Document the ConfigKit v0.8.0 release

README.md

@@ -26,6 +26,13 @@ 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).
+
 ```go
 type Config struct {
     Enabled bool `yaml:"enabled"`
@@ -62,17 +69,23 @@ 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.
+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.
 
 The [architecture goals](docs/policy/architecture.md#success-criteria) explain
 these priorities and how genuine new needs can justify extending the shared
-model. The [deferred scope](docs/roadmap/deferred.md) records deliberate exclusions.
+model. The [future feature list](docs/roadmap/future.md) preserves candidates for
+consideration in future releases.
 
 The [configuration contract](docs/config.md) describes implemented behavior,
 including [input destinations](docs/config.md#input-destinations). The
 [consumer adoption guide](docs/consumers/README.md) provides maintained simple,
-layered, and mounted-collection examples alongside the migration shapes. The
-accepted placement decision is recorded in [ADR 0011](docs/adr/0011-input-destinations.md).
+layered, mounted-collection, and identified-prompt examples alongside the
+migration shapes. Input placement decisions are recorded in
+[ADR 0011](docs/adr/0011-input-destinations.md) and sequence placement and
+composition in [ADR 0014](docs/adr/0014-sequence-placement-and-composition.md).
 See the [development guide](docs/development.md) for repository orientation and
 the [release procedure](docs/release.md) for maintainer-only candidate and
 publication steps.

conversion.go

@@ -43,10 +43,11 @@ func toEngineLocations(values []Location) []engine.Location {
 	return result
 }
 func fromEngineSourceInfo(value engine.SourceInfo) SourceInfo {
-	return SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Destination: fromEnginePath(value.Destination), Layer: Layer(value.Layer), Kind: SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: fromEngineLocationPointer(value.ImportedFrom), VersionCheck: fromEngineSourceVersionCheck(value.VersionCheck)}
+	return SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Destination: fromEnginePath(value.Destination), PlacementMode: PlacementMode(engine.SourceInfoPlacementModeForFacade(value)), DestinationUnresolved: engine.SourceInfoDestinationUnresolvedForFacade(value), Layer: Layer(value.Layer), Kind: SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: fromEngineLocationPointer(value.ImportedFrom), VersionCheck: fromEngineSourceVersionCheck(value.VersionCheck)}
 }
 func toEngineSourceInfo(value SourceInfo) engine.SourceInfo {
-	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Destination: toEnginePath(value.Destination), Layer: engine.Layer(value.Layer), Kind: engine.SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: toEngineLocationPointer(value.ImportedFrom), VersionCheck: toEngineSourceVersionCheck(value.VersionCheck)}
+	result := engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Destination: toEnginePath(value.Destination), Layer: engine.Layer(value.Layer), Kind: engine.SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: toEngineLocationPointer(value.ImportedFrom), VersionCheck: toEngineSourceVersionCheck(value.VersionCheck)}
+	return engine.SourceInfoWithPlacement(result, uint8(value.PlacementMode), value.DestinationUnresolved)
 }
 func fromEngineSourceVersionCheck(value *engine.SourceVersionCheck) *SourceVersionCheck {
 	if value == nil {

docs/adr/0014-sequence-placement-and-composition.md

@@ -0,0 +1,77 @@
+# ADR 0014: Place and Compose Sequence Items Explicitly
+
+## Status
+
+Accepted and implemented. The [configuration contract](../config.md#strict-document-composition)
+defines current behavior, and the [consumer guide](../consumers/README.md#identified-prompt-sequences)
+shows the maintained example. The completed roadmap, implementation plan, and
+resolved remediation record are preserved in Git history at `008d2ba` under
+`docs/roadmap/`; see the [development guide](../development.md) for retrieval
+commands.
+
+## Date
+
+2026-10-16
+
+## Context
+
+ConfigKit strictly composes independent documents by recursively combining
+mappings and rejecting other overlapping values. This makes two independently
+authored sequence fragments conflict, even when their records are distinct.
+Precedence assembly has a different purpose: a higher-priority present sequence
+replaces the lower-priority sequence as one value. Those existing rules are
+predictable, but they do not express the separate operation of collecting
+independent records while rejecting duplicate identities.
+
+Likewise, mapping destinations place a source beneath literal mapping keys, but
+there is no explicit way to state that a source mapping represents one item in a
+sequence. Inferring that intent from document shape or fields would make source
+meaning implicit and change existing placement behavior.
+
+## Decision
+
+Keep mapping placement as the zero-value behavior and add an explicit item
+placement choice for a source whose mapping should become one element at a
+non-root sequence destination. Item placement provides structure only; it does
+not infer or synthesize identity values.
+
+Allow a definition to register literal mapping paths and one literal identity
+field for sequences at those paths. Within each strict composition tier, a
+registered sequence accepts mapping records with a present string identity,
+appends distinct records in contribution order, and rejects repeated exact
+identity strings. Identical registrations coalesce; conflicting identity fields
+for the same path are invalid. Registration does not recursively select paths
+inside sequence records.
+
+Keep precedence assembly unchanged. A present higher-priority sequence replaces
+the lower-priority value wholesale, including at a registered path. Registration
+changes strict composition only; it does not turn precedence into append or
+identity-based merging.
+
+This decision preserves ConfigKit's distinction between independent
+same-priority contributions and intentional precedence. Explicit source
+placement and definition-owned composition policy make the two required choices
+visible while keeping the existing zero-value load behavior.
+
+## Alternatives Considered
+
+- **Infer item placement from a source mapping or identity field.** Shape-based
+  inference would make ordinary mapping placement depend on document contents
+  and would leave the intended destination ambiguous.
+- **Append all overlapping sequences during strict composition.** This would
+  change existing conflict semantics at unregistered paths and would not detect
+  duplicate records.
+- **Merge matching records recursively by identity.** This would add a second
+  recursive merge rule with different conflict and precedence behavior from
+  ConfigKit's existing mapping composition and overlay operations.
+- **Use identity to merge sequences across precedence tiers.** This would
+  change established whole-value replacement semantics and conflate strict
+  composition with precedence.
+
+## Consequences
+
+Consumers can explicitly place mappings as sequence items and opt selected
+sequences into duplicate-rejecting strict composition. Unregistered sequences
+retain their current strict-conflict behavior, and higher-priority sequences
+continue to replace lower-priority ones. The implementation plan owns the exact
+API, YAML syntax, diagnostics, and implementation status.

docs/config.md

@@ -67,13 +67,18 @@ ID `source-1`; enabled imports continue in depth-first order as `source-2` onwar
 contributes no source, and continues through defaults, decode, and projection. A
 missing file remains an error; it is never interpreted as no input.
 
-The current `Options` boundary accepts a defaults factory, `VersionRule`, resolver,
-semantic validator, explicit path and glob imports, one selected overlay, and
-filesystem context. It also accepts explicit CLI override records, static secret
-tags, and a dynamic path predicate. `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.
+`Options` accepts the application definition, ordinary-source `Destination` and
+`PlacementMode`, explicit path and glob 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
+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
+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.
 
 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
@@ -149,6 +154,11 @@ have index zero. Explicit empty groups and `NoSource` consume no source ID.
 Byte, source, node, and glob-entry budgets are shared across the load, while
 the existing per-source and import-depth bounds still apply.
 
+When a source mapping is explicitly placed as an item, its effective destination
+is the sequence container. After assembly assigns an index, nested source
+destinations are projected through that item. Physical source locations and
+resource contexts continue to identify the declaring source.
+
 #### Input destinations
 
 `Input.Destination` places the source's application mapping beneath a path
@@ -176,12 +186,21 @@ follow the same strict composition within a tier and recursive precedence
 behavior described above. A selected overlay and CLI overrides remain rooted at
 the application root and are not moved beneath the main input's destination.
 
-`SourceInfo.Destination` reports the placement for each acquired input source,
-including imports. Placement does not change source IDs, acquisition identity,
-physical source locations, or passive resource contexts. Introduced ancestor
-mappings use the contributing document's root location as a source fallback;
-authored content retains its original source coordinates. Returned source
-records are detached copies.
+`SourceInfo.Destination` reports each acquired source's effective mapping or
+sequence-container path, including imports. `PlacementMode` reports how that
+source was placed; mapping is the zero value, and CLI/overlay sources report
+mapping unless item placement was applied. `DestinationUnresolved` is true only
+when an enclosing item index needed to name the source's destination has not yet
+been assigned. In that case `Destination` reports the nearest resolved enclosing
+collection container and omits the unresolved local suffix. A source's own item
+placement still has an exact destination at its sequence container. Completed
+loads project nested source destinations through their owning items. Callback and
+failed-load snapshots preserve the evidence available at that point; a fallback
+snapshot remains unresolved, while a later exact snapshot is false. Returned
+source records and paths are detached. Placement does not change source IDs,
+acquisition identity, physical source locations, or passive resource contexts.
+Introduced ancestor mappings use the contributing document's root location as a
+source fallback; authored content retains its original source coordinates.
 
 The maintained [layered consumer example](consumers/README.md#native-filesystem-and-layered-input)
 shows a main host file, two underlay tiers, embedded defaults, a mapping source,
@@ -397,6 +416,11 @@ 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.
+
 A mapping assembled from the selected file and imports has one composite base origin
 whose locations retain source traversal order. A mapping also visited by an overlay
 has an overlay-layer composite origin with the lower contributing locations followed
@@ -744,12 +768,47 @@ Application defaults do not participate in this operation.
 | Mapping | Mapping | Recurse by mapping key |
 | 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. Nulls and
-sequences are present atomic values, so they also conflict when repeated; sequences
-are never joined by index. Disjoint entries under shared mappings compose, making
+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.
 
+`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.
+
+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.
+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.
+
 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
@@ -884,6 +943,30 @@ walk failures, matched directories, invalid documents, escapes, and repeated tar
 remain errors. Final candidates are bounded by the remaining source limit before
 their contents are accumulated.
 
+Each path or glob declaration may include `placement: mapping` or
+`placement: sequence-item`. Omission means `mapping` for that declaration,
+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.
+
+An item import first opens a local mapping scope. Its body and ordinary helper
+imports compose within that record. An omitted helper destination or
+`destination: []` places the helper at the record root; other destinations are
+relative to the immediate importing source. A helper may supply fields such as
+the record's `id`. An explicit nested `sequence-item` import creates a sequence
+inside the record, and its placement is independent of the enclosing item.
+At the record root, an item import must name a nonempty nested destination.
+Physical paths continue to resolve from the file that declares each import.
+An actual empty mapping contributes an empty item, while an optional unmatched
+path or glob contributes nothing.
+
 For host files, the configuration root defaults to the selected file's canonical
 directory. A host file may supply `FilesystemContext.Root` to widen that root but
 may not supply `BaseDir` or a native `FS`. Byte, reader, and mapping inputs with host context

docs/consumers/README.md

@@ -21,6 +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 |
 
 The YAML under [`testdata`](../../examples/consumers/testdata) is loaded by the
 executable examples and ordinary tests. It contains no private application data,
@@ -163,6 +164,24 @@ relative resources, and strict peer conflicts. The
 [configuration contract](../config.md#input-destinations) owns shared placement,
 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 main
+file imports one prompt by path and others by glob, placing each source as an
+item. A nested ordinary helper import supplies an item's ID. The main tier
+strictly appends the distinct records in contribution order; its completed
+sequence replaces the lower-priority default collection as one value.
+
+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.
+
 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
 fields are known; validation selects which fields apply to each module. Runtime-

docs/development.md

@@ -43,7 +43,7 @@ public behavior.
 | Shared source-version checks | [Configuration contract](config.md#shared-source-version-checks), [ADR 0013](adr/0013-shared-source-version-checks.md), and the version/source-version tests below |
 | Documentation changes | The owning document |
 | Licensing, dependencies, attribution, or release-document changes | [Project grant](../COPYRIGHT) and [attribution](oss/attribution.md); release changes also follow the release procedure |
-| Architecture decisions or future work | [Deferred scope](roadmap/deferred.md) and relevant ADRs listed below |
+| Architecture decisions or future work | [Future features](roadmap/future.md) and relevant ADRs listed below |
 | Code and test changes | The owning contract and tests from the map below |
 | Examples or copyable assets | The owning implemented contract and [consumer adoption guide](consumers/README.md) |
 | Release preparation | [Release procedure](release.md) |
@@ -108,7 +108,7 @@ git show 8e91a2b:docs/roadmap/startup-benchmark.md
 ```
 
 Remaining optimization candidates belong to
-[deferred scope](roadmap/deferred.md#deferred-performance-experiments).
+[future performance experiments](roadmap/future.md#3-performance-experiments).
 
 The completed effective-configuration diff roadmap, implementation plan, and
 resolved review findings are preserved in Git history at `1c2610c` under
@@ -131,7 +131,24 @@ 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.
-Possible follow-ups remain in [deferred scope](roadmap/deferred.md).
+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.
+
+The completed sequence-composition roadmap, implementation plan, and resolved
+remediation record are preserved in Git history at `008d2ba` under
+`docs/roadmap/`. For example:
+
+```sh
+git show 008d2ba:docs/roadmap/sequence-composition.md
+git show 008d2ba:docs/roadmap/implementation.md
+git show 008d2ba: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.
 Acquire declared dependencies explicitly before entering an offline

docs/internal/overview.md

@@ -9,9 +9,12 @@ Implemented component owners:
 
 - `internal/engine` owns host and native-filesystem source acquisition,
   grouped strict composition, recursive precedence assembly, per-input mapping
-  placement and immediate-parent-relative import placement within that
-  assembly, destination-local sibling inheritance before typed decoding, and
-  shared source-version checking with sanitized declaration evidence.
+  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 destination-local sibling
+  inheritance before typed decoding, plus shared source-version checking with
+  sanitized declaration evidence.
   It retains each source's effective placement alongside physical source
   identity and owns schema
   inspection, lifecycle coordination, callback auditing,
@@ -48,6 +51,10 @@ The [configuration contract](../config.md) owns implemented YAML and lifecycle
 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.
 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
@@ -59,3 +66,5 @@ and [release procedure](../release.md) for source-candidate gates. [ADR
 post-assembly structural operation. [ADR 0012](../adr/0012-relative-import-destinations.md)
 records the relative-import extension; [ADR 0013](../adr/0013-shared-source-version-checks.md)
 records the source-version decision.
+[ADR 0014](../adr/0014-sequence-placement-and-composition.md) records explicit
+sequence placement and composition.

docs/policy/architecture.md

@@ -2,7 +2,7 @@
 
 **Base Policy Version:** 1.3
 
-**Last Updated**: 2026-09-29
+**Last Updated**: 2026-10-16
 
 **Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/architecture.md)
 
@@ -19,8 +19,8 @@ to ADRs for decision rationale.
 ConfigKit is an opinionated Go library for consistent YAML configuration loading
 and resolution across consuming applications. The [component overview](../internal/overview.md)
 owns implementation inventory, the [configuration contract](../config.md) owns
-implemented behavior, and the [deferred scope](../roadmap/deferred.md) preserves
-deferred scope.
+implemented behavior, and the [future feature list](../roadmap/future.md) preserves
+candidates for future consideration.
 
 ## Adopted Constraints And Implemented Structure
 
@@ -163,13 +163,23 @@ framework on this library.
 
 Within a configuration tier, independent contributions use strict composition:
 shared mappings combine recursively, while overlapping non-mapping definitions
-are conflicts, including equal values. Between precedence tiers, intentional
-overlay combines mappings recursively, retains omitted lower-priority children,
+are conflicts, including equal values, except that sequences at explicitly
+registered paths append records with distinct present string identities. Between
+precedence tiers, intentional overlay combines mappings recursively, retains
+omitted lower-priority children,
 and replaces other present values according to the existing overlay rules.
 Tier precedence is explicit; traversal order within a tier establishes stable
 reporting, not permission to override. The distinction and its rationale are
 recorded in [ADR 0001](../adr/0001-separate-composition-and-overlay.md).
 
+Sequence-item placement and sequence registration are explicit source and
+definition controls; registration does not change precedence replacement. The
+[configuration contract](../config.md#strict-document-composition) defines the
+behavior, [ADR 0014](../adr/0014-sequence-placement-and-composition.md) records
+the decision, and the [consumer guide](../consumers/README.md#identified-prompt-sequences)
+shows the implemented subset. This feature does not authorize arbitrary
+configurable merge strategies.
+
 The main document and its imports form a strictly composed tier. Explicit
 underlay groups compose strictly within each group, then combine recursively
 from lowest to highest beneath the main tier. Source acquisition kind does not
@@ -243,7 +253,7 @@ for safe messages supplied by their hooks.
 Do not introduce automatic directory discovery, network providers, arbitrary
 merge strategies, additional configuration languages, or application runtime
 management without concrete consumer evidence and an explicit design decision.
-The [deferred scope](../roadmap/deferred.md#explicit-non-goals-and-deferred-scope)
-owns the deferred-feature inventory. Significant boundary
-changes require an ADR and corresponding policy/contract updates; implementation
-convenience alone is not a reason to broaden the public API.
+Record significant decisions to exclude capabilities, and their rationale, in
+ADRs. Significant boundary changes require an ADR and corresponding
+policy/contract updates; implementation convenience alone is not a reason to
+broaden the public API.

docs/policy/documentation.md

@@ -2,7 +2,7 @@
 
 **Base Policy Version:** 1.11
 
-**Last Updated**: 2026-09-28
+**Last Updated**: 2026-10-16
 
 **Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/documentation.md)
 
@@ -38,9 +38,10 @@ roadmap owns implementation status and sequencing until the decision is
 implemented. Current architecture, user, operator, integration, and internal
 documentation are updated when the behavior lands.
 
-`docs/roadmap/deferred.md` owns deliberate exclusions; task-specific plans
-own execution order and progress. Completed plans retire during release
-preparation.
+`docs/roadmap/future.md` owns potential enhancements and their open questions;
+task-specific plans own execution order and progress. Record significant
+architectural exclusions and their rationale in ADRs. Do not maintain a roadmap
+inventory of rejected proposals. Completed plans retire during release preparation.
 
 ### Audience And Detail
 
@@ -91,7 +92,7 @@ secret values.
 | Inventory of implemented internal packages | `docs/internal/overview.md` | Implemented packages and components, responsibilities, and links to focused internal docs. | Normative architecture, contributor reading policy, external contracts. |
 | Behavior and architecture of internal packages | Other files under `docs/internal/` | Implementation flow, collaborators, state transitions, package-local guarantees and failures, and relevant tests. | Global architecture invariants, configuration definitions and defaults, external schemas, operator procedures. |
 | Architectural decision records | `docs/adr/` | Significant decisions, context, alternatives, rationale, consequences, and supersession history. | Current behavior reference, implementation status, task sequencing. |
-| Deferred scope | `docs/roadmap/deferred.md` | Deliberate exclusions and conditions for reconsideration. | Task execution and current behavior. |
+| Potential future features | `docs/roadmap/future.md` | Candidate enhancements and open questions for future consideration. | Rejected proposals, architectural decisions, task execution, and current behavior. |
 | Future implementation plans | Task-specific files under `docs/roadmap/` | Unfinished work, sequencing, acceptance, and progress; completed plans retire during release preparation. | Implemented behavior and ADR rationale. |
 | Release-note history | `docs/releases/` | One checked-in summary per source release under the procedure; the note at its immutable tag is the release record. | Current commands, behavior, contracts, and compatibility definitions. |
 | Complete examples | `examples/` and executable Go examples | Maintained copyable artifacts and Go API examples. | Configuration reference, symbol contracts, and prose explanation. |
@@ -156,8 +157,10 @@ Use sequentially numbered ADR filenames such as
 
 Treat the decision content of an accepted ADR as immutable. When a decision
 changes, create a new ADR and update the earlier ADR's status to superseded.
-Rejected architectural alternatives belong in the ADR; rejected product ideas
-belong in the roadmap. Routine choices need no ADR.
+Rejected architectural alternatives and significant architectural exclusions
+belong in ADRs. Roadmaps preserve plausible future work, not rejected product
+ideas. Routine choices and rejected suggestions need no ADR unless they
+represent a significant architectural decision.
 
 ## Maintenance
 

docs/releases/v0.6.0.md

@@ -30,7 +30,7 @@ adopts ReleaseKit for source publication, and raises the consumer Go minimum to
   successful-tag-push completion point remain unchanged; CI is not a gate.
 - Adopt release policy 1.6, distinguishing the consumer Go minimum from the
   validation toolchain. Retire completed planning and measurement records into
-  Git history while preserving [deferred work](../roadmap/deferred.md).
+  Git history while preserving [performance candidates](../roadmap/future.md#3-performance-experiments).
 
 ## Compatibility and upgrade
 

docs/releases/v0.8.0.md

@@ -0,0 +1,70 @@
+# ConfigKit v0.8.0
+
+This minor release adds explicit input placement, relative import destinations,
+shared source-version checking, and opt-in composition of identified sequence
+records. Applications can assemble independently authored configuration fragments
+into one final configuration while retaining source locations, provenance,
+history, and sensitivity information.
+
+## Changes
+
+- Add `Destination` to ordinary load options and layered inputs. A source mapping
+  can contribute beneath literal application keys without being rewritten by the
+  consumer. Imports inherit that destination or append a relative YAML
+  `destination`. Physical import paths and resource contexts remain relative to
+  their declaring sources. See [input destinations](../config.md#input-destinations),
+  [import syntax](../config.md#explicit-imports), and the
+  [mounted collection example](../consumers/README.md#input-destinations-and-mounted-collections).
+- Add `SourceVersionModeIfPresent` and `SourceVersionModeRequired` to apply one
+  application-owned version checker across main, underlay, imported, and selected
+  overlay documents. Enabled modes check source-local declarations and consume
+  accepted non-main headers before composition. Detached, sensitivity-aware
+  evidence is available through `SourceInfo.VersionCheck`. Acceptance remains
+  application policy; ConfigKit does not require declarations to equal the main
+  version. See [shared source-version checks](../config.md#shared-source-version-checks).
+- Add explicit `PlacementModeSequenceItem` placement for ordinary and layered
+  inputs, and `placement: sequence-item` for path and glob imports. A source
+  mapping becomes one item at its destination; ordinary helper imports compose
+  within that item and can supply its identity. See
+  [imports](../config.md#explicit-imports).
+- Add `Definition.SequenceCompositions` registrations for strict composition at
+  selected literal paths using one string identity field. Distinct records append
+  in contribution order, while duplicate identities conflict. Higher-priority
+  sequences still replace lower-priority sequences wholesale. Registration does
+  not add identity-based overlays or final identity validation after defaults and
+  application resolution. See the [composition contract](../config.md#strict-document-composition)
+  and [maintained prompt catalog](../consumers/README.md#identified-prompt-sequences).
+- Extend source inspection with destination and placement evidence, including
+  whether a nested destination is unresolved in an intermediate or failed-load
+  snapshot. Preserve source-specific history, consumed-header sensitivity, and
+  explanations of descendants removed by sequence replacement as records are
+  assembled. Source locations continue to identify authored coordinates.
+
+## Compatibility and upgrade
+
+The new behavior is opt-in: zero-valued destinations retain root placement,
+mapping placement remains the default, unregistered sequences retain strict
+conflict behavior, and the default source-version mode preserves the existing
+main-only version policy. Recursive mapping precedence, whole-sequence
+replacement between tiers, and application-owned final validation remain intact.
+
+Public option, definition, input, and source-information structs gain fields.
+Consumers using unkeyed literals for affected structs must switch to keyed
+literals or update them. `SourceInfo` now contains a `Path` and is no longer
+comparable with `==` or usable directly as a map key; compare the relevant fields
+or select an application-appropriate key instead. Existing keyed literals need
+no changes to retain the previous loading behavior.
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.8.0
+```
+
+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/v0.8.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.8.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.8.0/docs/oss/attribution.md)

docs/roadmap/deferred.md

@@ -1,69 +0,0 @@
-# Deferred Scope
-
-This document owns intentionally excluded features, not a committed delivery plan.
-The [architecture policy](../policy/architecture.md) governs changes to these
-boundaries. Implemented behavior belongs in the [configuration contract](../config.md)
-and consumer adoption in the [consumer guide](../consumers/README.md).
-
-## Explicit non-goals and deferred scope
-
-General environment-variable configuration overlays are not supported. Typed secret
-references and explicit directory bootstrap are the environment convention. Literal
-credential configuration and implicit secret-directory discovery are unsupported.
-
-The following remain deliberately excluded:
-
-- automatic `conf.d` discovery;
-- remote imports, arbitrary providers, and additional configuration formats;
-- custom merge strategies, sequence merging, reset operators, multiple-parent
-  inheritance, and cross-scope references;
-- named overlay inheritance;
-- parser-specific or custom structural-decoding extension APIs;
-- generic dynamic-subtree conversion and module-registry APIs;
-- automatic schema migrations;
-- hot reload, runtime resource replacement, and remote resource validation;
-- a bundled CLI framework or application commands; and
-- configuration fingerprints.
-
-Applications may build their own validate, effective-value, source, bootstrap, and
-explanation commands around the library. Broadening these boundaries requires
-concrete consumer evidence, compatibility review, and an explicit design change.
-
-Effective configuration diffs are implemented over frozen successful-load
-inspections; their current comparison and disclosure contract is in the
-[configuration contract](../config.md#effective-configuration-diffs), with
-consumer workflows in the [adoption guide](../consumers/README.md#effective-configuration-diffs).
-Fingerprints remain deferred.
-
-Built-in sibling inheritance using `extends` is implemented as described in the
-[configuration contract](../config.md#sibling-inheritance). It is distinct from
-inheritance between selected overlay files, which remains excluded above.
-
-## Possible source-version follow-ups
-
-Independent collection or subtree specification policies were outside the
-completed shared source-version feature and remain possible future work, with
-no committed delivery plan. Current [source-version checks](../config.md#shared-source-version-checks)
-use one application-owned policy across participating documents. Reconsider
-independent policies if concrete consumer needs justify a separate design
-decision; this is not a permanent prohibition. The original request and its
-selected scope remain at `e4270c9:docs/roadmap/promptkit-feature-request.md` in
-Git history. Existing sequence-composition and cross-scope inheritance deferrals
-above also remain unchanged.
-
-## Deferred performance experiments
-
-The completed performance audit left these internal optimization candidates for
-separate investigation:
-
-- branch-local explanation replay instead of the current whole-tree replay;
-- redesigning the private metadata-copy handoff while preserving detached public
-  records;
-- caching schema descriptors across loads; and
-- indexing complex contributor-trace membership while preserving its equality
-  rules and compact linked traces.
-
-These are not committed implementation work. Reconsider them with evidence that
-the benefit justifies the ownership, retained-memory, or equivalence risks. The
-completed audit and implementation evidence are preserved in Git history at
-`8e91a2b` under `docs/roadmap/`.

docs/roadmap/future.md

@@ -0,0 +1,171 @@
+# Potential Future Features
+
+This document lists potential enhancements for consideration in future ConfigKit
+releases. Entries are not accepted contracts, implementation plans, or release
+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
+
+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.
+
+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.
+
+### 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.
+
+## 2. Required version declarations for selected sources
+
+Status: deferred. PromptKit requested a way to require version declarations on
+a selected subset of source files while permitting other fragments to omit them.
+
+### Motivation and proposed behavior
+
+PromptKit could require each collection's root file (for example,
+`collection.yml` or `meta.yml`, with naming still undecided) to declare an
+accepted specification version. Other files would use the existing
+[check-when-present behavior](../config.md#shared-source-version-checks).
+This would enforce a rule that otherwise remains a documentation convention.
+
+All declarations would use the same application-owned `Definition.Version`
+path and acceptance policy. The proposal concerns where declarations are
+required; it does not introduce independently versioned collections or subtrees.
+A root declaration would not prove that headerless descendants declare a
+compatible version or replace final configuration validation.
+
+### Questions to resolve before implementation
+
+- **Selection API:** should callers mark explicit inputs, select sources by
+  path or predicate, or annotate imports? An input-level requirement is simpler,
+  but cannot by itself select a root reached only through an import.
+- **Scope:** does a requirement apply only to the selected document or also to
+  its imports? Applying it only to that document best fits PromptKit's request;
+  imported fragments could still declare versions checked by the shared policy.
+- **Mode interaction:** how should selection interact with disabled source
+  checks? Require an enabled mode, or activate checking for selected sources?
+  Selection would strengthen check-when-present behavior and add no requirement
+  beyond the load-wide required mode.
+- **Source handling and evidence:** how should selection work across file,
+  reader, byte, and mapping inputs, glob matches, and repeated imports? Define
+  behavior for absent optional inputs and preserve source-local diagnostics,
+  inspection evidence, and existing sensitivity handling.
+
+## 3. Performance experiments
+
+Status: deferred pending evidence. The completed performance audit identified
+four internal optimization candidates for separate investigation:
+
+- Branch-local explanation replay instead of the current whole-tree replay.
+- Redesigning the private metadata-copy handoff while preserving detached public
+  records.
+- Caching schema descriptors across loads.
+- Indexing complex contributor-trace membership while preserving its equality
+  rules and compact linked traces.
+
+These are experiments, not committed implementation work or promised speedups.
+Reconsider them with evidence that the benefit justifies the ownership,
+retained-memory, or equivalence risks. The completed audit and implementation
+evidence are preserved in Git history at `8e91a2b` under `docs/roadmap/`.
+
+### Questions to resolve before implementation
+
+- Which representative workloads show enough remaining cost to justify each
+  optimization, and what measurements would establish a useful improvement?
+- Can narrower replay and membership indexes preserve explanation results,
+  ordering, provenance, and sensitivity behavior?
+- Can metadata handoff changes preserve detached public records and ownership
+  across loads without introducing aliasing?
+- What cache lifetime and concurrency design would make schema reuse worthwhile
+  without excessive retained memory?

examples/consumers/prompt_catalog.go

@@ -0,0 +1,53 @@
+package consumers
+
+import (
+	"strings"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// PromptCatalogConfig is an application-owned prompt collection shape.
+type PromptCatalogConfig struct {
+	Prompts []PromptRecord `yaml:"prompts"`
+}
+
+// PromptRecord keeps its identity and message meaning in the consumer.
+type PromptRecord struct {
+	ID       string   `yaml:"id"`
+	Name     string   `yaml:"name"`
+	Messages []string `yaml:"messages"`
+}
+
+// LoadPromptCatalog composes explicitly registered prompt records in the main
+// tier and replaces the lower-priority default collection as a whole.
+func LoadPromptCatalog(mainPath string) (*configkit.Result[PromptCatalogConfig], error) {
+	return configkit.LoadLayered(configkit.Input{
+		Source:  configkit.File(mainPath),
+		Imports: true,
+	}, configkit.LayeredOptions[PromptCatalogConfig]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+			Source: configkit.Bytes("default prompts", []byte("prompts:\n  - id: default\n    name: Default prompt\n    messages: [Default message]\n")),
+		}}}},
+		Definition: configkit.Definition[PromptCatalogConfig]{
+			SequenceCompositions: []configkit.SequenceComposition{{
+				Path:          configkit.Keys("prompts"),
+				IdentityField: "id",
+			}},
+			Validate: validatePromptCatalog,
+		},
+	})
+}
+
+func validatePromptCatalog(configuration *PromptCatalogConfig, 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")
+	}
+}

examples/consumers/prompt_catalog_test.go

@@ -0,0 +1,82 @@
+package consumers_test
+
+import (
+	"errors"
+	"fmt"
+	"path/filepath"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func Example_identifiedPromptCatalog() {
+	result, err := consumers.LoadPromptCatalog(fixture("prompt-catalog", "config.yml"))
+	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)
+	}
+	fmt.Println(ids, err)
+	// Output: [main curated weather welcome] <nil>
+}
+
+func TestPromptCatalogImportsRegisteredItemsAndReplacesDefaults(t *testing.T) {
+	result, err := consumers.LoadPromptCatalog(fixture("prompt-catalog", "config.yml"))
+	if err != nil {
+		var loadError *configkit.LoadError
+		if errors.As(err, &loadError) {
+			t.Fatalf("load prompt catalog: %v (%#v)", err, loadError.Diagnostics())
+		}
+		t.Fatal(err)
+	}
+	if len(result.Config.Prompts) != 4 {
+		t.Fatalf("prompt records = %#v", result.Config.Prompts)
+	}
+	ids := []string{"main", "curated", "weather", "welcome"}
+	for index, id := range ids {
+		if result.Config.Prompts[index].ID != id {
+			t.Fatalf("prompt %d ID = %q, want %q", index, result.Config.Prompts[index].ID, id)
+		}
+	}
+	for _, prompt := range result.Config.Prompts {
+		if prompt.ID == "default" {
+			t.Fatalf("lower-priority default survived main sequence replacement: %#v", result.Config.Prompts)
+		}
+	}
+
+	var itemImports, helperImports int
+	for _, source := range result.Sources() {
+		switch filepath.Base(source.Path) {
+		case "curated.yml", "generated-weather.yml", "generated-welcome.yml":
+			if source.PlacementMode != configkit.PlacementModeSequenceItem || source.Destination.String() != "/prompts" {
+				t.Fatalf("item source placement = %#v", source)
+			}
+			itemImports++
+		case "curated-id.yml":
+			if source.PlacementMode != configkit.PlacementModeMapping || source.ImportedFrom == nil {
+				t.Fatalf("helper source = %#v", source)
+			}
+			helperImports++
+		}
+	}
+	if itemImports != 3 || helperImports != 1 {
+		t.Fatalf("item imports = %d, helper imports = %d, sources = %#v", itemImports, helperImports, result.Sources())
+	}
+	if result.Config.Prompts[1].ID != "curated" {
+		t.Fatalf("helper did not provide prompt ID: %#v", result.Config.Prompts[1])
+	}
+}
+
+func TestPromptCatalogLeavesDomainValidationWithConsumer(t *testing.T) {
+	_, err := consumers.LoadPromptCatalog(fixture("prompt-catalog", "invalid.yml"))
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 ||
+		loadError.Diagnostics()[0].Code != configkit.CodeValidateError ||
+		loadError.Diagnostics()[0].Path.String() != "/prompts/0/messages" {
+		t.Fatalf("consumer domain validation error = %v", err)
+	}
+}

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

@@ -0,0 +1,12 @@
+configkit:
+  imports:
+    - path: prompts/curated.yml
+      destination: [prompts]
+      placement: sequence-item
+    - glob: prompts/generated-*.yml
+      destination: [prompts]
+      placement: sequence-item
+prompts:
+  - id: main
+    name: Main authored prompt
+    messages: [Use the main prompt.]

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

@@ -0,0 +1,4 @@
+prompts:
+  - id: invalid
+    name: Invalid prompt
+    messages: []

examples/consumers/testdata/prompt-catalog/prompts/curated-id.yml

@@ -0,0 +1 @@
+id: curated

examples/consumers/testdata/prompt-catalog/prompts/curated.yml

@@ -0,0 +1,6 @@
+configkit:
+  imports:
+    - path: curated-id.yml
+name: Curated summary
+messages:
+  - Summarize the supplied content.

examples/consumers/testdata/prompt-catalog/prompts/generated-weather.yml

@@ -0,0 +1,4 @@
+id: weather
+name: Weather forecast
+messages:
+  - Describe today's forecast.

examples/consumers/testdata/prompt-catalog/prompts/generated-welcome.yml

@@ -0,0 +1,4 @@
+id: welcome
+name: Welcome
+messages:
+  - Welcome the user.

facade_sequence_test.go

@@ -0,0 +1,669 @@
+package configkit_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestFacadeNoSourcePreservesAbsenceWithSequencePolicy(t *testing.T) {
+	t.Parallel()
+	for _, defaults := range []bool{false, true} {
+		for _, registered := range []bool{false, true} {
+			definition := configkit.Definition[map[string]any]{}
+			if defaults {
+				definition.Defaults = func() map[string]any { return map[string]any{"fallback": true} }
+			}
+			if registered {
+				definition.SequenceCompositions = []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}}
+			}
+			ordinary, err := configkit.Load(configkit.NoSource(), configkit.Options[map[string]any]{Definition: definition})
+			if err != nil {
+				t.Fatal(err)
+			}
+			layered, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{Definition: definition})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !reflect.DeepEqual(ordinary.Config, layered.Config) {
+				t.Fatal("NoSource values differ between entry points")
+			}
+			for _, result := range []*configkit.Result[map[string]any]{ordinary, layered} {
+				if len(result.Sources()) != 0 || result.Metadata().IsExplicit(configkit.Keys()) {
+					t.Fatal("NoSource contributed an explicit root")
+				}
+				info, ok := result.Metadata().Lookup(configkit.Keys())
+				if !ok || info.Origin.Layer != configkit.LayerDefault {
+					t.Fatalf("NoSource root origin = %#v, %t", info, ok)
+				}
+			}
+			empty, err := configkit.Load(configkit.Mapping("empty", map[string]any{}), configkit.Options[map[string]any]{Definition: definition})
+			if err != nil || !empty.Metadata().IsExplicit(configkit.Keys()) {
+				t.Fatal("real empty mapping lost its presence")
+			}
+		}
+	}
+}
+
+func TestFacadeItemHelperHistoryPreservesTraversalOrder(t *testing.T) {
+	t.Parallel()
+	filesystem := configkit.NewFilesystem("history", fstest.MapFS{
+		"main.yml":         {Data: []byte("records: [{id: authored}]\nconfigkit: {imports: [{path: one.yml, destination: [records], placement: sequence-item}, {path: two.yml, destination: [records], placement: sequence-item}]}\n")},
+		"one.yml":          {Data: []byte("id: one\nconfigkit: {imports: [{path: helper-one.yml}, {path: child.yml, destination: [children], placement: sequence-item}]}\n")},
+		"two.yml":          {Data: []byte("id: two\nconfigkit: {imports: [{path: helper-two.yml}]}\n")},
+		"helper-one.yml":   {Data: []byte("text: first helper\n")},
+		"helper-two.yml":   {Data: []byte("text: second helper\n")},
+		"child.yml":        {Data: []byte("id: child\nconfigkit: {imports: [{path: child-helper.yml}]}\n")},
+		"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"}}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	sourceOrder := make(map[string]int)
+	sourceFiles := make(map[string]string)
+	for index, source := range result.Sources() {
+		sourceOrder[source.ID] = index
+		sourceFiles[source.ID] = source.Path
+	}
+	for _, test := range []struct {
+		path       configkit.Path
+		file, text string
+	}{
+		{configkit.Keys("records").Index(1).Key("text"), "helper-one.yml", "first helper\n"},
+		{configkit.Keys("records").Index(2).Key("text"), "helper-two.yml", "second helper\n"},
+		{configkit.Keys("records").Index(1).Key("children").Index(0).Key("text"), "child-helper.yml", "nested helper\n"},
+	} {
+		explanation, ok := result.Explain(test.path)
+		if !ok || explanation.Effective.YAML != test.text || len(explanation.History) != 1 {
+			t.Fatalf("helper history at %s = %#v", test.path.String(), explanation)
+		}
+		locations := explanation.History[0].Origin.Locations
+		if len(locations) != 1 || sourceFiles[locations[0].SourceID] != test.file {
+			t.Fatalf("helper origin at %s = %#v", test.path.String(), locations)
+		}
+		info, ok := result.Metadata().Lookup(test.path)
+		if !ok || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != locations[0].SourceID {
+			t.Fatal("metadata and explanation disagree")
+		}
+	}
+	explanation, ok := result.Explain(configkit.Keys())
+	if !ok {
+		t.Fatal("root explanation missing")
+	}
+	previous := -1
+	for _, change := range explanation.History {
+		for _, location := range change.Origin.Locations {
+			current := sourceOrder[location.SourceID]
+			if current < previous {
+				t.Fatalf("source %s replayed out of traversal order", location.SourceID)
+			}
+			previous = current
+		}
+	}
+}
+
+func TestFacadeRegisteredReplacementExplainsRemovedDescendants(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name  string
+		upper []any
+		path  configkit.Path
+	}{
+		{"shorter", []any{map[string]any{"id": "main"}}, configkit.Keys("records").Index(1).Key("id")},
+		{"empty", []any{}, configkit.Keys("records").Index(0).Key("id")},
+		{"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"}}},
+				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 {
+				t.Fatal(err)
+			}
+			explanation, ok := result.Explain(test.path)
+			if !ok || explanation.Effective.Present || len(explanation.History) < 2 {
+				t.Fatalf("removal not explained: %#v", explanation)
+			}
+			last := explanation.History[len(explanation.History)-1]
+			if !last.Before.Present || last.After.Present || last.Origin.Layer != configkit.LayerBase || len(last.Origin.Locations) != 1 || last.Origin.Locations[0].SourceID != result.Sources()[0].ID {
+				t.Fatalf("wrong removal transition/origin: %#v", last)
+			}
+		})
+	}
+}
+
+func TestFacadeItemConflictDiagnosticsFollowEnclosingIndexes(t *testing.T) {
+	t.Parallel()
+	filesystem := configkit.NewFilesystem("conflicts", fstest.MapFS{
+		"main.yml":   {Data: []byte("records: [{id: authored}]\nconfigkit: {imports: [{path: good.yml, destination: [records], placement: sequence-item}, {path: bad.yml, destination: [records], placement: sequence-item}]}\n")},
+		"group.yml":  {Data: []byte("records: [{id: authored}]\nconfigkit: {imports: [{path: bad.yml, destination: [records], placement: sequence-item}]}\n")},
+		"nested.yml": {Data: []byte("records: [{id: authored}]\nconfigkit: {imports: [{path: parent.yml, destination: [records], placement: sequence-item}]}\n")},
+		"parent.yml": {Data: []byte("id: parent\nchildren: [{id: authored-child}]\nconfigkit: {imports: [{path: bad.yml, destination: [children], placement: sequence-item}]}\n")},
+		"good.yml":   {Data: []byte("id: good\n")},
+		"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"}}}
+	for _, test := range []struct {
+		name, file, path  string
+		grouped, itemMain bool
+	}{
+		{"preceding authored and imported items", "main.yml", "/records/2/id", false, false},
+		{"nested item", "nested.yml", "/records/1/children/1/id", false, false},
+		{"preceding grouped input", "group.yml", "/records/2/id", true, false},
+		{"item main source", "bad.yml", "/records/0/id", false, true},
+		{"grouped item source", "bad.yml", "/records/1/id", true, true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			var err error
+			input := configkit.Input{Source: filesystem.File(test.file), Imports: true}
+			if test.itemMain {
+				input.Destination = configkit.Keys("records")
+				input.PlacementMode = configkit.PlacementModeSequenceItem
+			}
+			if test.grouped {
+				_, err = configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{
+					Definition: definition, Underlays: []configkit.Underlay{{Inputs: []configkit.Input{
+						{Source: configkit.Mapping("prior", map[string]any{"records": []any{map[string]any{"id": "prior"}}})},
+						input,
+					}}},
+				})
+			} else {
+				_, err = configkit.Load(input.Source, configkit.Options[map[string]any]{Definition: definition, Imports: true, Destination: input.Destination, PlacementMode: input.PlacementMode})
+			}
+			loadError, ok := err.(*configkit.LoadError)
+			if !ok || len(loadError.Diagnostics()) != 1 {
+				t.Fatalf("expected helper conflict: %v", err)
+			}
+			diagnostic := loadError.Diagnostics()[0]
+			if diagnostic.Code != configkit.CodeComposeConflict || diagnostic.Path.String() != test.path {
+				t.Fatalf("diagnostic = %s %s, want %s", diagnostic.Code, diagnostic.Path.String(), test.path)
+			}
+			files := make(map[string]string)
+			for _, source := range loadError.Sources() {
+				files[source.ID] = source.Path
+			}
+			if diagnostic.Primary == nil || files[diagnostic.Primary.SourceID] != "bad.yml" || len(diagnostic.Related) != 1 || files[diagnostic.Related[0].SourceID] != "helper.yml" {
+				t.Fatal("conflict lost physical source locations")
+			}
+		})
+	}
+}
+
+func TestFacadeSequencePlacementControlsAcrossLoadEntryPoints(t *testing.T) {
+	definition := configkit.Definition[map[string]any]{
+		SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("entries"), IdentityField: "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,
+	})
+	if err != nil {
+		t.Fatalf("ordinary item load: %v", err)
+	}
+	if got := ordinary.Config["entries"].([]any); len(got) != 1 || got[0].(map[string]any)["id"] != "one" {
+		t.Fatalf("ordinary item placement = %#v", ordinary.Config)
+	}
+	if sources := ordinary.Sources(); len(sources) != 1 || sources[0].PlacementMode != configkit.PlacementModeSequenceItem || sources[0].Destination.String() != "/entries" || sources[0].DestinationUnresolved {
+		t.Fatalf("ordinary item source = %#v", sources)
+	}
+
+	layered, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{
+		Definition: definition,
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{
+			{Source: configkit.Mapping("first", map[string]any{"id": "first"}), Destination: configkit.Keys("entries"), PlacementMode: configkit.PlacementModeSequenceItem},
+			{Source: configkit.Mapping("second", map[string]any{"id": "second"}), Destination: configkit.Keys("entries"), PlacementMode: configkit.PlacementModeSequenceItem},
+		}}},
+	})
+	if err != nil {
+		t.Fatalf("layered item load: %v", err)
+	}
+	items := layered.Config["entries"].([]any)
+	if len(items) != 2 || items[0].(map[string]any)["id"] != "first" || items[1].(map[string]any)["id"] != "second" {
+		t.Fatalf("layered item placement = %#v", layered.Config)
+	}
+	for _, source := range layered.Sources() {
+		if source.PlacementMode != configkit.PlacementModeSequenceItem || source.Destination.String() != "/entries" || source.DestinationUnresolved {
+			t.Errorf("layered item source = %#v", source)
+		}
+	}
+
+	precedence, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Mapping("main", map[string]any{
+		"entries": []any{map[string]any{"id": "main"}},
+	})}, configkit.LayeredOptions[map[string]any]{
+		Definition: definition,
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+			Source: configkit.Mapping("lower", map[string]any{"id": "lower"}), Destination: configkit.Keys("entries"), PlacementMode: configkit.PlacementModeSequenceItem,
+		}}}},
+	})
+	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)
+	}
+
+	for _, test := range []struct {
+		name string
+		load func(configkit.Source) error
+	}{
+		{name: "ordinary", load: func(source configkit.Source) error {
+			_, loadErr := configkit.Load(source, configkit.Options[map[string]any]{PlacementMode: configkit.PlacementMode(9)})
+			return loadErr
+		}},
+		{name: "layered", load: func(source configkit.Source) error {
+			_, loadErr := configkit.LoadLayered[map[string]any](configkit.Input{Source: source, PlacementMode: configkit.PlacementMode(9)})
+			return loadErr
+		}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			reader := &destinationCountingReader{}
+			err := test.load(configkit.Reader("unread", reader))
+			loadError, ok := err.(*configkit.LoadError)
+			if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeOptionsInvalid || reader.reads != 0 {
+				t.Fatalf("invalid placement result = %v, reads = %d", err, reader.reads)
+			}
+		})
+	}
+}
+
+func TestFacadeItemImportGlobComposesWithAuthoredSequence(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := filepath.Join(directory, "main.yml")
+	if err := os.WriteFile(mainPath, []byte(`configkit:
+  imports:
+    - glob: records/*.yml
+      destination: [items]
+      placement: sequence-item
+items:
+  - id: authored
+    name: authored item
+`), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	for name, id := range map[string]string{"b.yml": "imported-b", "a.yml": "imported-a"} {
+		if err := os.MkdirAll(filepath.Join(directory, "records"), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(filepath.Join(directory, "records", name), []byte("id: "+id+"\n"), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	var callbackSources []configkit.SourceInfo
+	var callbackContexts []configkit.ResourceContext
+	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"}},
+			Resolve: func(_ *map[string]any, resolution *configkit.Resolution) error {
+				for _, id := range []string{"source-2", "source-3"} {
+					source, ok := resolution.Source(id)
+					if !ok {
+						t.Errorf("resolver could not query %s", id)
+						continue
+					}
+					context, found := resolution.SourceContext(id)
+					if !found || context.FS == nil || context.BaseDir != "records" {
+						t.Errorf("resolver context for %s = %#v, %v", id, context, found)
+					}
+					callbackSources = append(callbackSources, source)
+					callbackContexts = append(callbackContexts, context)
+				}
+				return nil
+			},
+		},
+	})
+	if err != nil {
+		t.Fatalf("load item glob: %v", err)
+	}
+	items := result.Config["items"].([]any)
+	wantIDs := []string{"authored", "imported-a", "imported-b"}
+	for index, item := range items {
+		if got := item.(map[string]any)["id"]; got != wantIDs[index] {
+			t.Errorf("item %d ID = %#v, want %q", index, got, wantIDs[index])
+		}
+	}
+	if len(callbackSources) != 2 || len(callbackContexts) != 2 {
+		t.Fatalf("callback snapshots = %#v, contexts %#v", callbackSources, callbackContexts)
+	}
+	for index, source := range callbackSources {
+		if source.PlacementMode != configkit.PlacementModeSequenceItem || source.Destination.String() != "/items" || source.DestinationUnresolved {
+			t.Errorf("callback source = %#v", source)
+		}
+		if callbackContexts[index].FS == nil || callbackContexts[index].BaseDir != "records" {
+			t.Errorf("callback context = %#v", callbackContexts[index])
+		}
+	}
+	metadata := result.Metadata()
+	for index := 1; index < len(wantIDs); index++ {
+		path := configkit.Keys("items").Index(index).Key("id")
+		info, ok := metadata.Lookup(path)
+		if !ok || !info.Explicit || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != []string{"source-2", "source-3"}[index-1] {
+			t.Errorf("shifted item origin at %s = %#v, %v", path.String(), info, ok)
+		}
+	}
+	if got := result.Sources(); len(got) != 3 || got[1].Destination.String() != "/items" || got[1].DestinationUnresolved {
+		t.Fatalf("final item sources = %#v", got)
+	}
+}
+
+func TestFacadeItemDestinationFallbackEvidenceSurvivesFailures(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := filepath.Join(directory, "main.yml")
+	helperPath := filepath.Join(directory, "helper.yml")
+	if err := os.WriteFile(mainPath, []byte("configkit: {imports: [{path: helper.yml, destination: [metadata]}]}\nid: main\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(helperPath, []byte("value: [unterminated\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	_, err := configkit.Load(configkit.File(mainPath), configkit.Options[map[string]any]{
+		Imports: true, Destination: configkit.Keys("items"), PlacementMode: configkit.PlacementModeSequenceItem,
+	})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok {
+		t.Fatalf("early failure = %v", err)
+	}
+	sources := loadError.Sources()
+	if len(sources) != 2 || sources[0].Destination.String() != "/items" || sources[0].DestinationUnresolved ||
+		sources[0].PlacementMode != configkit.PlacementModeSequenceItem ||
+		sources[1].Destination.String() != "/items" || !sources[1].DestinationUnresolved ||
+		sources[1].PlacementMode != configkit.PlacementModeMapping {
+		t.Fatalf("early source snapshots = %#v", sources)
+	}
+
+	if err := os.WriteFile(helperPath, []byte("value: invalid-for-typed-schema\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	type item struct {
+		ID       string `yaml:"id"`
+		Metadata struct {
+			Value int `yaml:"value"`
+		} `yaml:"metadata"`
+	}
+	type configuration struct {
+		Items []item `yaml:"items"`
+	}
+	_, err = configkit.Load(configkit.File(mainPath), configkit.Options[configuration]{
+		Imports: true, Destination: configkit.Keys("items"), PlacementMode: configkit.PlacementModeSequenceItem,
+	})
+	loadError, ok = err.(*configkit.LoadError)
+	if !ok {
+		t.Fatalf("post-placement failure = %v", err)
+	}
+	sources = loadError.Sources()
+	if len(sources) != 2 || sources[0].Destination.String() != "/items" || sources[0].DestinationUnresolved ||
+		sources[1].Destination.String() != "/items/0/metadata" || sources[1].DestinationUnresolved {
+		t.Fatalf("post-placement source snapshots = %#v", sources)
+	}
+	firstSnapshot := sources[1]
+	sources[1].Destination = configkit.Keys("mutated")
+	if again := loadError.Sources(); len(again) != 2 || again[1].Destination.String() != firstSnapshot.Destination.String() || again[1].DestinationUnresolved {
+		t.Fatalf("failure source snapshots were aliased: %#v", again)
+	}
+
+	var callback configkit.SourceInfo
+	_, err = configkit.Load(configkit.File(mainPath), configkit.Options[map[string]any]{
+		Imports: true, Destination: configkit.Keys("items"), PlacementMode: configkit.PlacementModeSequenceItem,
+		Definition: configkit.Definition[map[string]any]{Resolve: func(_ *map[string]any, resolution *configkit.Resolution) error {
+			callback, _ = resolution.Source("source-2")
+			return nil
+		}},
+	})
+	if err != nil {
+		t.Fatalf("successful item load: %v", err)
+	}
+	if callback.Destination.String() != "/items/0/metadata" || callback.DestinationUnresolved || callback.PlacementMode != configkit.PlacementModeMapping {
+		t.Fatalf("callback source snapshot = %#v", callback)
+	}
+}
+
+func TestFacadeNestedItemFallbackUsesNearestResolvedContainer(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := filepath.Join(directory, "main.yml")
+	childPath := filepath.Join(directory, "child.yml")
+	helperPath := filepath.Join(directory, "helper.yml")
+	if err := os.WriteFile(mainPath, []byte("configkit: {imports: [{path: child.yml, destination: [children], placement: sequence-item}]}\nid: parent\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(childPath, []byte("configkit: {imports: [{path: helper.yml, destination: [metadata]}]}\nid: child\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(helperPath, []byte("value: [unterminated\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	_, err := configkit.Load(configkit.File(mainPath), configkit.Options[map[string]any]{
+		Imports: true, Destination: configkit.Keys("groups"), PlacementMode: configkit.PlacementModeSequenceItem,
+	})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok {
+		t.Fatalf("nested acquisition failure = %v", err)
+	}
+	sources := loadError.Sources()
+	if len(sources) != 3 {
+		t.Fatalf("nested source snapshots = %#v", sources)
+	}
+	if got := sources[0]; got.Destination.String() != "/groups" || got.DestinationUnresolved || got.PlacementMode != configkit.PlacementModeSequenceItem {
+		t.Errorf("outer item snapshot = %#v", got)
+	}
+	if got := sources[1]; got.Destination.String() != "/groups" || !got.DestinationUnresolved || got.PlacementMode != configkit.PlacementModeSequenceItem {
+		t.Errorf("nested item fallback = %#v", got)
+	}
+	if got := sources[2]; got.Destination.String() != "/groups" || !got.DestinationUnresolved || got.PlacementMode != configkit.PlacementModeMapping {
+		t.Errorf("nested helper fallback = %#v", got)
+	}
+}
+
+func TestFacadeSequenceCompositionRegistrationPathIsDetached(t *testing.T) {
+	registrations := []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}}
+	definition := configkit.Definition[map[string]any]{
+		SequenceCompositions: registrations,
+		Version: &configkit.VersionRule{
+			Path: configkit.Keys("version"),
+			Check: func(configkit.Version, *configkit.Validation) error {
+				registrations[0].Path = configkit.Keys("other")
+				registrations[0].IdentityField = "changed"
+				return nil
+			},
+		},
+	}
+	_, err := configkit.Load(configkit.Mapping("registration", map[string]any{
+		"version": "1",
+		"records": []any{map[string]any{"id": "same"}, map[string]any{"id": "same"}},
+	}), configkit.Options[map[string]any]{
+		Definition: definition, SourceVersionMode: configkit.SourceVersionModeIfPresent,
+	})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok {
+		t.Fatalf("duplicate registered identity = %v", err)
+	}
+	for _, diagnostic := range loadError.Diagnostics() {
+		if diagnostic.Code == configkit.CodeComposeConflict {
+			return
+		}
+	}
+	t.Fatalf("registration mutation changed the in-flight policy: %#v", loadError.Diagnostics())
+}
+
+func TestFacadeSourceInfoPlacementCopiesAndZeroValues(t *testing.T) {
+	zero := configkit.SourceInfo{}
+	if zero.PlacementMode != configkit.PlacementModeMapping || zero.DestinationUnresolved {
+		t.Fatalf("zero source info = %#v", zero)
+	}
+	loaded, err := configkit.Load[map[string]any](configkit.Mapping("mapping", map[string]any{"value": true}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	sources := loaded.Sources()
+	if len(sources) != 1 || sources[0].PlacementMode != configkit.PlacementModeMapping || sources[0].DestinationUnresolved {
+		t.Fatalf("mapping source evidence = %#v", sources)
+	}
+	first := sources[0]
+	first.Destination = configkit.Keys("caller")
+	if again := loaded.Sources(); len(again) != 1 || again[0].Destination.String() != "" || again[0].DestinationUnresolved {
+		t.Fatalf("source evidence changed after caller mutation: %#v", again)
+	}
+}
+
+func TestFacadeSequenceReplacementKeepsTypedAndHistoricalEvidenceAligned(t *testing.T) {
+	type record struct {
+		ID       string  `yaml:"id"`
+		Count    int     `yaml:"count"`
+		Enabled  bool    `yaml:"enabled"`
+		Empty    string  `yaml:"empty"`
+		Nullable *string `yaml:"nullable"`
+		Token    string  `yaml:"token"`
+		Omitted  string  `yaml:"omitted"`
+	}
+	type configuration struct {
+		Entries []record `yaml:"entries"`
+	}
+
+	directory := t.TempDir()
+	mainPath := filepath.Join(directory, "main.yml")
+	underlayPath := filepath.Join(directory, "underlay.yml")
+	if err := os.WriteFile(mainPath, []byte(`entries:
+  - id: current
+    count: 0
+    enabled: false
+    empty: ""
+    nullable: null
+    token: CURRENT-SECRET-CANARY
+`), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(underlayPath, []byte(`configkit:
+  imports:
+    - glob: records/*.yml
+      destination: [entries]
+      placement: sequence-item
+`), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	recordsDir := filepath.Join(directory, "records")
+	if err := os.MkdirAll(recordsDir, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	for name, body := range map[string]string{
+		"a.yml": "id: old-a\ntoken: OLD-SECRET-CANARY-A\n",
+		"b.yml": "id: old-b\ntoken: OLD-SECRET-CANARY-B\n",
+	} {
+		if err := os.WriteFile(filepath.Join(recordsDir, name), []byte(body), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	var callbackItems []configkit.SourceInfo
+	var callbackContexts []configkit.ResourceContext
+	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"}},
+			Secret: func(path configkit.Path) bool {
+				return path.String() == "/entries/0/token" || path.String() == "/entries/1/token"
+			},
+			Resolve: func(value *configuration, resolution *configkit.Resolution) error {
+				for _, field := range []string{"count", "enabled", "empty", "nullable"} {
+					path := configkit.Keys("entries").Index(0).Key(field)
+					if !resolution.IsExplicit(path) {
+						t.Errorf("resolver did not see explicit %s", path.String())
+					}
+				}
+				if resolution.IsExplicit(configkit.Keys("entries").Index(0).Key("omitted")) {
+					t.Error("resolver treated an omitted field as explicit")
+				}
+				info, found := resolution.InputInfo(configkit.Keys("entries").Index(0).Key("count"))
+				if !found || info.Origin.Layer != configkit.LayerBase {
+					t.Errorf("resolver current-record input info = %#v, %v", info, found)
+				}
+				for _, id := range []string{"source-3", "source-4"} {
+					source, ok := resolution.Source(id)
+					if !ok {
+						t.Errorf("resolver could not query %s", id)
+						continue
+					}
+					context, contextOK := resolution.SourceContext(id)
+					if !contextOK || context.FS == nil || context.BaseDir != "records" {
+						t.Errorf("resolver context for %s = %#v, %v", id, context, contextOK)
+					}
+					callbackItems = append(callbackItems, source)
+					callbackContexts = append(callbackContexts, context)
+				}
+				return nil
+			},
+		},
+	})
+	if err != nil {
+		if loadError, ok := err.(*configkit.LoadError); ok {
+			t.Fatalf("layered item replacement: %#v", loadError.Diagnostics())
+		}
+		t.Fatalf("layered item replacement: %v", err)
+	}
+	if len(result.Config.Entries) != 1 || result.Config.Entries[0].ID != "current" || result.Config.Entries[0].Count != 0 ||
+		result.Config.Entries[0].Enabled || result.Config.Entries[0].Empty != "" || result.Config.Entries[0].Nullable != nil {
+		t.Fatalf("typed replacement result = %#v", result.Config)
+	}
+	if len(callbackItems) != 2 || len(callbackContexts) != 2 {
+		t.Fatalf("callback sources = %#v; contexts = %#v", callbackItems, callbackContexts)
+	}
+	for _, source := range callbackItems {
+		if source.PlacementMode != configkit.PlacementModeSequenceItem || source.Destination.String() != "/entries" || source.DestinationUnresolved {
+			t.Errorf("callback item source = %#v", source)
+		}
+	}
+	metadata := result.Metadata()
+	for _, field := range []string{"count", "enabled", "empty", "nullable"} {
+		path := configkit.Keys("entries").Index(0).Key(field)
+		if info, ok := metadata.Lookup(path); !ok || !info.Explicit || (field == "nullable") != info.Null {
+			t.Errorf("explicit metadata at %s = %#v, %v", path.String(), info, ok)
+		}
+	}
+	if info, ok := metadata.Lookup(configkit.Keys("entries").Index(0).Key("omitted")); ok && info.Explicit {
+		t.Errorf("metadata marked an omitted zero-valued field explicit: %#v", info)
+	}
+	mainID := ""
+	for _, source := range result.Sources() {
+		if source.Path == mainPath {
+			mainID = source.ID
+		}
+	}
+	currentOrigin, ok := metadata.Lookup(configkit.Keys("entries").Index(0).Key("id"))
+	if !ok || len(currentOrigin.Origin.Locations) != 1 || currentOrigin.Origin.Locations[0].SourceID != mainID || mainID == "" {
+		t.Fatalf("replacement origin = %#v, main source %q", currentOrigin, mainID)
+	}
+	oldRecord, ok := result.Explain(configkit.Keys("entries").Index(1).Key("token"))
+	if !ok || oldRecord.Effective.Present {
+		t.Fatalf("removed appended item explanation = %#v, %v", oldRecord, ok)
+	}
+	maskedHistory := false
+	for _, change := range oldRecord.History {
+		maskedHistory = maskedHistory || change.Before.Redacted || change.After.Redacted
+	}
+	if !maskedHistory {
+		t.Fatal("removed appended item lost historical sensitivity")
+	}
+	var effective bytes.Buffer
+	if err := result.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	var explanation bytes.Buffer
+	if err := result.WriteExplanation(&explanation, configkit.Keys("entries").Index(1).Key("token")); err != nil {
+		t.Fatal(err)
+	}
+	for _, output := range []string{effective.String(), explanation.String()} {
+		for _, canary := range []string{"CURRENT-SECRET-CANARY", "OLD-SECRET-CANARY-A", "OLD-SECRET-CANARY-B"} {
+			if strings.Contains(output, canary) {
+				t.Fatalf("sensitive sequence value leaked: %s", output)
+			}
+		}
+	}
+}

facade_source_version_mode_test.go

@@ -3,14 +3,145 @@ package configkit_test
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
+	"testing/fstest"
 
 	configkit "gitea.maximumdirect.net/eric/ConfigKit"
 )
 
+func TestFacadeSourceVersionEvidenceFollowsAuthoredSequenceRanges(t *testing.T) {
+	t.Parallel()
+	for _, grouped := range []bool{false, true} {
+		for _, sensitivePath := range []string{"", "/records/1/version", "/records/1"} {
+			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 },
+				}
+				first := "records: [{id: first, version: first-version}]\n"
+				child := "records: [{id: child, version: PRIVATE-VERSION-CANARY}]\n"
+				var result *configkit.Result[map[string]any]
+				var err error
+				if grouped {
+					result, err = configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte(first))}, configkit.LayeredOptions[map[string]any]{
+						Definition: definition, SourceVersionMode: configkit.SourceVersionModeRequired,
+						Underlays: []configkit.Underlay{{Inputs: []configkit.Input{
+							{Source: configkit.Bytes("first", []byte(first))}, {Source: configkit.Bytes("child", []byte(child))},
+						}}},
+					})
+				} else {
+					filesystem := configkit.NewFilesystem("versions", fstest.MapFS{
+						"main.yml":  {Data: []byte("configkit: {imports: [{path: child.yml}]}\n" + first)},
+						"child.yml": {Data: []byte(child)},
+					})
+					result, err = configkit.Load(filesystem.File("main.yml"), configkit.Options[map[string]any]{
+						Definition: definition, Imports: true, SourceVersionMode: configkit.SourceVersionModeRequired,
+					})
+				}
+				if err != nil {
+					t.Fatal(err)
+				}
+				sources := result.Sources()
+				check := sources[len(sources)-1].VersionCheck
+				if check == nil || check.Path.String() != "/records/0/version" || check.Location == nil || check.Location.SourceID != sources[len(sources)-1].ID {
+					t.Fatalf("source-local evidence changed: %#v", check)
+				}
+				if sensitivePath != "" {
+					if !check.Redacted || check.Version.Text != "" {
+						t.Fatal("shifted version evidence was disclosed")
+					}
+				} else if check.Redacted || check.Version.Text != "PRIVATE-VERSION-CANARY" {
+					t.Fatal("unclassified version evidence was lost")
+				}
+			})
+		}
+	}
+}
+
+func TestFacadeSourceVersionOwnershipAcceptsContextualIndexes(t *testing.T) {
+	t.Parallel()
+	parsed, err := configkit.ParsePath("/records/0/version")
+	if err != nil {
+		t.Fatal(err)
+	}
+	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 }},
+			}
+			first := "records: [{id: first, version: first}]\n"
+			child := "records: [{id: child, version: child}]\n"
+			var result *configkit.Result[map[string]any]
+			if grouped {
+				result, err = configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte(first))}, configkit.LayeredOptions[map[string]any]{
+					Definition: definition, SourceVersionMode: configkit.SourceVersionModeRequired,
+					Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Bytes("lower", []byte(child))}}}},
+				})
+			} else {
+				filesystem := configkit.NewFilesystem("versions", fstest.MapFS{"main.yml": {Data: []byte("configkit: {imports: [{path: child.yml}]}\n" + first)}, "child.yml": {Data: []byte(child)}})
+				result, err = configkit.Load(filesystem.File("main.yml"), configkit.Options[map[string]any]{Definition: definition, SourceVersionMode: configkit.SourceVersionModeRequired, Imports: true})
+			}
+			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 {
+				t.Fatal("version path changed composition")
+			}
+		}
+	}
+}
+
+func TestFacadeItemMainKeepsDefaultVersionOwnership(t *testing.T) {
+	t.Parallel()
+	for _, layered := range []bool{false, true} {
+		for _, protected := range []bool{true, false} {
+			body, helper := "id: main\n", "version: imported-version\n"
+			if !protected {
+				body += "version: main-version\n"
+				helper = "other: {version: permitted}\n"
+			}
+			filesystem := configkit.NewFilesystem("item-main", fstest.MapFS{
+				"main.yml": {Data: []byte(body + "configkit: {imports: [{path: helper.yml}]}\n")}, "helper.yml": {Data: []byte(helper)},
+			})
+			checks := 0
+			definition := configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+				Path: configkit.Keys("records").Index(0).Key("version"),
+				Check: func(version configkit.Version, _ *configkit.Validation) error {
+					checks++
+					if version.Text != "main-version" {
+						t.Error("helper supplied main version")
+					}
+					return nil
+				},
+			}}
+			var err error
+			if layered {
+				_, err = configkit.LoadLayered[map[string]any](configkit.Input{Source: filesystem.File("main.yml"), Imports: true, Destination: configkit.Keys("records"), PlacementMode: configkit.PlacementModeSequenceItem}, configkit.LayeredOptions[map[string]any]{Definition: definition})
+			} else {
+				_, err = configkit.Load(filesystem.File("main.yml"), configkit.Options[map[string]any]{Definition: definition, Imports: true, Destination: configkit.Keys("records"), PlacementMode: configkit.PlacementModeSequenceItem})
+			}
+			if protected {
+				loadError, ok := err.(*configkit.LoadError)
+				if !ok || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != configkit.CodeImportVersion || loadError.Diagnostics()[0].Path.String() != "/records/0/version" || checks != 0 {
+					t.Fatalf("layered=%t ownership error = %v, checks=%d", layered, err, checks)
+				}
+			} else if err != nil || checks != 1 {
+				t.Fatalf("unrelated helper version rejected: %v, checks=%d", err, checks)
+			}
+		}
+	}
+}
+
 type sourceVersionModeReader struct {
 	*strings.Reader
 	reads int

internal/engine/compose.go

@@ -6,13 +6,29 @@ import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
 // 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
-	claims       []compositionClaim
-	origins      map[string]Origin
-	overlays     []overlayRecord
-	applications []historyApplication
-	inputBytes   int64
-	inputNodes   int
+	root           *document.Node
+	acquisition    []acquisitionScope
+	claims         []compositionClaim
+	sequenceRanges []sequenceRange
+	origins        map[string]Origin
+	overlays       []overlayRecord
+	applications   []historyApplication
+	inputBytes     int64
+	inputNodes     int
+}
+
+type sequenceHistoryMode uint8
+
+const (
+	sequenceHistoryReplace sequenceHistoryMode = iota + 1
+	sequenceHistoryAppend
+)
+
+type sequenceHistoryContribution struct {
+	path   Path
+	mode   sequenceHistoryMode
+	offset int
+	length int
 }
 
 // assemblyAccumulator owns one mutable working document. Acquired trees held by
@@ -29,18 +45,25 @@ func newAssemblyAccumulator(owned *composedDocument) *assemblyAccumulator {
 // cloneComposedDocument detaches mutable composition and provenance state;
 // history application trees are immutable and retain their existing sharing.
 func cloneComposedDocument(base *composedDocument) *composedDocument {
+	if base == nil {
+		return nil
+	}
 	return &composedDocument{
 		root: document.Clone(base.root), claims: cloneCompositionClaims(base.claims),
-		origins: cloneOriginIndex(base.origins), overlays: cloneOverlayRecords(base.overlays),
+		acquisition:    cloneAcquisitionScopes(base.acquisition),
+		sequenceRanges: cloneSequenceRanges(base.sequenceRanges),
+		origins:        cloneOriginIndex(base.origins), overlays: cloneOverlayRecords(base.overlays),
 		applications: cloneHistoryApplications(base.applications),
 		inputBytes:   base.inputBytes, inputNodes: base.inputNodes,
 	}
 }
 
 type compositionClaim struct {
-	path     Path
-	kind     document.Kind
-	location Location
+	node        *document.Node // borrowed immutable contribution, not the assembly tree
+	path        Path
+	kind        document.Kind
+	location    Location
+	sourceIndex int
 }
 
 type compositionClaims struct {
@@ -48,16 +71,32 @@ type compositionClaims struct {
 	byPath  map[string][]compositionClaim
 }
 
+// sequenceRange records where one source's sequence items were assigned in the
+// composed sequence at path. End is exclusive.
+type sequenceRange struct {
+	sourceIndex int
+	path        Path
+	start       int
+	end         int
+}
+
 // composeDocuments recursively joins disjoint mapping entries. Source order
 // controls only output traversal and diagnostics: every non-mapping overlap is
 // rejected before a result is assembled.
 func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic) {
+	return composeDocumentsWithPolicy(sequencePolicy{}, inputs...)
+}
+
+// composeDocumentsWithPolicy composes one strict tier using the registered
+// 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) {
 	if len(inputs) == 0 {
 		return nil, []Diagnostic{compositionInvalidDiagnostic("strict composition requires at least one document", nil)}
 	}
 
 	collected := compositionClaims{byPath: make(map[string][]compositionClaim)}
-	for _, input := range inputs {
+	for sourceIndex, input := range inputs {
 		if input == nil || input.Kind != document.Mapping {
 			var location *Location
 			if input != nil {
@@ -66,7 +105,7 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 			}
 			return nil, []Diagnostic{compositionInvalidDiagnostic("strict composition requires mapping document roots", location)}
 		}
-		collected.append(input, Path{})
+		collected.append(input, Path{}, sourceIndex)
 	}
 
 	checked := make(map[string]struct{}, len(collected.byPath))
@@ -77,28 +116,52 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 		}
 		checked[pointer] = struct{}{}
 		claims := collected.byPath[pointer]
-		if len(claims) < 2 || allMappingClaims(claims) {
+		if len(claims) < 2 || allMappingClaims(claims) || (allSequenceClaims(claims) && policy.registered(claim.path)) {
 			continue
 		}
 		return nil, []Diagnostic{compositionConflictDiagnostic(claim.path, claims)}
 	}
 
+	validated, ranges, diagnostics := validateRegisteredSequences(collected.ordered, policy)
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+
 	root := document.Clone(inputs[0])
 	index := new(mappingIndex)
-	for _, input := range inputs[1:] {
-		mergeDisjointMappings(root, input, index)
+	for sourceIndex, input := range inputs[1:] {
+		mergeComposedMappings(root, input, Path{}, index, sourceIndex+1, policy, validated)
 	}
 	applications := make([]historyApplication, 0, len(inputs))
-	for _, input := range inputs {
-		applications = append(applications, historyApplication{layer: LayerBase, value: input})
+	seenSequences := make(map[string]bool)
+	rangesBySource := make([][]sequenceRange, len(inputs))
+	for _, assigned := range ranges {
+		rangesBySource[assigned.sourceIndex] = append(rangesBySource[assigned.sourceIndex], assigned)
 	}
-	origins := compositionOrigins(collected.ordered)
+	for sourceIndex, input := range inputs {
+		application := historyApplication{layer: LayerBase, value: input}
+		for _, assigned := range rangesBySource[sourceIndex] {
+			pointer := assigned.path.String()
+			mode := sequenceHistoryAppend
+			if !seenSequences[pointer] {
+				mode = sequenceHistoryReplace
+			}
+			seenSequences[pointer] = true
+			application.sequences = append(application.sequences, sequenceHistoryContribution{
+				path: assigned.path.clone(), mode: mode, offset: assigned.start, length: assigned.end - assigned.start,
+			})
+		}
+		application.sequencesByPath = indexSequenceHistoryContributions(application.sequences)
+		applications = append(applications, application)
+	}
+	origins := projectedCompositionOrigins(inputs, ranges)
 	completeCompositionOrigins(root, Path{}, origins)
 	return &composedDocument{
-		root:         root,
-		claims:       cloneCompositionClaims(collected.ordered),
-		origins:      origins,
-		applications: applications,
+		root:           root,
+		claims:         cloneCompositionClaims(collected.ordered),
+		sequenceRanges: cloneSequenceRanges(ranges),
+		origins:        origins,
+		applications:   applications,
 	}, nil
 }
 
@@ -121,11 +184,13 @@ func completeCompositionOrigins(node *document.Node, path Path, origins map[stri
 	}
 }
 
-func (c *compositionClaims) append(node *document.Node, path Path) {
+func (c *compositionClaims) append(node *document.Node, path Path, sourceIndex int) {
 	claim := compositionClaim{
-		path:     path.clone(),
-		kind:     node.Kind,
-		location: publicLocation(node.Location),
+		node:        node,
+		path:        path.clone(),
+		kind:        node.Kind,
+		location:    publicLocation(node.Location),
+		sourceIndex: sourceIndex,
 	}
 	c.ordered = append(c.ordered, claim)
 	pointer := path.String()
@@ -135,10 +200,146 @@ func (c *compositionClaims) append(node *document.Node, path Path) {
 		return
 	}
 	for _, entry := range node.Mapping {
-		c.append(entry.Value, path.Key(entry.Key))
+		c.append(entry.Value, path.Key(entry.Key), sourceIndex)
 	}
 }
 
+func allSequenceClaims(claims []compositionClaim) bool {
+	for _, claim := range claims {
+		if claim.kind != document.Sequence {
+			return false
+		}
+	}
+	return true
+}
+
+type validatedSequence struct {
+	items []*document.Node
+}
+
+func validateRegisteredSequences(claims []compositionClaim, policy sequencePolicy) (map[string]map[int]validatedSequence, []sequenceRange, []Diagnostic) {
+	itemsByPath := make(map[string]map[int]validatedSequence)
+	identitiesByPath := make(map[string]map[string]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 {
+			continue
+		}
+		node := claim.node
+		pathKey := claim.path.String()
+		identities := identitiesByPath[pathKey]
+		if identities == nil {
+			identities = make(map[string]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)
+				return nil, nil, []Diagnostic{{
+					Code: CodeComposeConflict, Severity: SeverityError,
+					Path: claim.path.clone(), Message: "registered sequence contains duplicate identities",
+					Primary: &first, Related: []Location{current},
+				}}
+			}
+			identities[field.Text] = publicLocation(field.Location)
+		}
+		if itemsByPath[pathKey] == nil {
+			itemsByPath[pathKey] = make(map[int]validatedSequence)
+		}
+		itemsByPath[pathKey][claim.sourceIndex] = contribution
+		lengthsByPath[pathKey] = start + len(node.Sequence)
+		ranges = append(ranges, sequenceRange{
+			sourceIndex: claim.sourceIndex, path: claim.path.clone(), start: start, end: start + len(node.Sequence),
+		})
+	}
+	return itemsByPath, ranges, nil
+}
+
+func mergeComposedMappings(target, incoming *document.Node, path Path, index *mappingIndex, sourceIndex int, policy sequencePolicy, validated map[string]map[int]validatedSequence) {
+	for _, incomingEntry := range incoming.Mapping {
+		entryPath := path.Key(incomingEntry.Key)
+		position := index.find(target, incomingEntry.Key)
+		if position < 0 {
+			cloned := incomingEntry
+			cloned.Value = document.Clone(incomingEntry.Value)
+			index.append(target, cloned)
+			continue
+		}
+		current := target.Mapping[position].Value
+		if current.Kind == document.Sequence && incomingEntry.Value.Kind == document.Sequence && policy.registered(entryPath) {
+			contribution := validated[entryPath.String()][sourceIndex]
+			if contribution.items != nil {
+				for _, item := range contribution.items {
+					current.Sequence = append(current.Sequence, document.Clone(item))
+				}
+			}
+			continue
+		}
+		mergeDisjointMappingsAtPath(current, incomingEntry.Value, entryPath, index, sourceIndex, policy, validated)
+	}
+}
+
+func mergeDisjointMappingsAtPath(target, incoming *document.Node, path Path, index *mappingIndex, sourceIndex int, policy sequencePolicy, 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
+}
+
+func cloneSequenceRanges(ranges []sequenceRange) []sequenceRange {
+	cloned := make([]sequenceRange, len(ranges))
+	for i, assigned := range ranges {
+		cloned[i] = assigned
+		cloned[i].path = assigned.path.clone()
+	}
+	return cloned
+}
+
 func allMappingClaims(claims []compositionClaim) bool {
 	for _, claim := range claims {
 		if claim.kind != document.Mapping {
@@ -172,18 +373,64 @@ func documentEntryIndex(entries []document.Entry, key string) int {
 	return -1
 }
 
-func compositionOrigins(claims []compositionClaim) map[string]Origin {
-	origins := make(map[string]Origin, len(claims))
-	for _, claim := range claims {
-		pointer := claim.path.String()
-		origin := origins[pointer]
-		origin.Layer = LayerBase
-		origin.Locations = append(origin.Locations, claim.location)
-		origins[pointer] = origin
+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
+	}
+	for sourceIndex, input := range inputs {
+		collectProjectedCompositionOrigins(input, Path{}, Path{}, sourceIndex, ranges, rangeBySourcePath, origins)
 	}
 	return origins
 }
 
+type sequenceRangeKey struct {
+	sourceIndex int
+	path        string
+}
+
+type sequenceRangeIndex map[sequenceRangeKey]sequenceRange
+
+func indexSequenceRanges(ranges []sequenceRange) sequenceRangeIndex {
+	if len(ranges) == 0 {
+		return nil
+	}
+	index := make(sequenceRangeIndex, len(ranges))
+	for _, assigned := range ranges {
+		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

@@ -1,12 +1,60 @@
 package engine
 
 import (
+	"fmt"
 	"reflect"
+	"strings"
 	"testing"
 
 	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
 )
 
+func TestComposeWideRegisteredCollectionsPreservesEvidenceAndReplay(t *testing.T) {
+	policy := sequencePolicy{identityFields: make(map[string]string)}
+	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"
+			fmt.Fprintf(&text, "%s: [{id: source%d}]\n", key, source)
+		}
+		inputs = append(inputs, parseCompositionDocument(t, fmt.Sprintf("source-%d", source), text.String()))
+	}
+	composed, diagnostics := composeDocumentsWithPolicy(policy, inputs...)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	working := &document.Node{Kind: document.Mapping}
+	for _, application := range composed.applications {
+		applyHistoryApplication(&working, application, new(mappingIndex))
+	}
+	if !configurationValuesEqual(working, composed.root) {
+		t.Fatal("wide registered collections replay differs from assembly")
+	}
+	for collection := 0; collection < 128; collection++ {
+		path := Keys(fmt.Sprintf("records%d", collection)).Index(2).Key("id")
+		origin := composed.origins[path.String()]
+		if len(origin.Locations) != 1 || origin.Locations[0].SourceID != "source-2" {
+			t.Fatalf("shifted origin at %s = %#v", path.String(), origin)
+		}
+		local, ok := historyApplicationLocalPath(composed.applications[2], path)
+		if !ok || local.String() != Keys(fmt.Sprintf("records%d", collection)).Index(0).Key("id").String() {
+			t.Fatal("wide local path projection failed")
+		}
+	}
+	clone := cloneComposedDocument(composed)
+	for _, original := range composed.applications[0].sequences {
+		pointer := original.path.String()
+		copy := clone.applications[0].sequencesByPath[pointer]
+		copy.path.segments[0].key = "changed"
+		if composed.applications[0].sequencesByPath[pointer].path.String() != pointer {
+			t.Fatal("history path index aliases cloned state")
+		}
+		break
+	}
+}
+
 func TestComposeDocumentsJoinsDisjointMappingsAndTracksClaims(t *testing.T) {
 	base := parseCompositionDocument(t, "source-1", `services:
   api:
@@ -190,6 +238,268 @@ func TestComposeDocumentsRejectsInvalidBoundaries(t *testing.T) {
 	}
 }
 
+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"}}
+
+	composed, diagnostics := composeDocumentsWithPolicy(policy, first, second)
+	if len(diagnostics) != 0 {
+		t.Fatalf("composeDocumentsWithPolicy() diagnostics = %#v", diagnostics)
+	}
+	records, ok := documentAtPath(composed.root, Keys("records"))
+	if !ok || records.Kind != document.Sequence || len(records.Sequence) != 3 {
+		t.Fatalf("records = %#v, want three appended records", records)
+	}
+	if got := composed.sequenceRanges; !reflect.DeepEqual(got, []sequenceRange{
+		{sourceIndex: 0, path: Keys("records"), start: 0, end: 2},
+		{sourceIndex: 1, path: Keys("records"), start: 2, end: 3},
+	}) {
+		t.Fatalf("sequence ranges = %#v", got)
+	}
+	if !reflect.DeepEqual(first, firstBefore) || !reflect.DeepEqual(second, secondBefore) {
+		t.Fatal("registered composition mutated an input")
+	}
+	composed.root.Mapping[0].Value.Sequence[0].Mapping[1].Value.Text = "changed"
+	if !reflect.DeepEqual(first, firstBefore) || !reflect.DeepEqual(second, secondBefore) {
+		t.Fatal("composed sequence aliases an input")
+	}
+}
+
+func TestComposeDocumentsProjectsSequenceOriginsToAssignedIndexes(t *testing.T) {
+	first := parseCompositionDocument(t, "source-1", `mounted:
+  records:
+    - id: first
+      name: first-name
+escaped/~:
+  "7":
+    - id: escaped-first
+      name: escaped-first-name
+`)
+	second := parseCompositionDocument(t, "source-2", `mounted:
+  records:
+    - id: second
+      name: second-name
+escaped/~:
+  "7":
+    - id: escaped-second
+      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",
+	}}
+
+	composed, diagnostics := composeDocumentsWithPolicy(policy, first, second)
+	if len(diagnostics) != 0 {
+		t.Fatalf("composeDocumentsWithPolicy() diagnostics = %#v", diagnostics)
+	}
+	for _, test := range []struct {
+		path Path
+		want Location
+	}{
+		{path: Keys("mounted", "records").Index(1).Key("name"), want: Location{SourceID: "source-2", Line: 4, Column: 13}},
+		{path: Keys("escaped/~", "7").Index(1).Key("name"), want: Location{SourceID: "source-2", Line: 8, Column: 13}},
+	} {
+		path, want := test.path, test.want
+		origin, ok := composed.origins[path.String()]
+		if !ok || !reflect.DeepEqual(origin.Locations, []Location{want}) {
+			t.Errorf("origin at %s = %#v, want source-specific location %#v", path, origin, want)
+		}
+	}
+	for _, test := range []struct {
+		path        Path
+		wantSources []string
+	}{
+		{path: Keys("mounted", "records"), wantSources: []string{"source-1", "source-2"}},
+		{path: Keys("escaped/~", "7"), wantSources: []string{"source-1", "source-2"}},
+	} {
+		path, wantSources := test.path, test.wantSources
+		origin := composed.origins[path.String()]
+		gotSources := make([]string, len(origin.Locations))
+		for index, location := range origin.Locations {
+			gotSources[index] = location.SourceID
+		}
+		if !reflect.DeepEqual(gotSources, wantSources) {
+			t.Errorf("container origin at %s = %#v, want contributors %#v", path, gotSources, wantSources)
+		}
+	}
+	if !reflect.DeepEqual(first, firstBefore) || !reflect.DeepEqual(second, secondBefore) {
+		t.Fatal("composition projection mutated an input")
+	}
+
+	copy := cloneComposedDocument(composed)
+	copy.origins[Keys("mounted", "records").Index(1).Key("name").String()].Locations[0].SourceID = "changed"
+	for index := range copy.claims {
+		if copy.claims[index].path.String() == "/mounted/records" {
+			copy.claims[index].path.segments[0].key = "changed"
+			break
+		}
+	}
+	copy.sequenceRanges[0].path.segments[0].key = "changed"
+	if got := composed.origins[Keys("mounted", "records").Index(1).Key("name").String()].Locations[0].SourceID; got != "source-2" {
+		t.Fatalf("copied origin mutation changed source evidence: %q", got)
+	}
+	foundClaim := false
+	for _, claim := range composed.claims {
+		if claim.path.String() == "/mounted/records" {
+			foundClaim = true
+			break
+		}
+	}
+	if !foundClaim {
+		t.Fatal("copied claim mutation changed source path")
+	}
+	if got := composed.sequenceRanges[0].path.String(); got != "/mounted/records" {
+		t.Fatalf("copied range mutation changed assigned path: %q", got)
+	}
+}
+
+func TestComposeDocumentsValidatesRegisteredSequenceSingletons(t *testing.T) {
+	tests := []struct {
+		name     string
+		body     string
+		path     string
+		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},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			_, diagnostics := composeDocumentsWithPolicy(
+				sequencePolicy{identityFields: map[string]string{"/records": "id"}},
+				parseCompositionDocument(t, "source-1", test.body),
+			)
+			if len(diagnostics) != 1 || diagnostics[0].Code != test.wantCode || diagnostics[0].Path.String() != test.path {
+				t.Fatalf("diagnostics = %#v, want %s at %s", diagnostics, test.wantCode, test.path)
+			}
+			if diagnostics[0].Primary == nil {
+				t.Fatal("malformed sequence diagnostic has no source location")
+			}
+		})
+	}
+}
+
+func TestComposeDocumentsReportsAssignedPathForMalformedRegisteredSequenceContribution(t *testing.T) {
+	_, diagnostics := composeDocumentsWithPolicy(
+		sequencePolicy{identityFields: map[string]string{"/records": "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 diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-2" {
+		t.Fatalf("malformed item location = %#v, want source-2", diagnostics[0].Primary)
+	}
+}
+
+func TestComposeDocumentsRejectsRegisteredDuplicateIdentities(t *testing.T) {
+	tests := []struct {
+		name   string
+		first  string
+		second string
+		within bool
+	}{
+		{name: "within identical bodies", first: "records: [{id: '', value: same}, {id: '', value: same}]\n", within: true},
+		{name: "within disjoint bodies", first: "records: [{id: same, left: 1}, {id: same, right: 2}]\n", within: true},
+		{name: "across identical bodies", first: "records: [{id: same, value: same}]\n", second: "records: [{id: same, value: same}]\n"},
+		{name: "across disjoint bodies", first: "records: [{id: same, left: 1}]\n", second: "records: [{id: same, right: 2}]\n"},
+		{name: "empty identity across lists", first: "records: [{id: ''}]\n", second: "records: [{id: ''}]\n"},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			inputs := []*document.Node{parseCompositionDocument(t, "source-1", test.first)}
+			if !test.within {
+				inputs = append(inputs, parseCompositionDocument(t, "source-2", test.second))
+			}
+			_, diagnostics := composeDocumentsWithPolicy(
+				sequencePolicy{identityFields: map[string]string{"/records": "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)
+			}
+			if diagnostics[0].Primary == nil || len(diagnostics[0].Related) != 1 {
+				t.Fatalf("duplicate locations = %#v", diagnostics[0])
+			}
+			for _, secret := range []string{"same", "left", "right", "value"} {
+				if strings.Contains(diagnostics[0].Message, secret) {
+					t.Fatalf("diagnostic leaks %q: %q", secret, diagnostics[0].Message)
+				}
+			}
+		})
+	}
+}
+
+func TestComposeDocumentsSkipsMissingAndNonSequenceRegistrations(t *testing.T) {
+	policy := sequencePolicy{identityFields: map[string]string{
+		"/absent": "id", "/not-a-sequence": "id", "/catalog": "id", "/catalog/records": "id",
+	}}
+	composed, diagnostics := composeDocumentsWithPolicy(policy, parseCompositionDocument(t, "source-1", `
+not-a-sequence: null
+catalog:
+  records: [{id: present}]
+`))
+	if len(diagnostics) != 0 {
+		t.Fatalf("composeDocumentsWithPolicy() diagnostics = %#v", diagnostics)
+	}
+	if len(composed.sequenceRanges) != 1 || composed.sequenceRanges[0].path.String() != "/catalog/records" {
+		t.Fatalf("sequence ranges = %#v, want only the reached descendant sequence", composed.sequenceRanges)
+	}
+
+	composed, diagnostics = composeDocumentsWithPolicy(
+		sequencePolicy{identityFields: map[string]string{"/records": "id"}},
+		parseCompositionDocument(t, "source-1", "records: []\n"),
+		parseCompositionDocument(t, "source-2", "records: []\n"),
+	)
+	if len(diagnostics) != 0 || len(composed.root.Mapping[0].Value.Sequence) != 0 {
+		t.Fatalf("empty registered sequences = (%#v, %#v)", composed, diagnostics)
+	}
+	if got := composed.sequenceRanges; !reflect.DeepEqual(got, []sequenceRange{
+		{sourceIndex: 0, path: Keys("records"), start: 0, end: 0},
+		{sourceIndex: 1, path: Keys("records"), start: 0, end: 0},
+	}) {
+		t.Fatalf("empty sequence ranges = %#v", got)
+	}
+}
+
+func TestComposeDocumentsKeepsUnregisteredSequenceConflicts(t *testing.T) {
+	_, diagnostics := composeDocumentsWithPolicy(
+		sequencePolicy{identityFields: map[string]string{"/registered": "id"}},
+		parseCompositionDocument(t, "source-1", "unregistered: [one]\n"),
+		parseCompositionDocument(t, "source-2", "unregistered: [two]\n"),
+	)
+	assertCompositionConflict(t, diagnostics, "/unregistered", []string{"source-1", "source-2"})
+}
+
+func TestComposeDocumentsKeepsNonSequenceRegisteredOverlapsConflicting(t *testing.T) {
+	tests := []struct {
+		name   string
+		first  string
+		second string
+	}{
+		{name: "mapping and sequence", first: "records: {id: one}\n", second: "records: [{id: two}]\n"},
+		{name: "null and sequence", first: "records: null\n", second: "records: [{id: two}]\n"},
+		{name: "scalar and sequence", first: "records: value\n", second: "records: [{id: two}]\n"},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			_, diagnostics := composeDocumentsWithPolicy(
+				sequencePolicy{identityFields: map[string]string{"/records": "id"}},
+				parseCompositionDocument(t, "source-1", test.first),
+				parseCompositionDocument(t, "source-2", test.second),
+			)
+			assertCompositionConflict(t, diagnostics, "/records", []string{"source-1", "source-2"})
+		})
+	}
+}
+
 func parseCompositionDocument(t *testing.T, sourceID, input string) *document.Node {
 	t.Helper()
 	parsed, diagnostics := parseSource(Bytes(sourceID, []byte(input)), sourceID, defaultParseLimits())

internal/engine/destination.go

@@ -11,6 +11,33 @@ var (
 	errDestinationLimit   = errors.New("configkit: destination exceeds document limits")
 )
 
+type placementMode uint8
+
+const (
+	placementModeMapping placementMode = iota
+	placementModeSequenceItem
+)
+
+// PlacementModeFromFacade preserves root-owned numeric values until preflight
+// can reject unsupported modes.
+func PlacementModeFromFacade(value uint8) placementMode { return placementMode(value) }
+
+// PlacementModeForFacade converts private source evidence to the root-owned mode.
+func PlacementModeForFacade(value placementMode) uint8 { return uint8(value) }
+
+// InputWithPlacement copies the root-owned mode into a private engine input.
+func InputWithPlacement(input Input, value uint8) Input {
+	input.placementMode = placementMode(value)
+	return input
+}
+
+// OptionsWithPlacement copies root-owned controls into private engine options.
+func OptionsWithPlacement[T any](options Options[T], destination Path, value uint8) Options[T] {
+	options.destination = destination.clone()
+	options.placementMode = placementMode(value)
+	return options
+}
+
 func normalizeDestination(path Path) (Path, error) {
 	if !path.valid() {
 		return Path{}, errInvalidDestination
@@ -47,18 +74,39 @@ func invalidDestinationDiagnostic(destination Path) Diagnostic {
 // placeDocument checks all per-document bounds before allocating destination
 // wrappers. Aggregate accounting remains the caller's responsibility.
 func placeDocument(body *document.Node, destination Path, sourceRoot document.Location, maxDepth, maxNodes int) (Path, *document.Node, error) {
+	return placeDocumentWithMode(body, destination, placementModeMapping, sourceRoot, maxDepth, maxNodes)
+}
+
+// placeDocumentWithMode places a source mapping beneath its literal destination.
+// Sequence-item placement adds one sequence containing the complete mapping.
+func placeDocumentWithMode(body *document.Node, destination Path, mode placementMode, sourceRoot document.Location, maxDepth, maxNodes int) (Path, *document.Node, error) {
 	normalized, err := normalizeDestination(destination)
 	if err != nil {
 		return Path{}, nil, err
 	}
+	if mode != placementModeMapping && mode != placementModeSequenceItem {
+		return Path{}, nil, errInvalidDestination
+	}
+	if mode == placementModeSequenceItem && len(normalized.segments) == 0 {
+		return Path{}, nil, errInvalidDestination
+	}
 	if body == nil {
 		return normalized, nil, nil
 	}
-	if !destinationWithinLimits(body, normalized, maxDepth, maxNodes) {
+	if mode == placementModeSequenceItem && body.Kind != document.Mapping {
+		return normalized, nil, errInvalidDestination
+	}
+	if !destinationWithinLimitsWithMode(body, normalized, mode, maxDepth, maxNodes) {
 		return normalized, nil, errDestinationLimit
 	}
 
 	placed := body
+	if mode == placementModeSequenceItem {
+		placed = &document.Node{
+			Kind: document.Sequence, Location: sourceRoot,
+			Sequence: []*document.Node{placed},
+		}
+	}
 	for index := len(normalized.segments) - 1; index >= 0; index-- {
 		key := normalized.segments[index].key
 		placed = &document.Node{
@@ -77,28 +125,49 @@ func placeDocument(body *document.Node, destination Path, sourceRoot document.Lo
 
 // destinationWithinLimits expects a normalized mapping-key destination.
 func destinationWithinLimits(body *document.Node, destination Path, maxDepth, maxNodes int) bool {
+	return destinationWithinLimitsWithMode(body, destination, placementModeMapping, maxDepth, maxNodes)
+}
+
+func destinationWithinLimitsWithMode(body *document.Node, destination Path, mode placementMode, maxDepth, maxNodes int) bool {
 	if body == nil {
 		return true
 	}
-	if maxDepth < 0 || maxNodes < 0 || !destination.valid() {
+	if maxDepth < 0 || maxNodes < 0 || !destination.valid() || (mode != placementModeMapping && mode != placementModeSequenceItem) {
 		return false
 	}
 	segments := len(destination.segments)
-	if segments > maxDepth || segments > maxNodes/2 {
+	sequenceNodes := 0
+	sequenceDepth := 0
+	if mode == placementModeSequenceItem {
+		if segments == 0 {
+			return false
+		}
+		sequenceNodes = 1
+		sequenceDepth = 1
+	}
+	if segments > maxDepth-sequenceDepth || segments > (maxNodes-sequenceNodes)/2 {
 		return false
 	}
-	wrapperNodes := destinationWrapperNodeCount(destination)
+	wrapperNodes := destinationWrapperNodeCount(destination) + sequenceNodes
 	for _, segment := range destination.segments {
 		if segment.kind != pathSegmentKey {
 			return false
 		}
 	}
-	if documentDepth(body) > maxDepth-segments {
+	if documentDepth(body) > maxDepth-segments-sequenceDepth {
 		return false
 	}
 	return countInputNodes(body) <= maxNodes-wrapperNodes
 }
 
+func destinationWrapperNodeCountForMode(destination Path, mode placementMode) int {
+	count := destinationWrapperNodeCount(destination)
+	if mode == placementModeSequenceItem {
+		count++
+	}
+	return count
+}
+
 func destinationWrapperNodeCount(destination Path) int {
 	return 2 * len(destination.segments)
 }

internal/engine/destination_accounting_test.go

@@ -68,6 +68,66 @@ func TestMultipleMountedInputsSharePlacementCharges(t *testing.T) {
 	}
 }
 
+func TestNestedItemGlobsShareUnderlayNodeBudget(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - glob: children/*.yml
+      destination: [children]
+      placement: sequence-item
+id: parent
+`)
+	fragmentPath := writeImportFile(t, directory, "children/a.yml", "id: child-a\n")
+	parseNodes := func(path string) int {
+		t.Helper()
+		parsed, diagnostics := parseSource(File(path), "fixture", defaultParseLimits())
+		if len(diagnostics) != 0 {
+			t.Fatalf("parse %s: %#v", path, diagnostics)
+		}
+		return countInputNodes(parsed.document)
+	}
+	destination := Keys("groups")
+	childDestination := appendDestination(destination, Keys("children"))
+	otherDestination := Keys("other")
+	wantNodes := parseNodes(mainPath) + parseNodes(fragmentPath) + countInputNodes(&document.Node{Kind: document.Mapping}) +
+		destinationWrapperNodeCountForMode(destination, placementModeSequenceItem) +
+		destinationWrapperNodeCountForMode(childDestination, placementModeSequenceItem) +
+		destinationWrapperNodeCountForMode(otherDestination, placementModeSequenceItem)
+	limits := defaultImportLimits()
+
+	for _, test := range []struct {
+		name      string
+		delta     int
+		wantError bool
+	}{
+		{name: "at aggregate bound"},
+		{name: "one node below aggregate bound", delta: 1, wantError: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			accounting := &importAccounting{nodes: limits.nodes - wantNodes + test.delta}
+			composition, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{
+				source: File(mainPath), imports: true, destination: destination, placementMode: placementModeSequenceItem,
+			}, {
+				source: Mapping("other", map[string]any{"id": "other"}), destination: otherDestination, placementMode: placementModeSequenceItem,
+			}}}, 1, nil, accounting)
+			if test.wantError {
+				if composition != nil || len(diagnostics) == 0 || diagnostics[0].Code != CodeSourceLimitExceeded {
+					t.Fatalf("under-budget nested item glob = %#v, %#v", composition, diagnostics)
+				}
+				return
+			}
+			if composition == nil || len(diagnostics) != 0 || len(sources) != 3 {
+				t.Fatalf("bounded nested item glob = %#v, sources %#v, diagnostics %#v", composition, sources, diagnostics)
+			}
+			if composition.inputNodes != wantNodes || accounting.nodes != limits.nodes+test.delta {
+				t.Fatalf("nested item node charge = %d/%d, want %d/%d", composition.inputNodes, accounting.nodes, wantNodes, limits.nodes+test.delta)
+			}
+		})
+	}
+}
+
 func TestImportedEmptyBodiesChargeEachDestinationWrapper(t *testing.T) {
 	t.Parallel()
 

internal/engine/destination_import_test.go

@@ -4,6 +4,8 @@ import (
 	"path/filepath"
 	"testing"
 	"testing/fstest"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
 )
 
 type importDestinationConfig struct {
@@ -133,7 +135,11 @@ parent: true
 		}
 	}
 
-	composed, diagnostics := composeDocuments(state.documents...)
+	documents := make([]*document.Node, 0, len(state.scope.contributions))
+	for _, contribution := range state.scope.contributions {
+		documents = append(documents, contribution.document)
+	}
+	composed, diagnostics := composeDocuments(documents...)
 	if len(diagnostics) != 0 {
 		t.Fatalf("composition diagnostics = %#v", diagnostics)
 	}

internal/engine/destination_integration_test.go

@@ -193,6 +193,43 @@ func TestLayeredMountedVersionProtectionCoversInheritanceAndUpperTiers(t *testin
 	})
 }
 
+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"}},
+	}
+	item := func(label string, value map[string]any) Input {
+		return Input{Source: Mapping(label, value), Destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem}
+	}
+	options := func(inputs ...Input) LayeredOptions[map[string]any] {
+		return LayeredOptions[map[string]any]{Definition: definition, SourceVersionMode: sourceVersionModeIfPresent, Underlays: []Underlay{{Inputs: inputs}}}
+	}
+	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})))
+		if got := groupedErrorCode(t, err); got != CodeUnderlayVersion {
+			t.Fatalf("protected item claim code = %s, want %s", got, CodeUnderlayVersion)
+		}
+	})
+
+	t.Run("appended peer does not claim main path", func(t *testing.T) {
+		loaded, err := LoadLayered(main, testSecretBinding(), options(
+			item("first", map[string]any{"id": "first", "schema": map[string]any{"other": "value"}}),
+			item("peer", map[string]any{"id": "peer", "schema": nil}),
+		))
+		if err != nil {
+			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" {
+			t.Fatalf("main item projection = %#v", entries)
+		}
+	})
+}
+
 type destinationDecodeProbe string
 
 var destinationDecodeProbeValues []string

internal/engine/destination_load_test.go

@@ -12,6 +12,98 @@ type destinationLoadConfig struct {
 	Value       string                       `yaml:"value"`
 }
 
+type destinationItemConfig struct {
+	Catalog struct {
+		Entries []map[string]string `yaml:"entries"`
+	} `yaml:"catalog"`
+}
+
+func TestLayeredLoadPlacesCompletedMappingAsOneItem(t *testing.T) {
+	t.Parallel()
+
+	loaded, err := LoadLayered[destinationItemConfig](
+		Input{Source: NoSource(), Destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem},
+		testSecretBinding(),
+		LayeredOptions[destinationItemConfig]{Underlays: []Underlay{{Inputs: []Input{{
+			Source: Mapping("empty-item", map[string]any{}), Destination: Keys("catalog", "entries"),
+			placementMode: placementModeSequenceItem,
+		}}}}},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	if len(result.Config.Catalog.Entries) != 1 || result.Config.Catalog.Entries[0] == nil || len(result.Config.Catalog.Entries[0]) != 0 {
+		t.Fatalf("empty mapping item = %#v", result.Config.Catalog.Entries)
+	}
+	if _, ok := result.Metadata().Lookup(Keys("catalog", "entries", "0")); ok {
+		t.Fatal("sequence index was treated as a literal mapping key during metadata lookup")
+	}
+	sources := result.Sources()
+	if len(sources) != 1 || sources[0].placementMode != placementModeSequenceItem || sources[0].destinationUnresolved {
+		t.Fatalf("source placement evidence = %#v", sources)
+	}
+	if got := sources[0].Destination.String(); got != "/catalog/entries" {
+		t.Fatalf("source destination = %q", got)
+	}
+
+	absent, err := LoadLayered[destinationItemConfig](
+		Input{Source: NoSource(), Destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem},
+		testSecretBinding(),
+	)
+	if err != nil {
+		t.Fatalf("NoSource item load: %v", err)
+	}
+	if got := len(testLoaded(absent).Config.Catalog.Entries); got != 0 {
+		t.Fatalf("absent item created %d entries", got)
+	}
+}
+
+func TestLayeredItemPlacementPreflightsModeAndDestination(t *testing.T) {
+	t.Parallel()
+
+	for _, test := range []struct {
+		name string
+		mode placementMode
+		path Path
+	}{
+		{name: "root item", mode: placementModeSequenceItem},
+		{name: "unknown mode", mode: placementMode(255), path: Keys("catalog")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			reader := bytes.NewBufferString("value: unused\n")
+			_, err := LoadLayered[destinationItemConfig](
+				Input{Source: Reader("reader", reader), Destination: test.path, placementMode: test.mode},
+				testSecretBinding(),
+			)
+			if err == nil || reader.Len() == 0 {
+				t.Fatalf("preflight err %v, unread bytes %d", err, reader.Len())
+			}
+			loadError, ok := err.(*LoadError)
+			if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != CodeOptionsInvalid {
+				t.Fatalf("preflight error = %#v", err)
+			}
+		})
+	}
+}
+
+func TestLoadOptionsPlaceOneMappingItem(t *testing.T) {
+	t.Parallel()
+
+	loaded, err := Load[destinationItemConfig](
+		Mapping("ordinary", map[string]any{}), testSecretBinding(),
+		Options[destinationItemConfig]{destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem},
+	)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	result := testLoaded(loaded)
+	if len(result.Config.Catalog.Entries) != 1 || result.Config.Catalog.Entries[0] == nil {
+		t.Fatalf("ordinary item placement = %#v", result.Config.Catalog.Entries)
+	}
+}
+
 func TestLayeredLoadPlacesNonImportMainAndUnderlayInputs(t *testing.T) {
 	t.Parallel()
 

internal/engine/destination_test.go

@@ -2,6 +2,7 @@ package engine
 
 import (
 	"errors"
+	"reflect"
 	"testing"
 
 	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
@@ -39,6 +40,29 @@ func TestNormalizeDestinationUsesLiteralMappingKeys(t *testing.T) {
 	}
 }
 
+func TestMappingPlacementModeMatchesZeroValue(t *testing.T) {
+	t.Parallel()
+
+	body := &document.Node{
+		Kind: document.Mapping,
+		Mapping: []document.Entry{{
+			Key: "name", Value: &document.Node{Kind: document.String, Text: "entry"},
+		}},
+	}
+	destination := Keys("catalog", "entry")
+	path, zero, err := placeDocument(body, destination, body.Location, 5, 32)
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicitPath, explicit, err := placeDocumentWithMode(body, destination, placementModeMapping, body.Location, 5, 32)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if path.String() != explicitPath.String() || !reflect.DeepEqual(zero, explicit) {
+		t.Fatalf("zero placement (%q, %#v) differs from mapping mode (%q, %#v)", path.String(), zero, explicitPath.String(), explicit)
+	}
+}
+
 func TestAppendDestinationCopiesLiteralSegments(t *testing.T) {
 	t.Parallel()
 
@@ -204,3 +228,60 @@ func TestPlaceDocumentRootAndBounds(t *testing.T) {
 		t.Fatalf("absent body should not consume placement limits: node %p, error %v", absent, err)
 	}
 }
+
+func TestPlaceDocumentSequenceItemAndMountedBounds(t *testing.T) {
+	t.Parallel()
+
+	root := document.Location{SourceID: "item", Line: 1, Column: 1}
+	body := &document.Node{Kind: document.Mapping, Location: root}
+	destination := Keys("catalog", "entries")
+	placedPath, placed, err := placeDocumentWithMode(body, destination, placementModeSequenceItem, root, 3, 6)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if placedPath.String() != "/catalog/entries" || placed.Kind != document.Mapping {
+		t.Fatalf("placed root = %q %#v", placedPath.String(), placed)
+	}
+	container, ok := documentAtPath(placed, destination)
+	if !ok || container.Kind != document.Sequence || len(container.Sequence) != 1 || container.Sequence[0] != body {
+		t.Fatalf("item destination = %#v, present %v", container, ok)
+	}
+	if got, want := countInputNodes(placed), countInputNodes(body)+destinationWrapperNodeCount(destination)+1; got != want {
+		t.Fatalf("item node count = %d, want %d", got, want)
+	}
+	if container.Location != root || container.Sequence[0].Location != root {
+		t.Fatal("synthetic sequence or authored mapping lost source coordinates")
+	}
+
+	if _, _, err := placeDocumentWithMode(body, Path{}, placementModeSequenceItem, root, 3, 6); !errors.Is(err, errInvalidDestination) {
+		t.Fatalf("root item placement error = %v, want invalid destination", err)
+	}
+	if _, _, err := placeDocumentWithMode(&document.Node{Kind: document.String}, destination, placementModeSequenceItem, root, 3, 6); !errors.Is(err, errInvalidDestination) {
+		t.Fatalf("non-mapping item placement error = %v, want invalid destination", err)
+	}
+	if _, _, err := placeDocumentWithMode(body, destination, placementMode(255), root, 3, 6); !errors.Is(err, errInvalidDestination) {
+		t.Fatalf("unknown placement mode error = %v, want invalid destination", err)
+	}
+
+	for _, test := range []struct {
+		name       string
+		depth      int
+		nodes      int
+		wantAccept bool
+	}{
+		{name: "exact allowance", depth: 3, nodes: 6, wantAccept: true},
+		{name: "one too shallow", depth: 2, nodes: 6},
+		{name: "one node short", depth: 3, nodes: 5},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			_, _, err := placeDocumentWithMode(body, destination, placementModeSequenceItem, root, test.depth, test.nodes)
+			if test.wantAccept && err != nil {
+				t.Fatalf("exact bounds rejected: %v", err)
+			}
+			if !test.wantAccept && !errors.Is(err, errDestinationLimit) {
+				t.Fatalf("over-bound placement error = %v", err)
+			}
+		})
+	}
+}

internal/engine/directives.go

@@ -10,11 +10,14 @@ import (
 const directiveKey = "configkit"
 
 type importDeclaration struct {
-	path        string
-	glob        string
-	destination Path
-	optional    bool
-	location    Location
+	path              string
+	glob              string
+	destination       Path
+	placement         placementMode
+	placementPath     Path
+	placementLocation *Location
+	optional          bool
+	location          Location
 }
 
 func splitImportDirectives(root *document.Node) (*document.Node, []importDeclaration, []Diagnostic) {
@@ -70,7 +73,7 @@ func parseImportDeclaration(node *document.Node, declarationPath Path) (importDe
 	if node.Kind != document.Mapping {
 		return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, declarationPath, "import declaration must be a mapping", node)}
 	}
-	declaration := importDeclaration{location: publicLocation(node.Location)}
+	declaration := importDeclaration{location: publicLocation(node.Location), placementPath: declarationPath.clone()}
 	seenTarget := false
 	for _, entry := range node.Mapping {
 		fieldPath := declarationPath.Key(entry.Key)
@@ -112,6 +115,21 @@ func parseImportDeclaration(node *document.Node, declarationPath Path) (importDe
 				return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "destination must contain only non-reserved mapping keys", entry.Value)}
 			}
 			declaration.destination = normalized
+		case "placement":
+			declaration.placementPath = fieldPath.clone()
+			placementLocation := publicLocation(entry.Value.Location)
+			declaration.placementLocation = &placementLocation
+			if entry.Value.Kind != document.String {
+				return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "placement must be mapping or sequence-item", entry.Value)}
+			}
+			switch entry.Value.Text {
+			case "mapping":
+				declaration.placement = placementModeMapping
+			case "sequence-item":
+				declaration.placement = placementModeSequenceItem
+			default:
+				return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "placement must be mapping or sequence-item", entry.Value)}
+			}
 		default:
 			return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "import declaration field is not recognized", entry.Value)}
 		}
@@ -145,26 +163,54 @@ func nestedDirectiveDiagnostics(node *document.Node, current Path) []Diagnostic
 }
 
 func fragmentVersionClaim(root *document.Node, rule *VersionRule) *document.Node {
+	return fragmentVersionClaimProjected(root, rule, -1, nil)
+}
+
+func fragmentVersionClaimProjected(root *document.Node, rule *VersionRule, sourceIndex int, ranges []sequenceRange) *document.Node {
+	return fragmentVersionClaimWithIndex(root, rule, sourceIndex, indexSequenceRanges(ranges))
+}
+
+func fragmentVersionClaimWithIndex(root *document.Node, rule *VersionRule, sourceIndex int, ranges sequenceRangeIndex) *document.Node {
 	if rule == nil {
 		return nil
 	}
 	current := root
+	currentPath := Path{}
 	if len(rule.Path.segments) == 0 {
 		return current
 	}
 	for _, segment := range rule.Path.segments {
-		if current.Kind != document.Mapping {
+		switch current.Kind {
+		case document.Mapping:
+			if segment.kind == pathSegmentIndex {
+				return current
+			}
+			index := documentEntryIndex(current.Mapping, segment.key)
+			if index < 0 {
+				return nil
+			}
+			current = current.Mapping[index].Value
+			currentPath = currentPath.Key(segment.key)
+		case document.Sequence:
+			index, valid := schemaIndex(segment)
+			if !valid {
+				return current
+			}
+			if assigned, found := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: currentPath.String()}]; found {
+				if index < assigned.start || index >= assigned.end {
+					return nil
+				}
+				index -= assigned.start
+			}
+			if index < 0 || index >= len(current.Sequence) {
+				return nil
+			}
+			current = current.Sequence[index]
+			currentPath = currentPath.Index(index)
+		default:
+			// A scalar or null at an ancestor replaces the protected subtree.
 			return current
 		}
-		if segment.kind == pathSegmentIndex {
-			return nil
-		}
-		key := segment.key
-		index := documentEntryIndex(current.Mapping, key)
-		if index < 0 {
-			return nil
-		}
-		current = current.Mapping[index].Value
 	}
 	return current
 }

internal/engine/history.go

@@ -45,10 +45,12 @@ func (c *retainedNodeCounter) add(root *document.Node) bool {
 // Replay reconciles with canonical decoding before domain resolution so both
 // contracts agree on the effective result without retaining a tree per field.
 type historyApplication struct {
-	layer  Layer
-	path   Path
-	value  *document.Node
-	origin Origin
+	layer           Layer
+	path            Path
+	value           *document.Node
+	origin          Origin
+	sequences       []sequenceHistoryContribution
+	sequencesByPath map[string]sequenceHistoryContribution
 }
 
 func cloneHistoryApplications(values []historyApplication) []historyApplication {
@@ -59,33 +61,115 @@ func cloneHistoryApplications(values []historyApplication) []historyApplication
 	for index, value := range values {
 		cloned[index] = historyApplication{
 			layer: value.layer, path: value.path.clone(), value: value.value,
-			origin: cloneOrigin(value.origin),
+			origin: cloneOrigin(value.origin), sequences: cloneSequenceHistoryContributions(value.sequences),
 		}
+		cloned[index].sequencesByPath = indexSequenceHistoryContributions(cloned[index].sequences)
 	}
 	return cloned
 }
 
+// walkHistoryApplication visits a retained source tree with both its local
+// semantic path and its effective projected path. Sequence fragments keep
+// their original tree indexes, while the effective path uses the assigned
+// range recorded by strict composition.
+func walkHistoryApplication(application historyApplication, visit func(local, effective Path, node *document.Node)) {
+	if application.value == nil {
+		return
+	}
+	var walk func(*document.Node, Path, Path)
+	walk = func(node *document.Node, local, effective Path) {
+		if node == nil {
+			return
+		}
+		visit(local, appendDestination(application.path, effective), node)
+		switch node.Kind {
+		case document.Mapping:
+			for _, entry := range node.Mapping {
+				walk(entry.Value, local.Key(entry.Key), effective.Key(entry.Key))
+			}
+		case document.Sequence:
+			contribution, projected := sequenceHistoryAt(application.sequencesByPath, local)
+			for index, child := range node.Sequence {
+				effectiveIndex := index
+				if projected {
+					effectiveIndex += contribution.offset
+				}
+				walk(child, local.Index(index), effective.Index(effectiveIndex))
+			}
+		}
+	}
+	walk(application.value, Path{}, Path{})
+}
+
+// historyApplicationLocalPath translates an effective projected path back to
+// the retained source tree, including indexes shifted by enclosing appends.
+func historyApplicationLocalPath(application historyApplication, effective Path) (Path, bool) {
+	relative, ok := relativeHistoryPath(application.path, effective)
+	if !ok {
+		return Path{}, false
+	}
+	local := 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 {
+				return Path{}, false
+			}
+			local = local.Index(index - contribution.offset)
+			continue
+		}
+		local.segments = append(local.segments, segment)
+	}
+	return local, true
+}
+
 func applicationOperationPath(application historyApplication, query Path, lookup *documentLookup) (Path, bool) {
 	if !application.path.valid() || application.value == nil {
 		return Path{}, false
 	}
-	if application.path.String() != "" {
-		if pathsIntersect(application.path, query) {
+	if !pathContains(application.path, query) {
+		if pathContains(query, application.path) {
 			return application.path.clone(), true
 		}
 		return Path{}, false
 	}
-	if len(query.segments) == 0 {
-		return Path{}, true
+	sequencePathMatched := false
+	for _, contribution := range application.sequences {
+		container := appendDestination(application.path, contribution.path)
+		if !pathContains(container, query) && !pathContains(query, container) {
+			continue
+		}
+		sequencePathMatched = true
+		if pathContains(container, query) && len(query.segments) > len(container.segments) {
+			local, ok := historyApplicationLocalPath(application, query)
+			if contribution.mode == sequenceHistoryReplace {
+				if _, present := documentAtPathWithLookup(application.value, local, lookup); !ok || !present {
+					// A replacement also changes descendants missing from the
+					// incoming list. Attribute their removal to its container.
+					return container, true
+				}
+			}
+			if !ok {
+				continue
+			}
+		}
+		return query.clone(), true
 	}
-	for length := 1; length <= len(query.segments); length++ {
-		prefix := Path{segments: clonePathSegments(query.segments[:length], 0)}
+	if sequencePathMatched {
+		return Path{}, false
+	}
+	if len(query.segments) == len(application.path.segments) || application.value.Kind != document.Mapping {
+		return application.path.clone(), true
+	}
+	relative, _ := relativeHistoryPath(application.path, query)
+	for length := 1; length <= len(relative.segments); length++ {
+		prefix := Path{segments: clonePathSegments(relative.segments[:length], 0)}
 		node, present := documentAtPathWithLookup(application.value, prefix, lookup)
 		if !present {
 			return Path{}, false
 		}
-		if length == len(query.segments) || node.Kind != document.Mapping {
-			return prefix, true
+		if length == len(relative.segments) || node.Kind != document.Mapping {
+			return appendDestination(application.path, prefix), true
 		}
 	}
 	return Path{}, false
@@ -96,7 +180,7 @@ func historyApplicationOrigin(application historyApplication, operationPath Path
 		return cloneOrigin(application.origin)
 	}
 	origin := Origin{Layer: application.layer}
-	relative, ok := relativeHistoryPath(application.path, operationPath)
+	relative, ok := historyApplicationLocalPath(application, operationPath)
 	if !ok {
 		return origin
 	}
@@ -119,13 +203,17 @@ func pathsIntersect(left, right Path) bool {
 
 func applyHistoryApplication(root **document.Node, application historyApplication, index *mappingIndex) {
 	if application.path.String() == "" {
-		mergeHistoryDocument(root, application.value, index)
+		mergeHistoryDocumentAtPath(root, application.value, Path{}, index, application.sequencesByPath)
 		return
 	}
 	mergeHistoryAtPath(root, application.path, application.value, index)
 }
 
 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) {
 	if incoming == nil {
 		return
 	}
@@ -141,10 +229,56 @@ func mergeHistoryDocument(target **document.Node, incoming *document.Node, index
 			index.append(*target, cloned)
 			continue
 		}
-		mergeHistoryDocument(&(*target).Mapping[position].Value, entry.Value, index)
+		entryPath := path.Key(entry.Key)
+		if contribution, found := sequenceHistoryAt(sequences, entryPath); found {
+			applySequenceHistory(&(*target).Mapping[position].Value, entry.Value, contribution)
+			continue
+		}
+		mergeHistoryDocumentAtPath(&(*target).Mapping[position].Value, entry.Value, entryPath, index, sequences)
 	}
 }
 
+func sequenceHistoryAt(contributions map[string]sequenceHistoryContribution, path Path) (sequenceHistoryContribution, bool) {
+	contribution, found := contributions[path.String()]
+	return contribution, found
+}
+
+func indexSequenceHistoryContributions(values []sequenceHistoryContribution) map[string]sequenceHistoryContribution {
+	if len(values) == 0 {
+		return nil
+	}
+	index := make(map[string]sequenceHistoryContribution, len(values))
+	for _, value := range values {
+		index[value.path.String()] = value
+	}
+	return index
+}
+
+func applySequenceHistory(target **document.Node, incoming *document.Node, contribution sequenceHistoryContribution) {
+	if incoming == nil || incoming.Kind != document.Sequence {
+		return
+	}
+	if contribution.mode == sequenceHistoryReplace || *target == nil || (*target).Kind != document.Sequence {
+		*target = document.Clone(incoming)
+		return
+	}
+	for _, item := range incoming.Sequence {
+		(*target).Sequence = append((*target).Sequence, document.Clone(item))
+	}
+}
+
+func cloneSequenceHistoryContributions(values []sequenceHistoryContribution) []sequenceHistoryContribution {
+	if values == nil {
+		return nil
+	}
+	cloned := make([]sequenceHistoryContribution, len(values))
+	for index, value := range values {
+		cloned[index] = value
+		cloned[index].path = value.path.clone()
+	}
+	return cloned
+}
+
 func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node, index *mappingIndex) {
 	if root == nil || *root == nil || len(path.segments) == 0 {
 		mergeHistoryDocument(root, incoming, index)
@@ -152,7 +286,19 @@ func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node
 	}
 	current := *root
 	for segmentIndex, segment := range path.segments {
-		if current.Kind != document.Mapping || segment.kind == pathSegmentIndex {
+		if segment.kind == pathSegmentIndex {
+			position, ok := schemaIndex(segment)
+			if current.Kind != document.Sequence || !ok || position >= len(current.Sequence) {
+				return
+			}
+			if segmentIndex == len(path.segments)-1 {
+				mergeHistoryDocument(&current.Sequence[position], incoming, index)
+				return
+			}
+			current = current.Sequence[position]
+			continue
+		}
+		if current.Kind != document.Mapping {
 			return
 		}
 		position := index.find(current, segment.key)

internal/engine/history_sequence_test.go

@@ -0,0 +1,240 @@
+package engine
+
+import (
+	"bytes"
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestSequenceHistoryReplaysAppendAtAssignedIndexes(t *testing.T) {
+	policy := sequencePolicy{identityFields: map[string]string{"/records": "id", "/other": "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)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose diagnostics = %#v", diagnostics)
+	}
+	for applicationIndex, application := range composed.applications {
+		for path, want := range map[string]int{"/records": 1, "/other": 1} {
+			sequence, present := documentAtPath(application.value, mustParsePath(t, path))
+			if !present || sequence.Kind != document.Sequence || len(sequence.Sequence) != want {
+				t.Errorf("application %d retained %s = %#v, %v; want one source fragment item", applicationIndex, path, sequence, present)
+			}
+		}
+	}
+
+	working := parseCompositionDocument(t, "defaults", "records: [{id: default}]\nother: [{id: default}]\n")
+	index := new(mappingIndex)
+	for _, application := range composed.applications {
+		applyHistoryApplication(&working, application, index)
+	}
+	for path, want := range map[string][]string{
+		"/records": {"a", "b"},
+		"/other":   {"x", "y"},
+	} {
+		sequence, present := documentAtPath(working, mustParsePath(t, path))
+		if !present || sequence.Kind != document.Sequence || len(sequence.Sequence) != len(want) {
+			t.Fatalf("replayed %s = %#v", path, sequence)
+		}
+		for itemIndex, id := range want {
+			field := sequence.Sequence[itemIndex].Mapping[0].Value.Text
+			if field != id {
+				t.Errorf("replayed %s[%d].id = %q, want %q", path, itemIndex, field, id)
+			}
+		}
+	}
+
+	application := composed.applications[1]
+	cloned := cloneHistoryApplications(composed.applications)
+	cloned[1].sequences[0].path.segments[0].key = "mutated"
+	if composed.applications[1].sequences[0].path.String() != "/records" {
+		t.Fatal("cloned sequence history path aliases its source")
+	}
+	addedPath := Keys("records").Index(1).Key("value")
+	if operation, relevant := applicationOperationPath(application, addedPath, nil); !relevant || operation.String() != addedPath.String() {
+		t.Fatalf("appended value operation = %s, %v", operation.String(), relevant)
+	}
+	if _, relevant := applicationOperationPath(application, Keys("records").Index(0).Key("value"), nil); relevant {
+		t.Fatal("later contribution was reported as changing an earlier record")
+	}
+	origin := historyApplicationOrigin(application, addedPath, nil)
+	if len(origin.Locations) != 1 || origin.Locations[0].SourceID != "source-2" {
+		t.Fatalf("appended value origin = %#v", origin)
+	}
+	overlay := historyApplication{
+		layer: LayerOverlay,
+		value: parseCompositionDocument(t, "overlay", "records: []\n"),
+	}
+	applyHistoryApplication(&working, overlay, index)
+	replaced, present := documentAtPath(working, Keys("records"))
+	if !present || replaced.Kind != document.Sequence || len(replaced.Sequence) != 0 {
+		t.Fatalf("ordinary later list replacement = %#v, %v", replaced, present)
+	}
+}
+
+func TestSequenceHistoryPreservesTierReplacementAndEmptyLists(t *testing.T) {
+	policy := sequencePolicy{identityFields: map[string]string{"/records": "id", "/independent": "id"}}
+	first, diagnostics := composeDocumentsWithPolicy(policy,
+		parseCompositionDocument(t, "underlay-1a", "records: [{id: shared}]\nindependent: [{id: x}]\n"),
+		parseCompositionDocument(t, "underlay-1b", "records: [{id: lower}]\n"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("first tier diagnostics = %#v", diagnostics)
+	}
+	for index := range first.applications {
+		first.applications[index].layer = LayerUnderlay
+	}
+	second, diagnostics := composeDocumentsWithPolicy(policy,
+		parseCompositionDocument(t, "underlay-2", "records: [{id: shared}]\nindependent: []\n"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("second tier diagnostics = %#v", diagnostics)
+	}
+	main, diagnostics := composeDocumentsWithPolicy(policy,
+		parseCompositionDocument(t, "main", "records: []\n"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("main diagnostics = %#v", diagnostics)
+	}
+
+	assembly := newAssemblyAccumulator(first)
+	if diagnostics := assembly.applyOverlay(second.root, second.origins, second.applications, LayerUnderlay); len(diagnostics) != 0 {
+		t.Fatalf("second tier handoff diagnostics = %#v", diagnostics)
+	}
+	if diagnostics := assembly.applyOverlay(main.root, main.origins, main.applications, LayerBase); len(diagnostics) != 0 {
+		t.Fatalf("main handoff diagnostics = %#v", diagnostics)
+	}
+	if len(assembly.current.applications) != 4 {
+		t.Fatalf("retained source applications = %d, want 4", len(assembly.current.applications))
+	}
+
+	working := parseCompositionDocument(t, "defaults", "records: [{id: default}]\nindependent: [{id: default}]\n")
+	index := new(mappingIndex)
+	for _, application := range assembly.current.applications {
+		applyHistoryApplication(&working, application, index)
+	}
+	for path, want := range map[string][]string{
+		"/records":     nil,
+		"/independent": nil,
+	} {
+		sequence, present := documentAtPath(working, mustParsePath(t, path))
+		if !present || sequence.Kind != document.Sequence || len(sequence.Sequence) != len(want) {
+			t.Errorf("replayed replacement %s = %#v, want an explicit empty sequence", path, sequence)
+		}
+	}
+
+	// The same identity may appear in separate tiers, while each tier still
+	// retains its individual source applications for explanation replay.
+	if got := []Layer{assembly.current.applications[0].layer, assembly.current.applications[1].layer,
+		assembly.current.applications[2].layer, assembly.current.applications[3].layer}; !reflect.DeepEqual(got, []Layer{LayerUnderlay, LayerUnderlay, LayerUnderlay, LayerBase}) {
+		t.Fatalf("application layers = %#v", got)
+	}
+}
+
+func TestHistoryMergeSupportsSemanticIndexedPrefixes(t *testing.T) {
+	root := parseCompositionDocument(t, "base", "records: [{id: one, helper: {value: old}}]\n")
+	application := historyApplication{
+		path:  Keys("records").Index(0).Key("helper"),
+		value: parseCompositionDocument(t, "helper", "value: new\n"),
+	}
+	applyHistoryApplication(&root, application, new(mappingIndex))
+	value, present := documentAtPath(root, Keys("records").Index(0).Key("helper").Key("value"))
+	if !present || value.Text != "new" {
+		t.Fatalf("indexed helper replay = %#v, %v", value, present)
+	}
+}
+
+func TestProjectedSequenceHistoryAgreesAcrossMetadataAndExplanation(t *testing.T) {
+	policy := sequencePolicy{identityFields: map[string]string{"/records": "id"}}
+	composed, diagnostics := composeDocumentsWithPolicy(policy,
+		parseCompositionDocument(t, "first", "records: [{id: first, token: public}]"),
+		parseCompositionDocument(t, "second", "records: [{id: second, token: private}]"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose diagnostics = %#v", diagnostics)
+	}
+	path := Keys("records").Index(1).Key("token")
+	var metadata Metadata
+	finalizeMetadataPaths(&metadata, composed.root, composed.applications)
+	if !metadata.hasHistoricalPath(path) {
+		t.Fatalf("projected metadata omitted shifted path %s: %#v", path, metadata.historicalPaths)
+	}
+
+	inspection := &Inspection{
+		metadata: metadata, baseline: &document.Node{Kind: document.Mapping},
+		input: composed.root, final: composed.root, applications: composed.applications,
+	}
+	inspection.secrets = classifySecretPaths(nil, func(candidate Path) bool {
+		return reflect.DeepEqual(candidate, path)
+	}, metadata, composed.root, composed.applications)
+	explanation, ok := inspection.Explain(path)
+	if !ok || !explanation.Effective.Redacted {
+		t.Fatalf("shifted explanation = %#v, %v", explanation, ok)
+	}
+	var foundSecondOrigin bool
+	for _, change := range explanation.History {
+		for _, location := range change.Origin.Locations {
+			foundSecondOrigin = foundSecondOrigin || location.SourceID == "second"
+		}
+	}
+	if !foundSecondOrigin {
+		t.Fatalf("shifted history lost second source attribution: %#v", explanation.History)
+	}
+	var rendered bytes.Buffer
+	if err := inspection.WriteExplanation(&rendered, path); err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(rendered.String(), `path: "/records/1/token"`) || strings.Contains(rendered.String(), "private") {
+		t.Fatalf("shifted explanation rendering is incomplete or unsafe: %s", rendered.String())
+	}
+}
+
+func TestReplacedProjectedSensitiveRecordRemainsRedacted(t *testing.T) {
+	policy := sequencePolicy{identityFields: map[string]string{"/records": "id"}}
+	composed, diagnostics := composeDocumentsWithPolicy(policy,
+		parseCompositionDocument(t, "first", "records: [{id: first, token: public}]"),
+		parseCompositionDocument(t, "second", "records: [{id: second, token: PRIVATE-RECORD-CANARY}]"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose diagnostics = %#v", diagnostics)
+	}
+	applications := append([]historyApplication(nil), composed.applications...)
+	applications = append(applications, historyApplication{
+		layer: LayerOverlay,
+		value: parseCompositionDocument(t, "replacement", "records: [{id: replacement, token: visible}]"),
+	})
+	final := applications[len(applications)-1].value
+	path := Keys("records").Index(1).Key("token")
+	var metadata Metadata
+	finalizeMetadataPaths(&metadata, final, applications)
+	inspection := &Inspection{
+		metadata: metadata, baseline: &document.Node{Kind: document.Mapping},
+		input: final, final: final, applications: applications,
+	}
+	inspection.secrets = classifySecretPaths(nil, func(candidate Path) bool {
+		return reflect.DeepEqual(candidate, path)
+	}, metadata, final, applications)
+	explanation, ok := inspection.Explain(path)
+	if !ok || explanation.Effective.Present {
+		t.Fatalf("replaced shifted record explanation = %#v, %v", explanation, ok)
+	}
+	var rendered bytes.Buffer
+	if err := inspection.WriteExplanation(&rendered, path); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(rendered.String(), "PRIVATE-RECORD-CANARY") || !strings.Contains(rendered.String(), "[REDACTED]") {
+		t.Fatalf("replaced sensitive history leaked or was not redacted: %s", rendered.String())
+	}
+}
+
+func mustParsePath(t *testing.T, value string) Path {
+	t.Helper()
+	path, err := ParsePath(value)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return path
+}

internal/engine/imports.go

@@ -51,11 +51,14 @@ type importTraversal struct {
 	rule         *VersionRule
 	versionState *sourceVersionState
 
-	documents []*document.Node
-	sources   []SourceInfo
-	active    map[resourceIdentity]int
-	visited   map[resourceIdentity]importRoute
-	stack     []importRoute
+	scope        acquisitionScope
+	itemScope    bool
+	itemPrefix   Path
+	itemFallback Path
+	sources      []SourceInfo
+	active       map[resourceIdentity]int
+	visited      map[resourceIdentity]importRoute
+	stack        []importRoute
 }
 
 func loadImportedDocuments(source Source, context *FilesystemContext, main parsedSource, rule *VersionRule, limits importLimits) (*composedDocument, []SourceInfo, []Diagnostic) {
@@ -71,16 +74,16 @@ func loadImportedDocumentsTrackedWithVersion(source Source, context *FilesystemC
 	if versionState != nil && versionState.mode != sourceVersionModeDefault {
 		processing = versionState.mainProcessing
 	}
-	return loadImportedDocumentsTrackedWithProcessing(source, context, main, destination, rule, limits, accounting, visited, hostToken, versionState, processing)
+	return loadImportedDocumentsTrackedWithProcessing(source, context, main, destination, rule, limits, accounting, visited, hostToken, versionState, processing, sequencePolicy{})
 }
 
-func loadImportedDocumentsTrackedWithProcessing(source Source, context *FilesystemContext, main parsedSource, destination Path, rule *VersionRule, limits importLimits, accounting *importAccounting, visited map[resourceIdentity]importRoute, hostToken *namespaceToken, versionState *sourceVersionState, processing *sourceVersionProcessingResult) (*composedDocument, []SourceInfo, []Diagnostic) {
+func loadImportedDocumentsTrackedWithProcessing(source Source, context *FilesystemContext, main parsedSource, destination Path, rule *VersionRule, limits importLimits, accounting *importAccounting, visited map[resourceIdentity]importRoute, hostToken *namespaceToken, versionState *sourceVersionState, processing *sourceVersionProcessingResult, policy sequencePolicy) (*composedDocument, []SourceInfo, []Diagnostic) {
 	if nativeContext(source, context) {
 		access, basePath, mainPath, diagnostic := nativeFilesystemAccess(source, context)
 		if diagnostic != nil {
 			return nil, sourceInfos(main.source), []Diagnostic{*diagnostic}
 		}
-		return loadImportedWithAccessTrackedAndProcessing(access, basePath, mainPath, main, destination, rule, limits, accounting, visited, versionState, processing)
+		return loadImportedWithAccessTrackedAndProcessingPolicy(access, basePath, mainPath, main, destination, rule, limits, accounting, visited, versionState, processing, policy)
 	}
 	rootPath, basePath, mainPath, diagnostic := importFilesystem(source, context, main)
 	if diagnostic != nil {
@@ -95,7 +98,7 @@ func loadImportedDocumentsTrackedWithProcessing(source Source, context *Filesyst
 	if hostToken != nil {
 		access.token = hostToken
 	}
-	return loadImportedWithAccessTrackedAndProcessing(access, basePath, mainPath, main, destination, rule, limits, accounting, visited, versionState, processing)
+	return loadImportedWithAccessTrackedAndProcessingPolicy(access, basePath, mainPath, main, destination, rule, limits, accounting, visited, versionState, processing, policy)
 }
 
 func loadImportedWithAccess(access *filesystemAccess, basePath, mainPath string, main parsedSource, rule *VersionRule, limits importLimits, accounting *importAccounting) (*composedDocument, []SourceInfo, []Diagnostic) {
@@ -115,6 +118,10 @@ func loadImportedWithAccessTrackedWithVersion(access *filesystemAccess, basePath
 }
 
 func loadImportedWithAccessTrackedAndProcessing(access *filesystemAccess, basePath, mainPath string, main parsedSource, destination Path, rule *VersionRule, limits importLimits, accounting *importAccounting, visited map[resourceIdentity]importRoute, versionState *sourceVersionState, processing *sourceVersionProcessingResult) (*composedDocument, []SourceInfo, []Diagnostic) {
+	return loadImportedWithAccessTrackedAndProcessingPolicy(access, basePath, mainPath, main, destination, rule, limits, accounting, visited, versionState, processing, sequencePolicy{})
+}
+
+func loadImportedWithAccessTrackedAndProcessingPolicy(access *filesystemAccess, basePath, mainPath string, main parsedSource, destination Path, rule *VersionRule, limits importLimits, accounting *importAccounting, visited map[resourceIdentity]importRoute, versionState *sourceVersionState, processing *sourceVersionProcessingResult, policy sequencePolicy) (*composedDocument, []SourceInfo, []Diagnostic) {
 	if !access.contains(basePath) || (mainPath != "" && !access.contains(mainPath)) {
 		_ = access.close()
 		return nil, sourceInfos(main.source), []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "selected source and import base must stay inside the configuration root", nil)}
@@ -128,6 +135,7 @@ func loadImportedWithAccessTrackedAndProcessing(access *filesystemAccess, basePa
 	}
 	mainInfo := cloneSourceInfo(*main.source)
 	mainInfo.Destination = destination.clone()
+	mainIsItem := main.source.placementMode == placementModeSequenceItem
 	if mainPath != "" {
 		mainInfo.Path = mainPath
 	}
@@ -144,9 +152,16 @@ func loadImportedWithAccessTrackedAndProcessing(access *filesystemAccess, basePa
 	accounting.nodes += countInputNodes(main.document)
 	state := importTraversal{
 		access: access, accounting: accounting, limits: limits, rule: rule, versionState: versionState,
-		documents: make([]*document.Node, 0, 1),
-		sources:   []SourceInfo{mainInfo},
-		active:    make(map[resourceIdentity]int), visited: visited,
+		scope:   acquisitionScope{destination: destination.clone()},
+		sources: []SourceInfo{mainInfo},
+		active:  make(map[resourceIdentity]int), visited: visited,
+	}
+	if mainIsItem {
+		state.itemScope = true
+		state.itemPrefix = destination.clone()
+		state.itemFallback = destination.clone()
+		state.scope.destination = Path{}
+		state.sources[0] = cloneSourceInfo(mainInfo)
 	}
 	if state.visited == nil {
 		state.visited = make(map[resourceIdentity]importRoute)
@@ -174,28 +189,108 @@ func loadImportedWithAccessTrackedAndProcessing(access *filesystemAccess, basePa
 		body, declarations, diagnostics = splitImportDirectives(main.document)
 	}
 	if len(diagnostics) == 0 {
-		_, placed, err := placeDocument(body, destination, body.Location, maxDocumentDepth, maxDocumentNodes)
-		placementNodes := destinationWrapperNodeCount(destination)
-		if err != nil {
-			diagnostics = []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, main.source.ID, "configuration exceeds the source depth or node limit after destination placement")}
-		} else if accounting.nodes+placementNodes > limits.nodes {
-			diagnostics = []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", nil)}
+		if mainIsItem {
+			state.scope.contributions = append(state.scope.contributions, acquiredContribution{
+				source: cloneSourceInfo(mainInfo), payload: body, document: body, processing: processing,
+			})
+			diagnostics = state.walkDeclarations(basePath, declarations, 0, Path{})
 		} else {
-			accounting.nodes += placementNodes
-			state.documents = append(state.documents, placed)
-			diagnostics = state.walkDeclarations(basePath, declarations, 0, destination)
+			_, placed, err := placeDocument(body, destination, body.Location, maxDocumentDepth, maxDocumentNodes)
+			placementNodes := destinationWrapperNodeCount(destination)
+			if err != nil {
+				diagnostics = []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, main.source.ID, "configuration exceeds the source depth or node limit after destination placement")}
+			} else if accounting.nodes+placementNodes > limits.nodes {
+				diagnostics = []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", nil)}
+			} else {
+				accounting.nodes += placementNodes
+				state.scope.contributions = append(state.scope.contributions, acquiredContribution{
+					source: cloneSourceInfo(mainInfo), payload: body, document: placed,
+					destination: destination.clone(), processing: processing,
+				})
+				diagnostics = state.walkDeclarations(basePath, declarations, 0, destination)
+			}
 		}
 	}
 
 	closeErr := access.close()
 	if len(diagnostics) != 0 {
+		if mainIsItem {
+			diagnostics = prependCompositionDiagnosticPaths(diagnostics, destination.Index(0))
+		}
 		return nil, cloneSourceInfos(state.sources), diagnostics
 	}
 	if closeErr != nil {
 		return nil, cloneSourceInfos(state.sources), []Diagnostic{importDiagnostic(CodeImportRead, "configuration root could not be closed", nil)}
 	}
-	composed, diagnostics := composeDocuments(state.documents...)
+	if mainIsItem {
+		localScope := state.scope
+		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))
+		}
+		if versionState != nil {
+			protectionRule := versionState.localRule
+			if versionState.mode == sourceVersionModeDefault && protectionRule != nil {
+				prefix := destination.Index(0)
+				relative, inside := relativeHistoryPath(prefix, protectionRule.Path)
+				if !inside || !matchesPathShape(Path{segments: protectionRule.Path.segments[:len(prefix.segments)]}, prefix) {
+					protectionRule = nil
+				} else {
+					scoped := *protectionRule
+					scoped.Path = relative
+					protectionRule = &scoped
+				}
+			}
+			if diagnostic := importedVersionClaim(documents, completed, protectionRule, 0); diagnostic != nil {
+				if rule != nil {
+					diagnostic.Path = rule.Path.clone()
+				}
+				return nil, cloneSourceInfos(state.sources), []Diagnostic{*diagnostic}
+			}
+		}
+		completed.acquisition = []acquisitionScope{cloneAcquisitionScope(localScope)}
+		projectNestedEvidence(completed, localScope)
+		_, placed, err := placeDocumentWithMode(completed.root, destination, placementModeSequenceItem, body.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")}
+		}
+		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)}
+		}
+		accounting.nodes += placementNodes
+		composed, failures := composeDocumentsWithPolicy(policy, placed)
+		if composed != nil {
+			synthetic := acquisitionScope{contributions: []acquiredContribution{{
+				source: cloneSourceInfo(mainInfo), payload: completed.root, document: placed,
+				destination: destination.clone(), processing: processing, nested: completed,
+			}}}
+			projectNestedEvidence(composed, synthetic)
+			projectAcquiredDestinations(composed, synthetic, state.sources)
+			composed.acquisition = []acquisitionScope{cloneAcquisitionScope(synthetic)}
+			composed.inputBytes = accounting.bytes
+			composed.inputNodes = accounting.nodes
+		}
+		return composed, cloneSourceInfos(state.sources), failures
+	}
+	documents := make([]*document.Node, 0, len(state.scope.contributions))
+	for _, contribution := range state.scope.contributions {
+		documents = append(documents, contribution.document)
+	}
+	composed, diagnostics := composeDocumentsWithPolicy(policy, documents...)
 	if composed != nil {
+		if diagnostic := importedVersionClaim(documents, composed, rule, 0); diagnostic != nil {
+			return nil, cloneSourceInfos(state.sources), []Diagnostic{*diagnostic}
+		}
+	}
+	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
 	}
@@ -237,21 +332,29 @@ func (s *importTraversal) walkDeclarations(basePath string, declarations []impor
 				matched := declaration
 				matched.path = match
 				matched.glob = ""
-				if diagnostics := s.visitPath(basePath, matched, childDestination, depth+1); len(diagnostics) != 0 {
+				if diagnostics := s.visitPath(basePath, matched, childDestination, declaration.destination, depth+1); len(diagnostics) != 0 {
 					return diagnostics
 				}
 			}
 			continue
 		}
-		if diagnostics := s.visitPath(basePath, declaration, childDestination, depth+1); len(diagnostics) != 0 {
+		if diagnostics := s.visitPath(basePath, declaration, childDestination, declaration.destination, depth+1); len(diagnostics) != 0 {
 			return diagnostics
 		}
 	}
 	return nil
 }
 
-func (s *importTraversal) visitPath(basePath string, declaration importDeclaration, destination Path, depth int) []Diagnostic {
+func (s *importTraversal) visitPath(basePath string, declaration importDeclaration, destination, relativeDestination Path, depth int) []Diagnostic {
 	location := declaration.location
+	if declaration.placement == placementModeSequenceItem && len(destination.segments) == 0 {
+		return []Diagnostic{{
+			Code: CodeImportInvalid, Severity: SeverityError,
+			Path:    declaration.placementPath.clone(),
+			Message: "sequence-item placement requires a nonempty destination",
+			Primary: cloneLocation(declaration.placementLocation),
+		}}
+	}
 	if depth > s.limits.depth {
 		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the active import-depth limit", &location)}
 	}
@@ -304,7 +407,14 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 		kind = SourceKindFilesystem
 		label = s.access.label + "/" + canonical
 	}
-	info := SourceInfo{ID: sourceID, Label: label, Path: canonical, Destination: destination.clone(), Layer: LayerBase, Kind: kind, ImportedFrom: &location}
+	info := SourceInfo{ID: sourceID, Label: label, Path: canonical, Destination: s.sourceDestination(destination), Layer: LayerBase, Kind: kind, ImportedFrom: &location}
+	if declaration.placement == placementModeSequenceItem {
+		info.placementMode = placementModeSequenceItem
+	}
+	// A source's own item placement still has an exact sequence-container
+	// destination. Only an unresolved enclosing item index makes the path a
+	// fallback container for this snapshot.
+	info.destinationUnresolved = s.itemScope
 	if kind == SourceKindFilesystem {
 		info.namespace = s.access.token
 	}
@@ -334,11 +444,17 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 	var body *document.Node
 	var declarations []importDeclaration
 	var diagnostics []Diagnostic
+	var processing *sourceVersionProcessingResult
 	if s.versionState != nil && s.versionState.mode != sourceVersionModeDefault {
-		processed, failures := s.versionState.process(root, sourceVersionContribution, destination, sourceID)
+		versionDestination := destination
+		if s.itemScope {
+			versionDestination = appendDestination(s.itemPrefix.Index(0), destination)
+		}
+		processed, failures := s.versionState.process(root, sourceVersionContribution, versionDestination, sourceID, declaration.placement)
 		if len(failures) != 0 {
 			return failures
 		}
+		processing = &processed
 		body, declarations = processed.payload, processed.imports
 	} else {
 		body, declarations, diagnostics = splitImportDirectives(root)
@@ -346,29 +462,35 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 			return diagnostics
 		}
 	}
-	_, placed, err := placeDocument(body, destination, body.Location, maxDocumentDepth, maxDocumentNodes)
+	placementDestination := destination
+	mode := placementModeMapping
+	if declaration.placement == placementModeSequenceItem {
+		mode = placementModeSequenceItem
+	}
+	if mode == placementModeSequenceItem {
+		route := importRoute{canonical: canonical, locations: currentRoute.locations}
+		s.active[identity] = len(s.stack)
+		s.visited[identity] = route
+		s.stack = append(s.stack, route)
+		diagnostics := s.visitItemSource(canonical, declaration, destination, relativeDestination, depth, info, body, declarations, processing)
+		delete(s.active, identity)
+		s.stack = s.stack[:len(s.stack)-1]
+		return diagnostics
+	}
+	_, placed, err := placeDocument(body, placementDestination, body.Location, maxDocumentDepth, maxDocumentNodes)
 	if err != nil {
 		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the source depth or node limit after destination placement", &location)}
 	}
-	placementNodes := destinationWrapperNodeCount(destination)
+	placementNodes := destinationWrapperNodeCount(placementDestination)
 	if s.accounting.nodes+placementNodes > s.limits.nodes {
 		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", &location)}
 	}
 	s.accounting.nodes += placementNodes
-	protectionRule := s.rule
-	if s.versionState != nil && s.versionState.effectiveRule != nil {
-		protectionRule = s.versionState.effectiveRule
-	}
-	if versionNode := fragmentVersionClaim(placed, protectionRule); versionNode != nil {
-		versionLocation := publicLocation(versionNode.Location)
-		return []Diagnostic{{
-			Code: CodeImportVersion, Severity: SeverityError, Path: protectionRule.Path.clone(),
-			Message: "imported fragments cannot contribute to the schema-version path", Primary: &versionLocation,
-		}}
-	}
-
 	route := importRoute{canonical: canonical, locations: currentRoute.locations}
-	s.documents = append(s.documents, placed)
+	s.scope.contributions = append(s.scope.contributions, acquiredContribution{
+		source: cloneSourceInfo(info), payload: body, document: placed,
+		destination: placementDestination.clone(), relativeDestination: relativeDestination.clone(), processing: processing,
+	})
 	s.active[identity] = len(s.stack)
 	s.visited[identity] = route
 	s.stack = append(s.stack, route)
@@ -378,6 +500,280 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 	return diagnostics
 }
 
+func (s *importTraversal) sourceDestination(destination Path) Path {
+	if !s.itemScope {
+		return destination.clone()
+	}
+	// Until the enclosing item receives its assigned index, only the collection
+	// container is exact. Keep the local suffix in the acquisition projection.
+	return s.itemFallback.clone()
+}
+
+func (s *importTraversal) visitItemSource(canonical string, declaration importDeclaration, destination, relativeDestination Path, depth int, info SourceInfo, body *document.Node, declarations []importDeclaration, processing *sourceVersionProcessingResult) []Diagnostic {
+	parentScope := s.scope
+	parentItemScope, parentPrefix, parentFallback := s.itemScope, s.itemPrefix, s.itemFallback
+	s.scope = acquisitionScope{}
+	s.itemScope = true
+	if parentItemScope {
+		s.itemPrefix = appendDestination(parentPrefix, destination)
+	} else {
+		s.itemPrefix = destination.clone()
+		s.itemFallback = destination.clone()
+	}
+	s.scope.contributions = append(s.scope.contributions, acquiredContribution{
+		source: cloneSourceInfo(info), payload: body, document: body,
+		processing: processing,
+	})
+	diagnostics := s.walkDeclarations(s.access.dir(canonical), declarations, depth, Path{})
+	itemScope := s.scope
+	s.scope, s.itemScope, s.itemPrefix, s.itemFallback = parentScope, parentItemScope, parentPrefix, parentFallback
+	if len(diagnostics) != 0 {
+		return projectItemDiagnosticPaths(diagnostics, parentScope, destination)
+	}
+	documents := make([]*document.Node, 0, len(itemScope.contributions))
+	for _, contribution := range itemScope.contributions {
+		documents = append(documents, contribution.document)
+	}
+	completed, diagnostics := composeDocuments(documents...)
+	if len(diagnostics) != 0 {
+		return projectItemDiagnosticPaths(diagnostics, parentScope, destination)
+	}
+	itemScope.destination = Path{}
+	completed.acquisition = []acquisitionScope{cloneAcquisitionScope(itemScope)}
+	projectNestedEvidence(completed, itemScope)
+	pl := destination
+	_, placed, err := placeDocumentWithMode(completed.root, pl, placementModeSequenceItem, body.Location, maxDocumentDepth, maxDocumentNodes)
+	if err != nil {
+		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the source depth or node limit after destination placement", &declaration.location)}
+	}
+	placementNodes := destinationWrapperNodeCountForMode(pl, placementModeSequenceItem)
+	if s.accounting.nodes+placementNodes > s.limits.nodes {
+		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", &declaration.location)}
+	}
+	s.accounting.nodes += placementNodes
+	s.scope.contributions = append(s.scope.contributions, acquiredContribution{
+		source: cloneSourceInfo(info), payload: completed.root, document: placed,
+		destination: pl.clone(), relativeDestination: relativeDestination.clone(), processing: processing,
+		nested: completed,
+	})
+	return nil
+}
+
+func importedVersionClaim(documents []*document.Node, composed *composedDocument, rule *VersionRule, mainIndex int) *Diagnostic {
+	if composed == nil || rule == nil {
+		return nil
+	}
+	ranges := indexSequenceRanges(composed.sequenceRanges)
+	for index, contribution := range documents {
+		if index == mainIndex {
+			continue
+		}
+		if node := fragmentVersionClaimWithIndex(contribution, rule, index, ranges); node != nil {
+			location := publicLocation(node.Location)
+			return &Diagnostic{
+				Code: CodeImportVersion, Severity: SeverityError, Path: rule.Path.clone(),
+				Message: "imported fragments cannot contribute to the schema-version path", Primary: &location,
+			}
+		}
+	}
+	return nil
+}
+
+func prependCompositionDiagnosticPaths(diagnostics []Diagnostic, prefix Path) []Diagnostic {
+	cloned := append([]Diagnostic(nil), diagnostics...)
+	for index := range cloned {
+		if !isCompositionDiagnostic(cloned[index]) {
+			continue
+		}
+		cloned[index].Path = appendDestination(prefix, cloned[index].Path)
+		cloned[index].Primary = cloneLocation(cloned[index].Primary)
+		cloned[index].Related = cloneLocations(cloned[index].Related)
+	}
+	return cloned
+}
+
+func isCompositionDiagnostic(diagnostic Diagnostic) bool {
+	return diagnostic.Code == CodeComposeConflict || diagnostic.Code == CodeComposeInvalid
+}
+
+// On a composition failure only the earlier contributions are needed to locate
+// this item. Project one enclosing scope at a time as traversal unwinds; syntax
+// and access diagnostics keep their original source-local coordinates.
+func projectItemDiagnosticPaths(diagnostics []Diagnostic, parent acquisitionScope, destination Path) []Diagnostic {
+	for _, diagnostic := range diagnostics {
+		if isCompositionDiagnostic(diagnostic) {
+			index := priorSequenceLength([]acquisitionScope{parent}, destination)
+			return prependCompositionDiagnosticPaths(diagnostics, destination.Index(index))
+		}
+	}
+	return diagnostics
+}
+
+// This scan is confined to failed loads, not the successful construction path.
+func priorSequenceLength(scopes []acquisitionScope, path Path) int {
+	length := 0
+	for _, scope := range scopes {
+		for _, contribution := range scope.contributions {
+			if node, present := documentAtPath(contribution.document, path); present && node.Kind == document.Sequence {
+				length += len(node.Sequence)
+			}
+		}
+	}
+	return length
+}
+
+func projectPriorInputDiagnosticPaths(diagnostics []Diagnostic, scopes []acquisitionScope) []Diagnostic {
+	cloned := cloneDiagnostics(diagnostics)
+	for index := range cloned {
+		if !isCompositionDiagnostic(cloned[index]) {
+			continue
+		}
+		for position, segment := range cloned[index].Path.segments {
+			if segment.kind != pathSegmentIndex {
+				continue
+			}
+			container := Path{segments: cloned[index].Path.segments[:position]}
+			cloned[index].Path.segments[position].index += priorSequenceLength(scopes, container)
+			// Registration does not address paths through item indexes. A prior
+			// input shifts the outer collection, not its item-local sequences.
+			break
+		}
+	}
+	return cloned
+}
+
+func projectNestedEvidence(composed *composedDocument, scope acquisitionScope) {
+	if composed == nil {
+		return
+	}
+	applications := composed.applications
+	projectedApplications := make([]historyApplication, 0, len(applications))
+	ranges := indexSequenceRanges(composed.sequenceRanges)
+	for sourceIndex, contribution := range scope.contributions {
+		if contribution.nested == nil {
+			projectedApplications = append(projectedApplications, applications[sourceIndex])
+			continue
+		}
+		start := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}].start
+		prefix := contribution.destination.Index(start)
+		if sourceIndex < len(applications) {
+			outer := applications[sourceIndex]
+			structural := document.Clone(outer.value)
+			if node, found := documentAtPath(structural, contribution.destination); found && node.Kind == document.Sequence && len(node.Sequence) > 0 {
+				// The retained fragment is still a local singleton. Its assigned
+				// outer index belongs to the projection, not this source tree.
+				node.Sequence[0] = &document.Node{Kind: document.Mapping, Location: node.Sequence[0].Location}
+			}
+			outer.value = structural
+			projectedApplications = append(projectedApplications, outer)
+			for _, nested := range contribution.nested.applications {
+				projected := nested
+				projected.path = appendDestination(prefix, nested.path)
+				projected.sequences = cloneSequenceHistoryContributions(nested.sequences)
+				projected.sequencesByPath = indexSequenceHistoryContributions(projected.sequences)
+				projectedApplications = append(projectedApplications, projected)
+			}
+		}
+		projectNestedOrigins(composed.origins, contribution.nested.root, contribution.nested.origins, prefix)
+	}
+	composed.applications = projectedApplications
+}
+
+func projectNestedOrigins(target map[string]Origin, root *document.Node, origins map[string]Origin, prefix Path) {
+	var visit func(*document.Node, Path)
+	visit = func(node *document.Node, path Path) {
+		if node == nil {
+			return
+		}
+		if origin, ok := origins[path.String()]; ok {
+			target[appendDestination(prefix, path).String()] = cloneOrigin(origin)
+		}
+		for _, entry := range node.Mapping {
+			visit(entry.Value, path.Key(entry.Key))
+		}
+		for index, child := range node.Sequence {
+			visit(child, path.Index(index))
+		}
+	}
+	visit(root, Path{})
+}
+
+func projectAcquiredDestinations(composed *composedDocument, scope acquisitionScope, sources []SourceInfo) {
+	byID := make(map[string]int, len(sources))
+	for index := range sources {
+		byID[sources[index].ID] = index
+	}
+	var projectScope func(*composedDocument, acquisitionScope, Path, bool, string)
+	projectScope = func(currentComposition *composedDocument, current acquisitionScope, prefix Path, insideItem bool, parentSourceID string) {
+		ranges := indexSequenceRanges(currentComposition.sequenceRanges)
+		for sourceIndex, contribution := range current.contributions {
+			if contribution.source.ID == parentSourceID {
+				continue
+			}
+			contributionPrefix := prefix
+			contributionInsideItem := insideItem
+			start := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: contribution.destination.String()}].start
+			if contribution.source.placementMode == placementModeSequenceItem {
+				container := appendDestination(contributionPrefix, contribution.destination)
+				contribution.source.Destination = container.clone()
+				contribution.source.destinationUnresolved = false
+				current.contributions[sourceIndex].source = cloneSourceInfo(contribution.source)
+				if position, ok := byID[contribution.source.ID]; ok {
+					sources[position].Destination = container
+					sources[position].destinationUnresolved = false
+				}
+				contributionPrefix = appendDestination(container, Path{}.Index(start))
+				contributionInsideItem = true
+				projectSourceVersionEvidence(contribution.processing, contributionPrefix, nil, sourceIndex)
+			} else if contributionInsideItem {
+				exactDestination := appendDestination(contributionPrefix, contribution.destination)
+				contribution.source.Destination = exactDestination.clone()
+				contribution.source.destinationUnresolved = false
+				current.contributions[sourceIndex].source = cloneSourceInfo(contribution.source)
+				if position, ok := byID[contribution.source.ID]; ok {
+					sources[position].Destination = exactDestination
+					sources[position].destinationUnresolved = false
+				}
+				projectSourceVersionEvidence(contribution.processing, exactDestination, nil, sourceIndex)
+			} else {
+				projectSourceVersionEvidence(contribution.processing, contribution.destination, ranges, sourceIndex)
+			}
+			if contribution.nested != nil && contribution.nested.acquisition != nil {
+				for _, nestedScope := range contribution.nested.acquisition {
+					projectScope(contribution.nested, nestedScope, contributionPrefix, contributionInsideItem, contribution.source.ID)
+				}
+			}
+		}
+	}
+	projectScope(composed, scope, Path{}, false, "")
+}
+
+func projectSourceVersionEvidence(processing *sourceVersionProcessingResult, destination Path, ranges sequenceRangeIndex, sourceIndex int) {
+	if processing != nil && processing.evidence != nil {
+		evidence := processing.evidence
+		evidence.destination = destination.clone()
+		// Rebuild from source-local paths on each handoff: an input can be
+		// composed first on its own and then at a different offset in its tier.
+		evidence.projectedPaths = nil
+		paths := evidence.sensitivityPaths()
+		for index, path := range paths {
+			for position := range path.segments {
+				prefix := Path{segments: path.segments[:position]}
+				assigned, found := ranges[sequenceRangeKey{sourceIndex: sourceIndex, path: prefix.String()}]
+				if !found {
+					continue
+				}
+				item, ok := schemaIndex(path.segments[position])
+				if ok && path.segments[position].kind != pathSegmentKey {
+					path.segments[position] = pathSegment{kind: pathSegmentIndex, index: assigned.start + item}
+				}
+			}
+			paths[index] = path
+		}
+		evidence.projectedPaths = paths
+	}
+}
+
 func (s *importTraversal) readImport(name, sourceID string, optional bool) ([]byte, bool, *Diagnostic) {
 	remaining := s.limits.bytes - s.accounting.bytes
 	limit := min(int64(maxSourceBytes), remaining)

internal/engine/imports_test.go

@@ -9,6 +9,8 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
 )
 
 type importTestConfig struct {
@@ -155,6 +157,8 @@ func TestLoadImportDirectiveValidationAndTemporaryGlobGate(t *testing.T) {
 		{name: "two targets", body: "configkit: {imports: [{path: a.yml, glob: '*.yml'}]}\n", code: CodeImportInvalid},
 		{name: "unknown declaration field", body: "configkit: {imports: [{path: a.yml, extra: true}]}\n", code: CodeImportInvalid},
 		{name: "optional kind", body: "configkit: {imports: [{path: a.yml, optional: 'yes'}]}\n", code: CodeImportInvalid},
+		{name: "placement kind", body: "configkit: {imports: [{path: a.yml, placement: true}]}\n", code: CodeImportInvalid},
+		{name: "placement value", body: "configkit: {imports: [{path: a.yml, placement: item}]}\n", code: CodeImportInvalid},
 		{name: "nested directive", body: "nested: {configkit: {}}\n", code: CodeImportInvalid},
 	}
 	for _, test := range tests {
@@ -166,6 +170,322 @@ func TestLoadImportDirectiveValidationAndTemporaryGlobGate(t *testing.T) {
 	}
 }
 
+func TestLoadItemImportComposesHelpersAndNestedItemsInDeclaringScope(t *testing.T) {
+	t.Parallel()
+	type nested struct {
+		Value string `yaml:"value"`
+	}
+	type record struct {
+		ID       string            `yaml:"id"`
+		Name     string            `yaml:"name"`
+		Label    string            `yaml:"label"`
+		Metadata map[string]string `yaml:"metadata"`
+		Children []nested          `yaml:"children"`
+	}
+	type config struct {
+		Items []record `yaml:"items"`
+	}
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: records/item.yml
+      destination: [items]
+      placement: sequence-item
+`)
+	itemPath := writeImportFile(t, directory, "records/item.yml", `configkit:
+  imports:
+    - path: nested/helper.yml
+    - path: child.yml
+      destination: [children]
+      placement: sequence-item
+name: authored
+`)
+	helperPath := writeImportFile(t, directory, "records/nested/helper.yml", `configkit:
+  imports:
+    - path: more.yml
+      destination: [metadata]
+id: supplied
+label: helper
+`)
+	morePath := writeImportFile(t, directory, "records/nested/more.yml", "flavor: transitive\n")
+	childPath := writeImportFile(t, directory, "records/child.yml", "value: nested\n")
+
+	result, err := testLoad[config](File(mainPath), Options[config]{Imports: true})
+	if err != nil {
+		t.Fatalf("load item import: %v", err)
+	}
+	if len(result.Config.Items) != 1 {
+		t.Fatalf("items = %#v", result.Config.Items)
+	}
+	item := result.Config.Items[0]
+	if item.ID != "supplied" || item.Name != "authored" || item.Label != "helper" || item.Metadata["flavor"] != "transitive" || len(item.Children) != 1 || item.Children[0].Value != "nested" {
+		t.Fatalf("composed item = %#v", item)
+	}
+	if got := result.Sources(); len(got) != 5 || got[1].Path != canonicalTestPath(t, itemPath) || got[2].Path != canonicalTestPath(t, helperPath) || got[3].Path != canonicalTestPath(t, morePath) || got[4].Path != canonicalTestPath(t, childPath) {
+		t.Fatalf("source traversal = %#v", got)
+	}
+	for _, test := range []struct {
+		index int
+		want  string
+	}{
+		{index: 1, want: "/items"},
+		{index: 2, want: "/items/0"},
+		{index: 3, want: "/items/0/metadata"},
+		{index: 4, want: "/items/0/children"},
+	} {
+		if got := result.Sources()[test.index].Destination.String(); got != test.want {
+			t.Errorf("source %d destination = %q, want %q", test.index+1, got, test.want)
+		}
+	}
+	for _, test := range []struct {
+		path     Path
+		sourceID string
+	}{
+		{path: Keys("items").Index(0).Key("id"), sourceID: "source-3"},
+		{path: Keys("items").Index(0).Key("metadata").Key("flavor"), sourceID: "source-4"},
+	} {
+		info, ok := result.Metadata().Lookup(test.path)
+		if !ok || !info.Explicit || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != test.sourceID {
+			t.Errorf("origin at %s = %#v, %v, want source %s", test.path.String(), info, ok, test.sourceID)
+		}
+		foundInHistory := false
+		for _, change := range result.Metadata().History(test.path) {
+			for _, location := range change.Origin.Locations {
+				foundInHistory = foundInHistory || location.SourceID == test.sourceID
+			}
+		}
+		if !foundInHistory {
+			t.Errorf("history at %s does not retain source %s", test.path.String(), test.sourceID)
+		}
+	}
+
+	t.Run("duplicate helper identity remains a mapping conflict", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: item.yml
+      destination: [items]
+      placement: sequence-item
+`)
+		writeImportFile(t, directory, "item.yml", `configkit:
+  imports:
+    - path: helper.yml
+id: authored
+`)
+		writeImportFile(t, directory, "helper.yml", "id: supplied\n")
+		_, err := testLoad[config](File(mainPath), Options[config]{Imports: true})
+		var loadError *LoadError
+		if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 {
+			t.Fatalf("duplicate identity error = %v", err)
+		}
+		diagnostic := loadError.Diagnostics()[0]
+		if diagnostic.Code != CodeComposeConflict || diagnostic.Path.String() != "/items/0/id" || diagnostic.Primary == nil || len(diagnostic.Related) != 1 {
+			t.Fatalf("duplicate identity diagnostic = %#v", diagnostic)
+		}
+	})
+}
+
+func TestLayeredUnderlayItemImportRetainsNestedEvidenceAndDestinations(t *testing.T) {
+	t.Parallel()
+	type nested struct {
+		Value string `yaml:"value"`
+	}
+	type record struct {
+		ID       string            `yaml:"id"`
+		Name     string            `yaml:"name"`
+		Label    string            `yaml:"label"`
+		Metadata map[string]string `yaml:"metadata"`
+		Children []nested          `yaml:"children"`
+	}
+	type config struct {
+		Catalog struct {
+			Entries []record `yaml:"entries"`
+		} `yaml:"catalog"`
+	}
+
+	directory := t.TempDir()
+	itemPath := writeImportFile(t, directory, "records/item.yml", `configkit:
+  imports:
+    - path: nested/helper.yml
+    - path: children/child.yml
+      destination: [children]
+      placement: sequence-item
+name: authored
+`)
+	helperPath := writeImportFile(t, directory, "records/nested/helper.yml", `configkit:
+  imports:
+    - path: more.yml
+      destination: [metadata]
+id: supplied
+label: helper
+`)
+	morePath := writeImportFile(t, directory, "records/nested/more.yml", "flavor: transitive\n")
+	childPath := writeImportFile(t, directory, "records/children/child.yml", "value: nested\n")
+
+	loaded, err := LoadLayered(
+		Input{Source: NoSource()}, testSecretBinding(),
+		LayeredOptions[config]{Underlays: []Underlay{{Inputs: []Input{{
+			Source: File(itemPath), Imports: true, Destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem,
+		}}}}},
+	)
+	if err != nil {
+		if loadError, ok := err.(*LoadError); ok {
+			t.Fatalf("LoadLayered: %v, diagnostics %#v", err, loadError.Diagnostics())
+		}
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	if len(result.Config.Catalog.Entries) != 1 || result.Config.Catalog.Entries[0].ID != "supplied" || result.Config.Catalog.Entries[0].Name != "authored" ||
+		result.Config.Catalog.Entries[0].Label != "helper" || result.Config.Catalog.Entries[0].Metadata["flavor"] != "transitive" ||
+		len(result.Config.Catalog.Entries[0].Children) != 1 || result.Config.Catalog.Entries[0].Children[0].Value != "nested" {
+		t.Fatalf("underlay item = %#v", result.Config.Catalog.Entries)
+	}
+
+	sources := result.Sources()
+	wantSources := []struct {
+		path        string
+		destination string
+	}{
+		{path: itemPath, destination: "/catalog/entries"},
+		{path: helperPath, destination: "/catalog/entries/0"},
+		{path: morePath, destination: "/catalog/entries/0/metadata"},
+		{path: childPath, destination: "/catalog/entries/0/children"},
+	}
+	if len(sources) != len(wantSources) {
+		t.Fatalf("sources = %#v", sources)
+	}
+	for index, want := range wantSources {
+		if sources[index].Path != canonicalTestPath(t, want.path) || sources[index].Destination.String() != want.destination {
+			t.Errorf("source %d = %#v, want path %q and destination %q", index+1, sources[index], want.path, want.destination)
+		}
+	}
+
+	for _, test := range []struct {
+		path     Path
+		sourceID string
+	}{
+		{path: Keys("catalog", "entries").Index(0).Key("id"), sourceID: "source-2"},
+		{path: Keys("catalog", "entries").Index(0).Key("metadata").Key("flavor"), sourceID: "source-3"},
+		{path: Keys("catalog", "entries").Index(0).Key("children").Index(0).Key("value"), sourceID: "source-4"},
+	} {
+		info, ok := result.Metadata().Lookup(test.path)
+		if !ok || !info.Explicit || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != test.sourceID || info.Origin.Layer != LayerUnderlay {
+			t.Errorf("origin at %s = %#v, %v, want %s in underlay", test.path.String(), info, ok, test.sourceID)
+		}
+		foundInHistory := false
+		for _, change := range result.Metadata().History(test.path) {
+			for _, location := range change.Origin.Locations {
+				foundInHistory = foundInHistory || location.SourceID == test.sourceID
+			}
+		}
+		if !foundInHistory {
+			t.Errorf("history at %s does not retain source %s", test.path.String(), test.sourceID)
+		}
+	}
+}
+
+func TestLoadItemGlobKeepsDefaultModeIndependentAndOptionalMissesAbsent(t *testing.T) {
+	t.Parallel()
+	type item struct {
+		Value string `yaml:"value"`
+	}
+	type config struct {
+		Items []item            `yaml:"items"`
+		Other map[string]string `yaml:"other"`
+	}
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - glob: records/*.yml
+      destination: [items]
+      placement: sequence-item
+    - path: absent/*.yml
+      destination: [items]
+      optional: true
+      placement: sequence-item
+    - path: other.yml
+      destination: [other]
+`)
+	emptyPath := writeImportFile(t, directory, "records/empty.yml", "{}\n")
+	writeImportFile(t, directory, "other.yml", "value: mapping-default\n")
+	result, err := testLoad[config](File(mainPath), Options[config]{Imports: true})
+	if err != nil {
+		t.Fatalf("load item glob: %v", err)
+	}
+	if len(result.Config.Items) != 1 || result.Config.Items[0].Value != "" || result.Config.Other["value"] != "mapping-default" {
+		t.Fatalf("config = %#v", result.Config)
+	}
+	sources := result.Sources()
+	if len(sources) != 3 || sources[1].Path != canonicalTestPath(t, emptyPath) || sources[1].Destination.String() != "/items" || sources[1].placementMode != placementModeSequenceItem || sources[2].placementMode != placementModeMapping || sources[2].Destination.String() != "/other" {
+		t.Fatalf("source placement evidence = %#v", sources)
+	}
+}
+
+func TestLoadMainItemPlacementKeepsItsHelpersInTheItem(t *testing.T) {
+	t.Parallel()
+	type item struct {
+		ID    string `yaml:"id"`
+		Value string `yaml:"value"`
+	}
+	type config struct {
+		Items []item `yaml:"items"`
+	}
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", "configkit:\n  imports:\n    - path: helper.yml\nid: main\n")
+	helperPath := writeImportFile(t, directory, "helper.yml", "value: helper\n")
+	result, err := testLoad[config](File(mainPath), Options[config]{
+		Imports: true, destination: Keys("items"), placementMode: placementModeSequenceItem,
+	})
+	if err != nil {
+		t.Fatalf("load main item: %v", err)
+	}
+	if len(result.Config.Items) != 1 || result.Config.Items[0].ID != "main" || result.Config.Items[0].Value != "helper" {
+		t.Fatalf("main item = %#v", result.Config.Items)
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Destination.String() != "/items" || sources[0].placementMode != placementModeSequenceItem || sources[1].Destination.String() != "/items/0" || sources[1].placementMode != placementModeMapping {
+		t.Fatalf("main item sources = %#v", sources)
+	}
+	if sources[1].Path != canonicalTestPath(t, helperPath) {
+		t.Fatalf("helper path = %q", sources[1].Path)
+	}
+}
+
+func TestItemImportCanJoinAnAuthoredListUnderRegisteredComposition(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		Items []map[string]string `yaml:"items"`
+	}
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: item.yml
+      destination: [items]
+      placement: sequence-item
+`)
+	writeImportFile(t, directory, "item.yml", "configkit: {imports: [{path: helper.yml}]}\nname: imported\n")
+	writeImportFile(t, directory, "helper.yml", "id: helper-id\n")
+	input, diagnostics := assembleBaseInput(File(mainPath), Options[config]{Imports: true})
+	if len(diagnostics) != 0 {
+		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)
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose authored and imported lists: %#v", diagnostics)
+	}
+	items, ok := documentAtPath(composed.root, Keys("items"))
+	if !ok || items.Kind != document.Sequence || len(items.Sequence) != 2 {
+		t.Fatalf("composed items = %#v", items)
+	}
+	if id, ok := documentAtPath(composed.root, Keys("items").Index(1).Key("id")); !ok || id.Text != "helper-id" {
+		t.Fatalf("imported helper ID = %#v, %v", id, ok)
+	}
+	if origin := composed.origins[Keys("items").Index(1).Key("id").String()]; len(origin.Locations) != 1 || origin.Locations[0].SourceID != "source-3" {
+		t.Fatalf("registered composition origin = %#v", origin)
+	}
+}
+
 func TestParseImportDestinationUsesLiteralStringSegments(t *testing.T) {
 	t.Parallel()
 
@@ -201,6 +521,19 @@ func TestParseImportDestinationUsesLiteralStringSegments(t *testing.T) {
 	}
 }
 
+func TestParseImportPlacementDefaultsPerDeclaration(t *testing.T) {
+	t.Parallel()
+	declarations := parseImportDeclarationsYAML(t, `configkit:
+  imports:
+    - path: mapping.yml
+    - glob: records/*.yml
+      placement: sequence-item
+`)
+	if len(declarations) != 2 || declarations[0].placement != placementModeMapping || declarations[1].placement != placementModeSequenceItem {
+		t.Fatalf("declaration placements = %#v", declarations)
+	}
+}
+
 func TestParseImportDestinationRejectsInvalidShapesAndMembers(t *testing.T) {
 	t.Parallel()
 

internal/engine/layered.go

@@ -4,10 +4,11 @@ import "os"
 
 // Input selects one source, its destination, and its independent import policy.
 type Input struct {
-	Source      Source
-	Imports     bool
-	Filesystem  *FilesystemContext
-	Destination Path
+	Source        Source
+	Imports       bool
+	Filesystem    *FilesystemContext
+	Destination   Path
+	placementMode placementMode
 }
 
 // Underlay contains strictly composed contributions at one precedence tier.
@@ -36,7 +37,7 @@ func LoadLayered[T any](main Input, secrets SecretBinding, supplied ...LayeredOp
 	request := loadRequest[T]{
 		main: loadInput{
 			source: main.Source, imports: main.Imports, filesystem: main.Filesystem,
-			destination: main.Destination.clone(),
+			destination: main.Destination.clone(), placementMode: main.placementMode,
 		},
 		definition: options.Definition, sourceVersionMode: options.SourceVersionMode,
 		overlay:           options.Overlay,
@@ -49,7 +50,7 @@ func LoadLayered[T any](main Input, secrets SecretBinding, supplied ...LayeredOp
 		for inputIndex, input := range underlay.Inputs {
 			request.underlays[index].inputs[inputIndex] = loadInput{
 				source: input.Source, imports: input.Imports, filesystem: input.Filesystem,
-				destination: input.Destination.clone(),
+				destination: input.Destination.clone(), placementMode: input.placementMode,
 			}
 		}
 	}

internal/engine/load.go

@@ -16,6 +16,8 @@ type Definition[T any] struct {
 	Validate func(*T, *Validation)
 	Version  *VersionRule
 	Secret   func(Path) bool
+
+	sequenceRegistrations []sequenceRegistration
 }
 
 // Options contains the application definition and per-load input selections.
@@ -28,6 +30,9 @@ type Options[T any] struct {
 	Filesystem        *FilesystemContext
 	Overrides         []Override
 	SourceVersionMode sourceVersionMode
+	destination       Path
+	placementMode     placementMode
+	sequencePolicy    sequencePolicy
 }
 
 // FilesystemContext supplies filesystem roots for imports and overlays.
@@ -137,15 +142,19 @@ 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)
+	diagnostics = append(diagnostics, policyDiagnostics...)
 	if len(diagnostics) != 0 {
 		return nil, (&diagnosticAccumulator{diagnostics: diagnostics}).loadError()
 	}
+	definition := options.Definition
+	definition.sequenceRegistrations = nil
 	request := loadRequest[T]{
-		main:       loadInput{source: source, imports: options.Imports, filesystem: options.Filesystem},
-		definition: options.Definition, overlay: options.Overlay,
+		main:       loadInput{source: source, imports: options.Imports, filesystem: options.Filesystem, destination: options.destination.clone(), placementMode: options.placementMode},
+		definition: definition, sequencePolicy: policy, overlay: options.Overlay,
 		overrides: options.Overrides, ordinary: true,
 		sourceVersionMode: options.SourceVersionMode,
-		effectiveVersion:  effectiveSourceVersionRule(options.Definition.Version, Path{}, options.SourceVersionMode),
+		effectiveVersion:  effectiveSourceVersionRuleForPlacement(options.Definition.Version, options.destination, options.SourceVersionMode, options.placementMode),
 	}
 	return executeLoadRequest(request, secrets, lookup)
 }
@@ -153,7 +162,8 @@ func loadWithSecretLookup[T any](source Source, secrets SecretBinding, lookup se
 func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lookup secretLookup) (*Loaded[T], error) {
 	accumulator := diagnosticAccumulator{}
 	accounting := &importAccounting{}
-	mainOptions := Options[T]{Definition: request.definition, Imports: request.main.imports, Filesystem: request.main.filesystem}
+	mainOptions := Options[T]{Definition: request.definition, Imports: request.main.imports, Filesystem: request.main.filesystem, destination: request.main.destination.clone(), placementMode: request.main.placementMode}
+	mainOptions.sequencePolicy = request.sequencePolicy
 	mainOptions.SourceVersionMode = request.sourceVersionMode
 	mainOptions.Definition.Version = request.protectionVersion()
 	mainOptions.Overrides = request.overrides
@@ -197,6 +207,14 @@ func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lo
 			return nil, accumulator.loadError()
 		}
 	}
+	if request.ordinary && input.document != nil {
+		input.composition, diagnostics = compositionForInputWithPolicy(input, request.sequencePolicy)
+		if len(diagnostics) != 0 {
+			accumulator.add(diagnostics...)
+			return nil, accumulator.loadError()
+		}
+		input.document = input.composition.root
+	}
 	if request.ordinary {
 		input, diagnostics = applyInputLayers(request.main.source, mainOptions, input)
 	} else {

internal/engine/load_input.go

@@ -11,12 +11,60 @@ type assembledInput struct {
 	parsed       parsedSource
 	document     *document.Node
 	composition  *composedDocument
+	acquisition  []acquisitionScope
 	inheritance  *inheritanceScope
 	sources      []SourceInfo
 	contexts     map[string]ResourceContext
 	versionState *sourceVersionState
 }
 
+// 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
+	payload             *document.Node
+	document            *document.Node
+	destination         Path
+	relativeDestination Path
+	processing          *sourceVersionProcessingResult
+	nested              *composedDocument
+}
+
+// acquisitionScope is the ordered strict mapping scope produced by one input
+// and its ordinary imports.
+type acquisitionScope struct {
+	destination   Path
+	contributions []acquiredContribution
+}
+
+func cloneAcquisitionScope(scope acquisitionScope) acquisitionScope {
+	cloned := acquisitionScope{destination: scope.destination.clone()}
+	if scope.contributions != nil {
+		cloned.contributions = make([]acquiredContribution, len(scope.contributions))
+		for index, contribution := range scope.contributions {
+			cloned.contributions[index] = acquiredContribution{
+				source: cloneSourceInfo(contribution.source), payload: contribution.payload,
+				document: contribution.document, destination: contribution.destination.clone(),
+				relativeDestination: contribution.relativeDestination.clone(), processing: contribution.processing,
+				nested: cloneComposedDocument(contribution.nested),
+			}
+		}
+	}
+	return cloned
+}
+
+func cloneAcquisitionScopes(scopes []acquisitionScope) []acquisitionScope {
+	if scopes == nil {
+		return nil
+	}
+	cloned := make([]acquisitionScope, len(scopes))
+	for index, scope := range scopes {
+		cloned[index] = cloneAcquisitionScope(scope)
+	}
+	return cloned
+}
+
 func assembleBaseInput[T any](source Source, options Options[T]) (assembledInput, []Diagnostic) {
 	return assembleBaseInputTracked(source, options, Path{}, &importAccounting{}, nil, nil)
 }
@@ -36,6 +84,7 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 	parsed, diagnostics := parseSource(source, "source-"+strconv.Itoa(accounting.sources+1), parseLimits)
 	if parsed.source != nil {
 		parsed.source.Destination = destination.clone()
+		parsed.source.placementMode = options.placementMode
 	}
 	input := assembledInput{parsed: parsed, document: parsed.document, sources: sourceInfos(parsed.source), versionState: versionState}
 	if len(diagnostics) != 0 {
@@ -62,7 +111,7 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		if parsed.source != nil {
 			sourceID = parsed.source.ID
 		}
-		processed, failures := versionState.process(parsed.document, role, destination, sourceID)
+		processed, failures := versionState.process(parsed.document, role, destination, sourceID, options.placementMode)
 		if len(failures) != 0 {
 			return input, failures
 		}
@@ -72,10 +121,11 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		}
 	}
 	if options.Imports {
-		input.composition, input.sources, diagnostics = loadImportedDocumentsTrackedWithProcessing(source, options.Filesystem, parsed, destination, options.Definition.Version, limits, accounting, visited, hostToken, versionState, versionProcessing)
+		input.composition, input.sources, diagnostics = loadImportedDocumentsTrackedWithProcessing(source, options.Filesystem, parsed, destination, options.Definition.Version, limits, accounting, visited, hostToken, versionState, versionProcessing, options.sequencePolicy)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
+		input.acquisition = cloneAcquisitionScopes(input.composition.acquisition)
 		input.document = input.composition.root
 	} else {
 		input.sources = sourceInfos(parsed.source)
@@ -89,13 +139,21 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		}
 	}
 	if !options.Imports && input.document != nil {
-		_, placed, err := placeDocument(input.document, destination, input.document.Location, parseLimits.document.Depth, parseLimits.document.Nodes)
+		_, placed, err := placeDocumentWithMode(input.document, destination, options.placementMode, input.document.Location, parseLimits.document.Depth, parseLimits.document.Nodes)
 		if err != nil {
 			return input, []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, parsed.source.ID, "configuration exceeds the source depth or node limit after destination placement")}
 		}
+		localPayload := input.document
 		input.document = placed
+		input.acquisition = []acquisitionScope{{
+			destination: destination.clone(),
+			contributions: []acquiredContribution{{
+				source: cloneSourceInfo(*parsed.source), payload: localPayload,
+				document: placed, destination: destination.clone(), processing: versionProcessing,
+			}},
+		}}
 		rawNodes := countInputNodes(parsed.document)
-		placementNodes := destinationWrapperNodeCount(destination)
+		placementNodes := destinationWrapperNodeCountForMode(destination, options.placementMode)
 		if accounting.sources+1 > limits.sources || accounting.bytes+parsed.bytes > limits.bytes || accounting.nodes+rawNodes+placementNodes > limits.nodes {
 			return input, []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, parsed.source.ID, "configuration exceeds the aggregate input limit")}
 		}
@@ -138,7 +196,7 @@ func applyInputLayers[T any](source Source, options Options[T], input assembledI
 	if !activeOverlay(options.Overlay) && len(options.Overrides) == 0 {
 		return input, nil
 	}
-	composition, diagnostics := compositionForInput(input)
+	composition, diagnostics := compositionForInputWithPolicy(input, options.sequencePolicy)
 	if len(diagnostics) != 0 {
 		return input, diagnostics
 	}
@@ -173,17 +231,22 @@ func applyInputLayers[T any](source Source, options Options[T], input assembledI
 // compositionForInput initializes one handoff for overlays and CLI, including
 // defaults-only CLI input. 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) {
 	if input.composition != nil {
 		return input.composition, nil
 	}
 	if input.document == nil {
 		return &composedDocument{root: &document.Node{Kind: document.Mapping}, origins: make(map[string]Origin)}, nil
 	}
-	composition, diagnostics := composeDocuments(input.document)
+	composition, diagnostics := composeDocumentsWithPolicy(policy, input.document)
 	if len(diagnostics) != 0 {
 		return nil, diagnostics
 	}
 	composition.inputBytes = input.parsed.bytes
 	composition.inputNodes = countInputNodes(input.document)
+	composition.acquisition = cloneAcquisitionScopes(input.acquisition)
 	return composition, nil
 }

internal/engine/load_request.go

@@ -10,6 +10,7 @@ type loadRequest[T any] struct {
 	main              loadInput
 	underlays         []loadGroup
 	definition        Definition[T]
+	sequencePolicy    sequencePolicy
 	sourceVersionMode sourceVersionMode
 	effectiveVersion  *VersionRule
 	overlay           string
@@ -26,10 +27,11 @@ func (request loadRequest[T]) protectionVersion() *VersionRule {
 }
 
 type loadInput struct {
-	source      Source
-	imports     bool
-	filesystem  *FilesystemContext
-	destination Path
+	source        Source
+	imports       bool
+	filesystem    *FilesystemContext
+	destination   Path
+	placementMode placementMode
 }
 
 type loadGroup struct{ inputs []loadInput }
@@ -37,26 +39,22 @@ type loadGroup struct{ inputs []loadInput }
 func loadGroupedWithSecretLookup[T any](request loadRequest[T], secrets SecretBinding, lookup secretLookup) (*Loaded[T], error) {
 	request.ordinary = false
 	var diagnostics []Diagnostic
-	if normalized, err := normalizeDestination(request.main.destination); err != nil {
-		diagnostics = append(diagnostics, invalidDestinationDiagnostic(request.main.destination))
-	} else {
-		request.main.destination = normalized
-	}
-	request.effectiveVersion = effectiveSourceVersionRule(request.definition.Version, request.main.destination, request.sourceVersionMode)
-	_, mainDiagnostics := selectOptions(request.main.source, []Options[T]{{
-		Definition: request.definition, Imports: request.main.imports, Filesystem: request.main.filesystem,
-		SourceVersionMode: request.sourceVersionMode,
+	policy, policyDiagnostics := compileSequencePolicy(request.definition.sequenceRegistrations)
+	request.sequencePolicy = policy
+	diagnostics = append(diagnostics, policyDiagnostics...)
+	request.definition.sequenceRegistrations = 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,
 	}})
+	request.main.destination = mainOptions.destination
 	diagnostics = append(diagnostics, mainDiagnostics...)
+	request.effectiveVersion = effectiveSourceVersionRuleForPlacement(request.definition.Version, request.main.destination, request.sourceVersionMode, request.main.placementMode)
 	for groupIndex := range request.underlays {
 		for inputIndex := range request.underlays[groupIndex].inputs {
 			input := &request.underlays[groupIndex].inputs[inputIndex]
-			if normalized, err := normalizeDestination(input.destination); err != nil {
-				diagnostics = append(diagnostics, invalidDestinationDiagnostic(input.destination))
-			} else {
-				input.destination = normalized
-			}
-			_, inputDiagnostics := selectOptions(input.source, []Options[T]{{Imports: input.imports, Filesystem: input.filesystem}})
+			inputOptions, inputDiagnostics := selectOptions(input.source, []Options[T]{{Imports: input.imports, Filesystem: input.filesystem, destination: input.destination.clone(), placementMode: input.placementMode}})
+			input.destination = inputOptions.destination
 			diagnostics = append(diagnostics, inputDiagnostics...)
 		}
 	}
@@ -79,7 +77,7 @@ 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))
 	for index, group := range request.underlays {
-		composition, sources, contexts, diagnostics := acquireUnderlayGroupWithVersion(group, index+1, request.protectionVersion(), accounting, input.versionState)
+		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)
 		if len(diagnostics) != 0 {
@@ -101,7 +99,7 @@ func assembleLayeredInput[T any](request loadRequest[T], input assembledInput, a
 		}
 	}
 	if input.document != nil {
-		mainComposition, diagnostics := compositionForInput(input)
+		mainComposition, diagnostics := compositionForInputWithPolicy(input, request.sequencePolicy)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
@@ -153,14 +151,22 @@ func acquireUnderlayGroup(group loadGroup, index int, rule *VersionRule, account
 }
 
 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) {
 	initialBytes, initialNodes := accounting.bytes, accounting.nodes
 	visited := make(map[resourceIdentity]importRoute)
 	hostToken := &namespaceToken{}
-	var documents []*document.Node
+	var scopes []acquisitionScope
 	var sources []SourceInfo
 	var contexts map[string]ResourceContext
 	for _, contribution := range group.inputs {
-		options := Options[struct{}]{Imports: contribution.imports, Filesystem: contribution.filesystem, Definition: Definition[struct{}]{Version: rule}}
+		options := Options[struct{}]{
+			Imports: contribution.imports, Filesystem: contribution.filesystem,
+			Definition: Definition[struct{}]{Version: rule}, destination: contribution.destination.clone(),
+			placementMode: contribution.placementMode, sequencePolicy: policy,
+		}
 		acquired, diagnostics := assembleBaseInputTrackedForRole(contribution.source, options, contribution.destination, accounting, visited, hostToken, versionState, sourceVersionContribution)
 		for _, source := range acquired.sources {
 			source.Layer = LayerUnderlay
@@ -169,35 +175,47 @@ func acquireUnderlayGroupWithVersion(group loadGroup, index int, rule *VersionRu
 		}
 		mergeResourceContexts(&contexts, acquired.contexts)
 		if len(diagnostics) != 0 {
-			return nil, sources, contexts, diagnostics
+			return nil, sources, contexts, projectPriorInputDiagnosticPaths(diagnostics, scopes)
 		}
 		if acquired.document == nil {
 			continue
 		}
-		if node := fragmentVersionClaim(acquired.document, rule); node != nil {
-			location := publicLocation(node.Location)
-			return nil, sources, contexts, []Diagnostic{{
-				Code: CodeUnderlayVersion, Severity: SeverityError, Path: rule.Path.clone(),
-				Message: "underlay inputs cannot contribute to the schema-version path", Primary: &location,
-			}}
-		}
-		if acquired.composition != nil {
-			for _, application := range acquired.composition.applications {
-				documents = append(documents, application.value)
-			}
-		} else {
-			documents = append(documents, acquired.document)
+		scopes = append(scopes, cloneAcquisitionScopes(acquired.acquisition)...)
+	}
+	var documents []*document.Node
+	for _, scope := range scopes {
+		for _, contribution := range scope.contributions {
+			documents = append(documents, contribution.document)
 		}
 	}
 	if len(documents) == 0 {
 		return nil, sources, contexts, nil
 	}
-	composition, diagnostics := composeDocuments(documents...)
+	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 {
+			if node := fragmentVersionClaimWithIndex(document, rule, sourceIndex, ranges); node != nil {
+				location := publicLocation(node.Location)
+				return nil, sources, contexts, []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
+	composition.acquisition = cloneAcquisitionScopes(scopes)
+	var projectionScope acquisitionScope
+	for _, scope := range scopes {
+		projectionScope.contributions = append(projectionScope.contributions, scope.contributions...)
+	}
+	projectNestedEvidence(composition, projectionScope)
+	projectAcquiredDestinations(composition, projectionScope, sources)
 	for pointer, origin := range composition.origins {
 		origin.Layer = LayerUnderlay
 		composition.origins[pointer] = origin

internal/engine/metadata.go

@@ -149,12 +149,9 @@ func finalizeMetadataPaths(m *Metadata, final *document.Node, applications []his
 		walkDocument(tree, Path{}, add)
 	}
 	for _, application := range applications {
-		// Overrides have mapping-only prefixes; parsed tokens are keys in this layer.
-		prefix := Path{}
-		for _, segment := range application.path.segments {
-			prefix = prefix.Key(segment.key)
-		}
-		walkDocument(application.value, prefix, add)
+		walkHistoryApplication(application, func(_ Path, effective Path, _ *document.Node) {
+			add(effective, nil)
+		})
 	}
 	for _, operation := range m.inheritance {
 		if operation.control != nil {

internal/engine/native_filesystem_test.go

@@ -42,6 +42,34 @@ items: {main: main}
 	}
 }
 
+func TestNativeFilesystemItemImportUsesSharedScopedAssembly(t *testing.T) {
+	t.Parallel()
+	type item struct {
+		ID   string `yaml:"id"`
+		Name string `yaml:"name"`
+	}
+	type config struct {
+		Items []item `yaml:"items"`
+	}
+	filesystem := fstest.MapFS{
+		"main.yml":           &fstest.MapFile{Data: []byte("configkit:\n  imports:\n    - path: records/item.yml\n      destination: [items]\n      placement: sequence-item\n")},
+		"records/item.yml":   &fstest.MapFile{Data: []byte("configkit:\n  imports:\n    - path: helper.yml\nid: native\n")},
+		"records/helper.yml": &fstest.MapFile{Data: []byte("name: helper\n")},
+	}
+	namespace := newNativeNamespace("bundle", filesystem)
+	result, err := testLoad[config](filesystemFile(namespace, "main.yml"), Options[config]{Imports: true})
+	if err != nil {
+		t.Fatalf("native item import: %v", err)
+	}
+	if len(result.Config.Items) != 1 || result.Config.Items[0].ID != "native" || result.Config.Items[0].Name != "helper" {
+		t.Fatalf("native item = %#v", result.Config.Items)
+	}
+	sources := result.Sources()
+	if len(sources) != 3 || sources[1].Destination.String() != "/items" || sources[2].Destination.String() != "/items/0" || sources[1].Path != "records/item.yml" || sources[2].Path != "records/helper.yml" {
+		t.Fatalf("native item sources = %#v", sources)
+	}
+}
+
 func TestNativeFilesystemNamespaceAndPathFailures(t *testing.T) {
 	first := fstest.MapFS{"main.yml": &fstest.MapFile{Data: []byte("items: {name: first}\n")}}
 	second := fstest.MapFS{"main.yml": &fstest.MapFile{Data: []byte("items: {name: second}\n")}}

internal/engine/options.go

@@ -9,6 +9,17 @@ func selectOptions[T any](source Source, supplied []Options[T]) (Options[T], []D
 		options = supplied[0]
 	}
 	var diagnostics []Diagnostic
+	if options.placementMode != placementModeMapping && options.placementMode != placementModeSequenceItem {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "input placement mode is invalid"))
+	}
+	if normalized, err := normalizeDestination(options.destination); err != nil {
+		diagnostics = append(diagnostics, invalidDestinationDiagnostic(options.destination))
+	} else {
+		options.destination = normalized
+	}
+	if options.placementMode == placementModeSequenceItem && len(options.destination.segments) == 0 {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "sequence-item placement requires a nonempty destination"))
+	}
 	definition := options.Definition
 	if !validOverlayName(options.Overlay) {
 		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "overlay name must contain only ASCII letters, digits, underscore, or hyphen"))

internal/engine/records.go

@@ -183,16 +183,36 @@ type Location struct {
 // namespace-relative filesystem path; synthetic sources have no Path.
 // ImportedFrom identifies the declaration that included this source.
 type SourceInfo struct {
-	ID            string
-	Label         string
-	Path          string
-	Destination   Path
-	Layer         Layer
-	Kind          SourceKind
-	UnderlayIndex int
-	ImportedFrom  *Location
-	VersionCheck  *SourceVersionCheck
-	namespace     *namespaceToken
+	ID                    string
+	Label                 string
+	Path                  string
+	Destination           Path
+	Layer                 Layer
+	Kind                  SourceKind
+	UnderlayIndex         int
+	ImportedFrom          *Location
+	VersionCheck          *SourceVersionCheck
+	namespace             *namespaceToken
+	placementMode         placementMode
+	destinationUnresolved bool
+}
+
+// SourceInfoPlacementModeForFacade converts private placement evidence to its
+// root-owned numeric representation.
+func SourceInfoPlacementModeForFacade(source SourceInfo) uint8 {
+	return uint8(source.placementMode)
+}
+
+// SourceInfoDestinationUnresolvedForFacade returns the private fallback marker.
+func SourceInfoDestinationUnresolvedForFacade(source SourceInfo) bool {
+	return source.destinationUnresolved
+}
+
+// SourceInfoWithPlacement copies facade-owned source evidence into private fields.
+func SourceInfoWithPlacement(source SourceInfo, mode uint8, unresolved bool) SourceInfo {
+	source.placementMode = placementMode(mode)
+	source.destinationUnresolved = unresolved
+	return source
 }
 
 // SourceVersionCheck is a detached, public-safe record of a source-local

internal/engine/resource_context_test.go

@@ -142,6 +142,7 @@ func TestLoadingClosesNativeFilesAcrossPipelineOutcomes(t *testing.T) {
 		name     string
 		main     string
 		child    string
+		helper   string
 		failRead string
 		options  Options[config]
 		wantErr  bool
@@ -150,6 +151,16 @@ func TestLoadingClosesNativeFilesAcrossPipelineOutcomes(t *testing.T) {
 		{name: "acquisition", main: "value: retained\n", failRead: "main.yml", wantErr: true},
 		{name: "parse", main: "value: [broken\n", wantErr: true},
 		{name: "composition", main: "configkit: {imports: [{path: child.yml}]}\nvalue: first\n", child: "value: second\n", options: Options[config]{Imports: true}, wantErr: true},
+		{
+			name:   "nested item composition",
+			main:   "configkit: {imports: [{path: child.yml, destination: [children], placement: sequence-item}]}\nid: parent\n",
+			child:  "configkit: {imports: [{path: helper.yml}]}\nid: child\nvalue: child\n",
+			helper: "value: helper\n",
+			options: Options[config]{
+				Imports: true, destination: Keys("groups"), placementMode: placementModeSequenceItem,
+			},
+			wantErr: true,
+		},
 		{name: "version", main: "version: wrong\n", options: Options[config]{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version"), Check: func(Version, *Validation) error { return fs.ErrInvalid }}}}, wantErr: true},
 		{name: "validation", main: "value: retained\n", options: Options[config]{Definition: Definition[config]{Validate: func(_ *config, validation *Validation) { validation.Error(Keys("value"), "rejected") }}}, wantErr: true},
 	} {
@@ -158,6 +169,9 @@ func TestLoadingClosesNativeFilesAcrossPipelineOutcomes(t *testing.T) {
 			if test.child != "" {
 				files["child.yml"] = &fstest.MapFile{Data: []byte(test.child)}
 			}
+			if test.helper != "" {
+				files["helper.yml"] = &fstest.MapFile{Data: []byte(test.helper)}
+			}
 			filesystem := &observedFilesystem{files: files, failRead: test.failRead}
 			_, err := testLoad(filesystemFile(newNativeNamespace("bundle", filesystem), "main.yml"), test.options)
 			if (err != nil) != test.wantErr {

internal/engine/secret.go

@@ -1,6 +1,10 @@
 package engine
 
-import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+import (
+	"slices"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
 
 type secretDependency struct {
 	target     Path
@@ -13,18 +17,40 @@ func classifySecretPaths(schema *schemaDescriptor, predicate func(Path) bool, me
 
 func classifySecretPathsWithEvidence(schema *schemaDescriptor, predicate func(Path) bool, metadata Metadata, final *document.Node, applications []historyApplication, evidence []*sourceVersionEvidence, projections ...*document.Node) map[string]bool {
 	paths := make(map[string]Path)
+	candidateShapes := make(map[string][]Path)
+	var candidates []Path
+	addCandidate := func(path Path) {
+		pointer := path.String()
+		for _, prior := range candidateShapes[pointer] {
+			if slices.Equal(path.segments, prior.segments) {
+				return
+			}
+		}
+		cloned := path.clone()
+		candidateShapes[pointer] = append(candidateShapes[pointer], cloned)
+		candidates = append(candidates, cloned)
+	}
 	for _, application := range applications {
-		walkDocument(application.value, application.path, func(path Path, _ *document.Node) {
+		walkHistoryApplication(application, func(_ Path, path Path, _ *document.Node) {
+			if len(application.sequences) != 0 {
+				addCandidate(path)
+			} else if parsed, err := ParsePath(path.String()); err == nil {
+				addCandidate(parsed)
+			}
 			addSecretPath(paths, path.String())
 		})
 	}
 	var dependencies []secretDependency
 	for pointer, history := range metadata.history {
 		addSecretPath(paths, pointer)
+		if path, err := ParsePath(pointer); err == nil {
+			addCandidate(path)
+		}
 		for _, change := range history {
 			paths[change.Path.String()] = change.Path.clone()
 			for _, dependency := range change.Dependencies {
 				paths[dependency.String()] = dependency.clone()
+				addCandidate(dependency)
 				dependencies = append(dependencies, secretDependency{
 					target: change.Path.clone(), dependency: dependency.clone(),
 				})
@@ -36,8 +62,10 @@ func classifySecretPathsWithEvidence(schema *schemaDescriptor, predicate func(Pa
 		if operation.control != nil {
 			control := operation.control.path
 			paths[control.String()] = control.clone()
+			addCandidate(control)
 			for _, contributor := range operation.controlOrigin.contributors {
 				paths[contributor.source.String()] = contributor.source.clone()
+				addCandidate(contributor.source)
 				inherited = append(inherited, secretDependency{target: control.clone(), dependency: contributor.source.clone()})
 			}
 			continue
@@ -57,6 +85,8 @@ func classifySecretPathsWithEvidence(schema *schemaDescriptor, predicate func(Pa
 			destination := appendInheritancePath(operation.transfer.to, relative)
 			paths[pointer] = source
 			paths[destination.String()] = destination.clone()
+			addCandidate(source)
+			addCandidate(destination)
 			inherited = append(inherited, secretDependency{target: destination, dependency: source})
 		}
 	}
@@ -66,6 +96,9 @@ func classifySecretPathsWithEvidence(schema *schemaDescriptor, predicate func(Pa
 		walkDocument(tree, Path{}, func(path Path, node *document.Node) {
 			pointer := path.String()
 			addSecretPath(paths, pointer)
+			if parsed, err := ParsePath(pointer); err == nil {
+				addCandidate(parsed)
+			}
 			if node.Sensitive {
 				secret[pointer] = true
 			}
@@ -83,7 +116,7 @@ func classifySecretPathsWithEvidence(schema *schemaDescriptor, predicate func(Pa
 			secret[path.String()] = true
 		}
 	}
-	for _, path := range paths {
+	for _, path := range candidates {
 		classifyPath(path)
 	}
 	// Classify each original evidence shape before folding results into pointer
@@ -92,13 +125,10 @@ func classifySecretPathsWithEvidence(schema *schemaDescriptor, predicate func(Pa
 		if item == nil {
 			continue
 		}
-		candidate := item.resolvedPath
-		if !candidate.valid() {
-			candidate = item.path
-		}
-		classifyPath(addSecretEvidencePath(paths, item.destination, candidate))
-		for _, ancestor := range item.ancestors {
-			classifyPath(addSecretEvidencePath(paths, item.destination, ancestor.path))
+		for _, resolved := range item.sensitivityPaths() {
+			addSecretPath(paths, resolved.String())
+			addCandidate(resolved)
+			classifyPath(resolved)
 		}
 	}
 	return closeSecretDependencies(paths, secret, dependencies, inherited)

internal/engine/selected_overlay.go

@@ -136,7 +136,7 @@ func loadSelectedOverlayWithAccessAndVersion(access *filesystemAccess, base *ass
 	}
 	overlayPayload := overlayRoot
 	if versionState != nil && versionState.mode != sourceVersionModeDefault {
-		processed, failures := versionState.process(overlayRoot, sourceVersionContribution, Path{}, sourceID)
+		processed, failures := versionState.process(overlayRoot, sourceVersionContribution, Path{}, sourceID, placementModeMapping)
 		if len(failures) != 0 {
 			return sources, failures
 		}

internal/engine/sequence_registration.go

@@ -0,0 +1,54 @@
+package engine
+
+type sequenceRegistration struct {
+	path          Path
+	identityField string
+}
+
+type sequencePolicy struct {
+	identityFields map[string]string
+}
+
+// DefinitionWithSequenceRegistrations attaches facade-owned registrations to a
+// detached engine definition without exposing the registration representation.
+func DefinitionWithSequenceRegistrations[T any](definition Definition[T], paths []Path, identityFields []string) Definition[T] {
+	if paths == nil {
+		definition.sequenceRegistrations = nil
+		return definition
+	}
+	definition.sequenceRegistrations = make([]sequenceRegistration, len(paths))
+	for index := range paths {
+		definition.sequenceRegistrations[index] = sequenceRegistration{
+			path: paths[index].clone(), identityField: identityFields[index],
+		}
+	}
+	return definition
+}
+
+func compileSequencePolicy(registrations []sequenceRegistration) (sequencePolicy, []Diagnostic) {
+	policy := sequencePolicy{identityFields: make(map[string]string, len(registrations))}
+	var diagnostics []Diagnostic
+	for _, registration := range registrations {
+		path, err := normalizeDestination(registration.path)
+		if err != nil {
+			diagnostics = append(diagnostics, Diagnostic{
+				Code: CodeOptionsInvalid, Severity: SeverityError,
+				Path: registration.path.clone(), Message: "sequence registration path must contain only non-reserved mapping keys",
+			})
+			continue
+		}
+
+		key := path.String()
+		if current, ok := policy.identityFields[key]; ok {
+			if current != registration.identityField {
+				diagnostics = append(diagnostics, Diagnostic{
+					Code: CodeOptionsInvalid, Severity: SeverityError,
+					Path: path, Message: "sequence registrations at one path must use the same identity field",
+				})
+			}
+			continue
+		}
+		policy.identityFields[key] = registration.identityField
+	}
+	return policy, canonicalDiagnostics(diagnostics)
+}

internal/engine/sequence_registration_test.go

@@ -0,0 +1,306 @@
+package engine
+
+import (
+	"errors"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+func TestCompileSequencePolicyNormalizesRegistrations(t *testing.T) {
+	parsed, err := ParsePath("/items/0/a~1b")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tests := []struct {
+		name          string
+		registrations []sequenceRegistration
+		want          map[string]string
+		wantError     bool
+	}{
+		{
+			name: "identical registrations coalesce",
+			registrations: []sequenceRegistration{
+				{path: Keys("items"), identityField: "id"},
+				{path: Keys("items"), identityField: "id"},
+			},
+			want: map[string]string{"/items": "id"},
+		},
+		{
+			name: "different identity fields conflict",
+			registrations: []sequenceRegistration{
+				{path: Keys("items"), identityField: "id"},
+				{path: Keys("items"), identityField: "name"},
+			},
+			wantError: true,
+		},
+		{
+			name:          "empty identity field is literal",
+			registrations: []sequenceRegistration{{path: Keys("items"), identityField: ""}},
+			want:          map[string]string{"/items": ""},
+		},
+		{
+			name:          "numeric and escaped tokens remain literal keys",
+			registrations: []sequenceRegistration{{path: parsed, identityField: "id"}},
+			want:          map[string]string{"/items/0/a~1b": "id"},
+		},
+		{
+			name:          "explicit index is invalid",
+			registrations: []sequenceRegistration{{path: Keys("items").Index(0), identityField: "id"}},
+			wantError:     true,
+		},
+		{
+			name: "unused and root registrations are accepted",
+			registrations: []sequenceRegistration{
+				{path: Path{}, identityField: "id"},
+				{path: Keys("unused"), identityField: "name"},
+			},
+			want: map[string]string{"": "id", "/unused": "name"},
+		},
+		{name: "zero option policy", want: map[string]string{}},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			policy, diagnostics := compileSequencePolicy(test.registrations)
+			if (len(diagnostics) != 0) != test.wantError {
+				t.Fatalf("diagnostics = %#v, wantError = %t", diagnostics, test.wantError)
+			}
+			if test.wantError {
+				if diagnostics[0].Code != CodeOptionsInvalid {
+					t.Fatalf("diagnostic code = %q", diagnostics[0].Code)
+				}
+				return
+			}
+			if len(policy.identityFields) != len(test.want) {
+				t.Fatalf("compiled fields = %#v, want %#v", policy.identityFields, test.want)
+			}
+			for path, field := range test.want {
+				if got := policy.identityFields[path]; got != field {
+					t.Errorf("identity field at %q = %q, want %q", path, got, field)
+				}
+			}
+		})
+	}
+}
+
+func TestSequenceRegistrationPreflightDoesNotReadSources(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		Value string `yaml:"value"`
+	}
+
+	ordinaryReader := strings.NewReader("value: ordinary\n")
+	_, err := Load[config](Reader("ordinary.yml", ordinaryReader), SecretBinding{}, Options[config]{
+		Definition: Definition[config]{sequenceRegistrations: []sequenceRegistration{{path: Keys("items").Index(0), identityField: "id"}}},
+	})
+	requireSequenceRegistrationOptionError(t, err)
+	if ordinaryReader.Len() == 0 {
+		t.Fatal("ordinary reader was consumed before invalid registration was rejected")
+	}
+
+	mainReader := strings.NewReader("value: main\n")
+	underlayReader := strings.NewReader("value: underlay\n")
+	_, err = LoadLayered[config](Input{Source: Reader("main.yml", mainReader)}, SecretBinding{}, LayeredOptions[config]{
+		Definition: Definition[config]{sequenceRegistrations: []sequenceRegistration{{path: Keys("items").Index(0), identityField: "id"}}},
+		Underlays:  []Underlay{{Inputs: []Input{{Source: Reader("underlay.yml", underlayReader)}}}},
+	})
+	requireSequenceRegistrationOptionError(t, err)
+	if mainReader.Len() == 0 || underlayReader.Len() == 0 {
+		t.Fatal("layered source was consumed before invalid registration was rejected")
+	}
+}
+
+func requireSequenceRegistrationOptionError(t *testing.T, err error) {
+	t.Helper()
+	if err == nil {
+		t.Fatal("expected invalid sequence registration error")
+	}
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("error type = %T, want *LoadError", err)
+	}
+	diagnostics := loadErr.Diagnostics()
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeOptionsInvalid {
+		t.Fatalf("diagnostics = %#v, want one options diagnostic", diagnostics)
+	}
+}
+
+func TestRegisteredSequencePolicyAcrossLoadingLifecycle(t *testing.T) {
+	t.Parallel()
+	type item struct {
+		ID string `yaml:"id"`
+	}
+	type config struct {
+		Items []item `yaml:"items"`
+	}
+	definition := Definition[config]{sequenceRegistrations: []sequenceRegistration{{path: Keys("items"), identityField: "id"}}}
+	registered := Options[config]{Definition: definition}
+	loadCode := func(t *testing.T, err error) Code {
+		t.Helper()
+		var loadErr *LoadError
+		if !errors.As(err, &loadErr) || len(loadErr.Diagnostics()) == 0 {
+			t.Fatalf("error = %v, want a diagnostic load error", err)
+		}
+		return loadErr.Diagnostics()[0].Code
+	}
+
+	t.Run("ordinary authored list is checked with one source", func(t *testing.T) {
+		if _, err := testLoad[config](Bytes("main", []byte("items: [{id: same}, {id: same}]\n")), registered); err == nil || loadCode(t, err) != CodeComposeConflict {
+			t.Fatalf("duplicate identity error = %v", err)
+		}
+		result, err := testLoad[config](Bytes("main", []byte("items: [{id: one}]\n")), registered)
+		if err != nil || len(result.Config.Items) != 1 || result.Config.Items[0].ID != "one" {
+			t.Fatalf("single-source load = %#v, %v", result, err)
+		}
+	})
+
+	t.Run("ordinary imports retain registered composition", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: fragment.yml}]}\nitems: [{id: authored}]\n")
+		writeImportFile(t, directory, "fragment.yml", "items: [{id: imported}]\n")
+		result, err := testLoad[config](File(mainPath), Options[config]{Imports: true, Definition: definition})
+		if err != nil || len(result.Config.Items) != 2 || result.Config.Items[1].ID != "imported" {
+			t.Fatalf("registered imported list = %#v, %v", result, err)
+		}
+	})
+
+	t.Run("strict underlay group appends while separate tiers reset identities", func(t *testing.T) {
+		request := loadRequest[config]{
+			main: loadInput{source: Bytes("main", []byte("items: [{id: shared}]\n"))},
+			underlays: []loadGroup{{inputs: []loadInput{
+				{source: Bytes("first", []byte("items: [{id: lower}]\n"))},
+				{source: Bytes("second", []byte("items: [{id: other}]\n"))},
+			}}},
+			definition: definition, sequencePolicy: mustCompileSequencePolicy(t, definition.sequenceRegistrations),
+		}
+		result, err := testGroupedLoad(request)
+		if err != nil || len(result.Config.Items) != 1 || result.Config.Items[0].ID != "shared" {
+			t.Fatalf("grouped load = %#v, %v", result, err)
+		}
+
+		request.underlays = []loadGroup{{inputs: []loadInput{
+			{source: Bytes("first", []byte("items: [{id: shared}]\n"))},
+			{source: Bytes("second", []byte("items: [{id: lower}]\n"))},
+		}}}
+		request.main.source = Bytes("main", []byte("items: [{id: shared}]\n"))
+		if _, err := testGroupedLoad(request); err != nil {
+			t.Fatalf("same identity in a separate tier: %v", err)
+		}
+	})
+
+	t.Run("empty registered contribution clears lower sequence", func(t *testing.T) {
+		result, err := testLoad[config](Bytes("main", []byte("items: []\n")), registered)
+		if err != nil || len(result.Config.Items) != 0 {
+			t.Fatalf("empty sequence load = %#v, %v", result, err)
+		}
+		request := loadRequest[config]{
+			main:       loadInput{source: Bytes("main", []byte("items: []\n"))},
+			underlays:  []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte("items: [{id: lower}]\n"))}}}},
+			definition: definition, sequencePolicy: mustCompileSequencePolicy(t, definition.sequenceRegistrations),
+		}
+		result, err = testGroupedLoad(request)
+		if err != nil || len(result.Config.Items) != 0 {
+			t.Fatalf("empty higher tier = %#v, %v", result, err)
+		}
+	})
+
+	t.Run("unregistered list overlap remains atomic", func(t *testing.T) {
+		request := loadRequest[config]{
+			main: loadInput{source: Bytes("main", []byte("items: [{id: main}]\n"))},
+			underlays: []loadGroup{{inputs: []loadInput{
+				{source: Bytes("first", []byte("items: [{id: lower}]\n"))},
+				{source: Bytes("second", []byte("items: [{id: other}]\n"))},
+			}}},
+		}
+		_, err := testGroupedLoad(request)
+		if got := loadCode(t, err); got != CodeComposeConflict {
+			t.Fatalf("unregistered overlap error = %s", got)
+		}
+	})
+
+	t.Run("malformed lower tier fails before precedence replacement", func(t *testing.T) {
+		request := loadRequest[config]{
+			main:       loadInput{source: Bytes("main", []byte("items: [{id: main}]\n"))},
+			underlays:  []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte("items: [{id: same}, {id: same}]\n"))}}}},
+			definition: definition, sequencePolicy: mustCompileSequencePolicy(t, definition.sequenceRegistrations),
+		}
+		if got := loadCode(t, func() error { _, err := testGroupedLoad(request); return err }()); got != CodeComposeConflict {
+			t.Fatalf("hidden lower-tier error = %s", got)
+		}
+
+		overlayFilesystem := NewFilesystem("overlay", fstest.MapFS{
+			"overlays/testing.yml": &fstest.MapFile{Data: []byte("items: [{id: overlay}]\n")},
+		})
+		request.overlay = "testing"
+		request.overlayFilesystem = &FilesystemContext{Root: ".", BaseDir: ".", FS: &overlayFilesystem}
+		if got := loadCode(t, func() error { _, err := testGroupedLoad(request); return err }()); got != CodeComposeConflict {
+			t.Fatalf("selected replacement overlay hid lower-tier error: %s", got)
+		}
+	})
+
+	t.Run("replacement overlay and CLI may introduce duplicates", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "items: [{id: base}]\n")
+		writeImportFile(t, directory, "overlays/testing.yml", "items: [{id: duplicate}, {id: duplicate}]\n")
+		result, err := testLoad[config](File(mainPath), Options[config]{Definition: definition, Overlay: "testing"})
+		if err != nil {
+			t.Fatalf("replacement overlay error code = %s", loadCode(t, err))
+		}
+		if len(result.Config.Items) != 2 {
+			t.Fatalf("replacement overlay = %#v, %v", result, err)
+		}
+
+		result, err = testLoad[config](Bytes("main", []byte("items: [{id: base}]\n")), Options[config]{
+			Definition: definition,
+			Overrides:  []Override{{Path: Keys("items"), Flag: "--items", Value: []any{map[string]any{"id": "duplicate"}, map[string]any{"id": "duplicate"}}}},
+		})
+		if err != nil || len(result.Config.Items) != 2 {
+			t.Fatalf("replacement CLI value = %#v, %v", result, err)
+		}
+
+		resolved := definition
+		resolved.Resolve = func(value *config, resolution *Resolution) error {
+			value.Items[1].ID = value.Items[0].ID
+			resolution.Record(Keys("items"), ChangeNormalization, "registered identities collide")
+			return nil
+		}
+		result, err = testLoad[config](Bytes("main", []byte("items: [{id: first}, {id: second}]\n")), Options[config]{Definition: resolved})
+		if err != nil {
+			t.Fatalf("resolver replacement error code = %s", loadCode(t, err))
+		}
+		if len(result.Config.Items) != 2 {
+			t.Fatalf("resolver replacement = %#v, %v", result, err)
+		}
+	})
+
+	t.Run("application validation can reject replacement duplicates", func(t *testing.T) {
+		checked := definition
+		checked.Validate = func(value *config, validation *Validation) {
+			seen := make(map[string]bool)
+			for index, entry := range value.Items {
+				if seen[entry.ID] {
+					validation.Error(Keys("items").Index(index).Key("id"), "identity must be unique")
+				}
+				seen[entry.ID] = true
+			}
+		}
+		_, err := testLoad[config](Bytes("main", []byte("items: [{id: base}]\n")), Options[config]{
+			Definition: checked,
+			Overrides:  []Override{{Path: Keys("items"), Flag: "--items", Value: []any{map[string]any{"id": "duplicate"}, map[string]any{"id": "duplicate"}}}},
+		})
+		if got := loadCode(t, err); got != CodeValidateError {
+			t.Fatalf("application validation error = %s", got)
+		}
+	})
+}
+
+func mustCompileSequencePolicy(t *testing.T, registrations []sequenceRegistration) sequencePolicy {
+	t.Helper()
+	policy, diagnostics := compileSequencePolicy(registrations)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	return policy
+}

internal/engine/source_version.go

@@ -30,16 +30,17 @@ type sourceVersionAncestor struct {
 }
 
 type sourceVersionEvidence struct {
-	sourceID     string
-	path         Path
-	resolvedPath Path
-	destination  Path
-	version      Version
-	location     *Location
-	ancestors    []sourceVersionAncestor
-	absence      sourceVersionPathAbsence
-	checked      bool
-	redacted     bool
+	sourceID       string
+	path           Path
+	resolvedPath   Path
+	destination    Path
+	projectedPaths []Path
+	version        Version
+	location       *Location
+	ancestors      []sourceVersionAncestor
+	absence        sourceVersionPathAbsence
+	checked        bool
+	redacted       bool
 }
 
 type sourceVersionState struct {
@@ -80,7 +81,7 @@ func sourceVersionCheckForEvidence(evidence *sourceVersionEvidence, sensitivity
 	redacted := evidence.redacted
 	if conservative {
 		redacted = redacted || version.Text != ""
-	} else if sensitivity.isSensitiveAtOrAbove(appendDestination(evidence.destination, path).String()) {
+	} else if sensitivity.isSensitiveAtOrAbove(evidence.sensitivityPaths()[0].String()) {
 		redacted = true
 	}
 	if redacted {
@@ -91,11 +92,33 @@ func sourceVersionCheckForEvidence(evidence *sourceVersionEvidence, sensitivity
 	}
 }
 
-func (state *sourceVersionState) process(root *document.Node, role sourceVersionRole, destination Path, sourceID string) (sourceVersionProcessingResult, []Diagnostic) {
+// sensitivityPaths preserves source-local evidence while describing its assigned
+// application coordinates. Before assembly, the original mount is the fallback.
+func (evidence *sourceVersionEvidence) sensitivityPaths() []Path {
+	if evidence.projectedPaths != nil {
+		return evidence.projectedPaths
+	}
+	local := evidence.resolvedPath
+	if !local.valid() {
+		local = evidence.path
+	}
+	paths := make([]Path, 0, len(evidence.ancestors)+1)
+	paths = append(paths, appendDestination(evidence.destination, local))
+	for _, ancestor := range evidence.ancestors {
+		paths = append(paths, appendDestination(evidence.destination, ancestor.path))
+	}
+	return paths
+}
+
+func (state *sourceVersionState) process(root *document.Node, role sourceVersionRole, destination Path, sourceID string, placement placementMode) (sourceVersionProcessingResult, []Diagnostic) {
 	if state == nil || state.mode == sourceVersionModeDefault {
 		return sourceVersionProcessingResult{}, nil
 	}
-	result := processSourceVersion(root, state.localRule, role, destination, state.mode)
+	projectedDestination := destination.clone()
+	if placement == placementModeSequenceItem {
+		projectedDestination = projectedDestination.Index(0)
+	}
+	result := processSourceVersion(root, state.localRule, role, projectedDestination, state.mode)
 	if result.evidence != nil {
 		result.evidence.sourceID = sourceID
 		state.evidence = append(state.evidence, result.evidence)
@@ -119,6 +142,10 @@ type sourceVersionProcessingResult struct {
 }
 
 func effectiveSourceVersionRule(local *VersionRule, destination Path, mode sourceVersionMode) *VersionRule {
+	return effectiveSourceVersionRuleForPlacement(local, destination, mode, placementModeMapping)
+}
+
+func effectiveSourceVersionRuleForPlacement(local *VersionRule, destination Path, mode sourceVersionMode, placement placementMode) *VersionRule {
 	if local == nil {
 		return nil
 	}
@@ -126,9 +153,13 @@ func effectiveSourceVersionRule(local *VersionRule, destination Path, mode sourc
 		return local
 	}
 	effective := *local
+	prefix := destination.clone()
+	if placement == placementModeSequenceItem {
+		prefix = prefix.Index(0)
+	}
 	effective.Path = Path{
-		segments: append(clonePathSegments(destination.segments, len(local.Path.segments)), local.Path.segments...),
-		invalid:  destination.invalid || local.Path.invalid,
+		segments: append(clonePathSegments(prefix.segments, len(local.Path.segments)), local.Path.segments...),
+		invalid:  prefix.invalid || local.Path.invalid,
 	}
 	return &effective
 }

internal/engine/source_version_test.go

@@ -37,6 +37,40 @@ func TestSourceVersionAbsentPathKeepsUnresolvedSegmentIntent(t *testing.T) {
 	}
 }
 
+func TestFragmentVersionClaimUsesAssignedSequenceProjection(t *testing.T) {
+	t.Parallel()
+
+	fragment := sourceVersionTestDocument(t, "items: [{version: v1}]\n")
+	rangeForPeer := []sequenceRange{{sourceIndex: 1, path: Keys("items"), start: 1, end: 2}}
+	if node := fragmentVersionClaimProjected(fragment, &VersionRule{Path: Keys("items").Index(0).Key("version")}, 1, rangeForPeer); node != nil {
+		t.Fatalf("peer append claimed main index 0: %#v", node)
+	}
+	if node := fragmentVersionClaimProjected(fragment, &VersionRule{Path: Keys("items").Index(1).Key("version")}, 1, rangeForPeer); node == nil || node.Text != "v1" {
+		t.Fatalf("assigned peer index 1 claim = %#v, want its version node", node)
+	}
+	nested := sourceVersionTestDocument(t, "items: [{nested: [{version: nested-v}]}]\n")
+	nestedRanges := []sequenceRange{
+		{sourceIndex: 1, path: Keys("items"), start: 1, end: 2},
+		{sourceIndex: 1, path: Keys("items").Index(0).Key("nested"), start: 2, end: 3},
+	}
+	if node := fragmentVersionClaimProjected(nested, &VersionRule{Path: Keys("items").Index(1).Key("nested").Index(2).Key("version")}, 1, nestedRanges); node == nil || node.Text != "nested-v" {
+		t.Fatalf("nested assigned claim = %#v, want nested version", node)
+	}
+	ancestor := sourceVersionTestDocument(t, "items: {version: replacement}\n")
+	if node := fragmentVersionClaim(ancestor, &VersionRule{Path: Keys("items").Index(0).Key("version")}); node == nil || node.Kind != document.Mapping {
+		t.Fatalf("destructive sequence ancestor claim = %#v, want mapping node", node)
+	}
+
+	effective := effectiveSourceVersionRuleForPlacement(&VersionRule{Path: Keys("version")}, Keys("catalog", "entries"), sourceVersionModeRequired, placementModeSequenceItem)
+	if got, want := effective.Path.String(), "/catalog/entries/0/version"; got != want {
+		t.Fatalf("item effective version path = %q, want %q", got, want)
+	}
+	defaultRule := effectiveSourceVersionRuleForPlacement(&VersionRule{Path: Keys("version")}, Keys("catalog", "entries"), sourceVersionModeDefault, placementModeSequenceItem)
+	if got, want := defaultRule.Path.String(), "/version"; got != want {
+		t.Fatalf("default effective version path = %q, want %q", got, want)
+	}
+}
+
 type sourceVersionMountedService struct {
 	Version string `yaml:"version"`
 }
@@ -473,6 +507,83 @@ func TestSourceVersionLoadChecksMountedMainLocallyAndProtectsEffectivePath(t *te
 	}
 }
 
+func TestSourceVersionItemPlacementKeepsLocalChecksAndProtectsProjectedPath(t *testing.T) {
+	t.Parallel()
+
+	for _, test := range []struct {
+		name string
+		load func(Definition[map[string]any]) error
+	}{
+		{
+			name: "ordinary",
+			load: func(definition Definition[map[string]any]) error {
+				_, err := Load(Mapping("main", map[string]any{"version": "v1", "name": "ordinary"}), testSecretBinding(), Options[map[string]any]{
+					Definition: definition, SourceVersionMode: sourceVersionModeRequired,
+					destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem,
+					Overrides: []Override{{Path: Keys("catalog", "entries"), Value: []any{}, Flag: "--entries"}},
+				})
+				return err
+			},
+		},
+		{
+			name: "layered",
+			load: func(definition Definition[map[string]any]) error {
+				_, err := LoadLayered(Input{Source: Mapping("main", map[string]any{"version": "v1", "name": "layered"}), Destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem}, testSecretBinding(), LayeredOptions[map[string]any]{
+					Definition: definition, SourceVersionMode: sourceVersionModeRequired,
+					Overrides: []Override{{Path: Keys("catalog", "entries"), Value: []any{}, Flag: "--entries"}},
+				})
+				return err
+			},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			checks := 0
+			definition := Definition[map[string]any]{Version: &VersionRule{
+				Path: Keys("version"),
+				Check: func(version Version, _ *Validation) error {
+					checks++
+					if !version.Present || version.Text != "v1" {
+						t.Errorf("source-local version = %#v, want v1", version)
+					}
+					return nil
+				},
+			}}
+			err := test.load(definition)
+			var loadErr *LoadError
+			if !errors.As(err, &loadErr) {
+				t.Fatalf("item placement override error = %T %v, want *LoadError", err, err)
+			}
+			requireSourceVersionDiagnostic(t, loadErr.Diagnostics(), CodeOverrideVersion, "/catalog/entries")
+			if checks != 1 {
+				t.Fatalf("source-version checker called %d times, want once", checks)
+			}
+		})
+	}
+}
+
+func TestSourceVersionItemEvidenceUsesProjectedSensitivityPath(t *testing.T) {
+	t.Parallel()
+
+	const canary = "PROJECTED-VERSION-EVIDENCE-CANARY"
+	secretPath := Keys("catalog", "entries").Index(0).Key("version")
+	loaded, err := Load(Mapping("main", map[string]any{"version": canary}), testSecretBinding(), Options[map[string]any]{
+		Definition: Definition[map[string]any]{
+			Version: &VersionRule{Path: Keys("version"), Check: func(Version, *Validation) error { return nil }},
+			Secret:  func(path Path) bool { return path.String() == secretPath.String() },
+		},
+		SourceVersionMode: sourceVersionModeRequired,
+		destination:       Keys("catalog", "entries"), placementMode: placementModeSequenceItem,
+	})
+	if err != nil {
+		t.Fatalf("item source-version load: %v", err)
+	}
+	sources := loaded.Inspection.Sources()
+	if len(sources) != 1 || sources[0].VersionCheck == nil || !sources[0].VersionCheck.Redacted || sources[0].VersionCheck.Version.Text != "" {
+		t.Fatalf("projected source-version evidence was not redacted: %#v", sources)
+	}
+}
+
 func TestSourceVersionLoadChecksNullAndNoSourceMain(t *testing.T) {
 	t.Parallel()
 

layered.go

@@ -12,8 +12,12 @@ type Input struct {
 	// path or append a relative destination declared in YAML. Its zero value
 	// keeps root placement.
 	Destination Path
-	Imports     bool
-	Filesystem  *FilesystemContext
+	// PlacementMode defaults to mapping placement. Sequence-item placement wraps
+	// the source mapping as one record; unknown values and an empty destination
+	// fail option validation before acquisition.
+	PlacementMode PlacementMode
+	Imports       bool
+	Filesystem    *FilesystemContext
 }
 
 // Underlay contains inputs that must compose strictly within one tier. Tiers
@@ -55,10 +59,11 @@ func LoadLayered[T any](main Input, supplied ...LayeredOptions[T]) (*Result[T],
 }
 
 func toEngineInput(input Input) engine.Input {
-	return engine.Input{
+	result := engine.Input{
 		Source: input.Source.toEngine(), Imports: input.Imports,
 		Filesystem: toEngineFilesystemContext(input.Filesystem), Destination: toEnginePath(input.Destination),
 	}
+	return engine.InputWithPlacement(result, uint8(input.PlacementMode))
 }
 
 func toEngineLayeredOptions[T any](options LayeredOptions[T]) engine.LayeredOptions[T] {

lifecycle.go

@@ -15,6 +15,38 @@ type Definition[T any] struct {
 	Validate func(*T, *Validation)
 	Version  *VersionRule
 	Secret   func(Path) bool
+	// SequenceCompositions registers mapping paths whose sequence contributions
+	// strictly append distinct records by the selected literal identity field.
+	// Invalid or conflicting registrations fail option validation. Repeated input
+	// identities fail strict composition after source and helper processing.
+	// Registration paths are copied when a load begins; definitions are read-only
+	// after being shared.
+	SequenceCompositions []SequenceComposition
+}
+
+// PlacementMode selects how an input mapping is placed at its Destination.
+// The zero value, PlacementModeMapping, preserves ordinary mapping placement.
+// Unknown values and sequence-item placement at the root fail option validation.
+type PlacementMode uint8
+
+const (
+	// PlacementModeMapping places a source mapping at Destination.
+	PlacementModeMapping PlacementMode = iota
+	// PlacementModeSequenceItem wraps a source mapping as one item at Destination.
+	PlacementModeSequenceItem
+)
+
+// SequenceComposition registers a sequence path for strict identity-based
+// composition. Path is copied at load start; IdentityField is one literal key,
+// including the empty string. Invalid or conflicting registrations fail option
+// validation.
+type SequenceComposition struct {
+	// Path selects a literal mapping path; registration paths are copied at load
+	// start. Invalid paths fail option validation; the root path is harmless.
+	Path Path
+	// IdentityField is one literal mapping key and may be empty. Records must
+	// contain a string at this key; malformed records fail strict composition.
+	IdentityField string
 }
 
 // SourceVersionMode controls schema-version declarations across one complete
@@ -44,10 +76,17 @@ const (
 // Imports enables explicit path and glob imports; Overlay selects one fixed overlay;
 // Overrides contains only values explicitly supplied by an application CLI.
 // Enabled source-version modes interpret Definition.Version.Path in each
-// document's local coordinates. Main protection remains at that path prefixed
-// by the main input's Destination.
+// document's local coordinates. Placement projects checks and main protection
+// through the input destination; item placement adds the source's item
+// coordinate to that projection. See docs/config.md for the full contract.
 type Options[T any] struct {
-	Definition        Definition[T]
+	Definition Definition[T]
+	// Destination places the ordinary source beneath literal mapping keys. Its
+	// zero value means the application root. Invalid paths fail before acquisition.
+	Destination Path
+	// PlacementMode defaults to mapping placement; unknown values and root item
+	// placement fail before acquisition.
+	PlacementMode     PlacementMode
 	Imports           bool
 	Overlay           string
 	Filesystem        *FilesystemContext
@@ -261,6 +300,7 @@ func toEngineOptions[T any](options Options[T]) engine.Options[T] {
 		Overlay:           options.Overlay,
 		SourceVersionMode: engine.SourceVersionModeFromFacade(uint8(options.SourceVersionMode)),
 	}
+	result = engine.OptionsWithPlacement(result, toEnginePath(options.Destination.clone()), uint8(options.PlacementMode))
 	if options.Filesystem != nil {
 		result.Filesystem = toEngineFilesystemContext(options.Filesystem)
 	}
@@ -289,6 +329,15 @@ func toEngineFilesystemContext(context *FilesystemContext) *engine.FilesystemCon
 
 func toEngineDefinition[T any](definition Definition[T]) engine.Definition[T] {
 	result := engine.Definition[T]{Defaults: definition.Defaults}
+	if definition.SequenceCompositions != nil {
+		paths := make([]engine.Path, len(definition.SequenceCompositions))
+		identityFields := make([]string, len(definition.SequenceCompositions))
+		for index, registration := range definition.SequenceCompositions {
+			paths[index] = toEnginePath(registration.Path.clone())
+			identityFields[index] = registration.IdentityField
+		}
+		result = engine.DefinitionWithSequenceRegistrations(result, paths, identityFields)
+	}
 	if definition.Resolve != nil {
 		result.Resolve = func(configuration *T, resolution *engine.Resolution) error {
 			return definition.Resolve(configuration, &Resolution{value: resolution})

records.go

@@ -189,14 +189,21 @@ type SourceInfo struct {
 	ID    string
 	Label string
 	Path  string
-	// Destination is the effective mapping path. Imports inherit their declaring
-	// document's path unless they append a YAML destination; overlay and CLI
-	// sources use the root path.
-	Destination   Path
-	Layer         Layer
-	Kind          SourceKind
-	UnderlayIndex int
-	ImportedFrom  *Location
+	// Destination is the effective mapping or sequence-container path. It is a
+	// detached snapshot. A fallback container is marked by DestinationUnresolved.
+	Destination Path
+	// PlacementMode describes how this source was placed. Mapping is the zero
+	// value; CLI and overlay sources report mapping placement unless item placement
+	// was applied to that source.
+	PlacementMode PlacementMode
+	// DestinationUnresolved marks Destination as the nearest resolved enclosing
+	// container when an enclosing item index is not yet assigned. It is false for
+	// exact destinations, including an item source's own sequence container.
+	DestinationUnresolved bool
+	Layer                 Layer
+	Kind                  SourceKind
+	UnderlayIndex         int
+	ImportedFrom          *Location
 	// VersionCheck is sanitized evidence for source-local declaration processing
 	// in an enabled source-version load. It is nil when that processing did not
 	// occur, including default mode and synthetic CLI sources. During callbacks

records_test.go

@@ -1,6 +1,10 @@
 package configkit
 
-import "testing"
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+)
 
 func TestOverlayPublicValues(t *testing.T) {
 	if LayerOverlay != "overlay" {
@@ -23,14 +27,17 @@ func TestOverlayPublicValues(t *testing.T) {
 
 func TestSourceInfoDestinationConversionsAndCopies(t *testing.T) {
 	destination := Keys("collections", "a/b", "0")
-	public := SourceInfo{ID: "source-1", Destination: destination}
+	public := SourceInfo{
+		ID: "source-1", Destination: destination,
+		PlacementMode: PlacementModeSequenceItem, DestinationUnresolved: true,
+	}
 	internal := toEngineSourceInfo(public)
-	if internal.Destination.String() != destination.String() {
-		t.Fatalf("engine destination = %q, want %q", internal.Destination.String(), destination.String())
+	if internal.Destination.String() != destination.String() || engine.SourceInfoPlacementModeForFacade(internal) != uint8(PlacementModeSequenceItem) || !engine.SourceInfoDestinationUnresolvedForFacade(internal) {
+		t.Fatalf("engine source evidence = %#v", internal)
 	}
 	converted := fromEngineSourceInfo(internal)
-	if converted.Destination.String() != destination.String() {
-		t.Fatalf("public destination = %q, want %q", converted.Destination.String(), destination.String())
+	if converted.Destination.String() != destination.String() || converted.PlacementMode != PlacementModeSequenceItem || !converted.DestinationUnresolved {
+		t.Fatalf("public source evidence = %#v", converted)
 	}
 	copy := cloneSourceInfo(converted)
 	copy.Destination = Path{}