {
+		t.Fatal("localized policy shares mutable template state")
+	}
+	if len(index.localize(Keys("records", "0")).collections) != 0 || len(index.localize(Keys("numeric").Index(0)).collections) != 0 {
+		t.Fatal("mapping keys and sequence indexes were conflated")
+	}
+	if len(index.localize(Keys("numeric", "0")).collections) != 1 {
+		t.Fatal("numeric mapping key did not retain its policy")
+	}
+	left := index.localize(Keys("records"))
+	right := index.localize(Keys("records"))
+	leftOwner := left.collections[""].policy.recordOrder[0]
+	rightOwner := right.collections[""].policy.recordOrder[0]
+	if leftOwner == rightOwner || leftOwner == record.owner || rightOwner == record.owner {
+		t.Fatal("independent templates share a record owner")
+	}
+}
+
+func TestInheritancePolicyIndexReplacesOnlyTheChangedScope(t *testing.T) {
+	scope := collectionPolicyScope{}
+	policy := collectionPolicy{explicitMode: sequenceModeAppend, explicitModePresent: true}
+	for index := range 128 {
+		scope.set(Keys(fmt.Sprintf("group-%03d", index), "pending"), policy)
+	}
+	index := newInheritancePolicyIndex(scope)
+	changed := Keys("group-064")
+	if len(index.absoluteScope(changed).collections) != 1 {
+		t.Fatal("scope includes unrelated groups")
+	}
+	replacement := collectionPolicyScope{}
+	replacement.set(changed.Key("introduced"), policy)
+	index.replace(&scope, changed, replacement)
+	if len(scope.collections) != 128 {
+		t.Fatal("replacement lost unrelated pending declarations")
+	}
+	local := index.localize(changed)
+	if _, found := local.collections["/introduced"]; !found || len(local.collections) != 1 {
+		t.Fatal("retired policy survived or introduced policy was not indexed")
+	}
+	index.replace(&scope, changed, collectionPolicyScope{})
+	if len(index.localize(changed).collections) != 0 || len(scope.collections) != 127 {
+		t.Fatal("descendant retirement did not update the table and index together")
+	}
+	if len(index.localize(Keys("group-065")).collections) != 1 {
+		t.Fatal("retirement changed another scope")
+	}
+}
+
+func TestInheritanceWideIndependentYamlCollectionScopes(t *testing.T) {
+	var body strings.Builder
+	body.WriteString("configkit:\n  sequences:\n")
+	for index := range 64 {
+		fmt.Fprintf(&body, "    - {destination: [groups, group-%03d, base, items], key: id, mode: append}\n", index)
+	}
+	body.WriteString("groups:\n")
+	for index := range 64 {
+		fmt.Fprintf(&body, "  group-%03d:\n    base: {items: [{id: shared, value: parent}]}\n    child: {extends: base, items: [{id: shared, value: child}]}\n", index)
+	}
+	type item struct {
+		ID    string `yaml:"id"`
+		Value string `yaml:"value"`
+	}
+	type definition struct {
+		Items []item `yaml:"items"`
+	}
+	type config struct {
+		Groups map[string]map[string]definition `yaml:"groups"`
+	}
+	loaded, err := testLoad[config](Bytes("wide-inheritance", []byte(body.String())))
+	if err != nil {
+		t.Fatal(err)
+	}
+	metadata := loaded.Metadata()
+	for name, group := range loaded.Config.Groups {
+		if len(group["base"].Items) != 1 || group["base"].Items[0].Value != "parent" || len(group["child"].Items) != 1 || group["child"].Items[0].Value != "child" {
+			t.Fatalf("independent scope %s composed incorrectly: %#v", name, group)
+		}
+		path := Keys("groups", name, "child", "items").Index(0).Key("value")
+		if info, exists := metadata.Lookup(path); !exists || !info.Explicit || info.Origin.Layer != LayerBase {
+			t.Fatalf("inherited destination lost source metadata at %s", path)
+		}
+	}
+}

internal/engine/inheritance_schedule.go

@@ -7,28 +7,39 @@ import (
 )
 
 type inheritanceTemplate struct {
-	name         string
-	entry        document.Entry
-	path         Path
-	parent       int
-	hasParent    bool
-	hasDirective bool
-	cancelled    bool
-	directive    document.Entry
+	name               string
+	entry              document.Entry
+	path               Path
+	collectionPolicies collectionPolicyScope
+	parent             int
+	hasParent          bool
+	hasDirective       bool
+	cancelled          bool
+	directive          document.Entry
 }
 
 type inheritanceGraph struct {
-	scope     *document.Node
-	scopePath Path
-	nodes     []inheritanceTemplate
-	edges     []dependencyEdge
-	byName    map[string]int
+	scope              *document.Node
+	scopePath          Path
+	collectionPolicies collectionPolicyScope
+	applicationPolicy  sequencePolicy
+	nodes              []inheritanceTemplate
+	edges              []dependencyEdge
+	byName             map[string]int
 }
 
 // analyzeInheritanceGraph retains only direct mapping siblings as graph nodes.
 // It validates declarations separately from applying their templates, leaving
 // nested scopes for the existing destination-local walk.
 func analyzeInheritanceGraph(scope *document.Node, scopePath Path) (*inheritanceGraph, []Diagnostic) {
+	return analyzeInheritanceGraphWithPolicies(scope, scopePath, collectionPolicyScope{})
+}
+
+func analyzeInheritanceGraphWithPolicies(scope *document.Node, scopePath Path, policies collectionPolicyScope) (*inheritanceGraph, []Diagnostic) {
+	return analyzeInheritanceGraphWithCollectionRules(scope, scopePath, policies, sequencePolicy{})
+}
+
+func analyzeInheritanceGraphWithCollectionRules(scope *document.Node, scopePath Path, policies collectionPolicyScope, applicationPolicy sequencePolicy) (*inheritanceGraph, []Diagnostic) {
 	if scope == nil || scope.Kind != document.Mapping {
 		return nil, []Diagnostic{{
 			Code: CodeInheritanceInvalid, Severity: SeverityError, Path: scopePath,
@@ -36,8 +47,10 @@ func analyzeInheritanceGraph(scope *document.Node, scopePath Path) (*inheritance
 		}}
 	}
 
+	policyIndex := newInheritancePolicyIndex(policies)
 	graph := &inheritanceGraph{
 		scope: scope, scopePath: scopePath,
+		collectionPolicies: policyIndex.absoluteScope(scopePath), applicationPolicy: applicationPolicy,
 		byName: make(map[string]int, len(scope.Mapping)),
 	}
 	entries := make(map[string]document.Entry, len(scope.Mapping))
@@ -57,6 +70,7 @@ func analyzeInheritanceGraph(scope *document.Node, scopePath Path) (*inheritance
 		graph.byName[name] = index
 		graph.nodes = append(graph.nodes, inheritanceTemplate{
 			name: name, entry: entries[name], path: scopePath.Key(name),
+			collectionPolicies: policyIndex.localize(scopePath.Key(name)),
 		})
 	}
 
@@ -115,7 +129,15 @@ func (graph *inheritanceGraph) schedule() ([]int, []Diagnostic) {
 }
 
 func expandSiblingDefinitionsWithOrigins(scope *document.Node, scopePath Path, limits projectionLimits, inputOrigins map[string]Origin, budget ...*retainedNodeCounter) (*inheritanceScope, []Diagnostic) {
-	graph, diagnostics := analyzeInheritanceGraph(scope, scopePath)
+	return expandSiblingDefinitionsWithPolicies(scope, scopePath, limits, inputOrigins, collectionPolicyScope{}, budget...)
+}
+
+func expandSiblingDefinitionsWithPolicies(scope *document.Node, scopePath Path, limits projectionLimits, inputOrigins map[string]Origin, policies collectionPolicyScope, budget ...*retainedNodeCounter) (*inheritanceScope, []Diagnostic) {
+	return expandSiblingDefinitionsWithCollectionRules(scope, scopePath, limits, inputOrigins, policies, sequencePolicy{}, budget...)
+}
+
+func expandSiblingDefinitionsWithCollectionRules(scope *document.Node, scopePath Path, limits projectionLimits, inputOrigins map[string]Origin, policies collectionPolicyScope, applicationPolicy sequencePolicy, budget ...*retainedNodeCounter) (*inheritanceScope, []Diagnostic) {
+	graph, diagnostics := analyzeInheritanceGraphWithCollectionRules(scope, scopePath, policies, applicationPolicy)
 	if len(diagnostics) != 0 {
 		return nil, diagnostics
 	}
@@ -123,44 +145,56 @@ func expandSiblingDefinitionsWithOrigins(scope *document.Node, scopePath Path, l
 	if len(diagnostics) != 0 {
 		return nil, diagnostics
 	}
-	return applyInheritanceTemplates(graph, order, limits, inputOrigins, budget...)
+	result, diagnostics := applyInheritanceTemplates(graph, order, limits, inputOrigins, budget...)
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+	return result, nil
 }
 
 // applyInheritanceTemplates builds detached templates in the supplied
 // dependency-ready order while retaining the existing origins, merge history,
 // and node-accounting evidence. The root keeps its authored mapping order.
 func applyInheritanceTemplates(graph *inheritanceGraph, order []int, limits projectionLimits, inputOrigins map[string]Origin, budget ...*retainedNodeCounter) (*inheritanceScope, []Diagnostic) {
-	resolved := make([]*document.Node, len(graph.nodes))
-	resolvedOrigins := make([]map[string]Origin, len(graph.nodes))
+	return applyInheritanceScopeTemplates(mappingInheritanceScope{graph: graph}, order, limits, inputOrigins, budget...)
+}
+
+func applyInheritanceScopeTemplates(adapter inheritanceScopeAdapter, order []int, limits projectionLimits, inputOrigins map[string]Origin, budget ...*retainedNodeCounter) (*inheritanceScope, []Diagnostic) {
+	templates := adapter.templates()
+	resolved := make([]*document.Node, len(templates))
+	resolvedOrigins := make([]map[string]Origin, len(templates))
+	resolvedPolicyTemplates := make([][]collectionPolicyScope, len(templates))
+	resolvedPolicies := make([]collectionPolicyScope, len(templates))
 	result := &inheritanceScope{}
 	counter := newRetainedNodeCounter(historyNodeLimit)
 	if len(budget) != 0 {
 		counter = budget[0]
 	}
-	retained := 1 // scope mapping node
-	for _, entry := range graph.scope.Mapping {
-		if entry.Value.Kind == document.Mapping {
+	retained := 1 // scope container node
+	for _, slot := range adapter.slots() {
+		if slot.hasTemplate || slot.value.Kind == document.Mapping {
 			continue
 		}
 		remaining := limits.nodes - retained - 1
 		if limits.nodes >= 0 && remaining < 0 {
-			return nil, []Diagnostic{inheritanceBudgetDiagnostic(graph.scopePath.Key(entry.Key), entry.Value.Location)}
+			return nil, []Diagnostic{inheritanceBudgetDiagnostic(slot.path, slot.value.Location)}
 		}
-		count, within := inheritanceLowerBoundCount(entry.Value, remaining)
+		count, within := inheritanceLowerBoundCount(slot.value, remaining)
 		if !within {
-			return nil, []Diagnostic{inheritanceBudgetDiagnostic(graph.scopePath.Key(entry.Key), entry.Value.Location)}
+			return nil, []Diagnostic{inheritanceBudgetDiagnostic(slot.path, slot.value.Location)}
 		}
 		retained += 1 + count
 	}
 
 	for _, index := range order {
-		node := graph.nodes[index]
+		node := templates[index]
+		templatePolicies := cloneCollectionPolicyScopeForInheritance(node.collectionPolicies)
 		payload := withoutInheritanceDirective(node.entry.Value)
 		operationStart, replacementStart := len(result.operations), len(result.replacements)
 		childOrigins := directInheritanceOrigins(payload, node.path, inputOrigins)
 		if node.hasDirective {
 			control := inheritanceControl{
-				path:      node.path.Key(inheritanceKey),
+				path:      node.directivePath,
 				locations: []Location{publicLocation(node.directive.Value.Location)},
 				cancelled: node.cancelled,
 			}
@@ -171,7 +205,7 @@ func applyInheritanceTemplates(graph *inheritanceGraph, order []int, limits proj
 			})
 		}
 		if limits.nodes >= 0 {
-			remaining := limits.nodes - retained - 1 // the sibling key
+			remaining := limits.nodes - retained - adapter.slotOverhead()
 			within := inheritanceLowerBoundWithin(payload, remaining)
 			if node.hasParent {
 				within = inheritanceOverlayLowerBoundWithin(resolved[node.parent], payload, remaining)
@@ -183,14 +217,46 @@ func applyInheritanceTemplates(graph *inheritanceGraph, order []int, limits proj
 
 		if node.hasParent {
 			parent := resolved[node.parent]
-			merged := document.Clone(parent)
+			parentPolicies := cloneInheritancePolicyTemplates(resolvedPolicyTemplates[node.parent])
+			effectivePolicies := append(cloneInheritancePolicyTemplates(parentPolicies), cloneCollectionPolicyScopeForInheritance(templatePolicies))
+			parentPolicyScope := projectInheritanceCollectionPolicies(resolvedPolicies[node.parent], node.path)
+			childPolicyScope := projectCurrentInheritanceCollectionPolicies(templatePolicies, node.path)
+			var replaced collectionReplacementPaths
+			if len(parentPolicyScope.collections) != 0 || len(childPolicyScope.collections) != 0 {
+				claims := compositionClaims{byPath: make(map[string][]compositionClaim)}
+				claims.append(payload, node.path, 0)
+				declarations := make(map[string][]sequenceDeclarationSpec, len(childPolicyScope.collections))
+				for key, entry := range childPolicyScope.collections {
+					declarations[key] = inheritanceDeclarationsAt(entry.destination, entry.policy)
+				}
+				parentPolicyScope, replaced = lowerCollectionPoliciesForClaims(parentPolicyScope, claims.ordered, declarations, adapter.applicationPolicy())
+			}
+			merged := inheritanceParentPayload(parent, node.identityFields)
+			historyParent := parent
+			if len(node.identityFields) != 0 {
+				// The retained parent snapshot is the inherited payload, before
+				// child fields are applied, with parent identity excluded.
+				historyParent = document.Clone(merged)
+			}
+			mergedPolicyScope, policyDiagnostics := mergeInheritanceCollectionPolicies(
+				parentPolicyScope, childPolicyScope, node.path, parent, payload, adapter.applicationPolicy(), replaced,
+			)
+			if len(policyDiagnostics) != 0 {
+				return nil, policyDiagnostics
+			}
 			transfer := inheritanceTransfer{
-				from: graph.nodes[node.parent].path, to: node.path,
-				reference: node.path.Key(inheritanceKey),
-				locations: []Location{publicLocation(node.directive.Value.Location)},
+				from: templates[node.parent].path, to: node.path,
+				reference:       node.directivePath,
+				locations:       []Location{publicLocation(node.directive.Value.Location)},
+				policyTemplates: parentPolicies, templatePolicies: cloneCollectionPolicyScopeForInheritance(templatePolicies),
+				projectedPolicies: projectInheritancePolicyTemplates(
+					[]collectionPolicyScope{resolvedPolicies[node.parent]}, node.path,
+				),
 			}
 			mergedOrigins := transferredInheritanceOrigins(resolvedOrigins[node.parent], transfer.from, node.path, transfer)
 			mergeInheritanceOrigin(mergedOrigins, node.path, childOrigins[node.path.String()])
+			var sequenceContributions []sequenceHistoryContribution
+			var combinationDiagnostics []Diagnostic
 			mergeRecursiveMappings(merged, payload, node.path, new(mappingIndex), recursiveMergeHooks{
 				added: func(childPath Path, incoming *document.Node) {
 					copyInheritanceSubtree(mergedOrigins, childOrigins, incoming, childPath)
@@ -207,21 +273,74 @@ func applyInheritanceTemplates(graph *inheritanceGraph, order []int, limits proj
 					deleteOriginSubtree(mergedOrigins, childPath, existing)
 					copyInheritanceSubtree(mergedOrigins, childOrigins, incoming, childPath)
 				},
+				merge: func(childPath Path, existing, incoming *document.Node) (*document.Node, bool) {
+					resolution, _, _, active, issues := inheritanceCollectionResolution(
+						childPath, parentPolicyScope, childPolicyScope, adapter.applicationPolicy(), incoming,
+					)
+					if len(issues) != 0 {
+						combinationDiagnostics = append(combinationDiagnostics, issues...)
+						return existing, false
+					}
+					if !active {
+						return nil, false
+					}
+					lowerValue := existing
+					combined, problems := combineSequence(lowerValue, incoming, resolution, sequenceCombinationContext{destination: childPath})
+					if len(problems) != 0 {
+						combinationDiagnostics = append(combinationDiagnostics, problems...)
+						return existing, false
+					}
+					policy := applyInheritanceSequencePolicy(
+						childPath, combined, resolution, parentPolicyScope, childPolicyScope,
+					)
+					mergedPolicyScope.set(childPath, policy)
+					applyInheritanceSequenceOrigins(
+						result, mergedOrigins, childOrigins, childPath, lowerValue, incoming, combined, resolution.operation,
+					)
+					history := sequenceHistoryContribution{path: childPath.clone(), mode: sequenceHistoryMapped}
+					if resolution.operation == sequenceModeReplace {
+						history.mode = sequenceHistoryReplace
+						if incoming != nil && incoming.Kind == document.Sequence {
+							history.length = len(incoming.Sequence)
+						}
+					} else {
+						history.items = append([]sequenceItemProjection(nil), combined.higherItems...)
+						history.replacements = append([]sequenceItemReplacement(nil), combined.replacements...)
+					}
+					sequenceContributions = append(sequenceContributions, history)
+					return combined.value, true
+				},
 			})
+			preserveInheritanceIdentityFields(merged, payload, node.identityFields)
+			if len(combinationDiagnostics) != 0 {
+				return nil, canonicalDiagnostics(combinationDiagnostics)
+			}
+			if problems := validateRegisteredInheritanceSequences(merged, node.path, adapter.applicationPolicy()); len(problems) != 0 {
+				return nil, problems
+			}
 			merged.Location = node.entry.Value.Location
 			resolved[index], resolvedOrigins[index] = merged, mergedOrigins
+			resolvedPolicies[index] = inheritancePolicyScopeAtPath(mergedPolicyScope, node.path)
 			result.transfers = append(result.transfers, transfer)
 			result.operations = append(result.operations, inheritanceOperation{
-				transfer: &transfer, parent: parent, effective: merged,
+				transfer: &transfer, parent: historyParent, effective: merged,
 				parentOrigins: resolvedOrigins[node.parent], effectiveOrigins: mergedOrigins,
+				parentPolicies:    cloneInheritancePolicyTemplates(parentPolicies),
+				templatePolicies:  cloneCollectionPolicyScopeForInheritance(templatePolicies),
+				effectivePolicies: cloneInheritancePolicyTemplates(effectivePolicies),
+				sequences:         cloneSequenceHistoryContributions(sequenceContributions),
+				identityFields:    cloneStrings(node.identityFields),
 			})
+			resolvedPolicyTemplates[index] = effectivePolicies
 		} else {
 			resolved[index] = document.Clone(payload)
 			resolvedOrigins[index] = childOrigins
+			resolvedPolicyTemplates[index] = []collectionPolicyScope{templatePolicies}
+			resolvedPolicies[index] = cloneCollectionPolicyScopeForInheritance(templatePolicies)
 		}
 
 		count, _ := inheritanceLowerBoundCount(resolved[index], limits.nodes)
-		retained += 1 + count
+		retained += adapter.slotOverhead() + count
 		for _, operation := range result.operations[operationStart:] {
 			if !counter.add(operation.parent) || !counter.add(operation.effective) {
 				return nil, []Diagnostic{inheritanceHistoryBudgetDiagnostic(node.path, node.entry.Value.Location)}
@@ -234,26 +353,28 @@ func applyInheritanceTemplates(graph *inheritanceGraph, order []int, limits proj
 		}
 	}
 
-	root := *graph.scope
-	root.Mapping = make([]document.Entry, len(graph.scope.Mapping))
-	result.origins = directInheritanceOrigins(&document.Node{Kind: document.Mapping, Location: graph.scope.Location}, graph.scopePath, inputOrigins)
-	for i, entry := range graph.scope.Mapping {
-		root.Mapping[i] = entry
-		if index, found := graph.byName[entry.Key]; found {
-			root.Mapping[i].Value = resolved[index]
+	root := adapter.rebuild(resolved)
+	result.collectionPolicies = collectionPolicyScope{}
+	result.origins = directInheritanceOrigins(&document.Node{Kind: root.Kind, Location: root.Location}, adapter.scopePath(), inputOrigins)
+	policyIndex := newInheritancePolicyIndex(adapter.collectionPolicies())
+	for _, slot := range adapter.slots() {
+		if slot.hasTemplate {
+			index := slot.template
+			projectInheritancePolicyRoots(&result.collectionPolicies, resolvedPolicies[index], templates[index].path)
 			for pointer, origin := range resolvedOrigins[index] {
 				result.origins[pointer] = origin
 			}
 			continue
 		}
-		root.Mapping[i].Value = document.Clone(entry.Value)
-		for pointer, origin := range directInheritanceOrigins(entry.Value, graph.scopePath.Key(entry.Key), inputOrigins) {
+		localPolicies := policyIndex.localize(slot.path)
+		projectInheritancePolicyRoots(&result.collectionPolicies, localPolicies, slot.path)
+		for pointer, origin := range directInheritanceOrigins(slot.value, slot.path, inputOrigins) {
 			result.origins[pointer] = origin
 		}
 	}
-	result.root = &root
+	result.root = root
 	if !inheritanceLowerBoundWithin(result.root, limits.nodes) {
-		return nil, []Diagnostic{inheritanceBudgetDiagnostic(graph.scopePath, graph.scope.Location)}
+		return nil, []Diagnostic{inheritanceBudgetDiagnostic(adapter.scopePath(), adapter.scope().Location)}
 	}
 	return result, nil
 }

internal/engine/inheritance_schedule_test.go

@@ -110,6 +110,91 @@ beta:
 	}
 }
 
+func TestInheritanceTransportsReusableCollectionPolicyTemplates(t *testing.T) {
+	root := parseItemPolicyDocument(t, "templates", `base:
+  prompts:
+    - id: shared
+      children:
+        derived:
+          extends: nested-base
+          own: destination-local
+        nested-base:
+          inherited: source-value
+first:
+  extends: base
+second:
+  extends: base
+`)
+
+	policyLocation := Location{SourceID: "base-resource", Line: 2, Column: 3}
+	nestedLocation := Location{SourceID: "nested-resource", Line: 5, Column: 11}
+	nested := collectionPolicy{identityFields: []string{"key"}, identityPresent: true, identityLocation: &nestedLocation}
+	recordScope := collectionPolicyScope{}
+	recordScope.set(Keys("children", "prompts"), nested)
+	basePolicy := collectionPolicy{identityFields: []string{"id"}, identityPresent: true, identityLocation: &policyLocation}
+	basePolicy.setRecordPolicyAt(0, collectionRecordPolicy{owner: newCollectionRecordPolicy().owner, scope: recordScope})
+	policies := collectionPolicyScope{}
+	policies.set(Keys("base", "prompts"), basePolicy)
+
+	got, diagnostics := expandSiblingDefinitionsWithPolicies(root, Path{}, defaultProjectionLimits(), nil, policies)
+	if len(diagnostics) != 0 {
+		t.Fatalf("expand sibling templates: %#v", diagnostics)
+	}
+	withoutPolicies, diagnostics := expandSiblingDefinitionsWithOrigins(root, Path{}, defaultProjectionLimits(), nil)
+	if len(diagnostics) != 0 {
+		t.Fatalf("expand sibling templates without policy: %#v", diagnostics)
+	}
+	if !reflect.DeepEqual(got.root, withoutPolicies.root) {
+		t.Fatalf("policy transport changed inherited values:\n got %#v\nwant %#v", got.root, withoutPolicies.root)
+	}
+	var transfers []inheritanceOperation
+	for _, operation := range got.operations {
+		if operation.transfer != nil {
+			transfers = append(transfers, operation)
+		}
+	}
+	if len(transfers) != 2 {
+		t.Fatalf("transfer operations = %d, want two destination transfers", len(transfers))
+	}
+
+	for index, destination := range []string{"/first", "/second"} {
+		operation := transfers[index]
+		if operation.transfer == nil || len(operation.transfer.projectedPolicies) != 2 {
+			t.Fatalf("transfer %d projected policies = %#v, want collection and record-local policy", index, operation.transfer)
+		}
+		projected := operation.transfer.projectedPolicies
+		if projected[0].path.String() != destination+"/prompts" || projected[1].path.String() != destination+"/prompts/0/children/prompts" {
+			t.Fatalf("transfer %d policy paths = %q, %q", index, projected[0].path, projected[1].path)
+		}
+		if got := projected[0].policy.identityLocation; got == nil || *got != policyLocation {
+			t.Fatalf("transfer %d collection declaration location = %#v", index, got)
+		}
+		if got := projected[1].policy.identityLocation; got == nil || *got != nestedLocation {
+			t.Fatalf("transfer %d record-local declaration location = %#v", index, got)
+		}
+		if len(operation.templatePolicies.collections) != 0 {
+			t.Fatalf("transfer %d unexpectedly copied parent YAML policy onto child template", index)
+		}
+	}
+
+	first, second := transfers[0].transfer.projectedPolicies[0].policy, transfers[1].transfer.projectedPolicies[0].policy
+	if first.recordOrder[0] == second.recordOrder[0] {
+		t.Fatal("destination policy templates share a mutable record owner")
+	}
+	first.identityFields[0] = "changed"
+	if second.identityFields[0] != "id" {
+		t.Fatalf("mutating one destination changed another: %v", second.identityFields)
+	}
+	if basePolicy.identityFields[0] != "id" {
+		t.Fatalf("mutating a destination changed the source template: %v", basePolicy.identityFields)
+	}
+
+	firstValue, found := documentAtPath(got.root.Mapping[1].Value, Keys("prompts").Index(0).Key("children").Key("derived").Key("extends"))
+	if !found || firstValue.Text != "nested-base" {
+		t.Fatalf("nested template instruction = %#v, found %t; want original instruction retained", firstValue, found)
+	}
+}
+
 func TestInheritanceExpansionInterleavesUnequalBranches(t *testing.T) {
 	root := parseCompositionDocument(t, "branches", `a-grandchild:
   extends: z-child

internal/engine/inheritance_scope.go

@@ -0,0 +1,198 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// inheritanceScopeAdapter keeps relationship execution independent from the
+// container that owns the records. Adapters are private preparation machinery;
+// public loading continues to use mapping scopes until record inheritance is
+// activated separately.
+type inheritanceScopeAdapter interface {
+	scope() *document.Node
+	scopePath() Path
+	collectionPolicies() collectionPolicyScope
+	applicationPolicy() sequencePolicy
+	templates() []inheritanceExecutionTemplate
+	slots() []inheritanceScopeSlot
+	slotOverhead() int
+	rebuild(resolved []*document.Node) *document.Node
+}
+
+type inheritanceExecutionTemplate struct {
+	entry              document.Entry
+	path               Path
+	collectionPolicies collectionPolicyScope
+	parent             int
+	hasParent          bool
+	hasDirective       bool
+	cancelled          bool
+	directive          document.Entry
+	directivePath      Path
+	identityFields     []string
+}
+
+type inheritanceScopeSlot struct {
+	value       *document.Node
+	path        Path
+	template    int
+	hasTemplate bool
+}
+
+type mappingInheritanceScope struct{ graph *inheritanceGraph }
+
+func (scope mappingInheritanceScope) scope() *document.Node { return scope.graph.scope }
+func (scope mappingInheritanceScope) scopePath() Path       { return scope.graph.scopePath }
+func (scope mappingInheritanceScope) collectionPolicies() collectionPolicyScope {
+	return scope.graph.collectionPolicies
+}
+func (scope mappingInheritanceScope) applicationPolicy() sequencePolicy {
+	return scope.graph.applicationPolicy
+}
+func (scope mappingInheritanceScope) slotOverhead() int { return 1 }
+func (scope mappingInheritanceScope) templates() []inheritanceExecutionTemplate {
+	result := make([]inheritanceExecutionTemplate, len(scope.graph.nodes))
+	for index, node := range scope.graph.nodes {
+		var directive document.Entry
+		if node.hasDirective {
+			directive = node.directive
+		}
+		result[index] = inheritanceExecutionTemplate{
+			entry: node.entry, path: node.path,
+			collectionPolicies: node.collectionPolicies, parent: node.parent,
+			hasParent: node.hasParent, hasDirective: node.hasDirective,
+			cancelled: node.cancelled, directive: directive,
+			directivePath: node.path.Key(inheritanceKey),
+		}
+	}
+	return result
+}
+func (scope mappingInheritanceScope) slots() []inheritanceScopeSlot {
+	result := make([]inheritanceScopeSlot, len(scope.graph.scope.Mapping))
+	for index, entry := range scope.graph.scope.Mapping {
+		slot := inheritanceScopeSlot{
+			value: entry.Value, path: scope.graph.scopePath.Key(entry.Key),
+		}
+		if template, ok := scope.graph.byName[entry.Key]; ok {
+			slot.template, slot.hasTemplate = template, true
+		}
+		result[index] = slot
+	}
+	return result
+}
+func (scope mappingInheritanceScope) rebuild(resolved []*document.Node) *document.Node {
+	root := *scope.graph.scope
+	root.Mapping = make([]document.Entry, len(scope.graph.scope.Mapping))
+	for index, entry := range scope.graph.scope.Mapping {
+		root.Mapping[index] = entry
+		if template, ok := scope.graph.byName[entry.Key]; ok {
+			root.Mapping[index].Value = resolved[template]
+		} else {
+			root.Mapping[index].Value = document.Clone(entry.Value)
+		}
+	}
+	return &root
+}
+
+type identifiedSequenceInheritanceScope struct {
+	graph       *identifiedInheritanceGraph
+	policies    collectionPolicyScope
+	application sequencePolicy
+}
+
+func (scope identifiedSequenceInheritanceScope) scope() *document.Node { return scope.graph.scope }
+func (scope identifiedSequenceInheritanceScope) scopePath() Path       { return scope.graph.scopePath }
+func (scope identifiedSequenceInheritanceScope) collectionPolicies() collectionPolicyScope {
+	return scope.policies
+}
+func (scope identifiedSequenceInheritanceScope) applicationPolicy() sequencePolicy {
+	return scope.application
+}
+func (scope identifiedSequenceInheritanceScope) slotOverhead() int { return 0 }
+func (scope identifiedSequenceInheritanceScope) templates() []inheritanceExecutionTemplate {
+	result := make([]inheritanceExecutionTemplate, len(scope.graph.nodes))
+	policyIndex := newInheritancePolicyIndex(scope.policies)
+	for index, node := range scope.graph.nodes {
+		result[index] = inheritanceExecutionTemplate{
+			entry: document.Entry{Value: node.item}, path: node.path,
+			collectionPolicies: policyIndex.localize(node.path),
+			parent:             node.parent, hasParent: node.hasParent,
+			hasDirective: node.directive != nil, cancelled: node.cancelled,
+			directive: document.Entry{Value: node.directive}, directivePath: node.directivePath,
+			identityFields: scope.graph.fields,
+		}
+	}
+	return result
+}
+func (scope identifiedSequenceInheritanceScope) slots() []inheritanceScopeSlot {
+	result := make([]inheritanceScopeSlot, len(scope.graph.nodes))
+	for index, node := range scope.graph.nodes {
+		result[index] = inheritanceScopeSlot{
+			value: node.item, path: node.path, template: index, hasTemplate: true,
+		}
+	}
+	return result
+}
+func (scope identifiedSequenceInheritanceScope) rebuild(resolved []*document.Node) *document.Node {
+	root := *scope.graph.scope
+	root.Sequence = make([]*document.Node, len(scope.graph.nodes))
+	for index := range root.Sequence {
+		root.Sequence[index] = resolved[index]
+	}
+	return &root
+}
+
+func preserveInheritanceIdentityFields(merged, child *document.Node, fields []string) {
+	if len(fields) == 0 || merged == nil || merged.Kind != document.Mapping || child == nil || child.Kind != document.Mapping {
+		return
+	}
+	for _, field := range fields {
+		for _, entry := range child.Mapping {
+			if entry.Key != field {
+				continue
+			}
+			for index := range merged.Mapping {
+				if merged.Mapping[index].Key == field {
+					merged.Mapping[index] = document.Entry{Key: field, Value: document.Clone(entry.Value)}
+					break
+				}
+			}
+			break
+		}
+	}
+}
+
+func inheritanceParentPayload(parent *document.Node, identityFields []string) *document.Node {
+	cloned := document.Clone(parent)
+	if len(identityFields) == 0 || cloned == nil || cloned.Kind != document.Mapping {
+		return cloned
+	}
+	excluded := make(map[string]struct{}, len(identityFields))
+	for _, field := range identityFields {
+		excluded[field] = struct{}{}
+	}
+	kept := cloned.Mapping[:0]
+	for _, entry := range cloned.Mapping {
+		if _, omit := excluded[entry.Key]; !omit {
+			kept = append(kept, entry)
+		}
+	}
+	cloned.Mapping = kept
+	return cloned
+}
+
+func applyIdentifiedInheritanceTemplates(
+	graph *identifiedInheritanceGraph,
+	limits projectionLimits,
+	inputOrigins map[string]Origin,
+	policies collectionPolicyScope,
+	application sequencePolicy,
+	budget ...*retainedNodeCounter,
+) (*inheritanceScope, []Diagnostic) {
+	order, diagnostics := graph.schedule()
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+	adapter := identifiedSequenceInheritanceScope{
+		graph: graph, policies: policies, application: application,
+	}
+	return applyInheritanceScopeTemplates(adapter, order, limits, inputOrigins, budget...)
+}

internal/engine/inheritance_walk.go

@@ -8,14 +8,17 @@ import (
 )
 
 type inheritanceWalker struct {
-	nodes        int // logical non-directive nodes in the entire working tree
-	origins      map[string]Origin
-	limits       projectionLimits
-	retained     *retainedNodeCounter
-	transfers    []inheritanceTransfer
-	controls     []inheritanceControl
-	replacements []inheritanceReplacement
-	operations   []inheritanceOperation
+	nodes              int // logical non-directive nodes in the entire working tree
+	origins            map[string]Origin
+	collectionPolicies collectionPolicyScope
+	policyIndex        *inheritancePolicyIndex
+	applicationPolicy  sequencePolicy
+	limits             projectionLimits
+	retained           *retainedNodeCounter
+	transfers          []inheritanceTransfer
+	controls           []inheritanceControl
+	replacements       []inheritanceReplacement
+	operations         []inheritanceOperation
 }
 
 // expandInheritanceTree resolves each destination scope after its enclosing
@@ -25,10 +28,26 @@ func expandInheritanceTree(input *document.Node, limits projectionLimits) (*inhe
 }
 
 func expandInheritanceWithOrigins(input *document.Node, inputOrigins map[string]Origin, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
-	return expandInheritanceWithBudgets(input, inputOrigins, limits, historyNodeLimit)
+	return expandInheritanceWithPolicies(input, inputOrigins, collectionPolicyScope{}, limits)
 }
 
 func expandInheritanceWithBudgets(input *document.Node, inputOrigins map[string]Origin, limits projectionLimits, historyLimit int) (*inheritanceScope, []Diagnostic) {
+	return expandInheritanceWithPolicyBudgets(input, inputOrigins, collectionPolicyScope{}, limits, historyLimit)
+}
+
+func expandInheritanceWithPolicies(input *document.Node, inputOrigins map[string]Origin, policies collectionPolicyScope, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
+	return expandInheritanceWithCollectionRules(input, inputOrigins, policies, sequencePolicy{}, limits)
+}
+
+func expandInheritanceWithPolicyBudgets(input *document.Node, inputOrigins map[string]Origin, policies collectionPolicyScope, limits projectionLimits, historyLimit int) (*inheritanceScope, []Diagnostic) {
+	return expandInheritanceWithCollectionRuleBudgets(input, inputOrigins, policies, sequencePolicy{}, limits, historyLimit)
+}
+
+func expandInheritanceWithCollectionRules(input *document.Node, inputOrigins map[string]Origin, policies collectionPolicyScope, applicationPolicy sequencePolicy, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
+	return expandInheritanceWithCollectionRuleBudgets(input, inputOrigins, policies, applicationPolicy, limits, historyNodeLimit)
+}
+
+func expandInheritanceWithCollectionRuleBudgets(input *document.Node, inputOrigins map[string]Origin, policies collectionPolicyScope, applicationPolicy sequencePolicy, limits projectionLimits, historyLimit int) (*inheritanceScope, []Diagnostic) {
 	if input == nil || input.Kind != document.Mapping {
 		return nil, []Diagnostic{{Code: CodeInheritanceInvalid, Severity: SeverityError,
 			Message: "inheritance requires a mapping document root"}}
@@ -44,8 +63,11 @@ func expandInheritanceWithBudgets(input *document.Node, inputOrigins map[string]
 	}
 	walker := inheritanceWalker{
 		nodes: nodes, limits: limits, origins: origins,
-		retained: newRetainedNodeCounter(historyLimit),
+		collectionPolicies: cloneCollectionPolicyScopeForInheritance(policies),
+		applicationPolicy:  applicationPolicy,
+		retained:           newRetainedNodeCounter(historyLimit),
 	}
+	walker.policyIndex = newInheritancePolicyIndex(walker.collectionPolicies)
 	if problem := walker.visit(&root, Path{}, true); problem != nil {
 		return nil, []Diagnostic{*problem}
 	}
@@ -53,8 +75,10 @@ func expandInheritanceWithBudgets(input *document.Node, inputOrigins map[string]
 		return nil, []Diagnostic{*problem}
 	}
 	return &inheritanceScope{
-		root: root, origins: walker.origins, transfers: walker.transfers,
-		controls: walker.controls, replacements: walker.replacements, operations: walker.operations,
+		root: root, origins: walker.origins,
+		collectionPolicies: cloneCollectionPolicyScopeForInheritance(walker.collectionPolicies),
+		transfers:          walker.transfers,
+		controls:           walker.controls, replacements: walker.replacements, operations: walker.operations,
 	}, nil
 }
 
@@ -90,11 +114,14 @@ func (w *inheritanceWalker) visit(node **document.Node, path Path, unscoped bool
 			if limits.nodes >= 0 {
 				limits.nodes -= w.nodes - before
 			}
-			resolved, diagnostics := expandSiblingDefinitionsWithOrigins(*node, path, limits, w.origins, w.retained)
+			resolved, diagnostics := expandSiblingDefinitionsWithCollectionRules(
+				*node, path, limits, w.origins, w.policyIndex.absoluteScope(path), w.applicationPolicy, w.retained,
+			)
 			if len(diagnostics) != 0 {
 				return &diagnostics[0]
 			}
 			*node = resolved.root
+			w.policyIndex.replace(&w.collectionPolicies, path, resolved.collectionPolicies)
 			deleteOriginSubtree(w.origins, path, original)
 			for pointer, origin := range resolved.origins {
 				w.origins[pointer] = origin
@@ -126,6 +153,11 @@ func (w *inheritanceWalker) visit(node **document.Node, path Path, unscoped bool
 			}
 		}
 	case document.Sequence:
+		if sequenceHasIdentifiedInheritanceReferences(*node) {
+			if problem := w.expandIdentifiedSequence(node, path); problem != nil {
+				return problem
+			}
+		}
 		working := **node
 		working.Sequence = slices.Clone(working.Sequence)
 		*node = &working
@@ -139,6 +171,53 @@ func (w *inheritanceWalker) visit(node **document.Node, path Path, unscoped bool
 	return nil
 }
 
+func (w *inheritanceWalker) expandIdentifiedSequence(node **document.Node, path Path) *Diagnostic {
+	policy, hasPolicy := w.collectionPolicies.at(path)
+	applicationFields, _ := w.applicationPolicy.identityFieldsAt(path)
+	var lower *collectionPolicy
+	if hasPolicy {
+		lower = &policy
+	}
+	resolution, policyDiagnostics := resolveCollectionPolicy(path, nil, applicationFields, lower)
+	if len(policyDiagnostics) != 0 {
+		return &policyDiagnostics[0]
+	}
+	// Without an identity rule, retain the existing diagnostic for a reference
+	// that has no enclosing sibling scope. A null cancellation needs no lookup.
+	if len(resolution.effectiveIdentityFields) == 0 {
+		return nil
+	}
+	original := *node
+	graph, diagnostics := analyzeIdentifiedInheritanceGraph(*node, path, resolution.effectiveIdentityFields)
+	if len(diagnostics) != 0 {
+		return &diagnostics[0]
+	}
+	before, _ := inheritanceLowerBoundCount(*node, -1)
+	limits := w.limits
+	if limits.nodes >= 0 {
+		limits.nodes -= w.nodes - before
+	}
+	resolved, diagnostics := applyIdentifiedInheritanceTemplates(
+		graph, limits, w.origins, w.policyIndex.absoluteScope(path), w.applicationPolicy, w.retained,
+	)
+	if len(diagnostics) != 0 {
+		return &diagnostics[0]
+	}
+	*node = resolved.root
+	w.policyIndex.replace(&w.collectionPolicies, path, resolved.collectionPolicies)
+	deleteOriginSubtree(w.origins, path, original)
+	for pointer, origin := range resolved.origins {
+		w.origins[pointer] = origin
+	}
+	w.transfers = append(w.transfers, resolved.transfers...)
+	w.controls = append(w.controls, resolved.controls...)
+	w.replacements = append(w.replacements, resolved.replacements...)
+	w.operations = append(w.operations, resolved.operations...)
+	after, _ := inheritanceLowerBoundCount(*node, -1)
+	w.nodes += after - before
+	return nil
+}
+
 func scopeHasInheritanceDefinitions(scope *document.Node) bool {
 	for _, entry := range scope.Mapping {
 		if entry.Value.Kind != document.Mapping {

internal/engine/item_policy_test.go

@@ -0,0 +1,471 @@
+package engine
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+func parseItemPolicyDocument(t *testing.T, sourceID, input string) *document.Node {
+	t.Helper()
+	root, issues := yamladapter.Parse(strings.NewReader(input), sourceID, yamladapter.Limits{
+		Depth: maxDocumentDepth, Nodes: maxDocumentNodes,
+	})
+	if len(issues) != 0 {
+		t.Fatalf("parse document: %#v", issues)
+	}
+	return root
+}
+
+func TestPendingItemCompositionKeepsLocalDeclarationsAndHelperIdentity(t *testing.T) {
+	t.Parallel()
+
+	declaration := sequenceDeclarationSpec{
+		destination: Keys("prompts"), identityFields: []string{"id"}, identityPresent: true,
+	}
+	itemScope := acquisitionScope{contributions: []acquiredContribution{
+		{
+			document:             parseItemPolicyDocument(t, "item", "name: authored\nprompts:\n  - id: authored\n"),
+			sequenceDeclarations: []sequenceDeclarationSpec{declaration},
+		},
+		{
+			document: parseItemPolicyDocument(t, "helper", "id: supplied\nprompts:\n  - id: helper\n"),
+		},
+	}}
+	outer := acquisitionScope{contributions: []acquiredContribution{{
+		source:      SourceInfo{ID: "item"},
+		payload:     parseItemPolicyDocument(t, "outer", "{}\n"),
+		destination: Keys("items"), pendingItem: &itemScope,
+	}}}
+
+	composed, diagnostics := composeAcquiredScopeWithPolicy(
+		&outer, collectionPolicyScope{}, sequencePolicy{}, newImportInclusionGraph(), nil, nil, nil, &importAccounting{},
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose pending item scope: %#v", diagnostics)
+	}
+
+	item := outer.contributions[0].payload
+	if got, ok := documentAtPath(item, Keys("id")); !ok || got == nil || got.Text != "supplied" {
+		t.Fatalf("helper-supplied record identity = %#v", got)
+	}
+	if got, ok := documentAtPath(item, Keys("prompts")); !ok || got == nil || got.Kind != document.Sequence || len(got.Sequence) != 2 {
+		t.Fatalf("composed prompts = %#v, want two identified entries", got)
+	}
+	itemPolicy, ok := composed.collectionPolicies.at(Keys("items"))
+	if !ok {
+		t.Fatalf("projected item collection policy missing")
+	}
+	owned, ok := itemPolicy.recordPolicyAt(0)
+	if !ok {
+		t.Fatalf("projected item owner missing: %#v", itemPolicy)
+	}
+	if policy, ok := owned.scope.at(Keys("prompts")); !ok || !policy.identityPresent || len(policy.identityFields) != 1 || policy.identityFields[0] != "id" {
+		t.Fatalf("item-owned prompt policy = %#v, %v", policy, ok)
+	}
+	if _, ok := composed.collectionPolicies.at(Keys("prompts")); ok {
+		t.Fatal("item-local declaration escaped its owning record")
+	}
+}
+
+func TestPendingItemCompositionProjectsNestedIdentityDiagnostics(t *testing.T) {
+	t.Parallel()
+
+	childScope := acquisitionScope{contributions: []acquiredContribution{
+		{document: parseItemPolicyDocument(t, "child", "prompts:\n  - id: same\n"), sequenceDeclarations: []sequenceDeclarationSpec{{
+			destination: Keys("prompts"), identityFields: []string{"id"}, identityPresent: true,
+		}}},
+		{document: parseItemPolicyDocument(t, "child-helper", "prompts:\n  - id: same\n")},
+	}}
+	itemScope := acquisitionScope{contributions: []acquiredContribution{{
+		document: parseItemPolicyDocument(t, "item", "name: parent\n"),
+	}, {
+		source: SourceInfo{ID: "child"}, payload: parseItemPolicyDocument(t, "child", "{}\n"),
+		destination: Keys("children"), pendingItem: &childScope,
+	}}}
+	outer := acquisitionScope{contributions: []acquiredContribution{{
+		source: SourceInfo{ID: "item"}, payload: parseItemPolicyDocument(t, "outer", "{}\n"),
+		destination: Keys("items"), pendingItem: &itemScope,
+	}}}
+
+	traversal := importTraversal{accounting: &importAccounting{}, limits: defaultImportLimits()}
+	diagnostics := traversal.composePendingItems(&outer)
+	if len(diagnostics) != 1 {
+		t.Fatalf("diagnostics = %#v, want one conflict", diagnostics)
+	}
+	if diagnostics[0].Code != CodeComposeConflict || diagnostics[0].Path.String() != "/items/0/children/0/prompts" {
+		t.Fatalf("diagnostic = %#v, want projected collection path", diagnostics[0])
+	}
+	if strings.Contains(fmt.Sprint(diagnostics[0]), "same") {
+		t.Fatalf("diagnostic disclosed identity value: %v", diagnostics[0])
+	}
+}
+
+func TestPendingItemCompositionAppliesOuterApplicationIdentityConstraints(t *testing.T) {
+	t.Parallel()
+
+	itemScope := acquisitionScope{contributions: []acquiredContribution{{
+		document: parseItemPolicyDocument(t, "item", "prompts:\n  - text: missing-id\n"),
+	}}}
+	outer := acquisitionScope{contributions: []acquiredContribution{{
+		source: SourceInfo{ID: "item"}, payload: parseItemPolicyDocument(t, "outer", "{}\n"),
+		destination: Keys("items"), pendingItem: &itemScope,
+	}}}
+	policy, diagnostics := compileSequenceIdentityPolicy([]sequenceIdentityRegistration{{
+		selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"},
+	}})
+	if len(diagnostics) != 0 {
+		t.Fatalf("compile application identity: %#v", diagnostics)
+	}
+
+	_, diagnostics = composeAcquiredScopeWithPolicy(&outer, collectionPolicyScope{}, policy,
+		newImportInclusionGraph(), nil, nil, nil, &importAccounting{})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeInvalid || diagnostics[0].Path.String() != "/items/0/id" {
+		t.Fatalf("application identity diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestReplacingOuterCollectionRetiresDiscardedItemIdentity(t *testing.T) {
+	t.Parallel()
+
+	outerPath := Keys("items")
+	nestedPath := Keys("items").Index(0).Key("prompts")
+	lower := collectionPolicyScope{}
+	outerPolicy := collectionPolicy{}
+	outerPolicy.setRecordPolicy(newCollectionRecordPolicy())
+	lower.set(outerPath, outerPolicy)
+	lower.set(nestedPath, collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+
+	declarations := []sequenceDeclarationSpec{{
+		destination: outerPath, mode: sequenceModeReplace, modePresent: true,
+	}}
+	input := parseItemPolicyDocument(t, "replacement", "items:\n  - prompts:\n      - text: new\n")
+	composed, diagnostics := composeDocumentsWithCollectionPolicy(sequencePolicy{}, lower, declarations, input)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose replacement: %#v", diagnostics)
+	}
+	policy, ok := composed.collectionPolicies.at(nestedPath)
+	if !ok || policy.identityPresent || len(policy.recordOrder) != 0 {
+		t.Fatalf("discarded nested owner policy = %#v, %v", policy, ok)
+	}
+}
+
+func TestNestedItemPolicyProjectionUsesStableOutputIndexes(t *testing.T) {
+	t.Parallel()
+
+	itemsPath := Keys("items")
+	outerPolicy := collectionPolicy{}
+	retainedScope := collectionPolicyScope{}
+	retainedScope.set(Keys("retained"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+	outerPolicy.setRecordPolicy(collectionRecordPolicy{owner: newCollectionRecordPolicy().owner, scope: retainedScope})
+	composed := &composedDocument{collectionPolicies: collectionPolicyScope{}}
+	composed.collectionPolicies.set(itemsPath, outerPolicy)
+
+	firstScope := collectionPolicyScope{}
+	firstScope.set(Keys("prompts"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+	secondScope := collectionPolicyScope{}
+	secondScope.set(Keys("prompts"), collectionPolicy{identityFields: []string{"slug"}, identityPresent: true})
+	scope := acquisitionScope{contributions: []acquiredContribution{
+		{document: parseItemPolicyDocument(t, "first", "items:\n  - id: first\n"), destination: itemsPath, nested: &composedDocument{collectionPolicies: firstScope}},
+		{document: parseItemPolicyDocument(t, "second", "items:\n  - id: second\n"), destination: itemsPath, nested: &composedDocument{collectionPolicies: secondScope}},
+	}}
+
+	projectNestedCollectionPolicies(composed, scope, nil)
+	policy, ok := composed.collectionPolicies.at(itemsPath)
+	if !ok || len(policy.recordOrder) != 3 {
+		t.Fatalf("record-owned policies = %#v, %v; want retained plus two item scopes", policy, ok)
+	}
+	for index, wantField := range []string{"id", "id", "slug"} {
+		record, ok := policy.recordPolicyAt(index)
+		if !ok {
+			t.Fatalf("record policy %d missing", index)
+		}
+		path := Keys("retained")
+		if index != 0 {
+			path = Keys("prompts")
+		}
+		nested, ok := record.scope.at(path)
+		if !ok || !nested.identityPresent || len(nested.identityFields) != 1 || nested.identityFields[0] != wantField {
+			t.Errorf("record policy %d = %#v, %v; want %q", index, nested, ok, wantField)
+		}
+	}
+}
+
+func TestItemPolicyProjectionCountsContributionsWithoutLocalPolicy(t *testing.T) {
+	t.Parallel()
+	items := Keys("items")
+	for _, assignments := range []map[int]int{nil, {1: 2, 3: 4}, {1: 2}, {3: 4}} {
+		t.Run(fmt.Sprint(assignments), func(t *testing.T) {
+			local := collectionPolicyScope{}
+			local.set(Keys("steps"), collectionPolicy{identityFields: []string{"name"}, identityPresent: true})
+			scope := acquisitionScope{contributions: []acquiredContribution{
+				{document: parseItemPolicyDocument(t, "inline", "items: [{id: inline-a}, {id: inline-b}]\n")},
+				{document: parseItemPolicyDocument(t, "first", "items: [{id: first}]\n"), destination: items, nested: &composedDocument{collectionPolicies: local}},
+				{document: parseItemPolicyDocument(t, "gap", "items: [{id: gap}]\n"), destination: items},
+				{document: parseItemPolicyDocument(t, "second", "items: [{id: second}]\n"), destination: items, nested: &composedDocument{collectionPolicies: local}},
+			}}
+			ranges := make(map[sequenceRangeKey]sequenceRange)
+			for sourceIndex, output := range assignments {
+				ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: items.String()}] = sequenceRange{start: output, end: output + 1}
+			}
+			composed := &composedDocument{}
+			projectNestedCollectionPolicies(composed, scope, ranges)
+			policy, _ := composed.collectionPolicies.at(items)
+			if len(policy.recordOrder) != 5 {
+				t.Fatalf("record policies = %d, want 5 positions", len(policy.recordOrder))
+			}
+			for index := range 5 {
+				record, _ := policy.recordPolicyAt(index)
+				_, found := record.scope.at(Keys("steps"))
+				if found != (index == 2 || index == 4) {
+					t.Errorf("local policy at item %d = %t", index, found)
+				}
+			}
+		})
+	}
+}
+
+func TestItemPolicyProjectionDetachesRepeatedScopes(t *testing.T) {
+	t.Parallel()
+	local := collectionPolicyScope{}
+	local.set(Keys("steps"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+	scope := acquisitionScope{}
+	ranges := make(map[sequenceRangeKey]sequenceRange)
+	for index, destination := range []string{"items", "other", "items"} {
+		path := Keys(destination)
+		scope.contributions = append(scope.contributions, acquiredContribution{
+			document: parseItemPolicyDocument(t, "item", destination+": [{}]\n"), destination: path,
+			nested: &composedDocument{collectionPolicies: local},
+		})
+		output := 0
+		if index == 2 {
+			output = 1
+		}
+		ranges[sequenceRangeKey{sourceIndex: index, path: path.String()}] = sequenceRange{start: output, end: output + 1}
+	}
+	first, second := &composedDocument{}, &composedDocument{}
+	projectNestedCollectionPolicies(first, scope, ranges)
+	projectNestedCollectionPolicies(second, scope, ranges)
+	policy := first.collectionPolicies.collections[Keys("items").String()].policy
+	if len(policy.recordOrder) != 2 || policy.recordOrder[0] == policy.recordOrder[1] {
+		t.Fatal("separately placed records did not retain distinct owners")
+	}
+	other := second.collectionPolicies.collections[Keys("items").String()].policy
+	if policy.recordOrder[0] == other.recordOrder[0] {
+		t.Fatal("independent projections reused a record owner")
+	}
+	// Mutate stored state directly: a detached query result would conceal an
+	// accidental alias between incrementally constructed record scopes.
+	steps := Keys("steps").String()
+	firstRecord := policy.recordPolicies[policy.recordOrder[0]]
+	firstRecord.collections[steps].policy.identityFields[0] = "changed"
+	for _, check := range []struct {
+		composed *composedDocument
+		path     Path
+		index    int
+	}{
+		{first, Keys("items"), 1}, {first, Keys("other"), 0},
+		{second, Keys("items"), 0}, {second, Keys("items"), 1}, {second, Keys("other"), 0},
+	} {
+		outer, _ := check.composed.collectionPolicies.at(check.path)
+		record, found := outer.recordPolicyAt(check.index)
+		identity, present := record.scope.at(Keys("steps"))
+		if !found || !present || !reflect.DeepEqual(identity.identityFields, []string{"id"}) {
+			t.Fatalf("mutation leaked into %s item %d: %#v", check.path, check.index, identity)
+		}
+	}
+	if local.collections[steps].policy.identityFields[0] != "id" {
+		t.Fatal("projected state mutated the incoming scope")
+	}
+	local.collections[steps].policy.identityFields[0] = "source-changed"
+	if other.recordPolicies[other.recordOrder[0]].collections[steps].policy.identityFields[0] != "id" {
+		t.Fatal("incoming scope remained aliased to projected state")
+	}
+}
+
+func TestOverlaySequenceTransfersIncomingItemPolicyOwnership(t *testing.T) {
+	path := Keys("records")
+	registered := mustCompileSequenceIdentityPolicy(t, []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("records")}, identityFields: []string{"id"}}})
+	makePolicies := func(childKey string) collectionPolicyScope {
+		local := collectionPolicyScope{}
+		local.set(Keys("steps"), collectionPolicy{
+			identityFields: []string{childKey}, identityPresent: true,
+			explicitMode: sequenceModeAppend, explicitModePresent: true,
+		})
+		record := newCollectionRecordPolicy()
+		record.scope = local
+		outer := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+		outer.setRecordPolicy(record)
+		scope := collectionPolicyScope{}
+		scope.set(path, outer)
+		return scope
+	}
+	checkPolicy := func(t *testing.T, scope collectionPolicyScope, index int, key string) {
+		t.Helper()
+		outer, ok := scope.at(path)
+		if !ok {
+			t.Fatal("outer collection policy missing")
+		}
+		record, ok := outer.recordPolicyAt(index)
+		if !ok {
+			t.Fatalf("record policy %d missing", index)
+		}
+		local, ok := record.scope.at(Keys("steps"))
+		if !ok || !local.identityPresent || !reflect.DeepEqual(local.identityFields, []string{key}) || !local.explicitModePresent || local.explicitMode != sequenceModeAppend {
+			t.Fatalf("record %d nested policy = %#v, %v; want key %q and append", index, local, ok, key)
+		}
+	}
+
+	tests := []struct {
+		name         string
+		lower        string
+		higher       string
+		declarations []sequenceDeclarationSpec
+		wantRecords  int
+	}{
+		{
+			name:        "upserting the same outer identity replaces its local policy",
+			lower:       "records: [{id: same, steps: [{id: old}]}]\n",
+			higher:      "records: [{id: same, steps: [{slug: new}]}]\n",
+			wantRecords: 1,
+		},
+		{
+			name:        "retained and appended outer records keep independent policies",
+			lower:       "records: [{id: lower, steps: [{id: old}]}]\n",
+			higher:      "records: [{id: higher, steps: [{slug: new}]}]\n",
+			wantRecords: 2,
+		},
+		{
+			name:   "whole sequence replacement transfers new local policy",
+			lower:  "records: [{id: same, steps: [{id: old}]}]\n",
+			higher: "records: [{id: same, steps: [{slug: new}]}]\n",
+			declarations: []sequenceDeclarationSpec{{
+				destination: path, mode: sequenceModeReplace, modePresent: true,
+			}},
+			wantRecords: 1,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			lowerRoot := parseItemPolicyDocument(t, "lower", test.lower)
+			incomingRoot := parseItemPolicyDocument(t, "higher", test.higher)
+			assembly := newAssemblyAccumulator(&composedDocument{
+				root: lowerRoot, collectionPolicies: makePolicies("id"), origins: make(map[string]Origin),
+			})
+			if diagnostics := assembly.applyOverlayWithCollectionPolicy(
+				incomingRoot, nil, makePolicies("slug"), nil, LayerBase, registered, test.declarations,
+			); len(diagnostics) != 0 {
+				t.Fatalf("overlay diagnostics = %#v", diagnostics)
+			}
+			outer, ok := assembly.current.collectionPolicies.at(path)
+			if !ok || len(outer.recordOrder) != test.wantRecords {
+				t.Fatalf("outer record policies = %#v, %v", outer, ok)
+			}
+			if test.wantRecords == 2 {
+				checkPolicy(t, assembly.current.collectionPolicies, 0, "id")
+				checkPolicy(t, assembly.current.collectionPolicies, 1, "slug")
+			} else {
+				checkPolicy(t, assembly.current.collectionPolicies, 0, "slug")
+			}
+		})
+	}
+}
+
+func TestAncestorReplacementRequiresIdentityWhenRetainedModeReturns(t *testing.T) {
+	path := Keys("catalog", "prompts")
+	newAssembly := func(t *testing.T) *assemblyAccumulator {
+		t.Helper()
+		policy := collectionPolicy{
+			identityFields: []string{"id"}, identityPresent: true,
+			explicitMode: sequenceModeAppend, explicitModePresent: true,
+		}
+		scope := collectionPolicyScope{}
+		scope.set(path, policy)
+		return newAssemblyAccumulator(&composedDocument{
+			root:               parseItemPolicyDocument(t, "base", "catalog:\n  prompts: [{id: old}]\n"),
+			collectionPolicies: scope, origins: make(map[string]Origin),
+		})
+	}
+	removeAncestor := func(t *testing.T, assembly *assemblyAccumulator) {
+		t.Helper()
+		if diagnostics := assembly.applyOverlayWithCollectionPolicy(
+			parseItemPolicyDocument(t, "remove", "catalog: null\n"), nil, collectionPolicyScope{}, nil,
+			LayerBase, sequencePolicy{}, nil,
+		); len(diagnostics) != 0 {
+			t.Fatalf("ancestor removal diagnostics = %#v", diagnostics)
+		}
+	}
+
+	t.Run("retained append mode requires a renewed identity", func(t *testing.T) {
+		assembly := newAssembly(t)
+		removeAncestor(t, assembly)
+		policy, ok := assembly.current.collectionPolicies.at(path)
+		if !ok || policy.identityPresent || !policy.explicitModePresent || policy.explicitMode != sequenceModeAppend {
+			t.Fatalf("retired collection policy = %#v, %v", policy, ok)
+		}
+		diagnostics := assembly.applyOverlayWithCollectionPolicy(
+			parseItemPolicyDocument(t, "reintroduction", "catalog:\n  prompts: [{id: restored, name: restored}]\n"), nil,
+			collectionPolicyScope{}, nil, LayerBase, sequencePolicy{}, nil,
+		)
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid {
+			t.Fatalf("append without renewed identity diagnostics = %#v", diagnostics)
+		}
+	})
+
+	t.Run("a renewed key restores the retained append mode", func(t *testing.T) {
+		assembly := newAssembly(t)
+		removeAncestor(t, assembly)
+		declaration := sequenceDeclarationSpec{
+			destination: path, identityFields: []string{"name"}, identityPresent: true,
+		}
+		diagnostics := assembly.applyOverlayWithCollectionPolicy(
+			parseItemPolicyDocument(t, "keyed", "catalog:\n  prompts: [{name: next}]\n"), nil,
+			collectionPolicyScope{}, nil, LayerBase, sequencePolicy{}, []sequenceDeclarationSpec{declaration},
+		)
+		if len(diagnostics) != 0 {
+			t.Fatalf("renewed identity diagnostics = %#v", diagnostics)
+		}
+		policy, ok := assembly.current.collectionPolicies.at(path)
+		if !ok || !policy.identityPresent || !reflect.DeepEqual(policy.identityFields, []string{"name"}) || !policy.explicitModePresent || policy.explicitMode != sequenceModeAppend {
+			t.Fatalf("renewed collection policy = %#v, %v", policy, ok)
+		}
+	})
+}
+
+func TestItemSourceVersionProcessingConsumesDeclarationsBeforeComposition(t *testing.T) {
+	t.Parallel()
+
+	root := parseItemPolicyDocument(t, "item", `version: v1
+configkit:
+  sequences:
+    - destination: [prompts]
+      key: id
+prompts:
+  - id: example
+`)
+	rule := &VersionRule{Path: Keys("version"), Check: func(version Version, _ *Validation) error {
+		if !version.Present || version.Text != "v1" {
+			t.Errorf("version = %#v, want v1", version)
+		}
+		return nil
+	}}
+	processed := processSourceVersionWithDirectiveContext(
+		root, rule, sourceVersionContribution, Keys("items").Index(0), sourceVersionModeRequired, rootDirectiveContextSource,
+	)
+	if len(processed.diagnostics) != 0 {
+		t.Fatalf("process item source version: %#v", processed.diagnostics)
+	}
+	if len(processed.sequences) != 1 || !processed.sequences[0].identityPresent {
+		t.Fatalf("extracted item declarations = %#v", processed.sequences)
+	}
+	if _, ok := documentAtPath(processed.payload, Keys("version")); ok {
+		t.Fatalf("consumed version header remains in item payload: %#v", processed.payload)
+	}
+	if _, ok := documentAtPath(processed.payload, Keys("configkit")); ok {
+		t.Fatalf("consumed controls remain in item payload: %#v", processed.payload)
+	}
+}

internal/engine/load.go

@@ -17,7 +17,7 @@ type Definition[T any] struct {
 	Version  *VersionRule
 	Secret   func(Path) bool
 
-	sequenceRegistrations []sequenceRegistration
+	sequenceIdentityRegistrations []sequenceIdentityRegistration
 }
 
 // Options contains the application definition and per-load input selections.
@@ -35,6 +35,7 @@ type Options[T any] struct {
 	placementMode           placementMode
 	sequencePolicy          sequencePolicy
 	deferAssembly           bool
+	directiveContext        rootDirectiveContext
 	importDefaultsValidated bool
 }
 
@@ -92,6 +93,10 @@ type Inspection struct {
 	baseline         *document.Node
 	input            *document.Node
 	inputOrigins     map[string]Origin
+	structuralInput  *document.Node
+	preCLI           *document.Node
+	preCLIOrigins    map[string]Origin
+	postDecodeApps   []historyApplication
 	final            *document.Node
 	immutableLookup  *documentLookup
 	resolution       []resolutionSnapshot
@@ -145,13 +150,13 @@ func Load[T any](source Source, secrets SecretBinding, supplied ...Options[T]) (
 
 func loadWithSecretLookup[T any](source Source, secrets SecretBinding, lookup secretLookup, supplied ...Options[T]) (*Loaded[T], error) {
 	options, diagnostics := selectOptions(source, supplied)
-	policy, policyDiagnostics := compileSequencePolicy(options.Definition.sequenceRegistrations)
+	policy, policyDiagnostics := compileSequenceIdentityPolicy(options.Definition.sequenceIdentityRegistrations)
 	diagnostics = append(diagnostics, policyDiagnostics...)
 	if len(diagnostics) != 0 {
 		return nil, (&diagnosticAccumulator{diagnostics: diagnostics}).loadError()
 	}
 	definition := options.Definition
-	definition.sequenceRegistrations = nil
+	definition.sequenceIdentityRegistrations = nil
 	request := loadRequest[T]{
 		main:       loadInput{source: source, imports: options.Imports, filesystem: options.Filesystem, destination: options.destination.clone(), placementMode: options.placementMode},
 		definition: definition, importDefaults: snapshotImportDefaults(options.importDefaults), sequencePolicy: policy, overlay: options.Overlay,
@@ -168,11 +173,14 @@ func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lo
 	mainOptions := Options[T]{Definition: request.definition, Imports: request.main.imports, Filesystem: request.main.filesystem, destination: request.main.destination.clone(), placementMode: request.main.placementMode}
 	mainOptions.importDefaults = snapshotImportDefaults(request.importDefaults)
 	mainOptions.sequencePolicy = request.sequencePolicy
+	mainOptions.deferAssembly = true
 	mainOptions.SourceVersionMode = request.sourceVersionMode
 	mainOptions.Definition.Version = request.protectionVersion()
-	mainOptions.Overrides = request.overrides
 	if request.ordinary {
 		mainOptions.Overlay = request.overlay
+		mainOptions.directiveContext = rootDirectiveContextSource
+	} else {
+		mainOptions.directiveContext = rootDirectiveContextLayered
 	}
 
 	var schema *schemaDescriptor
@@ -205,14 +213,7 @@ func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lo
 			return nil, accumulator.loadError()
 		}
 	}
-	if request.sourceVersionMode == sourceVersionModeDefault {
-		versionDiagnostics := checkVersion(request.definition.Version, input.document)
-		accumulator.add(versionDiagnostics...)
-		if hasErrors(versionDiagnostics) {
-			return nil, accumulator.loadError()
-		}
-	}
-	if request.ordinary && input.document != nil {
+	if request.ordinary && (input.document != nil || input.composition != nil) {
 		input.composition, diagnostics = compositionForInputWithPolicy(input, request.sequencePolicy)
 		if len(diagnostics) != 0 {
 			accumulator.add(diagnostics...)
@@ -220,7 +221,16 @@ func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lo
 		}
 		input.document = input.composition.root
 	}
+	if request.sourceVersionMode == sourceVersionModeDefault && request.ordinary {
+		versionDiagnostics := checkVersion(request.definition.Version, input.document)
+		accumulator.add(versionDiagnostics...)
+		if hasErrors(versionDiagnostics) {
+			return nil, accumulator.loadError()
+		}
+	}
 	if request.ordinary {
+		versionState.locateMainVersion(input.document)
+		mainOptions.Definition.Version = versionState.effectiveRule
 		input, diagnostics = applyInputLayers(request.main.source, mainOptions, input)
 	} else {
 		input, diagnostics = assembleLayeredInput(request, input, accounting)
@@ -228,9 +238,10 @@ func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lo
 	accumulator.add(versionState.diagnostics[reportedVersionDiagnostics:]...)
 	accumulator.sources = input.sources
 	accumulator.add(diagnostics...)
-	if len(diagnostics) != 0 {
+	if hasErrors(diagnostics) {
 		return nil, accumulator.loadError()
 	}
+	request.effectiveVersion = versionState.effectiveRule
 	if request.ordinary && input.parsed.source != nil {
 		baseSources := make([]SourceInfo, 0, len(input.sources))
 		for _, source := range input.sources {
@@ -253,20 +264,38 @@ func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lo
 			accumulator.add(problems...)
 			return nil, accumulator.loadError()
 		}
-		expanded, problems := expandInheritanceWithOrigins(composition.root, composition.origins, defaultProjectionLimits())
+		expanded, problems := expandInheritanceWithCollectionRules(
+			composition.root, composition.origins, composition.collectionPolicies, request.sequencePolicy, defaultProjectionLimits(),
+		)
 		if len(problems) != 0 {
 			accumulator.add(problems...)
 			return nil, accumulator.loadError()
 		}
-		if problem := inheritanceVersionDiagnostic(request.protectionVersion(), expanded); problem != nil {
+		effectiveVersion, problem := projectInheritanceVersion(request.protectionVersion(), expanded)
+		if problem != nil {
 			accumulator.add(*problem)
 			return nil, accumulator.loadError()
 		}
+		request.effectiveVersion = effectiveVersion
+		versionState.effectiveRule = effectiveVersion
 		input.composition = composition
 		input.inheritance = expanded
 		input.document = expanded.root
 	}
 
+	base := overridePreparationInput{root: input.document}
+	if input.composition != nil {
+		base.inputBytes = input.composition.inputBytes
+		base.inputNodes = input.composition.inputNodes
+	}
+	preparedOverrides, diagnostics := preparePostStructuralOverrides(base, input.sources, request.overrides, request.protectionVersion(), defaultImportLimits())
+	accumulator.add(diagnostics...)
+	if hasErrors(diagnostics) {
+		return nil, accumulator.loadError()
+	}
+	input.sources = preparedOverrides.sources
+	accumulator.sources = input.sources
+
 	decodeDefinition := request.definition
 	decodeDefinition.Version = request.protectionVersion()
 	decoded, diagnostics := decodeLoadInput(schema, decodeDefinition, input)
@@ -274,12 +303,25 @@ func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lo
 	if len(diagnostics) != 0 {
 		return nil, accumulator.loadError()
 	}
-	resolved, diagnostics := resolveLoadInput(schema, request.definition, input, decoded)
+	decodeContext := decodeContext{}
+	if decodeDefinition.Version != nil {
+		decodeContext.versionPath = &decodeDefinition.Version.Path
+	}
+	assigned, issues := applyDecodedOverrides(schema, decoded, preparedOverrides, decodeContext)
+	if len(issues) != 0 {
+		for _, issue := range issues {
+			accumulator.add(diagnosticFromDecodeIssue(issue))
+		}
+		return nil, accumulator.loadError()
+	}
+	postDecode := preparePostDecodeOverrideContext(input, decoded, assigned)
+	resolved, diagnostics := resolveLoadInput(schema, request.definition, input, postDecode)
 	accumulator.add(diagnostics...)
 	if hasErrors(diagnostics) {
 		return nil, accumulator.loadError()
 	}
-	if !historyNodesWithinLimit(decoded.baseline, decoded.final, resolved.final, resolved.applications, resolved.snapshots, input.inheritance) {
+	historyApplications := append(cloneHistoryApplications(resolved.applications), cloneHistoryApplications(resolved.postDecodeApps)...)
+	if !historyNodesWithinLimit(decoded.baseline, decoded.final, resolved.final, historyApplications, resolved.snapshots, input.inheritance) {
 		accumulator.add(Diagnostic{Code: CodeExplainLimitExceeded, Severity: SeverityError, Message: "retained configuration history exceeds the node limit"})
 		return nil, accumulator.loadError()
 	}
@@ -290,13 +332,18 @@ func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lo
 		accumulator.add(*ownershipIssue)
 		return nil, accumulator.loadError()
 	}
-	secretResolution := resolveSecretReferences(reflect.ValueOf(&configuration).Elem(), schema, input.document, lookup)
+	secretResolution := resolveSecretReferences(reflect.ValueOf(&configuration).Elem(), schema, resolved.referenceInput, lookup)
 	accumulator.add(secretResolution.diagnostics...)
 	if hasErrors(secretResolution.diagnostics) {
 		return nil, accumulator.loadError()
 	}
+	identityDiagnostics := validateFinalSequenceIdentities(resolved.referenceInput, request.sequencePolicy)
+	accumulator.add(identityDiagnostics...)
+	if hasErrors(identityDiagnostics) {
+		return nil, accumulator.loadError()
+	}
 	if request.definition.Validate != nil {
-		diagnostics = runValidation(&configuration, request.definition.Validate, schema, resolved.final, input.document, secretResolution.lookups)
+		diagnostics = runValidation(&configuration, request.definition.Validate, schema, resolved.final, resolved.referenceInput, secretResolution.lookups)
 		accumulator.add(diagnostics...)
 		if hasErrors(diagnostics) {
 			return nil, accumulator.loadError()

internal/engine/load_input.go

@@ -16,36 +16,42 @@ type assembledInput struct {
 	sources      []SourceInfo
 	contexts     map[string]ResourceContext
 	versionState *sourceVersionState
+	accounting   *importAccounting
 }
 
 // acquiredContribution keeps a source-local payload beside its placed document.
 // The latter is used for composition; the former preserves the acquisition
 // boundary for later scoped assembly and evidence.
 type acquiredContribution struct {
-	source              SourceInfo
-	identity            importNodeKey
-	role                importContributionRole
-	scopeOwner          importNodeKey
-	payload             *document.Node
-	document            *document.Node
-	destination         Path
-	relativeDestination Path
-	processing          *sourceVersionProcessingResult
-	nested              *composedDocument
-	pendingItem         *acquisitionScope
-	placementCharged    bool
+	source               SourceInfo
+	identity             importNodeKey
+	role                 importContributionRole
+	scopeOwner           importNodeKey
+	payload              *document.Node
+	document             *document.Node
+	destination          Path
+	relativeDestination  Path
+	processing           *sourceVersionProcessingResult
+	sequenceDeclarations []sequenceDeclarationSpec
+	nested               *composedDocument
+	pendingItem          *acquisitionScope
+	placementCharged     bool
 }
 
 // acquisitionScope is the ordered strict mapping scope produced by one input
 // and its ordinary imports.
 type acquisitionScope struct {
-	owner         importNodeKey
-	destination   Path
-	contributions []acquiredContribution
+	owner              importNodeKey
+	destination        Path
+	contributions      []acquiredContribution
+	collectionPolicies collectionPolicyScope
 }
 
 func cloneAcquisitionScope(scope acquisitionScope) acquisitionScope {
-	cloned := acquisitionScope{owner: scope.owner, destination: scope.destination.clone()}
+	cloned := acquisitionScope{
+		owner: scope.owner, destination: scope.destination.clone(),
+		collectionPolicies: cloneCollectionPolicyScope(scope.collectionPolicies),
+	}
 	if scope.contributions != nil {
 		cloned.contributions = make([]acquiredContribution, len(scope.contributions))
 		for index, contribution := range scope.contributions {
@@ -54,7 +60,8 @@ func cloneAcquisitionScope(scope acquisitionScope) acquisitionScope {
 				role: contribution.role, scopeOwner: contribution.scopeOwner, payload: contribution.payload,
 				document: contribution.document, destination: contribution.destination.clone(),
 				relativeDestination: contribution.relativeDestination.clone(), processing: contribution.processing,
-				nested: cloneComposedDocument(contribution.nested), pendingItem: cloneAcquisitionScopePointer(contribution.pendingItem),
+				sequenceDeclarations: cloneSequenceDeclarations(contribution.sequenceDeclarations),
+				nested:               cloneComposedDocument(contribution.nested), pendingItem: cloneAcquisitionScopePointer(contribution.pendingItem),
 				placementCharged: contribution.placementCharged,
 			}
 		}
@@ -102,7 +109,7 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		parsed.source.Destination = destination.clone()
 		parsed.source.placementMode = options.placementMode
 	}
-	input := assembledInput{parsed: parsed, document: parsed.document, sources: sourceInfos(parsed.source), versionState: versionState}
+	input := assembledInput{parsed: parsed, document: parsed.document, sources: sourceInfos(parsed.source), versionState: versionState, accounting: accounting}
 	if len(diagnostics) != 0 {
 		input.sources = sourceInfos(parsed.source)
 		// Mapping value issues already identify application content. Other early
@@ -116,13 +123,70 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		}
 		return input, diagnostics
 	}
+	directiveContext := options.directiveContext
+	if directiveContext == 0 {
+		directiveContext = rootDirectiveContextImportsOnly
+	}
 	if !options.Imports && parsed.document != nil {
-		if diagnostics := disabledDirectiveDiagnostics(parsed.document, Path{}); len(diagnostics) != 0 {
-			return input, diagnostics
+		var directiveDiagnostics []Diagnostic
+		if directiveContext == rootDirectiveContextSource || directiveContext == rootDirectiveContextLayered {
+			directiveDiagnostics = disabledRootImportDiagnostics(parsed.document)
+		} else {
+			directiveDiagnostics = disabledDirectiveDiagnostics(parsed.document, Path{})
+		}
+		if len(directiveDiagnostics) != 0 {
+			return input, directiveDiagnostics
 		}
 	}
 	var versionProcessing *sourceVersionProcessingResult
-	if versionState != nil && versionState.mode != sourceVersionModeDefault {
+	if directiveContext == rootDirectiveContextSource {
+		var sourceID string
+		if parsed.source != nil {
+			sourceID = parsed.source.ID
+		}
+		var processed sourceVersionProcessingResult
+		var failures []Diagnostic
+		if versionState != nil {
+			processed, failures = versionState.processWithDirectiveContext(parsed.document, role, destination, sourceID, options.placementMode, directiveContext)
+		} else {
+			processed = processSourceVersionWithDirectiveContext(parsed.document, options.Definition.Version, role, destination, sourceVersionModeDefault, directiveContext)
+			failures = processed.diagnostics
+		}
+		if len(failures) != 0 {
+			return input, failures
+		}
+		versionProcessing = &processed
+		if role == sourceVersionMain && versionState != nil {
+			versionState.mainProcessing = &processed
+		}
+	} else if directiveContext == rootDirectiveContextLayered {
+		extracted, failures := extractRootDirectives(parsed.document, rootDirectiveContextLayered)
+		if len(failures) != 0 {
+			return input, failures
+		}
+		processed := sourceVersionProcessingResult{
+			payload: extracted.body, imports: extracted.imports, sequences: extracted.sequences,
+			directiveContext: rootDirectiveContextLayered,
+		}
+		if versionState != nil && versionState.mode != sourceVersionModeDefault {
+			var sourceID string
+			if parsed.source != nil {
+				sourceID = parsed.source.ID
+			}
+			versionProcessed, versionFailures := versionState.process(extracted.body, role, destination, sourceID, options.placementMode)
+			if len(versionFailures) != 0 {
+				return input, versionFailures
+			}
+			versionProcessed.imports = extracted.imports
+			versionProcessed.sequences = append(cloneSequenceDeclarations(extracted.sequences), versionProcessed.sequences...)
+			versionProcessed.directiveContext = rootDirectiveContextLayered
+			processed = versionProcessed
+			if role == sourceVersionMain {
+				versionState.mainProcessing = &processed
+			}
+		}
+		versionProcessing = &processed
+	} else if versionState != nil && versionState.mode != sourceVersionModeDefault {
 		var sourceID string
 		if parsed.source != nil {
 			sourceID = parsed.source.ID
@@ -168,6 +232,10 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		localPayload := input.document
 		input.document = placed
 		identity := importKeyForMainSource(parsed.source)
+		var sequenceDeclarations []sequenceDeclarationSpec
+		if versionProcessing != nil {
+			sequenceDeclarations = cloneSequenceDeclarations(versionProcessing.sequences)
+		}
 		input.acquisition = []acquisitionScope{{
 			owner:       identity,
 			destination: destination.clone(),
@@ -175,6 +243,7 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 				source: cloneSourceInfo(*parsed.source), payload: localPayload,
 				identity: identity, role: importRoleMain, scopeOwner: identity,
 				document: placed, destination: destination.clone(), processing: versionProcessing,
+				sequenceDeclarations: sequenceDeclarations,
 			}},
 		}}
 		rawNodes := countInputNodes(parsed.document)
@@ -218,7 +287,7 @@ func registerAcquiredSource(source Source, context *FilesystemContext, info *Sou
 }
 
 func applyInputLayers[T any](source Source, options Options[T], input assembledInput) (assembledInput, []Diagnostic) {
-	if !activeOverlay(options.Overlay) && len(options.Overrides) == 0 {
+	if !activeOverlay(options.Overlay) {
 		return input, nil
 	}
 	composition, diagnostics := compositionForInputWithPolicy(input, options.sequencePolicy)
@@ -229,7 +298,7 @@ func applyInputLayers[T any](source Source, options Options[T], input assembledI
 	if activeOverlay(options.Overlay) {
 		input.sources, diagnostics = loadSelectedOverlayIntoWithVersion(
 			source, options.Filesystem, input.parsed, assembly, input.sources,
-			options.Overlay, options.Definition.Version, defaultImportLimits(), input.versionState,
+			options.Overlay, options.Definition.Version, defaultImportLimits(), input.versionState, options.sequencePolicy,
 		)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
@@ -240,33 +309,44 @@ func applyInputLayers[T any](source Source, options Options[T], input assembledI
 		}
 		mergeResourceContexts(&input.contexts, contexts)
 	}
-	if len(options.Overrides) != 0 {
-		input.sources, diagnostics = assembly.applyOverrides(
-			input.sources, options.Overrides, options.Definition.Version, defaultImportLimits(),
-		)
-		if len(diagnostics) != 0 {
-			return input, diagnostics
-		}
-	}
 	input.composition = assembly.current
 	input.document = assembly.current.root
 	return input, nil
 }
 
-// compositionForInput initializes one handoff for overlays and CLI, including
-// defaults-only CLI input. Raw source bytes and mapping keys retain their budgets.
+// compositionForInput initializes the structural handoff used by inheritance
+// expansion. Raw source bytes and mapping keys retain their budgets.
 func compositionForInput(input assembledInput) (*composedDocument, []Diagnostic) {
 	return compositionForInputWithPolicy(input, sequencePolicy{})
 }
 
 func compositionForInputWithPolicy(input assembledInput, policy sequencePolicy) (*composedDocument, []Diagnostic) {
+	return compositionForInputWithPolicies(input, collectionPolicyScope{}, policy)
+}
+
+func compositionForInputWithPolicies(input assembledInput, lower collectionPolicyScope, policy sequencePolicy) (*composedDocument, []Diagnostic) {
 	if input.composition != nil {
+		if input.composition.root == nil && len(input.composition.acquisition) != 0 {
+			return composeAcquiredInputWithPolicies(input, lower, policy)
+		}
 		return input.composition, nil
 	}
+	if len(input.acquisition) != 0 {
+		acquired := &composedDocument{acquisition: cloneAcquisitionScopes(input.acquisition)}
+		if input.accounting != nil {
+			acquired.inputBytes = input.accounting.bytes
+			acquired.inputNodes = input.accounting.nodes
+		} else {
+			acquired.inputBytes = input.parsed.bytes
+			acquired.inputNodes = countInputNodes(input.parsed.document)
+		}
+		input.composition = acquired
+		return composeAcquiredInputWithPolicies(input, lower, policy)
+	}
 	if input.document == nil {
 		return &composedDocument{root: &document.Node{Kind: document.Mapping}, origins: make(map[string]Origin)}, nil
 	}
-	composition, diagnostics := composeDocumentsWithPolicy(policy, input.document)
+	composition, diagnostics := composeDocumentsWithCollectionPolicy(policy, lower, nil, input.document)
 	if len(diagnostics) != 0 {
 		return nil, diagnostics
 	}

internal/engine/load_request.go

@@ -37,15 +37,25 @@ type loadInput struct {
 
 type loadGroup struct{ inputs []loadInput }
 
+type acquiredUnderlayGroup struct {
+	index       int
+	scopes      []acquisitionScope
+	importGraph importInclusionGraph
+	sources     []SourceInfo
+	contexts    map[string]ResourceContext
+	inputBytes  int64
+	inputNodes  int
+}
+
 func loadGroupedWithSecretLookup[T any](request loadRequest[T], secrets SecretBinding, lookup secretLookup) (*Loaded[T], error) {
 	request.ordinary = false
 	var diagnostics []Diagnostic
 	request.importDefaults = snapshotImportDefaults(request.importDefaults)
 	diagnostics = append(diagnostics, validateImportDefaults(request.importDefaults)...)
-	policy, policyDiagnostics := compileSequencePolicy(request.definition.sequenceRegistrations)
+	policy, policyDiagnostics := compileSequenceIdentityPolicy(request.definition.sequenceIdentityRegistrations)
 	request.sequencePolicy = policy
 	diagnostics = append(diagnostics, policyDiagnostics...)
-	request.definition.sequenceRegistrations = nil
+	request.definition.sequenceIdentityRegistrations = nil
 	mainOptions, mainDiagnostics := selectOptions(request.main.source, []Options[T]{{
 		Definition: request.definition, Imports: request.main.imports, Filesystem: request.main.filesystem, destination: request.main.destination.clone(),
 		SourceVersionMode: request.sourceVersionMode, placementMode: request.main.placementMode,
@@ -82,46 +92,114 @@ func loadGroupedWithSecretLookup[T any](request loadRequest[T], secrets SecretBi
 }
 
 func assembleLayeredInput[T any](request loadRequest[T], input assembledInput, accounting *importAccounting) (assembledInput, []Diagnostic) {
-	groups := make([]*composedDocument, 0, len(request.underlays))
+	var assembly *assemblyAccumulator
+	mainSourceCount := len(input.sources)
+	mainEvidence := input.versionState.evidenceSince(0)
+	projectVersion := sequenceVersionPath(request.protectionVersion())
 	for index, group := range request.underlays {
-		composition, sources, contexts, diagnostics := acquireUnderlayGroupWithPolicyVersion(group, index+1, request.protectionVersion(), accounting, input.versionState, request.sequencePolicy)
-		input.sources = append(input.sources, sources...)
-		mergeResourceContexts(&input.contexts, contexts)
+		evidenceStart := len(input.versionState.evidenceSince(0))
+		acquired, diagnostics := acquireUnderlayGroupScopes(group, index+1, request.protectionVersion(), accounting, input.versionState, request.sequencePolicy)
+		mergeResourceContexts(&input.contexts, acquired.contexts)
+		if len(diagnostics) != 0 {
+			input.sources = append(input.sources, acquired.sources...)
+			return input, diagnostics
+		}
+		var lower collectionPolicyScope
+		if assembly != nil && assembly.current != nil {
+			lower = assembly.current.collectionPolicies
+		}
+		protectionRule := request.protectionVersion()
+		if projectVersion {
+			// The main item's final position is known only after tier assembly.
+			// Source-local checks still run during acquisition above.
+			protectionRule = nil
+		}
+		composition, diagnostics := composeUnderlayGroup(acquired, protectionRule, request.sequencePolicy, lower)
+		sourceStart := len(input.sources)
+		input.sources = append(input.sources, acquired.sources...)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
 		if composition != nil {
-			groups = append(groups, composition)
+			if assembly == nil {
+				assembly = newAssemblyAccumulator(composition)
+			} else {
+				var declarations []sequenceDeclarationSpec
+				for _, scope := range composition.acquisition {
+					declarations = append(declarations, acquiredSequenceDeclarations(scope)...)
+				}
+				operations, diagnostics := assembly.applyOverlayWithCollectionProjection(
+					composition.root, composition.origins, composition.collectionPolicies, composition.applications, LayerUnderlay,
+					request.sequencePolicy, declarations,
+				)
+				if len(diagnostics) != 0 {
+					return input, diagnostics
+				}
+				projectTierSourceDestinations(input.sources[sourceStart:], input.versionState.evidenceSince(evidenceStart), operations)
+			}
 		}
 	}
 
-	var assembly *assemblyAccumulator
-	for _, group := range groups {
-		if assembly == nil {
-			assembly = newAssemblyAccumulator(group)
-			continue
+	mainPresent := input.document != nil || input.composition != nil || len(input.acquisition) != 0
+	var mainComposition *composedDocument
+	if mainPresent {
+		var lower collectionPolicyScope
+		if assembly != nil && assembly.current != nil {
+			lower = assembly.current.collectionPolicies
 		}
-		if diagnostics := assembly.applyOverlay(group.root, group.origins, group.applications, LayerUnderlay); len(diagnostics) != 0 {
-			return input, diagnostics
-		}
-	}
-	if input.document != nil {
-		mainComposition, diagnostics := compositionForInputWithPolicy(input, request.sequencePolicy)
+		var diagnostics []Diagnostic
+		mainComposition, diagnostics = compositionForInputWithPolicies(input, lower, request.sequencePolicy)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
-		if assembly == nil {
-			assembly = newAssemblyAccumulator(mainComposition)
-		} else if diagnostics := assembly.applyOverlay(mainComposition.root, mainComposition.origins, mainComposition.applications, LayerBase); len(diagnostics) != 0 {
-			return input, diagnostics
+	}
+	var versionDiagnostics []Diagnostic
+	if request.sourceVersionMode == sourceVersionModeDefault {
+		var mainRoot *document.Node
+		if mainComposition != nil {
+			mainRoot = mainComposition.root
+		} else if input.document != nil {
+			mainRoot = input.document
+		}
+		versionDiagnostics = checkVersion(request.definition.Version, mainRoot)
+		if hasErrors(versionDiagnostics) {
+			return input, versionDiagnostics
 		}
 	}
-	if assembly == nil && (activeOverlay(request.overlay) || len(request.overrides) != 0) {
+	if mainComposition != nil {
+		input.versionState.locateMainVersion(mainComposition.root)
+		if assembly == nil {
+			assembly = newAssemblyAccumulator(mainComposition)
+		} else {
+			var declarations []sequenceDeclarationSpec
+			for _, scope := range mainComposition.acquisition {
+				declarations = append(declarations, acquiredSequenceDeclarations(scope)...)
+			}
+			operations, diagnostics := assembly.applyOverlayWithCollectionProjection(
+				mainComposition.root, mainComposition.origins, mainComposition.collectionPolicies, mainComposition.applications, LayerBase,
+				request.sequencePolicy, declarations,
+			)
+			if len(diagnostics) != 0 {
+				return input, append(versionDiagnostics, diagnostics...)
+			}
+			projectTierSourceDestinations(input.sources[:mainSourceCount], mainEvidence, operations)
+			input.versionState.projectMainVersion(operations)
+		}
+	}
+	if input.versionState != nil {
+		request.effectiveVersion = input.versionState.effectiveRule
+	}
+	if projectVersion && assembly != nil {
+		if diagnostic := projectedUnderlayVersionDiagnostic(assembly.current.applications, input.sources, request.protectionVersion()); diagnostic != nil {
+			return input, append(versionDiagnostics, *diagnostic)
+		}
+	}
+	if assembly == nil && activeOverlay(request.overlay) {
 		composition, _ := compositionForInput(assembledInput{})
 		assembly = newAssemblyAccumulator(composition)
 	}
 	if assembly == nil {
-		return input, nil
+		return input, versionDiagnostics
 	}
 	assembly.current.inputBytes = accounting.bytes
 	assembly.current.inputNodes = accounting.nodes
@@ -131,37 +209,23 @@ func assembleLayeredInput[T any](request loadRequest[T], input assembledInput, a
 		selection := Source{kind: sourceBytes}
 		selected := parsedSource{document: &document.Node{Kind: document.Mapping}, source: &SourceInfo{}}
 		var diagnostics []Diagnostic
-		input.sources, diagnostics = loadSelectedOverlayIntoWithVersion(selection, request.overlayFilesystem, selected, assembly, input.sources, request.overlay, request.protectionVersion(), defaultImportLimits(), input.versionState)
+		overlayPolicy := []sequencePolicy{request.sequencePolicy}
+		input.sources, diagnostics = loadSelectedOverlayIntoWithVersion(selection, request.overlayFilesystem, selected, assembly, input.sources, request.overlay, request.protectionVersion(), defaultImportLimits(), input.versionState, overlayPolicy...)
 		if len(diagnostics) != 0 {
-			return input, diagnostics
+			return input, append(versionDiagnostics, diagnostics...)
 		}
 		contexts, diagnostic := contextsForAcquiredSources(selection, request.overlayFilesystem, selected.source, input.sources[len(input.sources)-1:])
 		if diagnostic != nil {
-			return input, []Diagnostic{*diagnostic}
+			return input, append(versionDiagnostics, *diagnostic)
 		}
 		mergeResourceContexts(&input.contexts, contexts)
 	}
-	if len(request.overrides) != 0 {
-		var diagnostics []Diagnostic
-		input.sources, diagnostics = assembly.applyOverrides(input.sources, request.overrides, request.protectionVersion(), defaultImportLimits())
-		if len(diagnostics) != 0 {
-			return input, diagnostics
-		}
-	}
 	input.composition = assembly.current
 	input.document = assembly.current.root
-	return input, nil
+	return input, versionDiagnostics
 }
 
-func acquireUnderlayGroup(group loadGroup, index int, rule *VersionRule, accounting *importAccounting) (*composedDocument, []SourceInfo, map[string]ResourceContext, []Diagnostic) {
-	return acquireUnderlayGroupWithVersion(group, index, rule, accounting, nil)
-}
-
-func acquireUnderlayGroupWithVersion(group loadGroup, index int, rule *VersionRule, accounting *importAccounting, versionState *sourceVersionState) (*composedDocument, []SourceInfo, map[string]ResourceContext, []Diagnostic) {
-	return acquireUnderlayGroupWithPolicyVersion(group, index, rule, accounting, versionState, sequencePolicy{})
-}
-
-func acquireUnderlayGroupWithPolicyVersion(group loadGroup, index int, rule *VersionRule, accounting *importAccounting, versionState *sourceVersionState, policy sequencePolicy) (*composedDocument, []SourceInfo, map[string]ResourceContext, []Diagnostic) {
+func acquireUnderlayGroupScopes(group loadGroup, index int, rule *VersionRule, accounting *importAccounting, versionState *sourceVersionState, policy sequencePolicy) (*acquiredUnderlayGroup, []Diagnostic) {
 	initialBytes, initialNodes := accounting.bytes, accounting.nodes
 	visited := make(map[resourceIdentity]importRoute)
 	hostToken := &namespaceToken{}
@@ -173,15 +237,14 @@ func acquireUnderlayGroupWithPolicyVersion(group loadGroup, index int, rule *Ver
 			}
 		}
 	}
-	var scopes []acquisitionScope
+	acquiredGroup := &acquiredUnderlayGroup{index: index}
 	inclusionGraph := newImportInclusionGraph()
-	var sources []SourceInfo
-	var contexts map[string]ResourceContext
 	for inputIndex, contribution := range group.inputs {
 		options := Options[struct{}]{
 			Imports: contribution.imports, Filesystem: contribution.filesystem,
 			Definition: Definition[struct{}]{Version: rule}, destination: contribution.destination.clone(),
 			placementMode: contribution.placementMode, sequencePolicy: policy, deferAssembly: true,
+			directiveContext: rootDirectiveContextLayered,
 			importDefaults: func() importDefaults {
 				if versionState == nil {
 					return importDefaults{}
@@ -193,11 +256,11 @@ func acquireUnderlayGroupWithPolicyVersion(group loadGroup, index int, rule *Ver
 		for _, source := range acquired.sources {
 			source.Layer = LayerUnderlay
 			source.UnderlayIndex = index
-			sources = append(sources, source)
+			acquiredGroup.sources = append(acquiredGroup.sources, source)
 		}
-		mergeResourceContexts(&contexts, acquired.contexts)
+		mergeResourceContexts(&acquiredGroup.contexts, acquired.contexts)
 		if len(diagnostics) != 0 {
-			return nil, sources, contexts, projectPriorInputDiagnosticPaths(diagnostics, scopes)
+			return acquiredGroup, projectPriorInputDiagnosticPaths(diagnostics, acquiredGroup.scopes)
 		}
 		if acquired.document == nil && len(acquired.acquisition) == 0 {
 			continue
@@ -219,59 +282,66 @@ func acquireUnderlayGroupWithPolicyVersion(group loadGroup, index int, rule *Ver
 		for scopeIndex := range inputScopes {
 			reanchorAcquisitionScope(&inputScopes[scopeIndex], keyMap, anchors)
 		}
-		scopes = append(scopes, inputScopes...)
+		acquiredGroup.scopes = append(acquiredGroup.scopes, inputScopes...)
 		mergeImportInclusionGraph(&inclusionGraph, inputGraph)
 	}
-	groupScope := scheduleAcquisitionGroup(scopes, inclusionGraph)
-	traversal := importTraversal{accounting: accounting, limits: defaultImportLimits(), rule: rule, importGraph: inclusionGraph}
-	if diagnostics := traversal.composePendingItems(&groupScope); len(diagnostics) != 0 {
-		return nil, sources, contexts, diagnostics
+	acquiredGroup.importGraph = inclusionGraph.clone()
+	acquiredGroup.inputBytes = accounting.bytes - initialBytes
+	acquiredGroup.inputNodes = accounting.nodes - initialNodes
+	return acquiredGroup, nil
+}
+
+func composeUnderlayGroup(acquired *acquiredUnderlayGroup, rule *VersionRule, policy sequencePolicy, lower collectionPolicyScope) (*composedDocument, []Diagnostic) {
+	if acquired == nil {
+		return nil, nil
+	}
+	groupScope := scheduleAcquisitionGroup(acquired.scopes, acquired.importGraph)
+	if len(groupScope.contributions) == 0 {
+		return nil, nil
+	}
+	groupScope.owner = importNodeKey{namespaceKind: "group", namespaceAnchor: explicitNamespaceAnchor(acquired.index, 0), resourceKind: "scope"}
+	for contributionIndex := range groupScope.contributions {
+		groupScope.contributions[contributionIndex].scopeOwner = groupScope.owner
+	}
+	accounting := &importAccounting{
+		sources: len(acquired.sources), bytes: acquired.inputBytes, nodes: acquired.inputNodes,
+	}
+	composition, diagnostics := composeAcquiredScopeWithPolicy(
+		&groupScope, lower, policy, acquired.importGraph, acquired.sources, nil, nil, accounting,
+	)
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
 	}
 	var documents []*document.Node
 	for _, contribution := range groupScope.contributions {
 		documents = append(documents, contribution.document)
 	}
-	if len(documents) == 0 {
-		return nil, sources, contexts, nil
-	}
-	composition, diagnostics := composeDocumentsWithPolicy(policy, documents...)
-	if len(diagnostics) != 0 {
-		return nil, sources, contexts, diagnostics
-	}
 	if composition != nil && rule != nil {
 		ranges := indexSequenceRanges(composition.sequenceRanges)
-		for sourceIndex, document := range documents {
+		for sourceIndex, input := range documents {
 			if groupScope.contributions[sourceIndex].role == importRoleMain {
 				continue
 			}
-			if node := fragmentVersionClaimWithIndex(document, rule, sourceIndex, ranges); node != nil {
+			if node := fragmentVersionClaimWithIndex(input, rule, sourceIndex, ranges); node != nil {
 				location := publicLocation(node.Location)
-				return nil, sources, contexts, []Diagnostic{{
+				return nil, []Diagnostic{{
 					Code: CodeImportVersion, Severity: SeverityError, Path: rule.Path.clone(),
 					Message: "imported fragments cannot contribute to the schema-version path", Primary: &location,
 				}}
 			}
 		}
-		for sourceIndex, document := range documents {
-			if node := fragmentVersionClaimWithIndex(document, rule, sourceIndex, ranges); node != nil {
+		for sourceIndex, input := range documents {
+			if node := fragmentVersionClaimWithIndex(input, rule, sourceIndex, ranges); node != nil {
 				location := publicLocation(node.Location)
-				return nil, sources, contexts, []Diagnostic{{
+				return nil, []Diagnostic{{
 					Code: CodeUnderlayVersion, Severity: SeverityError, Path: rule.Path.clone(),
 					Message: "underlay inputs cannot contribute to the schema-version path", Primary: &location,
 				}}
 			}
 		}
 	}
-	composition.inputBytes = accounting.bytes - initialBytes
-	composition.inputNodes = accounting.nodes - initialNodes
-	groupScope.owner = importNodeKey{namespaceKind: "group", namespaceAnchor: explicitNamespaceAnchor(index, 0), resourceKind: "scope"}
-	for contributionIndex := range groupScope.contributions {
-		groupScope.contributions[contributionIndex].scopeOwner = groupScope.owner
-	}
-	composition.acquisition = []acquisitionScope{cloneAcquisitionScope(groupScope)}
-	composition.importGraph = inclusionGraph.clone()
-	projectNestedEvidence(composition, groupScope)
-	projectAcquiredDestinations(composition, groupScope, sources)
+	composition.inputBytes = acquired.inputBytes
+	composition.inputNodes = acquired.inputNodes
 	for pointer, origin := range composition.origins {
 		origin.Layer = LayerUnderlay
 		composition.origins[pointer] = origin
@@ -279,7 +349,7 @@ func acquireUnderlayGroupWithPolicyVersion(group loadGroup, index int, rule *Ver
 	for index := range composition.applications {
 		composition.applications[index].layer = LayerUnderlay
 	}
-	return composition, sources, contexts, nil
+	return composition, nil
 }
 
 func nativeTokenForLoadInput(input loadInput) *namespaceToken {

internal/engine/load_request_test.go

@@ -46,7 +46,7 @@ items: [{id: b}]
 			{source: File(zRoot), imports: true},
 			{source: File(bRoot), imports: true},
 		}}},
-		definition: Definition[map[string]any]{sequenceRegistrations: []sequenceRegistration{{path: Keys("items"), identityField: "id"}}},
+		definition: Definition[map[string]any]{sequenceIdentityRegistrations: []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"}}}},
 	}
 	result, err := testGroupedLoad(request)
 	if err != nil {
@@ -73,6 +73,50 @@ items: [{id: b}]
 	}
 }
 
+func TestUnderlayGroupAcquisitionRetainsScopesUntilComposition(t *testing.T) {
+	filesystem := NewFilesystem("handoff", fstest.MapFS{
+		"main.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: child.yml}]}\nmain: retained\n")},
+		"child.yml": &fstest.MapFile{Data: []byte("child: retained\n")},
+	})
+	accounting := &importAccounting{}
+	group := loadGroup{inputs: []loadInput{{source: filesystem.File("main.yml"), imports: true}}}
+	acquired, diagnostics := acquireUnderlayGroupScopes(group, 2, nil, accounting, nil, sequencePolicy{})
+	if len(diagnostics) != 0 {
+		t.Fatalf("acquire underlay group: %#v", diagnostics)
+	}
+	if acquired == nil || acquired.index != 2 || len(acquired.scopes) != 1 || len(acquired.scopes[0].contributions) != 2 {
+		t.Fatalf("acquisition handoff lost its source scope: %#v", acquired)
+	}
+	if len(acquired.sources) != 2 || acquired.sources[0].UnderlayIndex != 2 || acquired.sources[1].ID != "source-2" {
+		t.Fatalf("acquired source evidence = %#v", acquired.sources)
+	}
+	if acquired.inputBytes != int64(len("configkit: {imports: [{path: child.yml}]}\nmain: retained\n")+len("child: retained\n")) {
+		t.Fatalf("group input byte accounting = %d", acquired.inputBytes)
+	}
+
+	composition, diagnostics := composeUnderlayGroup(acquired, nil, sequencePolicy{}, collectionPolicyScope{})
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose acquired group: %#v", diagnostics)
+	}
+	for key, want := range map[string]string{"main": "retained", "child": "retained"} {
+		value, present := documentAtPath(composition.root, Keys(key))
+		if !present || value.Text != want {
+			t.Errorf("composed %s = %#v, %v; want %q", key, value, present, want)
+		}
+	}
+	if composition.inputBytes != acquired.inputBytes || len(composition.acquisition) != 1 || len(composition.acquisition[0].contributions) != 2 {
+		t.Fatalf("composed group handoff = %#v", composition)
+	}
+
+	empty, diagnostics := acquireUnderlayGroupScopes(loadGroup{}, 3, nil, accounting, nil, sequencePolicy{})
+	if len(diagnostics) != 0 || len(empty.scopes) != 0 {
+		t.Fatalf("empty group acquisition = %#v, %#v", empty, diagnostics)
+	}
+	if composition, diagnostics := composeUnderlayGroup(empty, nil, sequencePolicy{}, collectionPolicyScope{}); len(diagnostics) != 0 || composition != nil {
+		t.Fatalf("empty group composition = %#v, %#v", composition, diagnostics)
+	}
+}
+
 func TestUnderlayNamespaceAnchorsUseInputPositionsAndFilesystemIdentity(t *testing.T) {
 	firstNamespace := NewFilesystem("same-label", fstest.MapFS{
 		"root.yml":  &fstest.MapFile{Data: []byte("items: [{id: first}]\n")},
@@ -88,7 +132,7 @@ func TestUnderlayNamespaceAnchorsUseInputPositionsAndFilesystemIdentity(t *testi
 			{source: secondNamespace.File("root.yml")},
 			{source: firstNamespace.File("later.yml")},
 		}}},
-		definition: Definition[map[string]any]{sequenceRegistrations: []sequenceRegistration{{path: Keys("items"), identityField: "id"}}},
+		definition: Definition[map[string]any]{sequenceIdentityRegistrations: []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"}}}},
 	}
 	result, err := testGroupedLoad(request)
 	if err != nil {
@@ -119,7 +163,7 @@ func TestUnderlayItemConflictUsesScheduledGroupIndex(t *testing.T) {
 			{source: File(zItem), destination: Keys("items"), placementMode: placementModeSequenceItem},
 			{source: File(aItem), destination: Keys("items"), placementMode: placementModeSequenceItem},
 		}}},
-		definition: Definition[map[string]any]{sequenceRegistrations: []sequenceRegistration{{path: Keys("items"), identityField: "id"}}},
+		definition: Definition[map[string]any]{sequenceIdentityRegistrations: []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"}}}},
 	}
 	_, err := testGroupedLoad(request)
 	loadError, ok := err.(*LoadError)
@@ -163,7 +207,7 @@ func TestUnderlayItemHelperConflictUsesScheduledGroupIndex(t *testing.T) {
 			{source: File(zBad), imports: true, destination: Keys("items"), placementMode: placementModeSequenceItem},
 			{source: File(aGood), destination: Keys("items"), placementMode: placementModeSequenceItem},
 		}}},
-		definition: Definition[map[string]any]{sequenceRegistrations: []sequenceRegistration{{path: Keys("items"), identityField: "id"}}},
+		definition: Definition[map[string]any]{sequenceIdentityRegistrations: []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"}}}},
 	}
 	_, err := testGroupedLoad(request)
 	loadError, ok := err.(*LoadError)
@@ -311,6 +355,69 @@ func TestGroupedLoadAbsentMainAndVersionProtection(t *testing.T) {
 	}
 }
 
+func TestLayeredCollectionPolicyFlowsBetweenTiers(t *testing.T) {
+	t.Parallel()
+
+	policyOnly := Bytes("lower-policy", []byte("configkit:\n  sequences:\n    - destination: [items]\n      key: id\n      mode: append\n"))
+	duplicate := func(name string) loadInput {
+		return loadInput{source: Bytes(name, []byte("items: [{id: repeated}]\n"))}
+	}
+	request := loadRequest[map[string]any]{
+		main: loadInput{source: NoSource()},
+		underlays: []loadGroup{
+			{inputs: []loadInput{{source: policyOnly}}},
+			{inputs: []loadInput{duplicate("first-peer"), duplicate("second-peer")}},
+		},
+	}
+	if _, err := testGroupedLoad(request); err != nil && groupedErrorCode(t, err) != CodeComposeConflict {
+		loadErr, _ := err.(*LoadError)
+		t.Fatalf("higher strict group code = %s, want %s: %#v", groupedErrorCode(t, err), CodeComposeConflict, loadErr.Diagnostics())
+	} else if err == nil {
+		t.Fatal("higher strict group accepted duplicate inherited identities")
+	}
+
+	request = loadRequest[map[string]any]{
+		main: loadInput{source: NoSource()},
+		underlays: []loadGroup{
+			{inputs: []loadInput{{source: Bytes("lower", []byte("configkit:\n  sequences:\n    - destination: [items]\n      key: id\n      mode: append\nitems: [{id: repeated}]\n"))}}},
+			{inputs: []loadInput{duplicate("higher")}},
+		},
+	}
+	if _, err := testGroupedLoad(request); err == nil || groupedErrorCode(t, err) != CodeSequenceConflict {
+		t.Fatalf("higher tier did not inherit explicit append mode: %v", err)
+	}
+
+	request = loadRequest[map[string]any]{
+		main:      loadInput{source: Bytes("main", []byte("configkit:\n  sequences:\n    - destination: [items]\n      key: name\n      mode: replace\nitems: [{name: higher}]\n"))},
+		underlays: []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte("configkit:\n  sequences:\n    - destination: [items]\n      key: id\n      mode: append\nitems: [{id: lower}]\n"))}}}},
+	}
+	result, err := testGroupedLoad(request)
+	if err != nil || !reflect.DeepEqual(result.Config["items"], []any{map[string]any{"name": "higher"}}) {
+		var diagnostics []Diagnostic
+		if loadErr, ok := err.(*LoadError); ok {
+			diagnostics = loadErr.Diagnostics()
+		}
+		t.Fatalf("replacement did not retire lower identity: %#v, %v (%#v)", result, err, diagnostics)
+	}
+
+	filesystem := NewFilesystem("overlay", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("configkit:\n  sequences:\n    - destination: [items]\n      key: id\n      mode: append\nitems: [{id: overlay}]\n")},
+	})
+	definition := Definition[map[string]any]{sequenceIdentityRegistrations: []sequenceIdentityRegistration{{selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"}}}}
+	request = loadRequest[map[string]any]{
+		main:       loadInput{source: Bytes("main", []byte("items: [{id: main}]\n"))},
+		underlays:  []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte("items: [{id: lower}]\n"))}}}},
+		definition: definition, sequencePolicy: mustCompileSequenceIdentityPolicy(t, definition.sequenceIdentityRegistrations),
+		overlay: "testing", overlayFilesystem: &FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem},
+	}
+	result, err = testGroupedLoad(request)
+	if err != nil || !reflect.DeepEqual(result.Config["items"], []any{
+		map[string]any{"id": "lower"}, map[string]any{"id": "main"}, map[string]any{"id": "overlay"},
+	}) {
+		t.Fatalf("layered selected overlay sequence policy = %#v, %v", result, err)
+	}
+}
+
 func TestGroupedLoadStrictConflictsRepetitionAndBudgets(t *testing.T) {
 	main := loadInput{source: Bytes("main", []byte("port: 5\n"))}
 	request := loadRequest[groupedConfig]{
@@ -349,18 +456,27 @@ func TestGroupedLoadStrictConflictsRepetitionAndBudgets(t *testing.T) {
 
 	accounting := &importAccounting{sources: maxImportSources - 1}
 	group := loadGroup{inputs: []loadInput{{source: Mapping("first", map[string]any{})}, {source: Mapping("second", map[string]any{})}}}
-	_, _, _, diagnostics := acquireUnderlayGroup(group, 1, nil, accounting)
+	_, _, _, diagnostics := testAcquireUnderlayGroup(group, 1, nil, accounting)
 	if len(diagnostics) == 0 || diagnostics[0].Code != CodeSourceLimitExceeded {
 		t.Fatalf("shared source budget = %#v", diagnostics)
 	}
 	accounting = &importAccounting{bytes: maxTotalBytes - 2}
 	group.inputs = []loadInput{{source: Mapping("first", map[string]any{"port": "one"})}}
-	_, _, _, diagnostics = acquireUnderlayGroup(group, 1, nil, accounting)
+	_, _, _, diagnostics = testAcquireUnderlayGroup(group, 1, nil, accounting)
 	if len(diagnostics) == 0 || diagnostics[0].Code != CodeSourceLimitExceeded {
 		t.Fatalf("shared byte budget = %#v", diagnostics)
 	}
 }
 
+func testAcquireUnderlayGroup(group loadGroup, index int, rule *VersionRule, accounting *importAccounting) (*composedDocument, []SourceInfo, map[string]ResourceContext, []Diagnostic) {
+	acquired, diagnostics := acquireUnderlayGroupScopes(group, index, rule, accounting, nil, sequencePolicy{})
+	if len(diagnostics) != 0 {
+		return nil, acquired.sources, acquired.contexts, diagnostics
+	}
+	composition, diagnostics := composeUnderlayGroup(acquired, rule, sequencePolicy{}, collectionPolicyScope{})
+	return composition, acquired.sources, acquired.contexts, diagnostics
+}
+
 func TestGroupedLoadInvalidIntermediateTypeCanBeReplaced(t *testing.T) {
 	request := loadRequest[groupedConfig]{
 		main:      loadInput{source: Bytes("main", []byte("port: 9\n"))},
@@ -402,11 +518,11 @@ func TestGroupedAccountingChargesMixedOccurrencesOnce(t *testing.T) {
 		"child-two.yml": &fstest.MapFile{Data: []byte(child)},
 	})
 	accounting := &importAccounting{}
-	first, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{source: filesystem.File("one.yml"), imports: true}}}, 1, nil, accounting)
+	first, sources, _, diagnostics := testAcquireUnderlayGroup(loadGroup{inputs: []loadInput{{source: filesystem.File("one.yml"), imports: true}}}, 1, nil, accounting)
 	if len(diagnostics) != 0 || first == nil || len(sources) != 2 || accounting.sources != 2 || accounting.bytes != int64(len(one)+len(child)) {
 		t.Fatalf("first acquisition = %#v, %#v, %#v, accounting %#v", first, sources, diagnostics, accounting)
 	}
-	second, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{source: filesystem.File("two.yml"), imports: true}}}, 2, nil, accounting)
+	second, sources, _, diagnostics := testAcquireUnderlayGroup(loadGroup{inputs: []loadInput{{source: filesystem.File("two.yml"), imports: true}}}, 2, nil, accounting)
 	if len(diagnostics) != 0 || second == nil || len(sources) != 2 || sources[0].ID != "source-3" || sources[1].ID != "source-4" || accounting.sources != 4 || accounting.bytes != int64(len(one)+len(two)+2*len(child)) {
 		t.Fatalf("second acquisition = %#v, %#v, %#v, accounting %#v", second, sources, diagnostics, accounting)
 	}
@@ -433,7 +549,7 @@ func TestGroupedGlobEntryBudgetIsShared(t *testing.T) {
 		"parts/b.yml": &fstest.MapFile{Data: []byte("right: two\n")},
 	})
 	accounting := &importAccounting{entries: maxGlobEntries - 1}
-	_, _, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{source: filesystem.File("main.yml"), imports: true}}}, 1, nil, accounting)
+	_, _, _, diagnostics := testAcquireUnderlayGroup(loadGroup{inputs: []loadInput{{source: filesystem.File("main.yml"), imports: true}}}, 1, nil, accounting)
 	if len(diagnostics) == 0 || diagnostics[0].Code != CodeImportLimitExceeded || accounting.entries <= maxGlobEntries {
 		t.Fatalf("shared glob budget = %#v, accounting %#v", diagnostics, accounting)
 	}

internal/engine/load_result.go

@@ -15,6 +15,11 @@ type resolvedLoad[T any] struct {
 	final           *document.Node
 	snapshots       []resolutionSnapshot
 	applications    []historyApplication
+	postDecodeApps  []historyApplication
+	structuralInput *document.Node
+	preCLI          *document.Node
+	preCLIOrigins   map[string]Origin
+	referenceInput  *document.Node
 	versionEvidence []*sourceVersionEvidence
 	inheritance     *inheritanceScope
 	contexts        map[string]ResourceContext
@@ -46,24 +51,16 @@ func decodeLoadInput[T any](schema *schemaDescriptor, definition Definition[T],
 	return decoded, nil
 }
 
-func resolveLoadInput[T any](schema *schemaDescriptor, definition Definition[T], input assembledInput, decoded decodedConfiguration) (resolvedLoad[T], []Diagnostic) {
+func resolveLoadInput[T any](schema *schemaDescriptor, definition Definition[T], input assembledInput, prepared postDecodeOverrideContext) (resolvedLoad[T], []Diagnostic) {
 	var diagnostics []Diagnostic
+	decoded := prepared.decoded
 	configuration := decoded.value.Interface().(T)
-	metadata := metadataFromDecoded(decoded)
-	if input.inheritance != nil {
-		applyInheritanceMetadata(&metadata, input.composition.root, input.composition.origins, input.composition.overlays, input.inheritance)
-	} else if input.composition != nil {
-		applyOverlayMetadata(&metadata, input.composition.overlays)
-	}
+	metadata := prepared.metadata
 	final := decoded.final
+	referenceInput := prepared.referenceInput
 	var resolutionSnapshots []resolutionSnapshot
 	if definition.Resolve != nil {
-		sources := input.sources
-		if input.versionState != nil {
-			sources = sourceInfosWithVersionEvidence(sources, input.versionState.evidence, nil, true)
-		}
-		entry := newResolutionInput(metadata, decoded.final, sources, input.contexts)
-		resolved := runResolution(&configuration, definition.Resolve, schema, decoded, input.document, entry)
+		resolved := runResolution(&configuration, definition.Resolve, schema, decoded, referenceInput, prepared.resolution)
 		diagnostics = append(diagnostics, resolved.diagnostics...)
 		if hasErrors(resolved.diagnostics) {
 			return resolvedLoad[T]{}, diagnostics
@@ -71,6 +68,7 @@ func resolveLoadInput[T any](schema *schemaDescriptor, definition Definition[T],
 		final = resolved.final
 		resolutionSnapshots = resolved.snapshots
 		applyResolutionMetadata(&metadata, resolved)
+		referenceInput = documentWithMetadataLocations(final, metadata, input.document)
 	}
 	applications := []historyApplication(nil)
 	if input.composition != nil {
@@ -82,7 +80,13 @@ func resolveLoadInput[T any](schema *schemaDescriptor, definition Definition[T],
 	if input.versionState != nil {
 		versionEvidence = append(versionEvidence, input.versionState.evidence...)
 	}
-	return resolvedLoad[T]{configuration: configuration, decoded: decoded, metadata: metadata, final: final, snapshots: resolutionSnapshots, applications: applications, versionEvidence: versionEvidence, inheritance: input.inheritance, contexts: input.contexts}, diagnostics
+	return resolvedLoad[T]{
+		configuration: configuration, decoded: decoded, metadata: metadata, final: final,
+		snapshots: resolutionSnapshots, applications: applications,
+		postDecodeApps: cloneHistoryApplications(prepared.postDecodeApps), structuralInput: document.Clone(prepared.structuralInput),
+		preCLI: document.Clone(prepared.preCLI), preCLIOrigins: cloneOrigins(prepared.preCLIOrigins), referenceInput: referenceInput,
+		versionEvidence: versionEvidence, inheritance: input.inheritance, contexts: input.contexts,
+	}, diagnostics
 }
 
 func detachLoadConfiguration[T any](schema *schemaDescriptor, configuration T, mode secretCloneMode) (T, *Diagnostic) {
@@ -105,12 +109,15 @@ func publishLoadResult[T any](schema *schemaDescriptor, definition Definition[T]
 	}
 	decoded, metadata := resolved.decoded, resolved.metadata
 	final, applications, resolutionSnapshots := resolved.final, resolved.applications, resolved.snapshots
-	finalizeMetadataPaths(&metadata, final, applications, decoded.baseline, decoded.final)
-	secrets := classifySecretPathsWithEvidence(schema, definition.Secret, metadata, final, applications, resolved.versionEvidence, decoded.baseline, decoded.final)
+	structuralInput := document.Clone(resolved.structuralInput)
+	preCLI := document.Clone(resolved.preCLI)
+	allApplications := append(cloneHistoryApplications(applications), cloneHistoryApplications(resolved.postDecodeApps)...)
+	finalizeMetadataPaths(&metadata, final, allApplications, decoded.baseline, resolved.structuralInput, resolved.preCLI, decoded.final)
+	secrets := classifySecretPathsWithEvidence(schema, definition.Secret, metadata, final, allApplications, resolved.versionEvidence, decoded.baseline, resolved.structuralInput, resolved.preCLI, decoded.final)
 	sources := sourceInfosWithVersionEvidence(accumulator.sources, resolved.versionEvidence, secrets, false)
-	retainedTrees := make([]*document.Node, 0, len(applications)+2)
-	retainedTrees = append(retainedTrees, decoded.final, final)
-	for _, application := range applications {
+	retainedTrees := make([]*document.Node, 0, len(allApplications)+5)
+	retainedTrees = append(retainedTrees, decoded.final, final, structuralInput, preCLI)
+	for _, application := range allApplications {
 		retainedTrees = append(retainedTrees, application.value)
 	}
 	inspection := &Inspection{
@@ -121,6 +128,10 @@ func publishLoadResult[T any](schema *schemaDescriptor, definition Definition[T]
 		baseline:         decoded.baseline,
 		input:            decoded.final,
 		inputOrigins:     cloneOriginIndex(decoded.origins),
+		structuralInput:  structuralInput,
+		preCLI:           preCLI,
+		preCLIOrigins:    cloneOrigins(resolved.preCLIOrigins),
+		postDecodeApps:   cloneHistoryApplications(resolved.postDecodeApps),
 		final:            final,
 		immutableLookup:  buildDocumentLookup(retainedTrees...),
 		resolution:       resolutionSnapshots,

internal/engine/metadata_locations_test.go

@@ -0,0 +1,79 @@
+package engine
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestMetadataLocationsAcrossBroadAuthoredAndDefaultMappings(t *testing.T) {
+	type config map[string]map[string]string
+	defaults := config{}
+	var body strings.Builder
+	for index := range 96 {
+		name := fmt.Sprintf("authored-%03d", index)
+		fmt.Fprintf(&body, "%s: {present: source}\n", name)
+		defaults[name] = map[string]string{"absent": "default"}
+		defaults[fmt.Sprintf("default-%03d", index)] = map[string]string{"absent": "default"}
+	}
+	input := parseDocumentForDecode(t, body.String())
+	decoded, issues := decodeConfiguration(mustSchemaFor[config](t), reflect.ValueOf(defaults), input)
+	if len(issues) != 0 {
+		t.Fatal(issues)
+	}
+	located := documentWithMetadataLocations(decoded.final, metadataFromDecoded(decoded), input)
+	inputIndex := buildDocumentLookup(input)
+	locatedIndex := buildDocumentLookup(located)
+	for index := range 96 {
+		authored := Keys(fmt.Sprintf("authored-%03d", index))
+		mapping, _ := documentAtPathWithLookup(input, authored, inputIndex)
+		present, _ := documentAtPathWithLookup(input, authored.Key("present"), inputIndex)
+		for _, test := range []struct {
+			path Path
+			want document.Location
+		}{
+			{authored.Key("present"), present.Location},
+			{authored.Key("absent"), mapping.Location},
+			{Keys(fmt.Sprintf("default-%03d", index), "absent"), input.Location},
+		} {
+			node, found := documentAtPathWithLookup(located, test.path, locatedIndex)
+			if !found || node.Location != test.want {
+				t.Fatalf("location at %s = %#v, want %#v", test.path, node, test.want)
+			}
+		}
+	}
+	walkDocument(located, Path{}, func(_ Path, node *document.Node) {
+		for _, entry := range node.Mapping {
+			if entry.KeyLocation != entry.Value.Location || entry.ValueLocation != entry.Value.Location {
+				t.Fatal("entry and value locations disagree")
+			}
+		}
+	})
+	before := decoded.final.Mapping[0].Value.Mapping[0].Value.Text
+	located.Mapping[0].Value.Mapping[0].Value.Text = "changed snapshot"
+	if decoded.final.Mapping[0].Value.Mapping[0].Value.Text != before || input.Mapping[0].Value.Mapping[0].Value.Text != "source" {
+		t.Fatal("location projection changed a borrowed input")
+	}
+}
+
+func TestIndexedNearestLocationsPreserveMissingAndSyntheticCoordinates(t *testing.T) {
+	root := parseDocumentForDecode(t, "mapping: {'0': value}\nitems: [{value: source}]\nscalar: stop\n")
+	lookup := buildDocumentLookup(root)
+	for _, path := range []Path{Path{}, Keys("missing"), Keys("mapping", "missing"), Keys("mapping", "0"), Keys("mapping").Index(0), Keys("items").Index(4), Keys("items").Index(0).Key("missing"), Keys("scalar", "child"), Path{}.Index(-1)} {
+		want, wantOK := nearestDocumentLocation(root, path)
+		got, gotOK := nearestDocumentLocationWithLookup(root, path, lookup)
+		if got != want || gotOK != wantOK {
+			t.Fatalf("nearest location changed for %s: %#v/%t, want %#v/%t", path, got, gotOK, want, wantOK)
+		}
+	}
+	final := parseDocumentForDecode(t, "value: cli\n")
+	synthetic := Location{SourceID: "cli-1"}
+	metadata := Metadata{fields: map[string]FieldInfo{"/value": {Origin: Origin{Layer: LayerCLI, Locations: []Location{synthetic}}}}}
+	located := documentWithMetadataLocations(final, metadata, root)
+	if location := located.Mapping[0].Value.Location; location.SourceID != "cli-1" || location.Line != 0 || location.Column != 0 {
+		t.Fatalf("synthetic CLI location was replaced by fallback: %#v", location)
+	}
+}

internal/engine/ordinary_sequence_test.go

@@ -0,0 +1,210 @@
+package engine
+
+import (
+	"errors"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type ordinarySequenceItem struct {
+	ID string `yaml:"id"`
+}
+
+type ordinarySequenceRecord struct {
+	ID       string                 `yaml:"id"`
+	Children []ordinarySequenceItem `yaml:"children"`
+}
+
+type ordinarySequenceConfig struct {
+	Items   []ordinarySequenceItem `yaml:"items"`
+	Catalog struct {
+		Items []ordinarySequenceItem `yaml:"items"`
+	} `yaml:"catalog"`
+	Records []ordinarySequenceRecord `yaml:"records"`
+}
+
+func TestOrdinaryLoadAcceptsDeclarationsWithoutImports(t *testing.T) {
+	result, err := testLoad[ordinarySequenceConfig](Bytes("catalog.yml", []byte(`configkit:
+  sequences:
+    - destination: [items]
+      key: id
+items:
+  - id: prompt
+`)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(result.Config.Items, []ordinarySequenceItem{{ID: "prompt"}}) {
+		t.Fatalf("items = %#v", result.Config.Items)
+	}
+	if _, ok := result.Metadata().Lookup(Keys("configkit")); ok {
+		t.Fatal("collection controls reached application metadata")
+	}
+	items, ok := result.Metadata().Lookup(Keys("items"))
+	if !ok || !items.Explicit || len(items.Origin.Locations) != 1 {
+		t.Fatalf("items metadata = %#v, %v", items, ok)
+	}
+}
+
+func TestOrdinaryLoadKeepsDisabledImportRejection(t *testing.T) {
+	_, err := testLoad[ordinarySequenceConfig](Bytes("disabled.yml", []byte(`configkit:
+  imports: [{path: fragment.yml}]
+  sequences: [{destination: [items], key: id}]
+`)))
+	if err == nil {
+		t.Fatal("source with disabled imports succeeded")
+	}
+	var loadError *LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != CodeImportDisabled {
+		t.Fatalf("error = %#v", err)
+	}
+}
+
+func TestOrdinaryLoadValidatesSingletonCollectionIdentity(t *testing.T) {
+	_, err := testLoad[ordinarySequenceConfig](Bytes("missing-identity.yml", []byte(`configkit:
+  sequences:
+    - destination: [items]
+      key: id
+items: [{}]
+`)))
+	if err == nil {
+		t.Fatal("singleton sequence without identity succeeded")
+	}
+	var loadError *LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != CodeComposeInvalid {
+		t.Fatalf("error = %#v", err)
+	}
+}
+
+func TestOrdinaryLoadCombinesImportedCollectionPeers(t *testing.T) {
+	tests := []struct {
+		name     string
+		second   string
+		want     []ordinarySequenceItem
+		wantCode Code
+	}{
+		{name: "distinct identities", second: "items: [{id: second}]\n", want: []ordinarySequenceItem{{ID: "first"}, {ID: "second"}}},
+		{name: "duplicate peer identity", second: "items: [{id: first}]\n", wantCode: CodeComposeConflict},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: first.yml
+    - path: second.yml
+  sequences:
+    - destination: [items]
+      key: id
+      mode: append
+`)
+			writeImportFile(t, directory, "first.yml", "items: [{id: first}]\n")
+			writeImportFile(t, directory, "second.yml", test.second)
+
+			result, err := testLoad[ordinarySequenceConfig](File(mainPath), Options[ordinarySequenceConfig]{Imports: true})
+			if test.wantCode != "" {
+				if result != nil || err == nil {
+					t.Fatalf("result, err = %#v, %v", result, err)
+				}
+				var loadError *LoadError
+				if !errors.As(err, &loadError) || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != test.wantCode {
+					t.Fatalf("error = %#v", err)
+				}
+				return
+			}
+			if err != nil {
+				var loadError *LoadError
+				if errors.As(err, &loadError) {
+					t.Fatalf("diagnostics = %#v", loadError.Diagnostics())
+				}
+				t.Fatal(err)
+			}
+			if !reflect.DeepEqual(result.Config.Items, test.want) {
+				t.Fatalf("items = %#v, want %#v", result.Config.Items, test.want)
+			}
+			inline, err := testLoad[ordinarySequenceConfig](Bytes("inline.yml", []byte(`configkit:
+  sequences:
+    - destination: [items]
+      key: id
+      mode: append
+items: [{id: first}, {id: second}]
+`)))
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !reflect.DeepEqual(result.Config.Items, inline.Config.Items) {
+				t.Fatalf("imported items %#v differ from inline items %#v", result.Config.Items, inline.Config.Items)
+			}
+		})
+	}
+}
+
+func TestImportedCollectionDeclarationsUseRelativePlacement(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: first.yml
+      destination: [catalog]
+    - path: second.yml
+      destination: [catalog]
+`)
+	fragment := `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+      mode: append
+items: [{id: ITEM}]
+`
+	writeImportFile(t, directory, "first.yml", strings.ReplaceAll(fragment, "ITEM", "first"))
+	writeImportFile(t, directory, "second.yml", strings.ReplaceAll(fragment, "ITEM", "second"))
+
+	result, err := testLoad[ordinarySequenceConfig](File(mainPath), Options[ordinarySequenceConfig]{Imports: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []ordinarySequenceItem{{ID: "first"}, {ID: "second"}}
+	if !reflect.DeepEqual(result.Config.Catalog.Items, want) {
+		t.Fatalf("catalog items = %#v, want %#v", result.Config.Catalog.Items, want)
+	}
+}
+
+func TestSequenceItemSourceAppliesItsCollectionDeclarations(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: item.yml
+      destination: [records]
+      placement: sequence-item
+`)
+	writeImportFile(t, directory, "item.yml", `configkit:
+  sequences:
+    - destination: [children]
+      key: id
+      mode: append
+id: parent
+children: [{id: child}]
+`)
+	result, err := testLoad[ordinarySequenceConfig](File(mainPath), Options[ordinarySequenceConfig]{Imports: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []ordinarySequenceRecord{{ID: "parent", Children: []ordinarySequenceItem{{ID: "child"}}}}
+	if !reflect.DeepEqual(result.Config.Records, want) {
+		t.Fatalf("records = %#v, want %#v", result.Config.Records, want)
+	}
+}
+
+func TestOrdinaryLoadAllowsDeclarationsWithoutValues(t *testing.T) {
+	result, err := testLoad[ordinarySequenceConfig](Bytes("policy.yml", []byte(`configkit:
+  sequences:
+    - destination: [items]
+      key: id
+`)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Config.Items != nil {
+		t.Fatalf("items = %#v, want absent", result.Config.Items)
+	}
+}

internal/engine/overlay.go

@@ -52,8 +52,221 @@ func (a *assemblyAccumulator) applyOverlay(overlay *document.Node, incomingOrigi
 	return nil
 }
 
+// applyOverlayWithCollectionPolicy is the shared recursive-precedence entry for
+// overlays. The caller supplies only the policy and declarations owned by this
+// contribution; layered loading can use the same operation when its lifecycle
+// reaches this boundary.
+func (a *assemblyAccumulator) applyOverlayWithCollectionPolicy(
+	overlay *document.Node,
+	incomingOrigins map[string]Origin,
+	incomingPolicies collectionPolicyScope,
+	applications []historyApplication,
+	layer Layer,
+	sequencePolicy sequencePolicy,
+	declarations []sequenceDeclarationSpec,
+) []Diagnostic {
+	_, diagnostics := a.applyOverlayWithCollectionProjection(overlay, incomingOrigins, incomingPolicies, applications, layer, sequencePolicy, declarations)
+	return diagnostics
+}
+
+// applyOverlayWithCollectionProjection also returns the incoming tier's item
+// assignments so acquisition evidence can follow the same operation as values.
+func (a *assemblyAccumulator) applyOverlayWithCollectionProjection(
+	overlay *document.Node,
+	incomingOrigins map[string]Origin,
+	incomingPolicies collectionPolicyScope,
+	applications []historyApplication,
+	layer Layer,
+	sequencePolicy sequencePolicy,
+	declarations []sequenceDeclarationSpec,
+) ([]sequenceHistoryContribution, []Diagnostic) {
+	if a == nil || a.current == nil || a.current.root == nil || a.current.root.Kind != document.Mapping || overlay == nil || overlay.Kind != document.Mapping {
+		return nil, invalidOverlayDocument()
+	}
+	mergeOriginContribution(a.current.origins, Path{}, incomingOrigin(incomingOrigins, Path{}, overlay, layer))
+	sequences, diagnostics := mergeOverlayMappingsWithCollectionPolicy(a, overlay, incomingOrigins, incomingPolicies, layer, sequencePolicy, declarations)
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+	if len(applications) == 0 {
+		application := historyApplication{layer: layer, value: overlay, sequences: sequences}
+		application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
+		a.current.applications = append(a.current.applications, application)
+	} else {
+		for _, application := range projectTierHistoryApplications(applications, sequences) {
+			application.layer = layer
+			a.current.applications = append(a.current.applications, application)
+		}
+	}
+	return sequences, nil
+}
+
+func mergeOverlayMappingsWithCollectionPolicy(
+	assembly *assemblyAccumulator,
+	overlay *document.Node,
+	incomingOrigins map[string]Origin,
+	incomingPolicies collectionPolicyScope,
+	layer Layer,
+	sequencePolicy sequencePolicy,
+	declarations []sequenceDeclarationSpec,
+) ([]sequenceHistoryContribution, []Diagnostic) {
+	result := assembly.current
+	claims := compositionClaims{byPath: make(map[string][]compositionClaim)}
+	claims.append(overlay, Path{}, 0)
+	identityFieldsByPath, resolvedPolicies, diagnostics := resolveStrictCollectionPolicies(
+		claims.ordered, sequencePolicy, result.collectionPolicies, declarations,
+	)
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+	declarationsByPath := make(map[string][]sequenceDeclarationSpec, len(declarations))
+	registeredPaths := make(map[string]Path)
+	for _, declaration := range declarations {
+		key := declaration.destination.String()
+		declarationsByPath[key] = append(declarationsByPath[key], declaration)
+		registeredPaths[key] = declaration.destination.clone()
+	}
+	registered := make(map[string]bool)
+	for _, claim := range claims.ordered {
+		if claim.kind == document.Sequence {
+			if _, ok := sequencePolicy.identityFieldsAt(claim.path); ok {
+				registered[claim.path.String()] = true
+				registeredPaths[claim.path.String()] = claim.path.clone()
+			}
+		}
+	}
+	for key := range declarationsByPath {
+		registered[key] = true
+	}
+	var overlayLookup *documentLookup
+	if len(result.collectionPolicies.collections) != 0 {
+		overlayLookup = buildDocumentLookup(overlay)
+	}
+	for key := range result.collectionPolicies.collections {
+		if node, found := documentAtPathWithLookup(overlay, result.collectionPolicies.collections[key].destination, overlayLookup); found && node != nil {
+			registered[key] = true
+			registeredPaths[key] = result.collectionPolicies.collections[key].destination.clone()
+		}
+	}
+
+	resolutions := make(map[string]collectionPolicyResolution, len(registered))
+	for key := range registered {
+		path := registeredPaths[key]
+		policy, hasPolicy := resolvedPolicies.at(path)
+		if !hasPolicy {
+			continue
+		}
+		operation := sequenceModeReplace
+		if policy.explicitModePresent {
+			operation = policy.explicitMode
+		} else if len(identityFieldsByPath[key]) != 0 {
+			operation = sequenceModeUpsert
+		}
+		resolutions[key] = collectionPolicyResolution{
+			policy: policy, operation: operation,
+			effectiveIdentityFields: cloneStrings(identityFieldsByPath[key]),
+		}
+	}
+
+	sequenceApplications := make(map[string]sequenceHistoryContribution)
+	policyValues := make(map[string]bool)
+	var combinationDiagnostics []Diagnostic
+	mergeOverlayMappingsWithHooks(result.root, overlay, Path{}, &assembly.index,
+		recursiveMergeHooks{
+			added: func(path Path, incoming *document.Node) {
+				collectOverlayOriginsFrom(result.origins, incomingOrigins, incoming, path, layer)
+			},
+			combined: func(path Path, incoming *document.Node) {
+				mergeOriginContribution(result.origins, path, incomingOrigin(incomingOrigins, path, incoming, layer))
+			},
+			replaced: func(path Path, existing, incoming *document.Node) {
+				assembly.current.collectionPolicies.retireCollectionPoliciesAtOrBelow(path)
+				recordOverlayReplacement(result, path, existing, incoming, layer, true, incomingOrigins)
+			},
+			merge: func(path Path, existing, incoming *document.Node) (*document.Node, bool) {
+				resolution, active := resolutions[path.String()]
+				if !active {
+					return nil, false
+				}
+				combined, issues := combineSequence(existing, incoming, resolution, sequenceCombinationContext{destination: path})
+				if len(issues) != 0 {
+					combinationDiagnostics = append(combinationDiagnostics, issues...)
+					return existing, false
+				}
+				applyOverlaySequenceEvidence(result, path, existing, incoming, combined, resolution, incomingOrigins, incomingPolicies, layer)
+				policyValues[path.String()] = true
+				history := sequenceHistoryContribution{path: path.clone(), mode: sequenceHistoryMapped}
+				if resolution.operation == sequenceModeReplace {
+					history.mode = sequenceHistoryReplace
+					if incoming != nil && incoming.Kind == document.Sequence {
+						history.length = len(incoming.Sequence)
+					}
+				} else {
+					history.items = append([]sequenceItemProjection(nil), combined.higherItems...)
+					history.replacements = append([]sequenceItemReplacement(nil), combined.replacements...)
+				}
+				sequenceApplications[path.String()] = history
+				return combined.value, true
+			},
+		},
+	)
+	if len(combinationDiagnostics) != 0 {
+		return nil, canonicalDiagnostics(combinationDiagnostics)
+	}
+
+	// Values are now complete. Index lazily for declaration-only checks;
+	// this lookup is never retained across the value-changing merge above.
+	var retainedLookup *documentLookup
+	for key, grouped := range declarationsByPath {
+		if policyValues[key] {
+			continue
+		}
+		path := grouped[0].destination
+		applicationIdentity, _ := sequencePolicy.identityFieldsAt(path)
+		current, diagnostics := resolveCollectionPolicy(path, grouped, applicationIdentity, nil)
+		if len(diagnostics) != 0 {
+			combinationDiagnostics = append(combinationDiagnostics, diagnostics...)
+			continue
+		}
+		lower, found := assembly.current.collectionPolicies.at(path)
+		var lowerPolicy *collectionPolicy
+		if found {
+			lowerPolicy = &lower
+		}
+		if retainedLookup == nil {
+			retainedLookup = buildDocumentLookup(assembly.current.root)
+		}
+		retained, _ := documentAtPathWithLookup(assembly.current.root, path, retainedLookup)
+		if retained != nil && retained.Kind == document.Sequence && len(current.effectiveIdentityFields) != 0 {
+			if _, issues := validateSequenceIdentity(retained, path, current.effectiveIdentityFields, sequenceIdentityDiagnosticCodes{invalid: CodeSequenceInvalid, conflict: CodeSequenceConflict}); len(issues) != 0 {
+				combinationDiagnostics = append(combinationDiagnostics, issues...)
+				continue
+			}
+		}
+		policy, issues := collectionPolicyWithoutValue(lowerPolicy, current.policy, retained, path)
+		if len(issues) != 0 {
+			combinationDiagnostics = append(combinationDiagnostics, issues...)
+			continue
+		}
+		assembly.current.collectionPolicies.set(path, policy)
+	}
+	if len(combinationDiagnostics) != 0 {
+		return nil, canonicalDiagnostics(combinationDiagnostics)
+	}
+	keys := make([]string, 0, len(sequenceApplications))
+	for key := range sequenceApplications {
+		keys = append(keys, key)
+	}
+	slices.Sort(keys)
+	sequences := make([]sequenceHistoryContribution, 0, len(keys))
+	for _, key := range keys {
+		sequences = append(sequences, sequenceApplications[key])
+	}
+	return sequences, nil
+}
+
 func mergeOverlayMappings(result *composedDocument, target, overlay *document.Node, current Path, layer Layer, recordReplacements bool, incomingOrigins map[string]Origin, index *mappingIndex) {
-	mergeRecursiveMappings(target, overlay, current, index, recursiveMergeHooks{
+	mergeOverlayMappingsWithHooks(target, overlay, current, index, recursiveMergeHooks{
 		added: func(path Path, incoming *document.Node) {
 			collectOverlayOriginsFrom(result.origins, incomingOrigins, incoming, path, layer)
 		},
@@ -78,22 +291,165 @@ func mergeOverlayMappings(result *composedDocument, target, overlay *document.No
 	})
 }
 
+func mergeOverlayMappingsWithHooks(
+	target, overlay *document.Node,
+	current Path,
+	index *mappingIndex,
+	hooks recursiveMergeHooks,
+) {
+	mergeRecursiveMappings(target, overlay, current, index, hooks)
+}
+
+func recordOverlayReplacement(
+	result *composedDocument,
+	path Path,
+	existing, incoming *document.Node,
+	layer Layer,
+	recordReplacement bool,
+	incomingOrigins map[string]Origin,
+) {
+	var displaced []displacedDefinition
+	var previous Origin
+	if existing != nil {
+		displaced = displacedOrigins(existing, result.origins, path)
+		previous = cloneOrigin(result.origins[path.String()])
+		deleteOriginSubtree(result.origins, path, existing)
+	}
+	collectOverlayOriginsFrom(result.origins, incomingOrigins, incoming, path, layer)
+	if recordReplacement && existing != nil {
+		result.overlays = append(result.overlays, overlayRecord{
+			change: Change{
+				Path: path.clone(), Origin: incomingOrigin(incomingOrigins, path, incoming, layer),
+				Previous: previousOrigins(previous),
+			},
+			displaced: displaced,
+		})
+	}
+}
+
+func applyOverlaySequenceEvidence(
+	result *composedDocument,
+	path Path,
+	lower, higher *document.Node,
+	combined sequenceCombinationResult,
+	resolution collectionPolicyResolution,
+	incomingOrigins map[string]Origin,
+	incomingPolicies collectionPolicyScope,
+	layer Layer,
+) {
+	lowerPolicy, hadLowerPolicy := result.collectionPolicies.at(path)
+	if resolution.operation == sequenceModeReplace {
+		var lowerValue *collectionPolicy
+		if hadLowerPolicy {
+			lowerValue = &lowerPolicy
+		}
+		result.collectionPolicies.retireCollectionPoliciesAtOrBelow(path)
+		resolution.policy = collectionPolicyForReplacement(lowerValue, resolution.policy)
+	}
+
+	if combined.value == nil || combined.value.Kind != document.Sequence {
+		recordOverlayReplacement(result, path, lower, higher, layer, true, incomingOrigins)
+		result.collectionPolicies.set(path, resolution.policy)
+		return
+	}
+
+	if resolution.operation == sequenceModeReplace {
+		var displaced []displacedDefinition
+		var previous Origin
+		if lower != nil {
+			displaced = displacedOrigins(lower, result.origins, path)
+			previous = cloneOrigin(result.origins[path.String()])
+			deleteOriginSubtree(result.origins, path, lower)
+		}
+		mergeOriginContribution(result.origins, path, incomingOrigin(incomingOrigins, path, higher, layer))
+		for _, projection := range combined.higherItems {
+			if higher != nil && higher.Kind == document.Sequence && projection.inputIndex >= 0 && projection.inputIndex < len(higher.Sequence) {
+				source := higher.Sequence[projection.inputIndex]
+				collectOverlayOriginsAt(result.origins, incomingOrigins, source, path.Index(projection.inputIndex), path.Index(projection.outputIndex), layer)
+			}
+		}
+		if lower != nil {
+			result.overlays = append(result.overlays, overlayRecord{
+				change: Change{
+					Path: path.clone(), Origin: incomingOrigin(incomingOrigins, path, higher, layer),
+					Previous: previousOrigins(previous),
+				}, displaced: displaced,
+			})
+		}
+	} else {
+		mergeOriginContribution(result.origins, path, incomingOrigin(incomingOrigins, path, higher, layer))
+		for _, replacement := range combined.replacements {
+			lowerPath := path.Index(replacement.lowerIndex)
+			higherPath := path.Index(replacement.higherIndex)
+			outputPath := path.Index(replacement.outputIndex)
+			if lower == nil || lower.Kind != document.Sequence || replacement.lowerIndex < 0 || replacement.lowerIndex >= len(lower.Sequence) ||
+				higher == nil || higher.Kind != document.Sequence || replacement.higherIndex < 0 || replacement.higherIndex >= len(higher.Sequence) {
+				continue
+			}
+			lowerItem := lower.Sequence[replacement.lowerIndex]
+			higherItem := higher.Sequence[replacement.higherIndex]
+			displaced := displacedOrigins(lowerItem, result.origins, lowerPath)
+			previous := cloneOrigin(result.origins[lowerPath.String()])
+			deleteOriginSubtree(result.origins, lowerPath, lowerItem)
+			collectOverlayOriginsAt(result.origins, incomingOrigins, higherItem, higherPath, outputPath, layer)
+			result.overlays = append(result.overlays, overlayRecord{
+				change: Change{
+					Path: outputPath.clone(), Origin: incomingOrigin(incomingOrigins, higherPath, higherItem, layer),
+					Previous: previousOrigins(previous),
+				},
+				displaced: displaced,
+			})
+		}
+		for _, projection := range combined.higherItems {
+			if higher != nil && higher.Kind == document.Sequence && projection.inputIndex >= 0 && projection.inputIndex < len(higher.Sequence) {
+				higherItem := higher.Sequence[projection.inputIndex]
+				collectOverlayOriginsAt(result.origins, incomingOrigins, higherItem, path.Index(projection.inputIndex), path.Index(projection.outputIndex), layer)
+			}
+		}
+	}
+
+	policy := cloneCollectionPolicy(resolution.policy)
+	policy.recordOrder = nil
+	policy.recordPolicies = nil
+	lowerProjection := newSequenceItemProjectionIndex(combined.lowerItems)
+	higherProjection := newSequenceItemProjectionIndex(combined.higherItems)
+	incomingCollection, hasIncomingCollection := incomingPolicies.at(path)
+	for outputIndex := range combined.value.Sequence {
+		if higherIndex, replaced := higherProjection.inputForOutput(outputIndex); replaced {
+			if record, found := incomingCollection.recordPolicyAt(higherIndex); hasIncomingCollection && found {
+				policy.setRecordPolicyAt(outputIndex, retainedCollectionRecordPolicy(record))
+			} else {
+				policy.setRecordPolicyAt(outputIndex, replacedCollectionRecordPolicy())
+			}
+			continue
+		}
+		if hadLowerPolicy && resolution.operation != sequenceModeReplace {
+			if lowerIndex, retained := lowerProjection.inputForOutput(outputIndex); retained {
+				if record, found := lowerPolicy.recordPolicyAt(lowerIndex); found {
+					policy.setRecordPolicyAt(outputIndex, retainedCollectionRecordPolicy(record))
+				}
+			}
+		} else {
+			policy.setRecordPolicyAt(outputIndex, replacedCollectionRecordPolicy())
+		}
+	}
+	result.collectionPolicies.set(path, policy)
+}
+
 func collectOverlayOrigins(origins map[string]Origin, node *document.Node, path Path, layer Layer) {
 	collectOverlayOriginsFrom(origins, nil, node, path, layer)
 }
 
 func collectOverlayOriginsFrom(origins, incomingOrigins map[string]Origin, node *document.Node, path Path, layer Layer) {
-	origins[path.String()] = incomingOrigin(incomingOrigins, path, node, layer)
-	switch node.Kind {
-	case document.Mapping:
-		for _, entry := range node.Mapping {
-			collectOverlayOriginsFrom(origins, incomingOrigins, entry.Value, path.Key(entry.Key), layer)
-		}
-	case document.Sequence:
-		for index, child := range node.Sequence {
-			collectOverlayOriginsFrom(origins, incomingOrigins, child, path.Index(index), layer)
-		}
-	}
+	collectOverlayOriginsAt(origins, incomingOrigins, node, path, path, layer)
+}
+
+func collectOverlayOriginsAt(origins, incomingOrigins map[string]Origin, node *document.Node, from, to Path, layer Layer) {
+	walkDocument(node, Path{}, func(relative Path, value *document.Node) {
+		source := appendDestination(from, relative)
+		destination := appendDestination(to, relative)
+		origins[destination.String()] = incomingOrigin(incomingOrigins, source, value, layer)
+	})
 }
 
 func mergeOverlayOrigin(origins map[string]Origin, path Path, location document.Location, layer Layer) {

internal/engine/override.go

@@ -19,9 +19,28 @@ type preparedOverride struct {
 	nodes  int
 }
 
-func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []Override, rule *VersionRule, limits importLimits) ([]preparedOverride, []SourceInfo, []Diagnostic) {
+// preparedOverrideBatch owns all override values and synthetic source records
+// needed to apply one CLI batch after its preparation checks succeed.
+type preparedOverrideBatch struct {
+	overrides []preparedOverride
+	sources   []SourceInfo
+}
+
+type overridePreparationInput struct {
+	root       *document.Node
+	inputBytes int64
+	inputNodes int
+}
+
+// preparePostStructuralOverrides snapshots CLI input for typed assignment after
+// structural expansion, when inheritance directives can no longer be supplied.
+func preparePostStructuralOverrides(base overridePreparationInput, sources []SourceInfo, overrides []Override, rule *VersionRule, limits importLimits) (preparedOverrideBatch, []Diagnostic) {
+	return prepareOverrideBatch(base, sources, overrides, rule, limits)
+}
+
+func prepareOverrideBatch(base overridePreparationInput, sources []SourceInfo, overrides []Override, rule *VersionRule, limits importLimits) (preparedOverrideBatch, []Diagnostic) {
 	if len(overrides) > maxOverrides || len(sources)+len(overrides) > limits.sources {
-		return nil, cloneSourceInfos(sources), []Diagnostic{{
+		return preparedOverrideBatch{sources: cloneSourceInfos(sources)}, []Diagnostic{{
 			Code: CodeOverrideLimitExceeded, Severity: SeverityError,
 			Message: "configuration overrides exceed the record or source-count limit",
 		}}
@@ -37,7 +56,7 @@ func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []
 		}
 	}
 	if len(diagnostics) != 0 {
-		return nil, cloneSourceInfos(sources), canonicalDiagnostics(diagnostics)
+		return preparedOverrideBatch{sources: cloneSourceInfos(sources)}, canonicalDiagnostics(diagnostics)
 	}
 
 	prepared := make([]preparedOverride, len(overrides))
@@ -89,7 +108,7 @@ func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []
 			})
 			continue
 		}
-		if pathContainsDirective(override.Path) {
+		if pathContainsDirective(override.Path) || pathContainsInheritanceDirective(override.Path) {
 			diagnostics = append(diagnostics, Diagnostic{
 				Code: CodeOverrideDirective, Severity: SeverityError, Path: override.Path.clone(),
 				Message: "configuration overrides cannot create configuration directives", Primary: location,
@@ -106,7 +125,7 @@ func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []
 			diagnostics = append(diagnostics, diagnostic)
 			continue
 		}
-		if relative, ok := overrideDirectivePath(value, Path{}); ok {
+		if relative, ok := overrideStructuralDirectivePath(value, Path{}); ok {
 			diagnostics = append(diagnostics, Diagnostic{
 				Code: CodeOverrideDirective, Severity: SeverityError, Path: appendOverridePath(override.Path, relative),
 				Message: "configuration overrides cannot create configuration directives", Primary: location,
@@ -136,10 +155,10 @@ func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []
 		usedNodes += nodes
 	}
 	if len(diagnostics) != 0 {
-		return nil, allSources, canonicalDiagnostics(diagnostics)
+		return preparedOverrideBatch{sources: allSources}, canonicalDiagnostics(diagnostics)
 	}
 
-	return prepared, allSources, nil
+	return preparedOverrideBatch{overrides: prepared, sources: allSources}, nil
 }
 
 func validOverrideLabel(label string) bool {
@@ -191,26 +210,42 @@ func pathContainsDirective(path Path) bool {
 	return false
 }
 
+func pathContainsInheritanceDirective(path Path) bool {
+	for _, segment := range path.segments {
+		if segment.key == inheritanceKey {
+			return true
+		}
+	}
+	return false
+}
+
 func appendOverridePath(base, relative Path) Path {
 	result := base.clone()
 	result.segments = append(result.segments, clonePathSegments(relative.segments, 0)...)
 	return result
 }
 
-func overrideDirectivePath(node *document.Node, path Path) (Path, bool) {
+func overrideStructuralDirectivePath(node *document.Node, path Path) (Path, bool) {
+	return overrideStructuralDirectivePathAt(node, path)
+}
+
+func overrideStructuralDirectivePathAt(node *document.Node, path Path) (Path, bool) {
+	if node == nil {
+		return Path{}, false
+	}
 	if node.Kind == document.Mapping {
 		for _, entry := range node.Mapping {
 			child := path.Key(entry.Key)
-			if entry.Key == directiveKey {
+			if entry.Key == directiveKey || entry.Key == inheritanceKey {
 				return child, true
 			}
-			if found, ok := overrideDirectivePath(entry.Value, child); ok {
+			if found, ok := overrideStructuralDirectivePathAt(entry.Value, child); ok {
 				return found, true
 			}
 		}
 	} else if node.Kind == document.Sequence {
 		for index, child := range node.Sequence {
-			if found, ok := overrideDirectivePath(child, path.Index(index)); ok {
+			if found, ok := overrideStructuralDirectivePathAt(child, path.Index(index)); ok {
 				return found, true
 			}
 		}

internal/engine/override_apply.go

@@ -1,110 +0,0 @@
-package engine
-
-import (
-	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
-)
-
-func applyOverrides(base *composedDocument, sources []SourceInfo, overrides []Override, rule *VersionRule, limits importLimits) (*composedDocument, []SourceInfo, []Diagnostic) {
-	if len(overrides) == 0 {
-		return base, cloneSourceInfos(sources), nil
-	}
-	assembly := newAssemblyAccumulator(cloneComposedDocument(base))
-	allSources, diagnostics := assembly.applyOverrides(sources, overrides, rule, limits)
-	if len(diagnostics) != 0 {
-		return nil, allSources, diagnostics
-	}
-	return assembly.current, allSources, nil
-}
-
-func (a *assemblyAccumulator) applyOverrides(sources []SourceInfo, overrides []Override, rule *VersionRule, limits importLimits) ([]SourceInfo, []Diagnostic) {
-	if len(overrides) == 0 {
-		return cloneSourceInfos(sources), nil
-	}
-	prepared, allSources, diagnostics := prepareOverrides(a.current, sources, overrides, rule, limits)
-	if len(diagnostics) != 0 {
-		return allSources, diagnostics
-	}
-	for _, override := range prepared {
-		applyPreparedOverride(a.current, override, &a.index)
-		a.current.inputBytes += override.bytes
-		a.current.inputNodes += override.nodes
-	}
-	return allSources, nil
-}
-
-func applyPreparedOverride(result *composedDocument, override preparedOverride, index *mappingIndex) {
-	location := document.Location{SourceID: override.source.ID}
-	mergeOverlayOrigin(result.origins, Path{}, location, LayerCLI)
-	current := result.root
-	currentPath := Path{}
-	for segmentIndex, segment := range override.path.segments {
-		path := currentPath.Key(segment.key)
-		position := index.find(current, segment.key)
-		if segmentIndex < len(override.path.segments)-1 {
-			if position < 0 {
-				child := &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{}}
-				index.append(current, document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: child})
-				collectOverlayOrigins(result.origins, child, path, LayerCLI)
-				current = child
-			} else {
-				current = current.Mapping[position].Value
-				mergeOverlayOrigin(result.origins, path, location, LayerCLI)
-			}
-			currentPath = path
-			continue
-		}
-
-		previous := cloneOrigin(result.origins[path.String()])
-		var displaced []displacedDefinition
-		if position < 0 {
-			incoming := document.Clone(override.value)
-			index.append(current, document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: incoming})
-			collectOverlayOrigins(result.origins, incoming, path, LayerCLI)
-		} else {
-			existing := current.Mapping[position].Value
-			displaced = affectedOverrideOrigins(existing, override.value, result.origins, path, index)
-			if existing.Kind == document.Mapping && override.value.Kind == document.Mapping {
-				mergeOverlayOrigin(result.origins, path, location, LayerCLI)
-				mergeOverlayMappings(result, existing, override.value, path, LayerCLI, false, nil, index)
-			} else {
-				deleteOriginSubtree(result.origins, path, existing)
-				current.Mapping[position] = document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: document.Clone(override.value)}
-				collectOverlayOrigins(result.origins, current.Mapping[position].Value, path, LayerCLI)
-			}
-		}
-		result.overlays = append(result.overlays, overlayRecord{change: Change{
-			Path: path.clone(), Origin: overlayOrigin(LayerCLI, location), Previous: previousOrigins(previous),
-		}, displaced: displaced})
-		result.applications = append(result.applications, historyApplication{
-			layer: LayerCLI, path: path.clone(), value: override.value,
-			origin: overlayOrigin(LayerCLI, location),
-		})
-	}
-}
-
-func affectedOverrideOrigins(existing, incoming *document.Node, origins map[string]Origin, path Path, index *mappingIndex) []displacedDefinition {
-	var displaced []displacedDefinition
-	collectAffectedOverrideOrigins(&displaced, existing, incoming, origins, path, index)
-	return sortDisplacedDefinitions(displaced)
-}
-
-func collectAffectedOverrideOrigins(displaced *[]displacedDefinition, existing, incoming *document.Node, origins map[string]Origin, path Path, index *mappingIndex) {
-	if existing.Kind != document.Mapping || incoming.Kind != document.Mapping {
-		collectDisplacedOrigins(displaced, existing, origins, path, Origin{})
-		return
-	}
-	*displaced = append(*displaced, displacedDefinition{path: path.clone(), origin: cloneOrigin(origins[path.String()])})
-	for _, entry := range incoming.Mapping {
-		position := index.find(existing, entry.Key)
-		if position < 0 {
-			continue
-		}
-		childPath := path.Key(entry.Key)
-		childExisting := existing.Mapping[position].Value
-		if childExisting.Kind == document.Mapping && entry.Value.Kind == document.Mapping {
-			collectAffectedOverrideOrigins(displaced, childExisting, entry.Value, origins, childPath, index)
-		} else {
-			collectDisplacedOrigins(displaced, childExisting, origins, childPath, Origin{})
-		}
-	}
-}

internal/engine/override_context.go

@@ -0,0 +1,246 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// postDecodeOverrideContext keeps the structural, typed pre-CLI, and effective
+// post-CLI views distinct while preparing the existing inspection machinery.
+type postDecodeOverrideContext struct {
+	structuralInput *document.Node
+	preCLI          *document.Node
+	preCLIOrigins   map[string]Origin
+	decoded         decodedConfiguration
+	metadata        Metadata
+	resolution      resolutionInput
+	sources         []SourceInfo
+	applications    []historyApplication
+	postDecodeApps  []historyApplication
+	referenceInput  *document.Node
+}
+
+func preparePostDecodeOverrideContext(input assembledInput, before decodedConfiguration, assigned decodedOverrideResult) postDecodeOverrideContext {
+	after := before
+	after.value = assigned.value
+	after.presence = cloneInputPresence(assigned.presence)
+	after.origins = cloneOrigins(assigned.origins)
+	after.final = document.Clone(assigned.final)
+
+	metadata := metadataFromDecoded(before)
+	cliRecords := overrideMetadataRecords(before, assigned.overrides)
+	var structuralOverlays []overlayRecord
+	if input.inheritance != nil && input.composition != nil {
+		structuralOverlays = input.composition.overlays
+		applyInheritanceMetadata(&metadata, input.composition.root, input.composition.origins, structuralOverlays, input.inheritance)
+	} else if input.composition != nil {
+		structuralOverlays = input.composition.overlays
+		applyOverlayMetadata(&metadata, structuralOverlays)
+	}
+	if len(cliRecords) != 0 {
+		metadata.fields = make(map[string]FieldInfo)
+		collectFieldInfo(after.final, Path{}, after, metadata.fields)
+		appendPostDecodeCLIHistory(&metadata, cliRecords)
+	}
+
+	sources := cloneSourceInfos(input.sources)
+	knownSources := make(map[string]bool, len(sources)+len(assigned.overrides))
+	for _, source := range sources {
+		knownSources[source.ID] = true
+	}
+	for _, override := range assigned.overrides {
+		if !knownSources[override.source.ID] {
+			sources = append(sources, cloneSourceInfo(override.source))
+			knownSources[override.source.ID] = true
+		}
+	}
+	if input.versionState != nil {
+		sources = sourceInfosWithVersionEvidence(sources, input.versionState.evidence, nil, true)
+	}
+	resolution := newResolutionInput(metadata, assigned.final, sources, input.contexts)
+
+	applications := structuralHistoryApplications(input)
+	var postDecodeApps []historyApplication
+	for _, override := range assigned.overrides {
+		postDecodeApps = append(postDecodeApps, historyApplication{
+			layer:  LayerCLI,
+			path:   override.path.clone(),
+			value:  document.Clone(override.value),
+			origin: overlayOrigin(LayerCLI, document.Location{SourceID: override.source.ID}),
+		})
+	}
+	allApplications := append(cloneHistoryApplications(applications), cloneHistoryApplications(postDecodeApps)...)
+	finalizeMetadataPaths(&metadata, assigned.final, allApplications, input.document, before.final)
+
+	return postDecodeOverrideContext{
+		structuralInput: document.Clone(input.document),
+		preCLI:          document.Clone(before.final),
+		preCLIOrigins:   cloneOrigins(before.origins),
+		decoded:         after,
+		metadata:        metadata,
+		resolution:      resolution,
+		sources:         sources,
+		applications:    applications,
+		postDecodeApps:  postDecodeApps,
+		referenceInput:  documentWithMetadataLocations(assigned.final, metadata, input.document),
+	}
+}
+
+// Source history is already assembled, with inheritance projected at query time.
+// Reuse overlay event construction for CLI changes without reapplying displaced
+// source definitions after those later assignments.
+func appendPostDecodeCLIHistory(metadata *Metadata, records []overlayRecord) {
+	cli := Metadata{fields: metadata.fields}
+	applyOverlayMetadata(&cli, records)
+	for pointer, history := range cli.history {
+		for _, change := range history {
+			if change.Origin.Layer == LayerCLI {
+				appendInputChangeEvent(metadata.history, nil, pointer, change)
+			}
+		}
+	}
+}
+
+func overrideMetadataRecords(before decodedConfiguration, overrides []preparedOverride) []overlayRecord {
+	if len(overrides) == 0 {
+		return nil
+	}
+	lookup := buildDocumentLookup(before.final)
+	records := make([]overlayRecord, 0, len(overrides))
+	for _, override := range overrides {
+		location := document.Location{SourceID: override.source.ID}
+		previous := cloneOrigin(before.origins[override.path.String()])
+		var displaced []displacedDefinition
+		if existing, ok := documentAtPathWithLookup(before.final, override.path, lookup); ok {
+			displaced = affectedOverrideOrigins(existing, override.value, before.origins, override.path, new(mappingIndex))
+		}
+		records = append(records, overlayRecord{change: Change{
+			Path: override.path.clone(), Origin: overlayOrigin(LayerCLI, location), Previous: previousOrigins(previous),
+		}, displaced: displaced})
+	}
+	return records
+}
+
+func affectedOverrideOrigins(existing, incoming *document.Node, origins map[string]Origin, path Path, index *mappingIndex) []displacedDefinition {
+	var displaced []displacedDefinition
+	collectAffectedOverrideOrigins(&displaced, existing, incoming, origins, path, index)
+	return sortDisplacedDefinitions(displaced)
+}
+
+func collectAffectedOverrideOrigins(displaced *[]displacedDefinition, existing, incoming *document.Node, origins map[string]Origin, path Path, index *mappingIndex) {
+	if existing.Kind != document.Mapping || incoming.Kind != document.Mapping {
+		collectDisplacedOrigins(displaced, existing, origins, path, Origin{})
+		return
+	}
+	*displaced = append(*displaced, displacedDefinition{path: path.clone(), origin: cloneOrigin(origins[path.String()])})
+	for _, entry := range incoming.Mapping {
+		position := index.find(existing, entry.Key)
+		if position < 0 {
+			continue
+		}
+		childPath := path.Key(entry.Key)
+		childExisting := existing.Mapping[position].Value
+		if childExisting.Kind == document.Mapping && entry.Value.Kind == document.Mapping {
+			collectAffectedOverrideOrigins(displaced, childExisting, entry.Value, origins, childPath, index)
+		} else {
+			collectDisplacedOrigins(displaced, childExisting, origins, childPath, Origin{})
+		}
+	}
+}
+
+func (context postDecodeOverrideContext) historyApplications() []historyApplication {
+	applications := cloneHistoryApplications(context.applications)
+	return append(applications, cloneHistoryApplications(context.postDecodeApps)...)
+}
+
+func structuralHistoryApplications(input assembledInput) []historyApplication {
+	if input.composition != nil {
+		return cloneHistoryApplications(input.composition.applications)
+	}
+	if input.document == nil {
+		return nil
+	}
+	return []historyApplication{{layer: LayerBase, value: document.Clone(input.document)}}
+}
+
+// documentWithMetadataLocations attaches effective origin coordinates to a
+// canonical typed projection for secret and validation diagnostics.
+func documentWithMetadataLocations(root *document.Node, metadata Metadata, fallback ...*document.Node) *document.Node {
+	located := document.Clone(root)
+	if located == nil {
+		return nil
+	}
+	var fallbackRoot *document.Node
+	if len(fallback) != 0 {
+		fallbackRoot = fallback[0]
+	}
+	var lookup *documentLookup
+	fallbackLocation := func(path Path) (document.Location, bool) {
+		if lookup == nil {
+			lookup = buildDocumentLookup(fallbackRoot)
+		}
+		return nearestDocumentLocationWithLookup(fallbackRoot, path, lookup)
+	}
+	var visit func(*document.Node, Path)
+	visit = func(node *document.Node, path Path) {
+		pointer := path.String()
+		if info, exists := metadata.fields[pointer]; exists {
+			if location, ok := metadataFieldLocation(metadata, pointer, info.Origin); ok {
+				node.Location = location
+			} else if location, ok := fallbackLocation(path); ok {
+				node.Location = location
+			}
+		} else if location, ok := fallbackLocation(path); ok {
+			node.Location = location
+		}
+		switch node.Kind {
+		case document.Mapping:
+			for index := range node.Mapping {
+				entry := &node.Mapping[index]
+				childPath := path.Key(entry.Key)
+				if info, exists := metadata.fields[childPath.String()]; exists {
+					if location, ok := metadataFieldLocation(metadata, childPath.String(), info.Origin); ok {
+						entry.KeyLocation = location
+						entry.ValueLocation = location
+					} else if location, ok := fallbackLocation(childPath); ok {
+						entry.KeyLocation = location
+						entry.ValueLocation = location
+					}
+				} else if location, ok := fallbackLocation(childPath); ok {
+					entry.KeyLocation = location
+					entry.ValueLocation = location
+				}
+				visit(entry.Value, childPath)
+			}
+		case document.Sequence:
+			for index, child := range node.Sequence {
+				visit(child, path.Index(index))
+			}
+		}
+	}
+	visit(located, Path{})
+	return located
+}
+
+func metadataFieldLocation(metadata Metadata, pointer string, origin Origin) (document.Location, bool) {
+	if location, ok := latestOriginDocumentLocation(origin); ok {
+		return location, true
+	}
+	for index := len(metadata.history[pointer]) - 1; index >= 0; index-- {
+		change := metadata.history[pointer][index]
+		if location, ok := latestOriginDocumentLocation(change.Origin); ok {
+			return location, true
+		}
+		for prior := len(change.Previous) - 1; prior >= 0; prior-- {
+			if location, ok := latestOriginDocumentLocation(change.Previous[prior]); ok {
+				return location, true
+			}
+		}
+	}
+	return document.Location{}, false
+}
+
+func latestOriginDocumentLocation(origin Origin) (document.Location, bool) {
+	if len(origin.Locations) == 0 {
+		return document.Location{}, false
+	}
+	location := origin.Locations[len(origin.Locations)-1]
+	return document.Location{SourceID: location.SourceID, Line: location.Line, Column: location.Column}, true
+}

internal/engine/override_context_test.go

@@ -0,0 +1,155 @@
+package engine
+
+import (
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestPostDecodeOverrideContextOrdersDecodeCLIAndResolutionEvidence(t *testing.T) {
+	t.Parallel()
+	type item struct {
+		Name    string `yaml:"name"`
+		Enabled bool   `yaml:"enabled"`
+	}
+	type config struct {
+		Version string  `yaml:"version"`
+		Rate    float64 `yaml:"rate"`
+		Count   int     `yaml:"count"`
+		Items   []item  `yaml:"items"`
+		APIKey  Secret  `yaml:"api_key"`
+		Derived int     `yaml:"derived"`
+	}
+	defaults := config{Version: "default", Rate: 2, Count: 7, Items: []item{{Name: "default"}}, APIKey: SecretEnv("DEFAULT_SECRET")}
+	structural := parseDocumentForDecode(t, "version: 1\nrate: 1.20\ncount: 5\nitems: [{name: old, enabled: true}]\napi_key: BASE_SECRET\n")
+	setOverrideTestDocumentSource(structural, "source-1")
+	versionPath := Keys("version")
+	schema, schemaIssue := buildSchema(reflect.TypeFor[config](), testSecretBinding())
+	if schemaIssue != nil {
+		t.Fatalf("buildSchema() issue = %#v", schemaIssue)
+	}
+	decoded, issues := decodeConfigurationWithContext(schema, reflect.ValueOf(defaults), structural, decodeContext{versionPath: &versionPath})
+	if len(issues) != 0 {
+		t.Fatalf("decodeConfigurationWithContext() issues = %#v", issues)
+	}
+	sources := []SourceInfo{{ID: "source-1", Label: "main input", Layer: LayerBase, Kind: SourceKindMapping}}
+	contexts := map[string]ResourceContext{"source-1": {BaseDir: "/config"}}
+	input := assembledInput{document: structural, sources: sources, contexts: contexts}
+	batch, diagnostics := preparePostStructuralOverrides(overridePreparationInput{root: structural}, sources, []Override{
+		{Path: Keys("rate"), Value: 1.2, Flag: "--rate"},
+		{Path: Keys("count"), Value: 5, Flag: "--count"},
+		{Path: Keys("items"), Value: []any{map[string]any{"name": "new"}}, Flag: "--items"},
+		{Path: Keys("api_key"), Value: "CLI_SECRET", Flag: "--api-key"},
+	}, nil, defaultImportLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("preparePostStructuralOverrides() diagnostics = %#v", diagnostics)
+	}
+	assigned, issues := applyDecodedOverrides(schema, decoded, batch, decodeContext{versionPath: &versionPath})
+	if len(issues) != 0 {
+		t.Fatalf("applyDecodedOverrides() issues = %#v", issues)
+	}
+	prepared := preparePostDecodeOverrideContext(input, decoded, assigned)
+	if prepared.structuralInput == structural || prepared.preCLI == decoded.final || prepared.decoded.final == assigned.preCLI ||
+		prepared.decoded.final == assigned.final || prepared.referenceInput == assigned.final {
+		t.Fatal("structural, pre-CLI, post-CLI, and located snapshots are not detached")
+	}
+	if raw, ok := documentAtPath(prepared.structuralInput, Keys("version")); !ok || raw.Kind != document.Number || raw.Text != "1" {
+		t.Fatalf("structural version = %#v", raw)
+	}
+	if typed, ok := documentAtPath(prepared.preCLI, Keys("version")); !ok || typed.Kind != document.String || typed.Text != "1" {
+		t.Fatalf("pre-CLI typed version = %#v", typed)
+	}
+	if prepared.metadata.fields["/count"].Origin.Layer != LayerCLI || !prepared.metadata.fields["/count"].Explicit {
+		t.Fatalf("equal-value CLI metadata = %#v", prepared.metadata.fields["/count"])
+	}
+	if prepared.decoded.origins["/count"].Layer != LayerCLI || prepared.decoded.presence["/count"].location.SourceID != batch.overrides[1].source.ID {
+		t.Fatalf("post-CLI metadata maps = %#v / %#v", prepared.decoded.origins["/count"], prepared.decoded.presence["/count"])
+	}
+	if _, explicit := prepared.decoded.presence["/items/0/enabled"]; explicit {
+		t.Fatal("replaced list retained discarded-child presence")
+	}
+	if len(prepared.applications) != 1 || len(prepared.postDecodeApps) != len(batch.overrides) || len(prepared.historyApplications()) != 1+len(batch.overrides) {
+		t.Fatalf("history applications = structural:%d CLI:%d all:%d", len(prepared.applications), len(prepared.postDecodeApps), len(prepared.historyApplications()))
+	}
+	if len(prepared.sources) != len(sources)+len(batch.overrides) || prepared.sources[len(prepared.sources)-1].Layer != LayerCLI {
+		t.Fatalf("prepared source order = %#v", prepared.sources)
+	}
+	if prepared.resolution.contexts["source-1"].BaseDir != "/config" {
+		t.Fatalf("resolution source contexts = %#v", prepared.resolution.contexts)
+	}
+	if info, ok := prepared.resolution.lookup(Keys("count")); !ok || !info.Explicit || info.Origin.Layer != LayerCLI {
+		t.Fatalf("resolution entry count = %#v, %v", info, ok)
+	}
+	cliSource := batch.overrides[3].source.ID
+	if source, ok := prepared.resolution.source(cliSource); !ok || source.Layer != LayerCLI || source.Label != "--api-key" {
+		t.Fatalf("resolution CLI source = %#v, %v", source, ok)
+	}
+
+	inspection := Inspection{
+		metadata: prepared.metadata, baseline: decoded.baseline, input: assigned.final, inputOrigins: assigned.origins,
+		structuralInput: prepared.structuralInput, preCLI: prepared.preCLI, preCLIOrigins: prepared.preCLIOrigins,
+		postDecodeApps: prepared.postDecodeApps, final: assigned.final,
+		applications: prepared.applications,
+		immutableLookup: buildDocumentLookup(prepared.structuralInput, prepared.preCLI, assigned.final,
+			prepared.applications[0].value, prepared.postDecodeApps[0].value, prepared.postDecodeApps[1].value,
+			prepared.postDecodeApps[2].value, prepared.postDecodeApps[3].value),
+	}
+	rateExplanation, ok := inspection.Explain(Keys("rate"))
+	if !ok {
+		t.Fatal("Explain(rate) did not return a history")
+	}
+	if len(rateExplanation.History) != 4 || rateExplanation.History[0].Origin.Layer != LayerDefault ||
+		rateExplanation.History[1].Origin.Layer != LayerBase || rateExplanation.History[2].Origin.Operation != ChangeDecoding ||
+		rateExplanation.History[3].Origin.Layer != LayerCLI {
+		t.Fatalf("rate history order = %#v", rateExplanation.History)
+	}
+
+	configuration := assigned.value.Interface().(config)
+	resolver := func(value *config, resolution *Resolution) error {
+		if info, found := resolution.InputInfo(Keys("count")); !found || !info.Explicit || info.Origin.Layer != LayerCLI {
+			t.Errorf("resolver input info = %#v, %v", info, found)
+		}
+		if source, found := resolution.Source(cliSource); !found || source.Layer != LayerCLI {
+			t.Errorf("resolver CLI source = %#v, %v", source, found)
+		}
+		Default(resolution, Keys("count"), &value.Count, 99, "fill missing count")
+		Default(resolution, Keys("derived"), &value.Derived, 42, "derive default")
+		value.APIKey = SecretEnv("RESOLVED_SECRET")
+		resolution.Record(Keys("api_key"), ChangeNormalization, "select effective secret reference")
+		return nil
+	}
+	resolved := runResolution(&configuration, resolver, schema, prepared.decoded, prepared.referenceInput, prepared.resolution)
+	if len(resolved.diagnostics) != 0 || configuration.Count != 5 || configuration.Derived != 42 {
+		t.Fatalf("resolution = %#v, configuration = %#v", resolved, configuration)
+	}
+	applyResolutionMetadata(&prepared.metadata, resolved)
+	effectiveReferenceInput := documentWithMetadataLocations(resolved.final, prepared.metadata)
+	if node, ok := documentAtPath(effectiveReferenceInput, Keys("api_key")); !ok || node.Text != "RESOLVED_SECRET" || node.Location.SourceID != cliSource {
+		t.Fatalf("effective reference/location = %#v", node)
+	}
+	missing := resolveSecretReferences(reflect.ValueOf(&configuration).Elem(), schema, effectiveReferenceInput, func(name string) (string, bool) {
+		if name != "RESOLVED_SECRET" {
+			t.Errorf("secret lookup name = %q", name)
+		}
+		return "", false
+	})
+	if len(missing.diagnostics) != 1 || missing.diagnostics[0].Code != CodeSecretUnset || missing.diagnostics[0].Primary == nil || missing.diagnostics[0].Primary.SourceID != cliSource {
+		t.Fatalf("secret failure handoff = %#v", missing.diagnostics)
+	}
+}
+
+func setOverrideTestDocumentSource(node *document.Node, sourceID string) {
+	if node == nil {
+		return
+	}
+	node.Location.SourceID = sourceID
+	for index := range node.Mapping {
+		node.Mapping[index].KeyLocation.SourceID = sourceID
+		node.Mapping[index].ValueLocation.SourceID = sourceID
+		setOverrideTestDocumentSource(node.Mapping[index].Value, sourceID)
+	}
+	for _, child := range node.Sequence {
+		setOverrideTestDocumentSource(child, sourceID)
+	}
+}

internal/engine/override_decode.go

@@ -0,0 +1,221 @@
+package engine
+
+import (
+	"reflect"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+// decodedOverrideResult keeps the typed pre-CLI projection and raw CLI
+// contributions alongside the effective typed value and its inspection data.
+type decodedOverrideResult struct {
+	value     reflect.Value
+	preCLI    *document.Node
+	final     *document.Node
+	presence  map[string]inputPresence
+	origins   map[string]Origin
+	overrides []preparedOverride
+}
+
+// applyDecodedOverrides assigns prepared CLI values to a detached, decoded
+// configuration. Each sparse mapping is decoded against the current typed
+// value, preserving omitted mapping fields while replacing supplied sequences.
+func applyDecodedOverrides(
+	schema *schemaDescriptor,
+	decoded decodedConfiguration,
+	batch preparedOverrideBatch,
+	context decodeContext,
+) (decodedOverrideResult, []decodeIssue) {
+	if schema == nil || schema.root == nil || !decoded.value.IsValid() || decoded.value.Type() != schema.root.typeOf || decoded.final == nil {
+		return decodedOverrideResult{}, []decodeIssue{{code: CodeSchemaUnsupportedType, message: "decoded configuration does not match the schema"}}
+	}
+
+	preCLI := document.Clone(decoded.final)
+	working, cloneIssue := cloneConfiguration(schema, decoded.value, defaultProjectionLimits())
+	if cloneIssue != nil {
+		return decodedOverrideResult{}, []decodeIssue{decodeIssueFromProjection(cloneIssue)}
+	}
+	presence := cloneInputPresence(decoded.presence)
+	origins := cloneOrigins(decoded.origins)
+	var lookup *documentLookup
+	if len(batch.overrides) != 0 {
+		lookup = buildDocumentLookup(decoded.final)
+	}
+
+	for _, override := range batch.overrides {
+		if len(override.path.segments) == 0 {
+			return decodedOverrideResult{}, []decodeIssue{{code: CodeOverrideInvalidPath, message: "configuration override path is invalid", location: document.Location{SourceID: override.source.ID}}}
+		}
+		input, issue := decodedOverrideInput(override)
+		if issue != nil {
+			return decodedOverrideResult{}, []decodeIssue{*issue}
+		}
+		updated, issues := decodeValue(input, working, schema.root, Path{}, schema, context)
+		if len(issues) != 0 {
+			return decodedOverrideResult{}, issues
+		}
+		working = updated
+
+		location := document.Location{SourceID: override.source.ID}
+		recordOverrideAncestors(presence, origins, override.path, location)
+		// Prepared paths cannot overlap, so other assignments cannot change
+		// this target's pre-CLI subtree.
+		existing, _ := documentAtPathWithLookup(decoded.final, override.path, lookup)
+		updateOverridePresence(presence, existing, override.value, override.path, location)
+		updateOverrideOrigins(origins, existing, override.value, override.path, location)
+	}
+	final, projectionIssue := projectConfiguration(schema, working, defaultProjectionLimits())
+	if projectionIssue != nil {
+		return decodedOverrideResult{}, []decodeIssue{decodeIssueFromProjection(projectionIssue)}
+	}
+
+	// New fields created by decoding a partial mapping into a previously nil
+	// pointer have no input origin. Fill those gaps as defaults while retaining
+	// all surviving source and CLI contributions already in the origin map.
+	fillDefaultOrigins(final, Path{}, origins)
+
+	return decodedOverrideResult{
+		value:     working,
+		preCLI:    preCLI,
+		final:     final,
+		presence:  presence,
+		origins:   origins,
+		overrides: clonePreparedOverrides(batch.overrides),
+	}, nil
+}
+
+func recordOverrideAncestors(presence map[string]inputPresence, origins map[string]Origin, path Path, location document.Location) {
+	mergeOverlayOrigin(origins, Path{}, location, LayerCLI)
+	prefix := Path{}
+	for index, segment := range path.segments {
+		prefix = prefix.Key(segment.key)
+		if index == len(path.segments)-1 {
+			break
+		}
+		pointer := prefix.String()
+		if _, exists := presence[pointer]; !exists {
+			presence[pointer] = inputPresence{location: location}
+		}
+		mergeOverlayOrigin(origins, prefix, location, LayerCLI)
+	}
+}
+
+func decodedOverrideInput(override preparedOverride) (*document.Node, *decodeIssue) {
+	location := document.Location{SourceID: override.source.ID}
+	value := document.Clone(override.value)
+	if value == nil {
+		return nil, &decodeIssue{code: CodeOverrideInvalidPath, path: override.path.clone(), message: "configuration override value is invalid", location: location}
+	}
+	for index := len(override.path.segments) - 1; index >= 0; index-- {
+		segment := override.path.segments[index]
+		if segment.kind == pathSegmentIndex {
+			return nil, &decodeIssue{code: CodeOverrideInvalidPath, path: override.path.clone(), message: "configuration override path is invalid", location: location}
+		}
+		value = &document.Node{
+			Kind:     document.Mapping,
+			Location: location,
+			Mapping: []document.Entry{{
+				Key: segment.key, KeyLocation: location, ValueLocation: location, Value: value,
+			}},
+		}
+	}
+	return value, nil
+}
+
+func cloneInputPresence(input map[string]inputPresence) map[string]inputPresence {
+	cloned := make(map[string]inputPresence, len(input))
+	for path, value := range input {
+		cloned[path] = value
+	}
+	return cloned
+}
+
+func cloneOrigins(input map[string]Origin) map[string]Origin {
+	cloned := make(map[string]Origin, len(input))
+	for path, value := range input {
+		cloned[path] = cloneOrigin(value)
+	}
+	return cloned
+}
+
+func clonePreparedOverrides(input []preparedOverride) []preparedOverride {
+	cloned := make([]preparedOverride, len(input))
+	for index, value := range input {
+		cloned[index] = preparedOverride{
+			path:   value.path.clone(),
+			value:  document.Clone(value.value),
+			source: value.source,
+			bytes:  value.bytes,
+			nodes:  value.nodes,
+		}
+	}
+	return cloned
+}
+
+func updateOverridePresence(presence map[string]inputPresence, existing, incoming *document.Node, path Path, location document.Location) {
+	if existing != nil && existing.Kind == document.Mapping && incoming.Kind == document.Mapping {
+		if _, exists := presence[path.String()]; !exists {
+			presence[path.String()] = inputPresence{location: location}
+		}
+		var index mappingIndex
+		for _, entry := range incoming.Mapping {
+			childPath := path.Key(entry.Key)
+			position := index.find(existing, entry.Key)
+			var previous *document.Node
+			if position >= 0 {
+				previous = existing.Mapping[position].Value
+			}
+			updateOverridePresence(presence, previous, entry.Value, childPath, location)
+		}
+		return
+	}
+
+	deletePresencePathSubtree(presence, existing, path)
+	collectPresence(incoming, path, presence)
+}
+
+func deletePresencePathSubtree(presence map[string]inputPresence, existing *document.Node, path Path) {
+	// The typed pre-CLI tree contains every effective descendant. Visit only
+	// the replaced subtree rather than searching unrelated metadata entries.
+	walkDocument(existing, path, func(path Path, _ *document.Node) {
+		delete(presence, path.String())
+	})
+}
+
+func updateOverrideOrigins(origins map[string]Origin, existing, incoming *document.Node, path Path, location document.Location) {
+	if existing != nil && existing.Kind == document.Mapping && incoming.Kind == document.Mapping {
+		mergeOverlayOrigin(origins, path, location, LayerCLI)
+		var index mappingIndex
+		for _, entry := range incoming.Mapping {
+			childPath := path.Key(entry.Key)
+			position := index.find(existing, entry.Key)
+			var previous *document.Node
+			if position >= 0 {
+				previous = existing.Mapping[position].Value
+			}
+			updateOverrideOrigins(origins, previous, entry.Value, childPath, location)
+		}
+		return
+	}
+
+	walkDocument(existing, path, func(path Path, _ *document.Node) {
+		delete(origins, path.String())
+	})
+	collectOverlayOrigins(origins, incoming, path, LayerCLI)
+}
+
+func fillDefaultOrigins(node *document.Node, path Path, origins map[string]Origin) {
+	if _, exists := origins[path.String()]; !exists {
+		origins[path.String()] = Origin{Layer: LayerDefault}
+	}
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			fillDefaultOrigins(entry.Value, path.Key(entry.Key), origins)
+		}
+	case document.Sequence:
+		for index, child := range node.Sequence {
+			fillDefaultOrigins(child, path.Index(index), origins)
+		}
+	}
+}

internal/engine/override_history_order_test.go

@@ -0,0 +1,93 @@
+package engine
+
+import "testing"
+
+func TestCLIHistoryFollowsInheritanceAndPrecedesResolution(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name                             string
+		layered, records, equal, resolve bool
+	}{
+		{name: "ordinary mapping"},
+		{name: "layered mapping equal with resolution", layered: true, equal: true, resolve: true},
+		{name: "ordinary records equal", records: true, equal: true},
+		{name: "layered records with resolution", layered: true, records: true, resolve: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			body := "base: {value: parent}\nchild: {extends: base, value: authored}\n"
+			query, parent := Keys("child", "value"), Keys("base", "value")
+			cliValue := "cli"
+			if test.equal {
+				cliValue = "authored"
+			}
+			override := Override{Path: query, Value: cliValue, Flag: "--value"}
+			if test.records {
+				body = "configkit:\n  sequences: [{destination: [items], key: id}]\nitems: [{id: base, value: parent}, {id: child, extends: {id: base}}]\n"
+				query, parent = Keys("items").Index(1).Key("value"), Keys("items").Index(0).Key("value")
+				if test.equal {
+					cliValue = "parent"
+				}
+				override = Override{Path: Keys("items"), Value: []any{map[string]any{"id": "base", "value": "parent"}, map[string]any{"id": "child", "value": cliValue}}, Flag: "--items"}
+			}
+			definition := Definition[map[string]any]{}
+			if test.resolve {
+				definition.Resolve = func(value *map[string]any, resolution *Resolution) error {
+					recorded := query
+					if test.records {
+						(*value)["items"].([]any)[1].(map[string]any)["value"] = "resolved"
+						recorded = Keys("items")
+					} else {
+						(*value)["child"].(map[string]any)["value"] = "resolved"
+					}
+					resolution.Record(recorded, ChangeNormalization, "normalize after CLI", parent)
+					return nil
+				}
+			}
+			source := Bytes("source", []byte(body))
+			var loaded *testResult[map[string]any]
+			var err error
+			if test.layered {
+				loaded, err = testGroupedLoad(loadRequest[map[string]any]{main: loadInput{source: source}, definition: definition, overrides: []Override{override}})
+			} else {
+				loaded, err = testLoad(source, Options[map[string]any]{Definition: definition, Overrides: []Override{override}})
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			history := loaded.Metadata().History(query)
+			inherited, cli, resolved := false, false, false
+			for _, change := range history {
+				if change.Origin.Operation == ChangeInheritance {
+					if cli {
+						t.Fatal("inheritance reported after CLI")
+					}
+					inherited = true
+				} else if change.Origin.Layer == LayerBase && cli {
+					t.Fatal("source definition reappeared after CLI")
+				}
+				if change.Origin.Layer == LayerCLI {
+					if !inherited || resolved {
+						t.Fatal("CLI reported outside its lifecycle phase")
+					}
+					cli = true
+				}
+				if change.Origin.Layer == LayerResolution {
+					if !cli {
+						t.Fatal("resolution reported before CLI")
+					}
+					resolved = true
+					if len(change.Dependencies) != 1 || change.Dependencies[0].String() != parent.String() {
+						t.Fatalf("resolution dependencies = %#v", change.Dependencies)
+					}
+				}
+			}
+			if !inherited || !cli || resolved != test.resolve {
+				t.Fatalf("missing lifecycle history: %#v", history)
+			}
+			info, _ := loaded.Metadata().Lookup(query)
+			if history[len(history)-1].Origin.Layer != info.Origin.Layer {
+				t.Fatal("last history origin disagrees with effective metadata")
+			}
+		})
+	}
+}

internal/engine/override_lifecycle_test.go

@@ -0,0 +1,268 @@
+package engine
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestNestedCLIOverridePreservesRetainedInputPresence(t *testing.T) {
+	t.Parallel()
+	type nested struct {
+		Changed int  `yaml:"changed"`
+		Count   int  `yaml:"count"`
+		Zero    int  `yaml:"zero"`
+		Enabled bool `yaml:"enabled"`
+	}
+	type settings struct {
+		Nested nested `yaml:"nested"`
+	}
+	type config struct {
+		Settings settings `yaml:"settings"`
+	}
+	for _, layered := range []bool{false, true} {
+		name := "ordinary"
+		if layered {
+			name = "layered"
+		}
+		t.Run(name, func(t *testing.T) {
+			source := Bytes("input", []byte("settings:\n  nested:\n    changed: 1\n    count: 2\n    zero: 0\n    enabled: false\n"))
+			definition := Definition[config]{Resolve: func(value *config, resolution *Resolution) error {
+				for _, key := range []string{"count", "zero", "enabled"} {
+					info, ok := resolution.InputInfo(Keys("settings", "nested", key))
+					if !ok || !info.Explicit || info.Origin.Layer != LayerBase {
+						t.Errorf("retained %s input info = %#v, %v", key, info, ok)
+					}
+				}
+				Default(resolution, Keys("settings", "nested", "count"), &value.Settings.Nested.Count, 99, "fill missing count")
+				Default(resolution, Keys("settings", "nested", "zero"), &value.Settings.Nested.Zero, 99, "fill missing zero")
+				Default(resolution, Keys("settings", "nested", "enabled"), &value.Settings.Nested.Enabled, true, "fill missing enabled")
+				return nil
+			}}
+			overrides := []Override{{Path: Keys("settings"), Value: map[string]any{"nested": map[string]any{"changed": 3}}, Flag: "--settings"}}
+			var result *testResult[config]
+			var err error
+			if layered {
+				result, err = testGroupedLoad(loadRequest[config]{main: loadInput{source: source}, definition: definition, overrides: overrides})
+			} else {
+				result, err = testLoad(source, Options[config]{Definition: definition, Overrides: overrides})
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got, want := result.Config.Settings.Nested, (nested{Changed: 3, Count: 2}); got != want {
+				t.Fatalf("resolved nested values = %#v, want %#v", got, want)
+			}
+			for _, key := range []string{"count", "zero", "enabled"} {
+				info, ok := result.Metadata().Lookup(Keys("settings", "nested", key))
+				if !ok || !info.Explicit || info.Origin.Layer != LayerBase || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != "source-1" {
+					t.Errorf("retained %s metadata = %#v, %v", key, info, ok)
+				}
+			}
+			info, ok := result.Metadata().Lookup(Keys("settings", "nested", "changed"))
+			if !ok || !info.Explicit || info.Origin.Layer != LayerCLI {
+				t.Errorf("changed metadata = %#v, %v", info, ok)
+			}
+		})
+	}
+}
+
+func TestPostDecodeOverrideLifecycleMatchesOrdinaryAndLayeredLoads(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		Count   int `yaml:"count"`
+		Derived int `yaml:"derived"`
+	}
+	input := Bytes("settings.yml", []byte("count: 4\n"))
+	defaultsCalls := 0
+	definition := Definition[config]{
+		Defaults: func() config {
+			defaultsCalls++
+			return config{Count: 9, Derived: 1}
+		},
+		Resolve: func(value *config, resolution *Resolution) error {
+			info, ok := resolution.InputInfo(Keys("count"))
+			if !ok || !info.Explicit || info.Origin.Layer != LayerCLI {
+				t.Errorf("resolution count info = %#v, %v", info, ok)
+			}
+			if source, ok := resolution.Source("source-2"); !ok || source.Layer != LayerCLI {
+				t.Errorf("resolution CLI source = %#v, %v", source, ok)
+			}
+			Default(resolution, Keys("count"), &value.Count, 99, "preserve explicit count")
+			Default(resolution, Keys("derived"), &value.Derived, value.Count*2, "derive from CLI count", Keys("count"))
+			Normalize(resolution, Keys("count"), &value.Count, value.Count+1, "normalize after CLI")
+			return nil
+		},
+	}
+	options := Options[config]{Definition: definition, Overrides: []Override{{Path: Keys("count"), Value: 4, Flag: "--count"}}}
+	ordinary, err := testLoad(input, options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	layered, err := testGroupedLoad(loadRequest[config]{
+		main: loadInput{source: input}, definition: definition,
+		overrides: []Override{{Path: Keys("count"), Value: 4, Flag: "--count"}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if defaultsCalls != 2 {
+		t.Fatalf("defaults factory calls = %d, want once per load", defaultsCalls)
+	}
+	for name, result := range map[string]*testResult[config]{"ordinary": ordinary, "layered": layered} {
+		if result.Config.Count != 5 || result.Config.Derived != 8 {
+			t.Errorf("%s resolved configuration = %#v", name, result.Config)
+		}
+		if got := historyLayers(result.Metadata().History(Keys("count"))); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerBase, LayerCLI, LayerResolution}) {
+			t.Errorf("%s count history layers = %#v", name, got)
+		}
+		info, ok := result.Metadata().Lookup(Keys("count"))
+		if !ok || info.Origin.Layer != LayerResolution {
+			t.Errorf("%s final count origin = %#v, %v", name, info, ok)
+		}
+	}
+	if !reflect.DeepEqual(ordinary.Config, layered.Config) ||
+		!reflect.DeepEqual(ordinary.Metadata().History(Keys("count")), layered.Metadata().History(Keys("count"))) {
+		t.Fatalf("ordinary and layered results differ: %#v / %#v", ordinary.Config, layered.Config)
+	}
+}
+
+func TestNoSourceOverridesReachResolutionAfterDefaults(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		Count   int `yaml:"count"`
+		Derived int `yaml:"derived"`
+	}
+	calls := 0
+	result, err := testLoad(NoSource(), Options[config]{
+		Definition: Definition[config]{
+			Defaults: func() config {
+				calls++
+				return config{Count: 2}
+			},
+			Resolve: func(value *config, resolution *Resolution) error {
+				info, ok := resolution.InputInfo(Keys("count"))
+				if !ok || !info.Explicit || info.Origin.Layer != LayerCLI {
+					t.Errorf("resolution count info = %#v, %v", info, ok)
+				}
+				Default(resolution, Keys("count"), &value.Count, 10, "preserve CLI value")
+				Default(resolution, Keys("derived"), &value.Derived, value.Count*3, "derive from CLI count")
+				return nil
+			},
+		},
+		Overrides: []Override{{Path: Keys("count"), Value: 4, Flag: "--count"}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if calls != 1 || result.Config.Count != 4 || result.Config.Derived != 12 {
+		t.Fatalf("defaults-only CLI result = %#v, defaults calls %d", result.Config, calls)
+	}
+	if sources := result.Sources(); len(sources) != 1 || sources[0].Layer != LayerCLI {
+		t.Fatalf("defaults-only CLI sources = %#v", sources)
+	}
+}
+
+func TestCLISequenceAssignmentReplacesAppendAndUpsertResults(t *testing.T) {
+	t.Parallel()
+	type item struct {
+		ID string `yaml:"id"`
+	}
+	type config struct {
+		Items []item `yaml:"items"`
+	}
+	declaration := func(mode string, id string) Source {
+		return Bytes(id, []byte("configkit:\n  sequences:\n    - destination: [items]\n      key: id\n      mode: "+mode+"\nitems: [{id: "+id+"}]\n"))
+	}
+	result, err := testGroupedLoad(loadRequest[config]{
+		main:      loadInput{source: declaration("upsert", "main")},
+		underlays: []loadGroup{{inputs: []loadInput{{source: declaration("append", "underlay")}}}},
+		overrides: []Override{{Path: Keys("items"), Value: []any{map[string]any{"id": "cli"}}, Flag: "--items"}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(result.Config.Items, []item{{ID: "cli"}}) {
+		t.Fatalf("CLI sequence assignment = %#v, want complete replacement", result.Config.Items)
+	}
+}
+
+func TestCLISecretReferenceUsesEffectiveLookupAndLocation(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		APIKey Secret `yaml:"api_key"`
+	}
+	options := Options[config]{
+		Definition: Definition[config]{Defaults: func() config { return config{APIKey: SecretEnv("DEFAULT_SECRET")} }},
+		Overrides:  []Override{{Path: Keys("api_key"), Value: "CLI_SECRET", Flag: "--api-key"}},
+	}
+	source := Bytes("secret-input", []byte("api_key: BASE_SECRET\n"))
+	t.Run("effective reference is looked up", func(t *testing.T) {
+		calls := 0
+		result, err := testLoadWithSecretLookup(source, func(name string) (string, bool) {
+			calls++
+			if name != "CLI_SECRET" {
+				t.Errorf("secret lookup name = %q", name)
+			}
+			return "effective-credential", true
+		}, options)
+		if err != nil || calls != 1 || result.Config.APIKey.Value() != "effective-credential" {
+			t.Fatalf("CLI secret result = %#v, calls %d, error %v", result, calls, err)
+		}
+		info, ok := result.Metadata().Lookup(Keys("api_key"))
+		if !ok || info.Origin.Layer != LayerCLI {
+			t.Fatalf("CLI secret origin = %#v, %v", info, ok)
+		}
+	})
+	t.Run("missing effective reference uses CLI location", func(t *testing.T) {
+		result, err := testLoadWithSecretLookup(source, func(name string) (string, bool) {
+			if name != "CLI_SECRET" {
+				t.Errorf("secret lookup name = %q", name)
+			}
+			return "", false
+		}, options)
+		if result != nil {
+			t.Fatalf("missing secret returned a result: %#v", result)
+		}
+		loadErr, ok := err.(*LoadError)
+		if !ok || len(loadErr.Diagnostics()) != 1 {
+			t.Fatalf("missing secret error = %v", err)
+		}
+		diagnostic := loadErr.Diagnostics()[0]
+		if diagnostic.Code != CodeSecretUnset || diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-2" {
+			t.Fatalf("missing secret diagnostic = %#v", diagnostic)
+		}
+	})
+}
+
+func TestCLIOverridesCannotHideEarlierFailuresOrAddInheritance(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		Count  int            `yaml:"count"`
+		Child  map[string]any `yaml:"child"`
+		Parent map[string]any `yaml:"parent"`
+	}
+	t.Run("decode error", func(t *testing.T) {
+		_, err := testLoad(Bytes("bad-count", []byte("count: invalid\n")), Options[config]{
+			Overrides: []Override{{Path: Keys("count"), Value: 3, Flag: "--count"}},
+		})
+		requireSecretLoadCode(t, err, CodeDecodeType)
+	})
+	t.Run("inheritance error", func(t *testing.T) {
+		_, err := testLoad(Bytes("missing-parent", []byte("child: {extends: missing, value: source}\n")), Options[config]{
+			Overrides: []Override{{Path: Keys("child"), Value: map[string]any{"value": "cli"}, Flag: "--child"}},
+		})
+		requireSecretLoadCode(t, err, CodeInheritanceMissing)
+	})
+	t.Run("cannot add relationship", func(t *testing.T) {
+		_, err := testLoad(Bytes("new-relationship", []byte("child: {value: source}\nparent: {value: parent}\n")), Options[config]{
+			Overrides: []Override{{Path: Keys("child"), Value: map[string]any{"extends": "parent"}, Flag: "--child"}},
+		})
+		requireSecretLoadCode(t, err, CodeOverrideDirective)
+	})
+	t.Run("cannot cancel relationship", func(t *testing.T) {
+		_, err := testLoad(Bytes("cancel-relationship", []byte("child: {extends: parent}\nparent: {value: parent}\n")), Options[config]{
+			Overrides: []Override{{Path: Keys("child", "extends"), Value: nil, Flag: "--extends"}},
+		})
+		requireSecretLoadCode(t, err, CodeOverrideDirective)
+	})
+}

internal/engine/override_metadata_test.go

@@ -0,0 +1,82 @@
+package engine
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestBroadPartialCLIOverrideRetainsUnchangedMetadata(t *testing.T) {
+	type child struct {
+		Name string `yaml:"name"`
+	}
+	type record struct {
+		Count   int     `yaml:"count"`
+		Zero    int     `yaml:"zero"`
+		Enabled bool    `yaml:"enabled"`
+		Items   []child `yaml:"items"`
+		Maybe   *child  `yaml:"maybe"`
+	}
+	type config struct {
+		Records map[string]record `yaml:"records"`
+	}
+	var body strings.Builder
+	body.WriteString("records:\n")
+	updates := make(map[string]any)
+	for index := range 96 {
+		name := fmt.Sprintf("key/%d~", index)
+		fmt.Fprintf(&body, "  %s: {count: 9, zero: 0, enabled: false, items: [{name: old}], maybe: {name: old}}\n", name)
+		if index%2 == 0 {
+			updates[name] = map[string]any{"count": 0, "items": []any{}, "maybe": nil}
+		}
+	}
+	input := parseDocumentForDecode(t, body.String())
+	schema := mustSchemaFor[config](t)
+	decoded, issues := decodeConfiguration(schema, reflect.ValueOf(config{}), input)
+	if len(issues) != 0 {
+		t.Fatal(issues)
+	}
+	batch, diagnostics := preparePostStructuralOverrides(overridePreparationInput{root: input}, nil,
+		[]Override{{Path: Keys("records"), Value: updates, Flag: "--records"}}, nil, defaultImportLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	result, issues := applyDecodedOverrides(schema, decoded, batch, decodeContext{})
+	if len(issues) != 0 {
+		t.Fatal(issues)
+	}
+	for index := range 96 {
+		name := fmt.Sprintf("key/%d~", index)
+		path := Keys("records", name)
+		value := result.value.Interface().(config).Records[name]
+		for _, key := range []string{"zero", "enabled"} {
+			pointer := path.Key(key).String()
+			if _, explicit := result.presence[pointer]; !explicit || result.origins[pointer].Layer != LayerBase {
+				t.Fatalf("retained explicit field lost metadata at %s", pointer)
+			}
+		}
+		if index%2 != 0 {
+			if value.Count != 9 || len(value.Items) != 1 || value.Maybe == nil || result.origins[path.Key("count").String()].Layer != LayerBase {
+				t.Fatalf("omitted record changed at %s", path)
+			}
+			continue
+		}
+		if value.Count != 0 || len(value.Items) != 0 || value.Maybe != nil {
+			t.Fatalf("CLI replacement failed at %s: %#v", path, value)
+		}
+		for _, removed := range []Path{path.Key("items").Index(0), path.Key("items").Index(0).Key("name"), path.Key("maybe").Key("name")} {
+			_, present := result.presence[removed.String()]
+			_, origin := result.origins[removed.String()]
+			if present || origin {
+				t.Fatalf("removed descendant kept metadata at %s", removed)
+			}
+		}
+		for _, key := range []string{"count", "items", "maybe"} {
+			pointer := path.Key(key).String()
+			if presence, ok := result.presence[pointer]; !ok || presence.location.SourceID != batch.overrides[0].source.ID || result.origins[pointer].Layer != LayerCLI {
+				t.Fatalf("CLI field lost metadata at %s", pointer)
+			}
+		}
+	}
+}

internal/engine/override_test.go

@@ -440,9 +440,91 @@ func TestOverrideAggregateLimitsFailBeforeApplication(t *testing.T) {
 		t.Fatal(diagnostics)
 	}
 	base.inputNodes = 1
-	result, sources, diagnostics := applyOverrides(base, nil, []Override{{Path: Keys("value"), Value: "x", Flag: "--value"}}, nil, importLimits{sources: 1, bytes: 1, nodes: 2})
-	if result != nil || len(sources) != 1 || len(diagnostics) != 1 || diagnostics[0].Code != CodeOverrideLimitExceeded || len(base.root.Mapping) != 0 {
-		t.Fatalf("bounded apply = %#v, %#v, %#v; base %#v", result, sources, diagnostics, base.root)
+	_, diagnostics = preparePostStructuralOverrides(overridePreparationInput{
+		root: base.root, inputBytes: base.inputBytes, inputNodes: base.inputNodes,
+	}, nil, []Override{{Path: Keys("value"), Value: "x", Flag: "--value"}}, nil, importLimits{sources: 1, bytes: 1, nodes: 2})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeOverrideLimitExceeded || len(base.root.Mapping) != 0 {
+		t.Fatalf("bounded preparation = %#v; base %#v", diagnostics, base.root)
+	}
+}
+
+func TestPreparedOverrideBatchDetachesInputsAndAllocatesSourcesInOrder(t *testing.T) {
+	base, diagnostics := composeDocuments(parseCompositionDocument(t, "base", "settings: {}\n"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	sources := []SourceInfo{{ID: "source-1", Label: "base", Layer: LayerBase, Kind: SourceKindMapping}}
+	firstValue := map[string]any{"nested": map[string]any{"name": "original"}}
+	secondValue := []any{map[string]any{"enabled": true}}
+	batch, diagnostics := preparePostStructuralOverrides(overridePreparationInput{
+		root: base.root, inputBytes: base.inputBytes, inputNodes: base.inputNodes,
+	}, sources, []Override{
+		{Path: Keys("settings", "first"), Value: firstValue, Flag: "--first"},
+		{Path: Keys("settings", "second"), Value: secondValue, Flag: "--second"},
+	}, nil, defaultImportLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("prepare post-structural overrides: %#v", diagnostics)
+	}
+	if len(batch.overrides) != 2 || len(batch.sources) != 3 {
+		t.Fatalf("prepared counts = %d overrides, %d sources", len(batch.overrides), len(batch.sources))
+	}
+	for index, want := range []string{"source-2", "source-3"} {
+		if batch.overrides[index].source.ID != want || batch.sources[index+1].ID != want {
+			t.Errorf("prepared source %d = %#v / %#v, want %s", index, batch.overrides[index].source, batch.sources[index+1], want)
+		}
+	}
+	if batch.sources[1].Label != "--first" || batch.sources[2].Label != "--second" {
+		t.Fatalf("synthetic source order = %#v", batch.sources)
+	}
+
+	firstValue["nested"].(map[string]any)["name"] = "changed after preparation"
+	preparedName, found := documentAtPath(batch.overrides[0].value, Keys("nested", "name"))
+	if !found || preparedName.Text != "original" {
+		t.Fatalf("prepared override aliases input value: %#v, %v", preparedName, found)
+	}
+	preparedName.Text = "changed prepared snapshot"
+	if firstValue["nested"].(map[string]any)["name"] != "changed after preparation" {
+		t.Fatal("prepared override mutation changed caller input")
+	}
+	batch.sources[0].Label = "changed batch source"
+	if sources[0].Label != "base" {
+		t.Fatal("prepared source list aliases the input source list")
+	}
+	if len(base.root.Mapping) != 1 || base.root.Mapping[0].Value.Kind != document.Mapping || len(base.root.Mapping[0].Value.Mapping) != 0 {
+		t.Fatalf("preparation mutated assembled input: %#v", base.root)
+	}
+}
+
+func TestPostStructuralOverridePreparationRejectsInheritanceDirectives(t *testing.T) {
+	base, diagnostics := composeDocuments(parseCompositionDocument(t, "base", "child: {}\nitems: []\n"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	tests := []struct {
+		name     string
+		override Override
+		path     string
+	}{
+		{
+			name:     "supplied path",
+			override: Override{Path: Keys("child", "extends"), Value: "parent", Flag: "--extends"},
+			path:     "/child/extends",
+		},
+		{
+			name:     "nested value",
+			override: Override{Path: Keys("items"), Value: []any{map[string]any{"extends": "parent"}}, Flag: "--items"},
+			path:     "/items/0/extends",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			_, diagnostics := preparePostStructuralOverrides(overridePreparationInput{
+				root: base.root, inputBytes: base.inputBytes, inputNodes: base.inputNodes,
+			}, nil, []Override{test.override}, nil, defaultImportLimits())
+			if len(diagnostics) != 1 || diagnostics[0].Code != CodeOverrideDirective || diagnostics[0].Path.String() != test.path {
+				t.Fatalf("post-structural diagnostics = %#v, want directive at %s", diagnostics, test.path)
+			}
+		})
 	}
 }
 
@@ -468,3 +550,139 @@ func fmtLoadError(loadError *LoadError) string {
 	_ = WriteDiagnostics(&output, loadError.Diagnostics())
 	return loadError.Error() + output.String()
 }
+
+func TestApplyDecodedOverridesUsesSchemaAndPreservesTypedOwnership(t *testing.T) {
+	t.Parallel()
+	type child struct {
+		Name    string `yaml:"name"`
+		Enabled bool   `yaml:"enabled"`
+	}
+	type config struct {
+		Nested  child            `yaml:"nested"`
+		Labels  map[string]child `yaml:"labels"`
+		Items   []child          `yaml:"items"`
+		Empty   []string         `yaml:"empty"`
+		Count   int              `yaml:"count"`
+		Enabled bool             `yaml:"enabled"`
+		Label   string           `yaml:"label"`
+		Small   namedSmall       `yaml:"small"`
+		Text    validatedText    `yaml:"text"`
+		Delay   time.Duration    `yaml:"delay"`
+		APIKey  Secret           `yaml:"api_key"`
+		Maybe   *string          `yaml:"maybe"`
+	}
+	defaultMaybe := "default"
+	defaults := config{
+		Nested: child{Name: "default", Enabled: true},
+		Labels: map[string]child{"default": {Name: "default", Enabled: true}},
+		Items:  []child{{Name: "default", Enabled: true}},
+		Empty:  []string{"default"},
+		Count:  8, Enabled: true, Label: "default", Small: 7, Text: "default", Delay: time.Second,
+		Maybe: &defaultMaybe,
+	}
+	input := parseDocumentForDecode(t, "nested: {name: source, enabled: true}\nlabels: {old: {name: source, enabled: true}}\nitems: [{name: discarded, enabled: true}]\nempty: [source]\ncount: 7\nenabled: true\nlabel: source\nsmall: 3\ntext: source\ndelay: 1s\napi_key: SOURCE_SECRET\nmaybe: source\n")
+	schema, schemaIssue := buildSchema(reflect.TypeFor[config](), testSecretBinding())
+	if schemaIssue != nil {
+		t.Fatalf("buildSchema() issue = %#v", schemaIssue)
+	}
+	decoded, decodeIssues := decodeConfiguration(schema, reflect.ValueOf(defaults), input)
+	if len(decodeIssues) != 0 {
+		t.Fatalf("decodeConfiguration() issues = %#v", decodeIssues)
+	}
+	batch, diagnostics := preparePostStructuralOverrides(overridePreparationInput{root: input}, nil, []Override{
+		{Path: Keys("nested"), Value: map[string]any{"enabled": false}, Flag: "--nested"},
+		{Path: Keys("labels"), Value: map[string]any{
+			"old": map[string]any{"enabled": false}, "new": map[string]any{"name": "cli"},
+		}, Flag: "--labels"},
+		{Path: Keys("items"), Value: []any{map[string]any{"name": "replacement"}}, Flag: "--items"},
+		{Path: Keys("empty"), Value: []any{}, Flag: "--empty"},
+		{Path: Keys("count"), Value: 0, Flag: "--count"},
+		{Path: Keys("enabled"), Value: false, Flag: "--enabled"},
+		{Path: Keys("label"), Value: "", Flag: "--label"},
+		{Path: Keys("small"), Value: 0, Flag: "--small"},
+		{Path: Keys("text"), Value: "cli text", Flag: "--text"},
+		{Path: Keys("delay"), Value: "250ms", Flag: "--delay"},
+		{Path: Keys("api_key"), Value: "CLI_SECRET", Flag: "--api-key"},
+		{Path: Keys("maybe"), Value: nil, Flag: "--maybe"},
+	}, nil, defaultImportLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("preparePostStructuralOverrides() diagnostics = %#v", diagnostics)
+	}
+	result, issues := applyDecodedOverrides(schema, decoded, batch, decodeContext{})
+	if len(issues) != 0 {
+		t.Fatalf("applyDecodedOverrides() issues = %#v", issues)
+	}
+	got := result.value.Interface().(config)
+	if got.Nested != (child{Name: "source", Enabled: false}) || !reflect.DeepEqual(got.Labels, map[string]child{
+		"default": {Name: "default", Enabled: true},
+		"old":     {Name: "source", Enabled: false},
+		"new":     {Name: "cli"},
+	}) || !reflect.DeepEqual(got.Items, []child{{Name: "replacement"}}) || got.Count != 0 || got.Enabled ||
+		got.Empty == nil || len(got.Empty) != 0 || got.Label != "" || got.Small != 0 ||
+		got.Text != "cli text" || got.Delay != 250*time.Millisecond || got.APIKey.name != "CLI_SECRET" || got.Maybe != nil {
+		t.Fatalf("post-decode configuration = %#v", got)
+	}
+	if result.preCLI == result.final || !reflect.DeepEqual(result.preCLI, decoded.final) || len(result.overrides) != len(batch.overrides) {
+		t.Fatalf("result snapshots/contributions = pre:%p final:%p overrides:%d", result.preCLI, result.final, len(result.overrides))
+	}
+	if _, explicit := result.presence["/items/0/enabled"]; explicit {
+		t.Fatalf("replacement list inherited presence from its discarded item: %#v", result.presence)
+	}
+	if result.presence["/items/0/name"].location.SourceID != batch.overrides[2].source.ID ||
+		result.presence["/nested"].location.SourceID != "source-1" ||
+		result.origins["/items/0/name"].Layer != LayerCLI || result.origins["/nested/name"].Layer != LayerBase ||
+		result.origins["/nested/enabled"].Layer != LayerCLI || result.origins["/labels/default/name"].Layer != LayerDefault {
+		t.Fatalf("presence/origins = %#v / %#v", result.presence, result.origins)
+	}
+	if node, ok := documentAtPath(result.final, Keys("items").Index(0).Key("enabled")); !ok || node.Kind != document.Boolean || node.Text != "false" {
+		t.Fatalf("replacement list default projection = %#v", node)
+	}
+
+	got.Labels["new"] = child{Name: "mutated"}
+	result.overrides[0].value.Mapping[0].Value.Text = "changed"
+	if decoded.value.Interface().(config).Labels["old"].Name != "source" || batch.overrides[0].value.Mapping[0].Value.Text != "false" {
+		t.Fatal("post-decode result mutation reached the pre-CLI value or prepared batch")
+	}
+}
+
+func TestApplyDecodedOverridesRejectsSchemaErrorsWithoutPartialResult(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		Nested struct {
+			Kept int `yaml:"kept"`
+		} `yaml:"nested"`
+		Count int `yaml:"count"`
+	}
+	input := parseDocumentForDecode(t, "nested: {kept: 1}\ncount: 2\n")
+	schema := mustSchemaFor[config](t)
+	decoded, issues := decodeConfiguration(schema, reflect.ValueOf(config{}), input)
+	if len(issues) != 0 {
+		t.Fatalf("decodeConfiguration() issues = %#v", issues)
+	}
+	for _, test := range []struct {
+		name     string
+		override Override
+		code     Code
+	}{
+		{name: "unknown nested field", override: Override{Path: Keys("nested"), Value: map[string]any{"unknown": 1}, Flag: "--nested"}, code: CodeDecodeUnknownField},
+		{name: "invalid type", override: Override{Path: Keys("count"), Value: "not an integer", Flag: "--count"}, code: CodeDecodeType},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			batch, diagnostics := preparePostStructuralOverrides(overridePreparationInput{root: input}, nil, []Override{test.override}, nil, defaultImportLimits())
+			if len(diagnostics) != 0 {
+				t.Fatalf("preparePostStructuralOverrides() diagnostics = %#v", diagnostics)
+			}
+			result, issues := applyDecodedOverrides(schema, decoded, batch, decodeContext{})
+			if len(issues) == 0 || issues[0].code != test.code {
+				t.Fatalf("decode issues = %#v, want code %q", issues, test.code)
+			}
+			if result.value.IsValid() || result.final != nil {
+				t.Fatalf("failed assignment returned a partial result: %#v", result)
+			}
+			original := decoded.value.Interface().(config)
+			if original.Count != 2 || original.Nested.Kept != 1 {
+				t.Fatal("failed assignment mutated the pre-CLI decoded value")
+			}
+		})
+	}
+}

internal/engine/records.go

@@ -105,6 +105,8 @@ const (
 	CodeImportRepeated      Code = "import.repeated"
 	CodeImportLimitExceeded Code = "import.limit_exceeded"
 	CodeImportVersion       Code = "import.version"
+	CodeSequenceInvalid     Code = "sequence.invalid"
+	CodeSequenceConflict    Code = "sequence.conflict"
 	CodeUnderlayVersion     Code = "underlay.version"
 
 	CodeOverlayInvalid       Code = "overlay.invalid"

internal/engine/recursive_merge.go

@@ -9,6 +9,7 @@ type recursiveMergeHooks struct {
 	added    func(Path, *document.Node)
 	combined func(Path, *document.Node)
 	replaced func(Path, *document.Node, *document.Node)
+	merge    func(Path, *document.Node, *document.Node) (*document.Node, bool)
 }
 
 // mergeRecursiveMappings changes an owned mapping and never aliases incoming
@@ -18,16 +19,31 @@ func mergeRecursiveMappings(target, incoming *document.Node, current Path, index
 		path := current.Key(entry.Key)
 		position := index.find(target, entry.Key)
 		if position < 0 {
-			cloned := entry
-			cloned.Value = document.Clone(entry.Value)
-			index.append(target, cloned)
+			if hooks.merge != nil {
+				if merged, handled := hooks.merge(path, nil, entry.Value); handled {
+					cloned := entry
+					cloned.Value = cloneMergeSubtree(merged, path, hooks.merge)
+					index.append(target, cloned)
+					continue
+				}
+			}
 			if hooks.added != nil {
 				hooks.added(path, entry.Value)
 			}
+			cloned := entry
+			cloned.Value = cloneMergeSubtree(entry.Value, path, hooks.merge)
+			index.append(target, cloned)
 			continue
 		}
 
 		existing := target.Mapping[position].Value
+		if hooks.merge != nil {
+			if merged, handled := hooks.merge(path, existing, entry.Value); handled {
+				target.Mapping[position] = entry
+				target.Mapping[position].Value = cloneMergeSubtree(merged, path, hooks.merge)
+				continue
+			}
+		}
 		if existing.Kind == document.Mapping && entry.Value.Kind == document.Mapping {
 			if hooks.combined != nil {
 				hooks.combined(path, entry.Value)
@@ -40,6 +56,28 @@ func mergeRecursiveMappings(target, incoming *document.Node, current Path, index
 			hooks.replaced(path, existing, entry.Value)
 		}
 		target.Mapping[position] = entry
-		target.Mapping[position].Value = document.Clone(entry.Value)
+		target.Mapping[position].Value = cloneMergeSubtree(entry.Value, path, hooks.merge)
 	}
 }
+
+// A copied mapping can contain collection operations even though its ancestor
+// has no retained mapping. Visit those operations with absent lower values.
+// Ordinary origin/replacement hooks already accounted for the whole subtree;
+// do not replay them for every descendant. Records inside sequences remain whole.
+func cloneMergeSubtree(incoming *document.Node, path Path, merge func(Path, *document.Node, *document.Node) (*document.Node, bool)) *document.Node {
+	if incoming == nil || incoming.Kind != document.Mapping || merge == nil {
+		return document.Clone(incoming)
+	}
+	cloned := *incoming
+	cloned.Mapping = make([]document.Entry, len(incoming.Mapping))
+	for index, entry := range incoming.Mapping {
+		childPath := path.Key(entry.Key)
+		value := entry.Value
+		if merged, handled := merge(childPath, nil, value); handled {
+			value = merged
+		}
+		entry.Value = cloneMergeSubtree(value, childPath, merge)
+		cloned.Mapping[index] = entry
+	}
+	return &cloned
+}

internal/engine/selected_overlay.go

@@ -41,7 +41,11 @@ func loadSelectedOverlayInto(source Source, context *FilesystemContext, main par
 	return loadSelectedOverlayIntoWithVersion(source, context, main, base, sources, name, rule, limits, nil)
 }
 
-func loadSelectedOverlayIntoWithVersion(source Source, context *FilesystemContext, main parsedSource, base *assemblyAccumulator, sources []SourceInfo, name string, rule *VersionRule, limits importLimits, versionState *sourceVersionState) ([]SourceInfo, []Diagnostic) {
+func loadSelectedOverlayIntoWithVersion(source Source, context *FilesystemContext, main parsedSource, base *assemblyAccumulator, sources []SourceInfo, name string, rule *VersionRule, limits importLimits, versionState *sourceVersionState, policies ...sequencePolicy) ([]SourceInfo, []Diagnostic) {
+	var policy sequencePolicy
+	if len(policies) != 0 {
+		policy = policies[0]
+	}
 	if nativeContext(source, context) {
 		access, basePath, mainPath, diagnostic := nativeFilesystemAccess(source, context)
 		if diagnostic != nil {
@@ -50,7 +54,10 @@ func loadSelectedOverlayIntoWithVersion(source Source, context *FilesystemContex
 		if diagnostic := validateNativeDirectories(access, basePath, mainPath); diagnostic != nil {
 			return cloneSourceInfos(sources), []Diagnostic{*diagnostic}
 		}
-		return loadSelectedOverlayWithAccessAndVersion(access, base, sources, name, rule, limits, versionState)
+		if len(policies) == 0 {
+			return loadSelectedOverlayWithAccessAndVersion(access, base, sources, name, rule, limits, versionState)
+		}
+		return loadSelectedOverlayWithAccessAndVersion(access, base, sources, name, rule, limits, versionState, policy)
 	}
 	rootPath, _, _, diagnostic := importFilesystem(source, context, main)
 	if diagnostic != nil {
@@ -62,14 +69,21 @@ func loadSelectedOverlayIntoWithVersion(source Source, context *FilesystemContex
 		value.Primary = nil
 		return cloneSourceInfos(sources), []Diagnostic{value}
 	}
-	return loadSelectedOverlayWithAccessAndVersion(access, base, sources, name, rule, limits, versionState)
+	if len(policies) == 0 {
+		return loadSelectedOverlayWithAccessAndVersion(access, base, sources, name, rule, limits, versionState)
+	}
+	return loadSelectedOverlayWithAccessAndVersion(access, base, sources, name, rule, limits, versionState, policy)
 }
 
 func loadSelectedOverlayWithAccess(access *filesystemAccess, base *assemblyAccumulator, sources []SourceInfo, name string, rule *VersionRule, limits importLimits) ([]SourceInfo, []Diagnostic) {
 	return loadSelectedOverlayWithAccessAndVersion(access, base, sources, name, rule, limits, nil)
 }
 
-func loadSelectedOverlayWithAccessAndVersion(access *filesystemAccess, base *assemblyAccumulator, sources []SourceInfo, name string, rule *VersionRule, limits importLimits, versionState *sourceVersionState) ([]SourceInfo, []Diagnostic) {
+func loadSelectedOverlayWithAccessAndVersion(access *filesystemAccess, base *assemblyAccumulator, sources []SourceInfo, name string, rule *VersionRule, limits importLimits, versionState *sourceVersionState, policies ...sequencePolicy) ([]SourceInfo, []Diagnostic) {
+	var policy sequencePolicy
+	if len(policies) != 0 {
+		policy = policies[0]
+	}
 	closed := false
 	defer func() {
 		if !closed {
@@ -131,18 +145,34 @@ func loadSelectedOverlayWithAccessAndVersion(access *filesystemAccess, base *ass
 	if base.current.inputNodes+nodeCount > limits.nodes {
 		return sources, []Diagnostic{importDiagnostic(CodeOverlayLimitExceeded, "selected overlay exceeds the aggregate node limit", overlayLocation)}
 	}
-	if diagnostics := overlayDirectiveDiagnostics(overlayRoot, Path{}); len(diagnostics) != 0 {
+	if diagnostics := overlayDirectiveDiagnostics(overlayRoot, Path{}, len(policies) != 0); len(diagnostics) != 0 {
 		return sources, diagnostics
 	}
-	overlayPayload := overlayRoot
-	if versionState != nil && versionState.mode != sourceVersionModeDefault {
-		processed, failures := versionState.process(overlayRoot, sourceVersionContribution, Path{}, sourceID, placementModeMapping)
-		if len(failures) != 0 {
-			return sources, failures
+	var processed sourceVersionProcessingResult
+	var failures []Diagnostic
+	if len(policies) != 0 {
+		if versionState != nil {
+			processed, failures = versionState.processWithDirectiveContext(
+				overlayRoot, sourceVersionContribution, Path{}, sourceID, placementModeMapping, rootDirectiveContextOverlay,
+			)
+		} else {
+			processed = processSourceVersionWithDirectiveContext(
+				overlayRoot, rule, sourceVersionContribution, Path{}, sourceVersionModeDefault, rootDirectiveContextOverlay,
+			)
+			failures = processed.diagnostics
+		}
+	} else {
+		if versionState != nil && versionState.mode != sourceVersionModeDefault {
+			processed, failures = versionState.process(overlayRoot, sourceVersionContribution, Path{}, sourceID, placementModeMapping)
+		} else {
+			processed = sourceVersionProcessingResult{payload: overlayRoot}
 		}
-		overlayPayload = processed.payload
 	}
-	if versionNode := fragmentVersionClaim(overlayPayload, rule); versionNode != nil {
+	if len(failures) != 0 {
+		return sources, failures
+	}
+	overlayPayload := processed.payload
+	if versionNode := fragmentVersionClaim(overlayPayload, rule); len(policies) == 0 && versionNode != nil {
 		location := publicLocation(versionNode.Location)
 		return sources, []Diagnostic{{
 			Code: CodeOverlayVersion, Severity: SeverityError, Path: rule.Path.clone(),
@@ -155,8 +185,26 @@ func loadSelectedOverlayWithAccessAndVersion(access *filesystemAccess, base *ass
 		return sources, []Diagnostic{importDiagnostic(CodeImportRead, "configuration root could not be closed after reading the selected overlay", nil)}
 	}
 
-	if diagnostics := base.applyOverlay(overlayPayload, nil, nil, LayerOverlay); len(diagnostics) != 0 {
-		return sources, diagnostics
+	var overlayDiagnostics []Diagnostic
+	if len(policies) != 0 {
+		overlayDiagnostics = base.applyOverlayWithCollectionPolicy(overlayPayload, nil, collectionPolicyScope{}, nil, LayerOverlay, policy, processed.sequences)
+	} else {
+		overlayDiagnostics = base.applyOverlay(overlayPayload, nil, nil, LayerOverlay)
+	}
+	if len(overlayDiagnostics) != 0 {
+		return sources, overlayDiagnostics
+	}
+	if len(policies) != 0 {
+		// The selected overlay retains one application with the actual item
+		// assignments. Raw list indexes cannot identify an upsert's destination.
+		application := base.current.applications[len(base.current.applications)-1]
+		if node := versionClaimFromApplication(application, rule); node != nil {
+			location := publicLocation(node.Location)
+			return sources, []Diagnostic{{
+				Code: CodeOverlayVersion, Severity: SeverityError, Path: rule.Path.clone(),
+				Message: "overlays cannot contribute to or replace the schema-version path", Primary: &location,
+			}}
+		}
 	}
 	base.current.inputBytes += int64(len(data))
 	base.current.inputNodes += nodeCount

internal/engine/selected_overlay_test.go

@@ -94,6 +94,263 @@ value: scalar
 	}
 }
 
+func TestLoadSelectedOverlayAppliesCollectionOperationsAndEvidence(t *testing.T) {
+	t.Parallel()
+	type item struct {
+		ID       string `yaml:"id"`
+		Value    string `yaml:"value"`
+		Children []item `yaml:"children"`
+	}
+	type config struct {
+		Items []item `yaml:"items"`
+	}
+	tests := []struct {
+		name    string
+		mode    string
+		overlay string
+		want    []item
+	}{
+		{
+			name: "append",
+			mode: "append",
+			overlay: `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+      mode: append
+items: [{id: c, value: added}]
+`,
+			want: []item{{ID: "a", Value: "old"}, {ID: "b", Value: "kept"}, {ID: "c", Value: "added"}},
+		},
+		{
+			name: "upsert replaces complete records at retained positions",
+			mode: "upsert",
+			overlay: `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+      mode: upsert
+items: [{id: a, value: new}]
+`,
+			want: []item{{ID: "a", Value: "new"}, {ID: "b", Value: "kept"}},
+		},
+		{
+			name: "replace",
+			mode: "replace",
+			overlay: `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+      mode: replace
+items: [{id: c, value: replacement}]
+`,
+			want: []item{{ID: "c", Value: "replacement"}},
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", "items: [{id: a, value: old, children: [{id: child}]}, {id: b, value: kept}]\n")
+			writeImportFile(t, directory, "overlays/testing.yml", test.overlay)
+			result, err := testLoad[config](File(mainPath), Options[config]{Overlay: "testing"})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if len(result.Config.Items) != len(test.want) {
+				t.Fatalf("items = %#v, want %#v", result.Config.Items, test.want)
+			}
+			for index, want := range test.want {
+				if result.Config.Items[index].ID != want.ID || result.Config.Items[index].Value != want.Value {
+					t.Fatalf("items[%d] = %#v, want id %q value %q", index, result.Config.Items[index], want.ID, want.Value)
+				}
+			}
+			if _, found := result.Metadata().Lookup(Keys("configkit")); found {
+				t.Fatal("overlay collection controls reached application metadata")
+			}
+			if test.mode == "append" {
+				base, _ := result.Metadata().Lookup(Keys("items").Index(0).Key("value"))
+				added, _ := result.Metadata().Lookup(Keys("items").Index(2).Key("value"))
+				if base.Origin.Layer != LayerBase || added.Origin.Layer != LayerOverlay {
+					t.Fatalf("append origins = base %#v added %#v", base.Origin, added.Origin)
+				}
+				history := result.Metadata().History(Keys("items").Index(2).Key("value"))
+				if len(history) == 0 || history[len(history)-1].Origin.Layer != LayerOverlay {
+					t.Fatalf("append history = %#v", history)
+				}
+			}
+			if test.mode == "upsert" {
+				if _, found := result.Metadata().Lookup(Keys("items").Index(0).Key("children").Index(0)); found {
+					t.Fatal("upsert retained a discarded child")
+				}
+				history := result.Metadata().History(Keys("items").Index(0).Key("children").Index(0))
+				if len(history) < 2 {
+					t.Fatalf("discarded child history = %#v", history)
+				}
+			}
+		})
+	}
+}
+
+func TestLoadSelectedOverlayReconcilesPendingAndRetainedCollectionPolicy(t *testing.T) {
+	t.Parallel()
+	requireCode := func(t *testing.T, main, overlay string, code Code) {
+		t.Helper()
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", main)
+		writeImportFile(t, directory, "overlays/testing.yml", overlay)
+		_, err := testLoad[ordinarySequenceConfig](File(mainPath), Options[ordinarySequenceConfig]{Overlay: "testing"})
+		var loadError *LoadError
+		if !errors.As(err, &loadError) || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != code {
+			t.Fatalf("load error = %#v, want %s", err, code)
+		}
+	}
+	t.Run("key only validates retained records", func(t *testing.T) {
+		requireCode(t, "items: [{id: duplicate}, {id: duplicate}]\n", `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+`, CodeSequenceConflict)
+	})
+
+	t.Run("mode only retains lower identity", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+      mode: append
+items: [{id: base}]
+`)
+		writeImportFile(t, directory, "overlays/testing.yml", `configkit:
+  sequences:
+    - destination: [items]
+      mode: append
+items: [{id: overlay}]
+`)
+		result, err := testLoad[ordinarySequenceConfig](File(mainPath), Options[ordinarySequenceConfig]{Overlay: "testing"})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !reflect.DeepEqual(result.Config.Items, []ordinarySequenceItem{{ID: "base"}, {ID: "overlay"}}) {
+			t.Fatalf("items = %#v", result.Config.Items)
+		}
+	})
+
+	t.Run("lower explicit mode remains active when overlay omits it", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+      mode: append
+items: [{id: base}]
+`)
+		writeImportFile(t, directory, "overlays/testing.yml", "items: [{id: overlay}]\n")
+		result, err := testLoad[ordinarySequenceConfig](File(mainPath), Options[ordinarySequenceConfig]{Overlay: "testing"})
+		if err != nil || !reflect.DeepEqual(result.Config.Items, []ordinarySequenceItem{{ID: "base"}, {ID: "overlay"}}) {
+			t.Fatalf("retained operation = %#v, %v", result, err)
+		}
+	})
+
+	t.Run("overlay identity must agree with retained identity", func(t *testing.T) {
+		requireCode(t, `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+items: [{id: base}]
+`, `configkit:
+  sequences:
+    - destination: [items]
+      key: slug
+items: [{id: overlay}]
+`, CodeSequenceConflict)
+	})
+
+	t.Run("replace accepts null and append rejects it", func(t *testing.T) {
+		for _, test := range []struct {
+			mode string
+			want Code
+		}{
+			{mode: "replace"},
+			{mode: "append", want: CodeSequenceInvalid},
+		} {
+			overlay := "configkit:\n  sequences:\n    - destination: [items]\n      key: id\n      mode: " + test.mode + "\nitems: null\n"
+			if test.want != "" {
+				requireCode(t, "items: [{id: base}]\n", overlay, test.want)
+				continue
+			}
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", "items: [{id: base}]\n")
+			writeImportFile(t, directory, "overlays/testing.yml", overlay)
+			result, err := testLoad[ordinarySequenceConfig](File(mainPath), Options[ordinarySequenceConfig]{Overlay: "testing"})
+			if err != nil || result.Config.Items != nil {
+				t.Fatalf("replace null = %#v, %v", result, err)
+			}
+		}
+	})
+
+	t.Run("empty contributions and kind mismatches", func(t *testing.T) {
+		for _, test := range []struct {
+			name, mode, value string
+			want              int
+			wantCode          Code
+		}{
+			{name: "append empty", mode: "append", value: "[]", want: 1},
+			{name: "upsert empty", mode: "upsert", value: "[]", want: 1},
+			{name: "replace empty", mode: "replace", value: "[]", want: 0},
+			{name: "append mapping", mode: "append", value: "{id: overlay}", wantCode: CodeSequenceInvalid},
+		} {
+			t.Run(test.name, func(t *testing.T) {
+				overlay := "configkit:\n  sequences:\n    - destination: [items]\n      key: id\n      mode: " + test.mode + "\nitems: " + test.value + "\n"
+				if test.wantCode != "" {
+					requireCode(t, "items: [{id: base}]\n", overlay, test.wantCode)
+					return
+				}
+				directory := t.TempDir()
+				mainPath := writeImportFile(t, directory, "main.yml", "items: [{id: base}]\n")
+				writeImportFile(t, directory, "overlays/testing.yml", overlay)
+				result, err := testLoad[ordinarySequenceConfig](File(mainPath), Options[ordinarySequenceConfig]{Overlay: "testing"})
+				if err != nil || len(result.Config.Items) != test.want {
+					t.Fatalf("%s result = %#v, %v", test.name, result, err)
+				}
+			})
+		}
+	})
+}
+
+func TestLoadSelectedOverlayKeepsCollectionPoliciesIndependent(t *testing.T) {
+	t.Parallel()
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+      mode: append
+    - destination: [catalog, items]
+      key: id
+      mode: append
+items: [{id: base}]
+catalog: {items: [{id: catalog-base}]}
+`)
+	writeImportFile(t, directory, "overlays/testing.yml", `configkit:
+  sequences:
+    - destination: [items]
+      mode: upsert
+    - destination: [catalog, items]
+      mode: append
+items: [{id: base}]
+catalog: {items: [{id: catalog-overlay}]}
+`)
+	result, err := testLoad[ordinarySequenceConfig](File(mainPath), Options[ordinarySequenceConfig]{Overlay: "testing"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(result.Config.Items, []ordinarySequenceItem{{ID: "base"}}) ||
+		!reflect.DeepEqual(result.Config.Catalog.Items, []ordinarySequenceItem{{ID: "catalog-base"}, {ID: "catalog-overlay"}}) {
+		t.Fatalf("independent overlay policies = items %#v, catalog %#v", result.Config.Items, result.Config.Catalog.Items)
+	}
+}
+
 func TestLoadOverlaySelectionUsesOneFixedYAMLFile(t *testing.T) {
 	directory := t.TempDir()
 	mainPath := writeImportFile(t, directory, "main.yml", "settings: {kept: base}\n")

internal/engine/sequence_combine.go

@@ -0,0 +1,256 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+type sequenceCombinationContext struct {
+	destination Path
+}
+
+type sequenceItemProjection struct {
+	inputIndex  int
+	outputIndex int
+}
+
+type sequenceItemReplacement struct {
+	lowerIndex  int
+	higherIndex int
+	outputIndex int
+}
+
+type sequenceCombinationResult struct {
+	value          *document.Node
+	lowerItems     []sequenceItemProjection
+	higherItems    []sequenceItemProjection
+	replacements   []sequenceItemReplacement
+	appendedHigher []int
+	discardedLower []int
+}
+
+func combineSequence(
+	lower, higher *document.Node,
+	policy collectionPolicyResolution,
+	context sequenceCombinationContext,
+) (sequenceCombinationResult, []Diagnostic) {
+	if higher == nil {
+		return unchangedSequenceValue(lower), nil
+	}
+	switch policy.operation {
+	case sequenceModeReplace:
+		return replaceSequence(lower, higher, policy.effectiveIdentityFields, context)
+	case sequenceModeAppend, sequenceModeUpsert:
+		return combineIdentifiedSequences(lower, higher, policy.operation, policy.effectiveIdentityFields, context)
+	default:
+		return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+			context.destination, "sequence operation is invalid", higher,
+		)}
+	}
+}
+
+func unchangedSequenceValue(lower *document.Node) sequenceCombinationResult {
+	result := sequenceCombinationResult{value: document.Clone(lower)}
+	if lower != nil && lower.Kind == document.Sequence {
+		result.lowerItems = sequenceItemProjections(len(lower.Sequence), 0)
+	}
+	return result
+}
+
+func replaceSequence(
+	lower, higher *document.Node,
+	identityFields []string,
+	context sequenceCombinationContext,
+) (sequenceCombinationResult, []Diagnostic) {
+	if higher.Kind == document.Sequence && len(identityFields) != 0 {
+		if _, diagnostics := validateSequenceIdentity(higher, context.destination, identityFields, sequenceCombinationIdentityCodes); len(diagnostics) != 0 {
+			return sequenceCombinationResult{}, diagnostics
+		}
+	}
+	result := sequenceCombinationResult{value: document.Clone(higher)}
+	if higher.Kind == document.Sequence {
+		result.higherItems = sequenceItemProjections(len(higher.Sequence), 0)
+	}
+	if lower != nil && lower.Kind == document.Sequence {
+		result.discardedLower = sequenceIndexes(len(lower.Sequence))
+	}
+	return result, nil
+}
+
+func combineIdentifiedSequences(
+	lower, higher *document.Node,
+	operation sequenceMode,
+	identityFields []string,
+	context sequenceCombinationContext,
+) (sequenceCombinationResult, []Diagnostic) {
+	if higher.Kind != document.Sequence {
+		return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+			context.destination, "append and upsert require a sequence contribution", higher,
+		)}
+	}
+	if lower != nil && lower.Kind != document.Sequence {
+		return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+			context.destination, "append and upsert require a lower sequence or an absent lower value", lower,
+		)}
+	}
+	if len(identityFields) == 0 {
+		return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+			context.destination, "append and upsert require an identity definition", higher,
+		)}
+	}
+
+	higherIndex, diagnostics := validateSequenceIdentity(higher, context.destination, identityFields, sequenceCombinationIdentityCodes)
+	if len(diagnostics) != 0 {
+		return sequenceCombinationResult{}, diagnostics
+	}
+	var lowerIndex map[string]sequenceIdentityRecord
+	if lower != nil {
+		lowerIndex, diagnostics = validateSequenceIdentity(lower, context.destination, identityFields, sequenceCombinationIdentityCodes)
+		if len(diagnostics) != 0 {
+			return sequenceCombinationResult{}, diagnostics
+		}
+	}
+	if operation == sequenceModeAppend {
+		if duplicate := duplicateAcrossSequenceContributions(lowerIndex, higherIndex, context.destination); duplicate != nil {
+			return sequenceCombinationResult{}, []Diagnostic{*duplicate}
+		}
+	}
+
+	if operation == sequenceModeAppend {
+		return appendSequenceItems(lower, higher), nil
+	}
+	if operation == sequenceModeUpsert {
+		return upsertSequenceItems(lower, higher, lowerIndex, higherIndex), nil
+	}
+	return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+		context.destination, "sequence operation is invalid", higher,
+	)}
+}
+
+func appendSequenceItems(lower, higher *document.Node) sequenceCombinationResult {
+	lowerCount := 0
+	if lower != nil {
+		lowerCount = len(lower.Sequence)
+	}
+	items := make([]*document.Node, 0, lowerCount+len(higher.Sequence))
+	if lower != nil {
+		items = append(items, lower.Sequence...)
+	}
+	items = append(items, higher.Sequence...)
+	return sequenceCombinationResult{
+		value:          cloneSequenceWithItems(higher, items),
+		lowerItems:     sequenceItemProjections(lowerCount, 0),
+		higherItems:    sequenceItemProjections(len(higher.Sequence), lowerCount),
+		appendedHigher: sequenceIndexes(len(higher.Sequence)),
+	}
+}
+
+func upsertSequenceItems(
+	lower, higher *document.Node,
+	lowerIndex, higherIndex map[string]sequenceIdentityRecord,
+) sequenceCombinationResult {
+	lowerCount := 0
+	var lowerItems []*document.Node
+	if lower != nil {
+		lowerCount = len(lower.Sequence)
+		lowerItems = lower.Sequence
+	}
+	matchedLower := make(map[int]sequenceIdentityRecord, len(higherIndex))
+	matchedLowerByHigher := make([]int, len(higher.Sequence))
+	for index := range matchedLowerByHigher {
+		matchedLowerByHigher[index] = -1
+	}
+	for tuple, incoming := range higherIndex {
+		if retained, exists := lowerIndex[tuple]; exists {
+			matchedLower[retained.index] = incoming
+			matchedLowerByHigher[incoming.index] = retained.index
+		}
+	}
+	items := make([]*document.Node, lowerCount, lowerCount+len(higher.Sequence))
+	result := sequenceCombinationResult{
+		lowerItems:  make([]sequenceItemProjection, 0, lowerCount-len(matchedLower)),
+		higherItems: sequenceItemProjections(len(higher.Sequence), 0),
+	}
+	for lowerPosition, item := range lowerItems {
+		if incoming, replaced := matchedLower[lowerPosition]; replaced {
+			items[lowerPosition] = higher.Sequence[incoming.index]
+			result.higherItems[incoming.index].outputIndex = lowerPosition
+			result.replacements = append(result.replacements, sequenceItemReplacement{
+				lowerIndex: lowerPosition, higherIndex: incoming.index, outputIndex: lowerPosition,
+			})
+			continue
+		}
+		items[lowerPosition] = item
+		result.lowerItems = append(result.lowerItems, sequenceItemProjection{inputIndex: lowerPosition, outputIndex: lowerPosition})
+	}
+	for higherPosition := range higher.Sequence {
+		if matchedLowerByHigher[higherPosition] >= 0 {
+			continue
+		}
+		outputIndex := len(items)
+		items = append(items, higher.Sequence[higherPosition])
+		result.appendedHigher = append(result.appendedHigher, higherPosition)
+		result.higherItems[higherPosition].outputIndex = outputIndex
+	}
+	result.value = cloneSequenceWithItems(higher, items)
+	return result
+}
+
+func duplicateAcrossSequenceContributions(
+	lowerIndex, higherIndex map[string]sequenceIdentityRecord,
+	path Path,
+) *Diagnostic {
+	higherTuples := make([]string, len(higherIndex))
+	for tuple, record := range higherIndex {
+		higherTuples[record.index] = tuple
+	}
+	for _, tuple := range higherTuples {
+		if lower, exists := lowerIndex[tuple]; exists {
+			higher := higherIndex[tuple]
+			primary := publicLocation(lower.location)
+			related := publicLocation(higher.location)
+			return &Diagnostic{
+				Code: CodeSequenceConflict, Severity: SeverityError, Path: path.clone(),
+				Message: "sequence contributions contain a duplicate identity",
+				Primary: &primary, Related: []Location{related},
+			}
+		}
+	}
+	return nil
+}
+
+func cloneSequenceWithItems(template *document.Node, items []*document.Node) *document.Node {
+	cloned := *template
+	cloned.Mapping = nil
+	cloned.Sequence = make([]*document.Node, len(items))
+	for index, item := range items {
+		cloned.Sequence[index] = document.Clone(item)
+	}
+	return &cloned
+}
+
+func sequenceItemProjections(length, outputOffset int) []sequenceItemProjection {
+	projections := make([]sequenceItemProjection, length)
+	for index := range projections {
+		projections[index] = sequenceItemProjection{inputIndex: index, outputIndex: index + outputOffset}
+	}
+	return projections
+}
+
+func sequenceIndexes(length int) []int {
+	indexes := make([]int, length)
+	for index := range indexes {
+		indexes[index] = index
+	}
+	return indexes
+}
+
+func sequenceCombinationDiagnostic(path Path, message string, node *document.Node) Diagnostic {
+	var primary *Location
+	if node != nil {
+		location := publicLocation(node.Location)
+		primary = &location
+	}
+	return Diagnostic{Code: CodeSequenceInvalid, Severity: SeverityError, Path: path.clone(), Message: message, Primary: primary}
+}
+
+var sequenceCombinationIdentityCodes = sequenceIdentityDiagnosticCodes{
+	invalid: CodeSequenceInvalid, conflict: CodeSequenceConflict,
+}

internal/engine/sequence_combine_bench_test.go

@@ -0,0 +1,42 @@
+package engine
+
+import (
+	"strconv"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func BenchmarkSequenceMixedUpsert(b *testing.B) {
+	for _, size := range []int{128, 512, 2048} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			lower := benchmarkSequenceRecords(0, size, "lower")
+			higher := benchmarkSequenceRecords(size/2, size, "higher")
+			context := sequenceCombinationContext{destination: Keys("items")}
+			identityFields := []string{"id"}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := combineIdentifiedSequences(
+					lower, higher, sequenceModeUpsert, identityFields, context,
+				); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
+func benchmarkSequenceRecords(start, count int, value string) *document.Node {
+	items := make([]*document.Node, count)
+	for index := range count {
+		items[index] = &document.Node{
+			Kind: document.Mapping,
+			Mapping: []document.Entry{
+				{Key: "id", Value: &document.Node{Kind: document.String, Text: "item-" + strconv.Itoa(start+index)}},
+				{Key: "value", Value: &document.Node{Kind: document.String, Text: value}},
+			},
+		}
+	}
+	return &document.Node{Kind: document.Sequence, Sequence: items}
+}

internal/engine/sequence_combine_test.go

@@ -0,0 +1,302 @@
+package engine
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestCombineSequenceOperationsAndItemEvidence(t *testing.T) {
+	tests := []struct {
+		name             string
+		lower            string
+		higher           string
+		operation        sequenceMode
+		want             []string
+		wantLower        []sequenceItemProjection
+		wantHigher       []sequenceItemProjection
+		wantReplacements []sequenceItemReplacement
+		wantAppended     []int
+		wantDiscarded    []int
+	}{
+		{
+			name:  "replace uses only higher records",
+			lower: "items: [{id: a}, {id: b}]\n", higher: "items: [{id: b, text: revised}]\n",
+			operation: sequenceModeReplace, want: []string{"b"},
+			wantHigher: []sequenceItemProjection{{inputIndex: 0, outputIndex: 0}}, wantDiscarded: []int{0, 1},
+		},
+		{
+			name:  "append retains then appends",
+			lower: "items: [{id: a}, {id: b}]\n", higher: "items: [{id: c}]\n",
+			operation: sequenceModeAppend, want: []string{"a", "b", "c"},
+			wantLower:  []sequenceItemProjection{{inputIndex: 0, outputIndex: 0}, {inputIndex: 1, outputIndex: 1}},
+			wantHigher: []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}}, wantAppended: []int{0},
+		},
+		{
+			name:      "upsert replaces whole records in place and appends new records",
+			lower:     "items: [{id: a, text: old}, {id: b}, {id: c, remove: true}]\n",
+			higher:    "items: [{id: c, text: revised}, {id: a, text: revised}, {id: d}]\n",
+			operation: sequenceModeUpsert, want: []string{"a", "b", "c", "d"},
+			wantLower:        []sequenceItemProjection{{inputIndex: 1, outputIndex: 1}},
+			wantHigher:       []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}, {inputIndex: 1, outputIndex: 0}, {inputIndex: 2, outputIndex: 3}},
+			wantReplacements: []sequenceItemReplacement{{lowerIndex: 0, higherIndex: 1, outputIndex: 0}, {lowerIndex: 2, higherIndex: 0, outputIndex: 2}},
+			wantAppended:     []int{2},
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			lower := sequenceFromYAML(t, test.lower)
+			higher := sequenceFromYAML(t, test.higher)
+			policy := sequenceCombinationPolicy(test.operation, "id")
+			result, diagnostics := combineSequence(lower, higher, policy, sequenceCombinationContext{destination: Keys("items")})
+			if len(diagnostics) != 0 {
+				t.Fatalf("combination diagnostics = %#v", diagnostics)
+			}
+			if got := sequenceIdentityValues(result.value, "id"); !reflect.DeepEqual(got, test.want) {
+				t.Fatalf("combined IDs = %#v, want %#v", got, test.want)
+			}
+			if !reflect.DeepEqual(result.lowerItems, test.wantLower) || !reflect.DeepEqual(result.higherItems, test.wantHigher) || !reflect.DeepEqual(result.replacements, test.wantReplacements) || !reflect.DeepEqual(result.appendedHigher, test.wantAppended) || !reflect.DeepEqual(result.discardedLower, test.wantDiscarded) {
+				t.Fatalf("item evidence = %#v", result)
+			}
+		})
+	}
+}
+
+func TestCombineSequenceDistinguishesAbsenceFromNullAndKindMismatch(t *testing.T) {
+	policy := sequenceCombinationPolicy(sequenceModeAppend, "id")
+	retained := sequenceFromYAML(t, "items: [{id: lower}]\n")
+	unchanged, diagnostics := combineSequence(retained, nil, policy, sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(unchanged.value, "id"), []string{"lower"}) || len(unchanged.lowerItems) != 1 {
+		t.Fatalf("absent higher value changed lower sequence: %#v, %#v", unchanged, diagnostics)
+	}
+
+	tests := []struct {
+		name   string
+		lower  string
+		higher string
+	}{
+		{name: "null higher value", lower: "items: [{id: lower}]\n", higher: "items: null\n"},
+		{name: "mapping higher value", lower: "items: [{id: lower}]\n", higher: "items: {id: higher}\n"},
+		{name: "null lower value", lower: "items: null\n", higher: "items: []\n"},
+		{name: "scalar lower value", lower: "items: blocked\n", higher: "items: []\n"},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			result, issues := combineSequence(sequenceFromYAML(t, test.lower), sequenceFromYAML(t, test.higher), policy, sequenceCombinationContext{destination: Keys("items")})
+			if result.value != nil || len(issues) != 1 || issues[0].Code != CodeSequenceInvalid || issues[0].Path.String() != "/items" {
+				t.Fatalf("kind mismatch result = %#v, %#v", result, issues)
+			}
+		})
+	}
+
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		result, issues := combineSequence(nil, sequenceFromYAML(t, "items: null\n"), sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if result.value != nil || len(issues) != 1 || issues[0].Code != CodeSequenceInvalid {
+			t.Fatalf("null incoming value under operation %d = %#v, %#v", operation, result, issues)
+		}
+	}
+}
+
+func TestReplaceSequenceAllowsNullAndNonSequenceValues(t *testing.T) {
+	for _, test := range []struct {
+		name     string
+		higher   string
+		wantKind document.Kind
+	}{
+		{name: "null", higher: "items: null\n", wantKind: document.Null},
+		{name: "mapping", higher: "items: {value: replacement}\n", wantKind: document.Mapping},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			lower := sequenceFromYAML(t, "items: [{id: lower}]\n")
+			higher := sequenceFromYAML(t, test.higher)
+			result, diagnostics := combineSequence(lower, higher, sequenceCombinationPolicy(sequenceModeReplace), sequenceCombinationContext{destination: Keys("items")})
+			if len(diagnostics) != 0 || result.value == nil || result.value.Kind != test.wantKind || !reflect.DeepEqual(result.discardedLower, []int{0}) {
+				t.Fatalf("replacement result = %#v, %#v", result, diagnostics)
+			}
+		})
+	}
+	result, diagnostics := combineSequence(sequenceFromYAML(t, "items: null\n"), sequenceFromYAML(t, "items: [{id: new}]\n"), sequenceCombinationPolicy(sequenceModeReplace), sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "id"), []string{"new"}) {
+		t.Fatalf("replacement over explicit lower null = %#v, %#v", result, diagnostics)
+	}
+}
+
+func TestCombineSequenceRequiresIdentityForAppendAndUpsertEvenWhenEmpty(t *testing.T) {
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		result, diagnostics := combineSequence(nil, sequenceFromYAML(t, "items: []\n"), sequenceCombinationPolicy(operation), sequenceCombinationContext{destination: Keys("items")})
+		if result.value != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid {
+			t.Fatalf("empty input bypassed identity requirement for operation %d: %#v, %#v", operation, result, diagnostics)
+		}
+	}
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		result, diagnostics := combineSequence(nil, sequenceFromYAML(t, "items: [{name: no-id}]\n"), sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if result.value != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid || diagnostics[0].Path.String() != "/items/0/id" {
+			t.Fatalf("missing identity for operation %d = %#v, %#v", operation, result, diagnostics)
+		}
+	}
+}
+
+func TestCombineSequenceValidatesDuplicatesWithinEachContributionInEveryMode(t *testing.T) {
+	duplicate := "items: [{id: same}, {id: same}]\n"
+	for _, operation := range []sequenceMode{sequenceModeReplace, sequenceModeAppend, sequenceModeUpsert} {
+		_, diagnostics := combineSequence(nil, sequenceFromYAML(t, duplicate), sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict {
+			t.Fatalf("higher duplicate under operation %d = %#v", operation, diagnostics)
+		}
+	}
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		_, diagnostics := combineSequence(sequenceFromYAML(t, duplicate), sequenceFromYAML(t, "items: [{id: other}]\n"), sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict {
+			t.Fatalf("lower duplicate under operation %d = %#v", operation, diagnostics)
+		}
+	}
+}
+
+func TestAppendSequenceRejectsDuplicatesAcrossContributions(t *testing.T) {
+	const identityCanary = "identity-canary-64"
+	lower := sequenceFromYAML(t, "items: [{id: "+identityCanary+"}]\n")
+	higher := sequenceFromYAML(t, "items: [{id: "+identityCanary+"}]\n")
+	_, diagnostics := combineSequence(lower, higher, sequenceCombinationPolicy(sequenceModeAppend, "id"), sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict || diagnostics[0].Path.String() != "/items" || diagnostics[0].Primary == nil || len(diagnostics[0].Related) != 1 {
+		t.Fatalf("cross-contribution duplicate = %#v", diagnostics)
+	}
+	if strings.Contains(diagnostics[0].Message, identityCanary) {
+		t.Fatalf("identity value leaked in diagnostic: %#v", diagnostics[0])
+	}
+}
+
+func TestCombineSequenceReplacementResetsLowerIdentity(t *testing.T) {
+	lower := sequenceFromYAML(t, "items: [{id: old}]\n")
+	higher := sequenceFromYAML(t, "items: [{name: new}]\n")
+	policy := sequenceCombinationPolicy(sequenceModeReplace, "name")
+	result, diagnostics := combineSequence(lower, higher, policy, sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "name"), []string{"new"}) || !reflect.DeepEqual(result.discardedLower, []int{0}) {
+		t.Fatalf("replacement applied obsolete identity: %#v, %#v", result, diagnostics)
+	}
+}
+
+func TestCombineSequenceDifferentIdentityDeclarationsConflict(t *testing.T) {
+	declarations := extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+    - destination: [items]
+      key: name
+`, rootDirectiveContextSource).sequences
+	if _, diagnostics := resolveCollectionPolicy(Keys("items"), declarations, nil, nil); len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict {
+		t.Fatalf("different collection key declarations = %#v", diagnostics)
+	}
+}
+
+func TestCombineSequenceEmptyInputsAndStableOrder(t *testing.T) {
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		lower := sequenceFromYAML(t, "items: [{id: a}, {id: b}]\n")
+		empty := sequenceFromYAML(t, "items: []\n")
+		result, diagnostics := combineSequence(lower, empty, sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "id"), []string{"a", "b"}) {
+			t.Fatalf("empty higher sequence under operation %d = %#v, %#v", operation, result, diagnostics)
+		}
+	}
+	result, diagnostics := combineSequence(nil, sequenceFromYAML(t, "items: []\n"), sequenceCombinationPolicy(sequenceModeReplace, "id"), sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || result.value == nil || result.value.Kind != document.Sequence || len(result.value.Sequence) != 0 {
+		t.Fatalf("explicit replacement with empty input = %#v, %#v", result, diagnostics)
+	}
+}
+
+func TestCombineSequenceInitializesAbsentLowerCollection(t *testing.T) {
+	higher := sequenceFromYAML(t, "items: [{id: first}, {id: second}]\n")
+	for _, operation := range []sequenceMode{sequenceModeReplace, sequenceModeAppend, sequenceModeUpsert} {
+		result, diagnostics := combineSequence(nil, higher, sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "id"), []string{"first", "second"}) {
+			t.Fatalf("absent lower collection under operation %d = %#v, %#v", operation, result, diagnostics)
+		}
+	}
+}
+
+func TestCombineSequenceClonesInputsAndDoesNotMutatePolicy(t *testing.T) {
+	for _, test := range []struct {
+		name      string
+		operation sequenceMode
+	}{
+		{name: "replace", operation: sequenceModeReplace},
+		{name: "append", operation: sequenceModeAppend},
+		{name: "upsert", operation: sequenceModeUpsert},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			lower := sequenceFromYAML(t, "items: [{id: a, text: original}]\n")
+			higherID := "b"
+			if test.operation == sequenceModeUpsert {
+				higherID = "a"
+			}
+			higher := sequenceFromYAML(t, "items: [{id: "+higherID+", text: incoming}]\n")
+			policy := sequenceCombinationPolicy(test.operation, "id")
+			policyCopy := append([]string(nil), policy.effectiveIdentityFields...)
+			result, diagnostics := combineSequence(lower, higher, policy, sequenceCombinationContext{destination: Keys("items")})
+			if len(diagnostics) != 0 {
+				t.Fatal(diagnostics)
+			}
+			for _, item := range result.value.Sequence {
+				for entryIndex := range item.Mapping {
+					if item.Mapping[entryIndex].Key == "text" {
+						item.Mapping[entryIndex].Value.Text = "changed"
+					}
+				}
+			}
+			if lower.Sequence[0].Mapping[1].Value.Text != "original" || higher.Sequence[0].Mapping[1].Value.Text != "incoming" {
+				t.Fatal("combined result aliases a mutable input node")
+			}
+			if !reflect.DeepEqual(policy.effectiveIdentityFields, policyCopy) {
+				t.Fatal("combination mutated the resolved policy")
+			}
+		})
+	}
+}
+
+func TestCombineSequenceEquivalentIdentityFieldSets(t *testing.T) {
+	lower := sequenceFromYAML(t, "items: [{id: a, namespace: one}]\n")
+	higher := sequenceFromYAML(t, "items: [{namespace: one, id: b}]\n")
+	policy, diagnostics := resolveCollectionPolicy(Keys("items"), extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [items]
+      key: [namespace, id, id]
+`, rootDirectiveContextSource).sequences, nil, nil)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	result, diagnostics := combineSequence(lower, higher, policy, sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "id"), []string{"a", "b"}) {
+		t.Fatalf("equivalent identity policy failed: %#v, %#v", result, diagnostics)
+	}
+}
+
+func sequenceCombinationPolicy(operation sequenceMode, fields ...string) collectionPolicyResolution {
+	normalized, _ := normalizeSequenceIdentityFields(fields)
+	return collectionPolicyResolution{operation: operation, effectiveIdentityFields: normalized}
+}
+
+func sequenceFromYAML(t *testing.T, input string) *document.Node {
+	t.Helper()
+	parsed := parsePolicyNode(t, input)
+	if len(parsed.Mapping) != 1 || parsed.Mapping[0].Key != "items" {
+		t.Fatalf("sequence fixture root = %#v", parsed.Mapping)
+	}
+	return parsed.Mapping[0].Value
+}
+
+func sequenceIdentityValues(sequence *document.Node, field string) []string {
+	if sequence == nil || sequence.Kind != document.Sequence {
+		return nil
+	}
+	values := make([]string, len(sequence.Sequence))
+	for index, item := range sequence.Sequence {
+		for _, entry := range item.Mapping {
+			if entry.Key == field {
+				values[index] = entry.Value.Text
+				break
+			}
+		}
+	}
+	return values
+}