Prepare ConfigKit v0.5.0 release

README.md

@@ -5,18 +5,22 @@ An opinionated Go library for applications that use YAML configuration files.
 Linux is release-validated; macOS support is best-effort. See the
 [platform support policy](docs/policy/release.md#platform-support) for details.
 
-ConfigKit's initial loader accepts one strict YAML
-source (or explicit defaults-only input), optionally traverses explicit path and glob
-imports, strictly composes their disjoint definitions, applies typed defaults, and
-can overlay one selected fixed overlay and check an application schema version. It
-returns detached metadata and safe diagnostics.
+ConfigKit's simple `Load` accepts one strict YAML source (or explicit
+defaults-only input). `LoadLayered` additionally combines explicit lower-priority
+source groups with an independently selected main input. Sources can be host
+files, native `fs.FS` files, labeled bytes or readers, or snapshotted mappings.
+Enabled path and glob imports compose strictly within their group; precedence
+between groups combines mappings recursively. Both entry points return a complete,
+validated configuration with detached metadata and safe diagnostics.
 Applications may perform audited defaulting and normalization through a resolver,
 then report domain errors and warnings through read-only semantic validation. Secret
 tags and a path predicate drive deterministic redacted effective YAML. Typed secret
 references resolve environment names before validation, and explicit
 secrets-directory bootstrap is available as a separate startup operation. Explicit
 CLI override records apply above overlays. Value explanation is available through
-detached structured records and deterministic redacted output.
+detached structured records and deterministic redacted output. Built-in
+`extends` relationships let a mapping inherit from a named sibling during the
+same load, retaining explicit presence and source/derivation history.
 
 ```go
 type Config struct {
@@ -33,11 +37,37 @@ func ReadConfig(path string) (Config, error) {
 }
 ```
 
+## Design philosophy
+
+ConfigKit favors one coherent, predictable configuration model over flexibility
+for reproducing each consumer's conventions. Its goal is to let applications
+load once, inspect where final values came from, and distinguish explicit values
+from omissions with minimal configuration bookkeeping. Applications retain their
+domain rules and execution responsibilities.
+
+Loading is synchronous and complete on return. Applications normally load once
+near startup and then use the final configuration and metadata; ConfigKit stays
+inactive until explicitly called again. Applications may initiate additional
+loads as needed. See the [loading lifecycle](docs/config.md#completion-and-resource-lifetime)
+for completion and ownership details.
+
+Within a configuration tier, independent contributions compose strictly: shared
+mappings combine, while overlapping non-mapping definitions are errors. Between
+precedence tiers, mappings combine recursively, retaining omitted lower-priority
+children; other values replace lower-priority values. The main document and its
+imports form one strictly composed tier. Explicit underlay groups sit below it
+and above typed defaults; a selected overlay and explicit CLI overrides sit
+above it. The [configuration contract](docs/config.md#strict-document-composition)
+owns the detailed combination rules and input-specific constraints.
+
+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.
+
 The [configuration contract](docs/config.md) describes implemented behavior. The
-[consumer adoption guide](docs/consumers/README.md) provides maintained Narratio-,
-Notarius-, WeatherReporter-, and Seriatim-shaped examples and migration guidance.
-The [deferred scope](docs/roadmap/deferred.md) records deliberate exclusions. See the
-[development guide](docs/development.md) for repository orientation and the
+[consumer adoption guide](docs/consumers/README.md) provides maintained simple
+native-filesystem, layered, and sibling-inheritance examples alongside the migration shapes.
+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

@@ -43,16 +43,36 @@ 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, Layer: Layer(value.Layer), ImportedFrom: fromEngineLocationPointer(value.ImportedFrom)}
+	return SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: Layer(value.Layer), Kind: SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: fromEngineLocationPointer(value.ImportedFrom)}
 }
 func toEngineSourceInfo(value SourceInfo) engine.SourceInfo {
-	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: engine.Layer(value.Layer), ImportedFrom: toEngineLocationPointer(value.ImportedFrom)}
+	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: engine.Layer(value.Layer), Kind: engine.SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: toEngineLocationPointer(value.ImportedFrom)}
 }
 func fromEngineOrigin(value engine.Origin) Origin {
-	return Origin{Layer: Layer(value.Layer), Locations: fromEngineLocations(value.Locations), Operation: ChangeKind(value.Operation)}
+	result := Origin{Layer: Layer(value.Layer), Locations: fromEngineLocations(value.Locations), Operation: ChangeKind(value.Operation)}
+	if value.Inheritance != nil {
+		result.Inheritance = make([]InheritanceStep, len(value.Inheritance))
+		for index, step := range value.Inheritance {
+			result.Inheritance[index] = InheritanceStep{
+				From: fromEnginePath(step.From), To: fromEnginePath(step.To), Reference: fromEnginePath(step.Reference),
+				Locations: fromEngineLocations(step.Locations),
+			}
+		}
+	}
+	return result
 }
 func toEngineOrigin(value Origin) engine.Origin {
-	return engine.Origin{Layer: engine.Layer(value.Layer), Locations: toEngineLocations(value.Locations), Operation: engine.ChangeKind(value.Operation)}
+	result := engine.Origin{Layer: engine.Layer(value.Layer), Locations: toEngineLocations(value.Locations), Operation: engine.ChangeKind(value.Operation)}
+	if value.Inheritance != nil {
+		result.Inheritance = make([]engine.InheritanceStep, len(value.Inheritance))
+		for index, step := range value.Inheritance {
+			result.Inheritance[index] = engine.InheritanceStep{
+				From: toEnginePath(step.From), To: toEnginePath(step.To), Reference: toEnginePath(step.Reference),
+				Locations: toEngineLocations(step.Locations),
+			}
+		}
+	}
+	return result
 }
 func fromEngineDiagnostic(value engine.Diagnostic) Diagnostic {
 	return Diagnostic{Code: Code(value.Code), Severity: Severity(value.Severity), Path: fromEnginePath(value.Path), Message: value.Message, Primary: fromEngineLocationPointer(value.Primary), Related: fromEngineLocations(value.Related)}

docs/adr/0007-source-backed-underlays.md

@@ -0,0 +1,118 @@
+# ADR 0007: Add Source-Backed Underlays with Consistent Recursive Precedence
+
+## Status
+
+Proposed draft for review. Completed planning and remediation records are
+preserved in Git history at `b604aa1` under `docs/roadmap/`; the
+[configuration contract](../config.md#layered-loading) owns current behavior.
+
+This extends [ADR 0001](0001-separate-composition-and-overlay.md) and preserves
+its distinction between strict composition and intentional overlay.
+
+## Date
+
+2026-09-29
+
+## Context
+
+Configuration-backed defaults provide lower-priority settings that applications
+can override. These sources carry imports, locations, and relative-resource
+contexts. Integrating them into ConfigKit's loading lifecycle gives their values
+the same presence tracking and inspectable provenance as other configuration
+inputs.
+
+The [architecture goals](../policy/architecture.md#success-criteria) prioritize
+one coherent, predictable configuration model with consistent rules across
+source representations and loading interfaces. Consumers should load once,
+receive a complete validated configuration, and inspect its final values' origins.
+Reducing integration code is important subject to those principles.
+
+## Decision
+
+### Add underlays to the existing precedence model
+
+Introduce explicitly supplied source-backed underlays between typed defaults
+and main configuration. The proposed precedence, from lowest to highest, is:
+
+**Typed application defaults < source underlays < strictly composed main
+configuration < selected overlay < explicit CLI overrides.**
+
+Typed defaults remain the Go-value baseline. Underlays supply lower-priority
+configuration with source provenance and, where applicable, imports and resource
+context. This ordering describes value precedence; typed defaults fill fields
+absent from the assembled input under the existing defaulting rules.
+
+Preserve strict composition of the main document and its imports before applying
+the resulting configuration over underlays. Independent contributions within an
+underlay tier also compose strictly. Explicitly ordered tiers express intentional
+precedence. Strict composition reports conflicting contributions within each tier.
+
+Across precedence tiers, reuse the existing recursive mapping-overlay semantics.
+Omitted children retain lower-priority input; existing scalar, sequence, null,
+and cross-kind replacement semantics remain unchanged. Reusing these operations
+keeps the combination rules consistent across the loading model.
+
+### Keep source representation separate from composition semantics
+
+Native `fs.FS` and programmatic inputs join the shared loading model.
+Equivalent inputs using the same
+operation have the same presence and precedence semantics regardless of medium.
+Programmatic values enter directly as labeled configuration sources.
+
+Retain the contributing source's identity and resource context through layering.
+A retained underlay value keeps its originating source's resource context.
+Consumers remain responsible for opening and interpreting their domain resources
+under the supplied filesystem's lifetime and access semantics. Exact
+context-access and input APIs remain roadmap decisions.
+
+### Deliver one final configuration and its inspection metadata
+
+Assemble effective input, then run the typed loading lifecycle once for the load.
+Preserve explicit zero,
+false, empty, and null input versus omission through configuration and metadata
+together.
+
+Extend existing provenance and history through underlays, defaults, and recorded
+application resolution. The returned result includes the completed combination,
+defaulting, and provenance work. Domain resolution and validation
+remain application-owned within the recorded lifecycle, consistent with
+[ADR 0003](0003-preserve-presence-and-resolution-history.md).
+
+Preserve the existing opt-in schema-version ownership, path protections, and
+check timing before precedence
+assembly and typed decoding. The roadmap owns how layered input identifies that
+owner when version checking is configured.
+
+Favor near-linear processing in input, retained history, and output size through
+incremental assembly of load-owned state. This is an engineering objective
+subordinate to clarity and semantic consistency. Preserve the root-owned facade and
+private implementation boundary established by [ADR 0006](0006-public-facade.md).
+
+## Alternatives considered
+
+- **Leave bundled defaults to consumers.** Avoids extending loading, but makes
+  consumers reproduce merging, presence tracking, and provenance reconciliation.
+  It does not meet the goal of obtaining a complete configuration in one load.
+- **Offer recursive or atomic combination per layer.** Makes both behaviors
+  explicit and could serve more consumers, but expands the semantic surface and
+  its interactions with presence and provenance without a demonstrated need that
+  outweighs a single consistent rule. A future concrete requirement may justify
+  reconsideration; this feature does not add the option.
+- **Flatten all documents into last-wins layers.** Simplifies ordering but turns
+  accidental main/import conflicts into overrides, weakening the intentional
+  distinction established by ADR 0001.
+
+## Consequences
+
+Consumers gain source-backed defaults within the shared loading lifecycle.
+Strict contributions and intentional precedence remain distinct, with consistent
+combination rules across source media. Presence and provenance remain
+part of the same loading lifecycle as the final values.
+
+Higher-priority mappings retain omitted lower-priority children, so application
+validation assesses the final combined configuration.
+
+Implementation must integrate source context and underlay history into the shared
+engine with attention to assembly and history-retention costs. Current behavior
+documentation will change when the feature lands. The roadmap retains ownership
+of the remaining API decisions.

docs/adr/0008-complete-on-return-loading.md

@@ -0,0 +1,82 @@
+# ADR 0008: Synchronous, Complete-on-Return Configuration Loading
+
+## Status
+
+Proposed draft for review. The
+[configuration contract](../config.md#completion-and-resource-lifetime) describes
+the implemented loading lifecycle and
+[source resource contexts](../config.md#source-resource-contexts).
+
+## Date
+
+2026-09-29
+
+## Context
+
+Applications need a clear point at which configuration is ready for use.
+Completing configuration processing and inspection metadata together gives
+applications a coherent result whose values and origins agree. A bounded loading
+lifecycle also makes resource ownership and startup failures straightforward to
+reason about.
+
+ConfigKit's [architecture goals](../policy/architecture.md#success-criteria)
+emphasize complete configuration, inspectable provenance, predictable behavior,
+and minimal consumer bookkeeping. Applications own their execution lifecycle and
+the timing of configuration acquisition.
+
+## Decision
+
+Make loading a synchronous, complete-on-return operation. Within each load,
+ConfigKit completes input assembly, defaulting, typed decoding, configured
+application resolution, secret lookup, validation, and snapshotting before
+returning the final configuration and metadata. Reported failures return
+diagnostics and a nil result.
+
+Release all library-owned loading handles before returning on success or failure.
+The resulting configuration and metadata are independently usable, caller-owned
+data with a retained inspection snapshot. Caller-supplied readers and filesystems
+remain under caller ownership and lifetime control.
+
+After the call completes, ConfigKit remains inactive until explicitly invoked
+again. Applications normally load once at or near startup and may initiate
+additional loads when needed. Each load follows the same lifecycle and source
+reuse contracts. Inspection, rendering, and separately requested operations run
+in response to explicit application calls.
+
+For the planned resource-context capability, retain source context as passive
+information or a borrowed caller-supplied filesystem. Subsequent resource access
+acquires its own handles with explicit ownership. This preserves a clear boundary
+between configuration completion and application resource use.
+
+These choices give consumers one completion boundary for configuration and
+provenance, predictable resource lifetimes, and control over when ConfigKit runs.
+Application callbacks remain responsible for their own effects and resources.
+
+## Alternatives considered
+
+- **Background watching or refresh.** Can automate responses to source changes,
+  but adds scheduling, shutdown, publication, and resource-lifetime concerns to
+  the library. Explicit calls keep those responsibilities with the application.
+- **Lazy completion after return.** Can defer work until particular values are
+  used, but moves configuration failures into application execution and makes
+  the readiness of values and metadata harder to establish.
+- **Transactional atomic loading.** Could imply simultaneous observation of all
+  external inputs and rollback of callback effects. Filesystems, environment
+  variables, and application callbacks do not share a transaction mechanism.
+  Synchronous completion provides the required usable-result boundary without
+  promising those external guarantees.
+- **Retain loading handles in results.** Can preserve open filesystem access,
+  but gives configuration results an ongoing resource lifecycle and cleanup
+  obligation. Passive source context supports later explicit resource access
+  while keeping loading resources scoped to the load call.
+
+## Consequences
+
+Applications can treat successful return as the point at which the final
+configuration and its metadata are ready for use. Startup loading remains simple,
+and additional loads have an explicit application-owned trigger.
+
+Configuration inspection operates on retained data. Resource access observes the
+backing resource when explicitly requested and has its own failure and ownership
+boundary. Implementations of new source kinds and layering features preserve the
+same completion boundary, including cleanup on reported load failures.

docs/adr/0009-sibling-inheritance.md

@@ -0,0 +1,76 @@
+# ADR 0009: Resolve Sibling Inheritance After Source Assembly
+
+## Status
+
+Accepted and implemented. The [configuration contract](../config.md#sibling-inheritance)
+owns current behavior. The completed implementation plan is preserved in Git
+history at `83c59ea`.
+
+## Date
+
+2026-09-29
+
+## Context
+
+Named configuration mappings often share settings while retaining individual
+overrides. ConfigKit already distinguishes strict composition of independent
+source contributions from recursive overlay between explicit precedence tiers.
+Doing another untracked merge in applications would lose omission, source
+context, and historical explanation information. A shared structural operation
+must preserve the same presence and provenance model as the rest of loading.
+
+Inheritance also interacts with source precedence, nested namespaces, version
+ownership, and secrets. Resolving it before source assembly would validate
+superseded references and copy values that a later tier should replace. Fully
+resolving nested relationships inside a reusable parent would bind them to the
+wrong sibling scope when copied into a child.
+
+## Decision
+
+Reserve `extends` as a built-in input key on mappings. A string names one exact
+sibling mapping; null cancels the relationship. Complete source acquisition,
+strict composition, selected overlay, and CLI precedence first. Then resolve
+each effective relationship in the parser-independent document tree, before
+typed defaults and decoding. A child recursively overlays its own payload on
+its resolved parent using the existing mapping-combination rules. This creates
+neither a source tier nor a configurable merge strategy.
+
+Resolve enclosing sibling chains before nested relationships. Reuse memoized
+parent templates with unresolved nested instructions; evaluate those instructions
+at each destination. Preserve original source applications separately from
+structural transfer/control evidence. Effective origins carry concrete
+source-to-destination derivation, and inspection projects relevant inheritance
+events between source replay and typed reconciliation. Inherited input counts as
+explicit presence, while direct replacement retains its own effective origin.
+
+The existing main-input version check remains at its original point. Reject
+structural contributions or directive consumption affecting its protected path.
+Classify sensitivity at corresponding source and destination paths and retain
+historical redaction. Secret references remain unresolved input until the
+existing lookup point. Keep all machinery behind the root-owned public facade.
+
+## Alternatives considered
+
+- **Application-owned inheritance after `Load`.** Leaves every consumer to
+  recreate ordering, presence, provenance, error reporting, and redaction after
+  ConfigKit has already declared a result complete.
+- **Resolve each source before precedence assembly.** Makes superseded
+  references active and can copy a parent definition before higher-priority
+  input finishes changing it.
+- **Resolve nested relationships in a cached parent subtree.** Shares more
+  ready-made output but binds nested sibling lookups to the parent's scope,
+  giving incorrect values at a child destination.
+- **Registered collections, configurable keywords, or multiple-parent merge
+  modes.** Increase public setup and semantic branches without a requirement
+  that warrants diverging from one structural rule.
+
+## Consequences
+
+Consumers can load a configuration once and inspect direct, inherited,
+defaulted, and application-derived values through the existing result surfaces.
+`extends` is no longer an ordinary application field, so existing configurations
+using that key as data must migrate. Expanding many children can materialize
+substantially more output than the input contains; projection and retained-history
+budgets bound that work without charging copies as newly acquired sources.
+Applications still own domain validation, entry selection, and runtime resource
+use.

docs/config.md

@@ -12,13 +12,21 @@ ConfigKit exposes opaque sources through these constructors:
 | Constructor | Current contract |
 | --- | --- |
 | `File(path)` | Requires a nonempty path, opens a regular file for each consumption, and closes only the library-owned file handle. Missing, inaccessible, unreadable, directory, and other nonregular inputs fail. |
+| `NewFilesystem(label, filesystem).File(name)` | Borrows a caller-owned `fs.FS` under a safe nonempty display label. `name` must be a non-root slash-relative `fs.ValidPath`. Each load reopens the selected regular file; copies of the `Filesystem` preserve namespace identity. Separate constructors create separate namespaces even for the same label or filesystem. |
 | `Bytes(label, data)` | Requires a nonempty, non-secret diagnostic label and snapshots bytes during construction. Later caller mutation cannot affect the source. |
 | `Reader(label, input)` | Requires a nonempty, non-secret label and non-nil reader. The reader is claimed once across all copies of the source, consumed but never closed, and cannot be reused. |
+| `Mapping(label, values)` | Requires a safe nonempty label and a non-nil root `map[string]any`. Snapshots supported built-in values into an owned mapping document at construction; `{}` is valid. Constructor conversion failures become safe deferred diagnostics. |
 | `NoSource()` | Selects defaults-only loading, creates no source record, and marks no paths explicit. |
 
 The zero `Source` is invalid. Constructor argument failures are retained inside the
 opaque source and become safe structured diagnostics when the source is consumed.
-File sources can observe file changes between consumptions; byte sources cannot.
+File and filesystem sources can observe changes between consumptions; byte and
+mapping sources cannot. Mapping accepts nil, built-in string, boolean, signed or
+unsigned integer, finite float, `[]any`, and `map[string]any` values recursively.
+Typed nil maps and slices represent explicit null; empty collections remain empty.
+Keys are sorted for stable provenance, integer decimal spelling stays exact, and
+cycles or unsupported types are rejected without exposing values. Mapping input
+uses the same directive and version rules as YAML, not CLI override restrictions.
 
 Each source is limited to 8 MiB. A load using imports or an active overlay is limited
 to 64 MiB across all accepted sources, 1,024 accepted sources, 64 active import
@@ -26,6 +34,8 @@ edges, and 1,000,000 input nodes in aggregate. Glob expansion separately permits
 1,000,000 visited directory entries, including nonmatches. Reads request at most the
 remaining allowance plus one byte so excess data is rejected rather than silently
 truncated. Data returned together with a read error is not accepted as partial input.
+For mapping sources, key and string bytes use the same accounting as programmatic
+overrides, and the converted tree uses the same node and depth bounds.
 
 ## Single-source loading
 
@@ -44,13 +54,14 @@ The current lifecycle is:
 ```text
 validate options → parse the selected source → traverse/compose enabled imports
   → build schema → check the optional application version → apply a selected overlay
-  → apply explicit CLI overrides → create and clone defaults → strictly decode
+  → apply explicit CLI overrides → expand sibling inheritance
+  → create and clone defaults → strictly decode
   → recorded application resolution
   → resolve typed secret references → semantic validation → classify secrets
   → snapshot configuration and metadata
 ```
 
-`File`, `Bytes`, and `Reader` each contribute the first base source with deterministic
+`File`, `Filesystem.File`, `Bytes`, `Reader`, and `Mapping` each contribute the first base source with deterministic
 ID `source-1`; enabled imports continue in depth-first order as `source-2` onward.
 `NoSource` skips parsing, supplies an absent version to a configured version checker,
 contributes no source, and continues through defaults, decode, and projection. A
@@ -66,11 +77,166 @@ 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
-`Metadata`, a defensive source list, and defensive nonfatal diagnostics. Mutating
+`Metadata`, a defensive source list, and defensive nonfatal diagnostics. Each source
+record's `Kind` identifies file, filesystem, bytes, reader, mapping, or CLI acquisition
+independently of its precedence `Layer`. Host file paths remain canonical;
+filesystem paths are slash-relative within their namespace; synthetic inputs have
+no path. Mutating
 the returned configuration or inspection records cannot alter ConfigKit's private
 snapshot. ConfigKit performs no logging. `Result.WriteEffective` exposes only the
 frozen redacted projection.
 
+### Completion and resource lifetime
+
+`Load` is synchronous and complete on return. Success means input assembly,
+defaulting, typed decoding, configured application resolution, secret lookup,
+validation, and configuration/metadata snapshotting have completed. A reported
+load failure returns diagnostics and a nil result, as described above.
+
+Before returning on success or failure, ConfigKit closes the filesystem handles
+it opened for loading, including files and directory/root handles. Caller-owned
+filesystems are borrowed, never closed. The returned
+configuration and inspection metadata require no close operation and remain
+usable independently of the source files. A supplied `Reader` remains
+caller-owned and is never closed by ConfigKit.
+
+ConfigKit performs no background work after returning: it does not watch inputs,
+refresh secrets, or mutate published results. Inspection and rendering run only
+when explicitly called and use the retained snapshot. Applications normally load
+once near startup, but may explicitly load again using sources that satisfy their
+documented reuse rules. Additional loads produce independently owned results.
+
+Completion is not a transaction over external state. Files and environment
+variables are observed during processing rather than as one simultaneous external
+snapshot. ConfigKit does not roll back application callback effects or manage work
+or resources created by application callbacks. The explicit secrets-directory
+bootstrap remains a separate operation invoked by the application.
+
+## Layered loading
+
+`LoadLayered[T](main, options...)` accepts zero or one `LayeredOptions[T]` value.
+Its `main` is an `Input` containing a source, import opt-in, and optional
+filesystem context. A zero source is invalid; `Input{Source: NoSource()}` omits
+the main document. Each `Underlay` contains inputs that compose strictly as
+one tier, including each input's enabled imports. Empty groups contribute
+nothing. Underlay groups are listed from lowest to highest priority. Repeating
+a file within one group remains an import/repetition error; the same reusable
+source may be selected again in another tier.
+
+ConfigKit acquires and composes the main input first, constructs the schema,
+and checks the optional application version from the main tier only. Underlay
+roots cannot claim that version path (`underlay.version`); imported fragments
+retain `import.version` protection. It then acquires and composes underlay
+groups in order, recursively combines the groups, and applies the main tier,
+one selected overlay, and explicit CLI overrides. Scalar, sequence, and null
+definitions replace lower values between tiers; mappings combine recursively.
+Syntax and strict-composition errors inside a tier cannot be masked by a
+higher-priority tier. Defaults, typed decoding, resolution, secret lookup,
+validation, and publication run once on the final assembled input.
+
+An active layered overlay requires `LayeredOptions.OverlayFilesystem` explicitly,
+independent of the main source's namespace. Its name and root-relative lookup
+follow the selected-overlay rules below; inactive overlays ignore the context.
+The same path in a different filesystem namespace is not a repeated file.
+Ordinary `Load` continues to infer its selected-overlay context from its source.
+
+Source IDs follow acquisition order—main and its depth-first imports, then each
+underlay tier's inputs and depth-first imports, then overlay and CLI—even though
+history follows precedence order. Source `Layer` describes precedence while
+`Kind` describes acquisition. Underlay records use `LayerUnderlay` and a
+one-based `UnderlayIndex` giving their original tier position; all other records
+have index zero. Explicit empty groups and `NoSource` consume no source ID.
+Byte, source, node, and glob-entry budgets are shared across the load, while
+the existing per-source and import-depth bounds still apply.
+
+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,
+and explicit resource access without a second configuration pass.
+
+### Sibling inheritance
+
+Any input mapping may declare `extends: name` to inherit from the mapping named
+`name` among its immediate siblings. Names are exact keys, including the empty
+string; lookup does not search ancestor or unrelated scopes. The document root
+and a sequence-element mapping have no sibling namespace for their own string
+relationship, though mappings nested inside them can have one. An `extends`
+value of null cancels a relationship without looking up a parent. Other value
+types, missing or non-mapping parents, and cycles fail the load with value-free
+diagnostics.
+
+`extends` is reserved structural input across ordinary and layered loads, every
+source kind, imports, selected overlays, and explicit CLI overrides. It is
+consumed before typed decoding and need not appear in the application's Go
+types. Applications previously using `extends` as an ordinary YAML field must
+rename that key and its field. No collection registration or keyword setup is
+required.
+
+ConfigKit completes strict composition within tiers and precedence assembly
+between tiers before resolving relationships. An omitted `extends` retains a
+lower-priority relationship; `extends: null` cancels it while preserving the
+child's own settings. Superseded references are historical evidence, not active
+relationships to validate. Each child recursively overlays its own settings
+over its resolved parent: explicit zero, false, empty string, sequence, and
+null values replace inherited values, while mappings combine recursively.
+The child retains its key. All effective relationships resolve, including
+entries the application may never select for execution. Partial parent
+definitions undergo normal typed decoding and application-owned validation,
+not a special structural exemption.
+
+Enclosing relationships resolve first. Nested instructions carried by a parent
+are then resolved separately at each destination, so a child's nested sibling
+override can change an inherited nested relationship. Typed defaults and
+application-derived values do not start another inheritance pass. Inherited
+input counts as explicit presence for `Metadata.IsExplicit`,
+`Resolution.IsExplicit`, and absence-based resolver defaults, even for false,
+zero, empty, or null input. `Origin.Inheritance` distinguishes it from a direct
+declaration with concrete source, destination, and controlling directive paths.
+A direct child replacement has its own effective origin; displaced ancestry
+remains inspectable in history. An inherited relative resource keeps the
+declaring source's passive `SourceContext`, not the child's directory.
+
+The optional schema-version checker still runs once against the main input at
+its original point. Inheritance cannot add, replace, or consume the protected
+version path; an unrelated relationship inside an enclosing mapping does not
+invalidate an unchanged version. Expansion uses the decoded projection
+node/depth limits without charging copied values as acquired sources;
+retained evidence uses the existing history-node limit. See the
+[consumer example](consumers/README.md#named-sibling-inheritance) for one-load
+usage.
+
+### Source resource contexts
+
+`Result.SourceContext(id)` and callback-scoped `Resolution.SourceContext(id)`
+return a passive `ResourceContext` for a source with filesystem context, or
+false for unknown and contextless IDs. The result query is nil-safe; the
+resolution query follows the callback's lifetime guard. Queries perform no
+filesystem access. Obtain IDs from exact origin locations rather than guessing
+one context for a composite origin or an application-derived value.
+
+`ResourceContext.FS` is rooted at the source's configured root, and `BaseDir`
+is its slash-relative directory inside that root. This includes imported
+sources, selected overlays, host files even without enabled imports, native
+files, and byte, reader, or mapping inputs with an explicit context. CLI and
+contextless synthetic inputs have no resource context. To open a relative
+resource, join `BaseDir` with the application resource name using slash-path
+operations and open that path through `FS`.
+
+For a resource-valued field, query `result.Metadata().Lookup(fieldPath)`, inspect
+its origin locations, choose a source ID only when the application's policy can
+identify the intended contributor, then call `result.SourceContext(id)`. The
+context belongs to that source even when the field survived from a lower-priority
+underlay. A composite origin has no automatically selected resource base.
+
+Native contexts borrow the caller's filesystem within the configured root.
+Host contexts retain a canonical root path, not an open root handle; each
+resource `Open` checks containment and opens a fresh handle that the returned
+file's `Close` releases. Applications own and close the files they open.
+Resources are application data, not configuration input, and do not consume
+configuration byte or node budgets. The loaded configuration and metadata stay
+usable if source files later change or disappear. Later resource operations
+observe the backing filesystem then and may fail if files move, disappear, or
+a caller-owned filesystem closes. This is not a snapshot or universal sandbox.
+
 ### Application schema versions
 
 `VersionRule` declares a semantic path and non-nil checker. Before defaults or typed
@@ -108,8 +274,9 @@ successful result. Callback panics remain programming failures.
 `Result.Metadata` describes every final canonical path. Queries retain explicit
 key/index distinctions using actual effective and historical container shapes;
 only parsed numeric tokens are resolved contextually. A descendant that never
-existed is not historical merely because an atomic ancestor was replaced. Exact surviving input paths
-are explicit and have a base or overlay origin; omitted paths have a default origin.
+existed is not historical merely because an atomic ancestor was replaced. Exact
+surviving direct or inherited input paths are explicit and retain their source
+layer and derivation; omitted paths have a default origin.
 An explicit parent mapping does not make omitted children explicit. Null state and
 available source coordinates are retained. `History` records the Go-zero or application
 default baseline followed by exact base or overlay definitions, including
@@ -124,8 +291,9 @@ domain ownership rules remain application responsibilities. The method's Go
 documentation in [metadata.go](../metadata.go) defines enumeration and path-ordering
 details.
 
-History is chronological: a path can have a synthetic default record, its exact base
-definition, an overlay replacement, and a recorded resolution operation. A descendant
+History is chronological: a path can have a synthetic default record, its exact
+base definition, an overlay replacement, structural inheritance, and a recorded
+resolution operation. A descendant
 query uses the ancestor's path for a whole-container or whole-sequence replacement;
 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
@@ -141,6 +309,15 @@ operation. Reasons and dependency paths retain first occurrence order with dupli
 removed. Source records remain ordered as selected source, depth-first imports, the
 selected overlay, then synthetic CLI records in supplied order.
 
+Inheritance adds no acquired source or precedence layer. `Origin.Inheritance`
+lists concrete transfers from contributor paths through destinations, with
+each step's controlling `extends` path and location. `History` retains source
+applications at their original paths and projects parent contributions, child
+replacements, and consumed controls at destination paths after source assembly
+and before typed decoding. `HistoricalPaths` includes consumed directives and
+displaced inherited descendants. A control can have history even when `Lookup`
+reports it absent. Queries return detached records.
+
 ### Secret classification and effective rendering
 
 Secret classification combines `configkit:"secret"` field tags with the optional
@@ -156,6 +333,11 @@ Propagation reaches a fixed point, so record/map iteration order does not affect
 result and dependency cycles terminate. A dependency on a classified descendant or
 on a container that contains one is treated conservatively as sensitive. Applications
 must classify secret-producing transformations whose dependencies are not recorded.
+Inherited value and control paths link source and destination classifications
+through chains and nested copies; unrelated sibling fields are not classified
+merely because another field is secret. Displaced historical evidence remains
+redacted. Secret references are copied only as unresolved names; credential
+lookup stays after decoding and application resolution.
 
 `Result.WriteEffective` writes the frozen resolved snapshot as deterministic redacted
 YAML. Mappings sort by decoded key string, sequences retain order, indentation is two
@@ -190,7 +372,10 @@ parser values. `Present=false` with empty YAML means absence or deletion; explic
 null is present and renders as `null`. The history starts with the canonical typed
 application-default or Go-zero baseline, then includes only relevant base sources in
 traversal order, the selected overlay, supplied CLI records, and audited net
-resolution operations. Overlapping parent/child resolution records are rendered
+resolution operations. Structural inheritance events appear after source replay
+and before typed reconciliation, including consumed or canceled `extends` input
+and equal-valued child replacements. Overlapping parent/child resolution records
+are rendered
 as one net subtree event with combined reasons and dependencies, rather than
 pretending that the recorded assignments ran in lexical order. Explicit equal
 writes remain visible. When input replay
@@ -442,8 +627,8 @@ input document.
 `[a-zA-Z0-9_-]+`; they are not paths. The empty name and exact lowercase name
 `default` select no overlay, and `overlays/default.yml` is never loaded implicitly.
 Every other name resolves only to `<configuration-root>/overlays/<name>.yml`, with
-no extension fallback, case normalization, inheritance, or overlay stack. A selected
-missing overlay is an error.
+no extension fallback, case normalization, inheritance between overlay files,
+or overlay stack. A selected missing overlay is an error.
 
 The overlay is a separately policy-validated YAML mapping. It is loaded after the
 base import graph is composed and its schema version is checked, but before defaults
@@ -557,18 +742,25 @@ 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.
 
-The configuration root defaults to a selected file's canonical directory. A file
-source may supply `FilesystemContext.Root` to widen that root but may not supply
-`BaseDir`. Whenever byte and reader sources supply filesystem context, they must
-provide both `BaseDir` and `Root` as absolute existing directories; their diagnostic
-labels, including URI-looking labels, never imply filesystem paths. The base and
-selected file must be inside the canonical root. Nonregular targets are rejected before opening to avoid blocking on FIFOs;
+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
+must provide both `BaseDir` and `Root` as absolute existing directories.
+For `Filesystem.File`, the default root is `.` in its namespace and the base is
+the selected file's directory. An explicit context must use the same `Filesystem`
+namespace and may set `Root` to contain the file and its base; it may not supply
+`BaseDir`. Byte, reader, and mapping inputs with native context provide both `BaseDir` and
+`Root` as slash-relative `fs.ValidPath` directories in `FS` (`.` means its root),
+with the base inside the root. Context paths are checked even when imports and
+overlays are inactive. Diagnostic labels, including URI-looking labels, never
+imply filesystem paths. Nonregular targets are rejected before opening to avoid blocking on FIFOs;
 post-open regular-file checks remain in place under the stable-input assumption.
 ConfigKit canonicalizes symlinks,
 checks containment with relative paths, and opens the canonical root-relative target
 through a library-owned root handle. Contained relative or absolute symlink targets
 are accepted; escapes are rejected. Canonical paths define identity, so hard links at
-distinct paths remain distinct sources.
+distinct paths remain distinct sources. Native filesystems use slash-path operations
+and their own symlink behavior; imports stay in their selected namespace and root.
 
 An active-stack repeat is `import.cycle` and reports the complete cycle's import
 sites. A previously visited canonical path is `import.repeated` and reports both

docs/consumers/README.md

@@ -7,7 +7,7 @@ validation, schema-version policy, CLI parsing, runtime matching and conversion,
 resource checks, remote acquisition, and service initialization.
 
 The maintained [`examples/consumers`](../../examples/consumers) package demonstrates
-four sanitized shapes using only ConfigKit and the Go standard library:
+sanitized shapes using only ConfigKit and the Go standard library:
 
 | Shape | Demonstrated fit | Application code deliberately retained |
 | --- | --- | --- |
@@ -15,6 +15,9 @@ four sanitized shapes using only ConfigKit and the Go standard library:
 | Notarius-like | Canonical object bindings, a string-backed schema version accepting string or integer YAML, strict dynamic options, and `int64` dynamic integers | Catalog selection, allowed option keys, range-checked conversion to runtime `int` |
 | WeatherReporter-like | Defaults-only loading, report maps, canonical module objects, typed option unions, explicit CLI records, and typed secret startup | Report/module compatibility, rejection of explicitly empty domain lists, CLI parsing, and startup ordering |
 | Seriatim-like | Independent strict ordered speaker and autocorrect rule files | First-match and ordered replacement algorithms; no central application configuration file |
+| Bundled file | Simple native `fs.FS` source through ordinary `Load` | Selection and lifetime of the application-owned embedded filesystem |
+| Layered catalog | Embedded defaults with an import, a mapping underlay, an independent host main file, explicit false/zero input, and source-relative resource access | Domain validation and opening/interpreting the template resource |
+| Named presets | One-load sibling inheritance, explicit false, nested destination resolution, and source/derivation inspection | Choosing and executing a preset |
 
 The YAML under [`testdata`](../../examples/consumers/testdata) is loaded by the
 executable examples and ordinary tests. It contains no private application data,
@@ -48,6 +51,43 @@ both without redefining their syntax. ConfigKit overlays are configuration overl
 they do not select or replace an application's LLM, model, pipeline, report, or
 execution profile concepts.
 
+## Named sibling inheritance
+
+[`LoadPresets`](../../examples/consumers/inheritance.go) loads the maintained
+[`presets.yml`](../../examples/consumers/testdata/inheritance/presets.yml) in one
+call. `custom` inherits `enabled: false` from `base`, while its nested
+`settings.primary` changes the inherited `settings.secondary` result at the
+custom destination. The application struct has no `extends` field; ConfigKit
+consumes the relationship before strict typed decoding.
+
+The [executable example](../../examples/consumers/consumers_test.go) reads
+`Metadata.IsExplicit` for the inherited false value, then follows
+`Origin.Inheritance` back to the parent path while `Origin.Locations` identifies
+the declaring source. Applications still choose which preset to execute and
+validate domain meaning. The [configuration contract](../config.md#sibling-inheritance)
+owns exact lookup, precedence, cancellation, and migration rules.
+
+## Native filesystem and layered input
+
+[`LoadBundled`](../../examples/consumers/layered.go) keeps the simple `Load` shape:
+it selects a file in an application-owned embedded filesystem and needs no
+underlay setup. [`LoadCatalog`](../../examples/consumers/layered.go) uses
+`LoadLayered` with an imported embedded-defaults group, a separate
+`Mapping` group, and a host main file supplied by its caller. The groups
+compose strictly within themselves and recursively across precedence boundaries;
+the main file's explicit `false` and `0` replace lower values without being
+mistaken for omissions. Its validator owns allowed modes and resource-name policy.
+
+[`ReadWelcome`](../../examples/consumers/layered.go) follows the effective field's
+origin location to a source ID, queries `Result.SourceContext`, and opens the
+resource relative to that context's `BaseDir`. It does not assume the main file
+owns a retained underlay value. A composite origin needs an application choice;
+ConfigKit does not guess a directory. The context query is passive, while later
+resource reads observe the caller-owned filesystem or a freshly opened host root
+and may fail independently of the completed configuration load. The
+[configuration contract](../config.md#source-resource-contexts) owns exact
+lifetime and precedence rules.
+
 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-
@@ -126,6 +166,8 @@ focused behavioral owner:
 | Source/import/selected-overlay pipeline rules and artifact defaults | Narratio-shaped executable example and fixture |
 | Canonical module objects and dynamic option handling | WeatherReporter- and Notarius-shaped examples and migration tests |
 | `LoadSecretsDir`, bootstrap reporting, typed lookup, CLI explanation | `Example_weatherStartup` |
+| Native filesystem, grouped underlays, programmatic mapping, retained resource context | `Example_nativeFilesystem`, `Example_layeredCatalog`, and their focused consumer tests |
+| Named sibling inheritance, presence, and derivation | `Example_inheritedPresets` and `TestPresetCatalogOneLoadAndDerivation` |
 | Results, metadata, diagnostics, effective output, and writer failures | Focused root-package load, provenance, rendering, and explanation tests |
 
 The package-level import-boundary test covers the consumer examples too: supported

docs/development.md

@@ -80,6 +80,14 @@ and current facade ownership belongs to the [component overview](internal/overvi
 
 ## Baseline validation
 
+The completed filesystem-source and underlay roadmap, original feature request,
+implementation plan, and resolved remediation record are preserved in Git history
+at `b604aa1` under `docs/roadmap/`. Current behavior belongs to the
+[configuration contract](config.md#layered-loading). Built-in sibling
+inheritance is now part of the [configuration contract](config.md#sibling-inheritance).
+The completed request, roadmap, plan, and resolved review findings are preserved
+in Git history at `83c59ea` under `docs/roadmap/`.
+
 The completed overlay rename plan and release-workflow simplification record are
 preserved in Git history at `73ce6e9` under `docs/roadmap/`. Current overlay
 behavior belongs to the [configuration contract](config.md#selected-overlays),
@@ -133,12 +141,18 @@ code changes. Use the component overview to find the corresponding implementatio
 | --- | --- | --- |
 | YAML syntax, diagnostics, parser limits | [Adapter tests](../internal/yaml/parse_test.go), [source translation](../internal/engine/parse_test.go) | `go test ./internal/yaml` and `go test ./internal/engine -run '^TestParseSource'` |
 | File/reader access, imports, globs, containment | [Sources](../internal/engine/source_test.go), [FIFO](../internal/engine/source_fifo_test.go), [imports](../internal/engine/imports_test.go), [globs](../internal/engine/glob_test.go) | `go test ./internal/engine -run 'Source\|Import\|Glob\|Filesystem\|Nonregular\|RegularFile'` |
+| Native filesystem and mapping sources | [Native acquisition](../internal/engine/native_filesystem_test.go), [filesystem facade](../facade_filesystem_test.go), [mapping facade](../facade_mapping_test.go) | `go test ./internal/engine -run 'NativeFilesystem'` and `go test . -run 'TestFacade'` |
 | Composition, selected overlays, CLI precedence | [Composition](../internal/engine/compose_test.go), [overlay](../internal/engine/overlay_test.go), [selected overlays](../internal/engine/selected_overlay_test.go), [overrides](../internal/engine/override_test.go) | `go test ./internal/engine -run 'Compose\|Overlay\|Override'` |
+| Sibling inheritance, provenance, history, sensitivity | [Expansion](../internal/engine/inheritance_test.go), [destination scopes](../internal/engine/inheritance_walk_test.go), [origins](../internal/engine/inheritance_origin_test.go), [history](../internal/engine/inheritance_history_test.go), [secrets](../internal/engine/inheritance_secret_test.go), [public loading](../facade_inheritance_test.go) | `go test ./internal/engine -run 'Inheritance\|Inherited'` and `go test . -run '^TestFacadeInheritance\|^TestFacadeInheritedSecrets'` |
+| Layered loading and source equivalence | [Grouped lifecycle](../internal/engine/load_request_test.go), [layered facade](../facade_layered_test.go), [equivalence](../facade_equivalence_test.go) | `go test ./internal/engine -run '^TestGrouped'`, `go test . -run '^TestFacadeLayered'`, and `go test . -run '^TestEquivalentSourceForms'` |
 | Schema, defaults, projection, cloning | [Schema](../internal/engine/schema_test.go), [decoding](../internal/engine/decode_test.go), [projection](../internal/engine/projection_test.go) | `go test ./internal/engine -run 'Schema\|Decode\|Projection\|Clone\|Inline\|CompareDocument'` |
 | Lifecycle, resolution, validation, callback ownership | [Loading](../internal/engine/load_test.go), [resolution](../internal/engine/resolution_test.go), [validation](../internal/engine/validation_test.go), [ownership](../internal/engine/callback_ownership_test.go), [independence](../internal/engine/independence_test.go) | `go test ./internal/engine -run 'Load\|Resolution\|Validation\|Validator\|Hook\|Callback\|Parallel'` |
 | Paths, metadata, provenance | [Paths](../internal/engine/path_test.go), [metadata](../internal/engine/metadata_test.go), [provenance](../internal/engine/provenance_test.go) | `go test ./internal/engine -run 'Path\|Metadata\|Provenance\|Queries'` |
+| Passive resource contexts | [Engine contexts](../internal/engine/resource_context_test.go), [public contexts](../facade_resource_context_test.go) | `go test ./internal/engine -run 'Resource'` and `go test . -run 'SourceContext'` |
 | Secrets, bootstrap, redaction, explanation | [References](../internal/engine/secret_reference_test.go), [bootstrap](../internal/engine/secrets_test.go), [rendering](../internal/engine/render_test.go), [explanation](../internal/engine/explain_test.go) | `go test ./internal/engine -run 'Secret\|Bootstrap\|Effective\|Explain\|Explanation\|History\|Sensitivity'` |
-| Consumer examples and migration fixtures | [Consumer tests](../examples/consumers/consumers_test.go), [API examples](../example_test.go) | `go test ./examples/consumers` and `go test . -run '^Example'` |
+| Consumer examples and migration fixtures | [Consumer tests](../examples/consumers/consumers_test.go), [native and layered examples](../examples/consumers/layered_test.go), [API examples](../example_test.go) | `go test ./examples/consumers` and `go test . -run '^Example'` |
+| Assembly and layered scaling evidence | [Assembly benchmarks](../internal/engine/assembly_bench_test.go), [layered benchmarks](../internal/engine/underlay_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkUnderlay' -benchmem`; repeat with `^BenchmarkCompose` and `^BenchmarkOverlay` |
+| Inheritance scaling evidence | [Expansion benchmarks](../internal/engine/inheritance_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkInheritance' -benchmem` |
 | Documentation links and dependency isolation | [Link checks](../documentation_test.go), [import boundary](../import_boundary_test.go), [public boundary](../public_boundary_test.go) | `go test . -run 'TestLocalDocumentationLinks\|TestDependencyImportBoundary\|TestPublicFacadeBoundary'` |
 | Development and release checker behavior | [Shell regression](../release_check_test.go) | `go test . -run 'TestCheckScripts\|TestDevelopmentCheck'` |
 | Guarded release publication and recovery | [Publication regression](../release_publish_test.go) | `go test . -run '^TestReleasePublication'` |

docs/internal/overview.md

@@ -7,23 +7,28 @@ private state; public signatures and fields do not expose engine types.
 
 Implemented component owners:
 
-- `internal/engine` owns source acquisition, composition, schema inspection,
-  decoding, lifecycle coordination, callback auditing, provenance/history,
-  secret resolution and bootstrap, and safe inspection rendering. It never
-  imports the root package.
-- Root `Source`, `Path`, `Secret`, records, lifecycle callbacks, results, and
-  bootstrap reports are root-defined wrappers or records. Callback wrappers pass
-  only live engine context during a callback; `Result.Config` is detached and its
-  private inspection remains frozen independently.
+- `internal/engine` owns host and native-filesystem source acquisition,
+  grouped strict composition, recursive precedence assembly, and destination-local
+  sibling inheritance before typed decoding. It also owns schema
+  inspection, lifecycle coordination, callback auditing,
+  provenance/history, passive source-context retention, secret resolution and
+  bootstrap, and safe inspection rendering. It never imports the root package.
+- Root `Source`, `Filesystem`, `Input`, `Underlay`, `Path`, `Secret`, records,
+  lifecycle callbacks, results, and bootstrap reports are root-defined wrappers
+  or records. Callback wrappers pass only live engine context during a callback;
+  `Result.Config` is detached and its private inspection remains frozen
+  independently.
 - Root conversion files explicitly cross the path, record, callback, secret, and
   error boundaries. There are no public aliases, anonymous engine embeddings, or
   public engine bridge methods.
 - `internal/document` owns the parser-independent ordered tree and cloning.
   `internal/yaml` is the sole upstream YAML adapter and parser/dumper boundary.
-- Engine mechanism tests own source, composition, decoding, callback, provenance,
-  secret, history, rendering, and bootstrap behavior. Root tests own public API
-  compatibility, facade integration, documentation links, and dependency/public
-  boundary checks. Consumer examples remain under `examples/consumers`.
+- Engine mechanism tests own source, composition, inheritance, decoding,
+  callback, provenance,
+  source-context, secret, history, rendering, and bootstrap behavior. Root tests
+  own public API compatibility, facade integration, documentation links, and
+  dependency/public boundary checks. Consumer examples remain under
+  `examples/consumers`.
 
 Dependency direction is root to engine to document/YAML support. The root may
 import only `internal/engine` among project internal production packages; engine
@@ -36,4 +41,6 @@ semantics. The [consumer adoption guide](../consumers/README.md) owns applicatio
 guidance. The [architecture policy](../policy/architecture.md) owns contributor
 boundaries, and [ADR 0006](../adr/0006-public-facade.md) records the facade
 decision. See the [development guide](../development.md) for validation commands
-and [release procedure](../release.md) for source-candidate gates.
+and [release procedure](../release.md) for source-candidate gates. [ADR
+0009](../adr/0009-sibling-inheritance.md) records why inheritance is a built-in
+post-assembly structural operation.

docs/policy/architecture.md

@@ -2,7 +2,7 @@
 
 **Base Policy Version:** 1.3
 
-**Last Updated**: 2026-09-28
+**Last Updated**: 2026-09-29
 
 **Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/architecture.md)
 
@@ -34,6 +34,23 @@ Do not promote incidental implementation choices into architectural invariants.
 
 ## Success Criteria
 
+ConfigKit prioritizes a coherent configuration model: consistency and
+predictability take precedence over flexibility for accommodating individual
+downstream conventions. Consumers should learn one set of rules, not different
+combination or presence semantics depending on the source format or consuming
+application. Distinct operations such as strict composition and intentional
+overlay express distinct purposes within that model.
+
+Consumers should be able to load once and receive a complete, validated
+configuration with inspectable provenance for its final values, including
+defaults and recorded application-derived values. Configuration and metadata
+together must distinguish explicit zero, false, empty, and null input from
+omission; ordinary Go values alone need not encode those distinctions.
+Application-owned resolution belongs within the recorded loading lifecycle, so
+consumers need no subsequent generic merging, defaulting, or provenance
+reconstruction. This does not transfer domain execution or runtime resource
+management into ConfigKit.
+
 1. **Reduce application infrastructure:** Can applications delete substantial
    configuration infrastructure while retaining clear ownership of their domain
    rules?
@@ -41,8 +58,11 @@ Do not promote incidental implementation choices into architectural invariants.
    configuration model, with only the flexibility needed to express genuine
    application requirements?
 
-Both criteria must hold. ConfigKit should not preserve divergent consumer
-conventions merely for compatibility, nor impose restrictions that prevent useful
+Both criteria must hold. Minimize consumer loading and processing code subject
+to the consistency, presence, and provenance principles above, rather than
+trading those principles for consumer-specific shortcuts. ConfigKit should not
+preserve divergent consumer conventions merely for compatibility, nor impose
+restrictions that prevent useful
 domain information from being expressed. Prefer a canonical representation using
 existing primitives before adding another mechanism. Extend the model when a
 concrete requirement cannot be accommodated cleanly, not simply because a
@@ -88,6 +108,22 @@ must never run implicitly as part of configuration loading.
 Remote data can enter through application-supplied input without introducing a
 network provider into the library.
 
+## Loading Lifecycle
+
+Configuration loading is a synchronous, complete-on-return operation. A successful
+load provides the final configuration and inspection metadata after the configured
+processing and validation phases complete. A failed load reports diagnostics and
+returns no usable partial result. The normal application pattern is one load near
+startup; applications may explicitly initiate additional loads.
+
+ConfigKit releases its library-owned loading handles before returning and remains
+inactive until the application explicitly invokes it again. It performs no
+background watching, refresh, or mutation of published results. Caller-owned
+resources and application callback effects remain the caller's responsibility.
+The [configuration contract](../config.md#completion-and-resource-lifetime) owns
+the precise completion and ownership behavior. The proposed decision record is
+[ADR 0008](../adr/0008-complete-on-return-loading.md).
+
 ## Public API And Dependency Direction
 
 Keep one small, coherent public surface for the supported lifecycle. Simple
@@ -116,18 +152,41 @@ framework on this library.
 
 ## Configuration Integrity
 
-Composition and intentional overlay are distinct operations. Composition must
-not silently become last-wins override through file ordering. The decision and
-alternatives are recorded in [ADR 0001](../adr/0001-separate-composition-and-overlay.md).
+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,
+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).
+
+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
+determine its precedence role. Input-specific constraints, including CLI path
+rules and opt-in version ownership, remain part of the configuration contract. Consistent combination
+semantics do not require identical acquisition or lifetime behavior from every
+source kind.
+
+After source precedence is complete, built-in sibling `extends` relationships
+resolve structurally in the same parser-independent representation, using the
+existing recursive overlay rules. This adds neither an acquired source nor a
+consumer registration mechanism. Enclosing relationships precede nested
+destination-local resolution; typed defaults, application resolution, and
+secret lookup retain their existing lifecycle roles. The [configuration
+contract](../config.md#sibling-inheritance) owns the exact syntax and presence
+rules, and [ADR 0009](../adr/0009-sibling-inheritance.md) records the rationale.
 
 ConfigKit must define supported YAML semantics explicitly rather than inherit
 parser defaults. Processing order and diagnostics must be deterministic for the
 same inputs and options. Invalid input must not produce a successful usable
 configuration. Do not run application hooks on invalid typed configuration.
 
-User-controlled precedence is application defaults, composed base, selected
-overlay, then explicitly supplied CLI overrides. Applications own flag parsing;
-ConfigKit owns overlay semantics and provenance. Domain resolution follows these
+User-controlled precedence is application defaults, ordered underlays, composed
+main input, selected overlay, then explicitly supplied CLI overrides.
+Applications own flag parsing; ConfigKit owns overlay semantics and provenance.
+Domain resolution follows these
 inputs and must remain recorded, not become a hidden override channel.
 
 Preserve node-based composition until the effective input is ready for typed

docs/releases/v0.5.0.md

@@ -0,0 +1,53 @@
+# ConfigKit v0.5.0
+
+This minor release adds source-backed layered configuration and built-in sibling
+inheritance, so applications can load one complete configuration while retaining
+presence, source context, and explainable derivation.
+
+## Summary
+
+- `LoadLayered` combines ordered underlay groups beneath the main input, selected
+  overlay, and explicit CLI overrides. Inputs within a group still compose
+  strictly; precedence tiers combine mappings recursively. The main input
+  remains the owner of the optional schema-version check.
+- `NewFilesystem(...).File(...)` accepts caller-owned `fs.FS` sources, and
+  `Mapping(...)` snapshots supported built-in values as a labeled source.
+  `Result.SourceContext` and `Resolution.SourceContext` expose passive resource
+  contexts for values from file-backed sources.
+- `extends` now resolves a mapping against one named sibling after source
+  precedence and before typed decoding. Inherited values count as explicit
+  input. Origins, history, explanations, and redacted rendering retain the
+  source and the structural path by which a value reached its destination.
+  Review corrections preserve immutable nested-history evidence and bounded
+  expansion work.
+
+See the [configuration contract](../config.md#layered-loading),
+[inheritance rules](../config.md#sibling-inheritance), and
+[consumer examples](../consumers/README.md) for current behavior.
+
+## Compatibility and upgrade
+
+This is an intentional pre-v1 incompatible minor release for configurations
+that use `extends` as ordinary data. That key is now reserved wherever an input
+mapping occurs; it is consumed before decoding. Rename any such YAML key and
+corresponding Go field, then rebuild. No collection registration or opt-in flag
+is available. Configurations without `extends` retain their existing meaning.
+
+Applications adopting the new features may move source-backed defaults into
+explicit `Underlay` groups, use `SourceContext` for relative resources, or
+replace application-owned sibling merging with one `extends` relationship per
+mapping. Adoption is optional. The maintained
+[named-preset example](../consumers/README.md#named-sibling-inheritance)
+shows inherited `false` and destination-local nested resolution.
+
+Select the new module version:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.5.0
+```
+
+The minimum Go line remains 1.25. Linux amd64 and arm64 remain the
+release-validated targets; macOS remains best-effort. The YAML dependency is
+unchanged at `go.yaml.in/yaml/v4 v4.0.0-rc.6`.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.5.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.5.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.5.0/docs/oss/attribution.md)

docs/roadmap/deferred.md

@@ -11,11 +11,13 @@ General environment-variable configuration overlays are not supported. Typed sec
 references and explicit directory bootstrap are the environment convention. Literal
 credential configuration and implicit secret-directory discovery are unsupported.
 
-The initial release deliberately excludes:
+The following remain deliberately excluded:
 
 - automatic `conf.d` discovery;
 - remote imports, arbitrary providers, and additional configuration formats;
-- custom merge strategies, sequence merging, reset operators, and overlay inheritance;
+- 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;
@@ -26,3 +28,7 @@ The initial release deliberately excludes:
 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.
+
+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.

examples/consumers/consumers_test.go

@@ -52,6 +52,22 @@ func Example_seriatim() {
 	// Output: visitor true the event occurred <nil> <nil>
 }
 
+func Example_inheritedPresets() {
+	result, err := consumers.LoadPresets(configkit.File(fixture("inheritance", "presets.yml")))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	path := configkit.Keys("presets", "custom", "enabled")
+	info, _ := result.Metadata().Lookup(path)
+	custom := result.Config.Presets["custom"]
+	fmt.Println(custom.Enabled, result.Metadata().IsExplicit(path), custom.Settings["secondary"]["width"])
+	fmt.Println(info.Origin.Locations[0].SourceID, info.Origin.Inheritance[0].From)
+	// Output:
+	// false true 20
+	// source-1 /presets/base/enabled
+}
+
 func Example_weatherStartup() {
 	const blockedName = "CONFIGKIT_WEATHER_EXAMPLE_BLOCKED"
 	const tokenName = "CONFIGKIT_WEATHER_EXAMPLE_TOKEN"

examples/consumers/inheritance.go

@@ -0,0 +1,20 @@
+package consumers
+
+import configkit "gitea.maximumdirect.net/eric/ConfigKit"
+
+// PresetCatalog is ordinary application data; no inheritance fields or
+// registration hooks are required in the typed configuration.
+type PresetCatalog struct {
+	Presets map[string]Preset `yaml:"presets"`
+}
+
+type Preset struct {
+	Enabled  bool                      `yaml:"enabled"`
+	Settings map[string]map[string]int `yaml:"settings"`
+}
+
+// LoadPresets performs one load; callers may inspect inherited presence and
+// source origins on the returned result before choosing a preset to execute.
+func LoadPresets(source configkit.Source) (*configkit.Result[PresetCatalog], error) {
+	return configkit.Load[PresetCatalog](source)
+}

examples/consumers/inheritance_test.go

@@ -0,0 +1,35 @@
+package consumers_test
+
+import (
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func TestPresetCatalogOneLoadAndDerivation(t *testing.T) {
+	result, err := consumers.LoadPresets(configkit.File(fixture("inheritance", "presets.yml")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Config.Presets["base"].Settings["secondary"]["width"] != 10 ||
+		result.Config.Presets["custom"].Settings["secondary"]["width"] != 20 ||
+		result.Config.Presets["custom"].Enabled {
+		t.Fatalf("expanded presets = %#v", result.Config.Presets)
+	}
+	enabled := configkit.Keys("presets", "custom", "enabled")
+	if !result.Metadata().IsExplicit(enabled) {
+		t.Fatal("inherited false lost explicit presence")
+	}
+	info, found := result.Metadata().Lookup(enabled)
+	if !found || info.Origin.Locations[0].SourceID != "source-1" ||
+		len(info.Origin.Inheritance) != 1 || info.Origin.Inheritance[0].From.String() != "/presets/base/enabled" {
+		t.Fatalf("origin and derivation = %#v, %v", info, found)
+	}
+	nested := configkit.Keys("presets", "custom", "settings", "secondary", "width")
+	info, found = result.Metadata().Lookup(nested)
+	if !found || len(info.Origin.Inheritance) != 1 ||
+		info.Origin.Inheritance[0].From.String() != "/presets/custom/settings/primary/width" {
+		t.Fatalf("destination-local derivation = %#v, %v", info, found)
+	}
+}

examples/consumers/layered.go

@@ -0,0 +1,95 @@
+package consumers
+
+import (
+	"embed"
+	"errors"
+	"io/fs"
+	"path"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+//go:embed testdata/native-source testdata/layered-bundle
+var bundledFiles embed.FS
+
+// BundledConfig is a small configuration loaded directly from an embedded file.
+type BundledConfig struct {
+	Name    string `yaml:"name"`
+	Enabled bool   `yaml:"enabled"`
+}
+
+// LoadBundled demonstrates a native filesystem source without layered options.
+func LoadBundled() (*configkit.Result[BundledConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/native-source")
+	if err != nil {
+		return nil, err
+	}
+	return configkit.Load[BundledConfig](configkit.NewFilesystem("bundled", files).File("config.yml"))
+}
+
+// CatalogConfig keeps resource names and domain rules in the application.
+type CatalogConfig struct {
+	Mode      string            `yaml:"mode"`
+	Enabled   bool              `yaml:"enabled"`
+	Workers   int               `yaml:"workers"`
+	Templates map[string]string `yaml:"templates"`
+	Labels    map[string]string `yaml:"labels"`
+}
+
+// LoadCatalog combines two underlay tiers beneath an independently located
+// host main file. ConfigKit assembles and validates the complete result once.
+func LoadCatalog(mainPath string) (*configkit.Result[CatalogConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/layered-bundle")
+	if err != nil {
+		return nil, err
+	}
+	bundle := configkit.NewFilesystem("catalog bundle", files)
+	return configkit.LoadLayered(configkit.Input{Source: configkit.File(mainPath)},
+		configkit.LayeredOptions[CatalogConfig]{
+			Underlays: []configkit.Underlay{
+				{Inputs: []configkit.Input{{Source: bundle.File("defaults/config.yml"), Imports: true}}},
+				{Inputs: []configkit.Input{{Source: configkit.Mapping("local policy", map[string]any{
+					"workers": 2,
+					"labels":  map[string]any{"tenant": "example"},
+				})}}},
+			},
+			Definition: configkit.Definition[CatalogConfig]{
+				Defaults: func() CatalogConfig { return CatalogConfig{Mode: "demo"} },
+				Validate: validateCatalog,
+			},
+		})
+}
+
+func validateCatalog(configuration *CatalogConfig, validation *configkit.Validation) {
+	if configuration.Mode != "demo" && configuration.Mode != "production" {
+		validation.Error(configkit.Keys("mode"), "must be demo or production")
+	}
+	if configuration.Workers < 0 {
+		validation.Error(configkit.Keys("workers"), "must be nonnegative")
+	}
+	if name := configuration.Templates["welcome"]; name == "." || !fs.ValidPath(name) {
+		validation.Error(configkit.Keys("templates", "welcome"), "must be a relative resource name")
+	}
+}
+
+// ReadWelcome follows the effective template field's one declaring source to
+// its retained resource context. Opening and interpreting the resource remain
+// application work; this does not reopen configuration or change metadata.
+func ReadWelcome(result *configkit.Result[CatalogConfig]) (string, error) {
+	if result == nil {
+		return "", errors.New("catalog result is nil")
+	}
+	info, found := result.Metadata().Lookup(configkit.Keys("templates", "welcome"))
+	if !found || len(info.Origin.Locations) != 1 {
+		return "", errors.New("welcome template has no single declaring source")
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found {
+		return "", errors.New("welcome template has no resource context")
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, result.Config.Templates["welcome"]))
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}

examples/consumers/layered_test.go

@@ -0,0 +1,71 @@
+package consumers_test
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func Example_nativeFilesystem() {
+	result, err := consumers.LoadBundled()
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(result.Config.Name, result.Config.Enabled)
+	// Output: embedded false
+}
+
+func Example_layeredCatalog() {
+	result, err := consumers.LoadCatalog(fixture("layered-main", "config.yml"))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	welcome, err := consumers.ReadWelcome(result)
+	fmt.Println(result.Config.Mode, result.Config.Enabled, result.Config.Workers,
+		result.Config.Labels["origin"], result.Config.Labels["tenant"], strings.TrimSpace(welcome), err)
+	// Output: production false 0 bundle example Welcome from the bundle. <nil>
+}
+
+func TestCatalogRetainsPresenceAndSourceContext(t *testing.T) {
+	result, err := consumers.LoadCatalog(fixture("layered-main", "config.yml"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, key := range []string{"enabled", "workers"} {
+		info, found := result.Metadata().Lookup(configkit.Keys(key))
+		if !found || !info.Explicit || info.Origin.Layer != configkit.LayerBase {
+			t.Fatalf("%s presence = %#v, %v", key, info, found)
+		}
+	}
+	info, found := result.Metadata().Lookup(configkit.Keys("templates", "welcome"))
+	if !found || info.Origin.Layer != configkit.LayerUnderlay || len(info.Origin.Locations) != 1 {
+		t.Fatalf("welcome origin = %#v, %v", info, found)
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found || context.BaseDir != "defaults" {
+		t.Fatalf("welcome context = %#v, %v", context, found)
+	}
+	welcome, err := consumers.ReadWelcome(result)
+	if err != nil || strings.TrimSpace(welcome) != "Welcome from the bundle." {
+		t.Fatalf("welcome = %q, %v", welcome, err)
+	}
+	if result.Config.Labels["tenant"] != "example" || result.Config.Labels["origin"] != "bundle" {
+		t.Fatalf("recursive underlay labels = %#v", result.Config.Labels)
+	}
+}
+
+func TestCatalogValidationRejectsInvalidMode(t *testing.T) {
+	_, err := consumers.LoadCatalog(fixture("layered-main", "invalid.yml"))
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) == 0 ||
+		loadError.Diagnostics()[0].Code != configkit.CodeValidateError ||
+		loadError.Diagnostics()[0].Path.String() != "/mode" {
+		t.Fatalf("invalid mode error = %v", err)
+	}
+}

examples/consumers/testdata/inheritance/presets.yml

@@ -0,0 +1,10 @@
+presets:
+  base:
+    enabled: false
+    settings:
+      primary: {width: 10}
+      secondary: {extends: primary}
+  custom:
+    extends: base
+    settings:
+      primary: {width: 20}

examples/consumers/testdata/layered-bundle/defaults/config.yml

@@ -0,0 +1,7 @@
+configkit:
+  imports:
+    - path: ../shared.yml
+enabled: true
+workers: 4
+templates:
+  welcome: messages/welcome.txt

examples/consumers/testdata/layered-bundle/defaults/messages/welcome.txt

@@ -0,0 +1 @@
+Welcome from the bundle.

examples/consumers/testdata/layered-bundle/shared.yml

@@ -0,0 +1,2 @@
+labels:
+  origin: bundle

examples/consumers/testdata/layered-main/config.yml

@@ -0,0 +1,3 @@
+mode: production
+enabled: false
+workers: 0

examples/consumers/testdata/layered-main/invalid.yml

@@ -0,0 +1 @@
+mode: unsupported

examples/consumers/testdata/native-source/config.yml

@@ -0,0 +1,2 @@
+name: embedded
+enabled: false

facade_candidate_test.go

@@ -31,6 +31,9 @@ func TestFacadeValuesRemainRootOwned(t *testing.T) {
 		reflect.TypeFor[configkit.Resolution](), reflect.TypeFor[configkit.Validation](),
 		reflect.TypeFor[configkit.Version](), reflect.TypeFor[configkit.VersionRule](),
 		reflect.TypeFor[configkit.FilesystemContext](), reflect.TypeFor[configkit.Override](),
+		reflect.TypeFor[configkit.Filesystem](), reflect.TypeFor[configkit.ResourceContext](),
+		reflect.TypeFor[configkit.Input](), reflect.TypeFor[configkit.Underlay](),
+		reflect.TypeFor[configkit.LayeredOptions[struct{}]](),
 		reflect.TypeFor[configkit.Definition[struct{}]](), reflect.TypeFor[configkit.Options[struct{}]](),
 		reflect.TypeFor[configkit.Result[struct{}]](),
 		reflect.TypeFor[configkit.ValueSnapshot](), reflect.TypeFor[configkit.ValueChange](),

facade_context_test.go

@@ -26,6 +26,7 @@ func TestFacadeInactiveContextsPreserveClosedFailure(t *testing.T) {
 				{"IsExplicit", func() { state.resolution.IsExplicit(path) }},
 				{"InputInfo", func() { state.resolution.InputInfo(path) }},
 				{"Source", func() { state.resolution.Source("source") }},
+				{"SourceContext", func() { state.resolution.SourceContext("source") }},
 				{"Record", func() { state.resolution.Record(path, configkit.ChangeNormalization, "reason") }},
 				{"Resolution.Error", func() { state.resolution.Error(path, "message") }},
 				{"Resolution.Warning", func() { state.resolution.Warning(path, "message") }},

facade_equivalence_test.go

@@ -0,0 +1,164 @@
+package configkit_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type equivalentConfig struct {
+	Version  uint64   `yaml:"version"`
+	Name     string   `yaml:"name"`
+	Enabled  bool     `yaml:"enabled"`
+	Count    int      `yaml:"count"`
+	Items    []string `yaml:"items"`
+	Optional *string  `yaml:"optional"`
+}
+
+func TestEquivalentSourceFormsPreserveInputSemantics(t *testing.T) {
+	const yaml = "version: 1\nname: retained\nenabled: false\ncount: 0\nitems: []\noptional: null\n"
+	hostPath := filepath.Join(t.TempDir(), "main.yml")
+	if err := os.WriteFile(hostPath, []byte(yaml), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{"main.yml": &fstest.MapFile{Data: []byte(yaml)}})
+	context := &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &namespace}
+	for _, test := range []struct {
+		name    string
+		source  configkit.Source
+		context *configkit.FilesystemContext
+		kind    configkit.SourceKind
+	}{
+		{"host", configkit.File(hostPath), nil, configkit.SourceKindFile},
+		{"native", namespace.File("main.yml"), nil, configkit.SourceKindFilesystem},
+		{"bytes", configkit.Bytes("memory", []byte(yaml)), context, configkit.SourceKindBytes},
+		{"reader", configkit.Reader("stream", strings.NewReader(yaml)), context, configkit.SourceKindReader},
+		{"mapping", configkit.Mapping("programmatic", map[string]any{
+			"version": uint64(1), "name": "retained", "enabled": false, "count": 0,
+			"items": []any{}, "optional": nil,
+		}), context, configkit.SourceKindMapping},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			checks := 0
+			result, err := configkit.LoadLayered(configkit.Input{Source: test.source, Filesystem: test.context}, configkit.LayeredOptions[equivalentConfig]{
+				Definition: configkit.Definition[equivalentConfig]{Version: &configkit.VersionRule{
+					Path: configkit.Keys("version"), Check: func(version configkit.Version, validation *configkit.Validation) error {
+						checks++
+						if !version.Present || version.Text != "1" {
+							t.Errorf("version = %#v", version)
+						}
+						return nil
+					},
+				}},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			want := equivalentConfig{Version: 1, Name: "retained", Items: []string{}}
+			if !reflect.DeepEqual(result.Config, want) || checks != 1 {
+				t.Fatalf("config = %#v, checks %d", result.Config, checks)
+			}
+			for _, field := range []string{"version", "name", "enabled", "count", "items", "optional"} {
+				info, ok := result.Metadata().Lookup(configkit.Keys(field))
+				if !ok || !info.Explicit || info.Origin.Layer != configkit.LayerBase || info.Null != (field == "optional") {
+					t.Errorf("%s input info = %#v, %v", field, info, ok)
+				}
+			}
+			sources := result.Sources()
+			if len(sources) != 1 || sources[0].Kind != test.kind || sources[0].ID != "source-1" || sources[0].UnderlayIndex != 0 {
+				t.Fatalf("sources = %#v", sources)
+			}
+			resource, ok := result.SourceContext("source-1")
+			if !ok || resource.BaseDir != "." || resource.FS == nil {
+				t.Fatalf("source context = %#v, %v", resource, ok)
+			}
+		})
+	}
+}
+
+func TestRepeatedImportsAcrossTiersAndReaderConsumption(t *testing.T) {
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"left.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: shared.yml}]}\n")},
+		"right.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: shared.yml}]}\n")},
+		"shared.yml": &fstest.MapFile{Data: []byte("name: retained\n")},
+	})
+	type config struct {
+		Name string `yaml:"name"`
+	}
+	left := configkit.Input{Source: namespace.File("left.yml"), Imports: true}
+	right := configkit.Input{Source: namespace.File("right.yml"), Imports: true}
+	_, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{left, right}}},
+	})
+	if got := layeredErrorCode(t, err); got != configkit.CodeImportRepeated {
+		t.Fatalf("same-tier repeated import = %s", got)
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{left}}, {Inputs: []configkit.Input{right}}},
+	})
+	if err != nil || result.Config.Name != "retained" || len(result.Sources()) != 4 {
+		t.Fatalf("cross-tier repeated import = %#v, %v", result, err)
+	}
+	reader := configkit.Reader("single-use", strings.NewReader("name: retained\n"))
+	_, err = configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: reader}}}, {Inputs: []configkit.Input{{Source: reader}}}},
+	})
+	if got := layeredErrorCode(t, err); got != configkit.CodeSourceReaderUsed {
+		t.Fatalf("reader re-consumption = %s", got)
+	}
+}
+
+func TestReplacedSecretRemainsAbsentFromSafeOutput(t *testing.T) {
+	type config struct {
+		Secret string `yaml:"secret"`
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte("secret: new-secret-canary\n"))}, configkit.LayeredOptions[config]{
+		Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"secret": "old-secret-canary"})}}}},
+		Definition: configkit.Definition[config]{Secret: func(path configkit.Path) bool { return path.String() == "/secret" }},
+	})
+	if err != nil || result.Config.Secret != "new-secret-canary" {
+		t.Fatalf("secret load = %#v, %v", result, err)
+	}
+	var effective, history bytes.Buffer
+	if err := result.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if err := result.WriteExplanation(&history, configkit.Keys("secret")); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String()+history.String(), "old-secret-canary") || strings.Contains(effective.String()+history.String(), "new-secret-canary") {
+		t.Fatalf("secret output leaked: %q %q", effective.String(), history.String())
+	}
+	_, err = configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte("secret: new-secret-canary\n"))}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"secret": "old-secret-canary"})}}}},
+		Definition: configkit.Definition[config]{Validate: func(config *config, validation *configkit.Validation) {
+			validation.Error(configkit.Keys("secret"), "invalid secret")
+		}},
+	})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok {
+		t.Fatalf("validation error = %#v", err)
+	}
+	var diagnostics bytes.Buffer
+	if err := configkit.WriteDiagnostics(&diagnostics, loadError.Diagnostics()); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(diagnostics.String(), "secret-canary") {
+		t.Fatalf("diagnostic leaked secret: %q", diagnostics.String())
+	}
+}
+
+func layeredErrorCode(t *testing.T, err error) configkit.Code {
+	t.Helper()
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 {
+		t.Fatalf("load error = %#v", err)
+	}
+	return loadError.Diagnostics()[0].Code
+}

facade_filesystem_test.go

@@ -0,0 +1,142 @@
+package configkit_test
+
+import (
+	"embed"
+	"io/fs"
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+//go:embed testdata/filesystem
+var filesystemFixtures embed.FS
+
+type filesystemFacadeConfig struct {
+	Items map[string]string `yaml:"items"`
+}
+
+func TestFacadeFilesystemSourceImportsOverlayAndReuse(t *testing.T) {
+	fixture, err := fs.Sub(filesystemFixtures, "testdata/filesystem")
+	if err != nil {
+		t.Fatal(err)
+	}
+	namespace := configkit.NewFilesystem("embedded", fixture)
+	copyOfNamespace := namespace
+	source := namespace.File("config/main.yml")
+	for range 2 {
+		result, err := configkit.Load(source, configkit.Options[filesystemFacadeConfig]{
+			Imports: true, Overlay: "testing", Filesystem: &configkit.FilesystemContext{FS: &copyOfNamespace},
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if want := (map[string]string{"main": "selected", "shared": "default"}); !reflect.DeepEqual(result.Config.Items, want) {
+			t.Fatalf("items = %#v, want %#v", result.Config.Items, want)
+		}
+		sources := result.Sources()
+		if len(sources) != 3 {
+			t.Fatalf("sources = %#v", sources)
+		}
+		for index, want := range []string{"config/main.yml", "shared.yml", "overlays/testing.yml"} {
+			if sources[index].ID != "source-"+strconv.Itoa(index+1) || sources[index].Path != want || sources[index].Kind != configkit.SourceKindFilesystem {
+				t.Errorf("source %d = %#v", index, sources[index])
+			}
+			if sources[index].Label != "embedded/"+want {
+				t.Errorf("source label = %q", sources[index].Label)
+			}
+		}
+		if sources[1].ImportedFrom == nil || sources[1].ImportedFrom.SourceID != sources[0].ID || sources[1].ImportedFrom.Line == 0 {
+			t.Fatalf("import location = %#v", sources[1].ImportedFrom)
+		}
+	}
+}
+
+func TestFacadeFilesystemContextsAndIdentity(t *testing.T) {
+	first := configkit.NewFilesystem("same", fstest.MapFS{
+		"root/config/main.yml":      &fstest.MapFile{Data: []byte("configkit: {imports: [{path: ../shared.yml}]}\n")},
+		"root/shared.yml":           &fstest.MapFile{Data: []byte("items: {value: first}\n")},
+		"root/overlays/testing.yml": &fstest.MapFile{Data: []byte("items: {overlay: first}\n")},
+	})
+	second := configkit.NewFilesystem("same", fstest.MapFS{
+		"root/config/main.yml": &fstest.MapFile{Data: []byte("items: {value: second}\n")},
+	})
+	context := &configkit.FilesystemContext{Root: "root", FS: &first}
+	result, err := configkit.Load(first.File("root/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Imports: true, Overlay: "testing", Filesystem: context})
+	if err != nil || result.Config.Items["value"] != "first" || result.Config.Items["overlay"] != "first" {
+		t.Fatalf("contained load = %#v, %v", result, err)
+	}
+	result, err = configkit.Load[filesystemFacadeConfig](second.File("root/config/main.yml"))
+	if err != nil || result.Config.Items["value"] != "second" {
+		t.Fatalf("same-label independent namespace = %#v, %v", result, err)
+	}
+	checkCode(t, second.File("root/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Filesystem: context}, configkit.CodeOptionsInvalid)
+	checkCode(t, first.File("root/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Filesystem: &configkit.FilesystemContext{Root: "root/config", FS: &first}, Imports: true}, configkit.CodeImportOutsideRoot)
+	checkCode(t, configkit.File("testdata/filesystem/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Filesystem: context}, configkit.CodeOptionsInvalid)
+
+	for _, input := range []struct {
+		name   string
+		source configkit.Source
+		kind   configkit.SourceKind
+	}{
+		{"bytes", configkit.Bytes("memory", []byte("configkit: {imports: [{path: shared.yml}]}\n")), configkit.SourceKindBytes},
+		{"reader", configkit.Reader("stream", strings.NewReader("configkit: {imports: [{path: shared.yml}]}\n")), configkit.SourceKindReader},
+	} {
+		t.Run(input.name, func(t *testing.T) {
+			result, err := configkit.Load(input.source, configkit.Options[filesystemFacadeConfig]{Imports: true, Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: "root", FS: &first}})
+			if err != nil || result.Config.Items["value"] != "first" {
+				t.Fatalf("synthetic import = %#v, %v", result, err)
+			}
+			if got := result.Sources(); len(got) != 2 || got[0].Kind != input.kind || got[0].Path != "" || got[1].Kind != configkit.SourceKindFilesystem {
+				t.Fatalf("sources = %#v", got)
+			}
+		})
+	}
+	checkCode(t, configkit.Bytes("memory", []byte("items: {}\n")), configkit.Options[filesystemFacadeConfig]{Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: ".", FS: &first}}, configkit.CodeImportOutsideRoot)
+	result, err = configkit.Load(configkit.Bytes("memory", []byte("items: {value: main}\n")), configkit.Options[filesystemFacadeConfig]{
+		Overlay: "testing", Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: "root", FS: &first},
+	})
+	if err != nil || result.Config.Items["value"] != "main" || result.Config.Items["overlay"] != "first" {
+		t.Fatalf("synthetic selected overlay = %#v, %v", result, err)
+	}
+}
+
+func TestFacadeSourceKindsIncludeHostAndCLI(t *testing.T) {
+	result, err := configkit.Load(configkit.File("testdata/filesystem/shared.yml"), configkit.Options[filesystemFacadeConfig]{
+		Overrides: []configkit.Override{{Path: configkit.Keys("items", "shared"), Value: "changed", Flag: "--shared"}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Kind != configkit.SourceKindFile || sources[0].Path == "" || sources[1].Kind != configkit.SourceKindCLI || sources[1].Path != "" {
+		t.Fatalf("host and CLI sources = %#v", sources)
+	}
+}
+
+func TestFacadeFilesystemConstructorDiagnostics(t *testing.T) {
+	var nilFilesystem fs.FS
+	var zero configkit.Filesystem
+	for _, source := range []configkit.Source{
+		zero.File("main.yml"),
+		configkit.NewFilesystem("", fstest.MapFS{}).File("main.yml"),
+		configkit.NewFilesystem("bad\nlabel", fstest.MapFS{}).File("main.yml"),
+		configkit.NewFilesystem("good", nilFilesystem).File("main.yml"),
+		configkit.NewFilesystem("good", fstest.MapFS{}).File("."),
+		configkit.NewFilesystem("good", fstest.MapFS{}).File("../outside.yml"),
+	} {
+		checkCode(t, source, configkit.Options[filesystemFacadeConfig]{}, configkit.CodeSourceInvalid)
+	}
+}
+
+func checkCode(t *testing.T, source configkit.Source, options configkit.Options[filesystemFacadeConfig], want configkit.Code) {
+	t.Helper()
+	_, err := configkit.Load(source, options)
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != want {
+		t.Fatalf("error = %#v, want %s", err, want)
+	}
+}

facade_inheritance_test.go

@@ -0,0 +1,284 @@
+package configkit_test
+
+import (
+	"bytes"
+	"io/fs"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type inheritedEntry struct {
+	Enabled  bool                         `yaml:"enabled"`
+	Count    int                          `yaml:"count"`
+	Resource string                       `yaml:"resource"`
+	Settings map[string]map[string]string `yaml:"settings"`
+}
+
+type inheritedConfiguration struct {
+	Version string                    `yaml:"version"`
+	Entries map[string]inheritedEntry `yaml:"entries"`
+}
+
+func TestFacadeInheritancePreservesPresenceOriginsAndHistory(t *testing.T) {
+	input := []byte(`entries:
+  parent:
+    enabled: false
+    count: 0
+    settings:
+      primary: {value: original}
+      secondary: {extends: primary}
+  child:
+    extends: parent
+    settings:
+      primary: {value: custom}
+`)
+	defaultRan := false
+	result, err := configkit.Load(configkit.Bytes("config.yml", input), configkit.Options[inheritedConfiguration]{Definition: configkit.Definition[inheritedConfiguration]{
+		Resolve: func(config *inheritedConfiguration, resolution *configkit.Resolution) error {
+			if !resolution.IsExplicit(configkit.Keys("entries", "child", "enabled")) {
+				t.Error("inherited false is not explicit")
+			}
+			info, found := resolution.InputInfo(configkit.Keys("entries", "child", "enabled"))
+			if !found || len(info.Origin.Inheritance) != 1 {
+				t.Errorf("resolver inherited origin = %#v, %v", info, found)
+			}
+			child := config.Entries["child"]
+			configkit.Default(resolution, configkit.Keys("entries", "child", "enabled"), &child.Enabled, true, "use fallback")
+			defaultRan = child.Enabled
+			config.Entries["child"] = child
+			return nil
+		},
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	child := result.Config.Entries["child"]
+	if defaultRan || child.Enabled || child.Count != 0 || child.Settings["secondary"]["value"] != "custom" {
+		t.Fatalf("inherited values = %#v, default ran %v", child, defaultRan)
+	}
+	path := configkit.Keys("entries", "child", "settings", "secondary", "value")
+	info, found := result.Metadata().Lookup(path)
+	if !found || !info.Explicit || len(info.Origin.Inheritance) != 1 ||
+		info.Origin.Inheritance[0].From.String() != "/entries/child/settings/primary/value" ||
+		info.Origin.Inheritance[0].To.String() != path.String() {
+		t.Fatalf("nested destination origin = %#v, %v", info, found)
+	}
+	info.Origin.Inheritance[0].Locations[0].SourceID = "mutated"
+	info, _ = result.Metadata().Lookup(path)
+	if info.Origin.Inheritance[0].Locations[0].SourceID == "mutated" {
+		t.Fatal("public inheritance record aliases inspection state")
+	}
+	if _, found := result.Metadata().Lookup(configkit.Keys("entries", "child", "extends")); found {
+		t.Fatal("consumed directive remained effective")
+	}
+	control, found := result.Explain(configkit.Keys("entries", "child", "extends"))
+	if !found || control.Effective.Present || len(control.History) < 2 ||
+		control.History[len(control.History)-1].Origin.Operation != configkit.ChangeInheritance {
+		t.Fatalf("consumed directive = %#v, %v", control, found)
+	}
+	explanation, found := result.Explain(path)
+	if !found || explanation.Effective.YAML != "custom\n" {
+		t.Fatalf("nested explanation = %#v, %v", explanation, found)
+	}
+	for _, change := range explanation.History {
+		if change.Origin.Operation == configkit.ChangeDecoding {
+			t.Fatalf("inherited value attributed to decoding: %#v", explanation.History)
+		}
+	}
+}
+
+func TestFacadeInheritanceLayeredPrecedenceAndSourceContext(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"config/main.yml":          &fstest.MapFile{Data: []byte("entries: {child: {extends: parent}}\n")},
+		"shared/parent.yml":        &fstest.MapFile{Data: []byte("entries: {parent: {resource: icon.txt, count: 1}}\n")},
+		"shared/flags.yml":         &fstest.MapFile{Data: []byte("entries: {parent: {enabled: false}}\n")},
+		"shared/icon.txt":          &fstest.MapFile{Data: []byte("parent-resource")},
+		"config/overlays/test.yml": &fstest.MapFile{Data: []byte("entries: {parent: {count: 2}}\n")},
+	})
+	result, err := configkit.LoadLayered(configkit.Input{Source: filesystem.File("config/main.yml")}, configkit.LayeredOptions[inheritedConfiguration]{
+		Underlays: []configkit.Underlay{
+			{Inputs: []configkit.Input{{Source: filesystem.File("shared/parent.yml")}}},
+			{Inputs: []configkit.Input{{Source: filesystem.File("shared/flags.yml")}}},
+		},
+		Overlay: "test", OverlayFilesystem: &configkit.FilesystemContext{Root: "config", BaseDir: "config", FS: &filesystem},
+		Overrides: []configkit.Override{{Path: configkit.Keys("entries", "child", "count"), Value: 3, Flag: "--count"}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	child := result.Config.Entries["child"]
+	if child.Count != 3 || child.Resource != "icon.txt" {
+		t.Fatalf("layered inheritance = %#v", child)
+	}
+	info, found := result.Metadata().Lookup(configkit.Keys("entries", "child", "resource"))
+	if !found || len(info.Origin.Inheritance) != 1 || len(info.Origin.Locations) != 1 {
+		t.Fatalf("resource origin = %#v, %v", info, found)
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found || context.BaseDir != "shared" || context.FS == nil {
+		t.Fatalf("source context = %#v, %v", context, found)
+	}
+	data, err := fs.ReadFile(context.FS, context.BaseDir+"/"+child.Resource)
+	if err != nil || string(data) != "parent-resource" {
+		t.Fatalf("source-relative resource = %q, %v", data, err)
+	}
+	count, found := result.Metadata().Lookup(configkit.Keys("entries", "child", "count"))
+	if !found || count.Origin.Layer != configkit.LayerCLI || len(count.Origin.Inheritance) != 0 {
+		t.Fatalf("CLI replacement origin = %#v, %v", count, found)
+	}
+	var rendered bytes.Buffer
+	if err := result.WriteEffective(&rendered); err != nil || strings.Contains(rendered.String(), "extends:") {
+		t.Fatalf("effective rendering = %q, %v", rendered.String(), err)
+	}
+}
+
+func TestFacadeInheritanceMappingMatchesBytes(t *testing.T) {
+	values := map[string]any{"entries": map[string]any{
+		"parent": map[string]any{"enabled": false, "count": 0},
+		"child":  map[string]any{"extends": "parent"},
+	}}
+	mapped, err := configkit.Load[inheritedConfiguration](configkit.Mapping("mapping", values))
+	if err != nil {
+		t.Fatal(err)
+	}
+	fromYAML, err := configkit.Load[inheritedConfiguration](configkit.Bytes("config.yml", []byte("entries: {parent: {enabled: false, count: 0}, child: {extends: parent}}\n")))
+	if err != nil || !reflect.DeepEqual(mapped.Config, fromYAML.Config) {
+		t.Fatalf("mapping and bytes differ: %#v, %#v, %v", mapped, fromYAML, err)
+	}
+}
+
+func TestFacadeInheritanceProtectsVersionOwnership(t *testing.T) {
+	for _, test := range []struct {
+		name, input string
+		path        configkit.Path
+		wantFailure bool
+	}{
+		{"inherited version", "entries: {parent: {count: 1}, child: {extends: parent}}\n", configkit.Keys("entries", "child", "count"), true},
+		{"overridden version", "entries: {parent: {count: 1}, child: {extends: parent, count: 1}}\n", configkit.Keys("entries", "child", "count"), true},
+		{"direct unchanged version", "entries: {parent: {enabled: false}, child: {extends: parent, count: 1}}\n", configkit.Keys("entries", "child", "count"), false},
+		{"consumed checked directive", "entries: {parent: {count: 1}, child: {extends: parent}}\n", configkit.Keys("entries", "child", "extends"), true},
+		{"independent top-level version", "version: v1\nentries: {parent: {count: 1}, child: {extends: parent}}\n", configkit.Keys("version"), false},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			checks := struct{ version, defaults, resolution, validation int }{}
+			result, err := configkit.Load(configkit.Bytes("version.yml", []byte(test.input)), configkit.Options[inheritedConfiguration]{Definition: configkit.Definition[inheritedConfiguration]{
+				Version: &configkit.VersionRule{Path: test.path, Check: func(configkit.Version, *configkit.Validation) error {
+					checks.version++
+					return nil
+				}},
+				Defaults: func() inheritedConfiguration { checks.defaults++; return inheritedConfiguration{} },
+				Resolve:  func(*inheritedConfiguration, *configkit.Resolution) error { checks.resolution++; return nil },
+				Validate: func(*inheritedConfiguration, *configkit.Validation) { checks.validation++ },
+			}})
+			if test.wantFailure {
+				if result != nil || err == nil {
+					t.Fatalf("protected version = %#v, %v", result, err)
+				}
+				loadError, ok := err.(*configkit.LoadError)
+				if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeInheritanceVersion ||
+					checks != (struct{ version, defaults, resolution, validation int }{version: 1}) {
+					t.Fatalf("version failure = %v, callbacks %#v", err, checks)
+				}
+			} else if err != nil || result == nil || checks != (struct{ version, defaults, resolution, validation int }{1, 1, 1, 1}) {
+				t.Fatalf("unchanged version = %#v, %v, callbacks %#v", result, err, checks)
+			}
+		})
+	}
+}
+
+func TestFacadeInheritanceFailureStopsLaterCallbacks(t *testing.T) {
+	checks := struct{ defaults, resolution, validation int }{}
+	result, err := configkit.Load(configkit.Bytes("invalid.yml", []byte("entries: {child: {extends: PRIVATE-REFERENCE-CANARY}}\n")), configkit.Options[inheritedConfiguration]{Definition: configkit.Definition[inheritedConfiguration]{
+		Defaults: func() inheritedConfiguration { checks.defaults++; return inheritedConfiguration{} },
+		Resolve:  func(*inheritedConfiguration, *configkit.Resolution) error { checks.resolution++; return nil },
+		Validate: func(*inheritedConfiguration, *configkit.Validation) { checks.validation++ },
+	}})
+	if result != nil || err == nil || checks != (struct{ defaults, resolution, validation int }{}) || strings.Contains(err.Error(), "PRIVATE-REFERENCE-CANARY") {
+		t.Fatalf("invalid inheritance = %#v, %v, callbacks %#v", result, err, checks)
+	}
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeInheritanceMissing {
+		t.Fatalf("missing reference diagnostics = %v", err)
+	}
+}
+
+func TestFacadeInheritanceResolvesAfterImportsAndCLIReferenceChange(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: parent.yml}]}\nentries: {child: {extends: first}}\n")},
+		"parent.yml": &fstest.MapFile{Data: []byte("entries: {first: {count: 1}, second: {count: 2}}\n")},
+	})
+	result, err := configkit.Load(filesystem.File("main.yml"), configkit.Options[inheritedConfiguration]{
+		Imports: true, Overrides: []configkit.Override{{Path: configkit.Keys("entries", "child", "extends"), Value: "second", Flag: "--extends"}},
+	})
+	if err != nil || result.Config.Entries["child"].Count != 2 {
+		t.Fatalf("import and CLI reference = %#v, %v", result, err)
+	}
+	history := result.Metadata().History(configkit.Keys("entries", "child", "extends"))
+	if len(history) < 3 || history[len(history)-1].Origin.Operation != configkit.ChangeInheritance {
+		t.Fatalf("superseded reference history = %#v", history)
+	}
+	conflicting := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: parent.yml}]}\nentries: {child: {extends: first}, first: {count: 3}}\n")},
+		"parent.yml": &fstest.MapFile{Data: []byte("entries: {first: {count: 1}}\n")},
+	})
+	_, err = configkit.Load[inheritedConfiguration](conflicting.File("main.yml"), configkit.Options[inheritedConfiguration]{Imports: true})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeComposeConflict {
+		t.Fatalf("strict conflict before expansion = %v", err)
+	}
+}
+
+func TestFacadeInheritanceSelectedOverlayCancelsRelationship(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":          &fstest.MapFile{Data: []byte("entries: {parent: {count: 7}, child: {extends: parent, enabled: false}}\n")},
+		"overlays/test.yml": &fstest.MapFile{Data: []byte("entries: {child: {extends: null}}\n")},
+	})
+	result, err := configkit.Load(filesystem.File("main.yml"), configkit.Options[inheritedConfiguration]{
+		Overlay: "test", Filesystem: &configkit.FilesystemContext{Root: ".", FS: &filesystem},
+	})
+	if err != nil || result.Config.Entries["child"].Count != 0 || result.Config.Entries["child"].Enabled {
+		if loadError, ok := err.(*configkit.LoadError); ok {
+			t.Fatalf("cancelled relationship = %#v, %v, diagnostics %#v", result, err, loadError.Diagnostics())
+		}
+		t.Fatalf("cancelled relationship = %#v, %v", result, err)
+	}
+	history := result.Metadata().History(configkit.Keys("entries", "child", "extends"))
+	if len(history) < 3 || history[len(history)-1].Origin.Operation != configkit.ChangeInheritance {
+		t.Fatalf("cancellation history = %#v", history)
+	}
+}
+
+func TestFacadeInheritedSecretsStayRedactedAfterLookup(t *testing.T) {
+	const canary = "INHERITED-RELEASE-TEST-CREDENTIAL"
+	t.Setenv("CONFIGKIT_INHERITED_TEST_SECRET", canary)
+	type entry struct {
+		Credential configkit.Secret `yaml:"credential"`
+		Plain      string           `yaml:"plain"`
+	}
+	type configuration struct {
+		Entries map[string]entry `yaml:"entries"`
+	}
+	result, err := configkit.Load[configuration](configkit.Bytes("secret.yml", []byte("entries: {parent: {credential: CONFIGKIT_INHERITED_TEST_SECRET, plain: visible}, child: {extends: parent}}\n")), configkit.Options[configuration]{Definition: configkit.Definition[configuration]{
+		Secret: func(path configkit.Path) bool { return path.String() == "/entries/parent/credential" },
+	}})
+	if err != nil || result.Config.Entries["child"].Credential.Value() != canary {
+		t.Fatalf("inherited credential = %#v, %v", result, err)
+	}
+	path := configkit.Keys("entries", "child", "credential")
+	explanation, found := result.Explain(path)
+	if !found || !explanation.Effective.Redacted || strings.Contains(explanation.Effective.YAML, canary) {
+		t.Fatalf("credential explanation = %#v, %v", explanation, found)
+	}
+	var rendered bytes.Buffer
+	if err := result.WriteEffective(&rendered); err != nil || strings.Contains(rendered.String(), canary) || !strings.Contains(rendered.String(), "visible") {
+		t.Fatalf("effective secret rendering = %q, %v", rendered.String(), err)
+	}
+	rendered.Reset()
+	if err := result.WriteExplanation(&rendered, path); err != nil || strings.Contains(rendered.String(), canary) {
+		t.Fatalf("historical secret rendering = %q, %v", rendered.String(), err)
+	}
+}

facade_layered_test.go

@@ -0,0 +1,243 @@
+package configkit_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type layeredFacadeConfig struct {
+	Settings map[string]string `yaml:"settings"`
+	Count    int               `yaml:"count"`
+	Enabled  bool              `yaml:"enabled"`
+	List     []string          `yaml:"list"`
+	Optional *string           `yaml:"optional"`
+	Derived  string            `yaml:"derived"`
+}
+
+func TestFacadeLayeredMixedSourcesAndInspection(t *testing.T) {
+	mainPath := filepath.Join(t.TempDir(), "main.yml")
+	if err := os.WriteFile(mainPath, []byte("settings: {shared: main, secret: new-canary}\ncount: 0\nenabled: false\nlist: []\noptional: null\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	lower := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":             &fstest.MapFile{Data: []byte("configkit: {imports: [{path: defaults.yml}]}\nsettings: {shared: lower, native: retained, secret: old-canary}\nlist: [lower]\n")},
+		"defaults.yml":         &fstest.MapFile{Data: []byte("settings: {imported: retained}\n")},
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("settings: {from_lower_namespace: retained}\n")},
+	})
+	selected := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("settings: {shared: overlay}\n")},
+	})
+	checks := struct{ defaults, resolution, validation int }{}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(mainPath)}, configkit.LayeredOptions[layeredFacadeConfig]{
+		Underlays: []configkit.Underlay{
+			{Inputs: []configkit.Input{
+				{Source: lower.File("main.yml"), Imports: true},
+				{Source: configkit.Mapping("programmatic-low", map[string]any{"settings": map[string]any{"mapping": "retained"}, "count": 7, "enabled": true})},
+				{Source: lower.File("overlays/testing.yml")},
+			}},
+			{Inputs: []configkit.Input{{Source: configkit.Mapping("programmatic-high", map[string]any{
+				"settings": map[string]any{"shared": "middle", "second": "retained"}, "list": []any{"middle"},
+			})}}},
+		},
+		Overlay: "testing", OverlayFilesystem: &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &selected},
+		Overrides: []configkit.Override{{Path: configkit.Keys("settings", "shared"), Value: "cli", Flag: "--shared"}},
+		Definition: configkit.Definition[layeredFacadeConfig]{
+			Defaults: func() layeredFacadeConfig { checks.defaults++; return layeredFacadeConfig{} },
+			Resolve: func(config *layeredFacadeConfig, resolution *configkit.Resolution) error {
+				checks.resolution++
+				if source, ok := resolution.Source("source-2"); !ok || source.Layer != configkit.LayerUnderlay || source.UnderlayIndex != 1 || source.Kind != configkit.SourceKindFilesystem {
+					t.Errorf("resolver source = %#v, %v", source, ok)
+				}
+				configkit.Default(resolution, configkit.Keys("derived"), &config.Derived, config.Settings["native"], "derive from native", configkit.Keys("settings", "native"))
+				return nil
+			},
+			Validate: func(config *layeredFacadeConfig, validation *configkit.Validation) { checks.validation++ },
+			Secret:   func(path configkit.Path) bool { return path.String() == "/settings/secret" },
+		},
+	})
+	if err != nil {
+		t.Fatalf("layered load: %v, diagnostics %#v", err, err.(*configkit.LoadError).Diagnostics())
+	}
+	wantSettings := map[string]string{
+		"shared": "cli", "secret": "new-canary", "native": "retained", "imported": "retained",
+		"mapping": "retained", "from_lower_namespace": "retained", "second": "retained",
+	}
+	if !reflect.DeepEqual(result.Config.Settings, wantSettings) || result.Config.Count != 0 || result.Config.Enabled || result.Config.List == nil || len(result.Config.List) != 0 || result.Config.Optional != nil || result.Config.Derived != "retained" {
+		t.Fatalf("configuration = %#v", result.Config)
+	}
+	if checks != (struct{ defaults, resolution, validation int }{1, 1, 1}) {
+		t.Fatalf("lifecycle calls = %#v", checks)
+	}
+	sources := result.Sources()
+	if len(sources) != 8 {
+		t.Fatalf("sources = %#v", sources)
+	}
+	wantLayers := []configkit.Layer{configkit.LayerBase, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerOverlay, configkit.LayerCLI}
+	wantIndexes := []int{0, 1, 1, 1, 1, 2, 0, 0}
+	wantKinds := []configkit.SourceKind{configkit.SourceKindFile, configkit.SourceKindFilesystem, configkit.SourceKindFilesystem, configkit.SourceKindMapping, configkit.SourceKindFilesystem, configkit.SourceKindMapping, configkit.SourceKindFilesystem, configkit.SourceKindCLI}
+	for index, source := range sources {
+		if source.ID != "source-"+strconv.Itoa(index+1) || source.Layer != wantLayers[index] || source.UnderlayIndex != wantIndexes[index] || source.Kind != wantKinds[index] {
+			t.Errorf("source %d = %#v", index, source)
+		}
+	}
+	if sources[4].Path != sources[6].Path || sources[4].Label != sources[6].Label {
+		t.Fatalf("namespace-collision fixture changed: %#v, %#v", sources[4], sources[6])
+	}
+	sources[1].Label = "changed"
+	if result.Sources()[1].Label != "bundle/main.yml" {
+		t.Fatal("source records alias retained metadata")
+	}
+	for _, test := range []struct {
+		path string
+		want configkit.Layer
+	}{
+		{"/settings/native", configkit.LayerUnderlay},
+		{"/settings/shared", configkit.LayerCLI},
+		{"/derived", configkit.LayerResolution},
+	} {
+		path, err := configkit.ParsePath(test.path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		info, ok := result.Metadata().Lookup(path)
+		if !ok || info.Origin.Layer != test.want {
+			t.Errorf("%s metadata = %#v, %v", test.path, info, ok)
+		}
+	}
+	container, ok := result.Metadata().Lookup(configkit.Keys("settings"))
+	if !ok || len(container.Origin.Locations) < 4 {
+		t.Fatalf("composite mapping origin = %#v, %v", container, ok)
+	}
+	explanation, ok := result.Explain(configkit.Keys("settings", "shared"))
+	if !ok || len(explanation.History) < 5 || explanation.Origin.Layer != configkit.LayerCLI {
+		t.Fatalf("shared history = %#v, %v", explanation, ok)
+	}
+	var effective, rendered bytes.Buffer
+	if err := result.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if err := result.WriteExplanation(&rendered, configkit.Keys("settings", "secret")); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String()+rendered.String(), "old-canary") || strings.Contains(effective.String()+rendered.String(), "new-canary") || !strings.Contains(rendered.String(), "redacted") {
+		t.Fatalf("secret rendering leaked: effective %q, explanation %q", effective.String(), rendered.String())
+	}
+}
+
+func TestFacadeLayeredDisplacedDescendantRemainsInspectable(t *testing.T) {
+	type config struct {
+		Choice string `yaml:"choice"`
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.Mapping("main", map[string]any{"choice": "flat"})}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{
+			"choice": map[string]any{"nested": "historical"},
+		})}}}},
+	})
+	if err != nil || result.Config.Choice != "flat" {
+		t.Fatalf("replacement load = %#v, %v", result, err)
+	}
+	path := configkit.Keys("choice", "nested")
+	explanation, ok := result.Explain(path)
+	if !ok || explanation.Effective.Present || len(explanation.History) == 0 || explanation.History[0].Origin.Layer != configkit.LayerUnderlay {
+		t.Fatalf("deleted descendant = %#v, %v", explanation, ok)
+	}
+}
+
+func TestFacadeLayeredZeroUnderlaysMatchesOrdinaryLoad(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("value: selected\n")},
+	})
+	context := &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem}
+	source := configkit.Bytes("main", []byte("value: base\n"))
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	ordinary, err := configkit.Load(source, configkit.Options[config]{Overlay: "testing", Filesystem: context})
+	if err != nil {
+		t.Fatal(err)
+	}
+	layered, err := configkit.LoadLayered(configkit.Input{Source: source}, configkit.LayeredOptions[config]{Overlay: "testing", OverlayFilesystem: context})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ordinary.Config != layered.Config || !reflect.DeepEqual(ordinary.Sources(), layered.Sources()) {
+		t.Fatalf("ordinary = %#v, %#v; layered = %#v, %#v", ordinary.Config, ordinary.Sources(), layered.Config, layered.Sources())
+	}
+	var first, second bytes.Buffer
+	if err := ordinary.WriteEffective(&first); err != nil {
+		t.Fatal(err)
+	}
+	if err := layered.WriteEffective(&second); err != nil {
+		t.Fatal(err)
+	}
+	if first.String() != second.String() {
+		t.Fatalf("effective ordinary %q, layered %q", first.String(), second.String())
+	}
+	if ordinaryExplanation, ok := ordinary.Explain(configkit.Keys("value")); !ok {
+		t.Fatal("ordinary explanation missing")
+	} else if layeredExplanation, ok := layered.Explain(configkit.Keys("value")); !ok || !reflect.DeepEqual(ordinaryExplanation, layeredExplanation) {
+		t.Fatalf("explanations differ: %#v, %#v", ordinaryExplanation, layeredExplanation)
+	}
+}
+
+func TestFacadeLayeredOptionAndVersionFailures(t *testing.T) {
+	type config struct {
+		Version string `yaml:"version"`
+	}
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("{}\n")},
+	})
+	for _, test := range []struct {
+		name    string
+		main    configkit.Input
+		options []configkit.LayeredOptions[config]
+		want    configkit.Code
+	}{
+		{"zero source", configkit.Input{}, nil, configkit.CodeSourceInvalid},
+		{"multiple options", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{}, {}}, configkit.CodeOptionsInvalid},
+		{"missing overlay context", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{Overlay: "testing"}}, configkit.CodeOptionsInvalid},
+		{"selected file already acquired", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{
+			Overlay: "testing", OverlayFilesystem: &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &namespace},
+			Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: namespace.File("overlays/testing.yml")}}}},
+		}}, configkit.CodeOverlayRepeated},
+		{"underlay version", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{
+			Definition: configkit.Definition[config]{Version: &configkit.VersionRule{Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil }}},
+			Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"version": "forbidden"})}}}},
+		}}, configkit.CodeUnderlayVersion},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			_, err := configkit.LoadLayered(test.main, test.options...)
+			loadError, ok := err.(*configkit.LoadError)
+			if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != test.want {
+				t.Fatalf("error = %#v, want %s", err, test.want)
+			}
+		})
+	}
+}
+
+func TestFacadeLayeredExplicitHostOverlayWithoutMain(t *testing.T) {
+	root := t.TempDir()
+	if err := os.Mkdir(filepath.Join(root, "overlays"), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(root, "overlays", "testing.yml"), []byte("value: selected\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Overlay: "testing", OverlayFilesystem: &configkit.FilesystemContext{Root: root, BaseDir: root},
+	})
+	if err != nil || result.Config.Value != "selected" || len(result.Sources()) != 1 || result.Sources()[0].Kind != configkit.SourceKindFile {
+		t.Fatalf("explicit host overlay = %#v, %v", result, err)
+	}
+}

facade_mapping_test.go

@@ -0,0 +1,133 @@
+package configkit_test
+
+import (
+	"math"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type mappingFacadeConfig struct {
+	Enabled  bool     `yaml:"enabled"`
+	Count    uint64   `yaml:"count"`
+	Optional *string  `yaml:"optional"`
+	Items    []string `yaml:"items"`
+	Name     string   `yaml:"name"`
+}
+
+func TestFacadeMappingSnapshotsAndMatchesYAML(t *testing.T) {
+	items := []any{"one", "two"}
+	values := map[string]any{
+		"name": "original", "enabled": false, "count": ^uint64(0),
+		"optional": nil, "items": items,
+	}
+	source := configkit.Mapping("programmatic", values)
+	values["name"] = "mutated"
+	items[0] = "mutated"
+	for range 2 {
+		result, err := configkit.Load[mappingFacadeConfig](source)
+		if err != nil {
+			t.Fatal(err)
+		}
+		want := mappingFacadeConfig{Count: ^uint64(0), Items: []string{"one", "two"}, Name: "original"}
+		if !reflect.DeepEqual(result.Config, want) {
+			t.Fatalf("mapping config = %#v, want %#v", result.Config, want)
+		}
+		if sources := result.Sources(); len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Kind != configkit.SourceKindMapping || sources[0].Layer != configkit.LayerBase || sources[0].Path != "" {
+			t.Fatalf("mapping sources = %#v", sources)
+		}
+		info, ok := result.Metadata().Lookup(configkit.Keys("name"))
+		if !ok || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != "source-1" || info.Origin.Locations[0].Line != 0 {
+			t.Fatalf("mapping name origin = %#v, %v", info, ok)
+		}
+		fromYAML, err := configkit.Load[mappingFacadeConfig](configkit.Bytes("equivalent", []byte("enabled: false\ncount: 18446744073709551615\noptional: null\nitems: [one, two]\nname: original\n")))
+		if err != nil || !reflect.DeepEqual(result.Config, fromYAML.Config) {
+			t.Fatalf("equivalent YAML = %#v, %v", fromYAML, err)
+		}
+	}
+}
+
+func TestFacadeMappingNullEmptyAndVersion(t *testing.T) {
+	result, err := configkit.Load[mappingFacadeConfig](configkit.Mapping("empty", map[string]any{}))
+	if err != nil || len(result.Sources()) != 1 {
+		t.Fatalf("empty mapping = %#v, %v", result, err)
+	}
+	var nilSequence []any
+	var nilMap map[string]any
+	var checked string
+	result, err = configkit.Load(configkit.Mapping("nullable", map[string]any{
+		"items": nilSequence, "optional": nilMap, "enabled": false, "count": uint64(0),
+	}), configkit.Options[mappingFacadeConfig]{Definition: configkit.Definition[mappingFacadeConfig]{
+		Version: &configkit.VersionRule{Path: configkit.Keys("count"), Check: func(version configkit.Version, validation *configkit.Validation) error {
+			checked = version.Text
+			return nil
+		}},
+	}})
+	if err != nil || checked != "0" || result.Config.Items != nil || result.Config.Optional != nil {
+		t.Fatalf("null and zero mapping = %#v, checked %q, %v", result, checked, err)
+	}
+	info, ok := result.Metadata().Lookup(configkit.Keys("items"))
+	if !ok || !info.Null || !info.Explicit {
+		t.Fatalf("null metadata = %#v, %v", info, ok)
+	}
+}
+
+func TestFacadeMappingImportsUseExplicitContext(t *testing.T) {
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"root/shared.yml": &fstest.MapFile{Data: []byte("name: imported\n")},
+	})
+	source := configkit.Mapping("programmatic", map[string]any{
+		"configkit": map[string]any{"imports": []any{map[string]any{"path": "shared.yml"}}},
+		"enabled":   true,
+	})
+	result, err := configkit.Load(source, configkit.Options[mappingFacadeConfig]{
+		Imports: true, Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: "root", FS: &namespace},
+	})
+	if err != nil || result.Config.Name != "imported" || !result.Config.Enabled {
+		t.Fatalf("mapping import = %#v, %v", result, err)
+	}
+	if sources := result.Sources(); len(sources) != 2 || sources[0].Kind != configkit.SourceKindMapping || sources[1].Kind != configkit.SourceKindFilesystem || sources[1].Layer != configkit.LayerBase {
+		t.Fatalf("mapping import sources = %#v", sources)
+	}
+	checkMappingCode(t, source, configkit.Options[mappingFacadeConfig]{Imports: true}, configkit.CodeOptionsInvalid)
+	checkMappingCode(t, source, configkit.Options[mappingFacadeConfig]{}, configkit.CodeImportDisabled)
+}
+
+func TestFacadeMappingDeferredValueDiagnostics(t *testing.T) {
+	cycle := map[string]any{}
+	cycle["secret"] = cycle
+	for _, test := range []struct {
+		name   string
+		source configkit.Source
+		want   configkit.Code
+		path   string
+	}{
+		{"nil root", configkit.Mapping("nil", nil), configkit.CodeSourceInvalid, ""},
+		{"bad label", configkit.Mapping("bad\nlabel", map[string]any{}), configkit.CodeSourceInvalid, ""},
+		{"unsupported", configkit.Mapping("bad", map[string]any{"secret": struct{ Value string }{"canary-value"}}), configkit.CodeSourceInvalid, "/secret"},
+		{"nonfinite", configkit.Mapping("bad", map[string]any{"count": math.NaN()}), configkit.CodeSourceInvalid, "/count"},
+		{"cycle", configkit.Mapping("bad", cycle), configkit.CodeSourceInvalid, "/secret"},
+		{"too many bytes", configkit.Mapping("bad", map[string]any{"name": strings.Repeat("x", 8<<20+1)}), configkit.CodeSourceLimitExceeded, "/name"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			loadError := checkMappingCode(t, test.source, configkit.Options[mappingFacadeConfig]{}, test.want)
+			diagnostic := loadError.Diagnostics()[0]
+			if diagnostic.Path.String() != test.path || diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || strings.Contains(diagnostic.Message, "canary-value") {
+				t.Fatalf("diagnostic = %#v", diagnostic)
+			}
+		})
+	}
+}
+
+func checkMappingCode(t *testing.T, source configkit.Source, options configkit.Options[mappingFacadeConfig], want configkit.Code) *configkit.LoadError {
+	t.Helper()
+	_, err := configkit.Load(source, options)
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != want {
+		t.Fatalf("error = %#v, want %s", err, want)
+	}
+	return loadError
+}

facade_resource_context_test.go

@@ -0,0 +1,221 @@
+package configkit_test
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type observedResourceFS struct {
+	files  fstest.MapFS
+	opens  int
+	closed bool
+}
+
+func (f *observedResourceFS) Open(name string) (fs.File, error) {
+	f.opens++
+	if f.closed {
+		return nil, fs.ErrClosed
+	}
+	return f.files.Open(name)
+}
+
+type resourceFacadeConfig struct {
+	Native   string `yaml:"native"`
+	Shared   string `yaml:"shared"`
+	Host     string `yaml:"host"`
+	Selected string `yaml:"selected"`
+}
+
+func TestFacadeSourceContextsFollowOriginsWithoutQueryIO(t *testing.T) {
+	host := t.TempDir()
+	if err := os.WriteFile(filepath.Join(host, "main.yml"), []byte("host: host.txt\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(host, "host.txt"), []byte("host-v1"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	borrowed := &observedResourceFS{files: fstest.MapFS{
+		"bundle/config/defaults.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: ../shared.yml}]}\nnative: icon.txt\n")},
+		"bundle/shared.yml":           &fstest.MapFile{Data: []byte("shared: shared.txt\n")},
+		"bundle/overlays/test.yml":    &fstest.MapFile{Data: []byte("selected: overlay.txt\n")},
+		"bundle/config/icon.txt":      &fstest.MapFile{Data: []byte("icon-v1")},
+		"bundle/shared.txt":           &fstest.MapFile{Data: []byte("shared-v1")},
+		"bundle/overlays/overlay.txt": &fstest.MapFile{Data: []byte("overlay-v1")},
+	}}
+	namespace := configkit.NewFilesystem("bundle", borrowed)
+	var resolverContext configkit.ResourceContext
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(filepath.Join(host, "main.yml"))}, configkit.LayeredOptions[resourceFacadeConfig]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+			Source: namespace.File("bundle/config/defaults.yml"), Imports: true,
+			Filesystem: &configkit.FilesystemContext{Root: "bundle", FS: &namespace},
+		}}}},
+		Overlay: "test", OverlayFilesystem: &configkit.FilesystemContext{Root: "bundle", BaseDir: "bundle", FS: &namespace},
+		Definition: configkit.Definition[resourceFacadeConfig]{Resolve: func(config *resourceFacadeConfig, resolution *configkit.Resolution) error {
+			before := borrowed.opens
+			var ok bool
+			resolverContext, ok = resolution.SourceContext("source-2")
+			if !ok || resolverContext.BaseDir != "config" || resolverContext.FS == nil {
+				t.Errorf("resolver context = %#v, %v", resolverContext, ok)
+			}
+			if _, ok := resolution.SourceContext("missing"); ok || borrowed.opens != before {
+				t.Error("resolution context query performed I/O or guessed an unknown source")
+			}
+			return nil
+		}},
+	})
+	if err != nil {
+		t.Fatalf("load: %v, diagnostics %#v", err, err.(*configkit.LoadError).Diagnostics())
+	}
+	queriesBefore := borrowed.opens
+	for _, test := range []struct {
+		field, wantBase, wantData string
+	}{
+		{"native", "config", "icon-v1"},
+		{"shared", ".", "shared-v1"},
+		{"selected", "overlays", "overlay-v1"},
+		{"host", ".", "host-v1"},
+	} {
+		origin, ok := result.Metadata().Lookup(configkit.Keys(test.field))
+		if !ok || len(origin.Origin.Locations) != 1 {
+			t.Fatalf("%s origin = %#v, %v", test.field, origin, ok)
+		}
+		context, ok := result.SourceContext(origin.Origin.Locations[0].SourceID)
+		if !ok || context.FS == nil || context.BaseDir != test.wantBase {
+			t.Fatalf("%s context = %#v, %v", test.field, context, ok)
+		}
+		if borrowed.opens != queriesBefore {
+			t.Fatal("result SourceContext query performed filesystem I/O")
+		}
+		resourceName := path.Join(context.BaseDir, map[string]string{
+			"native": result.Config.Native, "shared": result.Config.Shared,
+			"selected": result.Config.Selected, "host": result.Config.Host,
+		}[test.field])
+		data, err := fs.ReadFile(context.FS, resourceName)
+		if err != nil || string(data) != test.wantData {
+			t.Fatalf("%s resource = %q, %v", test.field, data, err)
+		}
+		queriesBefore = borrowed.opens
+	}
+	if _, ok := result.SourceContext(""); ok {
+		t.Fatal("empty source ID returned a context")
+	}
+	var nilResult *configkit.Result[resourceFacadeConfig]
+	if _, ok := nilResult.SourceContext("source-2"); ok {
+		t.Fatal("nil result returned a context")
+	}
+	if _, err := fs.ReadFile(resolverContext.FS, "../shared.txt"); !errors.Is(err, fs.ErrInvalid) {
+		t.Fatalf("native root escape = %v", err)
+	}
+	borrowed.files["bundle/config/icon.txt"].Data = []byte("icon-v2")
+	data, err := fs.ReadFile(resolverContext.FS, "config/icon.txt")
+	if err != nil || string(data) != "icon-v2" {
+		t.Fatalf("updated native resource = %q, %v", data, err)
+	}
+	borrowed.closed = true
+	if result.Config.Native != "icon.txt" {
+		t.Fatal("closing caller filesystem altered loaded configuration")
+	}
+	if _, err := fs.ReadFile(resolverContext.FS, "config/icon.txt"); !errors.Is(err, fs.ErrClosed) {
+		t.Fatalf("closed caller filesystem error = %v", err)
+	}
+}
+
+func TestFacadeHostContextSurvivesConfigurationDeletion(t *testing.T) {
+	root := t.TempDir()
+	main := filepath.Join(root, "main.yml")
+	resource := filepath.Join(root, "asset.txt")
+	if err := os.WriteFile(main, []byte("native: asset.txt\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(resource, []byte("first"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	result, err := configkit.Load[resourceFacadeConfig](configkit.File(main))
+	if err != nil {
+		t.Fatal(err)
+	}
+	context, ok := result.SourceContext("source-1")
+	if !ok || context.BaseDir != "." {
+		t.Fatalf("host context = %#v, %v", context, ok)
+	}
+	if err := os.Remove(main); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(resource, []byte("second"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, result.Config.Native))
+	if err != nil || string(data) != "second" || result.Config.Native != "asset.txt" {
+		t.Fatalf("resource after source deletion = %q, %v", data, err)
+	}
+	if _, err := fs.ReadFile(context.FS, "../asset.txt"); !errors.Is(err, fs.ErrInvalid) {
+		t.Fatalf("invalid host path = %v", err)
+	}
+	if err := os.Remove(resource); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := fs.ReadFile(context.FS, "asset.txt"); !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("missing later resource = %v", err)
+	}
+	outside := filepath.Join(t.TempDir(), "outside.txt")
+	if err := os.WriteFile(outside, []byte("outside"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(outside, filepath.Join(root, "escape")); err == nil {
+		if _, err := fs.ReadFile(context.FS, "escape"); !errors.Is(err, fs.ErrPermission) {
+			t.Fatalf("host symlink escape = %v", err)
+		}
+	}
+}
+
+func TestFacadeSyntheticSourceContextsAndContextlessSources(t *testing.T) {
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"bundle/config/icon.txt": &fstest.MapFile{Data: []byte("icon")},
+	})
+	context := &configkit.FilesystemContext{Root: "bundle", BaseDir: "bundle/config", FS: &namespace}
+	for _, test := range []struct {
+		name   string
+		source configkit.Source
+	}{
+		{"bytes", configkit.Bytes("memory", []byte("native: icon.txt\n"))},
+		{"reader", configkit.Reader("stream", strings.NewReader("native: icon.txt\n"))},
+		{"mapping", configkit.Mapping("values", map[string]any{"native": "icon.txt"})},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := configkit.Load(test.source, configkit.Options[resourceFacadeConfig]{Filesystem: context})
+			if err != nil {
+				t.Fatal(err)
+			}
+			resource, ok := result.SourceContext("source-1")
+			if !ok || resource.BaseDir != "config" {
+				t.Fatalf("synthetic context = %#v, %v", resource, ok)
+			}
+			data, err := fs.ReadFile(resource.FS, path.Join(resource.BaseDir, result.Config.Native))
+			if err != nil || string(data) != "icon" {
+				t.Fatalf("synthetic resource = %q, %v", data, err)
+			}
+		})
+	}
+	result, err := configkit.Load[resourceFacadeConfig](configkit.Bytes("plain", []byte("native: none\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := result.SourceContext("source-1"); ok {
+		t.Fatal("contextless byte source returned a context")
+	}
+	result, err = configkit.Load(configkit.NoSource(), configkit.Options[resourceFacadeConfig]{Overrides: []configkit.Override{{Path: configkit.Keys("native"), Value: "from-cli", Flag: "--native"}}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := result.SourceContext("source-1"); ok {
+		t.Fatal("CLI source returned a context")
+	}
+}

facade_underlay_origins_test.go

@@ -0,0 +1,107 @@
+package configkit_test
+
+import (
+	"reflect"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestUnderlaySequenceDescendantOrigins(t *testing.T) {
+	for _, later := range []bool{false, true} {
+		name := "first tier"
+		if later {
+			name = "later tier"
+		}
+		t.Run(name, func(t *testing.T) {
+			bundle := configkit.NewFilesystem("bundle", fstest.MapFS{
+				"main.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: items.yml}]}\n")},
+				"items.yml": &fstest.MapFile{Data: []byte("items: [{name: first, nested: [false]}]\n")},
+			})
+			tiers := []configkit.Underlay{}
+			if later {
+				tiers = append(tiers, configkit.Underlay{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"other": true})}}})
+			}
+			tiers = append(tiers, configkit.Underlay{Inputs: []configkit.Input{{Source: bundle.File("main.yml"), Imports: true}}})
+			paths := []string{"/items", "/items/0", "/items/0/name", "/items/0/nested", "/items/0/nested/0"}
+			entry := map[string]configkit.Origin{}
+			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Mapping("main", map[string]any{"main": true})}, configkit.LayeredOptions[map[string]any]{
+				Underlays: tiers,
+				Definition: configkit.Definition[map[string]any]{Resolve: func(_ *map[string]any, resolution *configkit.Resolution) error {
+					for _, pointer := range paths {
+						path, _ := configkit.ParsePath(pointer)
+						info, ok := resolution.InputInfo(path)
+						if !ok {
+							t.Errorf("missing resolver input %s", pointer)
+						}
+						entry[pointer] = info.Origin
+					}
+					return nil
+				}},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			var sourceID string
+			for _, source := range result.Sources() {
+				if source.Path == "items.yml" {
+					sourceID = source.ID
+				}
+			}
+			if sourceID == "" {
+				t.Fatal("missing imported source")
+			}
+			for _, pointer := range paths {
+				path, _ := configkit.ParsePath(pointer)
+				info, ok := result.Metadata().Lookup(path)
+				if !ok || info.Origin.Layer != configkit.LayerUnderlay || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != sourceID {
+					t.Errorf("%s origin = %+v", pointer, info.Origin)
+				}
+				if !reflect.DeepEqual(entry[pointer], info.Origin) {
+					t.Errorf("%s resolver origin differs", pointer)
+				}
+				explanation, ok := result.Explain(path)
+				if !ok || !reflect.DeepEqual(explanation.Origin, info.Origin) {
+					t.Errorf("%s explanation origin differs", pointer)
+				}
+				history := result.Metadata().History(path)
+				if len(history) == 0 || !reflect.DeepEqual(history[len(history)-1].Origin, info.Origin) {
+					t.Errorf("%s history = %+v", pointer, history)
+				}
+			}
+		})
+	}
+}
+
+func TestUnderlayHistoryRetainsLaterMappingContributors(t *testing.T) {
+	result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{Underlays: []configkit.Underlay{
+		{Inputs: []configkit.Input{{Source: configkit.Mapping("one", map[string]any{"x": 1})}}},
+		{Inputs: []configkit.Input{{Source: configkit.Mapping("two", map[string]any{"x": map[string]any{}})}}},
+		{Inputs: []configkit.Input{{Source: configkit.Mapping("three", map[string]any{"x": map[string]any{"y": 2}})}}},
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	path := configkit.Keys("x")
+	info, _ := result.Metadata().Lookup(path)
+	wantOrigin := configkit.Origin{Layer: configkit.LayerUnderlay, Locations: []configkit.Location{{SourceID: "source-2"}, {SourceID: "source-3"}}}
+	if !reflect.DeepEqual(info.Origin, wantOrigin) {
+		t.Fatalf("effective origin = %+v, want %+v", info.Origin, wantOrigin)
+	}
+	history := result.Metadata().History(path)
+	if len(history) != 3 || !reflect.DeepEqual(history[len(history)-1].Origin, info.Origin) {
+		t.Fatalf("history = %+v, effective origin = %+v", history, info.Origin)
+	}
+	explanation, ok := result.Explain(path)
+	if !ok || !reflect.DeepEqual(explanation.Origin, info.Origin) {
+		t.Fatal("explanation and metadata disagree")
+	}
+	if len(explanation.History) != 3 {
+		t.Fatalf("explanation history = %+v", explanation.History)
+	}
+	last := explanation.History[len(explanation.History)-1].Origin
+	if last.Layer != configkit.LayerUnderlay || len(last.Locations) != 1 || last.Locations[0].SourceID != "source-3" {
+		t.Fatalf("last explanation origin = %+v", last)
+	}
+}

inspection.go

@@ -39,7 +39,8 @@ type Explanation struct {
 }
 
 // Explain returns safe effective and chronological values for a known final or
-// historical path. Unknown and malformed paths return false.
+// historical path. Structural inheritance appears between source applications
+// and typed reconciliation. Unknown and malformed paths return false.
 func (r *Result[T]) Explain(path Path) (Explanation, bool) {
 	if r == nil {
 		return Explanation{}, false

internal/engine/assembly_bench_test.go

@@ -0,0 +1,119 @@
+package engine
+
+import (
+	"strconv"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func BenchmarkComposeWideMappings(b *testing.B) {
+	for _, width := range []int{128, 256, 512} {
+		b.Run(strconv.Itoa(width), func(b *testing.B) {
+			left := benchmarkCatalog("source-1", 0, width)
+			right := benchmarkCatalog("source-2", width, width)
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := composeDocuments(left, right); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkOverlayRecursiveMappings(b *testing.B) {
+	for _, width := range []int{128, 256, 512} {
+		b.Run(strconv.Itoa(width), func(b *testing.B) {
+			base, diagnostics := composeDocuments(benchmarkCatalog("source-1", 0, width))
+			if len(diagnostics) != 0 {
+				b.Fatal(diagnostics)
+			}
+			overlay := benchmarkCatalog("source-2", width/2, width)
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := overlayDocuments(base, overlay); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkOverlayManyApplications(b *testing.B) {
+	for _, count := range []int{32, 64, 128} {
+		inputs := make([]*document.Node, count)
+		for index := range inputs {
+			inputs[index] = benchmarkCatalog("source-2", index, 1)
+		}
+		benchmarkOverlayApplications(b, count, inputs)
+	}
+}
+
+func BenchmarkOverlayRepeatedReplacement(b *testing.B) {
+	for _, count := range []int{32, 64, 128} {
+		inputs := make([]*document.Node, count)
+		for index := range inputs {
+			location := document.Location{SourceID: "source-2"}
+			value := &document.Node{Kind: document.String, Text: "replacement", Location: location}
+			if index%2 == 0 {
+				value = &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+					Key: "enabled", Value: &document.Node{Kind: document.Boolean, Text: "false", Location: location},
+				}}}
+			}
+			inputs[index] = &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+				Key: "catalog", Value: &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+					Key: "item-0", Value: value,
+				}}},
+			}}}
+		}
+		benchmarkOverlayApplications(b, count, inputs)
+	}
+}
+
+func benchmarkOverlayApplications(b *testing.B, count int, inputs []*document.Node) {
+	base, diagnostics := composeDocuments(benchmarkCatalog("source-1", 0, 1))
+	if len(diagnostics) != 0 {
+		b.Fatal(diagnostics)
+	}
+	for _, mode := range []string{"owned", "copy-preserving"} {
+		b.Run(mode+"/"+strconv.Itoa(count), func(b *testing.B) {
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if mode == "owned" {
+					assembly := newAssemblyAccumulator(cloneComposedDocument(base))
+					for _, input := range inputs {
+						if diagnostics := assembly.applyOverlay(input, nil, nil, LayerOverlay); len(diagnostics) != 0 {
+							b.Fatal(diagnostics)
+						}
+					}
+					continue
+				}
+				current := base
+				for _, input := range inputs {
+					current, diagnostics = overlayDocuments(current, input)
+					if len(diagnostics) != 0 {
+						b.Fatal(diagnostics)
+					}
+				}
+			}
+		})
+	}
+}
+
+func benchmarkCatalog(source string, first, width int) *document.Node {
+	location := document.Location{SourceID: source}
+	entries := make([]document.Entry, width)
+	for index := range entries {
+		entries[index] = document.Entry{
+			Key:   "item-" + strconv.Itoa(first+index),
+			Value: &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{Key: "enabled", Value: &document.Node{Kind: document.Boolean, Text: "true", Location: location}}}},
+		}
+	}
+	return &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+		Key: "catalog", Value: &document.Node{Kind: document.Mapping, Location: location, Mapping: entries},
+	}}}
+}

internal/engine/compose.go

@@ -15,6 +15,17 @@ type composedDocument struct {
 	inputNodes   int
 }
 
+// assemblyAccumulator owns one mutable working document. Acquired trees held by
+// history applications are separate from its root and remain unchanged.
+type assemblyAccumulator struct {
+	current *composedDocument
+	index   mappingIndex
+}
+
+func newAssemblyAccumulator(owned *composedDocument) *assemblyAccumulator {
+	return &assemblyAccumulator{current: owned}
+}
+
 // cloneComposedDocument detaches mutable composition and provenance state;
 // history application trees are immutable and retain their existing sharing.
 func cloneComposedDocument(base *composedDocument) *composedDocument {
@@ -58,8 +69,14 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 		collected.append(input, Path{})
 	}
 
+	checked := make(map[string]struct{}, len(collected.byPath))
 	for _, claim := range collected.ordered {
-		claims := collected.byPath[claim.path.String()]
+		pointer := claim.path.String()
+		if _, seen := checked[pointer]; seen {
+			continue
+		}
+		checked[pointer] = struct{}{}
+		claims := collected.byPath[pointer]
 		if len(claims) < 2 || allMappingClaims(claims) {
 			continue
 		}
@@ -67,21 +84,43 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 	}
 
 	root := document.Clone(inputs[0])
+	index := new(mappingIndex)
 	for _, input := range inputs[1:] {
-		mergeDisjointMappings(root, input)
+		mergeDisjointMappings(root, input, index)
 	}
 	applications := make([]historyApplication, 0, len(inputs))
 	for _, input := range inputs {
 		applications = append(applications, historyApplication{layer: LayerBase, value: input})
 	}
+	origins := compositionOrigins(collected.ordered)
+	completeCompositionOrigins(root, Path{}, origins)
 	return &composedDocument{
 		root:         root,
 		claims:       cloneCompositionClaims(collected.ordered),
-		origins:      compositionOrigins(collected.ordered),
+		origins:      origins,
 		applications: applications,
 	}, nil
 }
 
+// Conflict claims treat sequences atomically, but inspection needs origins for
+// every descendant. Preserve composite mapping claims while filling those gaps.
+func completeCompositionOrigins(node *document.Node, path Path, origins map[string]Origin) {
+	pointer := path.String()
+	if _, present := origins[pointer]; !present {
+		origins[pointer] = Origin{Layer: LayerBase, Locations: []Location{publicLocation(node.Location)}}
+	}
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			completeCompositionOrigins(entry.Value, path.Key(entry.Key), origins)
+		}
+	case document.Sequence:
+		for index, child := range node.Sequence {
+			completeCompositionOrigins(child, path.Index(index), origins)
+		}
+	}
+}
+
 func (c *compositionClaims) append(node *document.Node, path Path) {
 	claim := compositionClaim{
 		path:     path.clone(),
@@ -109,18 +148,18 @@ func allMappingClaims(claims []compositionClaim) bool {
 	return true
 }
 
-func mergeDisjointMappings(target, incoming *document.Node) {
+func mergeDisjointMappings(target, incoming *document.Node, index *mappingIndex) {
 	for _, incomingEntry := range incoming.Mapping {
-		index := documentEntryIndex(target.Mapping, incomingEntry.Key)
-		if index < 0 {
+		position := index.find(target, incomingEntry.Key)
+		if position < 0 {
 			cloned := incomingEntry
 			cloned.Value = document.Clone(incomingEntry.Value)
-			target.Mapping = append(target.Mapping, cloned)
+			index.append(target, cloned)
 			continue
 		}
 		// Claim validation guarantees that the only remaining overlap is a
 		// mapping pair. Empty mappings naturally contribute no child entries.
-		mergeDisjointMappings(target.Mapping[index].Value, incomingEntry.Value)
+		mergeDisjointMappings(target.Mapping[position].Value, incomingEntry.Value, index)
 	}
 }
 

internal/engine/compose_test.go

@@ -116,6 +116,31 @@ func TestComposeDocumentsReportsAllClaimantsAtExactLocations(t *testing.T) {
 	assertCompositionConflict(t, diagnostics, "/group", []string{"source-1", "source-2", "source-3"})
 }
 
+func TestComposeDocumentsKeepsFirstConflictAndAppendedEntryOrder(t *testing.T) {
+	first := parseCompositionDocument(t, "source-1", "catalog:\n  first: 1\n  shared: one\n  later: one\n")
+	second := parseCompositionDocument(t, "source-2", "catalog:\n  second: 2\n  shared: two\n  later: two\n")
+	third := parseCompositionDocument(t, "source-3", "catalog:\n  third: 3\n  shared: three\n")
+	_, diagnostics := composeDocuments(first, second, third)
+	assertCompositionConflict(t, diagnostics, "/catalog/shared", []string{"source-1", "source-2", "source-3"})
+
+	composed, diagnostics := composeDocuments(
+		parseCompositionDocument(t, "source-1", "catalog:\n  first: 1\n"),
+		parseCompositionDocument(t, "source-2", "catalog:\n  second: 2\n"),
+		parseCompositionDocument(t, "source-3", "catalog:\n  third: 3\n"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("composeDocuments() diagnostics = %#v", diagnostics)
+	}
+	entries := composed.root.Mapping[0].Value.Mapping
+	got := make([]string, len(entries))
+	for index, entry := range entries {
+		got[index] = entry.Key
+	}
+	if want := []string{"first", "second", "third"}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("catalog keys = %#v, want %#v", got, want)
+	}
+}
+
 func TestComposeDocumentsTreatsEmptyMappingsAsChildlessContributions(t *testing.T) {
 	empty, diagnostics := composeDocuments(
 		parseCompositionDocument(t, "source-1", "{}\n"),

internal/engine/explain.go

@@ -51,13 +51,14 @@ func (r *Inspection) Explain(path Path) (Explanation, bool) {
 			After: valueSnapshot(working, path, r.secrets), Origin: Origin{Layer: LayerDefault},
 		})
 	}
+	replayIndex := new(mappingIndex)
 	for _, application := range r.applications {
 		operationPath, relevant := applicationOperationPath(application, path)
 		var before ValueSnapshot
 		if relevant {
 			before = valueSnapshot(working, path, r.secrets)
 		}
-		applyHistoryApplication(&working, application)
+		applyHistoryApplication(&working, application, replayIndex)
 		if !relevant {
 			continue
 		}
@@ -71,6 +72,7 @@ func (r *Inspection) Explain(path Path) (Explanation, bool) {
 			Origin: historyApplicationOrigin(application, operationPath),
 		})
 	}
+	appendInheritanceExplanation(&explanation, &working, path, r.inheritance, r.secrets)
 
 	// Intermediate input replacements do not permanently suppress defaults:
 	// decoding applies them to the final input tree. Show that reconciliation

internal/engine/explain_render.go

@@ -23,10 +23,14 @@ func (r *Inspection) WriteExplanation(output io.Writer, path Path) error {
 		return errExplanationUnknownPath
 	}
 	var rendered strings.Builder
+	labels := make(map[string]string, len(r.sources))
+	for _, source := range r.sources {
+		labels[source.ID] = source.Label
+	}
 	rendered.WriteString("path: ")
 	rendered.WriteString(strconv.Quote(explanation.Path.String()))
 	rendered.WriteByte('\n')
-	writeExplanationOrigin(&rendered, "effective source", explanation.Origin, r.sources)
+	writeExplanationOrigin(&rendered, "effective source", explanation.Origin, labels)
 	writeExplanationSnapshot(&rendered, "effective (canonical typed)", explanation.Effective, 0)
 	if explanation.SecretReference != nil {
 		rendered.WriteString("secret reference: ")
@@ -38,12 +42,14 @@ func (r *Inspection) WriteExplanation(output io.Writer, path Path) error {
 		rendered.WriteString("  - representation: ")
 		if change.Origin.Operation == ChangeDecoding {
 			rendered.WriteString("input to canonical typed\n")
-		} else if change.Origin.Layer == LayerBase || change.Origin.Layer == LayerOverlay || change.Origin.Layer == LayerCLI {
+		} else if change.Origin.Operation == ChangeInheritance {
+			rendered.WriteString("structural inheritance\n")
+		} else if change.Origin.Layer == LayerBase || change.Origin.Layer == LayerUnderlay || change.Origin.Layer == LayerOverlay || change.Origin.Layer == LayerCLI {
 			rendered.WriteString("policy-validated input\n")
 		} else {
 			rendered.WriteString("canonical typed\n")
 		}
-		writeExplanationOrigin(&rendered, "    source", change.Origin, r.sources)
+		writeExplanationOrigin(&rendered, "    source", change.Origin, labels)
 		writeExplanationSnapshot(&rendered, "    before", change.Before, 4)
 		writeExplanationSnapshot(&rendered, "    after", change.After, 4)
 		for _, reason := range change.Reasons {
@@ -65,7 +71,7 @@ func (r *Inspection) WriteExplanation(output io.Writer, path Path) error {
 	return err
 }
 
-func writeExplanationOrigin(output *strings.Builder, label string, origin Origin, sources []SourceInfo) {
+func writeExplanationOrigin(output *strings.Builder, label string, origin Origin, labels map[string]string) {
 	output.WriteString(label)
 	output.WriteString(": layer=")
 	output.WriteString(strconv.Quote(string(origin.Layer)))
@@ -77,7 +83,7 @@ func writeExplanationOrigin(output *strings.Builder, label string, origin Origin
 	for _, location := range origin.Locations {
 		output.WriteString("      location: source=")
 		output.WriteString(strconv.Quote(location.SourceID))
-		if label := sourceLabel(sources, location.SourceID); label != "" {
+		if label := labels[location.SourceID]; label != "" {
 			output.WriteString(" label=")
 			output.WriteString(strconv.Quote(label))
 		}
@@ -87,15 +93,15 @@ func writeExplanationOrigin(output *strings.Builder, label string, origin Origin
 		output.WriteString(strconv.Itoa(location.Column))
 		output.WriteByte('\n')
 	}
-}
-
-func sourceLabel(sources []SourceInfo, sourceID string) string {
-	for _, source := range sources {
-		if source.ID == sourceID {
-			return source.Label
-		}
+	for _, step := range origin.Inheritance {
+		output.WriteString("      inherited: from=")
+		output.WriteString(strconv.Quote(step.From.String()))
+		output.WriteString(" to=")
+		output.WriteString(strconv.Quote(step.To.String()))
+		output.WriteString(" reference=")
+		output.WriteString(strconv.Quote(step.Reference.String()))
+		output.WriteByte('\n')
 	}
-	return ""
 }
 
 func writeExplanationSnapshot(output *strings.Builder, label string, snapshot ValueSnapshot, indent int) {

internal/engine/filesystem.go

@@ -1,12 +1,21 @@
 package engine
 
 import (
+	"io/fs"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 )
 
 func validateFilesystemContext(source Source, context *FilesystemContext, main parsedSource) *Diagnostic {
+	if nativeContext(source, context) {
+		access, basePath, mainPath, diagnostic := nativeFilesystemAccess(source, context)
+		if diagnostic != nil {
+			return diagnostic
+		}
+		return validateNativeDirectories(access, basePath, mainPath)
+	}
 	rootPath, _, _, diagnostic := importFilesystem(source, context, main)
 	if diagnostic != nil {
 		return diagnostic
@@ -29,20 +38,87 @@ func filesystemOptionDiagnostics(source Source, context *FilesystemContext) []Di
 	case sourceNone:
 		return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "NoSource cannot use filesystem context")}
 	case sourceFile:
+		if context.FS != nil {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "host file sources cannot use a native filesystem context")}
+		}
 		if context.BaseDir != "" {
 			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "file sources determine their own import base directory")}
 		}
-	case sourceBytes, sourceReader:
+	case sourceFilesystem:
+		if context.FS == nil || context.FS.namespace == nil || source.namespace == nil || context.FS.namespace.token != source.namespace.token {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "filesystem file context must use the source namespace")}
+		}
+		if context.BaseDir != "" {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "file sources determine their own import base directory")}
+		}
+		if context.Root != "" && !fs.ValidPath(context.Root) {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "filesystem root must be a slash-relative valid path")}
+		}
+	case sourceBytes, sourceReader, sourceMapping:
 		if context.BaseDir == "" || context.Root == "" {
 			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "byte and reader filesystem context requires base and root directories")}
 		}
-		if !filepath.IsAbs(context.BaseDir) || !filepath.IsAbs(context.Root) {
+		if context.FS != nil {
+			if context.FS.namespace == nil || !fs.ValidPath(context.BaseDir) || !fs.ValidPath(context.Root) {
+				return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "filesystem base and root must be slash-relative valid paths")}
+			}
+		} else if !filepath.IsAbs(context.BaseDir) || !filepath.IsAbs(context.Root) {
 			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "byte and reader filesystem base and root directories must be absolute")}
 		}
 	}
 	return nil
 }
 
+func nativeContext(source Source, context *FilesystemContext) bool {
+	return source.kind == sourceFilesystem || context != nil && context.FS != nil
+}
+
+// nativeFilesystemAccess resolves namespace context without opening caller
+// resources. File inputs always derive their base from the selected file.
+func nativeFilesystemAccess(source Source, context *FilesystemContext) (*filesystemAccess, string, string, *Diagnostic) {
+	var namespace *nativeNamespace
+	rootPath := "."
+	basePath := "."
+	mainPath := ""
+	if source.kind == sourceFilesystem {
+		namespace = source.namespace
+		mainPath = source.path
+		basePath = path.Dir(source.path)
+		if context != nil && context.Root != "" {
+			rootPath = context.Root
+		}
+	} else if context != nil && context.FS != nil {
+		namespace = context.FS.namespace
+		rootPath, basePath = context.Root, context.BaseDir
+	}
+	if namespace == nil || nilFilesystem(namespace.filesystem) || !fs.ValidPath(rootPath) || !fs.ValidPath(basePath) {
+		value := optionDiagnostic(CodeOptionsInvalid, "filesystem context is invalid")
+		return nil, "", "", &value
+	}
+	access := newNativeAccess(namespace.filesystem, rootPath, namespace.token)
+	access.label = namespace.label
+	if !access.contains(basePath) || mainPath != "" && !access.contains(mainPath) {
+		value := importDiagnostic(CodeImportOutsideRoot, "selected source and import base must stay inside the configuration root", nil)
+		return nil, "", "", &value
+	}
+	return access, basePath, mainPath, nil
+}
+
+func validateNativeDirectories(access *filesystemAccess, basePath, mainPath string) *Diagnostic {
+	if !access.contains(basePath) || mainPath != "" && !access.contains(mainPath) {
+		value := importDiagnostic(CodeImportOutsideRoot, "selected source and import base must stay inside the configuration root", nil)
+		return &value
+	}
+	for _, directory := range []string{access.rootPath, basePath} {
+		info, err := access.stat(directory)
+		if err != nil || !info.IsDir() {
+			value := importDiagnostic(CodeImportInvalid, "configuration root and import base must be directories", nil)
+			return &value
+		}
+	}
+	return nil
+}
+
 func importFilesystem(source Source, context *FilesystemContext, main parsedSource) (rootPath, basePath, mainPath string, diagnostic *Diagnostic) {
 	if main.source == nil || main.document == nil {
 		value := importDiagnostic(CodeImportInvalid, "imports require a selected configuration source", nil)

internal/engine/filesystem_access.go

@@ -0,0 +1,185 @@
+package engine
+
+import (
+	"errors"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+)
+
+// namespaceToken identifies one caller-selected filesystem without comparing
+// arbitrary fs.FS values. A token can be shared by copies of a source wrapper.
+type namespaceToken struct{ marker byte }
+
+type nativeNamespace struct {
+	filesystem fs.FS
+	label      string
+	token      *namespaceToken
+}
+
+func newNativeNamespace(label string, filesystem fs.FS) *nativeNamespace {
+	return &nativeNamespace{filesystem: filesystem, label: label, token: &namespaceToken{}}
+}
+
+type resourceIdentity struct {
+	namespace  *namespaceToken
+	path       string
+	occurrence *namespaceToken
+}
+
+// filesystemAccess is a load-scoped view of either a contained host root or a
+// borrowed native filesystem. Names are canonical host paths or fs.FS paths.
+type filesystemAccess struct {
+	rootPath string
+	root     *os.Root
+	native   fs.FS
+	token    *namespaceToken
+	label    string
+}
+
+func openHostAccess(rootPath string) (*filesystemAccess, error) {
+	root, err := os.OpenRoot(rootPath)
+	if err != nil {
+		return nil, err
+	}
+	return &filesystemAccess{rootPath: rootPath, root: root, token: &namespaceToken{}}, nil
+}
+
+func newNativeAccess(filesystem fs.FS, root string, token *namespaceToken) *filesystemAccess {
+	return &filesystemAccess{rootPath: root, native: filesystem, token: token}
+}
+
+func (a *filesystemAccess) close() error {
+	if a.root != nil {
+		return a.root.Close()
+	}
+	return nil
+}
+
+func (a *filesystemAccess) identity(name string) resourceIdentity {
+	return resourceIdentity{namespace: a.token, path: name}
+}
+
+func (a *filesystemAccess) syntheticIdentity() resourceIdentity {
+	return resourceIdentity{namespace: a.token, occurrence: &namespaceToken{}}
+}
+
+func (a *filesystemAccess) join(base, name string) string {
+	if a.native != nil {
+		return path.Join(base, name)
+	}
+	return filepath.Join(base, filepath.FromSlash(path.Clean(name)))
+}
+
+func (a *filesystemAccess) dir(name string) string {
+	if a.native != nil {
+		return path.Dir(name)
+	}
+	return filepath.Dir(name)
+}
+
+func (a *filesystemAccess) contains(name string) bool {
+	if a.native == nil {
+		return containedPath(a.rootPath, filepath.Clean(name))
+	}
+	if !fs.ValidPath(name) || !fs.ValidPath(a.rootPath) {
+		return false
+	}
+	return a.rootPath == "." || name == a.rootPath || strings.HasPrefix(name, a.rootPath+"/")
+}
+
+func (a *filesystemAccess) canonical(name string) (string, error) {
+	if a.native == nil {
+		return canonicalExistingPath(name)
+	}
+	if !fs.ValidPath(name) {
+		return "", fs.ErrInvalid
+	}
+	_, err := fs.Stat(a.native, name)
+	if err != nil {
+		return "", err
+	}
+	return name, nil
+}
+
+func (a *filesystemAccess) relative(name string) (string, error) {
+	if a.native != nil {
+		if !a.contains(name) {
+			return "", fs.ErrInvalid
+		}
+		return name, nil
+	}
+	relative, err := filepath.Rel(a.rootPath, name)
+	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
+		return "", fs.ErrInvalid
+	}
+	return relative, nil
+}
+
+func (a *filesystemAccess) stat(name string) (fs.FileInfo, error) {
+	if a.native != nil {
+		return fs.Stat(a.native, name)
+	}
+	relative, err := a.relative(name)
+	if err != nil {
+		return nil, err
+	}
+	return a.root.Stat(relative)
+}
+
+func (a *filesystemAccess) open(name string) (fs.File, error) {
+	if a.native != nil {
+		return a.native.Open(name)
+	}
+	relative, err := a.relative(name)
+	if err != nil {
+		return nil, err
+	}
+	return a.root.Open(relative)
+}
+
+func (a *filesystemAccess) readDirectory(name string, visit func(fs.DirEntry) bool) error {
+	file, err := a.open(name)
+	if err != nil {
+		return err
+	}
+	if reader, ok := file.(fs.ReadDirFile); ok {
+		for {
+			entries, readErr := reader.ReadDir(globReadBatch)
+			for _, entry := range entries {
+				if !visit(entry) {
+					return file.Close()
+				}
+			}
+			if errors.Is(readErr, io.EOF) {
+				return file.Close()
+			}
+			if readErr != nil {
+				_ = file.Close()
+				return readErr
+			}
+			if len(entries) == 0 {
+				_ = file.Close()
+				return io.ErrNoProgress
+			}
+		}
+	}
+	if err := file.Close(); err != nil {
+		return err
+	}
+	// fs.ReadDir provides the standard fallback for implementations whose opened
+	// files do not implement fs.ReadDirFile. It may allocate all entries itself.
+	entries, err := fs.ReadDir(a.native, name)
+	if err != nil {
+		return err
+	}
+	for _, entry := range entries {
+		if !visit(entry) {
+			break
+		}
+	}
+	return nil
+}

internal/engine/glob.go

@@ -2,11 +2,8 @@ package engine
 
 import (
 	"errors"
-	"io"
 	"io/fs"
-	"os"
 	"path"
-	"path/filepath"
 	"slices"
 	"strings"
 )
@@ -42,29 +39,29 @@ func (s *importTraversal) expandImportGlob(basePath string, declaration importDe
 				names = []string{literal}
 			}
 			for _, name := range names {
-				absolute := filepath.Join(candidate.absolute, filepath.FromSlash(name))
+				absolute := s.access.join(candidate.absolute, name)
 				relative := path.Join(candidate.relative, name)
-				if !containedPath(s.rootPath, filepath.Clean(absolute)) {
+				if !s.access.contains(absolute) {
 					return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob traversal is outside the configuration root", &location)}
 				}
-				canonical, err := canonicalExistingPath(absolute)
+				canonical, err := s.access.canonical(absolute)
 				if err != nil {
 					if errors.Is(err, fs.ErrNotExist) && !patterned {
 						continue
 					}
 					return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob match could not be canonicalized", location)}
 				}
-				if !containedPath(s.rootPath, canonical) {
+				if !s.access.contains(canonical) {
 					return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob traversal resolves outside the configuration root", &location)}
 				}
 				if last {
 					next = append(next, globCandidate{absolute: canonical, relative: relative})
-					if len(s.sources)+len(next) > s.limits.sources {
+					if s.accounting.sources+len(next) > s.limits.sources {
 						return nil, []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "glob matches exceed the remaining source-count limit", &location)}
 					}
 					continue
 				}
-				info, err := os.Stat(canonical)
+				info, err := s.access.stat(canonical)
 				if err != nil {
 					return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob traversal target could not be inspected", location)}
 				}
@@ -88,56 +85,47 @@ func (s *importTraversal) expandImportGlob(basePath string, declaration importDe
 }
 
 func (s *importTraversal) readGlobMatches(directory, pattern string, location Location) ([]string, []Diagnostic) {
-	canonical, err := canonicalExistingPath(directory)
+	canonical, err := s.access.canonical(directory)
 	if err != nil {
 		if errors.Is(err, fs.ErrNotExist) {
 			return nil, nil
 		}
 		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be canonicalized", location)}
 	}
-	if !containedPath(s.rootPath, canonical) {
+	if !s.access.contains(canonical) {
 		return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob directory resolves outside the configuration root", &location)}
 	}
-	relative, err := filepath.Rel(s.rootPath, canonical)
-	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
+	if _, err := s.access.relative(canonical); err != nil {
 		return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob directory could not be addressed inside the configuration root", &location)}
 	}
-	directoryFile, err := s.root.Open(relative)
-	if err != nil {
-		if errors.Is(err, fs.ErrNotExist) {
-			return nil, nil
-		}
-		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be opened", location)}
-	}
-
 	var matches []string
-	for {
-		entries, readErr := directoryFile.ReadDir(globReadBatch)
-		for _, entry := range entries {
-			s.usedEntries++
-			if s.usedEntries > s.limits.entries {
-				_ = directoryFile.Close()
-				return nil, []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "glob traversal exceeds the directory-entry visit limit", &location)}
-			}
-			matched, matchErr := path.Match(pattern, entry.Name())
-			if matchErr != nil {
-				_ = directoryFile.Close()
-				return nil, []Diagnostic{importDiagnostic(CodeImportInvalid, "import glob pattern is invalid", &location)}
-			}
-			if matched {
-				matches = append(matches, entry.Name())
-			}
+	var problem *Diagnostic
+	err = s.access.readDirectory(canonical, func(entry fs.DirEntry) bool {
+		s.accounting.entries++
+		if s.accounting.entries > s.limits.entries {
+			value := importDiagnostic(CodeImportLimitExceeded, "glob traversal exceeds the directory-entry visit limit", &location)
+			problem = &value
+			return false
 		}
-		if errors.Is(readErr, io.EOF) {
-			break
+		matched, matchErr := path.Match(pattern, entry.Name())
+		if matchErr != nil {
+			value := importDiagnostic(CodeImportInvalid, "import glob pattern is invalid", &location)
+			problem = &value
+			return false
 		}
-		if readErr != nil {
-			_ = directoryFile.Close()
-			return nil, []Diagnostic{globFilesystemDiagnostic(readErr, "glob directory could not be read", location)}
+		if matched {
+			matches = append(matches, entry.Name())
 		}
+		return true
+	})
+	if problem != nil {
+		return nil, []Diagnostic{*problem}
 	}
-	if err := directoryFile.Close(); err != nil {
-		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be closed", location)}
+	if errors.Is(err, fs.ErrNotExist) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be traversed", location)}
 	}
 	slices.Sort(matches)
 	return matches, nil

internal/engine/history.go

@@ -6,6 +6,40 @@ import (
 
 const historyNodeLimit = 4_000_000
 
+type retainedNodeCounter struct {
+	seen  map[*document.Node]bool
+	count int
+	limit int
+}
+
+func newRetainedNodeCounter(limit int) *retainedNodeCounter {
+	return &retainedNodeCounter{seen: make(map[*document.Node]bool), limit: limit}
+}
+
+func (c *retainedNodeCounter) add(root *document.Node) bool {
+	if c == nil {
+		return true
+	}
+	stack := []*document.Node{root}
+	for len(stack) != 0 {
+		node := stack[len(stack)-1]
+		stack = stack[:len(stack)-1]
+		if node == nil || c.seen[node] {
+			continue
+		}
+		c.seen[node] = true
+		c.count++
+		if c.limit >= 0 && c.count > c.limit {
+			return false
+		}
+		for _, entry := range node.Mapping {
+			stack = append(stack, entry.Value)
+		}
+		stack = append(stack, node.Sequence...)
+	}
+	return true
+}
+
 // History applications borrow immutable policy-validated input trees. Metadata
 // records answer presence and origin queries; replay reconstructs value changes.
 // Replay reconciles with canonical decoding before domain resolution so both
@@ -83,15 +117,15 @@ func pathsIntersect(left, right Path) bool {
 	return pathContains(left, right) || pathContains(right, left)
 }
 
-func applyHistoryApplication(root **document.Node, application historyApplication) {
+func applyHistoryApplication(root **document.Node, application historyApplication, index *mappingIndex) {
 	if application.path.String() == "" {
-		mergeHistoryDocument(root, application.value)
+		mergeHistoryDocument(root, application.value, index)
 		return
 	}
-	mergeHistoryAtPath(root, application.path, application.value)
+	mergeHistoryAtPath(root, application.path, application.value, index)
 }
 
-func mergeHistoryDocument(target **document.Node, incoming *document.Node) {
+func mergeHistoryDocument(target **document.Node, incoming *document.Node, index *mappingIndex) {
 	if incoming == nil {
 		return
 	}
@@ -100,39 +134,39 @@ func mergeHistoryDocument(target **document.Node, incoming *document.Node) {
 		return
 	}
 	for _, entry := range incoming.Mapping {
-		position := documentEntryIndex((*target).Mapping, entry.Key)
+		position := index.find(*target, entry.Key)
 		if position < 0 {
 			cloned := entry
 			cloned.Value = document.Clone(entry.Value)
-			(*target).Mapping = append((*target).Mapping, cloned)
+			index.append(*target, cloned)
 			continue
 		}
-		mergeHistoryDocument(&(*target).Mapping[position].Value, entry.Value)
+		mergeHistoryDocument(&(*target).Mapping[position].Value, entry.Value, index)
 	}
 }
 
-func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node) {
+func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node, index *mappingIndex) {
 	if root == nil || *root == nil || len(path.segments) == 0 {
-		mergeHistoryDocument(root, incoming)
+		mergeHistoryDocument(root, incoming, index)
 		return
 	}
 	current := *root
-	for index, segment := range path.segments {
+	for segmentIndex, segment := range path.segments {
 		if current.Kind != document.Mapping || segment.kind == pathSegmentIndex {
 			return
 		}
-		position := documentEntryIndex(current.Mapping, segment.key)
-		if index == len(path.segments)-1 {
+		position := index.find(current, segment.key)
+		if segmentIndex == len(path.segments)-1 {
 			if position < 0 {
-				current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: document.Clone(incoming)})
+				index.append(current, document.Entry{Key: segment.key, Value: document.Clone(incoming)})
 			} else {
-				mergeHistoryDocument(&current.Mapping[position].Value, incoming)
+				mergeHistoryDocument(&current.Mapping[position].Value, incoming, index)
 			}
 			return
 		}
 		if position < 0 {
 			child := &document.Node{Kind: document.Mapping}
-			current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: child})
+			index.append(current, document.Entry{Key: segment.key, Value: child})
 			current = child
 		} else {
 			current = current.Mapping[position].Value
@@ -230,11 +264,11 @@ func replaceHistoryPath(root **document.Node, path Path, replacement *document.N
 	}
 }
 
-func historyNodesWithinLimit(baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
-	return historyNodesWithin(historyNodeLimit, baseline, input, final, applications, resolutions)
+func historyNodesWithinLimit(baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot, inherited ...*inheritanceScope) bool {
+	return historyNodesWithin(historyNodeLimit, baseline, input, final, applications, resolutions, inherited...)
 }
 
-func historyNodesWithin(limit int, baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
+func historyNodesWithin(limit int, baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot, inherited ...*inheritanceScope) bool {
 	seen := make(map[*document.Node]bool)
 	count := 0
 	var visit func(*document.Node) bool
@@ -272,5 +306,23 @@ func historyNodesWithin(limit int, baseline, input, final *document.Node, applic
 			return false
 		}
 	}
+	for _, expansion := range inherited {
+		if expansion == nil {
+			continue
+		}
+		if !visit(expansion.root) {
+			return false
+		}
+		for _, operation := range expansion.operations {
+			if !visit(operation.parent) || !visit(operation.effective) {
+				return false
+			}
+		}
+		for _, replacement := range expansion.replacements {
+			if !visit(replacement.before) || !visit(replacement.after) {
+				return false
+			}
+		}
+	}
 	return true
 }

internal/engine/imports.go

@@ -4,9 +4,6 @@ import (
 	"bytes"
 	"errors"
 	"io/fs"
-	"os"
-	"path"
-	"path/filepath"
 	"strconv"
 	"strings"
 
@@ -40,47 +37,96 @@ type importRoute struct {
 	locations []Location
 }
 
-type importTraversal struct {
-	rootPath string
-	root     *os.Root
-	limits   importLimits
-	rule     *VersionRule
+type importAccounting struct {
+	sources int
+	bytes   int64
+	nodes   int
+	entries int
+}
 
-	documents   []*document.Node
-	sources     []SourceInfo
-	active      map[string]int
-	visited     map[string]importRoute
-	stack       []importRoute
-	usedBytes   int64
-	usedNodes   int
-	usedEntries int
+type importTraversal struct {
+	access     *filesystemAccess
+	accounting *importAccounting
+	limits     importLimits
+	rule       *VersionRule
+
+	documents []*document.Node
+	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) {
+	return loadImportedDocumentsTracked(source, context, main, rule, limits, &importAccounting{}, nil, nil)
+}
+
+func loadImportedDocumentsTracked(source Source, context *FilesystemContext, main parsedSource, rule *VersionRule, limits importLimits, accounting *importAccounting, visited map[resourceIdentity]importRoute, hostToken *namespaceToken) (*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 loadImportedWithAccessTracked(access, basePath, mainPath, main, rule, limits, accounting, visited)
+	}
 	rootPath, basePath, mainPath, diagnostic := importFilesystem(source, context, main)
 	if diagnostic != nil {
 		return nil, sourceInfos(main.source), []Diagnostic{*diagnostic}
 	}
-	root, err := os.OpenRoot(rootPath)
+	access, err := openHostAccess(rootPath)
 	if err != nil {
 		diagnostic := globFilesystemDiagnostic(err, "configuration root could not be opened", Location{})
 		diagnostic.Primary = nil
 		return nil, sourceInfos(main.source), []Diagnostic{diagnostic}
 	}
+	if hostToken != nil {
+		access.token = hostToken
+	}
+	return loadImportedWithAccessTracked(access, basePath, mainPath, main, rule, limits, accounting, visited)
+}
 
+func loadImportedWithAccess(access *filesystemAccess, basePath, mainPath string, main parsedSource, rule *VersionRule, limits importLimits, accounting *importAccounting) (*composedDocument, []SourceInfo, []Diagnostic) {
+	return loadImportedWithAccessTracked(access, basePath, mainPath, main, rule, limits, accounting, nil)
+}
+
+func loadImportedWithAccessTracked(access *filesystemAccess, basePath, mainPath string, main parsedSource, rule *VersionRule, limits importLimits, accounting *importAccounting, visited map[resourceIdentity]importRoute) (*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)}
+	}
+	for _, directory := range []string{access.rootPath, basePath} {
+		info, err := access.stat(directory)
+		if err != nil || !info.IsDir() {
+			_ = access.close()
+			return nil, sourceInfos(main.source), []Diagnostic{importDiagnostic(CodeImportInvalid, "configuration root and import base must be directories", nil)}
+		}
+	}
 	mainInfo := cloneSourceInfo(*main.source)
 	if mainPath != "" {
 		mainInfo.Path = mainPath
 	}
+	mainKey := access.identity(mainPath)
+	if mainPath == "" {
+		mainKey = access.syntheticIdentity()
+	}
+	if previous, repeated := visited[mainKey]; repeated {
+		_ = access.close()
+		return nil, sourceInfos(main.source), []Diagnostic{importRepeatedDiagnostic(Location{SourceID: main.source.ID}, previous, importRoute{})}
+	}
+	accounting.sources++
+	accounting.bytes += main.bytes
+	accounting.nodes += countInputNodes(main.document)
 	state := importTraversal{
-		rootPath: rootPath, root: root, limits: limits, rule: rule,
+		access: access, accounting: accounting, limits: limits, rule: rule,
 		documents: make([]*document.Node, 0, 1),
 		sources:   []SourceInfo{mainInfo},
-		active:    make(map[string]int), visited: make(map[string]importRoute),
-		usedBytes: main.bytes, usedNodes: countInputNodes(main.document),
+		active:    make(map[resourceIdentity]int), visited: visited,
+	}
+	if state.visited == nil {
+		state.visited = make(map[resourceIdentity]importRoute)
 	}
 	if diagnostic := state.checkInitialLimits(); diagnostic != nil {
-		_ = root.Close()
+		_ = access.close()
 		return nil, cloneSourceInfos(state.sources), []Diagnostic{*diagnostic}
 	}
 
@@ -89,8 +135,8 @@ func loadImportedDocuments(source Source, context *FilesystemContext, main parse
 		mainIdentity = "<selected-source>"
 	}
 	mainRoute := importRoute{canonical: mainIdentity}
-	state.active[mainIdentity] = 0
-	state.visited[mainIdentity] = mainRoute
+	state.active[mainKey] = 0
+	state.visited[mainKey] = mainRoute
 	state.stack = append(state.stack, mainRoute)
 	body, declarations, diagnostics := splitImportDirectives(main.document)
 	if len(diagnostics) == 0 {
@@ -98,7 +144,7 @@ func loadImportedDocuments(source Source, context *FilesystemContext, main parse
 		diagnostics = state.walkDeclarations(basePath, declarations, 0)
 	}
 
-	closeErr := root.Close()
+	closeErr := access.close()
 	if len(diagnostics) != 0 {
 		return nil, cloneSourceInfos(state.sources), diagnostics
 	}
@@ -107,22 +153,22 @@ func loadImportedDocuments(source Source, context *FilesystemContext, main parse
 	}
 	composed, diagnostics := composeDocuments(state.documents...)
 	if composed != nil {
-		composed.inputBytes = state.usedBytes
-		composed.inputNodes = state.usedNodes
+		composed.inputBytes = accounting.bytes
+		composed.inputNodes = accounting.nodes
 	}
 	return composed, cloneSourceInfos(state.sources), diagnostics
 }
 
 func (s *importTraversal) checkInitialLimits() *Diagnostic {
-	if s.limits.sources < 1 {
+	if s.accounting.sources > s.limits.sources {
 		value := importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the source-count limit", nil)
 		return &value
 	}
-	if s.usedBytes > s.limits.bytes {
+	if s.accounting.bytes > s.limits.bytes {
 		value := importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate byte limit", nil)
 		return &value
 	}
-	if s.usedNodes > s.limits.nodes {
+	if s.accounting.nodes > s.limits.nodes {
 		value := importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", nil)
 		return &value
 	}
@@ -169,11 +215,11 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 		return []Diagnostic{importDiagnostic(CodeImportInvalid, "import path must be a nonempty slash-relative path without a network scheme", &location)}
 	}
 
-	candidate := filepath.Join(basePath, filepath.FromSlash(path.Clean(declaration.path)))
-	if !containedPath(s.rootPath, filepath.Clean(candidate)) {
+	candidate := s.access.join(basePath, declaration.path)
+	if !s.access.contains(candidate) {
 		return []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "import target is outside the configuration root", &location)}
 	}
-	canonical, err := canonicalExistingPath(candidate)
+	canonical, err := s.access.canonical(candidate)
 	if err != nil {
 		if declaration.optional && errors.Is(err, fs.ErrNotExist) {
 			return nil
@@ -187,30 +233,39 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 		}
 		return []Diagnostic{importDiagnostic(code, message, &location)}
 	}
-	if !containedPath(s.rootPath, canonical) {
+	if !s.access.contains(canonical) {
 		return []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "import target resolves outside the configuration root", &location)}
 	}
 
 	currentRoute := s.currentRoute(location)
-	if activeIndex, ok := s.active[canonical]; ok {
+	identity := s.access.identity(canonical)
+	if activeIndex, ok := s.active[identity]; ok {
 		cycle := append([]importRoute(nil), s.stack[activeIndex:]...)
 		return []Diagnostic{importCycleDiagnostic(canonical, location, cycle)}
 	}
-	if previous, ok := s.visited[canonical]; ok {
+	if previous, ok := s.visited[identity]; ok {
 		return []Diagnostic{importRepeatedDiagnostic(location, previous, currentRoute)}
 	}
-	if len(s.sources)+1 > s.limits.sources {
+	if s.accounting.sources+1 > s.limits.sources {
 		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the source-count limit", &location)}
 	}
 
-	relative, err := filepath.Rel(s.rootPath, canonical)
-	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
+	if _, err := s.access.relative(canonical); err != nil {
 		return []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "import target could not be addressed inside the configuration root", &location)}
 	}
-	sourceID := "source-" + strconv.Itoa(len(s.sources)+1)
-	info := SourceInfo{ID: sourceID, Label: declaration.path, Path: canonical, Layer: LayerBase, ImportedFrom: &location}
+	sourceID := "source-" + strconv.Itoa(s.accounting.sources+1)
+	kind := SourceKindFile
+	label := declaration.path
+	if s.access.native != nil {
+		kind = SourceKindFilesystem
+		label = s.access.label + "/" + canonical
+	}
+	info := SourceInfo{ID: sourceID, Label: label, Path: canonical, Layer: LayerBase, Kind: kind, ImportedFrom: &location}
+	if kind == SourceKindFilesystem {
+		info.namespace = s.access.token
+	}
 	s.sources = append(s.sources, info)
-	data, skipped, diagnostic := s.readImport(relative, sourceID, declaration.optional)
+	data, skipped, diagnostic := s.readImport(canonical, sourceID, declaration.optional)
 	if diagnostic != nil {
 		return []Diagnostic{*diagnostic}
 	}
@@ -218,6 +273,7 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 		s.sources = s.sources[:len(s.sources)-1]
 		return nil
 	}
+	s.accounting.sources++
 
 	root, issues := yamladapter.Parse(bytes.NewReader(data), sourceID, yamladapter.Limits{
 		Depth: maxDocumentDepth, Nodes: maxDocumentNodes,
@@ -226,10 +282,10 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 		return parserDiagnostics(sourceID, issues)
 	}
 	nodeCount := countInputNodes(root)
-	if s.usedNodes+nodeCount > s.limits.nodes {
+	if s.accounting.nodes+nodeCount > s.limits.nodes {
 		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", &location)}
 	}
-	s.usedNodes += nodeCount
+	s.accounting.nodes += nodeCount
 
 	body, declarations, diagnostics := splitImportDirectives(root)
 	if len(diagnostics) != 0 {
@@ -245,22 +301,22 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 
 	route := importRoute{canonical: canonical, locations: currentRoute.locations}
 	s.documents = append(s.documents, body)
-	s.active[canonical] = len(s.stack)
-	s.visited[canonical] = route
+	s.active[identity] = len(s.stack)
+	s.visited[identity] = route
 	s.stack = append(s.stack, route)
-	diagnostics = s.walkDeclarations(filepath.Dir(canonical), declarations, depth)
-	delete(s.active, canonical)
+	diagnostics = s.walkDeclarations(s.access.dir(canonical), declarations, depth)
+	delete(s.active, identity)
 	s.stack = s.stack[:len(s.stack)-1]
 	return diagnostics
 }
 
-func (s *importTraversal) readImport(relative, sourceID string, optional bool) ([]byte, bool, *Diagnostic) {
-	remaining := s.limits.bytes - s.usedBytes
+func (s *importTraversal) readImport(name, sourceID string, optional bool) ([]byte, bool, *Diagnostic) {
+	remaining := s.limits.bytes - s.accounting.bytes
 	limit := min(int64(maxSourceBytes), remaining)
 	if limit < 0 {
 		limit = 0
 	}
-	data, failure := readRegularFile(relative, s.root.Stat, s.root.Open, limit)
+	data, failure := readRegularFile(name, s.access.stat, s.access.open, limit)
 	if failure != nil {
 		if optional && failure.missingTarget {
 			return nil, true, nil
@@ -268,7 +324,7 @@ func (s *importTraversal) readImport(relative, sourceID string, optional bool) (
 		value := sourceReadDiagnostic(failure, sourceID, CodeImportLimitExceeded, "configuration exceeds a source or aggregate byte limit")
 		return nil, false, &value
 	}
-	s.usedBytes += int64(len(data))
+	s.accounting.bytes += int64(len(data))
 	return data, false, nil
 }
 

internal/engine/inheritance.go

@@ -0,0 +1,313 @@
+package engine
+
+import (
+	"slices"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+const inheritanceKey = "extends"
+
+// These private records keep structural evidence separate from acquired source
+// records. Later inspection can project a transfer onto individual descendants.
+type inheritanceTransfer struct {
+	from      Path
+	to        Path
+	reference Path
+	locations []Location
+}
+
+type inheritanceControl struct {
+	path      Path
+	locations []Location
+	cancelled bool
+}
+
+type inheritanceScope struct {
+	root         *document.Node
+	origins      map[string]Origin
+	transfers    []inheritanceTransfer
+	controls     []inheritanceControl
+	replacements []inheritanceReplacement
+	operations   []inheritanceOperation
+}
+
+type inheritanceOperation struct {
+	control          *inheritanceControl
+	transfer         *inheritanceTransfer
+	controlOrigin    Origin
+	parent           *document.Node
+	effective        *document.Node
+	parentOrigins    map[string]Origin
+	effectiveOrigins map[string]Origin
+}
+
+// A replacement retains the displaced source/derivation record while the
+// effective origin belongs to the child. Value trees are borrowed immutable
+// inputs or detached templates and are budgeted with retained history later.
+type inheritanceReplacement struct {
+	path      Path
+	before    *document.Node
+	after     *document.Node
+	previous  []displacedDefinition
+	effective Origin
+}
+
+// A link represents one shared relationship edge. Each contributor retains
+// its original semantic path independently of source line/column coordinates.
+type inheritanceTrace struct {
+	previous *inheritanceTrace
+	transfer *inheritanceTransfer
+}
+
+type inheritanceContributor struct {
+	source   Path
+	location Location
+	trace    *inheritanceTrace
+}
+
+type siblingFrame struct {
+	name      string
+	parent    string
+	hasParent bool
+	entered   bool
+}
+
+// expandSiblingDefinitions resolves only the mappings directly in scope.
+// Nested instructions remain in the returned templates for destination-local
+// resolution. This private entry point does not participate in public loading.
+func expandSiblingDefinitions(scope *document.Node, scopePath Path) (*inheritanceScope, []Diagnostic) {
+	return expandSiblingDefinitionsWithLimits(scope, scopePath, defaultProjectionLimits())
+}
+
+func expandSiblingDefinitionsWithLimits(scope *document.Node, scopePath Path, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
+	return expandSiblingDefinitionsWithOrigins(scope, scopePath, limits, nil)
+}
+
+func expandSiblingDefinitionsWithOrigins(scope *document.Node, scopePath Path, limits projectionLimits, inputOrigins map[string]Origin, budget ...*retainedNodeCounter) (*inheritanceScope, []Diagnostic) {
+	if scope == nil || scope.Kind != document.Mapping {
+		return nil, []Diagnostic{{
+			Code: CodeInheritanceInvalid, Severity: SeverityError, Path: scopePath,
+			Message: "inheritance scope must be a mapping",
+		}}
+	}
+
+	byName := make(map[string]document.Entry, len(scope.Mapping))
+	names := make([]string, 0, len(scope.Mapping))
+	for _, entry := range scope.Mapping {
+		byName[entry.Key] = entry
+		names = append(names, entry.Key)
+	}
+	slices.Sort(names)
+
+	resolved := make(map[string]*document.Node, len(byName))
+	resolvedOrigins := make(map[string]map[string]Origin, len(byName))
+	state := make(map[string]uint8, len(byName))
+	result := &inheritanceScope{}
+	counter := newRetainedNodeCounter(historyNodeLimit)
+	if len(budget) != 0 {
+		counter = budget[0]
+	}
+	retained := 1 // scope mapping node
+	for _, entry := range scope.Mapping {
+		if entry.Value.Kind == document.Mapping {
+			continue
+		}
+		remaining := limits.nodes - retained - 1
+		if limits.nodes >= 0 && remaining < 0 {
+			return nil, []Diagnostic{inheritanceBudgetDiagnostic(scopePath.Key(entry.Key), entry.Value.Location)}
+		}
+		count, within := inheritanceLowerBoundCount(entry.Value, remaining)
+		if !within {
+			return nil, []Diagnostic{inheritanceBudgetDiagnostic(scopePath.Key(entry.Key), entry.Value.Location)}
+		}
+		retained += 1 + count // key and value subtree
+	}
+	for _, name := range names {
+		if byName[name].Value.Kind != document.Mapping || state[name] == 2 {
+			continue
+		}
+		stack := []siblingFrame{{name: name}}
+		for len(stack) != 0 {
+			frame := &stack[len(stack)-1]
+			entry := byName[frame.name]
+			path := scopePath.Key(frame.name)
+			if !frame.entered {
+				state[frame.name] = 1
+				frame.entered = true
+				reference, found := inheritanceDirective(entry.Value)
+				if found {
+					referencePath := path.Key(inheritanceKey)
+					location := publicLocation(reference.Value.Location)
+					result.controls = append(result.controls, inheritanceControl{
+						path: referencePath, locations: []Location{location},
+						cancelled: reference.Value.Kind == document.Null,
+					})
+					switch reference.Value.Kind {
+					case document.Null:
+					case document.String:
+						frame.parent, frame.hasParent = reference.Value.Text, true
+						parent, exists := byName[frame.parent]
+						if !exists {
+							return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceMissing, referencePath, location, nil,
+								"referenced sibling does not exist")}
+						}
+						if parent.Value.Kind != document.Mapping {
+							related := []Location{publicLocation(parent.Value.Location)}
+							return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceInvalid, referencePath, location, related,
+								"referenced sibling is not a mapping")}
+						}
+						if state[frame.parent] == 1 {
+							related := []Location{publicLocation(parent.Value.Location)}
+							return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceCycle, referencePath, location, related,
+								"sibling inheritance contains a cycle")}
+						}
+						if state[frame.parent] == 0 {
+							stack = append(stack, siblingFrame{name: frame.parent})
+							continue
+						}
+					default:
+						return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceInvalid, referencePath, location, nil,
+							"inheritance reference must be a sibling name or null")}
+					}
+				}
+			}
+
+			payload := withoutInheritanceDirective(entry.Value)
+			operationStart, replacementStart := len(result.operations), len(result.replacements)
+			childOrigins := directInheritanceOrigins(payload, path, inputOrigins)
+			if reference, found := inheritanceDirective(entry.Value); found {
+				control := inheritanceControl{
+					path:      path.Key(inheritanceKey),
+					locations: []Location{publicLocation(reference.Value.Location)},
+					cancelled: reference.Value.Kind == document.Null,
+				}
+				controlOrigins := directInheritanceOrigins(reference.Value, control.path, inputOrigins)
+				result.operations = append(result.operations, inheritanceOperation{
+					control: &control, controlOrigin: controlOrigins[control.path.String()],
+				})
+			}
+			if limits.nodes >= 0 {
+				remaining := limits.nodes - retained - 1 // the sibling key
+				within := false
+				if frame.hasParent {
+					within = inheritanceOverlayLowerBoundWithin(resolved[frame.parent], payload, remaining)
+				} else {
+					within = inheritanceLowerBoundWithin(payload, remaining)
+				}
+				if !within {
+					return nil, []Diagnostic{inheritanceBudgetDiagnostic(path, entry.Value.Location)}
+				}
+			}
+			if frame.hasParent {
+				parent := resolved[frame.parent]
+				merged := document.Clone(parent)
+				control := inheritanceDirectiveLocation(entry.Value)
+				transfer := inheritanceTransfer{
+					from: scopePath.Key(frame.parent), to: path,
+					reference: path.Key(inheritanceKey), locations: []Location{control},
+				}
+				mergedOrigins := transferredInheritanceOrigins(resolvedOrigins[frame.parent], transfer.from, path, transfer)
+				mergeInheritanceOrigin(mergedOrigins, path, childOrigins[path.String()])
+				mergeRecursiveMappings(merged, payload, path, new(mappingIndex), recursiveMergeHooks{
+					added: func(childPath Path, incoming *document.Node) {
+						copyInheritanceSubtree(mergedOrigins, childOrigins, incoming, childPath)
+					},
+					combined: func(childPath Path, _ *document.Node) {
+						mergeInheritanceOrigin(mergedOrigins, childPath, childOrigins[childPath.String()])
+					},
+					replaced: func(childPath Path, existing, incoming *document.Node) {
+						previous := displacedOrigins(existing, mergedOrigins, childPath)
+						result.replacements = append(result.replacements, inheritanceReplacement{
+							path: childPath.clone(), before: existing, after: incoming,
+							previous: previous, effective: cloneOrigin(childOrigins[childPath.String()]),
+						})
+						deleteOriginSubtree(mergedOrigins, childPath, existing)
+						copyInheritanceSubtree(mergedOrigins, childOrigins, incoming, childPath)
+					},
+				})
+				merged.Location = entry.Value.Location
+				resolved[frame.name] = merged
+				resolvedOrigins[frame.name] = mergedOrigins
+				result.transfers = append(result.transfers, transfer)
+				result.operations = append(result.operations, inheritanceOperation{
+					transfer: &transfer, parent: parent, effective: merged,
+					parentOrigins: resolvedOrigins[frame.parent], effectiveOrigins: mergedOrigins,
+				})
+			} else {
+				resolved[frame.name] = document.Clone(payload)
+				resolvedOrigins[frame.name] = childOrigins
+			}
+			count, _ := inheritanceLowerBoundCount(resolved[frame.name], limits.nodes)
+			retained += 1 + count
+			for _, operation := range result.operations[operationStart:] {
+				if !counter.add(operation.parent) || !counter.add(operation.effective) {
+					return nil, []Diagnostic{inheritanceHistoryBudgetDiagnostic(path, entry.Value.Location)}
+				}
+			}
+			for _, replacement := range result.replacements[replacementStart:] {
+				if !counter.add(replacement.before) || !counter.add(replacement.after) {
+					return nil, []Diagnostic{inheritanceHistoryBudgetDiagnostic(path, entry.Value.Location)}
+				}
+			}
+			state[frame.name] = 2
+			stack = stack[:len(stack)-1]
+		}
+	}
+
+	root := *scope
+	root.Mapping = make([]document.Entry, len(scope.Mapping))
+	rootOrigin := directInheritanceOrigins(&document.Node{Kind: document.Mapping, Location: scope.Location}, scopePath, inputOrigins)
+	result.origins = rootOrigin
+	for index, entry := range scope.Mapping {
+		root.Mapping[index] = entry
+		if resolved[entry.Key] != nil {
+			root.Mapping[index].Value = resolved[entry.Key]
+			for pointer, origin := range resolvedOrigins[entry.Key] {
+				result.origins[pointer] = origin
+			}
+		} else {
+			root.Mapping[index].Value = document.Clone(entry.Value)
+			for pointer, origin := range directInheritanceOrigins(entry.Value, scopePath.Key(entry.Key), inputOrigins) {
+				result.origins[pointer] = origin
+			}
+		}
+	}
+	result.root = &root
+	if !inheritanceLowerBoundWithin(result.root, limits.nodes) {
+		return nil, []Diagnostic{inheritanceBudgetDiagnostic(scopePath, scope.Location)}
+	}
+	return result, nil
+}
+
+func inheritanceDirective(node *document.Node) (document.Entry, bool) {
+	for _, entry := range node.Mapping {
+		if entry.Key == inheritanceKey {
+			return entry, true
+		}
+	}
+	return document.Entry{}, false
+}
+
+func inheritanceDirectiveLocation(node *document.Node) Location {
+	entry, _ := inheritanceDirective(node)
+	return publicLocation(entry.Value.Location)
+}
+
+func withoutInheritanceDirective(node *document.Node) *document.Node {
+	payload := *node
+	payload.Mapping = make([]document.Entry, 0, len(node.Mapping))
+	for _, entry := range node.Mapping {
+		if entry.Key != inheritanceKey {
+			payload.Mapping = append(payload.Mapping, entry)
+		}
+	}
+	return &payload
+}
+
+func inheritanceDiagnostic(code Code, path Path, primary Location, related []Location, message string) Diagnostic {
+	return Diagnostic{
+		Code: code, Severity: SeverityError, Path: path, Message: message,
+		Primary: &primary, Related: related,
+	}
+}

internal/engine/inheritance_bench_test.go

@@ -0,0 +1,110 @@
+package engine
+
+import (
+	"strconv"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func BenchmarkInheritanceExpansion(b *testing.B) {
+	for _, shape := range []string{"chain", "wide", "nested", "replacement"} {
+		for _, size := range []int{16, 32, 64} {
+			b.Run(shape+"/"+strconv.Itoa(size), func(b *testing.B) {
+				input := inheritanceBenchmarkInput(shape, size)
+				b.ReportAllocs()
+				b.ResetTimer()
+				for range b.N {
+					if _, diagnostics := expandInheritanceWithOrigins(input, nil, defaultProjectionLimits()); len(diagnostics) != 0 {
+						b.Fatal(diagnostics)
+					}
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkInheritanceInspectedLoad(b *testing.B) {
+	type entry struct {
+		Value string `yaml:"value"`
+	}
+	type configuration struct {
+		Entries map[string]entry `yaml:"entries"`
+	}
+	for _, size := range []int{16, 32, 64} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			var input strings.Builder
+			input.WriteString("entries:\n  item-0: {value: base}\n")
+			for index := 1; index <= size; index++ {
+				input.WriteString("  item-")
+				input.WriteString(strconv.Itoa(index))
+				input.WriteString(": {extends: item-0}\n")
+			}
+			source := Bytes("inheritance-benchmark", []byte(input.String()))
+			query := Keys("entries", "item-"+strconv.Itoa(size), "value")
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				loaded, err := Load[configuration](source, testSecretBinding())
+				if err != nil {
+					b.Fatal(err)
+				}
+				if _, found := loaded.Inspection.Explain(query); !found {
+					b.Fatal("inherited value is not explainable")
+				}
+			}
+		})
+	}
+}
+
+// Each scope has constant-sized output and evidence. This distinguishes budget
+// accounting overhead from legitimate growth in expanded parent payloads.
+func BenchmarkInheritanceIndependentScopes(b *testing.B) {
+	for _, size := range []int{256, 512, 1024} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			input := &document.Node{Kind: document.Mapping}
+			for index := 0; index < size; index++ {
+				input.Mapping = append(input.Mapping, document.Entry{
+					Key: strconv.Itoa(index), Value: inheritanceBenchmarkInput("wide", 1),
+				})
+			}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := expandInheritanceTree(input, defaultProjectionLimits()); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
+func inheritanceBenchmarkInput(shape string, size int) *document.Node {
+	value := func(text string) *document.Node { return &document.Node{Kind: document.String, Text: text} }
+	mapping := func(entries ...document.Entry) *document.Node {
+		return &document.Node{Kind: document.Mapping, Mapping: entries}
+	}
+	entry := func(key string, node *document.Node) document.Entry { return document.Entry{Key: key, Value: node} }
+	base := mapping(entry("value", value("base")), entry("settings", mapping(entry("enabled", &document.Node{Kind: document.Boolean, Text: "false"}))))
+	if shape == "nested" {
+		base = mapping(entry("settings", mapping(entry("first", mapping(entry("value", value("base")))), entry("second", mapping(entry("extends", value("first")))))))
+	}
+	definitions := make([]document.Entry, 0, size+1)
+	definitions = append(definitions, entry("item-0", base))
+	for index := 1; index <= size; index++ {
+		parent := "item-0"
+		if shape == "chain" {
+			parent = "item-" + strconv.Itoa(index-1)
+		}
+		payload := []document.Entry{entry("extends", value(parent))}
+		if shape == "nested" {
+			payload = append(payload, entry("settings", mapping(entry("first", mapping(entry("value", value("custom")))))))
+		}
+		if shape == "replacement" {
+			payload = append(payload, entry("value", value("replacement")))
+		}
+		definitions = append(definitions, entry("item-"+strconv.Itoa(index), mapping(payload...)))
+	}
+	return mapping(entry("entries", mapping(definitions...)))
+}

internal/engine/inheritance_explain.go

@@ -0,0 +1,60 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+func appendInheritanceExplanation(explanation *Explanation, working **document.Node, query Path, operations []inheritanceOperation, secrets map[string]bool) {
+	for _, operation := range operations {
+		changes := projectedInheritanceChanges(query, operation)
+		if operation.control != nil {
+			if !pathsIntersect(operation.control.path, query) {
+				continue
+			}
+			before := valueSnapshot(*working, query, secrets)
+			replaceHistoryPath(working, operation.control.path, nil)
+			after := valueSnapshot(*working, query, secrets)
+			if len(changes) != 0 {
+				explanation.History = append(explanation.History, ValueChange{
+					Before: before, After: after, Origin: cloneOrigin(changes[0].Origin),
+					Dependencies: clonePaths(changes[0].Dependencies),
+				})
+			}
+			continue
+		}
+		if operation.transfer == nil {
+			continue
+		}
+		before := valueSnapshot(*working, query, secrets)
+		replaceHistoryPath(working, operation.transfer.to, operation.parent)
+		afterParent := valueSnapshot(*working, query, secrets)
+		parent, parentPresent := transferredOperationOrigin(operation, operationPathForQuery(operation.transfer.to, query))
+		if pathsIntersect(operation.transfer.to, query) && parentPresent {
+			parent.Operation = ChangeInheritance
+			parent.Inheritance = projectInheritanceSteps(parent)
+			explanation.History = append(explanation.History, ValueChange{
+				Before: before, After: afterParent, Origin: parent,
+				Dependencies: []Path{operation.transfer.from.clone(), operation.transfer.reference.clone()},
+			})
+		}
+		replaceHistoryPath(working, operation.transfer.to, operation.effective)
+		if !pathsIntersect(operation.transfer.to, query) {
+			continue
+		}
+		afterChild := valueSnapshot(*working, query, secrets)
+		if len(changes) != 0 {
+			last := changes[len(changes)-1]
+			if !parentPresent || len(changes) > 1 {
+				explanation.History = append(explanation.History, ValueChange{
+					Before: afterParent, After: afterChild, Origin: cloneOrigin(last.Origin),
+					Dependencies: clonePaths(last.Dependencies),
+				})
+			}
+		}
+	}
+}
+
+func operationPathForQuery(root, query Path) Path {
+	if pathContains(query, root) {
+		return root
+	}
+	return query
+}

internal/engine/inheritance_history.go

@@ -0,0 +1,119 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// Retain source history at original paths. Structural operations are held once
+// per relationship and projected onto a requested destination path only when
+// History or Explain is queried.
+func applyInheritanceMetadata(metadata *Metadata, raw *document.Node, rawOrigins map[string]Origin, overlays []overlayRecord, expansion *inheritanceScope) {
+	if metadata == nil || expansion == nil {
+		return
+	}
+	sourceView := Metadata{
+		fields:  make(map[string]FieldInfo),
+		history: metadata.history,
+	}
+	walkDocument(raw, Path{}, func(path Path, node *document.Node) {
+		origin, found := rawOrigins[path.String()]
+		if !found {
+			origin = Origin{Layer: LayerBase, Locations: []Location{publicLocation(node.Location)}}
+		}
+		sourceView.fields[path.String()] = FieldInfo{Explicit: true, Null: node.Kind == document.Null, Origin: cloneOrigin(origin)}
+	})
+	applyOverlayMetadata(&sourceView, overlays)
+	metadata.history = sourceView.history
+	metadata.inheritance = append([]inheritanceOperation(nil), expansion.operations...)
+}
+
+func historyWithInheritance(path Path, stored []Change, operations []inheritanceOperation) []Change {
+	split := len(stored)
+	for index, change := range stored {
+		if change.Origin.Layer == LayerResolution {
+			split = index
+			break
+		}
+	}
+	history := cloneChanges(stored[:split])
+	for _, operation := range operations {
+		for _, change := range projectedInheritanceChanges(path, operation) {
+			if len(history) != 0 {
+				change.Previous = []Origin{cloneOrigin(history[len(history)-1].Origin)}
+			}
+			history = append(history, change)
+		}
+	}
+	for _, change := range stored[split:] {
+		cloned := cloneChange(change)
+		if len(history) != 0 {
+			cloned.Previous = []Origin{cloneOrigin(history[len(history)-1].Origin)}
+		}
+		history = append(history, cloned)
+	}
+	return history
+}
+
+func projectedInheritanceChanges(query Path, operation inheritanceOperation) []Change {
+	if operation.control != nil {
+		if !pathsIntersect(operation.control.path, query) {
+			return nil
+		}
+		origin := cloneOrigin(operation.controlOrigin)
+		origin.Operation = ChangeInheritance
+		origin.Inheritance = projectInheritanceSteps(origin)
+		return []Change{{Path: operation.control.path.clone(), Origin: origin,
+			Dependencies: []Path{operation.control.path.clone()}}}
+	}
+	if operation.transfer == nil || !pathsIntersect(operation.transfer.to, query) {
+		return nil
+	}
+	path := query
+	if pathContains(query, operation.transfer.to) {
+		path = operation.transfer.to
+	}
+	var changes []Change
+	parent, parentFound := transferredOperationOrigin(operation, path)
+	if parentFound {
+		parent.Operation = ChangeInheritance
+		parent.Inheritance = projectInheritanceSteps(parent)
+		changes = append(changes, Change{
+			Path: path.clone(), Origin: parent,
+			Dependencies: []Path{operation.transfer.from.clone(), operation.transfer.reference.clone()},
+		})
+	}
+	effective, effectiveFound := operation.effectiveOrigins[path.String()]
+	if effectiveFound {
+		effective = cloneOrigin(effective)
+		effective.Operation = ChangeInheritance
+	}
+	if effectiveFound && (!parentFound || !originContainedBy(effective, parent)) {
+		effective.Inheritance = projectInheritanceSteps(effective)
+		changes = append(changes, Change{
+			Path: path.clone(), Origin: effective,
+			Dependencies: []Path{operation.transfer.reference.clone()},
+		})
+	} else if parentFound && !effectiveFound {
+		removed := cloneOrigin(operation.effectiveOrigins[operation.transfer.to.String()])
+		removed.Operation = ChangeInheritance
+		removed.Inheritance = projectInheritanceSteps(removed)
+		changes = append(changes, Change{
+			Path: operation.transfer.to.clone(), Origin: removed,
+			Dependencies: []Path{operation.transfer.reference.clone()},
+		})
+	}
+	return changes
+}
+
+func transferredOperationOrigin(operation inheritanceOperation, destination Path) (Origin, bool) {
+	relative, ok := relativeHistoryPath(operation.transfer.to, destination)
+	if !ok {
+		return Origin{}, false
+	}
+	source := appendInheritancePath(operation.transfer.from, relative)
+	origin, found := operation.parentOrigins[source.String()]
+	if !found {
+		return Origin{}, false
+	}
+	transferred := transferredInheritanceOrigins(map[string]Origin{source.String(): origin},
+		operation.transfer.from, operation.transfer.to, *operation.transfer)
+	return transferred[destination.String()], true
+}

internal/engine/inheritance_history_test.go

@@ -0,0 +1,363 @@
+package engine
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestNestedInheritanceHistoryRetainsUnresolvedTemplates(t *testing.T) {
+	input := `presets:
+  base:
+    settings:
+      primary: {width: 10}
+      secondary: {extends: primary}
+  custom:
+    extends: base
+    settings:
+      primary: {width: 20}
+  grandchild:
+    extends: custom
+    settings:
+      primary: {width: 30}
+`
+	loaded, err := Load[map[string]any](Bytes("nested", []byte(input)), testSecretBinding())
+	if err != nil {
+		t.Fatal(err)
+	}
+	for name, width := range map[string]string{"custom": "20", "grandchild": "30"} {
+		t.Run(name, func(t *testing.T) {
+			root := Keys("presets", name)
+			value, _ := loaded.Inspection.Explain(root.Key("settings").Key("secondary").Key("width"))
+			introduced := false
+			for _, change := range value.History {
+				if change.Origin.Operation == ChangeInheritance && !change.Before.Present && change.After.YAML == width+"\n" {
+					introduced = true
+				}
+			}
+			if !introduced || value.Effective.YAML != width+"\n" {
+				t.Fatalf("missing nested introduction: %#v", value)
+			}
+			controlPath := root.Key("settings").Key("secondary").Key("extends")
+			control, _ := loaded.Inspection.Explain(controlPath)
+			arrived, consumed := false, false
+			for _, change := range control.History {
+				arrived = arrived || change.After.YAML == "primary\n"
+				consumed = consumed || (change.Before.YAML == "primary\n" && !change.After.Present)
+			}
+			if !arrived || !consumed || control.Effective.Present {
+				t.Fatalf("missing control history: %#v", control)
+			}
+			container, _ := loaded.Inspection.Explain(root)
+			unresolved := false
+			for _, change := range container.History {
+				if change.Origin.Operation == ChangeInheritance && strings.Contains(change.After.YAML, "extends: primary") {
+					unresolved = true
+				}
+			}
+			if !unresolved || strings.Contains(container.Effective.YAML, "extends:") {
+				t.Fatalf("invalid enclosing snapshot: %#v", container)
+			}
+			if _, present := loaded.Inspection.Metadata().Lookup(controlPath); present {
+				t.Fatal("control is effective")
+			}
+			if len(loaded.Inspection.Metadata().History(controlPath)) != len(control.History) {
+				t.Fatal("metadata and explanation disagree")
+			}
+		})
+	}
+}
+
+func TestNestedInheritanceHistoryInsideSequence(t *testing.T) {
+	input := `base:
+  items:
+    - first: {value: inherited}
+      second: {extends: first}
+child: {extends: base}
+`
+	loaded, err := Load[map[string]any](Bytes("sequence", []byte(input)), testSecretBinding())
+	if err != nil {
+		t.Fatal(err)
+	}
+	path := Keys("child", "items").Index(0).Key("second")
+	control, _ := loaded.Inspection.Explain(path.Key("extends"))
+	consumed := false
+	for _, change := range control.History {
+		consumed = consumed || (change.Before.YAML == "first\n" && !change.After.Present)
+	}
+	if !consumed || control.Effective.Present {
+		t.Fatalf("sequence control history = %#v", control)
+	}
+	value, _ := loaded.Inspection.Explain(path.Key("value"))
+	introduced := false
+	for _, change := range value.History {
+		introduced = introduced || (!change.Before.Present && change.After.YAML == "inherited\n")
+	}
+	if !introduced || value.Effective.YAML != "inherited\n" {
+		t.Fatalf("sequence value history = %#v", value)
+	}
+}
+
+func inheritanceInspectionForTest(t *testing.T, sources ...*document.Node) (*Inspection, *inheritanceScope) {
+	t.Helper()
+	composed, diagnostics := composeDocuments(sources...)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	return inheritanceInspectionFromComposition(t, composed)
+}
+
+func inheritanceInspectionFromComposition(t *testing.T, composed *composedDocument) (*Inspection, *inheritanceScope) {
+	t.Helper()
+	expanded, diagnostics := expandInheritanceWithOrigins(composed.root, composed.origins, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	baseline := &document.Node{Kind: document.Mapping}
+	presence := make(map[string]inputPresence)
+	collectPresence(expanded.root, Path{}, presence)
+	decoded := decodedConfiguration{
+		baseline: baseline, final: expanded.root, presence: presence, origins: expanded.origins,
+	}
+	metadata := metadataFromDecoded(decoded)
+	applyInheritanceMetadata(&metadata, composed.root, composed.origins, composed.overlays, expanded)
+	finalizeMetadataPaths(&metadata, expanded.root, composed.applications, baseline, composed.root)
+	return &Inspection{
+		metadata: metadata, baseline: baseline, input: expanded.root,
+		inputOrigins: expanded.origins, final: expanded.root,
+		applications: composed.applications, inheritance: expanded.operations,
+	}, expanded
+}
+
+func TestInheritanceHistoryFollowsFinalParentAfterSourceOverlay(t *testing.T) {
+	base, diagnostics := composeDocuments(parseCompositionDocument(t, "source-1", "parent: {value: original}\nchild: {extends: parent}\n"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	assembly := newAssemblyAccumulator(base)
+	if diagnostics := assembly.applyOverlay(parseCompositionDocument(t, "source-2", "parent: {value: latest}\n"), nil, nil, LayerOverlay); len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	inspection, _ := inheritanceInspectionFromComposition(t, assembly.current)
+	path := Keys("child", "value")
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.YAML != "latest\n" ||
+		len(explanation.History) == 0 || explanation.History[len(explanation.History)-1].Origin.Locations[0].SourceID != "source-2" {
+		t.Fatalf("overlaid parent contribution = %#v, %v", explanation, found)
+	}
+	parentHistory := inspection.Metadata().History(Keys("parent", "value"))
+	if len(parentHistory) < 2 || parentHistory[0].Origin.Layer != LayerBase ||
+		parentHistory[len(parentHistory)-1].Origin.Layer != LayerOverlay {
+		t.Fatalf("parent source history = %#v", parentHistory)
+	}
+}
+
+func TestInheritanceHistoryPreservesCancellationAndSupersededDirective(t *testing.T) {
+	base, diagnostics := composeDocuments(parseCompositionDocument(t, "source-1", "parent: {value: one}\nchild: {extends: missing, value: own}\n"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	assembly := newAssemblyAccumulator(base)
+	if diagnostics := assembly.applyOverlay(parseCompositionDocument(t, "source-2", "child: {extends: null}\n"), nil, nil, LayerOverlay); len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	inspection, _ := inheritanceInspectionFromComposition(t, assembly.current)
+	control := Keys("child", inheritanceKey)
+	history := inspection.Metadata().History(control)
+	if len(history) < 3 || history[0].Origin.Locations[0].SourceID != "source-1" ||
+		history[1].Origin.Locations[0].SourceID != "source-2" ||
+		history[len(history)-1].Origin.Operation != ChangeInheritance {
+		t.Fatalf("superseded and cancelled directive history = %#v", history)
+	}
+	explanation, found := inspection.Explain(control)
+	if !found || explanation.Effective.Present || len(explanation.History) < 3 ||
+		explanation.History[len(explanation.History)-1].After.Present {
+		t.Fatalf("cancelled directive explanation = %#v, %v", explanation, found)
+	}
+	value, found := inspection.Explain(Keys("child", "value"))
+	if !found || value.Effective.YAML != "own\n" {
+		t.Fatalf("cancelled relationship changed own value = %#v, %v", value, found)
+	}
+}
+
+func TestInheritanceHistoryProjectsNestedDestinationValues(t *testing.T) {
+	inspection, _ := inheritanceInspectionForTest(t, parseCompositionDocument(t, "source-1", `base:
+  settings:
+    primary: {value: old}
+    secondary: {extends: primary}
+custom:
+  extends: base
+  settings:
+    primary: {value: new}
+`))
+	path := Keys("custom", "settings", "secondary", "value")
+	history := inspection.Metadata().History(path)
+	structural := 0
+	for _, change := range history {
+		if change.Origin.Operation == ChangeInheritance {
+			structural++
+		}
+	}
+	if structural == 0 {
+		t.Fatalf("nested destination history = %#v", history)
+	}
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.YAML != "new\n" ||
+		len(explanation.Origin.Inheritance) == 0 ||
+		explanation.Origin.Inheritance[len(explanation.Origin.Inheritance)-1].To.String() != path.String() {
+		t.Fatalf("nested destination explanation = %#v, %v", explanation, found)
+	}
+	for _, change := range explanation.History {
+		if change.Origin.Operation == ChangeDecoding {
+			t.Fatalf("nested inheritance was attributed to decoding: %#v", explanation.History)
+		}
+	}
+}
+
+func TestInheritanceHistoryShowsParentAndEqualChildContribution(t *testing.T) {
+	inspection, _ := inheritanceInspectionForTest(t,
+		parseCompositionDocument(t, "source-1", "parent: {value: same}\n"),
+		parseCompositionDocument(t, "source-2", "child: {extends: parent, value: same}\n"))
+	path := Keys("child", "value")
+	history := inspection.Metadata().History(path)
+	var structural []Change
+	for _, change := range history {
+		if change.Origin.Operation == ChangeInheritance {
+			structural = append(structural, change)
+		}
+	}
+	if len(structural) != 2 || structural[0].Origin.Locations[0].SourceID != "source-1" ||
+		structural[1].Origin.Locations[0].SourceID != "source-2" ||
+		len(structural[0].Dependencies) != 2 || structural[0].Dependencies[0].String() != "/parent" ||
+		len(structural[0].Origin.Inheritance) != 1 {
+		t.Fatalf("equal-valued structural history = %#v", history)
+	}
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.YAML != "same\n" {
+		t.Fatalf("explanation = %#v, %v", explanation, found)
+	}
+	var equalEvents int
+	for _, change := range explanation.History {
+		if change.Origin.Operation == ChangeDecoding {
+			t.Fatalf("inheritance was attributed to decoding: %#v", explanation.History)
+		}
+		if change.Origin.Operation == ChangeInheritance && change.Before.YAML == change.After.YAML {
+			equalEvents++
+		}
+	}
+	if equalEvents != 2 {
+		t.Fatalf("equal parent and child operations = %#v", explanation.History)
+	}
+	parentHistory := inspection.Metadata().History(Keys("parent", "value"))
+	for _, change := range parentHistory {
+		if change.Origin.Operation == ChangeInheritance {
+			t.Fatalf("source definition acquired a destination operation: %#v", parentHistory)
+		}
+	}
+}
+
+func TestInheritanceHistoryRetainsControlsAndDisplacedPaths(t *testing.T) {
+	inspection, _ := inheritanceInspectionForTest(t,
+		parseCompositionDocument(t, "source-1", "parent: {branch: {detail: inherited}}\n"),
+		parseCompositionDocument(t, "source-2", "child: {extends: parent, branch: replaced}\n"))
+	control := Keys("child", inheritanceKey)
+	controlHistory := inspection.Metadata().History(control)
+	if len(controlHistory) < 2 || controlHistory[len(controlHistory)-1].Origin.Operation != ChangeInheritance {
+		t.Fatalf("control history = %#v", controlHistory)
+	}
+	controlExplanation, found := inspection.Explain(control)
+	if !found || controlExplanation.Effective.Present ||
+		controlExplanation.History[len(controlExplanation.History)-1].After.Present {
+		t.Fatalf("consumed control = %#v, %v", controlExplanation, found)
+	}
+	path := Keys("child", "branch", "detail")
+	if _, found := inspection.Metadata().Lookup(path); found {
+		t.Fatal("displaced descendant remained effective")
+	}
+	listed := false
+	for _, historical := range inspection.Metadata().HistoricalPaths() {
+		if historical.String() == path.String() {
+			listed = true
+		}
+	}
+	if !listed {
+		t.Fatal("displaced inherited descendant was not enumerable")
+	}
+	history := inspection.Metadata().History(path)
+	if len(history) < 2 || history[len(history)-1].Origin.Operation != ChangeInheritance {
+		t.Fatalf("displaced descendant history = %#v", history)
+	}
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.Present {
+		t.Fatalf("displaced descendant explanation = %#v, %v", explanation, found)
+	}
+	var sawInherited, sawRemoval bool
+	for _, change := range explanation.History {
+		if change.Origin.Operation != ChangeInheritance {
+			continue
+		}
+		sawInherited = sawInherited || change.After.Present
+		sawRemoval = sawRemoval || (change.Before.Present && !change.After.Present)
+	}
+	if !sawInherited || !sawRemoval {
+		t.Fatalf("displaced value sequence = %#v", explanation.History)
+	}
+	var rendered bytes.Buffer
+	if err := inspection.WriteExplanation(&rendered, path); err != nil ||
+		!strings.Contains(rendered.String(), "structural inheritance") {
+		t.Fatalf("rendered structural explanation = %q, %v", rendered.String(), err)
+	}
+}
+
+func TestInheritanceHistoryPrecedesApplicationResolution(t *testing.T) {
+	inspection, _ := inheritanceInspectionForTest(t,
+		parseCompositionDocument(t, "source-1", "parent: {value: original}\nchild: {extends: parent}\n"))
+	path := Keys("child", "value")
+	replacement := &document.Node{Kind: document.String, Text: "derived"}
+	snapshot := resolutionSnapshot{
+		change: Change{Path: path, Origin: Origin{Layer: LayerResolution, Operation: ChangeNormalization}},
+		after:  replacement,
+	}
+	final := document.Clone(inspection.final)
+	replaceHistoryPath(&final, path, replacement)
+	inspection.final = final
+	inspection.resolution = []resolutionSnapshot{snapshot}
+	inspection.metadata.history[path.String()] = append(inspection.metadata.history[path.String()], snapshot.change)
+	inspection.metadata.fields[path.String()] = FieldInfo{Explicit: true, Origin: snapshot.change.Origin}
+	history := inspection.Metadata().History(path)
+	if len(history) < 2 || history[len(history)-2].Origin.Operation != ChangeInheritance ||
+		history[len(history)-1].Origin.Operation != ChangeNormalization {
+		t.Fatalf("resolution order = %#v", history)
+	}
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.YAML != "derived\n" ||
+		explanation.History[len(explanation.History)-1].Origin.Operation != ChangeNormalization {
+		t.Fatalf("resolved explanation = %#v, %v", explanation, found)
+	}
+}
+
+func TestHistoryNodeBudgetCountsSharedInheritanceEvidenceOnce(t *testing.T) {
+	shared := &document.Node{Kind: document.String, Text: "shared"}
+	extra := &document.Node{Kind: document.String, Text: "extra"}
+	expansion := &inheritanceScope{
+		root:         shared,
+		operations:   []inheritanceOperation{{parent: shared, effective: shared}},
+		replacements: []inheritanceReplacement{{before: shared, after: extra}},
+	}
+	if !historyNodesWithin(2, nil, nil, nil, nil, nil, expansion) ||
+		historyNodesWithin(1, nil, nil, nil, nil, nil, expansion) {
+		t.Fatal("shared evidence node accounting is incorrect")
+	}
+}
+
+func TestInheritanceExpansionEnforcesRetainedHistoryBudget(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", "parent: {value: original}\nchild: {extends: parent, value: replacement}\n")
+	if _, diagnostics := expandInheritanceWithBudgets(input, nil, defaultProjectionLimits(), 2); len(diagnostics) != 1 || diagnostics[0].Code != CodeExplainLimitExceeded {
+		t.Fatalf("retained history limit = %#v", diagnostics)
+	}
+	if _, diagnostics := expandInheritanceWithBudgets(input, nil, defaultProjectionLimits(), 100); len(diagnostics) != 0 {
+		t.Fatalf("sufficient retained history budget = %#v", diagnostics)
+	}
+}

internal/engine/inheritance_load.go

@@ -0,0 +1,53 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+func containsInheritanceDirective(root *document.Node) bool {
+	found := false
+	walkDocument(root, Path{}, func(path Path, _ *document.Node) {
+		if len(path.segments) != 0 && path.segments[len(path.segments)-1].kind == pathSegmentKey &&
+			path.segments[len(path.segments)-1].key == inheritanceKey {
+			found = true
+		}
+	})
+	return found
+}
+
+func inheritanceVersionDiagnostic(rule *VersionRule, expanded *inheritanceScope) *Diagnostic {
+	if rule == nil || expanded == nil {
+		return nil
+	}
+	for _, operation := range expanded.operations {
+		var location Location
+		var affected bool
+		if operation.control != nil {
+			affected = pathsIntersect(operation.control.path, rule.Path)
+			if len(operation.control.locations) != 0 {
+				location = operation.control.locations[0]
+			}
+		} else if operation.transfer != nil {
+			if relative, within := relativeHistoryPath(operation.transfer.to, rule.Path); within {
+				_, affected = documentAtPath(operation.parent, relative)
+			} else if pathContains(rule.Path, operation.transfer.to) {
+				affected = true
+			}
+			if len(operation.transfer.locations) != 0 {
+				location = operation.transfer.locations[0]
+			}
+		}
+		if affected {
+			return &Diagnostic{
+				Code: CodeInheritanceVersion, Severity: SeverityError, Path: rule.Path.clone(),
+				Message: "inheritance cannot change the schema-version path", Primary: &location,
+			}
+		}
+	}
+	return nil
+}
+
+func inheritanceOperations(expanded *inheritanceScope) []inheritanceOperation {
+	if expanded == nil {
+		return nil
+	}
+	return expanded.operations
+}

internal/engine/inheritance_origin.go

@@ -0,0 +1,127 @@
+package engine
+
+import (
+	"slices"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func directInheritanceOrigins(node *document.Node, root Path, input map[string]Origin) map[string]Origin {
+	result := make(map[string]Origin)
+	walkDocument(node, root, func(path Path, value *document.Node) {
+		origin, found := input[path.String()]
+		if !found {
+			origin = Origin{Layer: LayerBase, Locations: []Location{publicLocation(value.Location)}}
+		}
+		origin = cloneOrigin(origin)
+		if len(origin.contributors) == 0 {
+			origin.contributors = make([]inheritanceContributor, len(origin.Locations))
+			for index, location := range origin.Locations {
+				origin.contributors[index] = inheritanceContributor{
+					source: path.clone(), location: location,
+				}
+			}
+		}
+		result[path.String()] = origin
+	})
+	return result
+}
+
+func transferredInheritanceOrigins(parent map[string]Origin, from, to Path, control inheritanceTransfer) map[string]Origin {
+	result := make(map[string]Origin, len(parent))
+	transfer := control // one immutable edge shared by every matching contributor
+	links := make(map[*inheritanceTrace]*inheritanceTrace)
+	for pointer, origin := range parent {
+		path, err := ParsePath(pointer)
+		if err != nil {
+			continue
+		}
+		relative, ok := relativeHistoryPath(from, path)
+		if !ok {
+			continue
+		}
+		destination := appendInheritancePath(to, relative)
+		origin = cloneOrigin(origin)
+		for index, contributor := range origin.contributors {
+			link, found := links[contributor.trace]
+			if !found {
+				link = &inheritanceTrace{previous: contributor.trace, transfer: &transfer}
+				links[contributor.trace] = link
+			}
+			origin.contributors[index].trace = link
+		}
+		result[destination.String()] = origin
+	}
+	return result
+}
+
+func appendInheritancePath(root, relative Path) Path {
+	return Path{segments: append(clonePathSegments(root.segments, len(relative.segments)), relative.segments...)}
+}
+
+func mergeInheritanceOrigin(origins map[string]Origin, path Path, incoming Origin) {
+	pointer := path.String()
+	current := origins[pointer]
+	current.Layer = incoming.Layer
+	current.Locations = append(current.Locations, incoming.Locations...)
+	current.contributors = append(current.contributors, incoming.contributors...)
+	origins[pointer] = current
+}
+
+func copyInheritanceSubtree(target, incoming map[string]Origin, node *document.Node, root Path) {
+	walkDocument(node, root, func(path Path, _ *document.Node) {
+		target[path.String()] = cloneOrigin(incoming[path.String()])
+	})
+}
+
+// Materialize concrete steps only for an inspection record. The linked private
+// traces remain compact when one parent supplies many descendant values.
+func projectInheritanceSteps(origin Origin) []InheritanceStep {
+	steps := cloneInheritanceSteps(origin.Inheritance)
+	for _, contributor := range origin.contributors {
+		var reverse []*inheritanceTrace
+		for link := contributor.trace; link != nil; link = link.previous {
+			reverse = append(reverse, link)
+		}
+		current := contributor.source
+		for index := len(reverse) - 1; index >= 0; index-- {
+			transfer := reverse[index].transfer
+			relative, ok := relativeHistoryPath(transfer.from, current)
+			if !ok {
+				continue
+			}
+			next := appendInheritancePath(transfer.to, relative)
+			step := InheritanceStep{
+				From: current.clone(), To: next.clone(), Reference: transfer.reference.clone(),
+				Locations: cloneLocations(transfer.locations),
+			}
+			if !slices.ContainsFunc(steps, func(existing InheritanceStep) bool {
+				return existing.From.String() == step.From.String() &&
+					existing.To.String() == step.To.String() &&
+					existing.Reference.String() == step.Reference.String() &&
+					slices.Equal(existing.Locations, step.Locations)
+			}) {
+				steps = append(steps, step)
+			}
+			current = next
+		}
+	}
+	return steps
+}
+
+func sameInheritanceTrace(left, right *inheritanceTrace) bool {
+	for left != nil && right != nil {
+		if left == right {
+			return true
+		}
+		l, r := left.transfer, right.transfer
+		if !slices.Equal(l.from.segments, r.from.segments) ||
+			!slices.Equal(l.to.segments, r.to.segments) ||
+			!slices.Equal(l.reference.segments, r.reference.segments) ||
+			!slices.Equal(l.locations, r.locations) {
+			return false
+		}
+		left, right = left.previous, right.previous
+	}
+	return left == nil && right == nil
+}

internal/engine/inheritance_origin_test.go

@@ -0,0 +1,141 @@
+package engine
+
+import (
+	"io/fs"
+	"testing"
+	"testing/fstest"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestInheritedOriginsKeepSourcePathsPresenceAndIndependentReplacement(t *testing.T) {
+	parent := parseCompositionDocument(t, "source-1", `parent:
+  enabled: false
+  count: 0
+  optional: null
+  items: [{file: message.txt}]
+  settings: {shared: parent}
+`)
+	child := parseCompositionDocument(t, "source-2", `child:
+  extends: parent
+  count: 3
+  settings: {own: child}
+`)
+	composed, diagnostics := composeDocuments(parent, child)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	result, diagnostics := expandInheritanceWithOrigins(composed.root, composed.origins, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	for _, key := range []string{"enabled", "optional"} {
+		path := Keys("child", key)
+		value, found := documentAtPath(result.root, path)
+		origin, hasOrigin := result.origins[path.String()]
+		if !found || !hasOrigin || value == nil || len(origin.Locations) != 1 || origin.Locations[0].SourceID != "source-1" ||
+			len(projectInheritanceSteps(origin)) != 1 {
+			t.Fatalf("inherited %s = %#v, %#v", key, value, origin)
+		}
+	}
+	count := result.origins["/child/count"]
+	if len(count.Locations) != 1 || count.Locations[0].SourceID != "source-2" || len(projectInheritanceSteps(count)) != 0 {
+		t.Fatalf("direct replacement origin = %#v", count)
+	}
+	if len(result.replacements) != 1 || result.replacements[0].path.String() != "/child/count" ||
+		result.replacements[0].before.Text != "0" || result.replacements[0].after.Text != "3" ||
+		len(result.replacements[0].previous) != 1 ||
+		len(projectInheritanceSteps(result.replacements[0].previous[0].origin)) != 1 {
+		t.Fatalf("displaced inheritance = %#v", result.replacements)
+	}
+	settings := result.origins["/child/settings"]
+	if len(settings.Locations) != 2 || settings.Locations[0].SourceID != "source-1" ||
+		settings.Locations[1].SourceID != "source-2" || len(projectInheritanceSteps(settings)) != 1 {
+		t.Fatalf("composite mapping origin = %#v", settings)
+	}
+	if originContainedBy(result.origins["/child/enabled"], result.origins["/parent/enabled"]) ||
+		originContainedBy(result.origins["/parent/enabled"], result.origins["/child/enabled"]) {
+		t.Fatal("direct and inherited evidence collapsed despite matching source coordinates")
+	}
+	original := result.origins["/child/enabled"]
+	detached := cloneOrigin(original)
+	detached.Locations[0].SourceID = "changed"
+	detached.contributors[0].source.segments[0].key = "changed"
+	stepsCopy := projectInheritanceSteps(original)
+	stepsCopy[0].Locations[0].SourceID = "changed"
+	if original.Locations[0].SourceID != "source-1" ||
+		original.contributors[0].source.String() != "/parent/enabled" ||
+		projectInheritanceSteps(original)[0].Locations[0].SourceID != "source-2" {
+		t.Fatal("cloned origin or projected steps changed retained evidence")
+	}
+	resource := result.origins["/child/items/0/file"]
+	steps := projectInheritanceSteps(resource)
+	if len(steps) != 1 || steps[0].From.String() != "/parent/items/0/file" ||
+		steps[0].To.String() != "/child/items/0/file" || steps[0].Reference.String() != "/child/extends" ||
+		len(resource.Locations) != 1 || resource.Locations[0].SourceID != "source-1" {
+		t.Fatalf("sequence resource origin = %#v, steps = %#v", resource, steps)
+	}
+	contexts := map[string]fs.FS{
+		"source-1": fstest.MapFS{"message.txt": &fstest.MapFile{Data: []byte("from parent")}},
+		"source-2": fstest.MapFS{"message.txt": &fstest.MapFile{Data: []byte("from child")}},
+	}
+	data, err := fs.ReadFile(contexts[resource.Locations[0].SourceID], "message.txt")
+	if err != nil || string(data) != "from parent" {
+		t.Fatalf("inherited resource = %q, %v", data, err)
+	}
+}
+
+func TestInheritedOriginsRetainChainsAndNestedDestinations(t *testing.T) {
+	first := parseCompositionDocument(t, "source-1", `first:
+  enabled: false
+  settings:
+    primary: {value: original}
+    secondary: {extends: primary}
+`)
+	second := parseCompositionDocument(t, "source-2", `first: {other: present}
+middle: {extends: first}
+last:
+  extends: middle
+  settings:
+    primary: {value: replaced}
+`)
+	composed, diagnostics := composeDocuments(first, second)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	result, diagnostics := expandInheritanceWithOrigins(composed.root, composed.origins, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	chain := projectInheritanceSteps(result.origins["/last/enabled"])
+	if len(chain) != 2 || chain[0].From.String() != "/first/enabled" ||
+		chain[0].To.String() != "/middle/enabled" || chain[1].From.String() != "/middle/enabled" ||
+		chain[1].To.String() != "/last/enabled" {
+		t.Fatalf("chain = %#v", chain)
+	}
+	composite := result.origins["/last"]
+	if len(composite.Locations) < 2 || len(composite.contributors) != len(composite.Locations) {
+		t.Fatalf("composite contributors = %#v", composite)
+	}
+	seenSources := make(map[string]bool)
+	for _, contributor := range composite.contributors {
+		seenSources[contributor.location.SourceID] = true
+	}
+	if !seenSources["source-1"] || !seenSources["source-2"] {
+		t.Fatalf("same-layer source contributors = %#v", composite.contributors)
+	}
+	nested := projectInheritanceSteps(result.origins["/last/settings/secondary/value"])
+	if len(nested) != 1 || nested[0].From.String() != "/last/settings/primary/value" ||
+		nested[0].To.String() != "/last/settings/secondary/value" {
+		t.Fatalf("destination-specific nested transfer = %#v", nested)
+	}
+	if got := result.origins["/last/settings/secondary/value"].Locations[0].SourceID; got != "source-2" {
+		t.Fatalf("nested replacement source = %q", got)
+	}
+	if original, found := documentAtPath(first, Keys("first", "settings", "primary", "value")); !found || original.Text != "original" {
+		t.Fatal("expansion changed its input")
+	}
+	if inherited, found := documentAtPath(result.root, Keys("last", "settings", "secondary", "value")); !found || inherited.Kind != document.String || inherited.Text != "replaced" {
+		t.Fatalf("nested inherited value = %#v, %v", inherited, found)
+	}
+}

internal/engine/inheritance_secret_test.go

@@ -0,0 +1,136 @@
+package engine
+
+import (
+	"bytes"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestInheritanceSensitivityFollowsCopiesWithoutMaskingNeighbors(t *testing.T) {
+	const canary = "INHERITED-CANARY-CREDENTIAL"
+	input := parseCompositionDocument(t, "source-1", "parent: {token: "+canary+", plain: visible}\nchild: {extends: parent}\nnext: {extends: child}\nother: {extends: parent}\n")
+	for _, classified := range []Path{Keys("parent", "token"), Keys("next", "token")} {
+		t.Run(classified.String(), func(t *testing.T) {
+			inspection, _ := inheritanceInspectionForTest(t, input)
+			inspection.secrets = classifySecretPaths(nil, func(path Path) bool {
+				return path.String() == classified.String()
+			}, inspection.metadata, inspection.final, inspection.applications, inspection.input)
+			for _, path := range []Path{Keys("parent", "token"), Keys("child", "token"), Keys("next", "token"), Keys("other", "token")} {
+				explanation, found := inspection.Explain(path)
+				if !found || !explanation.Effective.Redacted || strings.Contains(explanation.Effective.YAML, canary) {
+					t.Fatalf("sensitive copy %s = %#v, %v", path, explanation, found)
+				}
+				for _, change := range explanation.History {
+					if strings.Contains(change.Before.YAML, canary) || strings.Contains(change.After.YAML, canary) {
+						t.Fatalf("historical canary at %s: %#v", path, change)
+					}
+				}
+			}
+			plain, found := inspection.Explain(Keys("other", "plain"))
+			if !found || plain.Effective.Redacted || plain.Effective.YAML != "visible\n" {
+				t.Fatalf("unrelated field = %#v, %v", plain, found)
+			}
+			var output bytes.Buffer
+			if err := inspection.WriteEffective(&output); err != nil || strings.Contains(output.String(), canary) || !strings.Contains(output.String(), "visible") {
+				t.Fatalf("effective rendering = %q, %v", output.String(), err)
+			}
+		})
+	}
+}
+
+func TestInheritanceSensitivityPreservesDisplacedHistoryAndControl(t *testing.T) {
+	const canary = "DISPLACED-CANARY-CREDENTIAL"
+	inspection, _ := inheritanceInspectionForTest(t, parseCompositionDocument(t, "source-1", "parent: {branch: {token: "+canary+"}}\nchild: {extends: parent, branch: public}\n"))
+	inspection.secrets = classifySecretPaths(nil, func(path Path) bool {
+		return path.String() == Keys("parent", "branch", "token").String() || path.String() == Keys("child", inheritanceKey).String()
+	}, inspection.metadata, inspection.final, inspection.applications, inspection.input)
+	path := Keys("child", "branch", "token")
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.Present {
+		t.Fatalf("displaced sensitive path = %#v, %v", explanation, found)
+	}
+	for _, change := range explanation.History {
+		if strings.Contains(change.Before.YAML, canary) || strings.Contains(change.After.YAML, canary) ||
+			(change.After.Present && !change.After.Redacted) {
+			t.Fatalf("displaced secret leaked: %#v", explanation.History)
+		}
+	}
+	var rendered bytes.Buffer
+	if err := inspection.WriteExplanation(&rendered, path); err != nil || strings.Contains(rendered.String(), canary) {
+		t.Fatalf("displaced explanation rendering = %q, %v", rendered.String(), err)
+	}
+	control, found := inspection.Explain(Keys("child", inheritanceKey))
+	if !found || control.Effective.Present || len(control.History) == 0 || !control.History[0].After.Redacted {
+		t.Fatalf("sensitive control history = %#v, %v", control, found)
+	}
+	info, found := inspection.Metadata().Lookup(Keys("child", "branch"))
+	if !found || len(info.Origin.Inheritance) != 0 {
+		t.Fatalf("independent replacement provenance = %#v, %v", info, found)
+	}
+}
+
+func TestInheritanceSensitivityIncludesSchemaTaggedSource(t *testing.T) {
+	type entry struct {
+		Token string `yaml:"token"`
+	}
+	type configuration struct {
+		Parent struct {
+			Token string `yaml:"token" configkit:"secret"`
+		} `yaml:"parent"`
+		Child entry `yaml:"child"`
+	}
+	schema, issue := buildSchema(reflect.TypeFor[configuration](), testSecretBinding())
+	if issue != nil {
+		t.Fatal(issue)
+	}
+	inspection, _ := inheritanceInspectionForTest(t, parseCompositionDocument(t, "source-1", "parent: {token: TAGGED-CANARY}\nchild: {extends: parent}\n"))
+	inspection.secrets = classifySecretPaths(schema, nil, inspection.metadata, inspection.final, inspection.applications, inspection.input)
+	child, found := inspection.Explain(Keys("child", "token"))
+	if !found || !child.Effective.Redacted || strings.Contains(child.Effective.YAML, "TAGGED-CANARY") {
+		t.Fatalf("tagged source copy = %#v, %v", child, found)
+	}
+}
+
+func TestInheritedSecretReferencesResolveOnlyAfterExpansion(t *testing.T) {
+	type entry struct {
+		Credential Secret `yaml:"credential"`
+	}
+	type configuration struct {
+		Entries map[string]entry `yaml:"entries"`
+	}
+	schema, issue := buildSchema(reflect.TypeFor[configuration](), testSecretBinding())
+	if issue != nil {
+		t.Fatal(issue)
+	}
+	composed, diagnostics := composeDocuments(parseCompositionDocument(t, "source-1", "entries:\n  parent: {credential: INHERITED_SECRET_NAME}\n  child: {extends: parent}\n"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	expanded, diagnostics := expandInheritanceWithOrigins(composed.root, composed.origins, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	lookupCount := 0
+	decoded, issues := decodeConfiguration(schema, reflect.Zero(reflect.TypeFor[configuration]()), expanded.root)
+	if len(issues) != 0 || lookupCount != 0 {
+		t.Fatalf("decoding and lookup order = %#v, %d", issues, lookupCount)
+	}
+	resolved := resolveSecretReferences(decoded.value, schema, expanded.root, func(name string) (string, bool) {
+		lookupCount++
+		if name != "INHERITED_SECRET_NAME" {
+			t.Fatalf("inherited reference name = %q", name)
+		}
+		return "RESOLVED-CREDENTIAL-CANARY", true
+	})
+	if len(resolved.diagnostics) != 0 || lookupCount != 1 ||
+		decoded.value.Interface().(configuration).Entries["child"].Credential.Value() != "RESOLVED-CREDENTIAL-CANARY" {
+		t.Fatalf("inherited reference resolution = %#v, %d", resolved.diagnostics, lookupCount)
+	}
+	inspection, _ := inheritanceInspectionFromComposition(t, composed)
+	inspection.secrets = classifySecretPaths(schema, nil, inspection.metadata, inspection.final, inspection.applications, inspection.input)
+	var rendered bytes.Buffer
+	if err := inspection.WriteEffective(&rendered); err != nil || strings.Contains(rendered.String(), "RESOLVED-CREDENTIAL-CANARY") {
+		t.Fatalf("reference rendering = %q, %v", rendered.String(), err)
+	}
+}

internal/engine/inheritance_test.go

@@ -0,0 +1,141 @@
+package engine
+
+import (
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestSiblingInheritanceResolvesReferencesAndKeepsInputs(t *testing.T) {
+	tests := []struct {
+		name, input string
+		path        Path
+		text        string
+		transfers   int
+		controls    int
+	}{
+		{
+			name: "forward reference", input: "child: {extends: parent, value: own}\nparent: {value: parent, kept: preserved}\n",
+			path: Keys("child", "kept"), text: "preserved", transfers: 1, controls: 1,
+		},
+		{
+			name: "backward reference", input: "parent: {value: parent, kept: preserved}\nchild: {extends: parent, value: own}\n",
+			path: Keys("child", "value"), text: "own", transfers: 1, controls: 1,
+		},
+		{
+			name: "chain and shared parent", input: "last: {extends: middle, enabled: false}\nfirst: {value: original}\nmiddle: {extends: first, extra: middle}\nother: {extends: first}\n",
+			path: Keys("last", "value"), text: "original", transfers: 3, controls: 3,
+		},
+		{
+			name: "exact empty sibling name", input: "child: {extends: \"\"}\n\"\": {value: empty}\n",
+			path: Keys("child", "value"), text: "empty", transfers: 1, controls: 1,
+		},
+		{
+			name: "exact unusual sibling name", input: "child: {extends: \"a/b~c\"}\n\"a/b~c\": {value: unusual}\n",
+			path: Keys("child", "value"), text: "unusual", transfers: 1, controls: 1,
+		},
+		{
+			name: "null cancellation", input: "child: {extends: null, value: own}\nparent: {value: parent}\n",
+			path: Keys("child", "value"), text: "own", controls: 1,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			input := parseCompositionDocument(t, "source-1", test.input)
+			before := document.Clone(input)
+			result, diagnostics := expandSiblingDefinitions(input, Path{})
+			if len(diagnostics) != 0 {
+				t.Fatalf("diagnostics = %#v", diagnostics)
+			}
+			if len(result.transfers) != test.transfers || len(result.controls) != test.controls {
+				t.Fatalf("evidence = %#v, %#v", result.transfers, result.controls)
+			}
+			node, found := documentAtPath(result.root, test.path)
+			if !found || node.Text != test.text {
+				t.Fatalf("%s = %#v, %v", test.path.String(), node, found)
+			}
+			if _, found := documentAtPath(result.root, Keys("child", inheritanceKey)); found {
+				t.Fatal("child directive remained in expanded payload")
+			}
+			if !reflect.DeepEqual(input, before) {
+				t.Fatal("resolver changed input")
+			}
+			node.Text = "changed"
+			if !reflect.DeepEqual(input, before) {
+				t.Fatal("expanded payload aliases input")
+			}
+		})
+	}
+}
+
+func TestSiblingInheritanceTracksTransferAndIndependentChildren(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", "first: {value: parent}\nsecond: {extends: first}\nthird: {extends: first}\n")
+	result, diagnostics := expandSiblingDefinitions(input, Keys("catalog"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	if len(result.transfers) != 2 || result.transfers[0].from.String() != "/catalog/first" ||
+		result.transfers[0].to.String() != "/catalog/second" ||
+		result.transfers[0].reference.String() != "/catalog/second/extends" ||
+		len(result.transfers[0].locations) != 1 || result.transfers[0].locations[0].SourceID != "source-1" {
+		t.Fatalf("transfers = %#v", result.transfers)
+	}
+	second, _ := documentAtPath(result.root, Keys("second", "value"))
+	third, _ := documentAtPath(result.root, Keys("third", "value"))
+	second.Text = "changed"
+	if third.Text != "parent" {
+		t.Fatal("siblings share mutable expanded values")
+	}
+}
+
+func TestSiblingInheritanceRejectsInvalidRelationships(t *testing.T) {
+	tests := []struct {
+		name, input, path string
+		code              Code
+		related           bool
+	}{
+		{"missing", "child: {extends: private-reference-canary}\n", "/child/extends", CodeInheritanceMissing, false},
+		{"nonmapping parent", "child: {extends: parent}\nparent: null\n", "/child/extends", CodeInheritanceInvalid, true},
+		{"boolean reference", "child: {extends: true}\n", "/child/extends", CodeInheritanceInvalid, false},
+		{"sequence reference", "child: {extends: [parent]}\n", "/child/extends", CodeInheritanceInvalid, false},
+		{"self reference", "child: {extends: child}\n", "/child/extends", CodeInheritanceCycle, true},
+		{"longer cycle", "a: {extends: b}\nb: {extends: c}\nc: {extends: a}\n", "/c/extends", CodeInheritanceCycle, true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			input := parseCompositionDocument(t, "source-1", test.input)
+			result, diagnostics := expandSiblingDefinitions(input, Path{})
+			if result != nil || len(diagnostics) != 1 {
+				t.Fatalf("result and diagnostics = %#v, %#v", result, diagnostics)
+			}
+			problem := diagnostics[0]
+			if problem.Code != test.code || problem.Path.String() != test.path || problem.Primary == nil ||
+				problem.Primary.SourceID != "source-1" || problem.Primary.Line < 1 ||
+				(len(problem.Related) != 0) != test.related || strings.Contains(problem.Message, "private-reference-canary") {
+				t.Fatalf("diagnostic = %#v", problem)
+			}
+		})
+	}
+}
+
+func TestSiblingInheritanceDetectsDeepCycleWithoutRecursiveDependencyWalk(t *testing.T) {
+	const count = 2048
+	entries := make([]document.Entry, count)
+	for index := range entries {
+		location := document.Location{SourceID: "source-1", Line: index + 1, Column: 1}
+		parent := strconv.Itoa((index + 1) % count)
+		entries[index] = document.Entry{
+			Key: strconv.Itoa(index), Value: &document.Node{Kind: document.Mapping, Location: location,
+				Mapping: []document.Entry{{Key: inheritanceKey, Value: &document.Node{
+					Kind: document.String, Text: parent, Location: location,
+				}}}},
+		}
+	}
+	result, diagnostics := expandSiblingDefinitions(&document.Node{Kind: document.Mapping, Mapping: entries}, Path{})
+	if result != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeInheritanceCycle {
+		t.Fatalf("deep cycle result = %#v, diagnostics = %#v", result, diagnostics)
+	}
+}

internal/engine/inheritance_walk.go

@@ -0,0 +1,309 @@
+package engine
+
+import (
+	"cmp"
+	"slices"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type inheritanceWalker struct {
+	nodes        int // logical non-directive nodes in the entire working tree
+	origins      map[string]Origin
+	limits       projectionLimits
+	retained     *retainedNodeCounter
+	transfers    []inheritanceTransfer
+	controls     []inheritanceControl
+	replacements []inheritanceReplacement
+	operations   []inheritanceOperation
+}
+
+// expandInheritanceTree resolves each destination scope after its enclosing
+// relationships. The loading lifecycle uses the origin-aware entry point.
+func expandInheritanceTree(input *document.Node, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
+	return expandInheritanceWithOrigins(input, nil, limits)
+}
+
+func expandInheritanceWithOrigins(input *document.Node, inputOrigins map[string]Origin, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
+	return expandInheritanceWithBudgets(input, inputOrigins, limits, historyNodeLimit)
+}
+
+func expandInheritanceWithBudgets(input *document.Node, inputOrigins map[string]Origin, limits projectionLimits, historyLimit int) (*inheritanceScope, []Diagnostic) {
+	if input == nil || input.Kind != document.Mapping {
+		return nil, []Diagnostic{{Code: CodeInheritanceInvalid, Severity: SeverityError,
+			Message: "inheritance requires a mapping document root"}}
+	}
+	root := document.Clone(input)
+	origins := cloneOriginIndex(inputOrigins)
+	if inputOrigins == nil {
+		origins = directInheritanceOrigins(input, Path{}, nil)
+	}
+	nodes, within := inheritanceLowerBoundCount(root, limits.nodes)
+	if !within {
+		return nil, []Diagnostic{inheritanceBudgetDiagnostic(Path{}, root.Location)}
+	}
+	walker := inheritanceWalker{
+		nodes: nodes, limits: limits, origins: origins,
+		retained: newRetainedNodeCounter(historyLimit),
+	}
+	if problem := walker.visit(&root, Path{}, true); problem != nil {
+		return nil, []Diagnostic{*problem}
+	}
+	if problem := inheritanceFinalBudget(root, limits); problem != nil {
+		return nil, []Diagnostic{*problem}
+	}
+	return &inheritanceScope{
+		root: root, origins: walker.origins, transfers: walker.transfers,
+		controls: walker.controls, replacements: walker.replacements, operations: walker.operations,
+	}, nil
+}
+
+func (w *inheritanceWalker) visit(node **document.Node, path Path, unscoped bool) *Diagnostic {
+	switch (*node).Kind {
+	case document.Mapping:
+		original := *node
+		if unscoped {
+			reference, found := inheritanceDirective(*node)
+			if found {
+				referencePath := path.Key(inheritanceKey)
+				location := publicLocation(reference.Value.Location)
+				if reference.Value.Kind != document.Null {
+					problem := inheritanceDiagnostic(CodeInheritanceInvalid, referencePath, location, nil,
+						"inheritance reference has no enclosing sibling scope")
+					return &problem
+				}
+				w.controls = append(w.controls, inheritanceControl{
+					path: referencePath, locations: []Location{location}, cancelled: true,
+				})
+				origin := directInheritanceOrigins(reference.Value, referencePath, w.origins)[referencePath.String()]
+				control := inheritanceControl{path: referencePath, locations: []Location{location}, cancelled: true}
+				w.operations = append(w.operations, inheritanceOperation{control: &control, controlOrigin: origin})
+				deleteOriginSubtree(w.origins, referencePath, reference.Value)
+				*node = withoutInheritanceDirective(*node)
+			}
+		}
+		if scopeHasInheritanceDefinitions(*node) {
+			// Only this scope can change the logical total. Reserve the nodes
+			// outside it so expansion checks the remaining budget before cloning.
+			before, _ := inheritanceLowerBoundCount(*node, -1)
+			limits := w.limits
+			if limits.nodes >= 0 {
+				limits.nodes -= w.nodes - before
+			}
+			resolved, diagnostics := expandSiblingDefinitionsWithOrigins(*node, path, limits, w.origins, w.retained)
+			if len(diagnostics) != 0 {
+				return &diagnostics[0]
+			}
+			*node = resolved.root
+			deleteOriginSubtree(w.origins, path, original)
+			for pointer, origin := range resolved.origins {
+				w.origins[pointer] = origin
+			}
+			w.transfers = append(w.transfers, resolved.transfers...)
+			w.controls = append(w.controls, resolved.controls...)
+			w.replacements = append(w.replacements, resolved.replacements...)
+			w.operations = append(w.operations, resolved.operations...)
+			after, _ := inheritanceLowerBoundCount(*node, -1)
+			w.nodes += after - before
+		}
+		// Operations retain immutable enclosing templates. Detach just this
+		// container before replacing descendants during destination resolution;
+		// untouched subtrees remain shared with the retained evidence.
+		working := **node
+		working.Mapping = slices.Clone(working.Mapping)
+		*node = &working
+		indices := make([]int, len((*node).Mapping))
+		for index := range indices {
+			indices[index] = index
+		}
+		slices.SortFunc(indices, func(left, right int) int {
+			return cmp.Compare((*node).Mapping[left].Key, (*node).Mapping[right].Key)
+		})
+		for _, index := range indices {
+			entry := &(*node).Mapping[index]
+			if problem := w.visit(&entry.Value, path.Key(entry.Key), false); problem != nil {
+				return problem
+			}
+		}
+	case document.Sequence:
+		working := **node
+		working.Sequence = slices.Clone(working.Sequence)
+		*node = &working
+		for index := range (*node).Sequence {
+			child := &(*node).Sequence[index]
+			if problem := w.visit(child, path.Index(index), (*child).Kind == document.Mapping); problem != nil {
+				return problem
+			}
+		}
+	}
+	return nil
+}
+
+func scopeHasInheritanceDefinitions(scope *document.Node) bool {
+	for _, entry := range scope.Mapping {
+		if entry.Value.Kind != document.Mapping {
+			continue
+		}
+		if _, found := inheritanceDirective(entry.Value); found {
+			return true
+		}
+	}
+	return false
+}
+
+// A directive may disappear during later destination resolution, so only
+// non-directive nodes form a safe early lower bound on final output size.
+func inheritanceLowerBoundWithin(root *document.Node, limit int) bool {
+	_, within := inheritanceLowerBoundCount(root, limit)
+	return within
+}
+
+func inheritanceLowerBoundCount(root *document.Node, limit int) (int, bool) {
+	if limit < 0 {
+		limit = int(^uint(0) >> 1)
+	}
+	stack := []*document.Node{root}
+	count := 0
+	for len(stack) != 0 {
+		node := stack[len(stack)-1]
+		stack = stack[:len(stack)-1]
+		if node == nil {
+			continue
+		}
+		count++
+		if count > limit {
+			return count, false
+		}
+		switch node.Kind {
+		case document.Mapping:
+			for _, entry := range node.Mapping {
+				if entry.Key == inheritanceKey {
+					continue
+				}
+				count++ // mapping key
+				stack = append(stack, entry.Value)
+			}
+		case document.Sequence:
+			stack = append(stack, node.Sequence...)
+		}
+		if count > limit {
+			return count, false
+		}
+	}
+	return count, true
+}
+
+// Count the materialized mapping shape before cloning a memoized parent.
+// Relationship fields are excluded because later destination resolution
+// consumes them; their omission makes this a safe lower bound.
+func inheritanceOverlayLowerBoundWithin(lower, upper *document.Node, limit int) bool {
+	if limit < 0 {
+		return false
+	}
+	index := new(mappingIndex)
+	count := 0
+	var visit func(*document.Node, *document.Node) bool
+	visit = func(base, incoming *document.Node) bool {
+		if base.Kind != document.Mapping || incoming.Kind != document.Mapping {
+			added, within := inheritanceLowerBoundCount(incoming, limit-count)
+			count += added
+			return within
+		}
+		count++ // mapping node
+		if count > limit {
+			return false
+		}
+		for _, entry := range base.Mapping {
+			if entry.Key == inheritanceKey {
+				continue
+			}
+			count++ // mapping key
+			if count > limit {
+				return false
+			}
+			position := index.find(incoming, entry.Key)
+			if position >= 0 {
+				if !visit(entry.Value, incoming.Mapping[position].Value) {
+					return false
+				}
+			} else {
+				added, within := inheritanceLowerBoundCount(entry.Value, limit-count)
+				count += added
+				if !within {
+					return false
+				}
+			}
+		}
+		for _, entry := range incoming.Mapping {
+			if entry.Key == inheritanceKey || index.find(base, entry.Key) >= 0 {
+				continue
+			}
+			count++ // mapping key
+			if count > limit {
+				return false
+			}
+			added, within := inheritanceLowerBoundCount(entry.Value, limit-count)
+			count += added
+			if !within || count > limit {
+				return false
+			}
+		}
+		return true
+	}
+	return visit(lower, upper)
+}
+
+func inheritanceFinalBudget(root *document.Node, limits projectionLimits) *Diagnostic {
+	type pending struct {
+		node *document.Node
+		path Path
+	}
+	stack := []pending{{node: root}}
+	count := 0
+	for len(stack) != 0 {
+		item := stack[len(stack)-1]
+		stack = stack[:len(stack)-1]
+		if len(item.path.segments) > limits.depth {
+			problem := inheritanceBudgetDiagnostic(item.path, item.node.Location)
+			return &problem
+		}
+		count++
+		if limits.nodes >= 0 && count > limits.nodes {
+			problem := inheritanceBudgetDiagnostic(item.path, item.node.Location)
+			return &problem
+		}
+		switch item.node.Kind {
+		case document.Mapping:
+			count += len(item.node.Mapping) // mapping keys
+			for index := len(item.node.Mapping) - 1; index >= 0; index-- {
+				entry := item.node.Mapping[index]
+				stack = append(stack, pending{node: entry.Value, path: item.path.Key(entry.Key)})
+			}
+		case document.Sequence:
+			for index := len(item.node.Sequence) - 1; index >= 0; index-- {
+				stack = append(stack, pending{node: item.node.Sequence[index], path: item.path.Index(index)})
+			}
+		}
+		if limits.nodes >= 0 && count > limits.nodes {
+			problem := inheritanceBudgetDiagnostic(item.path, item.node.Location)
+			return &problem
+		}
+	}
+	return nil
+}
+
+func inheritanceBudgetDiagnostic(path Path, location document.Location) Diagnostic {
+	primary := publicLocation(location)
+	return Diagnostic{
+		Code: CodeSchemaProjectionLimit, Severity: SeverityError, Path: path,
+		Message: "expanded configuration exceeds the projection limit", Primary: &primary,
+	}
+}
+
+func inheritanceHistoryBudgetDiagnostic(path Path, location document.Location) Diagnostic {
+	primary := publicLocation(location)
+	return Diagnostic{
+		Code: CodeExplainLimitExceeded, Severity: SeverityError, Path: path,
+		Message: "retained inheritance history exceeds the node limit", Primary: &primary,
+	}
+}

internal/engine/inheritance_walk_test.go

@@ -0,0 +1,219 @@
+package engine
+
+import (
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestNestedInheritanceUsesEachDestinationScope(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `presets:
+  base:
+    settings:
+      primary: {width: 10}
+      secondary: {extends: primary}
+  custom:
+    extends: base
+    settings:
+      primary: {width: 20}
+`)
+	before := document.Clone(input)
+	result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	for path, want := range map[string]string{
+		"/presets/base/settings/secondary/width":   "10",
+		"/presets/custom/settings/secondary/width": "20",
+	} {
+		parsed, _ := ParsePath(path)
+		node, found := documentAtPath(result.root, parsed)
+		if !found || node.Text != want {
+			t.Errorf("%s = %#v, found %v", path, node, found)
+		}
+	}
+	if len(result.transfers) != 3 || !reflect.DeepEqual(input, before) {
+		t.Fatalf("transfers = %#v; input changed = %v", result.transfers, !reflect.DeepEqual(input, before))
+	}
+	if node, found := documentAtPath(result.root, Keys("presets", "custom", "settings", "secondary", inheritanceKey)); found {
+		t.Fatalf("nested directive survived: %#v", node)
+	}
+}
+
+func TestNestedInheritanceSupportsChainsCancellationAndContainerReplacement(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `catalog:
+  alpha:
+    settings:
+      first: {value: one}
+      second: {extends: first}
+  beta:
+    extends: alpha
+    settings:
+      first: {value: two}
+  gamma:
+    extends: beta
+    settings:
+      first: {value: three}
+  cancelled:
+    extends: alpha
+    settings:
+      second: {extends: null, value: own}
+  replaced:
+    extends: alpha
+    settings: none
+`)
+	result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	checks := map[string]string{
+		"/catalog/alpha/settings/second/value":     "one",
+		"/catalog/beta/settings/second/value":      "two",
+		"/catalog/gamma/settings/second/value":     "three",
+		"/catalog/cancelled/settings/second/value": "own",
+		"/catalog/replaced/settings":               "none",
+	}
+	for pointer, want := range checks {
+		path, _ := ParsePath(pointer)
+		node, found := documentAtPath(result.root, path)
+		if !found || node.Text != want {
+			t.Errorf("%s = %#v, found %v", pointer, node, found)
+		}
+	}
+	if _, found := documentAtPath(result.root, Keys("catalog", "replaced", "settings", "second")); found {
+		t.Fatal("replaced container retained an active child")
+	}
+	if _, found := documentAtPath(result.root, Keys("catalog", "cancelled", "settings", "second", inheritanceKey)); found {
+		t.Fatal("cancelled nested directive survived")
+	}
+}
+
+func TestNestedInheritanceCanReplaceAnInheritedRelationship(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `base:
+  settings:
+    first: {value: old}
+    other: {value: new}
+    selected: {extends: first}
+custom:
+  extends: base
+  settings:
+    selected: {extends: other}
+`)
+	result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	base, found := documentAtPath(result.root, Keys("base", "settings", "selected", "value"))
+	if !found || base.Text != "old" {
+		t.Fatalf("base relationship = %#v, %v", base, found)
+	}
+	custom, found := documentAtPath(result.root, Keys("custom", "settings", "selected", "value"))
+	if !found || custom.Text != "new" {
+		t.Fatalf("replaced relationship = %#v, %v", custom, found)
+	}
+}
+
+func TestNestedInheritanceRejectsDestinationCycleAndInvalidPositions(t *testing.T) {
+	tests := []struct {
+		name, input, path string
+		code              Code
+	}{
+		{
+			name: "destination-only cycle", input: `base:
+  settings:
+    a: {value: one}
+    b: {extends: a}
+custom:
+  extends: base
+  settings:
+    a: {extends: b}
+`, path: "/custom/settings/b/extends", code: CodeInheritanceCycle,
+		},
+		{"root reference", "extends: anything\nentry: {value: one}\n", "/extends", CodeInheritanceInvalid},
+		{"list element reference", "items: [{extends: anything, value: one}]\n", "/items/0/extends", CodeInheritanceInvalid},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			input := parseCompositionDocument(t, "source-1", test.input)
+			result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+			if result != nil || len(diagnostics) != 1 || diagnostics[0].Code != test.code ||
+				diagnostics[0].Path.String() != test.path || diagnostics[0].Primary == nil {
+				t.Fatalf("result = %#v, diagnostics = %#v", result, diagnostics)
+			}
+		})
+	}
+}
+
+func TestNestedInheritanceResolvesScopesInsideSequences(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `items:
+  - primary: {value: first}
+    secondary: {extends: primary}
+  - extends: null
+    zeta: {extends: alpha}
+    alpha: {value: second}
+`)
+	result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	first, found := documentAtPath(result.root, Keys("items").Index(0).Key("secondary").Key("value"))
+	if !found || first.Text != "first" {
+		t.Fatalf("first sequence scope = %#v, %v", first, found)
+	}
+	second, found := documentAtPath(result.root, Keys("items").Index(1).Key("zeta").Key("value"))
+	if !found || second.Text != "second" {
+		t.Fatalf("second sequence scope = %#v, %v", second, found)
+	}
+	if _, found := documentAtPath(result.root, Keys("items").Index(1).Key(inheritanceKey)); found {
+		t.Fatal("sequence-element cancellation survived")
+	}
+}
+
+func TestInheritanceExpansionHonorsOutputNodeAndDepthBudgets(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", "base: {value: one}\nchild: {extends: base}\n")
+	for _, test := range []struct {
+		name   string
+		limits projectionLimits
+		valid  bool
+	}{
+		{name: "at node boundary", limits: projectionLimits{nodes: 9, depth: 2}, valid: true},
+		{name: "beyond node boundary", limits: projectionLimits{nodes: 8, depth: 2}},
+		{name: "at depth boundary", limits: projectionLimits{nodes: 20, depth: 2}, valid: true},
+		{name: "beyond depth boundary", limits: projectionLimits{nodes: 20, depth: 1}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, diagnostics := expandInheritanceTree(input, test.limits)
+			if test.valid {
+				if result == nil || len(diagnostics) != 0 {
+					t.Fatalf("result = %#v, diagnostics = %#v", result, diagnostics)
+				}
+			} else if result != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSchemaProjectionLimit {
+				t.Fatalf("result = %#v, diagnostics = %#v", result, diagnostics)
+			}
+		})
+	}
+}
+
+func TestInheritanceBudgetIncludesOtherScopesAndSequenceItems(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `extends: null
+groups:
+  - a: {x: one}
+    b: {extends: a}
+  - a: {x: two}
+    b: {extends: a}
+unchanged: {x: three}
+`)
+	// Root (1), groups key/list (2), two expanded scopes (18), and
+	// unchanged key/mapping/key/value (4). Consumed controls cost nothing.
+	for _, limit := range []int{24, 25, 26} {
+		result, diagnostics := expandInheritanceTree(input, projectionLimits{nodes: limit, depth: 5})
+		if limit < 25 {
+			if result != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSchemaProjectionLimit {
+				t.Fatalf("limit %d accepted or wrong error: %#v", limit, diagnostics)
+			}
+		} else if result == nil || len(diagnostics) != 0 {
+			t.Fatalf("limit %d rejected: %#v", limit, diagnostics)
+		}
+	}
+}

internal/engine/layered.go

@@ -0,0 +1,50 @@
+package engine
+
+import "os"
+
+// Input selects one source and its independent import policy.
+type Input struct {
+	Source     Source
+	Imports    bool
+	Filesystem *FilesystemContext
+}
+
+// Underlay contains strictly composed contributions at one precedence tier.
+type Underlay struct{ Inputs []Input }
+
+// LayeredOptions selects lower-priority tiers and final application rules.
+type LayeredOptions[T any] struct {
+	Definition        Definition[T]
+	Underlays         []Underlay
+	Overlay           string
+	OverlayFilesystem *FilesystemContext
+	Overrides         []Override
+}
+
+// LoadLayered loads a main input and lower-priority grouped inputs through one
+// final decode, resolution, and validation lifecycle.
+func LoadLayered[T any](main Input, secrets SecretBinding, supplied ...LayeredOptions[T]) (*Loaded[T], error) {
+	if len(supplied) > 1 {
+		return nil, (&diagnosticAccumulator{diagnostics: []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "LoadLayered accepts at most one options value")}}).loadError()
+	}
+	var options LayeredOptions[T]
+	if len(supplied) != 0 {
+		options = supplied[0]
+	}
+	request := loadRequest[T]{
+		main:       loadInput{source: main.Source, imports: main.Imports, filesystem: main.Filesystem},
+		definition: options.Definition, overlay: options.Overlay,
+		overlayFilesystem: options.OverlayFilesystem,
+		overrides:         append([]Override(nil), options.Overrides...),
+		underlays:         make([]loadGroup, len(options.Underlays)),
+	}
+	for index, underlay := range options.Underlays {
+		request.underlays[index].inputs = make([]loadInput, len(underlay.Inputs))
+		for inputIndex, input := range underlay.Inputs {
+			request.underlays[index].inputs[inputIndex] = loadInput{
+				source: input.Source, imports: input.Imports, filesystem: input.Filesystem,
+			}
+		}
+	}
+	return loadGroupedWithSecretLookup(request, secrets, os.LookupEnv)
+}

internal/engine/load.go

@@ -33,6 +33,7 @@ type Options[T any] struct {
 type FilesystemContext struct {
 	BaseDir string
 	Root    string
+	FS      *Filesystem
 }
 
 // Override describes one explicitly supplied application override. Value uses
@@ -77,6 +78,7 @@ type Loaded[T any] struct {
 type Inspection struct {
 	metadata         Metadata
 	sources          []SourceInfo
+	contexts         map[string]ResourceContext
 	diagnostics      []Diagnostic
 	baseline         *document.Node
 	input            *document.Node
@@ -84,6 +86,7 @@ type Inspection struct {
 	final            *document.Node
 	resolution       []resolutionSnapshot
 	applications     []historyApplication
+	inheritance      []inheritanceOperation
 	secrets          map[string]bool
 	secretReferences map[string]string
 }
@@ -104,6 +107,16 @@ func (r *Inspection) Sources() []SourceInfo {
 	return cloneSourceInfos(r.sources)
 }
 
+// SourceContext returns a passive filesystem view for an acquired source.
+// Unknown IDs and sources without a filesystem context return false.
+func (r *Inspection) SourceContext(id string) (ResourceContext, bool) {
+	if r == nil {
+		return ResourceContext{}, false
+	}
+	context, ok := r.contexts[id]
+	return context, ok
+}
+
 // Diagnostics returns detached nonfatal diagnostics from the load.
 func (r *Inspection) Diagnostics() []Diagnostic {
 	if r == nil {
@@ -120,14 +133,27 @@ 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) {
-	accumulator := diagnosticAccumulator{}
 	options, diagnostics := selectOptions(source, supplied)
-	accumulator.add(diagnostics...)
 	if len(diagnostics) != 0 {
-		return nil, accumulator.loadError()
+		return nil, (&diagnosticAccumulator{diagnostics: diagnostics}).loadError()
+	}
+	request := loadRequest[T]{
+		main:       loadInput{source: source, imports: options.Imports, filesystem: options.Filesystem},
+		definition: options.Definition, overlay: options.Overlay,
+		overrides: options.Overrides, ordinary: true,
+	}
+	return executeLoadRequest(request, secrets, lookup)
+}
+
+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}
+	if request.ordinary {
+		mainOptions.Overlay = request.overlay
 	}
 
-	input, diagnostics := assembleBaseInput(source, options)
+	input, diagnostics := assembleBaseInputTracked(request.main.source, mainOptions, accounting, nil, nil)
 	accumulator.sources = input.sources
 	accumulator.add(diagnostics...)
 	if len(diagnostics) != 0 {
@@ -138,29 +164,68 @@ func loadWithSecretLookup[T any](source Source, secrets SecretBinding, lookup se
 		accumulator.add(diagnosticFromSchemaIssue(schemaIssue))
 		return nil, accumulator.loadError()
 	}
-	versionDiagnostics := checkVersion(options.Definition.Version, input.document)
+	versionDiagnostics := checkVersion(request.definition.Version, input.document)
 	accumulator.add(versionDiagnostics...)
 	if hasErrors(versionDiagnostics) {
 		return nil, accumulator.loadError()
 	}
-	input, diagnostics = applyInputLayers(source, options, input)
+	if request.ordinary {
+		input, diagnostics = applyInputLayers(request.main.source, Options[T]{Definition: request.definition, Overlay: request.overlay, Filesystem: request.main.filesystem, Overrides: request.overrides}, input)
+	} else {
+		input, diagnostics = assembleLayeredInput(request, input, accounting)
+	}
 	accumulator.sources = input.sources
 	accumulator.add(diagnostics...)
 	if len(diagnostics) != 0 {
 		return nil, accumulator.loadError()
 	}
+	if request.ordinary && input.parsed.source != nil {
+		baseSources := make([]SourceInfo, 0, len(input.sources))
+		for _, source := range input.sources {
+			if source.Layer == LayerBase {
+				baseSources = append(baseSources, source)
+			}
+		}
+		if len(baseSources) != 0 && input.contexts[baseSources[0].ID].FS == nil {
+			contexts, diagnostic := contextsForAcquiredSources(request.main.source, request.main.filesystem, input.parsed.source, baseSources)
+			if diagnostic != nil {
+				accumulator.add(*diagnostic)
+				return nil, accumulator.loadError()
+			}
+			mergeResourceContexts(&input.contexts, contexts)
+		}
+	}
+	if input.document != nil && containsInheritanceDirective(input.document) {
+		composition, problems := compositionForInput(input)
+		if len(problems) != 0 {
+			accumulator.add(problems...)
+			return nil, accumulator.loadError()
+		}
+		expanded, problems := expandInheritanceWithOrigins(composition.root, composition.origins, defaultProjectionLimits())
+		if len(problems) != 0 {
+			accumulator.add(problems...)
+			return nil, accumulator.loadError()
+		}
+		if problem := inheritanceVersionDiagnostic(request.definition.Version, expanded); problem != nil {
+			accumulator.add(*problem)
+			return nil, accumulator.loadError()
+		}
+		input.composition = composition
+		input.inheritance = expanded
+		input.document = expanded.root
+	}
 
-	decoded, diagnostics := decodeLoadInput(schema, options.Definition, input)
+	decoded, diagnostics := decodeLoadInput(schema, request.definition, input)
 	accumulator.add(diagnostics...)
 	if len(diagnostics) != 0 {
 		return nil, accumulator.loadError()
 	}
-	resolved, diagnostics := resolveLoadInput(schema, options.Definition, input, decoded)
+	resolved, diagnostics := resolveLoadInput(schema, request.definition, input, decoded)
 	accumulator.add(diagnostics...)
 	if hasErrors(diagnostics) {
 		return nil, accumulator.loadError()
 	}
-	if !historyNodesWithinLimit(decoded.baseline, decoded.final, resolved.final, resolved.applications, resolved.snapshots) {
+	if !historyNodesWithinLimit(decoded.baseline, decoded.final, resolved.final, resolved.applications, resolved.snapshots, input.inheritance) {
 		accumulator.add(Diagnostic{Code: CodeExplainLimitExceeded, Severity: SeverityError, Message: "retained configuration history exceeds the node limit"})
 		return nil, accumulator.loadError()
 	}
@@ -176,8 +241,8 @@ func loadWithSecretLookup[T any](source Source, secrets SecretBinding, lookup se
 	if hasErrors(secretResolution.diagnostics) {
 		return nil, accumulator.loadError()
 	}
-	if options.Definition.Validate != nil {
-		diagnostics = runValidation(&configuration, options.Definition.Validate, schema, resolved.final, input.document, secretResolution.lookups)
+	if request.definition.Validate != nil {
+		diagnostics = runValidation(&configuration, request.definition.Validate, schema, resolved.final, input.document, secretResolution.lookups)
 		accumulator.add(diagnostics...)
 		if hasErrors(diagnostics) {
 			return nil, accumulator.loadError()
@@ -186,5 +251,5 @@ func loadWithSecretLookup[T any](source Source, secrets SecretBinding, lookup se
 	resolved.configuration = configuration
 	// Publication detaches equal-valued validator replacements, preserving the
 	// validated secret state, then finalizes metadata and sensitivity.
-	return publishLoadResult(schema, options.Definition, resolved, secretResolution.references, &accumulator)
+	return publishLoadResult(schema, request.definition, resolved, secretResolution.references, &accumulator)
 }

internal/engine/load_input.go

@@ -1,24 +1,37 @@
 package engine
 
-import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+import (
+	"strconv"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
 
 // assembledInput is the source/document handoff across schema and version checks.
 type assembledInput struct {
 	parsed      parsedSource
 	document    *document.Node
 	composition *composedDocument
+	inheritance *inheritanceScope
 	sources     []SourceInfo
+	contexts    map[string]ResourceContext
 }
 
 func assembleBaseInput[T any](source Source, options Options[T]) (assembledInput, []Diagnostic) {
-	parsed, diagnostics := parseSource(source, "source-1", defaultParseLimits())
+	return assembleBaseInputTracked(source, options, &importAccounting{}, nil, nil)
+}
+
+func assembleBaseInputTracked[T any](source Source, options Options[T], accounting *importAccounting, visited map[resourceIdentity]importRoute, hostToken *namespaceToken) (assembledInput, []Diagnostic) {
+	limits := defaultImportLimits()
+	parseLimits := defaultParseLimits()
+	parseLimits.source.remaining = limits.bytes - accounting.bytes
+	parsed, diagnostics := parseSource(source, "source-"+strconv.Itoa(accounting.sources+1), parseLimits)
 	input := assembledInput{parsed: parsed, document: parsed.document}
 	if len(diagnostics) != 0 {
 		input.sources = sourceInfos(parsed.source)
 		return input, diagnostics
 	}
 	if options.Imports {
-		input.composition, input.sources, diagnostics = loadImportedDocuments(source, options.Filesystem, parsed, options.Definition.Version, defaultImportLimits())
+		input.composition, input.sources, diagnostics = loadImportedDocumentsTracked(source, options.Filesystem, parsed, options.Definition.Version, limits, accounting, visited, hostToken)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
@@ -35,10 +48,44 @@ func assembleBaseInput[T any](source Source, options Options[T]) (assembledInput
 		if diagnostics := disabledDirectiveDiagnostics(input.document, Path{}); len(diagnostics) != 0 {
 			return input, diagnostics
 		}
+		if accounting.sources+1 > limits.sources || accounting.bytes+parsed.bytes > limits.bytes || accounting.nodes+countInputNodes(input.document) > limits.nodes {
+			return input, []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, parsed.source.ID, "configuration exceeds the aggregate input limit")}
+		}
+		if diagnostic := registerAcquiredSource(source, options.Filesystem, parsed.source, visited, hostToken); diagnostic != nil {
+			return input, []Diagnostic{*diagnostic}
+		}
+		accounting.sources++
+		accounting.bytes += parsed.bytes
+		accounting.nodes += countInputNodes(input.document)
+	}
+	if !activeOverlay(options.Overlay) || options.Imports {
+		contexts, diagnostic := contextsForAcquiredSources(source, options.Filesystem, parsed.source, input.sources)
+		if diagnostic != nil {
+			return input, []Diagnostic{*diagnostic}
+		}
+		input.contexts = contexts
 	}
 	return input, nil
 }
 
+func registerAcquiredSource(source Source, context *FilesystemContext, info *SourceInfo, visited map[resourceIdentity]importRoute, hostToken *namespaceToken) *Diagnostic {
+	if visited == nil || info == nil || info.Path == "" {
+		return nil
+	}
+	identity := resourceIdentity{namespace: hostToken, path: info.Path}
+	if source.kind == sourceFilesystem {
+		identity.namespace = source.namespace.token
+	} else if context != nil && context.FS != nil {
+		identity.namespace = context.FS.namespace.token
+	}
+	if previous, repeated := visited[identity]; repeated {
+		value := importRepeatedDiagnostic(Location{SourceID: info.ID}, previous, importRoute{})
+		return &value
+	}
+	visited[identity] = importRoute{canonical: info.Path}
+	return nil
+}
+
 func applyInputLayers[T any](source Source, options Options[T], input assembledInput) (assembledInput, []Diagnostic) {
 	if !activeOverlay(options.Overlay) && len(options.Overrides) == 0 {
 		return input, nil
@@ -47,25 +94,31 @@ func applyInputLayers[T any](source Source, options Options[T], input assembledI
 	if len(diagnostics) != 0 {
 		return input, diagnostics
 	}
+	assembly := newAssemblyAccumulator(composition)
 	if activeOverlay(options.Overlay) {
-		composition, input.sources, diagnostics = loadSelectedOverlayDocument(
-			source, options.Filesystem, input.parsed, composition, input.sources,
+		input.sources, diagnostics = loadSelectedOverlayInto(
+			source, options.Filesystem, input.parsed, assembly, input.sources,
 			options.Overlay, options.Definition.Version, defaultImportLimits(),
 		)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
+		contexts, diagnostic := contextsForAcquiredSources(source, options.Filesystem, input.parsed.source, input.sources[len(input.sources)-1:])
+		if diagnostic != nil {
+			return input, []Diagnostic{*diagnostic}
+		}
+		mergeResourceContexts(&input.contexts, contexts)
 	}
 	if len(options.Overrides) != 0 {
-		composition, input.sources, diagnostics = applyOverrides(
-			composition, input.sources, options.Overrides, options.Definition.Version, defaultImportLimits(),
+		input.sources, diagnostics = assembly.applyOverrides(
+			input.sources, options.Overrides, options.Definition.Version, defaultImportLimits(),
 		)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
 	}
-	input.composition = composition
-	input.document = composition.root
+	input.composition = assembly.current
+	input.document = assembly.current.root
 	return input, nil
 }
 

internal/engine/load_metadata.go

@@ -12,9 +12,10 @@ import (
 type resolutionInput struct {
 	metadata Metadata
 	sources  map[string]SourceInfo
+	contexts map[string]ResourceContext
 }
 
-func newResolutionInput(metadata Metadata, final *document.Node, sources []SourceInfo) resolutionInput {
+func newResolutionInput(metadata Metadata, final *document.Node, sources []SourceInfo, contexts ...map[string]ResourceContext) resolutionInput {
 	frozen := Metadata{
 		fields:         make(map[string]FieldInfo, len(metadata.fields)),
 		effectivePaths: effectivePathIndex(final),
@@ -32,7 +33,11 @@ func newResolutionInput(metadata Metadata, final *document.Node, sources []Sourc
 		}
 		indexed[source.ID] = cloneSourceInfo(source)
 	}
-	return resolutionInput{metadata: frozen, sources: indexed}
+	result := resolutionInput{metadata: frozen, sources: indexed}
+	if len(contexts) != 0 {
+		result.contexts = cloneResourceContexts(contexts[0])
+	}
+	return result
 }
 
 func (input resolutionInput) lookup(path Path) (FieldInfo, bool) {
@@ -47,6 +52,11 @@ func (input resolutionInput) source(id string) (SourceInfo, bool) {
 	return cloneSourceInfo(source), true
 }
 
+func (input resolutionInput) sourceContext(id string) (ResourceContext, bool) {
+	context, ok := input.contexts[id]
+	return context, ok
+}
+
 func metadataFromDecoded(decoded decodedConfiguration) Metadata {
 	metadata := Metadata{
 		fields:  make(map[string]FieldInfo),
@@ -115,9 +125,6 @@ func applyOverlayMetadata(metadata *Metadata, records []overlayRecord) {
 		if err != nil {
 			continue
 		}
-		if history := events[pointer]; len(history) != 0 && history[len(history)-1].Origin.Layer == info.Origin.Layer && history[len(history)-1].Origin.Operation == info.Origin.Operation {
-			continue
-		}
 		appendInputOriginEvent(events, baseline, pointer, path, info.Origin)
 	}
 	metadata.history = make(map[string][]Change, len(baseline)+len(events))
@@ -134,12 +141,37 @@ func appendInputOriginEvent(events map[string][]Change, baseline map[string][]Ch
 		return
 	}
 	history := events[pointer]
-	if len(history) != 0 && originContainedBy(history[len(history)-1].Origin, origin) {
+	if len(history) != 0 && originRepresentedByChange(path, origin, history[len(history)-1]) {
 		return
 	}
 	appendInputChangeEvent(events, baseline, pointer, Change{Path: path.clone(), Origin: cloneOrigin(origin)})
 }
 
+func originRepresentedByChange(path Path, origin Origin, change Change) bool {
+	if change.Path.String() == path.String() {
+		return originContainedBy(origin, change.Origin)
+	}
+	if !pathContains(change.Path, path) || origin.Layer != change.Origin.Layer || origin.Operation != change.Origin.Operation {
+		return false
+	}
+	// A subtree replacement already records its descendants' contributions.
+	// Their coordinates differ from the container's, but their sources must be
+	// covered by the operation; matching only the precedence role is insufficient.
+	for _, location := range origin.Locations {
+		found := false
+		for _, recorded := range change.Origin.Locations {
+			if location.SourceID == recorded.SourceID {
+				found = true
+				break
+			}
+		}
+		if !found {
+			return false
+		}
+	}
+	return true
+}
+
 func appendInputChangeEvent(events map[string][]Change, baseline map[string][]Change, pointer string, value Change) {
 	change := cloneChange(value)
 	history := events[pointer]
@@ -169,6 +201,23 @@ func originContainedBy(candidate, container Origin) bool {
 			return false
 		}
 	}
+	if (len(candidate.contributors) == 0) != (len(container.contributors) == 0) {
+		return false
+	}
+	for _, contributor := range candidate.contributors {
+		found := false
+		for _, available := range container.contributors {
+			if contributor.location == available.location &&
+				slices.Equal(contributor.source.segments, available.source.segments) &&
+				sameInheritanceTrace(contributor.trace, available.trace) {
+				found = true
+				break
+			}
+		}
+		if !found {
+			return false
+		}
+	}
 	return true
 }
 
@@ -205,8 +254,9 @@ func cloneMetadata(metadata Metadata) Metadata {
 		return Metadata{}
 	}
 	cloned := Metadata{
-		fields:  make(map[string]FieldInfo, len(metadata.fields)),
-		history: make(map[string][]Change, len(metadata.history)),
+		fields:      make(map[string]FieldInfo, len(metadata.fields)),
+		history:     make(map[string][]Change, len(metadata.history)),
+		inheritance: append([]inheritanceOperation(nil), metadata.inheritance...),
 	}
 	cloned.effectivePaths = make(map[string]Path, len(metadata.effectivePaths))
 	for pointer, path := range metadata.effectivePaths {

internal/engine/load_request.go

@@ -0,0 +1,182 @@
+package engine
+
+import (
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+// loadRequest keeps source acquisition separate from precedence assembly.
+// The ordinary entry point and grouped entry point share the final lifecycle.
+type loadRequest[T any] struct {
+	main              loadInput
+	underlays         []loadGroup
+	definition        Definition[T]
+	overlay           string
+	overlayFilesystem *FilesystemContext
+	overrides         []Override
+	ordinary          bool
+}
+
+type loadInput struct {
+	source     Source
+	imports    bool
+	filesystem *FilesystemContext
+}
+
+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
+	_, mainDiagnostics := selectOptions(request.main.source, []Options[T]{{
+		Definition: request.definition, Imports: request.main.imports, Filesystem: request.main.filesystem,
+	}})
+	diagnostics = append(diagnostics, mainDiagnostics...)
+	for _, group := range request.underlays {
+		for _, input := range group.inputs {
+			_, inputDiagnostics := selectOptions(input.source, []Options[T]{{Imports: input.imports, Filesystem: input.filesystem}})
+			diagnostics = append(diagnostics, inputDiagnostics...)
+		}
+	}
+	if !validOverlayName(request.overlay) {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "overlay name must contain only ASCII letters, digits, underscore, or hyphen"))
+	}
+	if activeOverlay(request.overlay) {
+		if request.overlayFilesystem == nil {
+			diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "selected overlay requires explicit filesystem context"))
+		} else {
+			diagnostics = append(diagnostics, filesystemOptionDiagnostics(Source{kind: sourceBytes}, request.overlayFilesystem)...)
+		}
+	}
+	if len(diagnostics) != 0 {
+		return nil, (&diagnosticAccumulator{diagnostics: canonicalDiagnostics(diagnostics)}).loadError()
+	}
+	return executeLoadRequest(request, secrets, lookup)
+}
+
+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 := acquireUnderlayGroup(group, index+1, request.definition.Version, accounting)
+		input.sources = append(input.sources, sources...)
+		mergeResourceContexts(&input.contexts, contexts)
+		if len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+		if composition != nil {
+			groups = append(groups, composition)
+		}
+	}
+
+	var assembly *assemblyAccumulator
+	for _, group := range groups {
+		if assembly == nil {
+			assembly = newAssemblyAccumulator(group)
+			continue
+		}
+		if diagnostics := assembly.applyOverlay(group.root, group.origins, group.applications, LayerUnderlay); len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+	}
+	if input.document != nil {
+		mainComposition, diagnostics := compositionForInput(input)
+		if len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+		if assembly == nil {
+			assembly = newAssemblyAccumulator(mainComposition)
+		} else if diagnostics := assembly.applyOverlay(mainComposition.root, mainComposition.origins, mainComposition.applications, LayerBase); len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+	}
+	if assembly == nil && (activeOverlay(request.overlay) || len(request.overrides) != 0) {
+		composition, _ := compositionForInput(assembledInput{})
+		assembly = newAssemblyAccumulator(composition)
+	}
+	if assembly == nil {
+		return input, nil
+	}
+	assembly.current.inputBytes = accounting.bytes
+	assembly.current.inputNodes = accounting.nodes
+	if activeOverlay(request.overlay) {
+		// The selected overlay context is independent of the main input. This
+		// synthetic descriptor creates no source record or input contribution.
+		selection := Source{kind: sourceBytes}
+		selected := parsedSource{document: &document.Node{Kind: document.Mapping}, source: &SourceInfo{}}
+		var diagnostics []Diagnostic
+		input.sources, diagnostics = loadSelectedOverlayInto(selection, request.overlayFilesystem, selected, assembly, input.sources, request.overlay, request.definition.Version, defaultImportLimits())
+		if len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+		contexts, diagnostic := contextsForAcquiredSources(selection, request.overlayFilesystem, selected.source, input.sources[len(input.sources)-1:])
+		if diagnostic != nil {
+			return input, []Diagnostic{*diagnostic}
+		}
+		mergeResourceContexts(&input.contexts, contexts)
+	}
+	if len(request.overrides) != 0 {
+		var diagnostics []Diagnostic
+		input.sources, diagnostics = assembly.applyOverrides(input.sources, request.overrides, request.definition.Version, defaultImportLimits())
+		if len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+	}
+	input.composition = assembly.current
+	input.document = assembly.current.root
+	return input, nil
+}
+
+func acquireUnderlayGroup(group loadGroup, index int, rule *VersionRule, accounting *importAccounting) (*composedDocument, []SourceInfo, map[string]ResourceContext, []Diagnostic) {
+	initialBytes, initialNodes := accounting.bytes, accounting.nodes
+	visited := make(map[resourceIdentity]importRoute)
+	hostToken := &namespaceToken{}
+	var documents []*document.Node
+	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}}
+		acquired, diagnostics := assembleBaseInputTracked(contribution.source, options, accounting, visited, hostToken)
+		for _, source := range acquired.sources {
+			source.Layer = LayerUnderlay
+			source.UnderlayIndex = index
+			sources = append(sources, source)
+		}
+		mergeResourceContexts(&contexts, acquired.contexts)
+		if len(diagnostics) != 0 {
+			return nil, sources, contexts, diagnostics
+		}
+		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)
+		}
+	}
+	if len(documents) == 0 {
+		return nil, sources, contexts, nil
+	}
+	composition, diagnostics := composeDocuments(documents...)
+	if len(diagnostics) != 0 {
+		return nil, sources, contexts, diagnostics
+	}
+	composition.inputBytes = accounting.bytes - initialBytes
+	composition.inputNodes = accounting.nodes - initialNodes
+	for pointer, origin := range composition.origins {
+		origin.Layer = LayerUnderlay
+		composition.origins[pointer] = origin
+	}
+	for index := range composition.applications {
+		composition.applications[index].layer = LayerUnderlay
+	}
+	return composition, sources, contexts, nil
+}

internal/engine/load_request_test.go

@@ -0,0 +1,280 @@
+package engine
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+type groupedConfig struct {
+	Version  string            `yaml:"version"`
+	Settings map[string]string `yaml:"settings"`
+	Port     int               `yaml:"port"`
+	Derived  string            `yaml:"derived"`
+}
+
+func testGroupedLoad[T any](request loadRequest[T]) (*testResult[T], error) {
+	loaded, err := loadGroupedWithSecretLookup(request, testSecretBinding(), func(string) (string, bool) { return "", false })
+	return testLoaded(loaded), err
+}
+
+func TestGroupedLoadPrecedenceSourceOrderAndSingleLifecycle(t *testing.T) {
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":          &fstest.MapFile{Data: []byte("configkit: {imports: [{path: imported.yml}]}\nversion: v1\nport: 5\nsettings: {shared: main}\n")},
+		"imported.yml":      &fstest.MapFile{Data: []byte("settings: {imported: included}\n")},
+		"overlays/test.yml": &fstest.MapFile{Data: []byte("settings: {shared: overlay}\n")},
+	})
+	checks := struct{ version, defaults, resolution, validation int }{}
+	request := loadRequest[groupedConfig]{
+		main: loadInput{source: filesystem.File("main.yml"), imports: true},
+		underlays: []loadGroup{
+			{inputs: []loadInput{
+				{source: Mapping("low-a", map[string]any{"settings": map[string]any{"shared": "low", "a": "one"}, "port": "invalid until replaced"})},
+				{source: Bytes("low-b", []byte("settings: {b: two}\n"))},
+			}},
+			{inputs: []loadInput{{source: Mapping("middle", map[string]any{"settings": map[string]any{"shared": "middle", "c": "three"}})}}},
+		},
+		definition: Definition[groupedConfig]{
+			Version: &VersionRule{Path: Keys("version"), Check: func(version Version, validation *Validation) error {
+				checks.version++
+				if version.Text != "v1" {
+					t.Errorf("version = %#v", version)
+				}
+				return nil
+			}},
+			Defaults: func() groupedConfig { checks.defaults++; return groupedConfig{Derived: "initial"} },
+			Resolve: func(config *groupedConfig, resolution *Resolution) error {
+				checks.resolution++
+				Default(resolution, Keys("derived"), &config.Derived, "resolved", "set derived")
+				return nil
+			},
+			Validate: func(config *groupedConfig, validation *Validation) { checks.validation++ },
+		},
+		overlay: "test", overlayFilesystem: &FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem},
+		overrides: []Override{{Path: Keys("settings", "shared"), Value: "cli", Flag: "--shared"}},
+	}
+	result, err := testGroupedLoad(request)
+	if err != nil {
+		t.Fatalf("load: %v, diagnostics %#v", err, err.(*LoadError).Diagnostics())
+	}
+	wantSettings := map[string]string{"shared": "cli", "a": "one", "b": "two", "c": "three", "imported": "included"}
+	if result.Config.Port != 5 || !reflect.DeepEqual(result.Config.Settings, wantSettings) {
+		t.Fatalf("config = %#v", result.Config)
+	}
+	if checks != (struct{ version, defaults, resolution, validation int }{1, 1, 1, 1}) {
+		t.Fatalf("lifecycle calls = %#v", checks)
+	}
+	sources := result.Sources()
+	if len(sources) != 7 {
+		t.Fatalf("sources = %#v", sources)
+	}
+	wantLayers := []Layer{LayerBase, LayerBase, LayerUnderlay, LayerUnderlay, LayerUnderlay, LayerOverlay, LayerCLI}
+	wantIndexes := []int{0, 0, 1, 1, 2, 0, 0}
+	for index, source := range sources {
+		if source.ID != "source-"+string(rune('1'+index)) || source.Layer != wantLayers[index] || source.UnderlayIndex != wantIndexes[index] {
+			t.Errorf("source %d = %#v", index, source)
+		}
+	}
+	info, ok := result.Metadata().Lookup(Keys("settings", "shared"))
+	if !ok || info.Origin.Layer != LayerCLI {
+		t.Fatalf("effective shared metadata = %#v, %v", info, ok)
+	}
+	explanation, ok := result.Explain(Keys("settings", "shared"))
+	if !ok || len(explanation.History) < 5 {
+		t.Fatalf("shared explanation = %#v, %v", explanation, ok)
+	}
+	var layers []Layer
+	for _, change := range explanation.History {
+		if change.Origin.Layer != LayerDefault && (len(layers) == 0 || layers[len(layers)-1] != change.Origin.Layer) {
+			layers = append(layers, change.Origin.Layer)
+		}
+	}
+	if !reflect.DeepEqual(layers, []Layer{LayerUnderlay, LayerBase, LayerOverlay, LayerCLI}) {
+		t.Fatalf("history layers = %#v", layers)
+	}
+}
+
+func TestGroupedLoadAbsentMainAndVersionProtection(t *testing.T) {
+	var observed Version
+	request := loadRequest[groupedConfig]{
+		main:      loadInput{source: NoSource()},
+		underlays: []loadGroup{{}, {inputs: []loadInput{{source: Bytes("underlay", []byte("port: 7\n"))}}}},
+		definition: Definition[groupedConfig]{Version: &VersionRule{Path: Keys("version"), Check: func(version Version, validation *Validation) error {
+			observed = version
+			return nil
+		}}},
+	}
+	result, err := testGroupedLoad(request)
+	if err != nil || result.Config.Port != 7 || observed.Present || len(result.Sources()) != 1 || result.Sources()[0].UnderlayIndex != 2 {
+		t.Fatalf("underlay-only load = %#v, version %#v, %v", result, observed, err)
+	}
+	request.underlays[1].inputs[0].source = Bytes("underlay", []byte("version: forbidden\n"))
+	_, err = testGroupedLoad(request)
+	if got := groupedErrorCode(t, err); got != CodeUnderlayVersion {
+		t.Fatalf("underlay version code = %s", got)
+	}
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":     &fstest.MapFile{Data: []byte("configkit: {imports: [{path: fragment.yml}]}\n")},
+		"fragment.yml": &fstest.MapFile{Data: []byte("version: forbidden\n")},
+	})
+	request.underlays[1].inputs[0] = loadInput{source: filesystem.File("main.yml"), imports: true}
+	_, err = testGroupedLoad(request)
+	if got := groupedErrorCode(t, err); got != CodeImportVersion {
+		t.Fatalf("import version code = %s", got)
+	}
+	request.underlays = []loadGroup{{}}
+	request.definition.Defaults = func() groupedConfig { return groupedConfig{Port: 11} }
+	result, err = testGroupedLoad(request)
+	if err != nil || result.Config.Port != 11 || len(result.Sources()) != 0 {
+		t.Fatalf("defaults-only load = %#v, %v", result, err)
+	}
+}
+
+func TestGroupedLoadStrictConflictsRepetitionAndBudgets(t *testing.T) {
+	main := loadInput{source: Bytes("main", []byte("port: 5\n"))}
+	request := loadRequest[groupedConfig]{
+		main: main,
+		underlays: []loadGroup{{inputs: []loadInput{
+			{source: Bytes("left", []byte("port: 1\n"))},
+			{source: Bytes("right", []byte("port: 2\n"))},
+		}}},
+	}
+	_, err := testGroupedLoad(request)
+	if got := groupedErrorCode(t, err); got != CodeComposeConflict {
+		t.Fatalf("strict conflict code = %s", got)
+	}
+	conflicting := NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":     &fstest.MapFile{Data: []byte("configkit: {imports: [{path: fragment.yml}]}\nport: 1\n")},
+		"fragment.yml": &fstest.MapFile{Data: []byte("port: 2\n")},
+	})
+	request.main = loadInput{source: conflicting.File("main.yml"), imports: true}
+	request.underlays = []loadGroup{{inputs: []loadInput{{source: Bytes("lower", []byte("port: 3\n"))}}}}
+	_, err = testGroupedLoad(request)
+	if got := groupedErrorCode(t, err); got != CodeComposeConflict {
+		t.Fatalf("main/import conflict code = %s", got)
+	}
+	request.main = main
+	filesystem := NewFilesystem("bundle", fstest.MapFS{"same.yml": &fstest.MapFile{Data: []byte("port: 1\n")}})
+	file := filesystem.File("same.yml")
+	request.underlays = []loadGroup{{inputs: []loadInput{{source: file}, {source: file}}}}
+	_, err = testGroupedLoad(request)
+	if got := groupedErrorCode(t, err); got != CodeImportRepeated {
+		t.Fatalf("same-group repetition code = %s", got)
+	}
+	request.underlays = []loadGroup{{inputs: []loadInput{{source: file}}}, {inputs: []loadInput{{source: file}}}}
+	if result, err := testGroupedLoad(request); err != nil || result.Config.Port != 5 || len(result.Sources()) != 3 {
+		t.Fatalf("cross-group reuse = %#v, %v", result, err)
+	}
+
+	accounting := &importAccounting{sources: maxImportSources - 1}
+	group := loadGroup{inputs: []loadInput{{source: Mapping("first", map[string]any{})}, {source: Mapping("second", map[string]any{})}}}
+	_, _, _, diagnostics := acquireUnderlayGroup(group, 1, nil, accounting)
+	if len(diagnostics) == 0 || diagnostics[0].Code != CodeSourceLimitExceeded {
+		t.Fatalf("shared source budget = %#v", diagnostics)
+	}
+	accounting = &importAccounting{bytes: maxTotalBytes - 2}
+	group.inputs = []loadInput{{source: Mapping("first", map[string]any{"port": "one"})}}
+	_, _, _, diagnostics = acquireUnderlayGroup(group, 1, nil, accounting)
+	if len(diagnostics) == 0 || diagnostics[0].Code != CodeSourceLimitExceeded {
+		t.Fatalf("shared byte budget = %#v", diagnostics)
+	}
+}
+
+func TestGroupedLoadInvalidIntermediateTypeCanBeReplaced(t *testing.T) {
+	request := loadRequest[groupedConfig]{
+		main:      loadInput{source: Bytes("main", []byte("port: 9\n"))},
+		underlays: []loadGroup{{inputs: []loadInput{{source: Bytes("low", []byte("port: wrong\n"))}}}},
+	}
+	result, err := testGroupedLoad(request)
+	if err != nil || result.Config.Port != 9 {
+		t.Fatalf("final typed decode = %#v, %v", result, err)
+	}
+	request.main.source = Bytes("main", []byte("port: wrong\n"))
+	_, err = testGroupedLoad(request)
+	if got := groupedErrorCode(t, err); got != CodeDecodeType && got != CodeDecodeRange {
+		t.Fatalf("final invalid type code = %s", got)
+	}
+}
+
+func groupedErrorCode(t *testing.T, err error) Code {
+	t.Helper()
+	loadError, ok := err.(*LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 {
+		t.Fatalf("error = %#v", err)
+	}
+	if strings.Contains(loadError.Diagnostics()[0].Message, "wrong") {
+		t.Fatal("raw value escaped diagnostics")
+	}
+	return loadError.Diagnostics()[0].Code
+}
+
+func TestGroupedAccountingChargesMixedOccurrencesOnce(t *testing.T) {
+	const (
+		one   = "configkit: {imports: [{path: child-one.yml}]}\nleft: first\n"
+		child = "extra: imported\n"
+		two   = "configkit: {imports: [{path: child-two.yml}]}\nright: second\n"
+	)
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"one.yml":       &fstest.MapFile{Data: []byte(one)},
+		"child-one.yml": &fstest.MapFile{Data: []byte(child)},
+		"two.yml":       &fstest.MapFile{Data: []byte(two)},
+		"child-two.yml": &fstest.MapFile{Data: []byte(child)},
+	})
+	accounting := &importAccounting{}
+	first, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{source: filesystem.File("one.yml"), imports: true}}}, 1, nil, accounting)
+	if len(diagnostics) != 0 || first == nil || len(sources) != 2 || accounting.sources != 2 || accounting.bytes != int64(len(one)+len(child)) {
+		t.Fatalf("first acquisition = %#v, %#v, %#v, accounting %#v", first, sources, diagnostics, accounting)
+	}
+	second, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{source: filesystem.File("two.yml"), imports: true}}}, 2, nil, accounting)
+	if len(diagnostics) != 0 || second == nil || len(sources) != 2 || sources[0].ID != "source-3" || sources[1].ID != "source-4" || accounting.sources != 4 || accounting.bytes != int64(len(one)+len(two)+2*len(child)) {
+		t.Fatalf("second acquisition = %#v, %#v, %#v, accounting %#v", second, sources, diagnostics, accounting)
+	}
+	var wantNodes int
+	for _, text := range []string{one, child, two, child} {
+		parsed, diagnostics := parseSource(Bytes("fixture", []byte(text)), "source-1", defaultParseLimits())
+		if len(diagnostics) != 0 {
+			t.Fatal(diagnostics)
+		}
+		wantNodes += countInputNodes(parsed.document)
+	}
+	if accounting.nodes != wantNodes {
+		t.Fatalf("charged nodes = %d, want %d", accounting.nodes, wantNodes)
+	}
+	if first.inputBytes != int64(len(one)+len(child)) || second.inputBytes != int64(len(two)+len(child)) {
+		t.Fatalf("composition costs = %d, %d, accounting %d", first.inputBytes, second.inputBytes, accounting.bytes)
+	}
+}
+
+func TestGroupedGlobEntryBudgetIsShared(t *testing.T) {
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":    &fstest.MapFile{Data: []byte("configkit: {imports: [{glob: 'parts/*.yml'}]}\n")},
+		"parts/a.yml": &fstest.MapFile{Data: []byte("left: one\n")},
+		"parts/b.yml": &fstest.MapFile{Data: []byte("right: two\n")},
+	})
+	accounting := &importAccounting{entries: maxGlobEntries - 1}
+	_, _, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{source: filesystem.File("main.yml"), imports: true}}}, 1, nil, accounting)
+	if len(diagnostics) == 0 || diagnostics[0].Code != CodeImportLimitExceeded || accounting.entries <= maxGlobEntries {
+		t.Fatalf("shared glob budget = %#v, accounting %#v", diagnostics, accounting)
+	}
+}
+
+func TestGroupedMixedSourcesShareCLIRecordLimit(t *testing.T) {
+	filesystem := NewFilesystem("bundle", fstest.MapFS{"main.yml": &fstest.MapFile{Data: []byte("value: base\n")}})
+	inputs := make([]Input, maxImportSources-1)
+	inputs[0] = Input{Source: Bytes("empty-bytes", []byte("{}\n"))}
+	for index := 1; index < len(inputs); index++ {
+		inputs[index] = Input{Source: Mapping("empty-map", map[string]any{})}
+	}
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	_, err := LoadLayered(Input{Source: filesystem.File("main.yml")}, testSecretBinding(), LayeredOptions[config]{
+		Underlays: []Underlay{{Inputs: inputs}},
+		Overrides: []Override{{Path: Keys("value"), Value: "cli", Flag: "--value"}},
+	})
+	loadError, ok := err.(*LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != CodeOverrideLimitExceeded || len(loadError.Sources()) != maxImportSources {
+		t.Fatalf("mixed source/CLI limit = %#v", err)
+	}
+}

internal/engine/load_result.go

@@ -15,6 +15,8 @@ type resolvedLoad[T any] struct {
 	final         *document.Node
 	snapshots     []resolutionSnapshot
 	applications  []historyApplication
+	inheritance   *inheritanceScope
+	contexts      map[string]ResourceContext
 }
 
 func decodeLoadInput[T any](schema *schemaDescriptor, definition Definition[T], input assembledInput) (decodedConfiguration, []Diagnostic) {
@@ -34,7 +36,9 @@ func decodeLoadInput[T any](schema *schemaDescriptor, definition Definition[T],
 		}
 		return decodedConfiguration{}, diagnostics
 	}
-	if input.composition != nil {
+	if input.inheritance != nil {
+		applyCompositionOrigins(&decoded, input.inheritance.origins)
+	} else if input.composition != nil {
 		applyCompositionOrigins(&decoded, input.composition.origins)
 	}
 
@@ -45,13 +49,15 @@ func resolveLoadInput[T any](schema *schemaDescriptor, definition Definition[T],
 	var diagnostics []Diagnostic
 	configuration := decoded.value.Interface().(T)
 	metadata := metadataFromDecoded(decoded)
-	if input.composition != nil {
+	if input.inheritance != nil {
+		applyInheritanceMetadata(&metadata, input.composition.root, input.composition.origins, input.composition.overlays, input.inheritance)
+	} else if input.composition != nil {
 		applyOverlayMetadata(&metadata, input.composition.overlays)
 	}
 	final := decoded.final
 	var resolutionSnapshots []resolutionSnapshot
 	if definition.Resolve != nil {
-		entry := newResolutionInput(metadata, decoded.final, input.sources)
+		entry := newResolutionInput(metadata, decoded.final, input.sources, input.contexts)
 		resolved := runResolution(&configuration, definition.Resolve, schema, decoded, input.document, entry)
 		diagnostics = append(diagnostics, resolved.diagnostics...)
 		if hasErrors(resolved.diagnostics) {
@@ -67,7 +73,7 @@ func resolveLoadInput[T any](schema *schemaDescriptor, definition Definition[T],
 	} else if input.document != nil {
 		applications = []historyApplication{{layer: LayerBase, value: input.document}}
 	}
-	return resolvedLoad[T]{configuration: configuration, decoded: decoded, metadata: metadata, final: final, snapshots: resolutionSnapshots, applications: applications}, diagnostics
+	return resolvedLoad[T]{configuration: configuration, decoded: decoded, metadata: metadata, final: final, snapshots: resolutionSnapshots, applications: applications, inheritance: input.inheritance, contexts: input.contexts}, diagnostics
 }
 
 func detachLoadConfiguration[T any](schema *schemaDescriptor, configuration T, mode secretCloneMode) (T, *Diagnostic) {
@@ -95,6 +101,7 @@ func publishLoadResult[T any](schema *schemaDescriptor, definition Definition[T]
 	inspection := &Inspection{
 		metadata:         metadata,
 		sources:          cloneSourceInfos(accumulator.sources),
+		contexts:         cloneResourceContexts(resolved.contexts),
 		diagnostics:      canonicalDiagnostics(accumulator.diagnostics),
 		baseline:         decoded.baseline,
 		input:            decoded.final,
@@ -102,6 +109,7 @@ func publishLoadResult[T any](schema *schemaDescriptor, definition Definition[T]
 		final:            final,
 		resolution:       resolutionSnapshots,
 		applications:     applications,
+		inheritance:      inheritanceOperations(resolved.inheritance),
 		secrets:          secrets,
 		secretReferences: cloneSecretReferences(references),
 	}

internal/engine/mapping_index.go

@@ -0,0 +1,37 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// mappingIndex keeps ordered document entries and their lookup positions in
+// sync while one composition or overlay is assembled.
+type mappingIndex struct {
+	byNode map[*document.Node]map[string]int
+}
+
+func (m *mappingIndex) entries(node *document.Node) map[string]int {
+	if m.byNode == nil {
+		m.byNode = make(map[*document.Node]map[string]int)
+	}
+	if entries, ok := m.byNode[node]; ok {
+		return entries
+	}
+	entries := make(map[string]int, len(node.Mapping))
+	for index, entry := range node.Mapping {
+		entries[entry.Key] = index
+	}
+	m.byNode[node] = entries
+	return entries
+}
+
+func (m *mappingIndex) find(node *document.Node, key string) int {
+	if index, ok := m.entries(node)[key]; ok {
+		return index
+	}
+	return -1
+}
+
+func (m *mappingIndex) append(node *document.Node, entry document.Entry) {
+	entries := m.entries(node)
+	entries[entry.Key] = len(node.Mapping)
+	node.Mapping = append(node.Mapping, entry)
+}

internal/engine/metadata.go

@@ -13,6 +13,7 @@ type Metadata struct {
 	historicalPaths map[string][]Path
 	fields          map[string]FieldInfo
 	history         map[string][]Change
+	inheritance     []inheritanceOperation
 }
 
 // Lookup returns information for an effective path. Unknown and invalid paths
@@ -25,17 +26,19 @@ func (m Metadata) Lookup(path Path) (FieldInfo, bool) {
 	if !ok {
 		return FieldInfo{}, false
 	}
-	return cloneFieldInfo(info), true
+	detached := cloneFieldInfo(info)
+	detached.Origin.Inheritance = projectInheritanceSteps(detached.Origin)
+	return detached, true
 }
 
-// IsExplicit reports whether a surviving input contribution explicitly defined
-// path.
+// IsExplicit reports whether a surviving input contribution, including one
+// inherited from a sibling, explicitly defined path.
 func (m Metadata) IsExplicit(path Path) bool {
 	info, ok := m.Lookup(path)
 	return ok && info.Explicit
 }
 
-// History returns detached definition and resolution records for path. A record's
+// History returns detached definition, inheritance, and resolution records for path. A record's
 // Path can be an ancestor when that container operation replaced or removed path.
 // Removed historical paths can have history even though Lookup returns false.
 // Unknown and invalid paths return nil.
@@ -43,19 +46,12 @@ func (m Metadata) History(path Path) []Change {
 	if !m.hasHistoricalPath(path) {
 		return nil
 	}
-	history, ok := m.history[path.String()]
-	if !ok {
-		return nil
-	}
-	cloned := make([]Change, len(history))
-	for i, change := range history {
-		cloned[i] = cloneChange(change)
-	}
-	return cloned
+	history := m.history[path.String()]
+	return historyWithInheritance(path, history, m.inheritance)
 }
 
 // HistoricalPaths returns the paths observed during loading, including effective
-// paths and paths removed by overlays or recorded resolution. It includes default
+// paths and paths removed by overlays, inheritance, or recorded resolution. It includes default
 // paths and the root, but not paths that never existed. The zero Metadata returns
 // an empty slice.
 //
@@ -136,6 +132,16 @@ func finalizeMetadataPaths(m *Metadata, final *document.Node, applications []his
 		}
 		walkDocument(application.value, prefix, add)
 	}
+	for _, operation := range m.inheritance {
+		if operation.control != nil {
+			add(operation.control.path, nil)
+			continue
+		}
+		if operation.transfer != nil {
+			walkDocument(operation.parent, operation.transfer.to, add)
+			walkDocument(operation.effective, operation.transfer.to, add)
+		}
+	}
 }
 
 // effectivePathIndex records document-derived paths with their contextual segment

internal/engine/native_filesystem_test.go

@@ -0,0 +1,284 @@
+package engine
+
+import (
+	"errors"
+	"io/fs"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+func TestNativeFilesystemImportsAndOverlay(t *testing.T) {
+	filesystem := fstest.MapFS{
+		"config/main.yml": &fstest.MapFile{Data: []byte(`configkit:
+  imports:
+    - path: ../shared.yml
+    - glob: fragments/*.yml
+items: {main: main}
+`)},
+		"shared.yml":             &fstest.MapFile{Data: []byte("items: {shared: shared}\n")},
+		"config/fragments/b.yml": &fstest.MapFile{Data: []byte("items: {beta: beta}\n")},
+		"config/fragments/a.yml": &fstest.MapFile{Data: []byte("items: {alpha: alpha}\n")},
+		"overlays/testing.yml":   &fstest.MapFile{Data: []byte("items: {main: overlaid}\n")},
+	}
+	namespace := newNativeNamespace("bundle", filesystem)
+	result, err := testLoad[importTestConfig](filesystemFile(namespace, "config/main.yml"), Options[importTestConfig]{Imports: true, Overlay: "testing"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := map[string]string{"main": "overlaid", "shared": "shared", "alpha": "alpha", "beta": "beta"}
+	if !reflect.DeepEqual(result.Config.Items, want) {
+		t.Fatalf("items = %#v, want %#v", result.Config.Items, want)
+	}
+	sources := result.Sources()
+	wantPaths := []string{"config/main.yml", "shared.yml", "config/fragments/a.yml", "config/fragments/b.yml", "overlays/testing.yml"}
+	gotPaths := make([]string, len(sources))
+	for index, source := range sources {
+		gotPaths[index] = source.Path
+	}
+	if !reflect.DeepEqual(gotPaths, wantPaths) {
+		t.Fatalf("source paths = %#v, want %#v", gotPaths, wantPaths)
+	}
+}
+
+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")}}
+	firstNamespace := newNativeNamespace("same", first)
+	secondNamespace := newNativeNamespace("same", second)
+	if reflect.TypeOf(first).Comparable() {
+		t.Fatal("test filesystem must have a non-comparable dynamic value")
+	}
+	if firstNamespace.token == secondNamespace.token {
+		t.Fatal("independent filesystem constructors shared identity")
+	}
+	if newNativeAccess(first, ".", firstNamespace.token).identity("main.yml") == newNativeAccess(second, ".", secondNamespace.token).identity("main.yml") {
+		t.Fatal("equal paths from separate filesystems have the same identity")
+	}
+	access := newNativeAccess(first, ".", firstNamespace.token)
+	if access.syntheticIdentity() == access.syntheticIdentity() || access.syntheticIdentity() == access.identity("<selected-source>") {
+		t.Fatal("independent synthetic sources shared a file identity")
+	}
+	for _, test := range []struct {
+		name      string
+		namespace *nativeNamespace
+		want      string
+	}{
+		{name: "first", namespace: firstNamespace, want: "first"},
+		{name: "second", namespace: secondNamespace, want: "second"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := testLoad[importTestConfig](filesystemFile(test.namespace, "main.yml"))
+			if err != nil || result.Config.Items["name"] != test.want {
+				t.Fatalf("result = %#v, %v", result, err)
+			}
+		})
+	}
+	for _, name := range []string{".", "../main.yml", "/main.yml", "missing.yml"} {
+		_, diagnostics := readSource(filesystemFile(firstNamespace, name), "source-1", defaultSourceLimits())
+		if len(diagnostics) != 1 {
+			t.Errorf("%q diagnostics = %#v", name, diagnostics)
+		}
+	}
+	missingImport := fstest.MapFS{"main.yml": &fstest.MapFile{Data: []byte("configkit: {imports: [{path: host-only.yml}]}\n")}}
+	_, err := testLoad[importTestConfig](filesystemFile(newNativeNamespace("bundle", missingImport), "main.yml"), Options[importTestConfig]{Imports: true})
+	if err == nil {
+		t.Fatal("missing virtual import fell back to the host filesystem")
+	}
+	loadError, ok := err.(*LoadError)
+	if !ok || loadError.Diagnostics()[0].Code != CodeImportMissing {
+		t.Fatalf("missing import error = %#v", err)
+	}
+}
+
+func TestNativeFilesystemClosesFilesOnSuccessAndFailure(t *testing.T) {
+	for _, fallback := range []bool{false, true} {
+		t.Run(map[bool]string{false: "ReadDirFile", true: "ReadDir fallback"}[fallback], func(t *testing.T) {
+			filesystem := &observedFilesystem{files: fstest.MapFS{
+				"main.yml":      &fstest.MapFile{Data: []byte("configkit: {imports: [{glob: 'parts/*.yml'}]}\nitems: {main: main}\n")},
+				"parts/one.yml": &fstest.MapFile{Data: []byte("items: {one: one}\n")},
+			}, fallback: fallback}
+			result, err := testLoad[importTestConfig](filesystemFile(newNativeNamespace("bundle", filesystem), "main.yml"), Options[importTestConfig]{Imports: true})
+			if err != nil || result.Config.Items["one"] != "one" {
+				t.Fatalf("result = %#v, %v", result, err)
+			}
+			filesystem.assertClosed(t)
+		})
+	}
+
+	for _, failure := range []struct {
+		name     string
+		file     string
+		failRead string
+		failStat string
+		failDir  string
+	}{
+		{name: "read", file: "items: {one: one}\n", failRead: "parts/one.yml"},
+		{name: "stat", file: "items: {one: one}\n", failStat: "parts/one.yml"},
+		{name: "parse", file: "items: [unterminated\n"},
+		{name: "directory", file: "items: {one: one}\n", failDir: "parts"},
+	} {
+		t.Run(failure.name, func(t *testing.T) {
+			filesystem := &observedFilesystem{files: fstest.MapFS{
+				"main.yml":      &fstest.MapFile{Data: []byte("configkit: {imports: [{glob: 'parts/*.yml'}]}\n")},
+				"parts/one.yml": &fstest.MapFile{Data: []byte(failure.file)},
+			}, failRead: failure.failRead, failStat: failure.failStat, failDir: failure.failDir}
+			_, err := testLoad[importTestConfig](filesystemFile(newNativeNamespace("bundle", filesystem), "main.yml"), Options[importTestConfig]{Imports: true})
+			if err == nil {
+				t.Fatal("expected a load failure")
+			}
+			if strings.Contains(err.Error(), "tracking failure canary") {
+				t.Fatal("unsafe filesystem error escaped")
+			}
+			filesystem.assertClosed(t)
+		})
+	}
+}
+
+func TestNativeFilesystemRootAndOptionalImports(t *testing.T) {
+	filesystem := fstest.MapFS{
+		"config/sub/main.yml": &fstest.MapFile{Data: []byte(`configkit:
+  imports:
+    - path: ../shared.yml
+    - path: absent.yml
+      optional: true
+    - glob: absent/*.yml
+      optional: true
+`)},
+		"config/shared.yml": &fstest.MapFile{Data: []byte("items: {shared: retained}\n")},
+		"outside.yml":       &fstest.MapFile{Data: []byte("items: {outside: blocked}\n")},
+	}
+	namespace := newNativeNamespace("bundle", filesystem)
+	source := filesystemFile(namespace, "config/sub/main.yml")
+	parsed, diagnostics := parseSource(source, "source-1", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	access := newNativeAccess(filesystem, "config", namespace.token)
+	composed, sources, diagnostics := loadImportedWithAccess(access, "config/sub", source.path, parsed, nil, defaultImportLimits(), &importAccounting{})
+	if len(diagnostics) != 0 || composed == nil || len(sources) != 2 || sources[1].Path != "config/shared.yml" {
+		t.Fatalf("contained imports = (%#v, %#v, %#v)", composed, sources, diagnostics)
+	}
+
+	outside := fstest.MapFS{
+		"config/sub/main.yml": &fstest.MapFile{Data: []byte("configkit: {imports: [{path: ../../outside.yml}]}\n")},
+		"outside.yml":         &fstest.MapFile{Data: []byte("items: {outside: blocked}\n")},
+	}
+	outsideNamespace := newNativeNamespace("bundle", outside)
+	parsed, diagnostics = parseSource(filesystemFile(outsideNamespace, "config/sub/main.yml"), "source-1", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	_, _, diagnostics = loadImportedWithAccess(newNativeAccess(outside, "config", outsideNamespace.token), "config/sub", "config/sub/main.yml", parsed, nil, defaultImportLimits(), &importAccounting{})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeImportOutsideRoot {
+		t.Fatalf("escaping import diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestNativeFilesystemDetectsRepeatedAndCyclicImports(t *testing.T) {
+	for _, test := range []struct {
+		name  string
+		files fstest.MapFS
+		code  Code
+	}{
+		{
+			name: "repeated",
+			files: fstest.MapFS{
+				"main.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: child.yml}, {path: child.yml}]}\n")},
+				"child.yml": &fstest.MapFile{Data: []byte("items: {child: child}\n")},
+			},
+			code: CodeImportRepeated,
+		},
+		{
+			name: "cycle",
+			files: fstest.MapFS{
+				"main.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: child.yml}]}\n")},
+				"child.yml": &fstest.MapFile{Data: []byte("configkit: {imports: [{path: main.yml}]}\n")},
+			},
+			code: CodeImportCycle,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			_, err := testLoad[importTestConfig](filesystemFile(newNativeNamespace("bundle", test.files), "main.yml"), Options[importTestConfig]{Imports: true})
+			loadError, ok := err.(*LoadError)
+			if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != test.code {
+				t.Fatalf("load error = %#v, want %q", err, test.code)
+			}
+		})
+	}
+}
+
+type observedFilesystem struct {
+	files    fstest.MapFS
+	opened   int
+	closed   int
+	fallback bool
+	failRead string
+	failStat string
+	failDir  string
+}
+
+func (f *observedFilesystem) Open(name string) (fs.File, error) {
+	inner, err := f.files.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	f.opened++
+	file := &observedFile{File: inner, owner: f, name: name}
+	if directory, ok := inner.(fs.ReadDirFile); ok && !f.fallback {
+		return &observedDirectoryFile{observedFile: file, directory: directory}, nil
+	}
+	return file, nil
+}
+
+func (f *observedFilesystem) ReadDir(name string) ([]fs.DirEntry, error) {
+	if name == f.failDir {
+		return nil, errors.New("tracking failure canary")
+	}
+	return fs.ReadDir(f.files, name)
+}
+
+func (f *observedFilesystem) assertClosed(t *testing.T) {
+	t.Helper()
+	if f.opened == 0 || f.opened != f.closed {
+		t.Fatalf("filesystem opens=%d closes=%d", f.opened, f.closed)
+	}
+}
+
+type observedFile struct {
+	fs.File
+	owner *observedFilesystem
+	name  string
+}
+
+func (f *observedFile) Read(data []byte) (int, error) {
+	if f.name == f.owner.failRead {
+		return 0, errors.New("tracking failure canary")
+	}
+	return f.File.Read(data)
+}
+
+func (f *observedFile) Stat() (fs.FileInfo, error) {
+	if f.name == f.owner.failStat {
+		return nil, errors.New("tracking failure canary")
+	}
+	return f.File.Stat()
+}
+
+func (f *observedFile) Close() error {
+	f.owner.closed++
+	return f.File.Close()
+}
+
+type observedDirectoryFile struct {
+	*observedFile
+	directory fs.ReadDirFile
+}
+
+func (f *observedDirectoryFile) ReadDir(count int) ([]fs.DirEntry, error) {
+	if f.name == f.owner.failDir {
+		return nil, errors.New("tracking failure canary")
+	}
+	return f.directory.ReadDir(count)
+}

internal/engine/options.go

@@ -22,8 +22,8 @@ func selectOptions[T any](source Source, supplied []Options[T]) (Options[T], []D
 	if source.kind == sourceNone && options.Overlay != "" && options.Overlay != "default" {
 		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "NoSource cannot select an active overlay"))
 	}
-	if options.Filesystem == nil && (source.kind == sourceBytes || source.kind == sourceReader) && (options.Imports || activeOverlay(options.Overlay)) {
-		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "byte and reader imports or overlays require filesystem context"))
+	if options.Filesystem == nil && (source.kind == sourceBytes || source.kind == sourceReader || source.kind == sourceMapping) && (options.Imports || activeOverlay(options.Overlay)) {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "synthetic-source imports or overlays require filesystem context"))
 	}
 	if definition.Version != nil {
 		if !definition.Version.Path.valid() {

internal/engine/overlay.go

@@ -18,78 +18,105 @@ type displacedDefinition struct {
 }
 
 func overlayDocuments(base *composedDocument, overlay *document.Node) (*composedDocument, []Diagnostic) {
-	if base == nil || base.root == nil || base.root.Kind != document.Mapping || overlay == nil || overlay.Kind != document.Mapping {
-		return nil, []Diagnostic{{
-			Code: CodeOverlayInvalid, Severity: SeverityError,
-			Message: "overlay requires mapping document roots",
-		}}
+	if base == nil {
+		return nil, invalidOverlayDocument()
 	}
-	result := cloneComposedDocument(base)
-	mergeOverlayOrigin(result.origins, Path{}, overlay.Location, LayerOverlay)
-	mergeOverlayMappings(result, result.root, overlay, Path{}, LayerOverlay, true)
-	result.applications = append(result.applications, historyApplication{layer: LayerOverlay, value: overlay})
-	return result, nil
+	assembly := newAssemblyAccumulator(cloneComposedDocument(base))
+	if diagnostics := assembly.applyOverlay(overlay, nil, nil, LayerOverlay); len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+	return assembly.current, nil
 }
 
-func mergeOverlayMappings(result *composedDocument, target, overlay *document.Node, current Path, layer Layer, recordReplacements bool) {
-	for _, incoming := range overlay.Mapping {
-		path := current.Key(incoming.Key)
-		index := documentEntryIndex(target.Mapping, incoming.Key)
-		if index < 0 {
-			cloned := incoming
-			cloned.Value = document.Clone(incoming.Value)
-			target.Mapping = append(target.Mapping, cloned)
-			collectOverlayOrigins(result.origins, incoming.Value, path, layer)
-			continue
-		}
+func invalidOverlayDocument() []Diagnostic {
+	return []Diagnostic{{
+		Code: CodeOverlayInvalid, Severity: SeverityError,
+		Message: "overlay requires mapping document roots",
+	}}
+}
 
-		existing := target.Mapping[index].Value
-		if existing.Kind == document.Mapping && incoming.Value.Kind == document.Mapping {
-			mergeOverlayOrigin(result.origins, path, incoming.Value.Location, layer)
-			mergeOverlayMappings(result, existing, incoming.Value, path, layer, recordReplacements)
-			continue
-		}
-
-		displaced := displacedOrigins(existing, result.origins, path)
-		previous := cloneOrigin(result.origins[path.String()])
-		deleteOriginSubtree(result.origins, path)
-		target.Mapping[index] = incoming
-		target.Mapping[index].Value = document.Clone(incoming.Value)
-		collectOverlayOrigins(result.origins, incoming.Value, path, layer)
-		if recordReplacements {
-			result.overlays = append(result.overlays, overlayRecord{
-				change: Change{
-					Path: path.clone(), Origin: overlayOrigin(layer, incoming.Value.Location),
-					Previous: previousOrigins(previous),
-				},
-				displaced: displaced,
-			})
+func (a *assemblyAccumulator) applyOverlay(overlay *document.Node, incomingOrigins map[string]Origin, applications []historyApplication, layer Layer) []Diagnostic {
+	if a == nil || a.current == nil || a.current.root == nil || a.current.root.Kind != document.Mapping || overlay == nil || overlay.Kind != document.Mapping {
+		return invalidOverlayDocument()
+	}
+	mergeOriginContribution(a.current.origins, Path{}, incomingOrigin(incomingOrigins, Path{}, overlay, layer))
+	mergeOverlayMappings(a.current, a.current.root, overlay, Path{}, layer, true, incomingOrigins, &a.index)
+	if len(applications) == 0 {
+		a.current.applications = append(a.current.applications, historyApplication{layer: layer, value: overlay})
+	} else {
+		for _, application := range applications {
+			application.layer = layer
+			a.current.applications = append(a.current.applications, application)
 		}
 	}
+	return nil
+}
+
+func mergeOverlayMappings(result *composedDocument, target, overlay *document.Node, current Path, layer Layer, recordReplacements bool, incomingOrigins map[string]Origin, index *mappingIndex) {
+	mergeRecursiveMappings(target, overlay, current, index, recursiveMergeHooks{
+		added: func(path Path, incoming *document.Node) {
+			collectOverlayOriginsFrom(result.origins, incomingOrigins, incoming, path, layer)
+		},
+		combined: func(path Path, incoming *document.Node) {
+			mergeOriginContribution(result.origins, path, incomingOrigin(incomingOrigins, path, incoming, layer))
+		},
+		replaced: func(path Path, existing, incoming *document.Node) {
+			displaced := displacedOrigins(existing, result.origins, path)
+			previous := cloneOrigin(result.origins[path.String()])
+			deleteOriginSubtree(result.origins, path, existing)
+			collectOverlayOriginsFrom(result.origins, incomingOrigins, incoming, path, layer)
+			if recordReplacements {
+				result.overlays = append(result.overlays, overlayRecord{
+					change: Change{
+						Path: path.clone(), Origin: incomingOrigin(incomingOrigins, path, incoming, layer),
+						Previous: previousOrigins(previous),
+					},
+					displaced: displaced,
+				})
+			}
+		},
+	})
 }
 
 func collectOverlayOrigins(origins map[string]Origin, node *document.Node, path Path, layer Layer) {
-	origins[path.String()] = overlayOrigin(layer, node.Location)
+	collectOverlayOriginsFrom(origins, nil, node, path, layer)
+}
+
+func collectOverlayOriginsFrom(origins, incomingOrigins map[string]Origin, node *document.Node, path Path, layer Layer) {
+	origins[path.String()] = incomingOrigin(incomingOrigins, path, node, layer)
 	switch node.Kind {
 	case document.Mapping:
 		for _, entry := range node.Mapping {
-			collectOverlayOrigins(origins, entry.Value, path.Key(entry.Key), layer)
+			collectOverlayOriginsFrom(origins, incomingOrigins, entry.Value, path.Key(entry.Key), layer)
 		}
 	case document.Sequence:
 		for index, child := range node.Sequence {
-			collectOverlayOrigins(origins, child, path.Index(index), layer)
+			collectOverlayOriginsFrom(origins, incomingOrigins, child, path.Index(index), layer)
 		}
 	}
 }
 
 func mergeOverlayOrigin(origins map[string]Origin, path Path, location document.Location, layer Layer) {
+	mergeOriginContribution(origins, path, overlayOrigin(layer, location))
+}
+
+func mergeOriginContribution(origins map[string]Origin, path Path, incoming Origin) {
 	pointer := path.String()
-	origin := cloneOrigin(origins[pointer])
-	origin.Layer = layer
-	origin.Locations = append(origin.Locations, publicLocation(location))
+	origin := origins[pointer]
+	origin.Layer = incoming.Layer
+	origin.Locations = append(origin.Locations, incoming.Locations...)
 	origins[pointer] = origin
 }
 
+func incomingOrigin(origins map[string]Origin, path Path, node *document.Node, layer Layer) Origin {
+	if origin, ok := origins[path.String()]; ok {
+		origin = cloneOrigin(origin)
+		origin.Layer = layer
+		return origin
+	}
+	return overlayOrigin(layer, node.Location)
+}
+
 func overlayOrigin(layer Layer, location document.Location) Origin {
 	return Origin{Layer: layer, Locations: []Location{publicLocation(location)}}
 }
@@ -129,11 +156,16 @@ func collectDisplacedOrigins(displaced *[]displacedDefinition, node *document.No
 	}
 }
 
-func deleteOriginSubtree(origins map[string]Origin, path Path) {
-	prefix := path.String()
-	for pointer := range origins {
-		if pointer == prefix || strings.HasPrefix(pointer, prefix+"/") {
-			delete(origins, pointer)
+func deleteOriginSubtree(origins map[string]Origin, path Path, node *document.Node) {
+	delete(origins, path.String())
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			deleteOriginSubtree(origins, path.Key(entry.Key), entry.Value)
+		}
+	case document.Sequence:
+		for index, child := range node.Sequence {
+			deleteOriginSubtree(origins, path.Index(index), child)
 		}
 	}
 }

internal/engine/overlay_test.go

@@ -76,3 +76,44 @@ to_scalar: scalar
 		t.Fatal("overlay result aliases an input document")
 	}
 }
+
+func TestOwnedOverlayKeepsCompositeContributorsAndAcquiredInputs(t *testing.T) {
+	baseInput := parseCompositionDocument(t, "source-1", "catalog:\n  base: true\n")
+	base, diagnostics := composeDocuments(baseInput)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	first := parseCompositionDocument(t, "source-2", "catalog:\n  first: true\n")
+	second := parseCompositionDocument(t, "source-3", "catalog:\n  second: true\n")
+	firstBefore, secondBefore := document.Clone(first), document.Clone(second)
+	incoming, diagnostics := composeDocuments(first, second)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	assembly := newAssemblyAccumulator(base)
+	if diagnostics := assembly.applyOverlay(incoming.root, incoming.origins, incoming.applications, LayerOverlay); len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	for _, key := range []string{"base", "first", "second"} {
+		if _, ok := documentAtPath(assembly.current.root, Keys("catalog", key)); !ok {
+			t.Fatalf("catalog is missing %q", key)
+		}
+	}
+	origin := assembly.current.origins[Keys("catalog").String()]
+	if got, want := origin.Locations, []Location{{SourceID: "source-1", Line: 2, Column: 3}, {SourceID: "source-2", Line: 2, Column: 3}, {SourceID: "source-3", Line: 2, Column: 3}}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("catalog origin locations = %#v, want %#v", got, want)
+	}
+	if len(assembly.current.applications) != 3 {
+		t.Fatalf("history applications = %d, want three source contributions", len(assembly.current.applications))
+	}
+	for _, application := range assembly.current.applications[1:] {
+		if application.layer != LayerOverlay {
+			t.Fatalf("incoming application layer = %q", application.layer)
+		}
+	}
+	assembly.current.root.Mapping[0].Value.Mapping[1].Value.Text = "false"
+	if !reflect.DeepEqual(first, firstBefore) || !reflect.DeepEqual(second, secondBefore) ||
+		!reflect.DeepEqual(first, assembly.current.applications[1].value) || !reflect.DeepEqual(second, assembly.current.applications[2].value) {
+		t.Fatal("working tree mutation changed an acquired input")
+	}
+}

internal/engine/override.go

@@ -44,7 +44,7 @@ func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []
 	allSources := cloneSourceInfos(sources)
 	for index, override := range overrides {
 		sourceID := "source-" + strconv.Itoa(len(sources)+index+1)
-		info := SourceInfo{ID: sourceID, Label: override.Flag, Layer: LayerCLI}
+		info := SourceInfo{ID: sourceID, Label: override.Flag, Layer: LayerCLI, Kind: SourceKindCLI}
 		allSources = append(allSources, info)
 		prepared[index].source = info
 	}
@@ -94,11 +94,12 @@ func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []
 		}
 		pathAccepted[index] = true
 
-		builder := overrideValueBuilder{sourceID: prepared[index].source.ID, active: make(map[valueVisit]bool)}
+		builder := valueBuilder{sourceID: prepared[index].source.ID, active: make(map[valueVisit]bool)}
 		value, issue := builder.build(override.Value, override.Path, len(override.Path.segments))
 		if issue != nil {
-			issue.Primary = location
-			diagnostics = append(diagnostics, *issue)
+			diagnostic := issue.overrideDiagnostic()
+			diagnostic.Primary = location
+			diagnostics = append(diagnostics, diagnostic)
 			continue
 		}
 		if relative, ok := overrideDirectivePath(value, Path{}); ok {

internal/engine/override_apply.go

@@ -11,31 +11,42 @@ func applyOverrides(base *composedDocument, sources []SourceInfo, overrides []Ov
 	if len(overrides) == 0 {
 		return base, cloneSourceInfos(sources), nil
 	}
-	prepared, allSources, diagnostics := prepareOverrides(base, sources, overrides, rule, limits)
+	assembly := newAssemblyAccumulator(cloneComposedDocument(base))
+	allSources, diagnostics := assembly.applyOverrides(sources, overrides, rule, limits)
 	if len(diagnostics) != 0 {
 		return nil, allSources, diagnostics
 	}
-	result := cloneComposedDocument(base)
-	for _, override := range prepared {
-		applyPreparedOverride(result, override)
-		result.inputBytes += override.bytes
-		result.inputNodes += override.nodes
-	}
-	return result, allSources, nil
+	return assembly.current, allSources, nil
 }
 
-func applyPreparedOverride(result *composedDocument, override preparedOverride) {
+func (a *assemblyAccumulator) applyOverrides(sources []SourceInfo, overrides []Override, rule *VersionRule, limits importLimits) ([]SourceInfo, []Diagnostic) {
+	if len(overrides) == 0 {
+		return cloneSourceInfos(sources), nil
+	}
+	prepared, allSources, diagnostics := prepareOverrides(a.current, sources, overrides, rule, limits)
+	if len(diagnostics) != 0 {
+		return allSources, diagnostics
+	}
+	for _, override := range prepared {
+		applyPreparedOverride(a.current, override, &a.index)
+		a.current.inputBytes += override.bytes
+		a.current.inputNodes += override.nodes
+	}
+	return allSources, nil
+}
+
+func applyPreparedOverride(result *composedDocument, override preparedOverride, index *mappingIndex) {
 	location := document.Location{SourceID: override.source.ID}
 	mergeOverlayOrigin(result.origins, Path{}, location, LayerCLI)
 	current := result.root
 	currentPath := Path{}
-	for index, segment := range override.path.segments {
+	for segmentIndex, segment := range override.path.segments {
 		path := currentPath.Key(segment.key)
-		position := documentEntryIndex(current.Mapping, segment.key)
-		if index < len(override.path.segments)-1 {
+		position := index.find(current, segment.key)
+		if segmentIndex < len(override.path.segments)-1 {
 			if position < 0 {
 				child := &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{}}
-				current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: child})
+				index.append(current, document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: child})
 				collectOverlayOrigins(result.origins, child, path, LayerCLI)
 				current = child
 			} else {
@@ -50,16 +61,16 @@ func applyPreparedOverride(result *composedDocument, override preparedOverride)
 		var displaced []displacedDefinition
 		if position < 0 {
 			incoming := document.Clone(override.value)
-			current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: incoming})
+			index.append(current, document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: incoming})
 			collectOverlayOrigins(result.origins, incoming, path, LayerCLI)
 		} else {
 			existing := current.Mapping[position].Value
-			displaced = affectedOverrideOrigins(existing, override.value, result.origins, path)
+			displaced = affectedOverrideOrigins(existing, override.value, result.origins, path, index)
 			if existing.Kind == document.Mapping && override.value.Kind == document.Mapping {
 				mergeOverlayOrigin(result.origins, path, location, LayerCLI)
-				mergeOverlayMappings(result, existing, override.value, path, LayerCLI, false)
+				mergeOverlayMappings(result, existing, override.value, path, LayerCLI, false, nil, index)
 			} else {
-				deleteOriginSubtree(result.origins, path)
+				deleteOriginSubtree(result.origins, path, existing)
 				current.Mapping[position] = document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: document.Clone(override.value)}
 				collectOverlayOrigins(result.origins, current.Mapping[position].Value, path, LayerCLI)
 			}
@@ -74,18 +85,18 @@ func applyPreparedOverride(result *composedDocument, override preparedOverride)
 	}
 }
 
-func affectedOverrideOrigins(existing, incoming *document.Node, origins map[string]Origin, path Path) []displacedDefinition {
+func affectedOverrideOrigins(existing, incoming *document.Node, origins map[string]Origin, path Path, index *mappingIndex) []displacedDefinition {
 	displaced := []displacedDefinition{{path: path.clone(), origin: cloneOrigin(origins[path.String()])}}
 	if existing.Kind == document.Mapping && incoming.Kind == document.Mapping {
 		for _, entry := range incoming.Mapping {
-			position := documentEntryIndex(existing.Mapping, entry.Key)
+			position := index.find(existing, entry.Key)
 			if position < 0 {
 				continue
 			}
 			childPath := path.Key(entry.Key)
 			childExisting := existing.Mapping[position].Value
 			if childExisting.Kind == document.Mapping && entry.Value.Kind == document.Mapping {
-				displaced = append(displaced, affectedOverrideOrigins(childExisting, entry.Value, origins, childPath)...)
+				displaced = append(displaced, affectedOverrideOrigins(childExisting, entry.Value, origins, childPath, index)...)
 			} else {
 				displaced = append(displaced, displacedOrigins(childExisting, origins, childPath)...)
 			}

internal/engine/override_value.go

@@ -1,150 +1,21 @@
 package engine
 
-import (
-	"math"
-	"reflect"
-	"slices"
-	"strconv"
-
-	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
-)
-
-type overrideValueBuilder struct {
-	sourceID string
-	active   map[valueVisit]bool
-	bytes    int64
-	nodes    int
-}
-
-func (b *overrideValueBuilder) build(value any, path Path, depth int) (*document.Node, *Diagnostic) {
-	if depth > maxDocumentDepth {
-		return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the depth limit")
-	}
-	b.nodes++
-	if b.nodes > maxDocumentNodes {
-		return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the node limit")
-	}
-	location := document.Location{SourceID: b.sourceID}
-	switch value := value.(type) {
-	case nil:
-		return &document.Node{Kind: document.Null, Location: location}, nil
-	case string:
-		if !b.addBytes(len(value)) {
-			return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the record string-byte limit")
-		}
-		return &document.Node{Kind: document.String, Text: value, Location: location}, nil
-	case bool:
-		return &document.Node{Kind: document.Boolean, Text: strconv.FormatBool(value), Location: location}, nil
-	case int:
-		return overrideNumber(strconv.FormatInt(int64(value), 10), location), nil
-	case int8:
-		return overrideNumber(strconv.FormatInt(int64(value), 10), location), nil
-	case int16:
-		return overrideNumber(strconv.FormatInt(int64(value), 10), location), nil
-	case int32:
-		return overrideNumber(strconv.FormatInt(int64(value), 10), location), nil
-	case int64:
-		return overrideNumber(strconv.FormatInt(value, 10), location), nil
-	case uint:
-		return overrideNumber(strconv.FormatUint(uint64(value), 10), location), nil
-	case uint8:
-		return overrideNumber(strconv.FormatUint(uint64(value), 10), location), nil
-	case uint16:
-		return overrideNumber(strconv.FormatUint(uint64(value), 10), location), nil
-	case uint32:
-		return overrideNumber(strconv.FormatUint(uint64(value), 10), location), nil
-	case uint64:
-		return overrideNumber(strconv.FormatUint(value, 10), location), nil
-	case float32:
-		if math.IsNaN(float64(value)) || math.IsInf(float64(value), 0) {
-			return nil, overrideValueDiagnostic(CodeOverrideInvalidValue, path, "configuration override float must be finite")
-		}
-		return overrideNumber(strconv.FormatFloat(float64(value), 'g', -1, 32), location), nil
-	case float64:
-		if math.IsNaN(value) || math.IsInf(value, 0) {
-			return nil, overrideValueDiagnostic(CodeOverrideInvalidValue, path, "configuration override float must be finite")
-		}
-		return overrideNumber(strconv.FormatFloat(value, 'g', -1, 64), location), nil
-	case []any:
-		if value == nil {
-			return &document.Node{Kind: document.Null, Location: location}, nil
-		}
-		leave, diagnostic := b.enter(reflect.ValueOf(value), path)
-		if diagnostic != nil {
-			return nil, diagnostic
-		}
-		defer leave()
-		if len(value) > maxDocumentNodes-b.nodes {
-			return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the node limit")
-		}
-		node := &document.Node{Kind: document.Sequence, Location: location, Sequence: make([]*document.Node, len(value))}
-		for index, child := range value {
-			built, diagnostic := b.build(child, path.Index(index), depth+1)
-			if diagnostic != nil {
-				return nil, diagnostic
-			}
-			node.Sequence[index] = built
-		}
-		return node, nil
-	case map[string]any:
-		if value == nil {
-			return &document.Node{Kind: document.Null, Location: location}, nil
-		}
-		leave, diagnostic := b.enter(reflect.ValueOf(value), path)
-		if diagnostic != nil {
-			return nil, diagnostic
-		}
-		defer leave()
-		if len(value) > (maxDocumentNodes-b.nodes)/2 {
-			return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the node limit")
-		}
-		keys := make([]string, 0, len(value))
-		for key := range value {
-			keys = append(keys, key)
-		}
-		slices.Sort(keys)
-		node := &document.Node{Kind: document.Mapping, Location: location, Mapping: make([]document.Entry, 0, len(keys))}
-		for _, key := range keys {
-			b.nodes++ // mapping key
-			if !b.addBytes(len(key)) {
-				return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path.Key(key), "configuration override exceeds the record string-byte limit")
-			}
-			built, diagnostic := b.build(value[key], path.Key(key), depth+1)
-			if diagnostic != nil {
-				return nil, diagnostic
-			}
-			node.Mapping = append(node.Mapping, document.Entry{Key: key, KeyLocation: location, ValueLocation: location, Value: built})
-		}
-		return node, nil
+func (issue *valueIssue) overrideDiagnostic() Diagnostic {
+	code := CodeOverrideInvalidValue
+	var message string
+	switch issue.kind {
+	case valueDepthLimit:
+		code, message = CodeOverrideLimitExceeded, "configuration override exceeds the depth limit"
+	case valueNodeLimit:
+		code, message = CodeOverrideLimitExceeded, "configuration override exceeds the node limit"
+	case valueByteLimit:
+		code, message = CodeOverrideLimitExceeded, "configuration override exceeds the record string-byte limit"
+	case valueNonFinite:
+		message = "configuration override float must be finite"
+	case valueCycle:
+		message = "configuration override values must not contain cycles"
 	default:
-		return nil, overrideValueDiagnostic(CodeOverrideInvalidValue, path, "configuration override value has an unsupported type")
+		message = "configuration override value has an unsupported type"
 	}
-}
-
-func (b *overrideValueBuilder) addBytes(count int) bool {
-	if count < 0 || int64(count) > maxSourceBytes-b.bytes {
-		return false
-	}
-	b.bytes += int64(count)
-	return true
-}
-
-func (b *overrideValueBuilder) enter(value reflect.Value, path Path) (func(), *Diagnostic) {
-	visit, ok := visitFor(value)
-	if !ok {
-		return func() {}, nil
-	}
-	if b.active[visit] {
-		return nil, overrideValueDiagnostic(CodeOverrideInvalidValue, path, "configuration override values must not contain cycles")
-	}
-	b.active[visit] = true
-	return func() { delete(b.active, visit) }, nil
-}
-
-func overrideValueDiagnostic(code Code, path Path, message string) *Diagnostic {
-	return &Diagnostic{Code: code, Severity: SeverityError, Path: path.clone(), Message: message}
-}
-
-func overrideNumber(text string, location document.Location) *document.Node {
-	return &document.Node{Kind: document.Number, Text: text, Location: location}
+	return Diagnostic{Code: code, Severity: SeverityError, Path: issue.path.clone(), Message: message}
 }

internal/engine/parse.go

@@ -53,6 +53,9 @@ func parseSource(source Source, sourceID string, limits parseLimits) (parsedSour
 	if len(diagnostics) != 0 || read.info == nil {
 		return parsedSource{source: read.info}, diagnostics
 	}
+	if read.document != nil {
+		return parsedSource{document: read.document, source: read.info, bytes: read.bytes}, nil
+	}
 
 	root, issues := yamladapter.Parse(bytes.NewReader(read.data), sourceID, limits.document)
 	if len(issues) != 0 {

internal/engine/records.go

@@ -6,11 +6,24 @@ type Layer string
 const (
 	LayerDefault    Layer = "default"
 	LayerBase       Layer = "base"
+	LayerUnderlay   Layer = "underlay"
 	LayerOverlay    Layer = "overlay"
 	LayerCLI        Layer = "cli"
 	LayerResolution Layer = "resolution"
 )
 
+// SourceKind identifies source acquisition independently of precedence.
+type SourceKind string
+
+const (
+	SourceKindFile       SourceKind = "file"
+	SourceKindFilesystem SourceKind = "filesystem"
+	SourceKindBytes      SourceKind = "bytes"
+	SourceKindReader     SourceKind = "reader"
+	SourceKindMapping    SourceKind = "mapping"
+	SourceKindCLI        SourceKind = "cli"
+)
+
 // ChangeKind identifies a recorded operation. The zero value is
 // used for input and default origins.
 type ChangeKind string
@@ -18,7 +31,10 @@ type ChangeKind string
 const (
 	// ChangeDecoding records the built-in transition to canonical typed values.
 	// It is inspection metadata, not an operation accepted by Resolution.Record.
-	ChangeDecoding       ChangeKind = "decoding"
+	ChangeDecoding ChangeKind = "decoding"
+	// ChangeInheritance records structural input expansion. It is inspection
+	// metadata, not an operation accepted by Resolution.Record.
+	ChangeInheritance    ChangeKind = "inheritance"
 	ChangeDerivedDefault ChangeKind = "derived_default"
 	ChangeNormalization  ChangeKind = "normalization"
 )
@@ -89,6 +105,7 @@ const (
 	CodeImportRepeated      Code = "import.repeated"
 	CodeImportLimitExceeded Code = "import.limit_exceeded"
 	CodeImportVersion       Code = "import.version"
+	CodeUnderlayVersion     Code = "underlay.version"
 
 	CodeOverlayInvalid       Code = "overlay.invalid"
 	CodeOverlayMissing       Code = "overlay.missing"
@@ -146,6 +163,11 @@ const (
 	CodeOverrideLimitExceeded Code = "override.limit_exceeded"
 
 	CodeExplainLimitExceeded Code = "explain.limit_exceeded"
+
+	CodeInheritanceInvalid Code = "inheritance.invalid"
+	CodeInheritanceMissing Code = "inheritance.missing"
+	CodeInheritanceCycle   Code = "inheritance.cycle"
+	CodeInheritanceVersion Code = "inheritance.version"
 )
 
 // Location identifies a position in a configuration source. Zero line and column
@@ -156,15 +178,19 @@ type Location struct {
 	Column   int
 }
 
-// SourceInfo describes a source discovered during a load. Path is a canonical
-// local path when one exists. ImportedFrom identifies the declaration that
-// included this source.
+// SourceInfo describes a source discovered during a load. Kind identifies
+// acquisition independently of Layer. Path is a canonical host path or a
+// 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
-	Layer        Layer
-	ImportedFrom *Location
+	ID            string
+	Label         string
+	Path          string
+	Layer         Layer
+	Kind          SourceKind
+	UnderlayIndex int
+	ImportedFrom  *Location
+	namespace     *namespaceToken
 }
 
 // Diagnostic is a safe structured problem report. Message and source labels must
@@ -178,13 +204,25 @@ type Diagnostic struct {
 	Related  []Location
 }
 
+// InheritanceStep identifies a concrete payload transfer controlled by a
+// relationship directive. Locations belong to that directive, not its value.
+type InheritanceStep struct {
+	From      Path
+	To        Path
+	Reference Path
+	Locations []Location
+}
+
 // Origin describes where an effective definition came from. Multiple locations
 // represent a composite origin; resolution origins are synthetic and identify their
-// operation instead of a file location.
+// operation instead of a file location. Inheritance is projected from private
+// contributor traces at inspection boundaries.
 type Origin struct {
-	Layer     Layer
-	Locations []Location
-	Operation ChangeKind
+	Layer        Layer
+	Locations    []Location
+	Operation    ChangeKind
+	Inheritance  []InheritanceStep
+	contributors []inheritanceContributor
 }
 
 // FieldInfo describes an effective configuration path.
@@ -238,9 +276,32 @@ func cloneSourceInfos(sources []SourceInfo) []SourceInfo {
 
 func cloneOrigin(origin Origin) Origin {
 	origin.Locations = cloneLocations(origin.Locations)
+	origin.Inheritance = cloneInheritanceSteps(origin.Inheritance)
+	if origin.contributors != nil {
+		contributors := make([]inheritanceContributor, len(origin.contributors))
+		for index, contributor := range origin.contributors {
+			contributors[index] = contributor
+			contributors[index].source = contributor.source.clone()
+		}
+		origin.contributors = contributors
+	}
 	return origin
 }
 
+func cloneInheritanceSteps(steps []InheritanceStep) []InheritanceStep {
+	if steps == nil {
+		return nil
+	}
+	cloned := make([]InheritanceStep, len(steps))
+	for index, step := range steps {
+		cloned[index] = InheritanceStep{
+			From: step.From.clone(), To: step.To.clone(), Reference: step.Reference.clone(),
+			Locations: cloneLocations(step.Locations),
+		}
+	}
+	return cloned
+}
+
 func cloneFieldInfo(info FieldInfo) FieldInfo {
 	info.Origin = cloneOrigin(info.Origin)
 	return info

internal/engine/recursive_merge.go

@@ -0,0 +1,45 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// recursiveMergeHooks lets an assembly owner account for the values changed by
+// a recursive mapping overlay without making source roles part of the merge.
+// Hooks run before replacement and receive the still-owned incoming subtree.
+type recursiveMergeHooks struct {
+	added    func(Path, *document.Node)
+	combined func(Path, *document.Node)
+	replaced func(Path, *document.Node, *document.Node)
+}
+
+// mergeRecursiveMappings changes an owned mapping and never aliases incoming
+// values into it. Entry order follows the target, then new incoming entries.
+func mergeRecursiveMappings(target, incoming *document.Node, current Path, index *mappingIndex, hooks recursiveMergeHooks) {
+	for _, entry := range incoming.Mapping {
+		path := current.Key(entry.Key)
+		position := index.find(target, entry.Key)
+		if position < 0 {
+			cloned := entry
+			cloned.Value = document.Clone(entry.Value)
+			index.append(target, cloned)
+			if hooks.added != nil {
+				hooks.added(path, entry.Value)
+			}
+			continue
+		}
+
+		existing := target.Mapping[position].Value
+		if existing.Kind == document.Mapping && entry.Value.Kind == document.Mapping {
+			if hooks.combined != nil {
+				hooks.combined(path, entry.Value)
+			}
+			mergeRecursiveMappings(existing, entry.Value, path, index, hooks)
+			continue
+		}
+
+		if hooks.replaced != nil {
+			hooks.replaced(path, existing, entry.Value)
+		}
+		target.Mapping[position] = entry
+		target.Mapping[position].Value = document.Clone(entry.Value)
+	}
+}

internal/engine/recursive_merge_test.go

@@ -0,0 +1,90 @@
+package engine
+
+import (
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestRecursiveMappingMergePreservesValuesPathsAndInputs(t *testing.T) {
+	type expectedNode struct {
+		path    Path
+		kind    document.Kind
+		text    string
+		entries int
+	}
+	tests := []struct {
+		name, base, incoming string
+		want                 []expectedNode
+		events               []string
+	}{
+		{
+			name: "nested mappings", base: "shared: {kept: base, changed: old}\n", incoming: "shared: {changed: new, added: child}\n",
+			want: []expectedNode{
+				{path: Keys("shared"), kind: document.Mapping, entries: 3},
+				{path: Keys("shared", "kept"), kind: document.String, text: "base"},
+				{path: Keys("shared", "changed"), kind: document.String, text: "new"},
+				{path: Keys("shared", "added"), kind: document.String, text: "child"},
+			},
+			events: []string{"combine:/shared", "replace:/shared/changed", "add:/shared/added"},
+		},
+		{
+			name: "sequence replacement", base: "value: [old]\n", incoming: "value: []\n",
+			want:   []expectedNode{{path: Keys("value"), kind: document.Sequence}},
+			events: []string{"replace:/value"},
+		},
+		{
+			name: "scalar replacement", base: "value: old\n", incoming: "value: new\n",
+			want:   []expectedNode{{path: Keys("value"), kind: document.String, text: "new"}},
+			events: []string{"replace:/value"},
+		},
+		{
+			name: "null replacement", base: "value: old\n", incoming: "value: null\n",
+			want:   []expectedNode{{path: Keys("value"), kind: document.Null}},
+			events: []string{"replace:/value"},
+		},
+		{
+			name: "empty mapping retains children", base: "shared: {kept: base}\n", incoming: "shared: {}\n",
+			want: []expectedNode{
+				{path: Keys("shared"), kind: document.Mapping, entries: 1},
+				{path: Keys("shared", "kept"), kind: document.String, text: "base"},
+			},
+			events: []string{"combine:/shared"},
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			base := parseCompositionDocument(t, "source-1", test.base)
+			incoming := parseCompositionDocument(t, "source-2", test.incoming)
+			baseBefore, incomingBefore := document.Clone(base), document.Clone(incoming)
+			owned := document.Clone(base)
+			var events []string
+			mergeRecursiveMappings(owned, incoming, Path{}, new(mappingIndex), recursiveMergeHooks{
+				added: func(path Path, _ *document.Node) { events = append(events, "add:"+path.String()) },
+				combined: func(path Path, _ *document.Node) {
+					events = append(events, "combine:"+path.String())
+				},
+				replaced: func(path Path, _, _ *document.Node) {
+					events = append(events, "replace:"+path.String())
+				},
+			})
+			if !reflect.DeepEqual(events, test.events) {
+				t.Fatalf("merge events = %#v, want %#v", events, test.events)
+			}
+			for _, expected := range test.want {
+				node, found := documentAtPath(owned, expected.path)
+				if !found || node.Kind != expected.kind || node.Text != expected.text || len(node.Mapping) != expected.entries {
+					t.Errorf("merged %s = %#v, found %v", expected.path.String(), node, found)
+				}
+			}
+			if !reflect.DeepEqual(base, baseBefore) || !reflect.DeepEqual(incoming, incomingBefore) {
+				t.Fatal("merge changed an input")
+			}
+			owned.Mapping[0].Value.Location.SourceID = "changed"
+			if !reflect.DeepEqual(base, baseBefore) || !reflect.DeepEqual(incoming, incomingBefore) {
+				t.Fatal("merged result aliases an input")
+			}
+		})
+	}
+}

internal/engine/resolution.go

@@ -41,7 +41,7 @@ type resolutionResult struct {
 	diagnostics []Diagnostic
 }
 
-// IsExplicit reports whether surviving input explicitly defined path.
+// IsExplicit reports whether surviving direct or inherited input defined path.
 func (r *Resolution) IsExplicit(path Path) bool {
 	state := r.activeState()
 	if !state.validPath(path) {
@@ -72,6 +72,12 @@ func (r *Resolution) Source(id string) (SourceInfo, bool) {
 	return r.activeState().input.source(id)
 }
 
+// SourceContext returns a passive context during the active resolution
+// callback. Retaining the callback context after return is unsupported.
+func (r *Resolution) SourceContext(id string) (ResourceContext, bool) {
+	return r.activeState().input.sourceContext(id)
+}
+
 // Record declares the intent for a resolver mutation. A container path covers
 // mutations to its descendants.
 func (r *Resolution) Record(path Path, kind ChangeKind, reason string, dependencies ...Path) {

internal/engine/resource_context.go

@@ -0,0 +1,133 @@
+package engine
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+)
+
+// ResourceContext is a passive copy of one acquired source's rooted filesystem
+// and slash-relative base directory. FS is borrowed and opens resources only
+// when the application explicitly requests them.
+type ResourceContext struct {
+	FS      fs.FS
+	BaseDir string
+}
+
+func cloneResourceContexts(contexts map[string]ResourceContext) map[string]ResourceContext {
+	if contexts == nil {
+		return nil
+	}
+	cloned := make(map[string]ResourceContext, len(contexts))
+	for id, context := range contexts {
+		cloned[id] = context
+	}
+	return cloned
+}
+
+// rootedNativeFS borrows a namespace without invoking SubFS or opening it at
+// query time. All resource names remain relative to its configured root.
+type rootedNativeFS struct {
+	filesystem fs.FS
+	root       string
+}
+
+func (r rootedNativeFS) name(name string) (string, error) {
+	if !fs.ValidPath(name) {
+		return "", &fs.PathError{Op: "open", Path: name, Err: fs.ErrInvalid}
+	}
+	joined := path.Join(r.root, name)
+	if !fs.ValidPath(joined) || r.root != "." && joined != r.root && !strings.HasPrefix(joined, r.root+"/") {
+		return "", &fs.PathError{Op: "open", Path: name, Err: fs.ErrPermission}
+	}
+	return joined, nil
+}
+
+func (r rootedNativeFS) Open(name string) (fs.File, error) {
+	joined, err := r.name(name)
+	if err != nil {
+		return nil, err
+	}
+	return r.filesystem.Open(joined)
+}
+
+func (r rootedNativeFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	joined, err := r.name(name)
+	if err != nil {
+		return nil, err
+	}
+	return fs.ReadDir(r.filesystem, joined)
+}
+
+// rootedHostFS stores only a canonical root path. No os.Root is retained after
+// loading or created by a SourceContext query.
+type hostResourceRoot interface {
+	Open(string) (*os.File, error)
+	Close() error
+}
+
+type rootedHostFS struct {
+	root     string
+	openRoot func(string) (hostResourceRoot, error)
+}
+
+func (r rootedHostFS) Open(name string) (fs.File, error) {
+	localName, err := filepath.Localize(name)
+	if err != nil {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrInvalid}
+	}
+	opener := r.openRoot
+	if opener == nil {
+		opener = func(name string) (hostResourceRoot, error) { return os.OpenRoot(name) }
+	}
+	root, err := opener(r.root)
+	if err != nil {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: err}
+	}
+	canonical, err := canonicalExistingPath(filepath.Join(r.root, localName))
+	if err != nil {
+		_ = root.Close()
+		return nil, &fs.PathError{Op: "open", Path: name, Err: err}
+	}
+	if !containedPath(r.root, canonical) {
+		_ = root.Close()
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrPermission}
+	}
+	relative, err := filepath.Rel(r.root, canonical)
+	if err != nil {
+		_ = root.Close()
+		return nil, &fs.PathError{Op: "open", Path: name, Err: err}
+	}
+	file, err := root.Open(relative)
+	if err != nil {
+		_ = root.Close()
+		return nil, &fs.PathError{Op: "open", Path: name, Err: err}
+	}
+	owned := rootedHostFile{File: file, root: root}
+	var opened fs.File = file
+	if directory, ok := opened.(fs.ReadDirFile); ok {
+		return &rootedHostDirFile{rootedHostFile: owned, directory: directory}, nil
+	}
+	return &owned, nil
+}
+
+type rootedHostFile struct {
+	fs.File
+	root hostResourceRoot
+}
+
+func (f *rootedHostFile) Close() error {
+	return errors.Join(f.File.Close(), f.root.Close())
+}
+
+type rootedHostDirFile struct {
+	rootedHostFile
+	directory fs.ReadDirFile
+}
+
+func (f *rootedHostDirFile) ReadDir(count int) ([]fs.DirEntry, error) {
+	return f.directory.ReadDir(count)
+}

internal/engine/resource_context_test.go

@@ -0,0 +1,169 @@
+package engine
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+	"testing/fstest"
+)
+
+func TestResourceContextsPreserveLiteralBackslashes(t *testing.T) {
+	const directory = `assets\literal`
+	const name = directory + `/text\file`
+	check := func(t *testing.T, filesystem fs.FS) {
+		t.Helper()
+		data, err := fs.ReadFile(filesystem, name)
+		if err != nil || string(data) != "content" {
+			t.Fatalf("read = %q, %v", data, err)
+		}
+		entries, err := fs.ReadDir(filesystem, directory)
+		if err != nil || len(entries) != 1 || entries[0].Name() != `text\file` {
+			t.Fatalf("entries = %v, %v", entries, err)
+		}
+		if _, err := filesystem.Open("../outside"); !errors.Is(err, fs.ErrInvalid) {
+			t.Fatalf("escape = %v", err)
+		}
+	}
+	t.Run("native", func(t *testing.T) {
+		check(t, rootedNativeFS{filesystem: fstest.MapFS{"bundle/" + name: &fstest.MapFile{Data: []byte("content")}}, root: "bundle"})
+	})
+	t.Run("host", func(t *testing.T) {
+		if runtime.GOOS == "windows" {
+			t.Skip("backslash is a host separator on Windows; native case remains covered")
+		}
+		root := t.TempDir()
+		if err := os.Mkdir(filepath.Join(root, directory), 0o700); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(filepath.Join(root, filepath.FromSlash(name)), []byte("content"), 0o600); err != nil {
+			t.Fatal(err)
+		}
+		check(t, rootedHostFS{root: root})
+	})
+}
+
+type trackedHostRoot struct {
+	root     *os.Root
+	closed   *int
+	failOpen bool
+}
+
+func (r *trackedHostRoot) Open(name string) (*os.File, error) {
+	if r.failOpen {
+		return nil, fs.ErrPermission
+	}
+	return r.root.Open(name)
+}
+func (r *trackedHostRoot) Close() error {
+	(*r.closed)++
+	return r.root.Close()
+}
+
+func TestRootedHostFSClosesRootOnSuccessfulAndFailedOpen(t *testing.T) {
+	rootPath := t.TempDir()
+	if err := os.WriteFile(filepath.Join(rootPath, "resource.txt"), []byte("resource"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	opened, closed := 0, 0
+	failOpen := false
+	filesystem := rootedHostFS{root: rootPath, openRoot: func(name string) (hostResourceRoot, error) {
+		root, err := os.OpenRoot(name)
+		if err != nil {
+			return nil, err
+		}
+		opened++
+		return &trackedHostRoot{root: root, closed: &closed, failOpen: failOpen}, nil
+	}}
+	if _, err := filesystem.Open("../resource.txt"); !errors.Is(err, fs.ErrInvalid) || opened != 0 {
+		t.Fatalf("invalid path = %v; roots opened %d", err, opened)
+	}
+	if _, err := filesystem.Open("missing.txt"); !errors.Is(err, fs.ErrNotExist) || opened != 1 || closed != 1 {
+		t.Fatalf("missing resource = %v; opened %d, closed %d", err, opened, closed)
+	}
+	failOpen = true
+	if _, err := filesystem.Open("resource.txt"); !errors.Is(err, fs.ErrPermission) || opened != 2 || closed != 2 {
+		t.Fatalf("failed inner open = %v; opened %d, closed %d", err, opened, closed)
+	}
+	failOpen = false
+	file, err := filesystem.Open("resource.txt")
+	if err != nil || opened != 3 || closed != 2 {
+		t.Fatalf("resource open = %v; opened %d, closed %d", err, opened, closed)
+	}
+	if err := file.Close(); err != nil || closed != 3 {
+		t.Fatalf("resource close = %v; opened %d, closed %d", err, opened, closed)
+	}
+	if _, err := file.Read(make([]byte, 1)); err == nil {
+		t.Fatal("closed resource file remained readable")
+	}
+	directory, err := filesystem.Open(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	reader, ok := directory.(fs.ReadDirFile)
+	if !ok {
+		t.Fatal("rooted host directory did not expose ReadDirFile")
+	}
+	entries, err := reader.ReadDir(-1)
+	if err != nil || len(entries) != 1 || entries[0].Name() != "resource.txt" {
+		t.Fatalf("directory entries = %#v, %v", entries, err)
+	}
+	if err := directory.Close(); err != nil || opened != 4 || closed != 4 {
+		t.Fatalf("directory close = %v; opened %d, closed %d", err, opened, closed)
+	}
+}
+
+func TestNativeResourceViewIsBorrowedAndRooted(t *testing.T) {
+	filesystem := &observedFilesystem{files: fstest.MapFS{
+		"bundle/config/resource.txt": &fstest.MapFile{Data: []byte("content")},
+	}}
+	view := rootedNativeFS{filesystem: filesystem, root: "bundle"}
+	if filesystem.opened != 0 {
+		t.Fatal("constructing a native resource view opened files")
+	}
+	data, err := fs.ReadFile(view, "config/resource.txt")
+	if err != nil || string(data) != "content" {
+		t.Fatalf("resource = %q, %v", data, err)
+	}
+	filesystem.assertClosed(t)
+	if _, err := fs.ReadFile(view, "../outside.txt"); !errors.Is(err, fs.ErrInvalid) {
+		t.Fatalf("resource escape = %v", err)
+	}
+}
+
+func TestLoadingClosesNativeFilesAcrossPipelineOutcomes(t *testing.T) {
+	type config struct {
+		Value   string `yaml:"value"`
+		Version string `yaml:"version"`
+	}
+	for _, test := range []struct {
+		name     string
+		main     string
+		child    string
+		failRead string
+		options  Options[config]
+		wantErr  bool
+	}{
+		{name: "success", main: "value: retained\n"},
+		{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: "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},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			files := fstest.MapFS{"main.yml": &fstest.MapFile{Data: []byte(test.main)}}
+			if test.child != "" {
+				files["child.yml"] = &fstest.MapFile{Data: []byte(test.child)}
+			}
+			filesystem := &observedFilesystem{files: files, failRead: test.failRead}
+			_, err := testLoad(filesystemFile(newNativeNamespace("bundle", filesystem), "main.yml"), test.options)
+			if (err != nil) != test.wantErr {
+				t.Fatalf("load error = %v, want failure %v", err, test.wantErr)
+			}
+			filesystem.assertClosed(t)
+		})
+	}
+}

internal/engine/secret.go

@@ -34,6 +34,35 @@ func classifySecretPaths(schema *schemaDescriptor, predicate func(Path) bool, me
 			}
 		}
 	}
+	var inherited []secretDependency
+	for _, operation := range metadata.inheritance {
+		if operation.control != nil {
+			control := operation.control.path
+			paths[control.String()] = control.clone()
+			for _, contributor := range operation.controlOrigin.contributors {
+				paths[contributor.source.String()] = contributor.source.clone()
+				inherited = append(inherited, secretDependency{target: control.clone(), dependency: contributor.source.clone()})
+			}
+			continue
+		}
+		if operation.transfer == nil {
+			continue
+		}
+		for pointer := range operation.parentOrigins {
+			source, err := ParsePath(pointer)
+			if err != nil {
+				continue
+			}
+			relative, ok := relativeHistoryPath(operation.transfer.from, source)
+			if !ok {
+				continue
+			}
+			destination := appendInheritancePath(operation.transfer.to, relative)
+			paths[pointer] = source
+			paths[destination.String()] = destination.clone()
+			inherited = append(inherited, secretDependency{target: destination, dependency: source})
+		}
+	}
 
 	secret := make(map[string]bool)
 	for _, tree := range append(projections, final) {
@@ -62,6 +91,16 @@ func classifySecretPaths(schema *schemaDescriptor, predicate func(Path) bool, me
 				changed = true
 			}
 		}
+		for _, link := range inherited {
+			if pathIsSecret(secret, link.dependency) && !secret[link.target.String()] {
+				secret[link.target.String()] = true
+				changed = true
+			}
+			if pathIsSecret(secret, link.target) && !secret[link.dependency.String()] {
+				secret[link.dependency.String()] = true
+				changed = true
+			}
+		}
 		if !changed {
 			return secret
 		}

internal/engine/selected_overlay.go

@@ -4,10 +4,7 @@ import (
 	"bytes"
 	"errors"
 	"io/fs"
-	"os"
-	"path/filepath"
 	"strconv"
-	"strings"
 
 	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
 )
@@ -32,12 +29,47 @@ func validOverlayName(name string) bool {
 }
 
 func loadSelectedOverlayDocument(source Source, context *FilesystemContext, main parsedSource, base *composedDocument, sources []SourceInfo, name string, rule *VersionRule, limits importLimits) (*composedDocument, []SourceInfo, []Diagnostic) {
+	assembly := newAssemblyAccumulator(cloneComposedDocument(base))
+	allSources, diagnostics := loadSelectedOverlayInto(source, context, main, assembly, sources, name, rule, limits)
+	if len(diagnostics) != 0 {
+		return nil, allSources, diagnostics
+	}
+	return assembly.current, allSources, nil
+}
+
+func loadSelectedOverlayInto(source Source, context *FilesystemContext, main parsedSource, base *assemblyAccumulator, sources []SourceInfo, name string, rule *VersionRule, limits importLimits) ([]SourceInfo, []Diagnostic) {
+	if nativeContext(source, context) {
+		access, basePath, mainPath, diagnostic := nativeFilesystemAccess(source, context)
+		if diagnostic != nil {
+			return cloneSourceInfos(sources), []Diagnostic{*diagnostic}
+		}
+		if diagnostic := validateNativeDirectories(access, basePath, mainPath); diagnostic != nil {
+			return cloneSourceInfos(sources), []Diagnostic{*diagnostic}
+		}
+		return loadSelectedOverlayWithAccess(access, base, sources, name, rule, limits)
+	}
 	rootPath, _, _, diagnostic := importFilesystem(source, context, main)
 	if diagnostic != nil {
-		return nil, cloneSourceInfos(sources), []Diagnostic{*diagnostic}
+		return cloneSourceInfos(sources), []Diagnostic{*diagnostic}
 	}
-	overlayPath := filepath.Join(rootPath, "overlays", name+".yml")
-	canonical, err := canonicalExistingPath(overlayPath)
+	access, err := openHostAccess(rootPath)
+	if err != nil {
+		value := globFilesystemDiagnostic(err, "configuration root could not be opened for the selected overlay", Location{})
+		value.Primary = nil
+		return cloneSourceInfos(sources), []Diagnostic{value}
+	}
+	return loadSelectedOverlayWithAccess(access, base, sources, name, rule, limits)
+}
+
+func loadSelectedOverlayWithAccess(access *filesystemAccess, base *assemblyAccumulator, sources []SourceInfo, name string, rule *VersionRule, limits importLimits) ([]SourceInfo, []Diagnostic) {
+	closed := false
+	defer func() {
+		if !closed {
+			_ = access.close()
+		}
+	}()
+	overlayPath := access.join(access.rootPath, "overlays/"+name+".yml")
+	canonical, err := access.canonical(overlayPath)
 	if err != nil {
 		code := CodeOverlayInvalid
 		message := "selected overlay could not be resolved"
@@ -46,83 +78,81 @@ func loadSelectedOverlayDocument(source Source, context *FilesystemContext, main
 		} else if errors.Is(err, fs.ErrPermission) {
 			code, message = CodeImportPermission, "selected overlay cannot be accessed"
 		}
-		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(code, message, nil)}
+		return cloneSourceInfos(sources), []Diagnostic{importDiagnostic(code, message, nil)}
 	}
-	if !containedPath(rootPath, canonical) {
-		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayOutsideRoot, "selected overlay resolves outside the configuration root", nil)}
+	if !access.contains(canonical) {
+		return cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayOutsideRoot, "selected overlay resolves outside the configuration root", nil)}
 	}
 	for _, existing := range sources {
-		if existing.Path != "" && existing.Path == canonical {
-			return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayRepeated, "selected overlay is already present in the base source graph", nil)}
+		if existing.Path != "" && existing.Path == canonical &&
+			((access.native == nil && existing.Kind == SourceKindFile) ||
+				(access.native != nil && existing.Kind == SourceKindFilesystem && existing.namespace == access.token)) {
+			return cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayRepeated, "selected overlay is already present in the base source graph", nil)}
 		}
 	}
 	if len(sources)+1 > limits.sources {
-		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayLimitExceeded, "selected overlay exceeds the source-count limit", nil)}
+		return cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayLimitExceeded, "selected overlay exceeds the source-count limit", nil)}
 	}
 
-	root, err := os.OpenRoot(rootPath)
-	if err != nil {
-		value := globFilesystemDiagnostic(err, "configuration root could not be opened for the selected overlay", Location{})
-		value.Primary = nil
-		return nil, cloneSourceInfos(sources), []Diagnostic{value}
-	}
-	rootClosed := false
-	defer func() {
-		if !rootClosed {
-			_ = root.Close()
-		}
-	}()
-	relative, err := filepath.Rel(rootPath, canonical)
-	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
-		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayOutsideRoot, "selected overlay could not be addressed inside the configuration root", nil)}
+	if _, err := access.relative(canonical); err != nil {
+		return cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayOutsideRoot, "selected overlay could not be addressed inside the configuration root", nil)}
 	}
 
 	sourceID := "source-" + strconv.Itoa(len(sources)+1)
 	overlayLocation := &Location{SourceID: sourceID}
-	overlaySource := SourceInfo{ID: sourceID, Label: filepath.ToSlash(filepath.Join("overlays", name+".yml")), Path: canonical, Layer: LayerOverlay}
+	kind := SourceKindFile
+	label := "overlays/" + name + ".yml"
+	if access.native != nil {
+		kind = SourceKindFilesystem
+		label = access.label + "/" + canonical
+	}
+	overlaySource := SourceInfo{ID: sourceID, Label: label, Path: canonical, Layer: LayerOverlay, Kind: kind}
+	if kind == SourceKindFilesystem {
+		overlaySource.namespace = access.token
+	}
 	sources = append(cloneSourceInfos(sources), overlaySource)
-	data, diagnostic := readOverlayFile(root, relative, sourceID, limits.bytes-base.inputBytes)
+	data, diagnostic := readOverlayFile(access, canonical, sourceID, limits.bytes-base.current.inputBytes)
 	if diagnostic != nil {
-		return nil, sources, []Diagnostic{*diagnostic}
+		return sources, []Diagnostic{*diagnostic}
 	}
 	overlayRoot, issues := yamladapter.Parse(bytes.NewReader(data), sourceID, yamladapter.Limits{Depth: maxDocumentDepth, Nodes: maxDocumentNodes})
 	if len(issues) != 0 {
-		return nil, sources, parserDiagnostics(sourceID, issues)
+		return sources, parserDiagnostics(sourceID, issues)
 	}
 	nodeCount := countInputNodes(overlayRoot)
-	if base.inputNodes+nodeCount > limits.nodes {
-		return nil, sources, []Diagnostic{importDiagnostic(CodeOverlayLimitExceeded, "selected overlay exceeds the aggregate node limit", overlayLocation)}
+	if base.current.inputNodes+nodeCount > limits.nodes {
+		return sources, []Diagnostic{importDiagnostic(CodeOverlayLimitExceeded, "selected overlay exceeds the aggregate node limit", overlayLocation)}
 	}
 	if diagnostics := overlayDirectiveDiagnostics(overlayRoot, Path{}); len(diagnostics) != 0 {
-		return nil, sources, diagnostics
+		return sources, diagnostics
 	}
 	if versionNode := fragmentVersionClaim(overlayRoot, rule); versionNode != nil {
 		location := publicLocation(versionNode.Location)
-		return nil, sources, []Diagnostic{{
+		return sources, []Diagnostic{{
 			Code: CodeOverlayVersion, Severity: SeverityError, Path: rule.Path.clone(),
 			Message: "overlays cannot contribute to or replace the schema-version path", Primary: &location,
 		}}
 	}
-	closeErr := root.Close()
-	rootClosed = true
+	closeErr := access.close()
+	closed = true
 	if closeErr != nil {
-		return nil, sources, []Diagnostic{importDiagnostic(CodeImportRead, "configuration root could not be closed after reading the selected overlay", nil)}
+		return sources, []Diagnostic{importDiagnostic(CodeImportRead, "configuration root could not be closed after reading the selected overlay", nil)}
 	}
 
-	overlayd, diagnostics := overlayDocuments(base, overlayRoot)
-	if overlayd != nil {
-		overlayd.inputBytes = base.inputBytes + int64(len(data))
-		overlayd.inputNodes = base.inputNodes + nodeCount
+	if diagnostics := base.applyOverlay(overlayRoot, nil, nil, LayerOverlay); len(diagnostics) != 0 {
+		return sources, diagnostics
 	}
-	return overlayd, sources, diagnostics
+	base.current.inputBytes += int64(len(data))
+	base.current.inputNodes += nodeCount
+	return sources, nil
 }
 
-func readOverlayFile(root *os.Root, relative, sourceID string, remaining int64) ([]byte, *Diagnostic) {
+func readOverlayFile(access *filesystemAccess, name, sourceID string, remaining int64) ([]byte, *Diagnostic) {
 	limit := min(int64(maxSourceBytes), remaining)
 	if limit < 0 {
 		limit = 0
 	}
-	data, failure := readRegularFile(relative, root.Stat, root.Open, limit)
+	data, failure := readRegularFile(name, access.stat, access.open, limit)
 	if failure != nil {
 		value := sourceReadDiagnostic(failure, sourceID, CodeOverlayLimitExceeded, "selected overlay exceeds a source or aggregate byte limit")
 		return nil, &value

internal/engine/source.go

@@ -9,6 +9,8 @@ import (
 	"path/filepath"
 	"reflect"
 	"sync/atomic"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
 )
 
 const (
@@ -16,14 +18,19 @@ const (
 	maxTotalBytes  = 64 << 20
 )
 
-// Source is an opaque configuration input. Construct sources with File, Bytes,
-// Reader, or NoSource; the zero value is invalid.
+// Source is an opaque configuration input. Construct sources with File,
+// Filesystem.File, Bytes, Reader, or NoSource; the zero value is invalid.
 type Source struct {
 	kind      sourceKind
 	path      string
 	label     string
+	namespace *nativeNamespace
 	data      []byte
 	reader    *readerSource
+	mapping   *document.Node
+	mapIssue  *valueIssue
+	mapBytes  int64
+	mapNodes  int
 	createErr sourceCreationError
 }
 
@@ -32,8 +39,10 @@ type sourceKind uint8
 const (
 	sourceInvalid sourceKind = iota
 	sourceFile
+	sourceFilesystem
 	sourceBytes
 	sourceReader
+	sourceMapping
 	sourceNone
 )
 
@@ -45,8 +54,53 @@ const (
 	creationInvalidLabel
 	creationInvalidReader
 	creationTooLarge
+	creationInvalidFilesystem
+	creationInvalidFilesystemLabel
+	creationInvalidFilesystemPath
+	creationInvalidMappingRoot
 )
 
+// Filesystem is a reusable opaque namespace for caller-owned fs.FS inputs.
+// Copies preserve the namespace identity used for explicit contexts.
+type Filesystem struct {
+	namespace *nativeNamespace
+	createErr sourceCreationError
+}
+
+// NewFilesystem records a borrowed filesystem without opening it. Invalid
+// arguments are reported when a File source is consumed.
+func NewFilesystem(label string, filesystem fs.FS) Filesystem {
+	result := Filesystem{}
+	if !validOverrideLabel(label) {
+		result.createErr = creationInvalidFilesystemLabel
+	} else if nilFilesystem(filesystem) {
+		result.createErr = creationInvalidFilesystem
+	} else {
+		result.namespace = newNativeNamespace(label, filesystem)
+	}
+	return result
+}
+
+// File selects a non-root slash-relative file within this namespace.
+func (f Filesystem) File(name string) Source {
+	if f.createErr != creationValid {
+		return Source{kind: sourceFilesystem, createErr: f.createErr}
+	}
+	return filesystemFile(f.namespace, name)
+}
+
+func nilFilesystem(filesystem fs.FS) bool {
+	if filesystem == nil {
+		return true
+	}
+	value := reflect.ValueOf(filesystem)
+	switch value.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
+		return value.IsNil()
+	}
+	return false
+}
+
 type readerSource struct {
 	input io.Reader
 	used  atomic.Bool
@@ -60,6 +114,16 @@ func File(path string) Source {
 	return Source{kind: sourceFile, path: path, label: path}
 }
 
+func filesystemFile(namespace *nativeNamespace, name string) Source {
+	if namespace == nil || nilFilesystem(namespace.filesystem) {
+		return Source{kind: sourceFilesystem, createErr: creationInvalidFilesystem}
+	}
+	if name == "." || !fs.ValidPath(name) {
+		return Source{kind: sourceFilesystem, createErr: creationInvalidFilesystemPath}
+	}
+	return Source{kind: sourceFilesystem, path: name, label: namespace.label + "/" + name, namespace: namespace}
+}
+
 // Bytes snapshots data immediately. Label must be nonempty and safe to display;
 // constructor problems are reported when the source is consumed.
 func Bytes(label string, data []byte) Source {
@@ -92,6 +156,24 @@ func Reader(label string, input io.Reader) Source {
 	return source
 }
 
+// Mapping snapshots supported built-in values into an owned document at
+// construction. Invalid values are retained as safe deferred diagnostics.
+func Mapping(label string, values map[string]any) Source {
+	source := Source{kind: sourceMapping, label: label}
+	if !validOverrideLabel(label) {
+		source.createErr = creationInvalidLabel
+		return source
+	}
+	if values == nil {
+		source.createErr = creationInvalidMappingRoot
+		return source
+	}
+	builder := valueBuilder{active: make(map[valueVisit]bool)}
+	source.mapping, source.mapIssue = builder.build(values, Path{}, 0)
+	source.mapBytes, source.mapNodes = builder.bytes, builder.nodes
+	return source
+}
+
 // NoSource explicitly selects a defaults-only input. It creates no source record.
 func NoSource() Source {
 	return Source{kind: sourceNone}
@@ -107,14 +189,24 @@ func defaultSourceLimits() sourceLimits {
 }
 
 type sourceRead struct {
-	data []byte
-	info *SourceInfo
+	data     []byte
+	document *document.Node
+	bytes    int64
+	info     *SourceInfo
 }
 
 func readSource(source Source, sourceID string, limits sourceLimits) (sourceRead, []Diagnostic) {
 	if diagnostic := sourceConstructorDiagnostic(source); diagnostic != nil {
+		if source.kind == sourceMapping {
+			diagnostic.Primary = &Location{SourceID: sourceID}
+			return sourceRead{info: mappingSourceInfo(source, sourceID)}, []Diagnostic{*diagnostic}
+		}
 		return sourceRead{}, []Diagnostic{*diagnostic}
 	}
+	if source.kind == sourceMapping && source.mapIssue != nil {
+		diagnostic := source.mapIssue.sourceDiagnostic(sourceID)
+		return sourceRead{info: mappingSourceInfo(source, sourceID)}, []Diagnostic{diagnostic}
+	}
 	if source.kind == sourceNone {
 		return sourceRead{}, nil
 	}
@@ -123,7 +215,7 @@ func readSource(source Source, sourceID string, limits sourceLimits) (sourceRead
 	if limit < 0 {
 		limit = 0
 	}
-	info := &SourceInfo{ID: sourceID, Label: source.label, Layer: LayerBase}
+	info := &SourceInfo{ID: sourceID, Label: source.label, Layer: LayerBase, Kind: sourceSourceKind(source.kind)}
 
 	var input io.Reader
 	switch source.kind {
@@ -143,6 +235,20 @@ func readSource(source Source, sourceID string, limits sourceLimits) (sourceRead
 			return sourceRead{info: info}, []Diagnostic{sourceReadDiagnostic(failure, sourceID, CodeSourceLimitExceeded, "source exceeds the configured byte limit")}
 		}
 		return sourceRead{data: bytes.Clone(data), info: info}, nil
+	case sourceFilesystem:
+		access := newNativeAccess(source.namespace.filesystem, ".", source.namespace.token)
+		info.Path = source.path
+		info.namespace = source.namespace.token
+		canonical, err := access.canonical(source.path)
+		if err != nil {
+			return sourceRead{info: info}, []Diagnostic{classifySourceError(err, sourceID)}
+		}
+		info.Path = canonical
+		data, failure := readRegularFile(canonical, access.stat, access.open, limit)
+		if failure != nil {
+			return sourceRead{info: info}, []Diagnostic{sourceReadDiagnostic(failure, sourceID, CodeSourceLimitExceeded, "source exceeds the configured byte limit")}
+		}
+		return sourceRead{data: bytes.Clone(data), info: info}, nil
 	case sourceBytes:
 		input = bytes.NewReader(source.data)
 	case sourceReader:
@@ -150,6 +256,13 @@ func readSource(source Source, sourceID string, limits sourceLimits) (sourceRead
 			return sourceRead{info: info}, []Diagnostic{sourceDiagnostic(CodeSourceReaderUsed, sourceID, "reader source has already been consumed")}
 		}
 		input = source.reader.input
+	case sourceMapping:
+		if source.mapBytes > limit || source.mapNodes > maxDocumentNodes {
+			return sourceRead{info: info}, []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, sourceID, "mapping source exceeds the configured input limit")}
+		}
+		owned := document.Clone(source.mapping)
+		setDocumentSourceID(owned, sourceID)
+		return sourceRead{document: owned, bytes: source.mapBytes, info: info}, nil
 	default:
 		return sourceRead{info: info}, []Diagnostic{sourceDiagnostic(CodeSourceInvalid, sourceID, "source is invalid")}
 	}
@@ -160,6 +273,14 @@ func readSource(source Source, sourceID string, limits sourceLimits) (sourceRead
 	return sourceRead{data: bytes.Clone(data), info: info}, nil
 }
 
+func mappingSourceInfo(source Source, sourceID string) *SourceInfo {
+	label := source.label
+	if !validOverrideLabel(label) {
+		label = "mapping"
+	}
+	return &SourceInfo{ID: sourceID, Label: label, Layer: LayerBase, Kind: SourceKindMapping}
+}
+
 func sourceConstructorDiagnostic(source Source) *Diagnostic {
 	var message string
 	code := CodeSourceInvalid
@@ -173,16 +294,60 @@ func sourceConstructorDiagnostic(source Source) *Diagnostic {
 		message = "source path must not be empty"
 	case creationInvalidLabel:
 		message = "source label must not be empty"
+		if source.kind == sourceMapping {
+			message = "mapping source label must be nonempty and safe to display"
+		}
 	case creationInvalidReader:
 		message = "source reader must not be nil"
 	case creationTooLarge:
 		code = CodeSourceLimitExceeded
 		message = "source exceeds the configured byte limit"
+	case creationInvalidFilesystem:
+		message = "filesystem must not be nil"
+	case creationInvalidFilesystemLabel:
+		message = "filesystem label must be nonempty and safe to display"
+	case creationInvalidFilesystemPath:
+		message = "filesystem source path must be a non-root slash-relative valid path"
+	case creationInvalidMappingRoot:
+		message = "mapping source root must be a non-nil mapping"
 	}
 	diagnostic := sourceDiagnostic(code, "", message)
 	return &diagnostic
 }
 
+func sourceSourceKind(kind sourceKind) SourceKind {
+	switch kind {
+	case sourceFile:
+		return SourceKindFile
+	case sourceFilesystem:
+		return SourceKindFilesystem
+	case sourceBytes:
+		return SourceKindBytes
+	case sourceReader:
+		return SourceKindReader
+	case sourceMapping:
+		return SourceKindMapping
+	default:
+		return ""
+	}
+}
+
+func setDocumentSourceID(node *document.Node, sourceID string) {
+	if node == nil {
+		return
+	}
+	node.Location.SourceID = sourceID
+	for index := range node.Mapping {
+		entry := &node.Mapping[index]
+		entry.KeyLocation.SourceID = sourceID
+		entry.ValueLocation.SourceID = sourceID
+		setDocumentSourceID(entry.Value, sourceID)
+	}
+	for _, child := range node.Sequence {
+		setDocumentSourceID(child, sourceID)
+	}
+}
+
 func classifySourceError(err error, sourceID string) Diagnostic {
 	switch {
 	case errors.Is(err, fs.ErrNotExist):

internal/engine/source_context.go

@@ -0,0 +1,112 @@
+package engine
+
+import (
+	"io/fs"
+	"path"
+	"path/filepath"
+	"strings"
+)
+
+func contextsForAcquiredSources(source Source, context *FilesystemContext, baseSource *SourceInfo, sources []SourceInfo) (map[string]ResourceContext, *Diagnostic) {
+	if len(sources) == 0 {
+		return nil, nil
+	}
+	var rootPath, basePath string
+	var filesystem fs.FS
+	if source.kind == sourceFilesystem {
+		filesystem = source.namespace.filesystem
+		rootPath, basePath = ".", path.Dir(source.path)
+		if context != nil && context.Root != "" {
+			rootPath = context.Root
+		}
+	} else if context != nil && context.FS != nil {
+		filesystem = context.FS.namespace.filesystem
+		rootPath, basePath = context.Root, context.BaseDir
+	} else if source.kind == sourceFile {
+		filePath := sources[0].Path
+		if baseSource != nil {
+			filePath = baseSource.Path
+		}
+		basePath = filepath.Dir(filePath)
+		rootPath = basePath
+		if context != nil && context.Root != "" {
+			canonical, err := canonicalExistingPath(context.Root)
+			if err != nil {
+				value := importDiagnostic(CodeImportInvalid, "configuration root could not be canonicalized", nil)
+				return nil, &value
+			}
+			rootPath = canonical
+		}
+	} else if context != nil {
+		var err error
+		rootPath, err = canonicalExistingPath(context.Root)
+		if err != nil {
+			value := importDiagnostic(CodeImportInvalid, "configuration root could not be canonicalized", nil)
+			return nil, &value
+		}
+		basePath, err = canonicalExistingPath(context.BaseDir)
+		if err != nil {
+			value := importDiagnostic(CodeImportInvalid, "import base directory could not be canonicalized", nil)
+			return nil, &value
+		}
+	} else {
+		return nil, nil
+	}
+	var rooted fs.FS
+	if filesystem != nil {
+		rooted = rootedNativeFS{filesystem: filesystem, root: rootPath}
+	} else {
+		rooted = rootedHostFS{root: rootPath}
+	}
+	result := make(map[string]ResourceContext, len(sources))
+	for _, acquired := range sources {
+		base := basePath
+		if acquired.Path != "" {
+			if filesystem != nil {
+				base = path.Dir(acquired.Path)
+			} else {
+				base = filepath.Dir(acquired.Path)
+			}
+		}
+		var relative string
+		if filesystem != nil {
+			if !fs.ValidPath(rootPath) || !fs.ValidPath(base) || rootPath != "." && base != rootPath && !strings.HasPrefix(base, rootPath+"/") {
+				value := importDiagnostic(CodeImportOutsideRoot, "source base is outside the configuration root", nil)
+				return nil, &value
+			}
+			relative = base
+			if rootPath != "." {
+				relative = strings.TrimPrefix(strings.TrimPrefix(base, rootPath), "/")
+				if relative == "" {
+					relative = "."
+				}
+			}
+		} else {
+			if !containedPath(rootPath, base) {
+				value := importDiagnostic(CodeImportOutsideRoot, "source base is outside the configuration root", nil)
+				return nil, &value
+			}
+			var err error
+			relative, err = filepath.Rel(rootPath, base)
+			if err != nil {
+				value := importDiagnostic(CodeImportInvalid, "source base could not be addressed in the configuration root", nil)
+				return nil, &value
+			}
+			relative = filepath.ToSlash(relative)
+		}
+		result[acquired.ID] = ResourceContext{FS: rooted, BaseDir: relative}
+	}
+	return result, nil
+}
+
+func mergeResourceContexts(target *map[string]ResourceContext, incoming map[string]ResourceContext) {
+	if len(incoming) == 0 {
+		return
+	}
+	if *target == nil {
+		*target = make(map[string]ResourceContext, len(incoming))
+	}
+	for id, context := range incoming {
+		(*target)[id] = context
+	}
+}

internal/engine/source_file.go

@@ -4,7 +4,6 @@ import (
 	"errors"
 	"io"
 	"io/fs"
-	"os"
 )
 
 type sourceReadFailureKind uint8
@@ -26,7 +25,7 @@ type sourceReadFailure struct {
 // readRegularFile borrows stat/open operations so root-bound callers retain their
 // containment policy. Preflight rejects FIFOs before open can block. The opened
 // handle is checked again and always closed; read-only close errors are ignored.
-func readRegularFile(name string, stat func(string) (fs.FileInfo, error), open func(string) (*os.File, error), limit int64) ([]byte, *sourceReadFailure) {
+func readRegularFile[F fs.File](name string, stat func(string) (fs.FileInfo, error), open func(string) (F, error), limit int64) ([]byte, *sourceReadFailure) {
 	info, err := stat(name)
 	if err != nil {
 		return nil, &sourceReadFailure{cause: err, missingTarget: errors.Is(err, fs.ErrNotExist)}

internal/engine/source_value.go

@@ -0,0 +1,19 @@
+package engine
+
+func (issue *valueIssue) sourceDiagnostic(sourceID string) Diagnostic {
+	code := CodeSourceInvalid
+	message := "mapping source has an unsupported value"
+	switch issue.kind {
+	case valueDepthLimit:
+		code, message = CodeSourceLimitExceeded, "mapping source exceeds the depth limit"
+	case valueNodeLimit:
+		code, message = CodeSourceLimitExceeded, "mapping source exceeds the node limit"
+	case valueByteLimit:
+		code, message = CodeSourceLimitExceeded, "mapping source exceeds the string-byte limit"
+	case valueNonFinite:
+		message = "mapping source float must be finite"
+	case valueCycle:
+		message = "mapping source values must not contain cycles"
+	}
+	return Diagnostic{Code: code, Severity: SeverityError, Path: issue.path.clone(), Message: message, Primary: &Location{SourceID: sourceID}}
+}

internal/engine/underlay_bench_test.go

@@ -0,0 +1,156 @@
+package engine
+
+import (
+	"io"
+	"strconv"
+	"testing"
+	"testing/fstest"
+)
+
+type benchmarkCatalogConfig struct {
+	Catalog map[string]string `yaml:"catalog"`
+}
+
+type benchmarkNodeConfig struct {
+	Node string `yaml:"node"`
+}
+
+func BenchmarkUnderlayBroadCatalog(b *testing.B) {
+	for _, width := range []int{128, 256, 512} {
+		b.Run(strconv.Itoa(width), func(b *testing.B) {
+			inputs := make([]Input, 2)
+			for half := range inputs {
+				catalog := make(map[string]any, width/2)
+				for index := range width / 2 {
+					catalog["item-"+strconv.Itoa(half*width/2+index)] = "value"
+				}
+				inputs[half] = Input{Source: Mapping("catalog-"+strconv.Itoa(half), map[string]any{"catalog": catalog})}
+			}
+			options := LayeredOptions[benchmarkCatalogConfig]{Underlays: []Underlay{{Inputs: inputs}}}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), options); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkUnderlayManySmallTiers(b *testing.B) {
+	for _, count := range []int{32, 64, 128} {
+		b.Run(strconv.Itoa(count), func(b *testing.B) {
+			groups := make([]Underlay, count)
+			for index := range groups {
+				groups[index].Inputs = []Input{{Source: Mapping("item", map[string]any{
+					"catalog": map[string]any{"item-" + strconv.Itoa(index): "value"},
+				})}}
+			}
+			options := LayeredOptions[benchmarkCatalogConfig]{Underlays: groups}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), options); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkUnderlaySubtreeReplacement(b *testing.B) {
+	for _, count := range []int{32, 64, 128} {
+		b.Run(strconv.Itoa(count), func(b *testing.B) {
+			groups := make([]Underlay, count)
+			for index := range groups {
+				var node any = "temporary"
+				if index%2 == 0 {
+					node = map[string]any{"left": "one", "right": "two", "nested": map[string]any{"enabled": false}}
+				}
+				groups[index].Inputs = []Input{{Source: Mapping("replacement", map[string]any{"node": node})}}
+			}
+			options := LayeredOptions[benchmarkNodeConfig]{Underlays: groups}
+			main := Input{Source: Mapping("main", map[string]any{"node": "final"})}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, err := LoadLayered(main, testSecretBinding(), options); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkUnderlayNativeImports(b *testing.B) {
+	for _, count := range []int{16, 32, 64} {
+		b.Run(strconv.Itoa(count), func(b *testing.B) {
+			files := fstest.MapFS{
+				"main.yml": &fstest.MapFile{Data: []byte("configkit: {imports: [{glob: 'defaults/*.yml'}]}\n")},
+			}
+			for index := range count {
+				name := "item-" + strconv.Itoa(index)
+				files["defaults/"+name+".yml"] = &fstest.MapFile{Data: []byte("catalog: {" + name + ": value}\n")}
+			}
+			filesystem := NewFilesystem("bundle", files)
+			main := Input{Source: filesystem.File("main.yml"), Imports: true}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, err := LoadLayered[benchmarkCatalogConfig](main, testSecretBinding()); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkUnderlayRetainedHistory(b *testing.B) {
+	for _, count := range []int{16, 32, 64} {
+		b.Run(strconv.Itoa(count), func(b *testing.B) {
+			groups := make([]Underlay, count)
+			for index := range groups {
+				groups[index].Inputs = []Input{{Source: Mapping("value", map[string]any{
+					"catalog": map[string]any{"shared": strconv.Itoa(index)},
+				})}}
+			}
+			options := LayeredOptions[benchmarkCatalogConfig]{Underlays: groups}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				result, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), options)
+				if err != nil {
+					b.Fatal(err)
+				}
+				if _, ok := result.Inspection.Explain(Keys("catalog", "shared")); !ok {
+					b.Fatal("retained path was not explained")
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkUnderlayRenderedHistory(b *testing.B) {
+	for _, count := range []int{32, 64, 128} {
+		b.Run(strconv.Itoa(count), func(b *testing.B) {
+			groups := make([]Underlay, count)
+			for index := range groups {
+				groups[index].Inputs = []Input{{Source: Mapping("value", map[string]any{
+					"catalog": map[string]any{"shared": strconv.Itoa(index)},
+				})}}
+			}
+			result, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), LayeredOptions[benchmarkCatalogConfig]{Underlays: groups})
+			if err != nil {
+				b.Fatal(err)
+			}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if err := result.Inspection.WriteExplanation(io.Discard, Keys("catalog", "shared")); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}

internal/engine/value.go

@@ -0,0 +1,166 @@
+package engine
+
+import (
+	"math"
+	"reflect"
+	"slices"
+	"strconv"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type valueBuilder struct {
+	sourceID string
+	active   map[valueVisit]bool
+	bytes    int64
+	nodes    int
+}
+
+type valueIssueKind uint8
+
+const (
+	valueDepthLimit valueIssueKind = iota
+	valueNodeLimit
+	valueByteLimit
+	valueNonFinite
+	valueUnsupported
+	valueCycle
+)
+
+type valueIssue struct {
+	kind valueIssueKind
+	path Path
+}
+
+func newValueIssue(kind valueIssueKind, path Path) *valueIssue {
+	return &valueIssue{kind: kind, path: path.clone()}
+}
+
+func (b *valueBuilder) build(value any, path Path, depth int) (*document.Node, *valueIssue) {
+	if depth > maxDocumentDepth {
+		return nil, newValueIssue(valueDepthLimit, path)
+	}
+	b.nodes++
+	if b.nodes > maxDocumentNodes {
+		return nil, newValueIssue(valueNodeLimit, path)
+	}
+	location := document.Location{SourceID: b.sourceID}
+	switch value := value.(type) {
+	case nil:
+		return &document.Node{Kind: document.Null, Location: location}, nil
+	case string:
+		if !b.addBytes(len(value)) {
+			return nil, newValueIssue(valueByteLimit, path)
+		}
+		return &document.Node{Kind: document.String, Text: value, Location: location}, nil
+	case bool:
+		return &document.Node{Kind: document.Boolean, Text: strconv.FormatBool(value), Location: location}, nil
+	case int:
+		return valueNumber(strconv.FormatInt(int64(value), 10), location), nil
+	case int8:
+		return valueNumber(strconv.FormatInt(int64(value), 10), location), nil
+	case int16:
+		return valueNumber(strconv.FormatInt(int64(value), 10), location), nil
+	case int32:
+		return valueNumber(strconv.FormatInt(int64(value), 10), location), nil
+	case int64:
+		return valueNumber(strconv.FormatInt(value, 10), location), nil
+	case uint:
+		return valueNumber(strconv.FormatUint(uint64(value), 10), location), nil
+	case uint8:
+		return valueNumber(strconv.FormatUint(uint64(value), 10), location), nil
+	case uint16:
+		return valueNumber(strconv.FormatUint(uint64(value), 10), location), nil
+	case uint32:
+		return valueNumber(strconv.FormatUint(uint64(value), 10), location), nil
+	case uint64:
+		return valueNumber(strconv.FormatUint(value, 10), location), nil
+	case float32:
+		if math.IsNaN(float64(value)) || math.IsInf(float64(value), 0) {
+			return nil, newValueIssue(valueNonFinite, path)
+		}
+		return valueNumber(strconv.FormatFloat(float64(value), 'g', -1, 32), location), nil
+	case float64:
+		if math.IsNaN(value) || math.IsInf(value, 0) {
+			return nil, newValueIssue(valueNonFinite, path)
+		}
+		return valueNumber(strconv.FormatFloat(value, 'g', -1, 64), location), nil
+	case []any:
+		if value == nil {
+			return &document.Node{Kind: document.Null, Location: location}, nil
+		}
+		leave, diagnostic := b.enter(reflect.ValueOf(value), path)
+		if diagnostic != nil {
+			return nil, diagnostic
+		}
+		defer leave()
+		if len(value) > maxDocumentNodes-b.nodes {
+			return nil, newValueIssue(valueNodeLimit, path)
+		}
+		node := &document.Node{Kind: document.Sequence, Location: location, Sequence: make([]*document.Node, len(value))}
+		for index, child := range value {
+			built, diagnostic := b.build(child, path.Index(index), depth+1)
+			if diagnostic != nil {
+				return nil, diagnostic
+			}
+			node.Sequence[index] = built
+		}
+		return node, nil
+	case map[string]any:
+		if value == nil {
+			return &document.Node{Kind: document.Null, Location: location}, nil
+		}
+		leave, diagnostic := b.enter(reflect.ValueOf(value), path)
+		if diagnostic != nil {
+			return nil, diagnostic
+		}
+		defer leave()
+		if len(value) > (maxDocumentNodes-b.nodes)/2 {
+			return nil, newValueIssue(valueNodeLimit, path)
+		}
+		keys := make([]string, 0, len(value))
+		for key := range value {
+			keys = append(keys, key)
+		}
+		slices.Sort(keys)
+		node := &document.Node{Kind: document.Mapping, Location: location, Mapping: make([]document.Entry, 0, len(keys))}
+		for _, key := range keys {
+			b.nodes++ // mapping key
+			if !b.addBytes(len(key)) {
+				return nil, newValueIssue(valueByteLimit, path.Key(key))
+			}
+			built, diagnostic := b.build(value[key], path.Key(key), depth+1)
+			if diagnostic != nil {
+				return nil, diagnostic
+			}
+			node.Mapping = append(node.Mapping, document.Entry{Key: key, KeyLocation: location, ValueLocation: location, Value: built})
+		}
+		return node, nil
+	default:
+		return nil, newValueIssue(valueUnsupported, path)
+	}
+}
+
+func (b *valueBuilder) addBytes(count int) bool {
+	if count < 0 || int64(count) > maxSourceBytes-b.bytes {
+		return false
+	}
+	b.bytes += int64(count)
+	return true
+}
+
+func (b *valueBuilder) enter(value reflect.Value, path Path) (func(), *valueIssue) {
+	visit, ok := visitFor(value)
+	if !ok {
+		return func() {}, nil
+	}
+	if b.active[visit] {
+		return nil, newValueIssue(valueCycle, path)
+	}
+	b.active[visit] = true
+	return func() { delete(b.active, visit) }, nil
+}
+
+func valueNumber(text string, location document.Location) *document.Node {
+	return &document.Node{Kind: document.Number, Text: text, Location: location}
+}

layered.go

@@ -0,0 +1,78 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+
+// Input selects one source, whether its imports are enabled, and any explicit
+// filesystem context. A zero Source is invalid; use NoSource() for an omitted
+// main input. Sources keep their documented reuse and reader ownership rules.
+type Input struct {
+	Source     Source
+	Imports    bool
+	Filesystem *FilesystemContext
+}
+
+// Underlay contains inputs that must compose strictly within one tier. Tiers
+// appear in lowest-to-highest precedence order; an empty tier is a no-op.
+type Underlay struct{ Inputs []Input }
+
+// LayeredOptions selects lower-priority tiers and final application rules.
+// An active Overlay requires an explicit OverlayFilesystem; an inactive overlay
+// ignores it. Overrides remain explicit CLI records, not general mappings.
+type LayeredOptions[T any] struct {
+	Definition        Definition[T]
+	Underlays         []Underlay
+	Overlay           string
+	OverlayFilesystem *FilesystemContext
+	Overrides         []Override
+}
+
+// LoadLayered acquires the main input first and strictly composes enabled
+// imports with each input's tier. It strictly composes contributions within
+// each underlay, then recursively combines underlays in listed order beneath
+// main, a selected overlay, and CLI overrides. Defaults, decoding, resolution,
+// and validation run once after sibling `extends` expansion of the assembled
+// input. Source IDs follow acquisition order, not precedence order. Zero or
+// one options value is accepted; any error
+// returns a nil result.
+func LoadLayered[T any](main Input, supplied ...LayeredOptions[T]) (*Result[T], error) {
+	options := make([]engine.LayeredOptions[T], len(supplied))
+	for index := range supplied {
+		options[index] = toEngineLayeredOptions(supplied[index])
+	}
+	loaded, err := engine.LoadLayered(toEngineInput(main), secretBinding(), options...)
+	return fromEngineLoaded(loaded, err)
+}
+
+func toEngineInput(input Input) engine.Input {
+	return engine.Input{
+		Source: input.Source.toEngine(), Imports: input.Imports,
+		Filesystem: toEngineFilesystemContext(input.Filesystem),
+	}
+}
+
+func toEngineLayeredOptions[T any](options LayeredOptions[T]) engine.LayeredOptions[T] {
+	result := engine.LayeredOptions[T]{
+		Definition: toEngineDefinition(options.Definition), Overlay: options.Overlay,
+		OverlayFilesystem: toEngineFilesystemContext(options.OverlayFilesystem),
+	}
+	if options.Underlays != nil {
+		result.Underlays = make([]engine.Underlay, len(options.Underlays))
+		for index, group := range options.Underlays {
+			if group.Inputs != nil {
+				result.Underlays[index].Inputs = make([]engine.Input, len(group.Inputs))
+				for inputIndex, input := range group.Inputs {
+					result.Underlays[index].Inputs[inputIndex] = toEngineInput(input)
+				}
+			}
+		}
+	}
+	if options.Overrides != nil {
+		result.Overrides = make([]engine.Override, len(options.Overrides))
+		for index, override := range options.Overrides {
+			result.Overrides[index] = engine.Override{
+				Path: toEnginePath(override.Path), Value: override.Value, Flag: override.Flag,
+			}
+		}
+	}
+	return result
+}

lifecycle.go

@@ -28,10 +28,14 @@ type Options[T any] struct {
 	Overrides  []Override
 }
 
-// FilesystemContext supplies filesystem roots for imports and overlays.
+// FilesystemContext supplies roots for imports and overlays. With FS nil,
+// BaseDir and Root use absolute host paths. With FS set, they use slash-relative
+// paths in that namespace ("." denotes its root). File sources derive BaseDir
+// from the selected file; a native file's explicit Root limits its scope.
 type FilesystemContext struct {
 	BaseDir string
 	Root    string
+	FS      *Filesystem
 }
 
 // Override describes one explicitly supplied application override. Value uses
@@ -75,14 +79,19 @@ type Result[T any] struct {
 }
 
 // Load parses and strictly decodes zero or one selected source, explicitly enabled
-// imports, and one selected overlay. It accepts at most one Options value and
-// returns no result when any error is reported.
+// imports, and one selected overlay. After source precedence, built-in sibling
+// `extends` relationships resolve before defaults and typed decoding. It accepts
+// at most one Options value and returns no result when any error is reported.
 func Load[T any](source Source, supplied ...Options[T]) (*Result[T], error) {
 	options := make([]engine.Options[T], len(supplied))
 	for index := range supplied {
 		options[index] = toEngineOptions(supplied[index])
 	}
 	loaded, err := engine.Load(source.toEngine(), secretBinding(), options...)
+	return fromEngineLoaded(loaded, err)
+}
+
+func fromEngineLoaded[T any](loaded *engine.Loaded[T], err error) (*Result[T], error) {
 	if err != nil {
 		var loadError *engine.LoadError
 		if errors.As(err, &loadError) {
@@ -133,7 +142,7 @@ func (r *Result[T]) Diagnostics() []Diagnostic {
 	return result
 }
 
-// IsExplicit reports whether surviving input explicitly defined path.
+// IsExplicit reports whether surviving direct or inherited input defined path.
 func (r *Resolution) IsExplicit(path Path) bool {
 	return r.engineContext().IsExplicit(toEnginePath(path))
 }
@@ -222,7 +231,7 @@ func toEngineOptions[T any](options Options[T]) engine.Options[T] {
 		Overlay:    options.Overlay,
 	}
 	if options.Filesystem != nil {
-		result.Filesystem = &engine.FilesystemContext{BaseDir: options.Filesystem.BaseDir, Root: options.Filesystem.Root}
+		result.Filesystem = toEngineFilesystemContext(options.Filesystem)
 	}
 	if options.Overrides != nil {
 		result.Overrides = make([]engine.Override, len(options.Overrides))
@@ -235,6 +244,18 @@ func toEngineOptions[T any](options Options[T]) engine.Options[T] {
 	return result
 }
 
+func toEngineFilesystemContext(context *FilesystemContext) *engine.FilesystemContext {
+	if context == nil {
+		return nil
+	}
+	result := &engine.FilesystemContext{BaseDir: context.BaseDir, Root: context.Root}
+	if context.FS != nil {
+		filesystem := context.FS.value
+		result.FS = &filesystem
+	}
+	return result
+}
+
 func toEngineDefinition[T any](definition Definition[T]) engine.Definition[T] {
 	result := engine.Definition[T]{Defaults: definition.Defaults}
 	if definition.Resolve != nil {

metadata.go

@@ -13,13 +13,13 @@ func (m Metadata) Lookup(path Path) (FieldInfo, bool) {
 	return fromEngineFieldInfo(value), ok
 }
 
-// IsExplicit reports whether a surviving input contribution explicitly defined
-// path.
+// IsExplicit reports whether a surviving input contribution, including one
+// inherited from a sibling, explicitly defined path.
 func (m Metadata) IsExplicit(path Path) bool {
 	return m.value.IsExplicit(toEnginePath(path))
 }
 
-// History returns detached definition and resolution records for path. A record's
+// History returns detached definition, inheritance, and resolution records for path. A record's
 // Path can be an ancestor when that container operation replaced or removed path.
 // Removed historical paths can have history even though Lookup returns false.
 // Unknown and invalid paths return nil.
@@ -36,7 +36,7 @@ func (m Metadata) History(path Path) []Change {
 }
 
 // HistoricalPaths returns the paths observed during loading, including effective
-// paths and paths removed by overlays or recorded resolution. It includes default
+// paths and paths removed by overlays, inheritance, or recorded resolution. It includes default
 // paths and the root, but not paths that never existed. The zero Metadata returns
 // an empty slice.
 //

records.go

@@ -6,11 +6,25 @@ type Layer string
 const (
 	LayerDefault    Layer = "default"
 	LayerBase       Layer = "base"
+	LayerUnderlay   Layer = "underlay"
 	LayerOverlay    Layer = "overlay"
 	LayerCLI        Layer = "cli"
 	LayerResolution Layer = "resolution"
 )
 
+// SourceKind identifies how a configuration source was acquired, independent
+// of its precedence layer.
+type SourceKind string
+
+const (
+	SourceKindFile       SourceKind = "file"
+	SourceKindFilesystem SourceKind = "filesystem"
+	SourceKindBytes      SourceKind = "bytes"
+	SourceKindReader     SourceKind = "reader"
+	SourceKindMapping    SourceKind = "mapping"
+	SourceKindCLI        SourceKind = "cli"
+)
+
 // ChangeKind identifies a recorded operation. The zero value is
 // used for input and default origins.
 type ChangeKind string
@@ -18,7 +32,10 @@ type ChangeKind string
 const (
 	// ChangeDecoding records the built-in transition to canonical typed values.
 	// It is inspection metadata, not an operation accepted by Resolution.Record.
-	ChangeDecoding       ChangeKind = "decoding"
+	ChangeDecoding ChangeKind = "decoding"
+	// ChangeInheritance records structural input expansion. It is inspection
+	// metadata, not an operation accepted by Resolution.Record.
+	ChangeInheritance    ChangeKind = "inheritance"
 	ChangeDerivedDefault ChangeKind = "derived_default"
 	ChangeNormalization  ChangeKind = "normalization"
 )
@@ -89,6 +106,7 @@ const (
 	CodeImportRepeated      Code = "import.repeated"
 	CodeImportLimitExceeded Code = "import.limit_exceeded"
 	CodeImportVersion       Code = "import.version"
+	CodeUnderlayVersion     Code = "underlay.version"
 
 	CodeOverlayInvalid       Code = "overlay.invalid"
 	CodeOverlayMissing       Code = "overlay.missing"
@@ -146,6 +164,11 @@ const (
 	CodeOverrideLimitExceeded Code = "override.limit_exceeded"
 
 	CodeExplainLimitExceeded Code = "explain.limit_exceeded"
+
+	CodeInheritanceInvalid Code = "inheritance.invalid"
+	CodeInheritanceMissing Code = "inheritance.missing"
+	CodeInheritanceCycle   Code = "inheritance.cycle"
+	CodeInheritanceVersion Code = "inheritance.version"
 )
 
 // Location identifies a position in a configuration source. Zero line and column
@@ -156,15 +179,19 @@ type Location struct {
 	Column   int
 }
 
-// SourceInfo describes a source discovered during a load. Path is a canonical
-// local path when one exists. ImportedFrom identifies the declaration that
-// included this source.
+// SourceInfo describes a source discovered during a load. Kind identifies its
+// acquisition, independent of Layer. Path is canonical for a host file and
+// namespace-relative for a filesystem file; synthetic sources have no Path.
+// UnderlayIndex is the one-based original underlay position, or zero for other
+// sources. ImportedFrom identifies the declaration that included this source.
 type SourceInfo struct {
-	ID           string
-	Label        string
-	Path         string
-	Layer        Layer
-	ImportedFrom *Location
+	ID            string
+	Label         string
+	Path          string
+	Layer         Layer
+	Kind          SourceKind
+	UnderlayIndex int
+	ImportedFrom  *Location
 }
 
 // Diagnostic is a safe structured problem report. Message and source labels must
@@ -178,13 +205,25 @@ type Diagnostic struct {
 	Related  []Location
 }
 
+// InheritanceStep identifies a concrete payload transfer. From and To name
+// semantic value paths; Reference names the controlling destination `extends`
+// path. Locations belong to that directive, not the transferred value.
+type InheritanceStep struct {
+	From      Path
+	To        Path
+	Reference Path
+	Locations []Location
+}
+
 // Origin describes where an effective definition came from. Multiple locations
 // represent a composite origin; resolution origins are synthetic and identify their
-// operation instead of a file location.
+// operation instead of a file location. Inheritance records the concrete
+// derivation path while Layer and Locations continue to identify value sources.
 type Origin struct {
-	Layer     Layer
-	Locations []Location
-	Operation ChangeKind
+	Layer       Layer
+	Locations   []Location
+	Operation   ChangeKind
+	Inheritance []InheritanceStep
 }
 
 // FieldInfo describes an effective configuration path.
@@ -238,6 +277,16 @@ func cloneSourceInfos(sources []SourceInfo) []SourceInfo {
 
 func cloneOrigin(origin Origin) Origin {
 	origin.Locations = cloneLocations(origin.Locations)
+	if origin.Inheritance != nil {
+		steps := make([]InheritanceStep, len(origin.Inheritance))
+		for index, step := range origin.Inheritance {
+			steps[index] = InheritanceStep{
+				From: step.From.clone(), To: step.To.clone(), Reference: step.Reference.clone(),
+				Locations: cloneLocations(step.Locations),
+			}
+		}
+		origin.Inheritance = steps
+	}
 	return origin
 }
 

resource_context.go

@@ -0,0 +1,33 @@
+package configkit
+
+import "io/fs"
+
+// ResourceContext provides a passive rooted filesystem view and a
+// slash-relative BaseDir for one acquired source. FS is borrowed: caller-owned
+// filesystems remain under caller control, while a host context opens a fresh
+// contained root for each resource open rather than retaining a loading handle.
+// Querying this record performs no I/O. Resource contents may change after
+// loading; callers must close files they open through FS.
+type ResourceContext struct {
+	FS      fs.FS
+	BaseDir string
+}
+
+// SourceContext returns the context of an acquired source ID. Unknown IDs and
+// sources without context return false. It is nil-safe and performs no I/O.
+// Follow individual origin locations when an origin has multiple sources.
+func (r *Result[T]) SourceContext(id string) (ResourceContext, bool) {
+	if r == nil || r.inspection == nil {
+		return ResourceContext{}, false
+	}
+	context, ok := r.inspection.SourceContext(id)
+	return ResourceContext{FS: context.FS, BaseDir: context.BaseDir}, ok
+}
+
+// SourceContext returns a passive source context during the active resolution
+// callback. Like other Resolution queries, it cannot be used after callback
+// return and performs no filesystem operations itself.
+func (r *Resolution) SourceContext(id string) (ResourceContext, bool) {
+	context, ok := r.engineContext().SourceContext(id)
+	return ResourceContext{FS: context.FS, BaseDir: context.BaseDir}, ok
+}

source.go

@@ -2,14 +2,31 @@ package configkit
 
 import (
 	"io"
+	"io/fs"
 
 	"gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
 )
 
-// Source is an opaque configuration input. Construct sources with File, Bytes,
-// Reader, or NoSource; the zero value is invalid.
+// Source is an opaque configuration input. Construct sources with File,
+// Filesystem.File, Bytes, Reader, Mapping, or NoSource; the zero value is invalid.
 type Source struct{ value engine.Source }
 
+// Filesystem is a reusable opaque namespace for a caller-owned fs.FS. Copies
+// preserve identity; separately constructed values are distinct even if their
+// labels or underlying filesystems match. The filesystem is never closed.
+type Filesystem struct{ value engine.Filesystem }
+
+// NewFilesystem borrows filesystem without opening it. Label must be nonempty
+// and safe to display; a nil filesystem or invalid label is reported when a
+// source from the returned namespace is consumed.
+func NewFilesystem(label string, filesystem fs.FS) Filesystem {
+	return Filesystem{value: engine.NewFilesystem(label, filesystem)}
+}
+
+// File selects a non-root fs.ValidPath name in this filesystem. Each load
+// reopens the file; constructor problems are reported when the source is used.
+func (f Filesystem) File(name string) Source { return Source{value: f.value.File(name)} }
+
 // File returns a source that reopens path for every load attempt.
 func File(path string) Source { return Source{value: engine.File(path)} }
 
@@ -21,6 +38,15 @@ func Bytes(label string, data []byte) Source { return Source{value: engine.Bytes
 // Copies of the returned Source share the same single-use claim.
 func Reader(label string, input io.Reader) Source { return Source{value: engine.Reader(label, input)} }
 
+// Mapping snapshots supported built-in values immediately. A nil root map is
+// invalid; an empty map is a valid empty document. Values may be nil, built-in
+// scalars, []any, or map[string]any recursively. Unsupported values and cycles
+// are reported when the source is consumed. Label must be nonempty and safe to
+// display. Later caller mutation cannot change the captured document.
+func Mapping(label string, values map[string]any) Source {
+	return Source{value: engine.Mapping(label, values)}
+}
+
 // NoSource explicitly selects a defaults-only input. It creates no source record.
 func NoSource() Source { return Source{value: engine.NoSource()} }
 

testdata/filesystem/config/main.yml

@@ -0,0 +1,5 @@
+configkit:
+  imports:
+    - path: ../shared.yml
+items:
+  main: base

testdata/filesystem/overlays/testing.yml

@@ -0,0 +1,2 @@
+items:
+  main: selected

testdata/filesystem/shared.yml

@@ -0,0 +1,2 @@
+items:
+  shared: default