Retire completed roadmaps before the next release

README.md

@@ -5,18 +5,26 @@ 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. The root
+`Diff` API compares frozen effective values from two successful same-type loads;
+the [configuration contract](docs/config.md#effective-configuration-diffs) and
+[consumer guide](docs/consumers/README.md#effective-configuration-diffs) describe
+its safe changed-region records and maintained workflows.
 
 ```go
 type Config struct {
@@ -33,13 +41,41 @@ func ReadConfig(path string) (Config, error) {
 }
 ```
 
-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
-[release procedure](docs/release.md) for maintainer-only candidate and publication
-steps.
+## 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,
+including [input destinations](docs/config.md#input-destinations). The
+[consumer adoption guide](docs/consumers/README.md) provides maintained simple,
+layered, and mounted-collection examples alongside the migration shapes. The
+accepted placement decision is recorded in [ADR 0011](docs/adr/0011-input-destinations.md).
+See the [development guide](docs/development.md) for repository orientation and
+the [release procedure](docs/release.md) for maintainer-only candidate and
+publication steps.
 
 ConfigKit code is licensed under the [LGPL v3.0 or later](COPYRIGHT).
 

conversion.go

@@ -43,16 +43,56 @@ 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, Destination: fromEnginePath(value.Destination), Layer: Layer(value.Layer), Kind: SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: fromEngineLocationPointer(value.ImportedFrom), VersionCheck: fromEngineSourceVersionCheck(value.VersionCheck)}
 }
 func toEngineSourceInfo(value SourceInfo) engine.SourceInfo {
-	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: engine.Layer(value.Layer), ImportedFrom: toEngineLocationPointer(value.ImportedFrom)}
+	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Destination: toEnginePath(value.Destination), Layer: engine.Layer(value.Layer), Kind: engine.SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: toEngineLocationPointer(value.ImportedFrom), VersionCheck: toEngineSourceVersionCheck(value.VersionCheck)}
+}
+func fromEngineSourceVersionCheck(value *engine.SourceVersionCheck) *SourceVersionCheck {
+	if value == nil {
+		return nil
+	}
+	return &SourceVersionCheck{
+		Path:     fromEnginePath(value.Path),
+		Version:  Version{Present: value.Version.Present, Null: value.Version.Null, Text: value.Version.Text},
+		Location: fromEngineLocationPointer(value.Location), Redacted: value.Redacted,
+	}
+}
+func toEngineSourceVersionCheck(value *SourceVersionCheck) *engine.SourceVersionCheck {
+	if value == nil {
+		return nil
+	}
+	return &engine.SourceVersionCheck{
+		Path:     toEnginePath(value.Path),
+		Version:  engine.Version{Present: value.Version.Present, Null: value.Version.Null, Text: value.Version.Text},
+		Location: toEngineLocationPointer(value.Location), Redacted: value.Redacted,
+	}
 }
 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)}

development_check_test.go

@@ -66,7 +66,7 @@ func TestDevelopmentCheckHandlesFilenamesAndStopsOnFailure(t *testing.T) {
 						t.Fatal(err)
 					}
 					cmd := exec.Command("/bin/bash", filepath.Join(root, "scripts", scriptName))
-					cmd.Env = append(os.Environ(), "PATH="+bin+string(os.PathListSeparator)+os.Getenv("PATH"), "CONFIGKIT_BASELINE_TOOLCHAIN=test-baseline", "CONFIGKIT_NEWER_TOOLCHAIN=", "CONFIGKIT_TEST_FMT_LOG="+log, "CONFIGKIT_TEST_GO_LOG="+calls)
+					cmd.Env = append(os.Environ(), "PATH="+bin+string(os.PathListSeparator)+os.Getenv("PATH"), "CONFIGKIT_TEST_FMT_LOG="+log, "CONFIGKIT_TEST_GO_LOG="+calls)
 					// Log only targets selected by the script, not inherited host settings.
 					cmd.Env = append(cmd.Env, "GOOS=", "GOARCH=")
 					if name == "failed-go-check" {
@@ -108,90 +108,3 @@ func TestDevelopmentCheckHandlesFilenamesAndStopsOnFailure(t *testing.T) {
 		})
 	}
 }
-
-func TestCheckScriptsReleaseGates(t *testing.T) {
-	script, err := os.ReadFile("scripts/release-check.sh")
-	if err != nil {
-		t.Fatal(err)
-	}
-	for _, scenario := range []string{"success", "failed-test", "missing-toolchain"} {
-		t.Run(scenario, func(t *testing.T) {
-			root := t.TempDir()
-			bin := t.TempDir()
-			for _, directory := range []string{"scripts", "nested"} {
-				if err := os.Mkdir(filepath.Join(root, directory), 0700); err != nil {
-					t.Fatal(err)
-				}
-			}
-			checker := filepath.Join(root, "scripts", "release-check.sh")
-			if err := os.WriteFile(checker, script, 0700); err != nil {
-				t.Fatal(err)
-			}
-			calls := filepath.Join(bin, "go.calls")
-			goStub := `#!/bin/sh
-printf '%s|%s|%s|%s|%s|%s\n' "$GOTOOLCHAIN" "$GOPROXY" "${CGO_ENABLED-}" "${GOOS-}" "${GOARCH-}" "$*" >> "$CONFIGKIT_TEST_GO_LOG"
-if [ "${CONFIGKIT_TEST_FAIL-}" = toolchain ] && [ "${1-}" = version ]; then exit 37; fi
-if [ "${CONFIGKIT_TEST_FAIL-}" = test ] && [ "${1-}" = test ]; then exit 37; fi
-`
-			if err := os.WriteFile(filepath.Join(bin, "go"), []byte(goStub), 0700); err != nil {
-				t.Fatal(err)
-			}
-			cmd := exec.Command("/bin/bash", checker)
-			cmd.Dir = filepath.Join(root, "nested")
-			cmd.Env = append(os.Environ(),
-				"PATH="+bin+string(os.PathListSeparator)+os.Getenv("PATH"),
-				"CONFIGKIT_BASELINE_TOOLCHAIN=test-baseline",
-				"CONFIGKIT_TEST_GO_LOG="+calls,
-				"GOOS=", "GOARCH=", "CGO_ENABLED=",
-			)
-			switch scenario {
-			case "failed-test":
-				cmd.Env = append(cmd.Env, "CONFIGKIT_TEST_FAIL=test")
-			case "missing-toolchain":
-				cmd.Env = append(cmd.Env, "CONFIGKIT_TEST_FAIL=toolchain")
-			}
-			output, err := cmd.CombinedOutput()
-			if scenario == "success" && err != nil {
-				t.Fatalf("checker: %v\n%s", err, output)
-			}
-			if scenario != "success" && err == nil {
-				t.Fatalf("checker accepted %s", scenario)
-			}
-			data, err := os.ReadFile(calls)
-			if err != nil {
-				t.Fatal(err)
-			}
-			lines := strings.Split(strings.TrimSpace(string(data)), "\n")
-			for _, line := range lines {
-				if !strings.HasPrefix(line, "test-baseline|off|") {
-					t.Fatalf("toolchain or offline setting lost: %q", line)
-				}
-			}
-			if scenario == "missing-toolchain" {
-				if len(lines) != 1 || !strings.HasSuffix(lines[0], "|version") {
-					t.Fatalf("checker ran after unavailable toolchain: %q", lines)
-				}
-				return
-			}
-			if len(lines) < 2 || lines[1] != "test-baseline|off||||test -mod=readonly -vet=off -race -v ./..." {
-				t.Fatalf("release suite command changed: %q", lines)
-			}
-			if scenario == "failed-test" {
-				if len(lines) != 2 {
-					t.Fatalf("checker continued after failure: %q", lines)
-				}
-				return
-			}
-			want := []string{
-				"test-baseline|off||||version",
-				"test-baseline|off||||test -mod=readonly -vet=off -race -v ./...",
-				"test-baseline|off||||vet -mod=readonly ./...",
-				"test-baseline|off|0|linux|amd64|build -mod=readonly ./...",
-				"test-baseline|off|0|linux|arm64|build -mod=readonly ./...",
-			}
-			if strings.Join(lines, "\n") != strings.Join(want, "\n") {
-				t.Fatalf("unexpected release gates:\n%s", data)
-			}
-		})
-	}
-}

diff.go

@@ -0,0 +1,53 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+
+// Difference describes one canonical changed region between two successful
+// loads. Left is the earlier value and Right is the later value. Each snapshot
+// is detached; Present distinguishes absence from an explicit null, and
+// Redacted reports whether combined sensitivity affected that side. Mappings
+// are compared recursively, while sequences are reported as atomic values.
+// Records are ordered by canonical JSON pointer. HiddenChanged indicates that
+// a value concealed by combined sensitivity changed, without revealing which
+// concealed member changed.
+type Difference struct {
+	Path          Path
+	Left          ValueSnapshot
+	Right         ValueSnapshot
+	HiddenChanged bool
+}
+
+// Diff compares the frozen effective values of two successful loads of the
+// same configuration type. It does not reread sources, environment variables,
+// or rerun defaults or resolvers. Equal values produce no records even when
+// their sources, defaults, or resolution histories differ. Credential contents
+// are not compared: typed secret references compare by reference name only.
+//
+// Both results must be non-nil successful loads with inspection state; nil or
+// zero results return an error and no records. Load errors are returned by
+// Load, so callers must handle those before calling Diff. Snapshots are safe
+// for presentation but are not a complete reloadable configuration.
+func Diff[T any](left, right *Result[T]) ([]Difference, error) {
+	var leftInspection, rightInspection *engine.Inspection
+	if left != nil {
+		leftInspection = left.inspection
+	}
+	if right != nil {
+		rightInspection = right.inspection
+	}
+
+	values, err := engine.Diff(leftInspection, rightInspection)
+	if err != nil {
+		return nil, err
+	}
+	differences := make([]Difference, len(values))
+	for index, value := range values {
+		differences[index] = Difference{
+			Path:          fromEnginePath(value.Path),
+			Left:          fromEngineValueSnapshot(value.Left),
+			Right:         fromEngineValueSnapshot(value.Right),
+			HiddenChanged: value.HiddenChanged,
+		}
+	}
+	return differences, nil
+}

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/adr/0010-effective-configuration-diff.md

@@ -0,0 +1,99 @@
+# ADR 0010: Compare Frozen Effective Configuration Snapshots
+
+## Status
+
+Accepted and implemented. The
+[configuration contract](../config.md#effective-configuration-diffs) owns current
+behavior. Completed planning and remediation records are preserved in Git
+history at `1c2610c` under `docs/roadmap/`.
+
+## Date
+
+2026-10-09
+
+## Context
+
+Applications need to inspect how two completed configurations differ, including
+the effective impact of an overlay after defaults and application resolution.
+ConfigKit already retains canonical typed inspection snapshots, semantic paths,
+presence information, and sensitivity classification for successful loads.
+
+Extending this inspection model lets applications share configuration comparison
+and safe disclosure while keeping their presentation and domain interpretation.
+The operation should follow ConfigKit's existing equality, ownership, and secret
+boundaries so consumers can reason about loading and inspection consistently.
+
+## Decision
+
+Provide structured effective-value comparison between successful results of the
+same configuration type. Compare their retained canonical inspection snapshots
+through the root-owned public facade. Each comparison uses the completed load
+state and returns detached records with a semantic path and safe left and right
+value snapshots.
+
+Use canonical value equality and the existing structural comparison rules.
+Mappings compare recursively, sequences report atomic changes, and scalar or
+kind replacements report the affected region. Presence distinguishes additions
+and removals from explicit null. Equal effective values produce an empty diff;
+source attribution, explicit-input status, and history remain separately
+inspectable through each result. Canonical pointer ordering makes reporting
+deterministic.
+
+Detect changes before masking values. Apply sensitivity from both loads to both
+returned sides, including historical and dependency-derived classification.
+Preserve public content where the shapes permit corresponding sensitive regions
+to be identified. When a structural replacement prevents safe correspondence,
+mask the complete reported replacement region on both present sides. Absence
+retains its own representation.
+
+Keep masking and hidden-change information distinct. Each record indicates
+whether an actual change is withheld by redaction, allowing applications to
+display a changed-sensitive-value indicator even when safe snapshots render
+identically. Public-only changes alongside unchanged masked content retain that
+distinction.
+
+Typed secret references participate through their configured environment names.
+Resolved credential bytes remain behind the existing explicit secret accessor.
+The comparison uses the same frozen inspection boundary as effective rendering
+and explanation.
+
+ConfigKit owns comparison and paired safe disclosure. Applications own labels,
+layout, and rendering of the returned changed regions. Reusing the existing
+snapshot representation and engine boundary keeps the extension consistent with
+the library's established inspection model.
+
+## Alternatives considered
+
+- **Compare input text or separately rendered YAML.** Supports familiar textual
+  displays, but input text precedes composition and resolution, and independently
+  redacted output can hide changes or apply different disclosure rules. Canonical
+  comparison with paired masking represents the completed configuration.
+- **Leave comparison to applications using returned Go values.** Offers direct
+  access to application types, but duplicates structural and redaction machinery
+  and observes caller mutation. Retained inspection snapshots preserve the
+  completed load's values and established ownership boundary.
+- **Align sequence elements and return context-rich rendering data.** Enables
+  finer-grained displays, but requires additional correspondence and disclosure
+  rules. Atomic sequence records and existing value snapshots satisfy the
+  selected changed-region workflows with a smaller public model.
+- **Compare resolved credentials or retain credential-derived tokens.** Could
+  detect rotation behind an unchanged reference, but expands retained sensitive
+  state and introduces a separate trusted comparison mechanism. Reference-name
+  comparison preserves the existing inspection boundary.
+
+## Consequences
+
+Applications can compare independently loaded configurations using ConfigKit's
+shared equality, presence, and disclosure rules. Existing composition and load
+semantics continue to determine the values being compared. Caller mutation of a
+returned configuration leaves its retained comparison snapshot unchanged.
+
+Paired redaction can conceal public content within a structurally incompatible
+replacement. The changed region and hidden-change indicator still communicate
+its effective impact. Credential rotation behind an unchanged environment name
+produces equal reference values for comparison.
+
+The implementation requires operation-local disclosure preparation and focused
+coverage for presence, shape changes, historical sensitivity, and detached
+results. Applications retain control of presentation; changed-region records
+supply the safe data needed for that responsibility.

docs/adr/0011-input-destinations.md

@@ -0,0 +1,74 @@
+# ADR 0011: Place Configuration Inputs at Mapping Destinations
+
+## Status
+
+Accepted and implemented. Current placement behavior is owned by the
+[configuration contract](../config.md#input-destinations). The accepted
+relative-import extension is recorded in
+[ADR 0012](0012-relative-import-destinations.md). The completed roadmap, consumer
+request, and review records are preserved at `e4270c9` under `docs/roadmap/` in
+Git history; the original input-destination implementation plan remains at
+`315f464:docs/roadmap/implementation.md`.
+
+## Date
+
+2026-10-14
+
+## Context
+
+Consumers assembling collections of independently authored configuration
+fragments need each source to contribute beneath a selected application path.
+Requiring consumers to rewrite source documents or merge completed values would
+separate placement from ConfigKit's existing composition, precedence, source
+identity, and inspection behavior. Placement should remain part of the ordinary
+loading lifecycle and preserve the root-owned public facade.
+
+## Decision
+
+Add an optional `Destination Path` to root-owned `Input` and expose the effective
+destination on existing `SourceInfo` records. The empty path means the existing
+application root. A non-empty destination is a sequence of literal mapping keys;
+explicit sequence-index segments are invalid. Existing reserved structural keys
+remain reserved, while keys that merely contain their spelling remain ordinary
+keys.
+
+Acquire and interpret each source using its original physical identity and
+source-local directives, then place its application body before strict
+composition and the existing final lifecycle. Imported application bodies
+inherit the input destination. Placement applies to main and underlay inputs;
+ordinary root loading, selected overlays, and CLI inputs retain root placement.
+The main-owned version rule continues to check its effective application path
+at its existing lifecycle point.
+
+Placement preserves authored node locations and source identity. Introduced
+mapping structure belongs to the contributing source and uses that document's
+root location as its source-coordinate fallback. A present empty mapping
+materializes its destination; an absent `NoSource` contributes nothing. The
+result continues through the existing strict composition, overlay, inheritance,
+typed loading, diagnostics, provenance, history, sensitivity, and resource
+context mechanisms.
+
+The added exported fields preserve keyed and zero-value struct use. Callers
+using unkeyed composite literals for `Input` or `SourceInfo` must migrate to
+keyed literals. Treat this feature as minor-release work; select the version and
+document the compatibility impact during separately authorized release
+preparation, consistent with the release policy.
+
+## Alternatives considered
+
+- **Rewrite or merge sources in consumers.** This makes each consumer reproduce
+  placement, strict composition, source attribution, and inspection semantics.
+- **Add a separate loading entry point or public wrapper type.** This creates a
+  parallel API for a behavior that belongs to existing per-input loading.
+- **Load each fragment as a complete application before placement.** This runs
+  defaults, decoding, resolution, and validation on incomplete fragments rather
+  than once on the assembled application.
+
+## Consequences
+
+Consumers can assemble contributions at effective mapping paths in one ordinary
+layered load and inspect each source's placement through existing source
+records. Paths, conflicts, and later application operations use the placed
+structure while source coordinates and relative-resource contexts continue to
+identify their original sources. Public field additions require release-note
+guidance for callers using unkeyed literals.

docs/adr/0012-relative-import-destinations.md

@@ -0,0 +1,78 @@
+# ADR 0012: Place Imported Configuration at Relative Destinations
+
+## Status
+
+Accepted and implemented. The
+[configuration contract](../config.md#input-destinations) owns current behavior.
+The completed roadmap and review records are preserved at `e4270c9` under
+`docs/roadmap/` in Git history; the relative-import implementation plan remains
+at `d9f08ff:docs/roadmap/implementation.md`.
+
+## Date
+
+2026-10-14
+
+## Context
+
+ConfigKit can place an input document at a literal mapping path while retaining
+source identity, composition, and inspection in its normal loading lifecycle.
+An importing document can currently bring in other documents, but those
+contributions inherit only the importing input's destination. Consumers that
+assemble a collection therefore cannot declare the layout of imported
+fragments alongside the import itself without rewriting source documents or
+performing another merge outside ConfigKit.
+
+The extension should reuse input placement and the existing import lifecycle.
+Logical application placement must remain independent from physical path
+resolution, source identity, precedence, and resource ownership.
+
+## Decision
+
+Add an optional `destination` field to each YAML import declaration. Its value
+is a sequence of strings representing literal mapping-key segments. Omission
+and an empty sequence add no segments. A non-empty destination appends to the
+effective destination of the immediately importing document; nested imports
+continue from the resulting child destination. Every match of one glob
+declaration receives the same computed destination.
+
+Use the existing input-destination validation and reserved-key rules. Do not
+interpret destination segments as filesystem paths, separators, sequence
+indices, or root-reset instructions. Physical import paths continue to resolve
+from their declaring source under existing containment rules.
+
+Imports remain in their existing strict-composition group. A destination does
+not grant precedence, bypass cycle or repeated-source rules, or change source
+identity. Existing placement, version ownership, source records, diagnostics,
+resource contexts, inheritance, history, and inspection mechanisms continue to
+govern the placed contributions. No public Go entry point or independent
+specification-header behavior is introduced.
+
+This is additive YAML functionality and is minor-release work. Version
+selection, release notes, roadmap retirement, and publication remain separate
+release-preparation work.
+
+## Alternatives Considered
+
+- **Require complete application-root destinations on every import.** This
+  avoids inherited context but repeats collection prefixes in nested source
+  declarations and does not express placement relative to the importing
+  document.
+- **Encode destinations as dotted or slash-delimited strings.** This introduces
+  separator and escaping rules even though existing paths already represent
+  literal mapping keys as segments.
+- **Rewrite or merge imported documents in consumers.** This duplicates source
+  placement, strict composition, attribution, and inspection behavior outside
+  ConfigKit's loading lifecycle.
+- **Add independent specification headers or collection-specific validation.**
+  These are separate design questions and are not required to declare relative
+  placement.
+
+## Consequences
+
+Import declarations can describe the logical placement of their contributions
+while preserving the physical source and existing composition lifecycle.
+Placement is immediate-parent-relative, so nested imports accumulate their
+declared segments and sibling declarations remain independent. The feature is
+implemented behind the existing public loading surface; detailed validation,
+diagnostic, accounting, and inspection requirements are tracked by the
+implementation roadmap.

docs/adr/0013-shared-source-version-checks.md

@@ -0,0 +1,92 @@
+# ADR 0013: Check Source Versions with One Shared Policy
+
+## Status
+
+Accepted and implemented. The
+[configuration contract](../config.md#shared-source-version-checks) owns current
+behavior. The completed roadmap, implementation plan, consumer request, and
+resolved review findings are preserved at `e4270c9` under `docs/roadmap/` in
+Git history; the [development guide](../development.md#baseline-validation)
+provides retrieval examples.
+
+## Date
+
+2026-10-15
+
+## Context
+
+Applications that assemble one configuration from independently authored
+documents need a way to require that each contributing document declares a
+version accepted by the application's existing `Definition.Version` checker.
+Checking only the eventual composed value lets another source hide a rejected
+declaration, while separate consumer-side parsing duplicates ConfigKit's source
+handling and diagnostics.
+
+The extension must preserve one application-owned acceptance policy and the
+existing loading lifecycle. Source declarations are compatibility evidence, not
+independent fragment schemas or values to compare for equality. Existing
+composition, placement, precedence, source identity, diagnostics, and
+sensitivity rules remain authoritative.
+
+## Decision
+
+Provide a load-wide source-version mode for ordinary and layered loading. The
+zero-value mode preserves current behavior. The enabled modes check declarations
+when present or require them, using the existing `Definition.Version` path and
+checker. Interpret the configured path relative to each participating source
+before placement or composition. Different declarations may be accepted by the
+same checker; ConfigKit adds no equality, SemVer, or expected-version rule.
+
+Apply the selected policy to the actual main document, direct underlays,
+imports, and selected overlays. Missing non-main declarations are skipped in
+if-present mode and rejected in required mode. Existing main absence behavior
+remains, with required mode additionally rejecting an absent declaration on an
+actual main document. `NoSource`, unmatched optional inputs, synthetic field
+operations, and empty groups do not become documents that need declarations.
+
+Reuse the existing version vocabulary and validation boundary: absence, null,
+exact strings, and strict decimal integers retain their established meanings;
+unsupported present values fail version-type validation. Warnings remain
+nonfatal, and source declarations do not replace final application decoding or
+domain validation.
+
+After an accepted non-main declaration is checked, consume exactly that node
+before its remaining application content is composed or overlaid. Preserve the
+main declaration and its value semantics. Keep the declaration, source-local
+path, physical location, and checked absence as detached source evidence,
+separate from effective application values and history. Apply established
+sensitivity rules to that evidence, including original mounted paths and
+ancestors; retain existing safe failure disclosure.
+
+In enabled modes, the main declaration remains source-local while its protected
+application path is derived from the main input's destination. This distinction
+does not change default-mode path interpretation or weaken protection of
+remaining payload and synthetic operations.
+
+Public option-field additions and the enabled-mode interpretation for mounted
+main inputs are compatibility considerations for separately authorized minor-
+release preparation. This decision does not select a release version or
+authorize release notes or publication.
+
+## Alternatives Considered
+
+- **Check only the final composed value.** Later sources can hide an earlier
+  rejected declaration, so this does not validate every contributing document.
+- **Let consumers parse and check each source.** This duplicates acquisition,
+  source attribution, safe diagnostics, and lifecycle coordination in every
+  application.
+- **Require declarations to be equal or impose a universal version grammar.**
+  Consumers already own the acceptance policy; ConfigKit need not invent an
+  expected value or a domain-specific compatibility rule.
+- **Keep accepted non-main declarations in application payload.** A selected
+  source-local declaration is metadata for this policy and cannot also remain
+  an ordinary field at the same path without ambiguity.
+
+## Consequences
+
+Consumers can apply one existing version policy across contributing documents
+and select gradual or required adoption per load. Accepted non-main headers no
+longer participate in application composition; their inspectable evidence is
+kept separate and subject to sensitivity classification. Default callers retain
+current behavior. The implementation plan records the concrete API names,
+integration order, and validation evidence.

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 +77,203 @@ 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.
+
+#### Input destinations
+
+`Input.Destination` places the source's application mapping beneath a path
+relative to the configuration root. Its zero or empty value means root, so
+existing inputs keep their behavior. Main inputs and underlay inputs use the
+same destination model. Destination segments are literal mapping keys: dots,
+slashes, and numeric-looking text are not split or interpreted as sequence
+indexes. Explicit index segments and paths that introduce the reserved
+`configkit` or mapping-valued `extends` structure are invalid input options and
+produce a structured `LoadError` before source acquisition.
+
+Source-root `configkit.imports` directives are processed before placement. An
+import without an explicit destination inherits the effective destination of
+its importing document, including through nested imports. An import may instead
+declare a relative destination as a sequence of literal mapping keys; its keys
+append to the importing document's effective destination. Omission and `[]`
+add no keys. Nested imports continue relative to their immediate importing
+document, each declaration is independent of its siblings, and all matches of
+one glob receive the same resulting destination. This logical placement does
+not change how the physical import path resolves from its declaring file.
+
+A valid destination on `NoSource()` contributes nothing; an actual empty
+mapping materializes its destination containers. Once placed, contributions
+follow the same strict composition within a tier and recursive precedence
+behavior described above. A selected overlay and CLI overrides remain rooted at
+the application root and are not moved beneath the main input's destination.
+
+`SourceInfo.Destination` reports the placement for each acquired input source,
+including imports. Placement does not change source IDs, acquisition identity,
+physical source locations, or passive resource contexts. Introduced ancestor
+mappings use the contributing document's root location as a source fallback;
+authored content retains its original source coordinates. Returned source
+records are detached copies.
+
+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.
+The accepted rationale for input placement and its relative-import extension
+is recorded in [ADR 0011](adr/0011-input-destinations.md) and
+[ADR 0012](adr/0012-relative-import-destinations.md).
+
+### 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
@@ -103,13 +306,75 @@ error without reporting one, ConfigKit emits a generic `version.hook_failed`
 diagnostic and suppresses the returned error text. Warnings alone are retained on a
 successful result. Callback panics remain programming failures.
 
+#### Shared source-version checks
+
+`Options.SourceVersionMode` and `LayeredOptions.SourceVersionMode` select how
+one `Definition.Version` policy applies across source documents. The zero value
+`SourceVersionModeDefault` preserves the existing main-only check at its
+effective path and existing protected-path behavior. `SourceVersionModeIfPresent`
+checks each participating document that declares a version and permits absent
+non-main declarations. `SourceVersionModeRequired` rejects a participating
+document without a declaration. Enabled modes require a usable `VersionRule`
+and checker; an unknown mode or unusable rule is an options error before source
+access or callbacks.
+
+In enabled modes, `Definition.Version.Path` is interpreted in each acquired
+document's own coordinates, before placement and composition. The main document,
+direct underlay documents, imported documents (including transitive and glob
+matches), and the selected overlay participate. `NoSource`, absent optional
+imports, empty underlay groups, defaults, resolution output, inheritance, and
+synthetic CLI overrides are not documents to check. An absent declaration in an
+actual main document still reaches the checker in if-present mode; required mode
+rejects it first. With `NoSource`, the existing single absent-main checker
+behavior remains in both enabled modes. If-present omissions in non-main
+documents skip the checker. Explicit null is present and reaches the checker;
+unsupported selected values produce `version.type`. No equality with the main
+declaration or non-null rule is imposed. Acceptance establishes only that the
+declaration is accepted by the consumer's checker; it does not establish that
+each fragment conforms to a complete application schema. Final assembled
+decoding and application validation still run once.
+
+A header that is absent below a structural control follows the same omission
+policy. Actual header/control overlap is rejected, and ordinary structural
+validation and main-version ownership protection continue independently.
+
+Accepted non-main declarations are consumed from their owned payload before
+composition; their authored ancestors remain, even if empty. The main
+declaration remains in the application. Ordinary release metadata should use a
+different path. Enabled main protection uses the main input's effective path:
+its `Destination` prefixed to the source-local `VersionRule.Path`. This protects
+the remaining payload against imports, underlays, inheritance, and CLI
+overrides. Thus a mounted main looks up a local header while ownership checks
+protect its destination-prefixed application path. Main checker diagnostics and
+declaration locations use source-local coordinates; ordinary payload
+diagnostics continue to use effective paths. Default-mode diagnostics are
+unchanged.
+
+`SourceInfo.VersionCheck` is nil when enabled declaration processing did not
+occur. Otherwise it records a source-local `Path`, the observed `Version` state
+(`Present`, `Null`, and, when disclosed, exact `Text`), an actual or nearest
+available `Location`, and whether text was redacted. A non-nil record with
+`Present == false` records observed absence; it does not say the checker ran or
+accepted a declaration. Absent evidence still identifies the full requested
+header path, while its location identifies the nearest available source node.
+During callbacks and in failed-load source records,
+text is conservatively withheld because final classification may not be
+available. Successful final source records reflect completed sensitivity
+classification at the declaration's mounted path and ancestors, including
+historical classification. Redaction empties `Text` while retaining presence
+and null state. Returned copies are detached; evidence does not reopen inputs.
+Consult [ADR 0013](adr/0013-shared-source-version-checks.md) for the accepted
+decision and the [development guide](development.md#baseline-validation) for
+the completed implementation and remediation records in Git history.
+
 ### Layered metadata
 
 `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 +389,9 @@ 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 +407,15 @@ 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 +431,11 @@ 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 +470,10 @@ 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
@@ -238,6 +521,46 @@ an explicit `LoadSecretsDir` bootstrap, then use `WriteDiagnostics`,
 ConfigKit does not infer bootstrap outcomes from a secret reference and does not
 bundle these independent reports into a command framework.
 
+## Effective configuration diffs
+
+`configkit.Diff(left, right)` compares two completed successful loads of the same
+configuration type. `left` is the earlier state and `right` is the later state.
+Applications load both sides with the sources and options they choose; to inspect
+an overlay's impact, load the same base once without the overlay and once with it
+selected. The [consumer guide](consumers/README.md#effective-configuration-diffs)
+links to maintained examples. The public API returns safe changed-region records;
+applications own their presentation.
+
+Comparison uses the frozen canonical effective inspection values, not mutable
+`Result.Config`, source text, provenance, or history. It does not reopen sources,
+reread environment variables, or rerun defaults or callbacks. Equal effective
+values produce no records even when sources, explicit-input status, defaults, or
+resolution histories differ; for example, explicit zero and defaulted zero are
+equal. Mappings compare recursively, sequences compare atomically, and scalar,
+kind, or null/container replacements report their affected region. Records use
+the existing comparator's canonical pointer ordering. `Present=false` with empty
+YAML means absence; explicit null is present and renders as `null`.
+
+Before either side is returned, sensitivity from both loads is applied to both
+snapshots, including historical and dependency-derived classification. A
+classification on either side protects the corresponding values on both sides.
+`Redacted` reports masking, while `HiddenChanged` reports an actual concealed
+value or presence change; it is not implied merely by redaction. A structural
+replacement that prevents safe correspondence masks the complete reported
+region, and an absent side remains absent. Record membership establishes that a
+change occurred even when safe snapshots render identically. Literal
+marker-looking input is ordinary input; consumers use the record fields rather
+than interpreting marker text.
+
+Typed secret references contribute their configured environment names to the
+frozen comparison, never resolved credential bytes. Credential rotation behind
+an unchanged reference is therefore not detected. The records contain changed
+regions only, not paired full-document context; independently calling
+`WriteEffective` on each result does not create cross-protected context. Nil or
+uninitialized results return an error with no records. A failed `Load` retains
+its normal nil-result/`LoadError` behavior, which callers handle before invoking
+`Diff`.
+
 ## Typed secret references
 
 Credential-bearing fields use the opaque `Secret` type and store an environment
@@ -442,8 +765,8 @@ 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
@@ -526,9 +849,13 @@ zero.
 Imports are opt-in. With `Options.Imports` enabled, a document may reserve its root
 `configkit` mapping for an `imports` sequence. ConfigKit removes that mapping before
 application decoding. Each sequence item is a mapping with exactly one nonempty
-string `path` or `glob` plus an optional boolean `optional`; all other shapes and
-fields are `import.invalid`. A `configkit` key below the root is invalid. With
-imports disabled, any occurrence remains `import.disabled`.
+string `path` or `glob`, an optional boolean `optional`, and an optional
+`destination` sequence of strings. Every destination string is one literal
+mapping key and follows the input-destination validation rules above; quote
+numeric-looking keys so YAML reads them as strings. Null, scalar destinations,
+and non-string members are `import.invalid`, as are all other unsupported shapes
+and fields. A `configkit` key below the root is invalid. With imports disabled,
+any occurrence remains `import.disabled`.
 
 The directive key is `configkit`, not the `_configkit` spelling used in v0.1.0.
 There is no legacy alias: `_configkit` is ordinary application data, subject to
@@ -557,18 +884,25 @@ 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 +15,12 @@ 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 |
+| Mounted collections | Entry-declared relative destinations, nested metadata/prompt/profile imports, local inheritance, and a version-only higher-tier override | Collection IDs, profile meaning, and opening/interpreting prompt resources |
+| Named presets | One-load sibling inheritance, explicit false, nested destination resolution, and source/derivation inspection | Choosing and executing a preset |
+| Effective configuration diff | Same-type frozen effective comparisons for two selected files or base-versus-overlay impact | Source/options selection and consumer-owned safe presentation |
+| Shared source-version checks | One `Definition.Version` policy for accepted declarations across participating source documents, with optional or required declarations and sanitized evidence | Choosing which declarations the application accepts; final assembled schema and domain validation |
 
 The YAML under [`testdata`](../../examples/consumers/testdata) is loaded by the
 executable examples and ordinary tests. It contains no private application data,
@@ -48,6 +54,115 @@ 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.
 
+## Shared source-version checks
+
+[`LoadVersionedService`](../../examples/consumers/source_version.go) demonstrates
+if-present and required modes with one `Definition.Version` rule across a mounted
+main input and an underlay. The example reads only sanitized
+`SourceInfo.VersionCheck` fields; its executable test is in
+[`layered_test.go`](../../examples/consumers/layered_test.go). The
+[configuration contract](../config.md#shared-source-version-checks) defines
+participation, source-local lookup and diagnostics, non-main header consumption,
+main-path protection, default behavior, and evidence disclosure. Accepted
+declarations do not replace final assembled decoding or application validation.
+
+## Effective configuration diffs
+
+[`CompareFiles`](../../examples/consumers/diff.go) loads two caller-selected files
+with one application definition, handles each load error, calls `configkit.Diff`,
+and presents the safe records through consumer-owned `FormatDifferences`.
+[`CompareRolloutOverlay`](../../examples/consumers/diff.go) loads the same base
+once without an overlay and once with the selected overlay, so omitted lower
+priority values remain present. Its resolver records a derived endpoint URL after
+overlay application, and its nested typed secret reference is classified before
+the resulting records are presented.
+
+The maintained [behavior tests and executable examples](../../examples/consumers/diff_test.go)
+use temporary files and disposable secret values. Presenters should use
+`ValueSnapshot.Present` for absence, record membership for change, and
+`HiddenChanged` for a withheld change indicator. Do not parse the redaction marker
+or add separately rendered full contexts that lack combined sensitivity. The
+[configuration contract](../config.md#effective-configuration-diffs) owns shared
+equality, granularity, and disclosure semantics.
+
+## 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.
+
+[`LoadPromptBundle`](../../examples/consumers/layered.go) is the ordinary-root
+counterpart: its entry opts into imports through `Load`, imports metadata at the
+root, and places a prompt beneath `prompts` using YAML. The executable example
+and behavior test are in
+[`layered_test.go`](../../examples/consumers/layered_test.go).
+
+## Input destinations and mounted collections
+
+Main and underlay inputs use the same destination field. An import without a
+`destination` inherits its declaring document's effective destination; an
+explicit sequence of key strings appends a relative suffix. Nested imports
+append to their immediate parent's effective destination, while filesystem
+paths remain relative to the declaring file. Destinations are application
+mapping paths, independent of filesystem context. The configuration contract
+owns the complete behavior and validation rules.
+
+The small [`Example_inputDestination`](../../examples/consumers/destinations_test.go)
+mounts one mapping source beneath literal collection/prompt keys and loads a
+second input without a destination to show that root placement remains the
+default. Consumers pass paths, not filenames or namespace strings for ConfigKit
+to interpret.
+
+[`LoadCollectionBundle`](../../examples/consumers/destinations.go) assembles an
+embedded collection manifest mounted beneath a literal collection path. The
+entry's YAML places nested metadata imports at the collection root and prompt
+and profile peers beneath their own keys. A higher-priority input changes only
+one prompt's ordinary `version` field. The main document alone supplies the
+application schema version. A prompt's local `extends` relationship resolves
+after all tiers, so `custom` inherits the final `brief` content and fields while
+retaining its own profile selection.
+
+The resolver follows each effective `content_file` origin to its declaring
+source context. This also works for `custom`'s inherited resource. The
+application-owned [`ReadCollectionPromptContent`](../../examples/consumers/destinations.go)
+demonstrates the same lookup after loading; ConfigKit does not open or interpret
+the resource. [`destinations_test.go`](../../examples/consumers/destinations_test.go)
+checks source destinations and authored coordinates, presence, inherited values,
+relative resources, and strict peer conflicts. The
+[configuration contract](../config.md#input-destinations) owns shared placement,
+import, source-location, and ownership behavior.
+
 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 +241,10 @@ 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 |
+| Literal input destinations, root-default compatibility, and mounted peer collections | `Example_inputDestination`, `LoadCollectionBundle`, and `destinations_test.go` |
+| Named sibling inheritance, presence, and derivation | `Example_inheritedPresets` and `TestPresetCatalogOneLoadAndDerivation` |
+| Effective configuration diffs and consumer-owned presentation | `ExampleDiff`, `ExampleCompareFiles`, and `ExampleCompareRolloutOverlay` |
 | 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

@@ -39,6 +39,8 @@ public behavior.
 | Task | Read |
 | --- | --- |
 | Repository orientation or component responsibility | [Component overview](internal/overview.md) |
+| Input destinations | [Configuration contract](config.md#input-destinations), [ADR 0011](adr/0011-input-destinations.md), and the destination tests below |
+| Shared source-version checks | [Configuration contract](config.md#shared-source-version-checks), [ADR 0013](adr/0013-shared-source-version-checks.md), and the version/source-version tests below |
 | Documentation changes | The owning document |
 | Licensing, dependencies, attribution, or release-document changes | [Project grant](../COPYRIGHT) and [attribution](oss/attribution.md); release changes also follow the release procedure |
 | Architecture decisions or future work | [Deferred scope](roadmap/deferred.md) and relevant ADRs listed below |
@@ -49,11 +51,14 @@ public behavior.
 Accepted decisions:
 
 - [Separate composition and overlay](adr/0001-separate-composition-and-overlay.md)
+- [Check source versions with one shared policy](adr/0013-shared-source-version-checks.md)
 - [Own YAML semantics behind a private parser boundary](adr/0002-own-yaml-semantics.md)
 - [Preserve presence and record application resolution](adr/0003-preserve-presence-and-resolution-history.md)
 - [Secret references and explicit bootstrap](adr/0004-secret-references-and-explicit-bootstrap.md)
 - [Explicit overrides and safe explanation](adr/0005-explicit-overrides-and-safe-explanation.md)
 - [Keep the public facade root-owned](adr/0006-public-facade.md)
+- [Compare frozen effective configuration snapshots](adr/0010-effective-configuration-diff.md)
+- [Place inputs at literal application mapping paths](adr/0011-input-destinations.md)
 
 For cross-cutting changes, follow every applicable row. Completed implementation
 and remediation plans are preserved in Git history at `cb5b435`; use current
@@ -64,8 +69,9 @@ git show cb5b435:docs/roadmap/remediation.md
 ```
 
 Release-candidate, publication, and verification work follows the operational
-[release procedure](release.md). Its single publication command validates the
-committed candidate and publishes only the intended refs.
+[release procedure](release.md). Its module-selected ReleaseKit `publish`
+invocation validates the committed candidate and publishes only the intended
+tag; those release checks remain distinct from the everyday development check.
 
 The completed string-and-integer schema-version implementation and remediation
 records are preserved in Git history at `244104f`. The
@@ -80,14 +86,56 @@ 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),
 and release operations belong to the [release procedure](release.md).
 
-Install Go 1.25.0 or a supported patched/newer Go toolchain. Acquire declared
-dependencies explicitly before entering an offline development or validation
-environment:
+The completed ReleaseKit adoption, startup performance request and experiments,
+benchmark evidence, performance audit, implementation plan, and remediation
+record are preserved in Git history at `8e91a2b` under `docs/roadmap/`. For example:
+
+```sh
+git show 8e91a2b:docs/roadmap/startup-benchmark.md
+```
+
+Remaining optimization candidates belong to
+[deferred scope](roadmap/deferred.md#deferred-performance-experiments).
+
+The completed effective-configuration diff roadmap, implementation plan, and
+resolved review findings are preserved in Git history at `1c2610c` under
+`docs/roadmap/`. The
+[configuration contract](config.md#effective-configuration-diffs) owns current
+comparison and disclosure behavior, and
+[ADR 0010](adr/0010-effective-configuration-diff.md) records the decision.
+
+The completed input-destination, relative-import-destination, and shared
+source-version roadmaps, associated consumer request, implementation plan, and
+resolved review findings are preserved in Git history at `e4270c9` under
+`docs/roadmap/`. For example:
+
+```sh
+git show e4270c9:docs/roadmap/implementation.md
+git show e4270c9:docs/roadmap/remediation.md
+```
+
+Current behavior belongs to the configuration contract's
+[input destinations](config.md#input-destinations),
+[imports](config.md#explicit-imports), and
+[shared source-version checks](config.md#shared-source-version-checks) sections.
+Possible follow-ups remain in [deferred scope](roadmap/deferred.md).
+
+Install the Go version required by `go.mod` or a compatible newer toolchain.
+Acquire declared dependencies explicitly before entering an offline
+development or validation environment:
 
 ```sh
 go mod download
@@ -117,11 +165,10 @@ Use focused checks while iterating, with validation proportionate to the change
 under the testing policy. Documentation-only work normally needs the local-link
 check below and whitespace review; behavior-bearing examples and commands also
 need execution. Use `go test -race ./...` for ownership or concurrency changes.
-The [publication command](release.md#4-publish-the-committed-candidate) invokes
-the release checker once. It performs one minimum-toolchain offline race-suite
-run, one full vet pass, and Linux cross-builds using readonly module mode.
-It already covers examples, links, and package boundaries; do not additionally
-run the everyday checker just to repeat those checks. CI never gates a release.
+The [release procedure](release.md#4-publish-the-committed-candidate) performs
+its selected validation through one `publish` invocation. The everyday checker
+remains a development gate and is not a separate release prerequisite. CI never
+gates a release.
 
 ## Task-to-test map
 
@@ -132,16 +179,25 @@ code changes. Use the component overview to find the corresponding implementatio
 | Task | Behavioral test owners | Focused command |
 | --- | --- | --- |
 | 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'` |
+| 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), [relative destinations](../internal/engine/destination_import_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'` |
+| Input and relative-import destinations, mounted sources, and source ownership | [Destination engine tests](../internal/engine/destination_test.go), [tier tests](../internal/engine/destination_tiers_test.go), [import traversal and version tests](../internal/engine/destination_import_test.go), [accounting tests](../internal/engine/destination_accounting_test.go), [lifecycle inspection tests](../internal/engine/destination_import_lifecycle_test.go), [public source/error tests](../facade_destination_test.go), [resource contexts](../facade_resource_context_test.go), and [consumer examples](../examples/consumers/destinations_test.go) | `go test ./internal/engine -run 'Destination|Import|Glob|Mounted'`, `go test . -run 'Destination|Import|SourceContext|FacadeMounted'`, and `go test ./examples/consumers` |
+| Shared source-version modes and evidence | [Engine source-version tests](../internal/engine/source_version_test.go), [public facade tests](../facade_source_version_mode_test.go), and [consumer example tests](../examples/consumers/layered_test.go) | `go test ./internal/engine -run 'SourceVersion'`, `go test . -run 'SourceVersion'`, and `go test ./examples/consumers` |
 | 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'` |
+| Effective configuration diff | [Engine comparison and masking tests](../internal/engine/diff_test.go), [engine operation tests](../internal/engine/diff_operation_test.go), [root facade tests](../facade_diff_test.go), and [consumer examples](../examples/consumers/diff_test.go) | `go test ./internal/engine -run 'Diff|Explain|History|Secret|Independent'`, `go test . -run 'Diff|FacadeInspection|PublicFacadeBoundary|DependencyImportBoundary'`, `go test ./examples/consumers`, and `go test . -run '^ExampleDiff$'` |
+| Consumer examples and migration fixtures | [Consumer tests](../examples/consumers/consumers_test.go), [native, layered, and ordinary relative-import examples](../examples/consumers/layered_test.go), [mounted collection examples](../examples/consumers/destinations_test.go), [API examples](../example_test.go) | `go test ./examples/consumers` and `go test . -run '^Example'` |
+| Assembly and layered scaling evidence | [Assembly benchmarks](../internal/engine/assembly_bench_test.go), [layered benchmarks](../internal/engine/underlay_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkUnderlay' -benchmem`; repeat with `^BenchmarkCompose` and `^BenchmarkOverlay` |
+| Inheritance scaling evidence | [Expansion benchmarks](../internal/engine/inheritance_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkInheritance' -benchmem` |
 | Documentation links and dependency isolation | [Link checks](../documentation_test.go), [import boundary](../import_boundary_test.go), [public boundary](../public_boundary_test.go) | `go test . -run 'TestLocalDocumentationLinks\|TestDependencyImportBoundary\|TestPublicFacadeBoundary'` |
-| Development and release checker behavior | [Shell regression](../release_check_test.go) | `go test . -run 'TestCheckScripts\|TestDevelopmentCheck'` |
-| Guarded release publication and recovery | [Publication regression](../release_publish_test.go) | `go test . -run '^TestReleasePublication'` |
+| Development check behavior | [Development checker test](../development_check_test.go) | `go test . -run '^TestDevelopmentCheckHandlesFilenamesAndStopsOnFailure$'` |
+| Release profile wiring | [Release profile test](../releasekit_profile_test.go) | `go test . -run '^TestReleaseKitProfile$'` |
 
 ## Fuzz validation
 

docs/internal/overview.md

@@ -7,23 +7,36 @@ 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, per-input mapping
+  placement and immediate-parent-relative import placement within that
+  assembly, destination-local sibling inheritance before typed decoding, and
+  shared source-version checking with sanitized declaration evidence.
+  It retains each source's effective placement alongside physical source
+  identity and owns schema
+  inspection, lifecycle coordination, callback auditing,
+  provenance/history, passive source-context retention, secret resolution and
+  bootstrap, safe inspection rendering, and effective-value comparison over
+  frozen inspection snapshots. 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.
+- Root `Diff` and `Difference` provide the public comparison boundary and
+  convert detached engine records to root-owned paths and snapshots; consumer
+  examples own presentation of the safe changed regions.
 - `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
@@ -32,8 +45,17 @@ these constraints and rejects exported aliases, fields, or signatures involving
 internal types.
 
 The [configuration contract](../config.md) owns implemented YAML and lifecycle
-semantics. The [consumer adoption guide](../consumers/README.md) owns application
-guidance. The [architecture policy](../policy/architecture.md) owns contributor
-boundaries, and [ADR 0006](../adr/0006-public-facade.md) records the facade
+semantics, including [input destinations](../config.md#input-destinations),
+[shared source-version checks](../config.md#shared-source-version-checks), and
+[effective configuration diffs](../config.md#effective-configuration-diffs).
+The [consumer adoption guide](../consumers/README.md) owns application guidance.
+The [architecture policy](../policy/architecture.md) owns contributor
+boundaries; [ADR 0006](../adr/0006-public-facade.md) records the facade decision
+and [ADR 0010](../adr/0010-effective-configuration-diff.md) records the diff
+decision; [ADR 0011](../adr/0011-input-destinations.md) records the placement
 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. [ADR 0012](../adr/0012-relative-import-destinations.md)
+records the relative-import extension; [ADR 0013](../adr/0013-shared-source-version-checks.md)
+records the source-version decision.

docs/oss/attribution.md

@@ -2,11 +2,12 @@
 
 ## Standard Repository Policies
 
-Material: `docs/policy/meta.md`, `docs/policy/oss.md`,
+Material: `docs/policy/release.md`, adapted from
+`e833faddf120c789a9b45348edaf75720de16348`; `docs/policy/meta.md`, `docs/policy/oss.md`,
 `docs/policy/documentation.md`, `docs/policy/agents.md`,
-`docs/policy/architecture.md`, `docs/policy/testing.md`, and
-`docs/policy/release.md`; root `AGENTS.md`; and the Policy Reading Guide in
-`docs/development.md`, adapted for this repository.
+`docs/policy/architecture.md`, `docs/policy/testing.md`; root `AGENTS.md`; and the
+Policy Reading Guide in `docs/development.md`, adapted from
+`d84a38ebab26f048cafbe59a2246700c35d29843` for this repository.
 
 Copyright (C) 2026 Eric Rakestraw
 
@@ -14,10 +15,14 @@ SPDX-License-Identifier: CC-BY-SA-4.0
 
 License and notices: [CC BY-SA 4.0](licenses/CC-BY-SA-4.0.txt) |
 [COPYRIGHT](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/COPYRIGHT) |
-[LICENSE](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/LICENSE).
+[LICENSE](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/LICENSE) |
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348/COPYRIGHT) |
+[LICENSE](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348/LICENSE).
 
 [Source](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843).
 
+[Source](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348).
+
 ## go-yaml v4
 
 Selected module: `go.yaml.in/yaml/v4` at `v4.0.0-rc.6`.

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
@@ -79,6 +99,15 @@ types, field meanings, defaults, normalization, semantic validation, and schema
 version policy. Applications also own main-file search conventions, CLI wiring,
 remote acquisition, runtime initialization, and resource checks.
 
+The engine applies each input's mapping destination within the existing source
+assembly, applies immediate-parent-relative YAML destinations to imports, and
+retains each effective placement with source identity and inspection records;
+the root facade owns the public fields and conversions. The
+[configuration contract](../config.md#input-destinations) defines the observable
+placement behavior. [ADR 0011](../adr/0011-input-destinations.md) records input
+placement, and [ADR 0012](../adr/0012-relative-import-destinations.md) records
+the import extension.
+
 Library code must not depend on consuming applications or initialize their
 runtime resources. It must not perform implicit network access, discover
 application-specific paths, or change process-global state during ordinary `Load`.
@@ -88,6 +117,22 @@ 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 +161,41 @@ 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
@@ -142,6 +210,13 @@ See [ADR 0005](../adr/0005-explicit-overrides-and-safe-explanation.md).
 Detailed syntax and operation tables belong to the
 [current configuration contract](../config.md).
 
+The frozen inspection boundary also supports comparing completed same-type
+results without reopening sources or rerunning the load lifecycle. The
+[configuration contract](../config.md#effective-configuration-diffs) owns
+comparison and disclosure behavior, the [component overview](../internal/overview.md)
+owns implementation responsibility, and [ADR 0010](../adr/0010-effective-configuration-diff.md)
+records the accepted decision.
+
 ## Ownership And Confidentiality
 
 Returned configuration and metadata must not share mutable state across loads.

docs/policy/release.md

@@ -1,10 +1,10 @@
 # Release Policy
 
-**Base Policy Version:** 1.4
+**Base Policy Version:** 1.6
 
-**Last Updated**: 2026-09-28
+**Last Updated**: 2026-10-09
 
-**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/release.md)
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348/docs/policy/release.md)
 
 ## Purpose
 
@@ -12,6 +12,9 @@ This policy defines a proportionate workflow for validating and publishing an
 intended source revision under a fresh version. Releases should require little
 routine intervention, avoid repeated work, and preserve published contents.
 
+Adopt stronger verification or reproducibility requirements through an explicit
+project-specific decision justified by the intended audience and support needs.
+
 The repository-specific commands belong in `docs/release.md`. Historical release
 summaries belong in `docs/releases/`. The procedure must agree with the adopted
 policy and link to it rather than introduce additional release requirements.
@@ -54,10 +57,11 @@ authoritative primary branch, completes the release. The source commit may be
 pushed before the tag or published together with it through an atomic push.
 The procedure must choose and document the supported approach.
 
-ConfigKit uses sequential explicit pushes: first the captured candidate to
-authoritative `main`, then the annotated tag. Its
-[release procedure](../release.md) names `origin` and checks that its fetch and
-push destinations resolve to the authoritative URL.
+ConfigKit uses sequential publication: first push the committed candidate to
+authoritative `main`, then invoke ReleaseKit once to validate that candidate and
+publish its annotated tag. The [release procedure](../release.md) names
+`origin`; ReleaseKit checks its exact effective push URL before validation and
+tag publication.
 
 Release commands must finish at successful publication. They must not wait for
 or poll CI, require successful CI status, inspect downloadable artifacts, or
@@ -76,8 +80,9 @@ the tagged source. The procedure must identify those deliverables and the
 responsibility of their automation. Published deliverables must be traceable
 to that source and identify the same version.
 
-ConfigKit currently declares no additional deliverables. Its source-tag push
-is the completion point.
+ConfigKit currently declares no additional deliverables. Confirmed publication
+of the intended source tag is the completion point. CI, macOS validation,
+hosted delivery, and consumer retrieval are outside the release gates.
 
 ## Versioning And Compatibility
 
@@ -96,7 +101,8 @@ requires a deliberate update to the policy and procedure where needed.
 For releases at or after `v1.0.0`:
 
 - a major release may make intentional incompatible changes;
-- a minor release may add backward-compatible behavior; and
+- a minor release may add backward-compatible behavior or raise the Go consumer
+  minimum as described below; and
 - a patch release may make backward-compatible fixes.
 
 Before `v1.0.0`, a minor release may change a documented public, operational,
@@ -108,13 +114,53 @@ Additional compatibility promises and deliberate exceptions belong in the
 adopted policy or canonical contract documentation. Release notes explain
 changes; they do not redefine current compatibility contracts.
 
+## Consumer Minimum And Release Toolchain
+
+Distinguish the minimum toolchain supported for consumers from the toolchain
+maintainers use to build and validate a release. Record the consumer minimum in
+the language's standard metadata and explain its support meaning in canonical
+documentation. Define the selected release toolchain and its setup commands in
+`docs/release.md`. Toolchain selection and upgrade cadence are repository-specific
+choices.
+
+For Go repositories, normally declare the initial stable release of the selected
+series as the consumer minimum, such as `go 1.26.0`. Use a higher patch minimum
+when a dependency or an identified correctness fix requires it, and document
+the reason.
+
+For release-policy purposes, successful build and release validation on the
+selected Go patch release are sufficient to presume support across that Go
+series, subject to the declared consumer minimum and concrete evidence of
+incompatibility. Investigate specific compatibility concerns as they arise.
+
+Use the selected release toolchain for candidate validation and release builds,
+including CI jobs performing those tasks. A newer validation patch version alone
+leaves the consumer minimum unchanged.
+
+A minor release may raise the Go consumer minimum, including from one Go series
+to the next, both before and after `v1.0.0`. This is an explicit exception to the
+backward-compatibility requirement for minor releases. Patch releases preserve
+the consumer minimum. Explain an increase's impact and required consumer action
+in the release note. Other incompatible changes remain subject to the versioning
+rules above.
+
+### ConfigKit Toolchain Selection
+
+For minor and major releases, select the newest stable Go series available in
+the development environment. For patch releases, retain the series recorded in
+`go.mod` and its consumer minimum. Use the latest locally available compatible
+patch in the selected series, with `GOTOOLCHAIN=local`, to build the
+module-selected ReleaseKit CLI and validate the candidate. The same Go
+executable builds the tool and runs its validation commands.
+
 ## Release Documentation And Preparation
 
 Every new release must include `docs/releases/<version>.md` in the tagged
 source tree. Use the complete version tag as the filename and begin with a
 title containing the product name and that exact version.
 
-ConfigKit requires the first line `# ConfigKit <version>`.
+ConfigKit requires the first line `# ConfigKit <version>`. ReleaseKit's
+publication guard checks the release-note H1 for the product and version.
 
 Include a short summary and any relevant compatibility impact or upgrade
 instructions. Choose sections suited to the release and keep the note
@@ -146,10 +192,9 @@ checks publication state, and publishes the intended refs. A validation-only
 mode or command may be provided for diagnosis, but must not be a required step
 before publication repeats the same checks.
 
-ConfigKit uses `./scripts/release.sh vMAJOR.MINOR.PATCH`. The separate
-`scripts/release-check.sh` remains an optional diagnostic command; the
-publication command invokes it once and does not use a persistent candidate
-record.
+ConfigKit uses the module-selected ReleaseKit CLI's `publish
+vMAJOR.MINOR.PATCH` command. Its `check` command is optional for diagnosis; it
+is not a required preliminary pass to publication.
 
 Run the complete deterministic, offline default test suite once for the final
 candidate, following the [testing policy](testing.md). For repositories that
@@ -157,13 +202,17 @@ publish builds, verify the required build targets. Select those targets for
 the application's actual support commitments; native testing on each platform
 and access to particular hardware are not default release prerequisites.
 
-ConfigKit's one maintained Go 1.25 baseline runs the full suite with the race
-detector and test-time vet disabled. This protects library behavior, ownership,
-and concurrent use; the suite already includes examples, consumer fixtures,
-documentation links, and package boundaries. One full vet pass adds static
-checks beyond the test-time subset. CGO-disabled Linux amd64 and arm64 builds
-cover supported configurations distinct from native race compilation. macOS
-and newer-toolchain checks are optional.
+Prepare dependencies and build the tool before offline validation; the
+publication command is the required validation run, with no separate successful
+test or CI run required before it.
+
+The selected profile runs the complete deterministic, offline default suite
+once with the race detector and test-time vet disabled. The suite already
+includes examples, consumer fixtures, documentation links, and package
+boundaries. One full vet pass adds static checks beyond the test-time subset.
+CGO-disabled Linux amd64 and arm64 package compilation covers supported
+configurations distinct from native race compilation. macOS and newer-toolchain
+checks are optional.
 
 Additional gates must address a concrete risk that the existing checks do not
 adequately cover. Do not automatically require linting, race testing, module
@@ -172,9 +221,10 @@ expanded platform matrices solely because they are possible. Retain such
 checks when their distinct protection justifies their cost. A one-time migration
 check must not silently become a permanent release requirement.
 
-The selected Go commands use readonly module mode with `GOPROXY=off`; acquire
-the baseline toolchain and dependencies before validation. Formatting and
-whitespace remain ordinary development checks in `scripts/check.sh`.
+ReleaseKit runs its selected Go commands with readonly module mode and
+`GOPROXY=off`; acquire dependencies and build the tool before candidate
+validation. Formatting and whitespace remain ordinary development checks in
+`scripts/check.sh`.
 Review module tidiness when dependencies change, not as a release gate.
 
 Release tests must not require real credentials, mutable application services,
@@ -206,16 +256,19 @@ Automate the checks needed to publish the intended source under a fresh version:
   authoritative remote;
 - the worktree and index are clean on the primary release branch;
 - the source being published is the exact candidate that passed validation,
-  including its matching release note;
+  including its matching release note H1;
 - the remote primary branch can be advanced to that candidate without
   discarding remote history, and is published before the tag; and
 - the tag identifies the validated candidate and only the intended refs are
   published.
 
-ConfigKit also checks that `origin` has exactly the authoritative fetch and
-push destination, including configured push URLs. The command checks the
-candidate and unused version again after validation, and checks local state
-before creating the tag. Ignored local files do not block publication.
+ReleaseKit checks that `origin`'s effective push URL exactly matches the
+authoritative URL before candidate validation. This does not assert the
+configured fetch URL or require the release-note H1 to be the first line; the
+required `# ConfigKit <version>` first-line format remains a content rule. The
+command checks the candidate and unused version again after validation, and
+checks local state before creating the tag. Ignored local files do not block
+publication.
 
 Fetch the authoritative branch and tag state as needed for these checks.
 Do not force branch or tag updates to bypass a failed guard. Publish explicit
@@ -248,10 +301,10 @@ release without replacing its contents or reusing its version.
 ## Repository Release Procedure
 
 Maintain `docs/release.md` as a short, executable maintainer runbook. It must
-identify prerequisites, the primary branch and authoritative remote, version
-and note preparation, one publication command with local checks and guards,
-source-only deliverables, the successful-tag-push completion point, and
-partial or unknown-state recovery.
+identify prerequisites, the selected release toolchain, the primary branch and
+authoritative remote, version and note preparation, one publication command with
+local checks and guards, source-only deliverables, the successful-tag-push
+completion point, and partial or unknown-state recovery.
 
 Use copyable commands where practical. Keep optional diagnostics and consumer
 verification clearly optional. The procedure must not turn preparation work,

docs/release.md

@@ -1,7 +1,7 @@
 # ConfigKit Release Procedure
 
 This is the maintainer runbook for ConfigKit's [release policy](policy/release.md).
-ConfigKit publishes one source-only Go module. The immutable annotated tag and
+ConfigKit publishes a source-only Go module. The immutable annotated tag and
 matching checked-in release note are the deliverables; there is no binary,
 container image, generated package, or separate registry publication.
 
@@ -9,183 +9,221 @@ container image, generated package, or separate registry publication.
 
 - Module: `gitea.maximumdirect.net/eric/ConfigKit`
 - Primary branch: `main`
-- Authoritative remote: `origin`, with fetch and push destination
-  `https://gitea.maximumdirect.net/eric/ConfigKit.git`
+- Authoritative remote: `origin`
+- Authoritative push URL: `https://gitea.maximumdirect.net/eric/ConfigKit.git`
 - Tags: stable Semantic Versions such as `v0.5.0`; no prereleases
-- Minimum Go line: Go 1.25, validated with one maintained patch
+- Minimum Go version: the first stable patch of the selected Go series, or a
+  higher patch when required by a dependency or specific correctness fix, as
+  recorded in `go.mod`; see the
+  [consumer minimum policy](policy/release.md#consumer-minimum-and-release-toolchain)
 - Release-gated targets: Linux amd64 and arm64
 - Best-effort targets: macOS amd64 and arm64, under the
   [platform support policy](policy/release.md#platform-support)
 
-The checker runs one offline race suite, one full vet pass, and CGO-disabled
-builds for both Linux targets. It reports skipped filesystem cases. macOS checks
-and newer-toolchain tests are optional. CI is never a readiness or completion
-gate; do not wait for or poll it.
+Release validation runs one offline race-enabled Go suite, one full vet pass,
+and CGO-disabled package compilation for both Linux targets. macOS and CI are
+not release gates; do not wait for or poll CI.
 
-## 1. Prepare access and tools
+## 1. Select Go and prepare the tool
 
-Use a clean checkout with Git, Bash 3.2 or newer, a maintained Go 1.25 patch,
-and a native compiler for race tests. Configure private-host access through the
-normal credential manager. Keep credentials and environment dumps outside the
-repository and command transcript.
+Use a clean checkout with Git, Bash 3.2 or newer, and a native compiler for race
+tests. Choose the release type before selecting Go. For a minor or major release,
+select the newest stable Go series available in the development environment.
+For a patch release, retain the series recorded in `go.mod` and leave its consumer
+minimum unchanged. Follow the
+[toolchain selection policy](policy/release.md#configkit-toolchain-selection).
 
-Verify access without displaying credentials:
+Put the selected series' latest locally available compatible patch first on
+`PATH`, ensuring it meets `go.mod`'s minimum. The same `go` executable must build
+ReleaseKit and run its child validation commands. Set local toolchain selection
+and verify it:
 
 ```sh
-git remote get-url origin
-git ls-remote origin HEAD
+export GOTOOLCHAIN=local
+go version
+go env GOVERSION
+command -v go
 ```
 
-The fetch URL and all configured push destinations must resolve to the
-authoritative URL above. The publication command checks this again. Acquire the
-selected toolchain and dependencies explicitly before offline validation:
+Only when a minor or major release advances the consumer minimum to a newer Go
+series, derive that series' initial stable release from the selected executable:
 
 ```sh
-export CONFIGKIT_BASELINE_TOOLCHAIN=go1.25.14
-GOTOOLCHAIN="$CONFIGKIT_BASELINE_TOOLCHAIN" go version
-GOTOOLCHAIN="$CONFIGKIT_BASELINE_TOOLCHAIN" go mod download
+selected_go=$(GOTOOLCHAIN=local go env GOVERSION)
+selected_series=${selected_go#go}
+selected_series=${selected_series%.*}
+release_minimum="$selected_series.0"
+go mod edit -go="$release_minimum"
 ```
 
-A maintained patch may replace the example pin. No per-release latest-version
-survey is required. Dependencies must be cached; the checker sets `GOPROXY=off`.
-Publication requires user authorization. Standing authorization for local
-commits does not authorize a push or release.
+If a dependency or identified correctness fix requires a higher patch minimum,
+use that required version instead and document the reason. When the series is
+unchanged, leave the minimum unchanged unless a minor or major release
+deliberately changes it under the
+[consumer minimum policy](policy/release.md#consumer-minimum-and-release-toolchain).
+For example, validating with Go 1.26.8 leaves `go 1.26.0` unchanged. There is no
+separate requirement to validate with Go 1.26.0.
 
-## 2. Select the version and review compatibility
+Prepare dependencies whether or not the minimum changes:
 
-Choose an unused stable version. Compare the candidate with the previous tag.
-Review exported Go signatures and ownership, YAML and scalar semantics,
-precedence, diagnostic codes, minimum Go version, and platform support. A
-signature-preserving change may still affect consumers. Before `v1.0.0`,
-intentional incompatible minor-release changes need a note explaining impact
-and consumer action; patch releases remain compatible within their minor line.
-Review the maintained consumer examples and migration guide.
+```sh
+go mod download
+```
 
-When dependencies change, review parser behavior and migration impact as part
-of that change. Module tidiness belongs to dependency-change review, not the
-release checker. Do not repeat a dependency-version survey for every release.
+Build the tool selected by `go.mod` into temporary storage outside the checkout.
+Ensure `$TMPDIR` names a system temporary location, not the repository. These
+commands never install another ReleaseKit version:
 
-## 3. Write and commit the release note
+```sh
+releasekit_tmp=$(mktemp -d "${TMPDIR:-/tmp}/configkit-releasekit.XXXXXX")
+releasekit_bin="$releasekit_tmp/releasekit"
+go build -o "$releasekit_bin" gitea.maximumdirect.net/eric/ReleaseKit/cmd/releasekit
+go list -m gitea.maximumdirect.net/eric/ReleaseKit
+"$releasekit_bin" version
+```
 
-Create `docs/releases/<version>.md` for the selected version. Its first line
-must be `# ConfigKit <version>`. Give a meaningful summary and include
-compatibility impact or upgrade instructions when applicable. Link to current
-contracts for exact behavior. Review the note and affected canonical documents
-as ordinary preparation, then commit the complete candidate.
+The module-selected executable is ReleaseKit v0.4.1. Use the built executable
+for publication to preserve its exit status and publication-state diagnostics.
+`go run` maps a nonzero child exit status to 1. ReleaseKit's child Go commands
+use the `go` found on `PATH` with `GOTOOLCHAIN=local`; keep the selected
+executable first on `PATH` through the publication command.
 
-End every new note with these links, replacing `VERSION` with the complete tag:
+Verify access to the authoritative remote without displaying credentials:
+
+```sh
+git ls-remote https://gitea.maximumdirect.net/eric/ConfigKit.git HEAD refs/heads/main
+```
+
+Publication requires explicit authorization. Local implementation commits do
+not authorize a push or release.
+
+## 2. Prepare and commit the candidate
+
+Choose an unused stable version locally and on the authoritative remote. Compare
+the candidate with the previous tag and review exported Go signatures and
+ownership, YAML and resolution semantics, diagnostic contracts, minimum Go
+version, and platform support. Review maintained consumer examples and the
+migration guidance. Explain any consumer-minimum increase and required consumer
+action under the
+[consumer minimum policy](policy/release.md#consumer-minimum-and-release-toolchain).
+
+Create `docs/releases/<version>.md`. Its first line must be exactly
+`# ConfigKit <version>`, and it must end with the license and attribution footer:
 
 ```markdown
 [COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/VERSION/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/VERSION/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/VERSION/docs/oss/attribution.md)
 ```
 
-Confirm the three target files exist in the candidate and review the version in
-the URLs. The future tag need not resolve remotely yet; this review adds no
-network gate. ConfigKit uses checked-in notes without hosted release objects.
-If a hosted page is used later, reuse the note and preserve its footer. Do not
-rewrite historical notes or add downloadable license assets.
+Replace `VERSION` with the full tag and confirm the three target files exist in
+the candidate. Retire completed roadmaps, preserve unfinished and deferred
+work, and repair affected links. Review the complete candidate for unintended
+files and credentials, then commit it on `main`. These are ordinary preparation
+tasks, not separate approval gates. Any `go.mod` change belongs in this
+candidate.
+
+## 3. Push the candidate to `main`
+
+Push only that committed candidate to the literal authoritative URL. This does
+not follow local tags:
+
+```sh
+git push --no-follow-tags https://gitea.maximumdirect.net/eric/ConfigKit.git HEAD:refs/heads/main
+```
+
+ReleaseKit requires authoritative `main` to equal this candidate before it
+validates. A failure before this push means ReleaseKit publication has not
+started. If the push result is uncertain, inspect authoritative `main` and
+continue only when it is confirmed at the candidate. If validation later fails,
+no release tag is published; correct the issue in a new commit, push the new
+candidate to `main`, and invoke validation again.
 
 ## 4. Publish the committed candidate
 
-Before publication, review compatibility, release-note accuracy, and the file
-list for unintended content or credentials. Retire completed roadmaps as part of
-release preparation, preserving unfinished and deferred work and repairing
-links. These are ordinary preparation tasks, not separate human sign-offs.
-
-With the release explicitly authorized, run the one publication command.
-The script resolves the repository root from its own path, so it may be invoked
-from another directory by using its path there. Replace the sample version with
-the selected version:
+With the authorized candidate already on authoritative `main`, invoke the
+module-selected native executable exactly once:
 
 ```sh
-./scripts/release.sh v0.5.0
+"$releasekit_bin" publish vMAJOR.MINOR.PATCH
 ```
 
-The command requires `main`, a clean index/worktree, an unused version locally
-and on the authoritative remote, and a matching note in the captured commit.
-Ignored local files do not block it. It checks remote ancestry, runs
-[`release-check.sh`](../scripts/release-check.sh) once against that commit,
-rechecks candidate and version state, pushes only that commit to `main`, then
-creates and pushes one annotated tag. The tag message is the checked-in note.
-It never commits or repairs the checkout.
-
-A successful intended tag push completes publication. The command reports the
-version and commit immediately. No CI, module retrieval, remote read-back, or
-hosted release object is required. The checker may be run separately for
-diagnosis, but it is not a prerequisite: a fresh publication invocation
-validates its candidate once.
-
-## 5. Recover from a failed or uncertain push
-
-The command performs one bounded authoritative-state check after a failed push.
-If the main push failed but authoritative main equals the candidate, it proceeds.
-If the tag push failed but its exact tag object reached the remote, it reports
-success. Otherwise it reports branch-only, conflicting, or unknown state and
-retains any local tag.
-
-Inspect exact refs before any retry. Set `release_version` to the attempted
-version and compare main with the candidate from the command's report:
+Replace the placeholder with the selected version. ReleaseKit checks the
+effective push URL against `expected_push_url`, verifies the candidate on
+`main`, and checks the release-note H1 for the product and version. It then
+runs the selected race-enabled standard suite once with test-time vet disabled,
+one full vet pass, and CGO-disabled package compilation for Linux amd64 and
+arm64. It publishes only the annotated tag for that candidate. Confirmed
+publication of the intended tag completes the release. The tag points to the
+captured commit and uses the complete checked-in note as its annotation message.
+The optional `check`
+command is available for diagnosis but is not a separate required pass:
 
 ```sh
-release_version=v0.5.0
-authoritative=https://gitea.maximumdirect.net/eric/ConfigKit.git
-if remote_main=$(git ls-remote --refs --exit-code "$authoritative" refs/heads/main); then
-  printf 'remote main: %s\n' "$remote_main"
-else
-  result=$?
-  if [ "$result" -eq 2 ]; then
-    printf 'authoritative main confirmed absent\n' >&2
-  else
-    printf 'remote main state unknown; inspect before retrying\n' >&2
-  fi
-fi
-ref="refs/tags/$release_version"
-if remote_tag=$(git ls-remote --refs --exit-code "$authoritative" "$ref"); then
-  printf 'remote tag: %s\n' "$remote_tag"
-else
-  result=$?
-  if [ "$result" -eq 2 ]; then
-    printf 'remote tag confirmed absent\n'
-  else
-    printf 'remote tag state unknown; do not reuse the version\n' >&2
-  fi
-fi
-if local_tag=$(git rev-parse --verify "refs/tags/$release_version" 2>/dev/null); then
-  printf 'local tag object: %s\n' "$local_tag"
-else
-  printf 'no local tag object\n'
-fi
+"$releasekit_bin" check vMAJOR.MINOR.PATCH
 ```
 
-A query failure is not proof that the tag is absent. Compare the remote tag
-object ID with the local annotated tag object ID; its peeled commit alone is
-insufficient to establish identical publication. A conflicting published tag
-must not be moved or replaced. If remote main advanced but the tag was not
-published, report that partial progress. Only after confirming the local tag
-was never published or used by another channel may a maintainer remove it:
+Status 0 confirms publication, status 1 reports a known publication failure or
+conflict, status 2 reports usage, profile, or version errors, and status 3 means
+the push outcome remains unknown. Preserve both the native exit status and its
+diagnostics.
+
+The command does not push or repair `main`, wait for CI, or publish binaries or
+a hosted release page. A successful intended-tag publication does not require
+remote read-back or consumer retrieval; those checks remain optional.
+
+## 5. Recover from a failed or uncertain tag push
+
+The executable reports the publication state, phase, version, candidate commit,
+tag-object ID, effective endpoint, and error on standard error. It performs one
+bounded authoritative reconciliation after a failed tag push. Exit status 3
+means the tag outcome remains unknown; do not blindly retry or move a tag.
+
+For an uncertain result, use the exact endpoint, candidate commit, version, and
+tag-object ID from the report. Copy the captured endpoint into the variable
+below using normal shell quoting. Check that Git still resolves it to itself,
+then query the authoritative refs once:
+
+```sh
+authoritative='COPY_CAPTURED_ENDPOINT'
+release_version='vMAJOR.MINOR.PATCH'
+test "$(git ls-remote --get-url -- "$authoritative")" = "$authoritative" &&
+  git ls-remote --refs -- "$authoritative" refs/heads/main "refs/tags/$release_version"
+```
+
+A failed URL-equality check or remote query leaves the outcome unresolved.
+Correct transport configuration before continuing; a rewritten destination
+cannot establish absence at the captured endpoint.
+
+Compare the `main` object ID with the reported candidate commit and the tag
+object ID with the reported tag-object ID. `--refs` returns the annotated tag
+object itself, not merely its peeled commit. If the tag object matches, the
+release is published. If the query fails, the result is conflicting, or the
+endpoint or candidate differs, stop and resolve the state before retrying. If
+the endpoint is reachable, the tag is confirmed absent, and `main` still equals
+the reported candidate, remove a retained local tag only after confirming it
+was not published or used by another channel; then rerun `publish`, which
+revalidates the candidate:
 
 ```sh
 git tag -d "$release_version"
+"$releasekit_bin" publish "$release_version"
 ```
 
-A subsequent invocation revalidates the candidate; a correctly published tag
-needs no rerun. Do not roll back main or automatically delete a tag. Transport
-or authentication failure alone does not require a new version. A defect in
-published source requires a corrected commit, a new semantically appropriate
-version and note, and the release procedure again.
+A validation failure after the candidate reached `main` leaves an untagged
+candidate there. Correct it with a new commit rather than rewriting or rolling
+back `main`. A conflicting published tag must never be moved or reused. A
+defect in published source requires a corrected commit, a new semantically
+appropriate version, and a matching release note.
 
 ## 6. Optional consumer verification
 
-Only when useful or requested, read back the remote tag or retrieve the exact
-version into a new module without a local `replace` directive. These checks
-are optional and non-gating. Do not poll CI or wait for module propagation.
-Private-host credentials must already be configured. For direct-origin
-verification, set the version to the release just published (replace the sample
-below), then use a temporary consumer module and check the exact module path
-and version reported by `go list -m`:
+When useful or requested, read back the remote tag or retrieve the exact
+version into a new module without a local `replace` directive. These checks are
+optional and non-gating. Do not poll CI or wait for module propagation. For
+direct-origin verification, use the exact published version in a temporary
+consumer module:
 
 ```sh
-release_version=v0.5.0
+release_version=vMAJOR.MINOR.PATCH
 consumer_dir=$(mktemp -d)
 (
   cd "$consumer_dir"

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/releases/v0.6.0.md

@@ -0,0 +1,63 @@
+# ConfigKit v0.6.0
+
+This minor release reduces avoidable configuration-loading and inspection work,
+adopts ReleaseKit for source publication, and raises the consumer Go minimum to
+1.26.0. The public configuration model remains unchanged from v0.5.0.
+
+## Changes
+
+- Reduce temporary path allocations by sizing pointer-formatting buffers,
+  combining final metadata traversal, reusing parsed sensitivity paths, and
+  avoiding path construction when detecting inheritance. Effective and
+  historical metadata indexes retain independently owned paths.
+- Replace repeated resolution-coverage, metadata-update, CLI-overlap, and
+  sensitivity-propagation scans with private indexes and incremental processing.
+  Preserve diagnostic ordering, history, contextual path shapes, and redaction.
+- Make presence-only metadata queries avoid materializing provenance. Prepare
+  resolution replay descriptors and immutable mapping and inheritance-history
+  lookups for inspection, and use ordered membership indexes for provenance
+  collections. Returned records remain detached; explanations still replay
+  complete trees and render on demand.
+- Add a maintained public-API startup construction benchmark. The initial path
+  formatting and metadata traversal experiments reproduced allocation reductions
+  across local and downstream workloads. Later algorithmic improvements were
+  validated behaviorally without a new benchmark campaign. Busy-host timings
+  were inconclusive; no fixed speedup or allocation threshold is promised.
+- Replace ConfigKit-owned release scripts with a small profile and the
+  module-selected ReleaseKit v0.3.0 tool. The
+  [release procedure](../release.md) documents candidate preparation, tag-only
+  publication, and failed-push recovery. Existing release checks and the
+  successful-tag-push completion point remain unchanged; CI is not a gate.
+- Adopt release policy 1.6, distinguishing the consumer Go minimum from the
+  validation toolchain. Retire completed planning and measurement records into
+  Git history while preserving [deferred work](../roadmap/deferred.md).
+
+## Compatibility and upgrade
+
+The consumer minimum increases from Go 1.25.0 to **Go 1.26.0**. Install Go 1.26.0
+or newer before upgrading, and update application build and CI environments that
+still use Go 1.25. The `go.mod` directive records the consumer minimum; using a
+newer validation patch does not itself raise that minimum. See the
+[consumer minimum policy](../policy/release.md#consumer-minimum-and-release-toolchain).
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.6.0
+```
+
+No application-code or configuration migration is required from v0.5.0. Public
+Go signatures, strict within-tier composition, recursive tier precedence,
+sibling inheritance, presence/null distinctions, provenance, secret handling,
+and synchronous completion retain their existing contracts. See the
+[configuration contract](../config.md) and
+[consumer guide](../consumers/README.md).
+
+The YAML dependency remains `go.yaml.in/yaml/v4 v4.0.0-rc.6`. ReleaseKit is a
+declared Go tool dependency for maintainer commands, not a library runtime
+import; its requirements participate in ConfigKit's module graph. Linux amd64
+and arm64 remain release-validated targets, and macOS remains best-effort.
+Maintainers should use the current release procedure instead of the retired
+`scripts/release.sh` and `scripts/release-check.sh` commands.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.6.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.6.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.6.0/docs/oss/attribution.md)

docs/releases/v0.7.0.md

@@ -0,0 +1,58 @@
+# ConfigKit v0.7.0
+
+This minor release adds structured comparison of completed configurations.
+Applications can inspect changes between two configuration files or the
+effective impact of a selected overlay, with ConfigKit handling comparison and
+safe value disclosure while applications own presentation.
+
+## Changes
+
+- Add `Diff[T](left, right *Result[T]) ([]Difference, error)` through the public
+  facade. It compares frozen effective inspection values from successful loads,
+  including defaults and recorded application resolution, rather than source
+  text or later mutations of `Result.Config`.
+- Return detached changed-region records containing a semantic path, safe left
+  and right `ValueSnapshot` values, and `HiddenChanged`. Mappings compare
+  recursively, sequences report atomic changes, and records follow canonical
+  pointer order. Presence distinguishes additions and removals from explicit
+  null; equal effective values produce no records regardless of provenance or
+  loading history.
+- Protect both snapshots with sensitivity from both loads, including historical
+  and dependency-derived classification. Compatible regions retain public
+  siblings; incompatible structural replacements use complete-region masking
+  where required. `HiddenChanged` identifies actual changes withheld by masking,
+  rather than merely indicating that a snapshot contains redacted content.
+- Preserve visible public scalar changes when only historical sensitive
+  descendants remain, including additions and removals. Historical explanation
+  redaction and masking of incompatible present shapes remain intact.
+- Add maintained file-comparison and base-versus-overlay examples with
+  consumer-owned formatting, plus an executable public API example. See the
+  [consumer guide](../consumers/README.md#effective-configuration-diffs) and
+  [configuration contract](../config.md#effective-configuration-diffs).
+
+## Compatibility and upgrade
+
+No application-code or configuration migration is required from v0.6.0.
+Adopting the new API is optional. Loading, composition, inheritance, provenance,
+existing inspection, and secret-handling contracts remain unchanged.
+
+Load both configurations successfully before calling `Diff`; nil or
+uninitialized results return an error with no records. Comparison does not
+reopen sources, rerun callbacks, or reread environment variables. Typed secret
+references compare by configured reference name, not resolved credential bytes,
+so credential rotation behind an unchanged name is not reported. Returned
+snapshots describe changed regions, not cross-protected full-document context.
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.7.0
+```
+
+The consumer minimum remains Go 1.26.0. The YAML dependency remains
+`go.yaml.in/yaml/v4 v4.0.0-rc.6`, and the maintainer tool dependency remains
+ReleaseKit v0.3.0. Linux amd64 and arm64 remain release-validated targets;
+macOS remains best-effort. Publication follows the existing
+[release procedure](../release.md).
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.0/docs/oss/attribution.md)

docs/releases/v0.7.1.md

@@ -0,0 +1,22 @@
+# ConfigKit v0.7.1
+
+This patch updates the maintainer release tool to ReleaseKit v0.4.1. ConfigKit's
+runtime behavior and public API are unchanged.
+
+## Compatibility and upgrade
+
+No application-code or configuration migration is required from v0.7.0. The
+consumer minimum remains Go 1.26.0, and the YAML dependency is unchanged.
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.7.1
+```
+
+ReleaseKit v0.4.1 is a maintainer tool dependency and does not change consumer
+configuration or runtime behavior. Linux amd64 and arm64 remain
+release-validated targets; macOS remains best-effort. Publication follows the
+existing [release procedure](../release.md).
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.1/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.1/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.1/docs/oss/attribution.md)

docs/roadmap/deferred.md

@@ -11,18 +11,59 @@ 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;
 - hot reload, runtime resource replacement, and remote resource validation;
 - a bundled CLI framework or application commands; and
-- configuration diffs and fingerprints.
+- configuration fingerprints.
 
 Applications may build their own validate, effective-value, source, bootstrap, and
 explanation commands around the library. Broadening these boundaries requires
 concrete consumer evidence, compatibility review, and an explicit design change.
+
+Effective configuration diffs are implemented over frozen successful-load
+inspections; their current comparison and disclosure contract is in the
+[configuration contract](../config.md#effective-configuration-diffs), with
+consumer workflows in the [adoption guide](../consumers/README.md#effective-configuration-diffs).
+Fingerprints remain deferred.
+
+Built-in sibling inheritance using `extends` is implemented as described in the
+[configuration contract](../config.md#sibling-inheritance). It is distinct from
+inheritance between selected overlay files, which remains excluded above.
+
+## Possible source-version follow-ups
+
+Independent collection or subtree specification policies were outside the
+completed shared source-version feature and remain possible future work, with
+no committed delivery plan. Current [source-version checks](../config.md#shared-source-version-checks)
+use one application-owned policy across participating documents. Reconsider
+independent policies if concrete consumer needs justify a separate design
+decision; this is not a permanent prohibition. The original request and its
+selected scope remain at `e4270c9:docs/roadmap/promptkit-feature-request.md` in
+Git history. Existing sequence-composition and cross-scope inheritance deferrals
+above also remain unchanged.
+
+## Deferred performance experiments
+
+The completed performance audit left these internal optimization candidates for
+separate investigation:
+
+- branch-local explanation replay instead of the current whole-tree replay;
+- redesigning the private metadata-copy handoff while preserving detached public
+  records;
+- caching schema descriptors across loads; and
+- indexing complex contributor-trace membership while preserving its equality
+  rules and compact linked traces.
+
+These are not committed implementation work. Reconsider them with evidence that
+the benefit justifies the ownership, retained-memory, or equivalence risks. The
+completed audit and implementation evidence are preserved in Git history at
+`8e91a2b` under `docs/roadmap/`.

example_test.go

@@ -30,6 +30,32 @@ func ExampleNoSource() {
 	// Output: 4 0 <nil>
 }
 
+func ExampleDiff() {
+	type Config struct {
+		Workers int `yaml:"workers"`
+	}
+	before, err := configkit.Load[Config](configkit.Mapping("before", map[string]any{"workers": 2}))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	after, err := configkit.Load[Config](configkit.Mapping("after", map[string]any{"workers": 4}))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	differences, err := configkit.Diff(before, after)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	for _, difference := range differences {
+		fmt.Printf("%s: %s -> %s\n", difference.Path.String(),
+			strings.TrimSpace(difference.Left.YAML), strings.TrimSpace(difference.Right.YAML))
+	}
+	// Output: /workers: 2 -> 4
+}
+
 func ExampleValidation_Error() {
 	type Config struct {
 		First  string `yaml:"first"`

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/destinations.go

@@ -0,0 +1,154 @@
+package consumers
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"path"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// CollectionBundleConfig is a representative application-owned collection
+// shape. Collection and prompt version labels remain ordinary domain fields.
+type CollectionBundleConfig struct {
+	Version     string                          `yaml:"version"`
+	Collections map[string]CollectionDefinition `yaml:"collections"`
+}
+
+type CollectionDefinition struct {
+	Name        string                       `yaml:"name"`
+	Version     string                       `yaml:"version"`
+	Description string                       `yaml:"description"`
+	Prompts     map[string]CollectionPrompt  `yaml:"prompts"`
+	Profiles    map[string]CollectionProfile `yaml:"profiles"`
+}
+
+type CollectionPrompt struct {
+	Version        string              `yaml:"version"`
+	DefaultProfile string              `yaml:"default_profile"`
+	ContentFile    string              `yaml:"content_file"`
+	ResourceText   string              `yaml:"resource_text"`
+	Messages       []CollectionMessage `yaml:"messages"`
+}
+
+type CollectionMessage struct {
+	Role    string `yaml:"role"`
+	Content string `yaml:"content"`
+}
+
+type CollectionProfile struct {
+	Format      string  `yaml:"format"`
+	Temperature float64 `yaml:"temperature"`
+}
+
+// LoadCollectionBundle demonstrates one layered collection load. Its main
+// document owns the application schema version; the mounted entry declares
+// relative destinations for collection metadata, prompt/profile peers, and
+// their nested imports. The operator version override remains a separate tier.
+func LoadCollectionBundle() (*configkit.Result[CollectionBundleConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/collection-bundle")
+	if err != nil {
+		return nil, err
+	}
+	bundle := configkit.NewFilesystem("collection bundle", files)
+	collectionDestination := configkit.Keys("collections", "weatherreporter")
+	promptDestination := collectionDestination.Key("prompts")
+	return configkit.LoadLayered(
+		configkit.Input{Source: configkit.Bytes("application main", []byte("version: 1\n"))},
+		configkit.LayeredOptions[CollectionBundleConfig]{
+			Definition: configkit.Definition[CollectionBundleConfig]{
+				Version: &configkit.VersionRule{Path: configkit.Keys("version"), Check: func(version configkit.Version, _ *configkit.Validation) error {
+					if !version.Present || version.Text != "1" {
+						return errors.New("unsupported application schema version")
+					}
+					return nil
+				}},
+				Resolve:  resolveCollectionResources,
+				Validate: validateCollectionBundle,
+			},
+			Underlays: []configkit.Underlay{
+				{Inputs: []configkit.Input{{
+					Source: bundle.File("collections/weather/collection.yml"), Imports: true, Destination: collectionDestination,
+				}}},
+				{Inputs: []configkit.Input{{
+					Source: bundle.File("operators/weather-prompt-version.yml"), Destination: promptDestination,
+				}}},
+			},
+		},
+	)
+}
+
+func resolveCollectionResources(configuration *CollectionBundleConfig, resolution *configkit.Resolution) error {
+	for collectionID, collection := range configuration.Collections {
+		for promptID, prompt := range collection.Prompts {
+			if prompt.ContentFile == "" {
+				continue
+			}
+			contentPath := configkit.Keys("collections", collectionID, "prompts", promptID, "content_file")
+			info, found := resolution.InputInfo(contentPath)
+			if !found || len(info.Origin.Locations) != 1 {
+				return fmt.Errorf("prompt resource has no single declaring source")
+			}
+			resourceSource, found := resolution.Source(info.Origin.Locations[0].SourceID)
+			if !found {
+				return fmt.Errorf("prompt resource source is unavailable")
+			}
+			context, found := resolution.SourceContext(resourceSource.ID)
+			if !found {
+				return fmt.Errorf("prompt resource context is unavailable")
+			}
+			data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, prompt.ContentFile))
+			if err != nil {
+				return fmt.Errorf("read prompt resource: %w", err)
+			}
+			prompt.ResourceText = string(data)
+			collection.Prompts[promptID] = prompt
+			resolution.Record(
+				configkit.Keys("collections", collectionID, "prompts", promptID, "resource_text"),
+				configkit.ChangeNormalization, "read source-relative prompt content", contentPath,
+			)
+		}
+		configuration.Collections[collectionID] = collection
+	}
+	return nil
+}
+
+func validateCollectionBundle(configuration *CollectionBundleConfig, validation *configkit.Validation) {
+	for collectionID, collection := range configuration.Collections {
+		for promptID, prompt := range collection.Prompts {
+			if prompt.DefaultProfile == "" {
+				continue
+			}
+			if _, found := collection.Profiles[prompt.DefaultProfile]; !found {
+				validation.Error(configkit.Keys("collections", collectionID, "prompts", promptID, "default_profile"), "must name a profile in this collection")
+			}
+		}
+	}
+}
+
+// ReadCollectionPromptContent follows the effective content field's physical
+// source, including when it was inherited from a peer prompt.
+func ReadCollectionPromptContent(result *configkit.Result[CollectionBundleConfig], collectionID, promptID string) (string, error) {
+	if result == nil {
+		return "", errors.New("collection result is nil")
+	}
+	contentPath := configkit.Keys("collections", collectionID, "prompts", promptID, "content_file")
+	info, found := result.Metadata().Lookup(contentPath)
+	if !found || len(info.Origin.Locations) != 1 {
+		return "", errors.New("prompt resource has no single declaring source")
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found {
+		return "", errors.New("prompt resource context is unavailable")
+	}
+	prompt, found := result.Config.Collections[collectionID].Prompts[promptID]
+	if !found {
+		return "", errors.New("prompt is unavailable")
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, prompt.ContentFile))
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}

examples/consumers/destinations_test.go

@@ -0,0 +1,137 @@
+package consumers_test
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func Example_inputDestination() {
+	mounted, err := configkit.LoadLayered[map[string]any](configkit.Input{
+		Source: configkit.Mapping("weather prompt", map[string]any{
+			"summarize.alerts": map[string]any{"version": "1"},
+		}),
+		Destination: configkit.Keys("collections", "weather.reporter", "prompts"),
+	})
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	root, err := configkit.LoadLayered[map[string]any](configkit.Input{
+		Source: configkit.Bytes("standalone", []byte("name: standalone\n")),
+	})
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	_, found := mounted.Config["collections"]
+	fmt.Println(found, root.Config["name"])
+	// Output: true standalone
+}
+
+func TestCollectionBundleAssemblesMountedPeersAndResources(t *testing.T) {
+	result, err := consumers.LoadCollectionBundle()
+	if err != nil {
+		t.Fatalf("LoadCollectionBundle: %v", err)
+	}
+	collection := result.Config.Collections["weatherreporter"]
+	if result.Config.Version != "1" || collection.Name != "Weather Reporter" || collection.Version != "2.4.0" {
+		t.Fatalf("main and collection versions = %q, %#v", result.Config.Version, collection)
+	}
+	brief := collection.Prompts["brief"]
+	custom := collection.Prompts["custom"]
+	if brief.Version != "2" || brief.DefaultProfile != "compact" || brief.ContentFile != "welcome.txt" || len(brief.Messages) != 1 {
+		t.Fatalf("version-only override replaced lower prompt fields: %#v", brief)
+	}
+	if custom.Version != "2" || custom.DefaultProfile != "weather" || custom.ContentFile != brief.ContentFile ||
+		len(custom.Messages) != 1 || custom.ResourceText != "Welcome to the weather collection.\n" {
+		t.Fatalf("inherited prompt = %#v", custom)
+	}
+	weatherProfile := collection.Profiles["weather"]
+	if weatherProfile.Format != "text" || weatherProfile.Temperature != 0.4 {
+		t.Fatalf("inherited profile = %#v", weatherProfile)
+	}
+	resource, err := consumers.ReadCollectionPromptContent(result, "weatherreporter", "custom")
+	if err != nil || resource != custom.ResourceText {
+		t.Fatalf("inherited prompt resource = %q, %v", resource, err)
+	}
+
+	versionInfo, ok := result.Metadata().Lookup(configkit.Keys("version"))
+	if !ok || !versionInfo.Explicit || versionInfo.Origin.Layer != configkit.LayerBase || len(versionInfo.Origin.Locations) != 1 {
+		t.Fatalf("main-owned version metadata = %#v, %v", versionInfo, ok)
+	}
+	collectionVersionPath := configkit.Keys("collections", "weatherreporter", "version")
+	collectionVersion, ok := result.Metadata().Lookup(collectionVersionPath)
+	if !ok || !collectionVersion.Explicit || collectionVersion.Origin.Layer != configkit.LayerUnderlay || len(collectionVersion.Origin.Locations) != 1 {
+		t.Fatalf("ordinary collection version metadata = %#v, %v", collectionVersion, ok)
+	}
+	var metadataSource configkit.SourceInfo
+	var collectionEntrySource configkit.SourceInfo
+	var versionSource configkit.SourceInfo
+	var parentPromptSource configkit.SourceInfo
+	var promptIndexSource configkit.SourceInfo
+	var profileIndexSource configkit.SourceInfo
+	var promptVersionSource configkit.SourceInfo
+	for _, source := range result.Sources() {
+		switch source.Path {
+		case "collections/weather/collection.yml":
+			collectionEntrySource = source
+		case "collections/weather/metadata.yml":
+			metadataSource = source
+		case "collections/weather/version.yml":
+			versionSource = source
+		case "collections/weather/prompts/index.yml":
+			promptIndexSource = source
+		case "collections/weather/prompts/brief.yml":
+			parentPromptSource = source
+		case "collections/weather/profiles/index.yml":
+			profileIndexSource = source
+		case "operators/weather-prompt-version.yml":
+			promptVersionSource = source
+		}
+	}
+	collectionVersionLocation := collectionVersion.Origin.Locations[0]
+	if collectionEntrySource.ID != "source-2" || collectionEntrySource.Destination.String() != "/collections/weatherreporter" ||
+		metadataSource.ID == "" || metadataSource.Destination.String() != "/collections/weatherreporter" ||
+		metadataSource.ImportedFrom == nil || metadataSource.ImportedFrom.SourceID != collectionEntrySource.ID ||
+		versionSource.ID == "" || collectionVersionLocation.SourceID != versionSource.ID || versionSource.Destination.String() != "/collections/weatherreporter" ||
+		versionSource.ImportedFrom == nil || versionSource.ImportedFrom.SourceID != metadataSource.ID || collectionVersionLocation.Line != 1 {
+		t.Fatalf("nested collection version provenance = entry %#v, metadata %#v, version %#v, location %#v", collectionEntrySource, metadataSource, versionSource, collectionVersionLocation)
+	}
+	if promptIndexSource.ID == "" || promptIndexSource.Destination.String() != "/collections/weatherreporter/prompts" ||
+		promptIndexSource.ImportedFrom == nil || promptIndexSource.ImportedFrom.SourceID != collectionEntrySource.ID ||
+		profileIndexSource.ID == "" || profileIndexSource.Destination.String() != "/collections/weatherreporter/profiles" ||
+		profileIndexSource.ImportedFrom == nil || profileIndexSource.ImportedFrom.SourceID != collectionEntrySource.ID {
+		t.Fatalf("nested prompt/profile placement = prompt %#v profile %#v", promptIndexSource, profileIndexSource)
+	}
+	if promptVersionSource.ID == "" || promptVersionSource.Destination.String() != "/collections/weatherreporter/prompts" || promptVersionSource.UnderlayIndex != 2 {
+		t.Fatalf("prompt version source = %#v", promptVersionSource)
+	}
+	contentPath := configkit.Keys("collections", "weatherreporter", "prompts", "custom", "content_file")
+	contentInfo, ok := result.Metadata().Lookup(contentPath)
+	if !ok || !contentInfo.Explicit || len(contentInfo.Origin.Inheritance) != 1 || len(contentInfo.Origin.Locations) != 1 {
+		t.Fatalf("inherited content field metadata = %#v, %v", contentInfo, ok)
+	}
+	contentLocation := contentInfo.Origin.Locations[0]
+	if parentPromptSource.ID == "" || contentLocation.SourceID != parentPromptSource.ID || contentLocation.Line != 4 ||
+		parentPromptSource.Destination.String() != "/collections/weatherreporter/prompts" {
+		t.Fatalf("inherited resource source coordinates = %#v, source %#v", contentLocation, parentPromptSource)
+	}
+}
+
+func TestCollectionPeerPromptConflictsRemainStrict(t *testing.T) {
+	destination := configkit.Keys("collections", "weatherreporter", "prompts")
+	_, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{
+			{Source: configkit.Bytes("peer-a", []byte("brief: {version: '1'}\n")), Destination: destination},
+			{Source: configkit.Bytes("peer-b", []byte("brief: {version: '1'}\n")), Destination: destination},
+		}}},
+	})
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeComposeConflict {
+		t.Fatalf("duplicate peer prompt error = %v", err)
+	}
+}

examples/consumers/diff.go

@@ -0,0 +1,136 @@
+package consumers
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// FileComparisonConfig is a small application-owned shape for comparing
+// caller-selected configuration files.
+type FileComparisonConfig struct {
+	Name    string         `yaml:"name"`
+	Entries map[string]any `yaml:"entries"`
+}
+
+// CompareFiles loads two caller-selected files with the same application
+// definition, then formats only the safe records returned by configkit.Diff.
+func CompareFiles(leftPath, rightPath string) (string, error) {
+	options := configkit.Options[FileComparisonConfig]{
+		Definition: configkit.Definition[FileComparisonConfig]{
+			Defaults: func() FileComparisonConfig {
+				return FileComparisonConfig{Entries: map[string]any{}}
+			},
+		},
+	}
+	left, err := configkit.Load(configkit.File(leftPath), options)
+	if err != nil {
+		return "", fmt.Errorf("load left configuration: %w", err)
+	}
+	right, err := configkit.Load(configkit.File(rightPath), options)
+	if err != nil {
+		return "", fmt.Errorf("load right configuration: %w", err)
+	}
+	differences, err := configkit.Diff(left, right)
+	if err != nil {
+		return "", err
+	}
+	return FormatDifferences(differences), nil
+}
+
+// RolloutConfig models a service endpoint whose URL is derived by the
+// application resolver and whose credential reference is classified.
+type RolloutConfig struct {
+	Service RolloutService `yaml:"service"`
+}
+
+// RolloutService contains endpoint settings and a classified credential.
+type RolloutService struct {
+	Endpoint   RolloutEndpoint   `yaml:"endpoint"`
+	Credential RolloutCredential `yaml:"credential"`
+}
+
+// RolloutEndpoint is the effective destination used by the application.
+type RolloutEndpoint struct {
+	Host string `yaml:"host"`
+	Port int    `yaml:"port"`
+	URL  string `yaml:"url"`
+}
+
+// RolloutCredential holds only a typed secret reference in configuration.
+type RolloutCredential struct {
+	Token configkit.Secret `yaml:"token" configkit:"secret"`
+}
+
+// LoadRollout loads a base file, optionally with one selected configuration
+// overlay. Resolution derives the effective URL after the overlay is applied.
+func LoadRollout(basePath, overlayName string) (*configkit.Result[RolloutConfig], error) {
+	return configkit.Load(configkit.File(basePath), configkit.Options[RolloutConfig]{
+		Overlay: overlayName,
+		Definition: configkit.Definition[RolloutConfig]{
+			Resolve: resolveRollout,
+		},
+	})
+}
+
+// CompareRolloutOverlay compares the effective base configuration with that
+// same file plus one selected overlay. It presents only safe diff records.
+func CompareRolloutOverlay(basePath, overlayName string) (string, error) {
+	base, err := LoadRollout(basePath, "")
+	if err != nil {
+		return "", fmt.Errorf("load base configuration: %w", err)
+	}
+	withOverlay, err := LoadRollout(basePath, overlayName)
+	if err != nil {
+		return "", fmt.Errorf("load overlay configuration: %w", err)
+	}
+	differences, err := configkit.Diff(base, withOverlay)
+	if err != nil {
+		return "", err
+	}
+	return FormatDifferences(differences), nil
+}
+
+func resolveRollout(configuration *RolloutConfig, resolution *configkit.Resolution) error {
+	endpoint := &configuration.Service.Endpoint
+	if endpoint.Host == "" || endpoint.Port <= 0 {
+		return nil
+	}
+	url := fmt.Sprintf("https://%s:%d", endpoint.Host, endpoint.Port)
+	if endpoint.URL != url {
+		endpoint.URL = url
+		resolution.Record(configkit.Keys("service", "endpoint", "url"),
+			configkit.ChangeNormalization, "derived endpoint URL",
+			configkit.Keys("service", "endpoint", "host"),
+			configkit.Keys("service", "endpoint", "port"))
+	}
+	return nil
+}
+
+// FormatDifferences is consumer-owned presentation. It distinguishes absence
+// through Present and reports withheld changes through HiddenChanged without
+// interpreting redaction marker text.
+func FormatDifferences(differences []configkit.Difference) string {
+	if len(differences) == 0 {
+		return "No effective changes.\n"
+	}
+	var output strings.Builder
+	for _, difference := range differences {
+		fmt.Fprintf(&output, "%s: left=%s right=%s", difference.Path.String(),
+			formatSnapshot(difference.Left), formatSnapshot(difference.Right))
+		if difference.HiddenChanged {
+			output.WriteString(" hidden-change")
+		}
+		output.WriteByte('\n')
+	}
+	return output.String()
+}
+
+func formatSnapshot(snapshot configkit.ValueSnapshot) string {
+	if !snapshot.Present {
+		return "<absent>"
+	}
+	return strconv.Quote(snapshot.YAML)
+}

examples/consumers/diff_test.go

@@ -0,0 +1,187 @@
+package consumers_test
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func ExampleCompareFiles() {
+	directory, err := os.MkdirTemp("", "configkit-diff-files-")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer os.RemoveAll(directory)
+	leftPath := filepath.Join(directory, "before.yml")
+	rightPath := filepath.Join(directory, "after.yml")
+	if err := os.WriteFile(leftPath, []byte("name: before\nentries: {}\n"), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	if err := os.WriteFile(rightPath, []byte("name: after\nentries:\n  added: null\n"), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	output, err := consumers.CompareFiles(leftPath, rightPath)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Print(output)
+	// Output:
+	// /entries/added: left=<absent> right="null\n"
+	// /name: left="before\n" right="after\n"
+}
+
+func ExampleCompareRolloutOverlay() {
+	const baseToken = "CONFIGKIT_DIFF_EXAMPLE_BASE_TOKEN"
+	const overlayToken = "CONFIGKIT_DIFF_EXAMPLE_OVERLAY_TOKEN"
+	defer preserveEnvironment(baseToken)()
+	defer preserveEnvironment(overlayToken)()
+	_ = os.Setenv(baseToken, "disposable-example-base-credential")
+	_ = os.Setenv(overlayToken, "disposable-example-overlay-credential")
+
+	directory, err := os.MkdirTemp("", "configkit-diff-overlay-")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer os.RemoveAll(directory)
+	basePath := filepath.Join(directory, "config.yml")
+	overlayPath := filepath.Join(directory, "overlays", "canary.yml")
+	if err := os.MkdirAll(filepath.Dir(overlayPath), 0o700); err != nil {
+		fmt.Println(err)
+		return
+	}
+	base := "service:\n  endpoint:\n    host: base.example\n    port: 8443\n  credential:\n    token: " + baseToken + "\n"
+	overlay := "service:\n  endpoint:\n    host: canary.example\n  credential:\n    token: " + overlayToken + "\n"
+	if err := os.WriteFile(basePath, []byte(base), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	if err := os.WriteFile(overlayPath, []byte(overlay), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	output, err := consumers.CompareRolloutOverlay(basePath, "canary")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Print(output)
+	// Output:
+	// /service/credential/token: left="'[REDACTED]'\n" right="'[REDACTED]'\n" hidden-change
+	// /service/endpoint/host: left="base.example\n" right="canary.example\n"
+	// /service/endpoint/url: left="https://base.example:8443\n" right="https://canary.example:8443\n"
+}
+
+func TestCompareFilesUsesCallerSelectedSourcesAndSafePresence(t *testing.T) {
+	directory := t.TempDir()
+	leftPath := filepath.Join(directory, "before.yml")
+	rightPath := filepath.Join(directory, "after.yml")
+	writeConfiguration(t, leftPath, "name: before\nentries: {}\n")
+	writeConfiguration(t, rightPath, "name: after\nentries:\n  added: null\n")
+
+	output, err := consumers.CompareFiles(leftPath, rightPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "/entries/added: left=<absent> right=\"null\\n\"\n" +
+		"/name: left=\"before\\n\" right=\"after\\n\"\n"
+	if output != want {
+		t.Fatalf("comparison output = %q, want %q", output, want)
+	}
+	if again, err := consumers.CompareFiles(leftPath, rightPath); err != nil || again != output {
+		t.Fatalf("repeat output = %q, %v", again, err)
+	}
+}
+
+func TestCompareFilesReturnsEachLoadError(t *testing.T) {
+	directory := t.TempDir()
+	leftPath := filepath.Join(directory, "before.yml")
+	rightPath := filepath.Join(directory, "after.yml")
+	writeConfiguration(t, leftPath, "name: before\n")
+	writeConfiguration(t, rightPath, "unknown: invalid\n")
+
+	output, err := consumers.CompareFiles(leftPath, rightPath)
+	if err == nil || output != "" || !strings.Contains(err.Error(), "load right configuration") {
+		t.Fatalf("right load failure = %q, %v", output, err)
+	}
+	if _, err := consumers.CompareFiles(filepath.Join(directory, "missing.yml"), rightPath); err == nil || !strings.Contains(err.Error(), "load left configuration") {
+		t.Fatalf("left load failure = %v", err)
+	}
+}
+
+func TestCompareRolloutOverlayShowsResolvedSafeChangesAndRetainsSiblings(t *testing.T) {
+	t.Setenv("CONFIGKIT_DIFF_BASE_TOKEN", "disposable-base-credential")
+	t.Setenv("CONFIGKIT_DIFF_OVERLAY_TOKEN", "disposable-overlay-credential")
+	directory := t.TempDir()
+	basePath := filepath.Join(directory, "config.yml")
+	overlayPath := filepath.Join(directory, "overlays", "canary.yml")
+	if err := os.MkdirAll(filepath.Dir(overlayPath), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	writeConfiguration(t, basePath, "service:\n  endpoint:\n    host: base.example\n    port: 8443\n  credential:\n    token: CONFIGKIT_DIFF_BASE_TOKEN\n")
+	writeConfiguration(t, overlayPath, "service:\n  endpoint:\n    host: canary.example\n  credential:\n    token: CONFIGKIT_DIFF_OVERLAY_TOKEN\n")
+
+	base, err := consumers.LoadRollout(basePath, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	withOverlay, err := consumers.LoadRollout(basePath, "canary")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if base.Config.Service.Endpoint.Port != 8443 || withOverlay.Config.Service.Endpoint.Port != 8443 {
+		t.Fatalf("lower-priority port was not retained: base=%#v overlay=%#v", base.Config.Service.Endpoint, withOverlay.Config.Service.Endpoint)
+	}
+	urlInfo, found := withOverlay.Metadata().Lookup(configkit.Keys("service", "endpoint", "url"))
+	if !found || urlInfo.Origin.Layer != configkit.LayerResolution || withOverlay.Config.Service.Endpoint.URL != "https://canary.example:8443" {
+		t.Fatalf("resolved endpoint = %#v, metadata=%#v, %v", withOverlay.Config.Service.Endpoint, urlInfo, found)
+	}
+
+	differences, err := configkit.Diff(base, withOverlay)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 3 {
+		t.Fatalf("differences = %#v", differences)
+	}
+	wantPaths := []string{"/service/credential/token", "/service/endpoint/host", "/service/endpoint/url"}
+	for index, difference := range differences {
+		if got := difference.Path.String(); got != wantPaths[index] {
+			t.Errorf("path[%d] = %q, want %q", index, got, wantPaths[index])
+		}
+	}
+	secret := differences[0]
+	if !secret.Left.Redacted || !secret.Right.Redacted || !secret.HiddenChanged || secret.Left.YAML != "'[REDACTED]'\n" || secret.Right.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("classified credential difference = %#v", secret)
+	}
+	if strings.Contains(consumers.FormatDifferences(differences), "disposable-") {
+		t.Fatal("formatted differences exposed credential contents")
+	}
+
+	output, err := consumers.CompareRolloutOverlay(basePath, "canary")
+	if err != nil || output != consumers.FormatDifferences(differences) {
+		t.Fatalf("formatted base/overlay comparison = %q, %v", output, err)
+	}
+}
+
+func TestFormatDifferencesUsesRecordMembershipForNoChange(t *testing.T) {
+	if got := consumers.FormatDifferences(nil); got != "No effective changes.\n" {
+		t.Fatalf("no-change output = %q", got)
+	}
+}
+
+func writeConfiguration(t *testing.T, filename, contents string) {
+	t.Helper()
+	if err := os.WriteFile(filename, []byte(contents), 0o600); err != nil {
+		t.Fatal(err)
+	}
+}

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 +1,119 @@
+package consumers
+
+import (
+	"embed"
+	"errors"
+	"io/fs"
+	"path"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+//go:embed testdata/native-source testdata/layered-bundle testdata/collection-bundle testdata/ordinary-imports testdata/source-version
+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"`
+}
+
+type PromptBundleConfig struct {
+	Name        string                  `yaml:"name"`
+	Version     string                  `yaml:"version"`
+	Description string                  `yaml:"description"`
+	Prompts     map[string]BundlePrompt `yaml:"prompts"`
+}
+
+type BundlePrompt struct {
+	Summary string `yaml:"summary"`
+}
+
+// LoadPromptBundle demonstrates ordinary Load with opt-in imports whose
+// destinations are declared relative to the entry document.
+func LoadPromptBundle() (*configkit.Result[PromptBundleConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/ordinary-imports")
+	if err != nil {
+		return nil, err
+	}
+	return configkit.Load[PromptBundleConfig](
+		configkit.NewFilesystem("prompt bundle", files).File("config.yml"),
+		configkit.Options[PromptBundleConfig]{Imports: true},
+	)
+}
+
+// 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 +1,151 @@
+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_rootRelativeImports() {
+	result, err := consumers.LoadPromptBundle()
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	source := result.Sources()[2]
+	fmt.Println(result.Config.Name, result.Config.Version, result.Config.Prompts["brief"].Summary, source.Destination.String(), err)
+	// Output: Root import example 1.0 A root-loaded prompt. /prompts <nil>
+}
+
+func TestPromptBundleUsesRelativeImportDestinations(t *testing.T) {
+	result, err := consumers.LoadPromptBundle()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Config.Name != "Root import example" || result.Config.Version != "1.0" || result.Config.Prompts["brief"].Summary != "A root-loaded prompt." {
+		t.Fatalf("root import result = %#v", result.Config)
+	}
+	sources := result.Sources()
+	if len(sources) != 3 || sources[0].Destination.String() != configkit.Keys().String() ||
+		sources[1].Path != "metadata.yml" || sources[1].Destination.String() != configkit.Keys().String() ||
+		sources[2].Path != "prompts/brief.yml" || sources[2].Destination.String() != "/prompts" ||
+		sources[2].ImportedFrom == nil || sources[2].ImportedFrom.SourceID != sources[0].ID {
+		t.Fatalf("root import sources = %#v", sources)
+	}
+}
+
+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 Example_sourceVersionAdoption() {
+	result, err := consumers.LoadVersionedService(configkit.SourceVersionModeIfPresent)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	checks, redacted := 0, true
+	for _, source := range result.Sources() {
+		if check := source.VersionCheck; check != nil {
+			checks++
+			redacted = redacted && check.Redacted && check.Version.Text == ""
+		}
+	}
+	required, requiredErr := consumers.LoadVersionedService(configkit.SourceVersionModeRequired)
+	fmt.Println(result.Config.Service.Version, result.Config.Service.Metadata.Release,
+		checks, redacted, required != nil && requiredErr == nil)
+	// Output: service-v2 application-2026.10 2 true true
+}
+
+func TestVersionedServiceModesAndMountedProtection(t *testing.T) {
+	for _, mode := range []configkit.SourceVersionMode{
+		configkit.SourceVersionModeIfPresent,
+		configkit.SourceVersionModeRequired,
+	} {
+		result, err := consumers.LoadVersionedService(mode)
+		if err != nil || result == nil || result.Config.Service.Version != "service-v2" ||
+			result.Config.Service.Name != "example service" || result.Config.Service.Endpoint != "https://service.example.invalid" ||
+			result.Config.Service.Metadata.Release != "application-2026.10" {
+			t.Fatalf("versioned service mode %d = %#v, %v", mode, result, err)
+		}
+		sources := result.Sources()
+		if len(sources) != 2 {
+			t.Fatalf("versioned service sources = %#v", sources)
+		}
+		for _, source := range sources {
+			check := source.VersionCheck
+			if check == nil || check.Path.String() != "/version" || !check.Version.Present ||
+				!check.Redacted || check.Version.Text != "" {
+				t.Fatalf("safe source version evidence = %#v", sources)
+			}
+		}
+	}
+	_, err := consumers.LoadVersionedService(configkit.SourceVersionModeIfPresent,
+		configkit.Override{Path: configkit.Keys("service", "version"), Value: "cli-v3", Flag: "--service-version"})
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 ||
+		loadError.Diagnostics()[0].Code != configkit.CodeOverrideVersion ||
+		loadError.Diagnostics()[0].Path.String() != "/service/version" {
+		t.Fatalf("mounted main version override error = %v", err)
+	}
+}
+
+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/source_version.go

@@ -0,0 +1,64 @@
+package consumers
+
+import (
+	"errors"
+	"io/fs"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// VersionedServiceConfig demonstrates a mounted main input whose source-local
+// schema version is retained at the effective service path.
+type VersionedServiceConfig struct {
+	Service struct {
+		Version  string `yaml:"version"`
+		Name     string `yaml:"name"`
+		Endpoint string `yaml:"endpoint"`
+		Metadata struct {
+			Release string `yaml:"release"`
+		} `yaml:"metadata"`
+	} `yaml:"service"`
+}
+
+// LoadVersionedService checks the main and underlay declarations with one
+// application policy. The local rule path is "version" for each source; the
+// main input's destination places its protected value at /service/version.
+// Optional CLI overrides are passed through to ConfigKit for normal protection.
+func LoadVersionedService(mode configkit.SourceVersionMode, overrides ...configkit.Override) (*configkit.Result[VersionedServiceConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/source-version")
+	if err != nil {
+		return nil, err
+	}
+	main, err := fs.ReadFile(files, "main.yml")
+	if err != nil {
+		return nil, err
+	}
+	underlay, err := fs.ReadFile(files, "underlay.yml")
+	if err != nil {
+		return nil, err
+	}
+	definition := configkit.Definition[VersionedServiceConfig]{
+		Version: &configkit.VersionRule{
+			Path: configkit.Keys("version"),
+			Check: func(version configkit.Version, _ *configkit.Validation) error {
+				if !version.Present || version.Null || version.Text == "" {
+					return errors.New("a nonempty source version is required")
+				}
+				return nil
+			},
+		},
+		Secret: func(path configkit.Path) bool {
+			return path.String() == "/service/version"
+		},
+	}
+	return configkit.LoadLayered(configkit.Input{
+		Source: configkit.Bytes("service main", main), Destination: configkit.Keys("service"),
+	}, configkit.LayeredOptions[VersionedServiceConfig]{
+		Definition:        definition,
+		SourceVersionMode: mode,
+		Overrides:         overrides,
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+			Source: configkit.Bytes("service defaults", underlay), Destination: configkit.Keys("service"),
+		}}}},
+	})
+}

examples/consumers/testdata/collection-bundle/collections/weather/collection.yml

@@ -0,0 +1,9 @@
+configkit:
+  imports:
+    - path: metadata.yml
+      destination: []
+    - path: prompts/index.yml
+      destination: [prompts]
+    - path: profiles/index.yml
+      destination: [profiles]
+name: Weather Reporter

examples/consumers/testdata/collection-bundle/collections/weather/metadata.yml

@@ -0,0 +1,4 @@
+configkit:
+  imports:
+    - path: version.yml
+description: Imported collection metadata.

examples/consumers/testdata/collection-bundle/collections/weather/profiles/compact.yml

@@ -0,0 +1,3 @@
+compact:
+  format: text
+  temperature: 0.2

examples/consumers/testdata/collection-bundle/collections/weather/profiles/index.yml

@@ -0,0 +1,4 @@
+configkit:
+  imports:
+    - path: compact.yml
+    - path: weather.yml

examples/consumers/testdata/collection-bundle/collections/weather/profiles/weather.yml

@@ -0,0 +1,3 @@
+weather:
+  extends: compact
+  temperature: 0.4

examples/consumers/testdata/collection-bundle/collections/weather/prompts/brief.yml

@@ -0,0 +1,7 @@
+brief:
+  version: "1"
+  default_profile: compact
+  content_file: welcome.txt
+  messages:
+    - role: user
+      content: Write a concise weather summary.

examples/consumers/testdata/collection-bundle/collections/weather/prompts/custom.yml

@@ -0,0 +1,3 @@
+custom:
+  extends: brief
+  default_profile: weather

examples/consumers/testdata/collection-bundle/collections/weather/prompts/index.yml

@@ -0,0 +1,4 @@
+configkit:
+  imports:
+    - path: brief.yml
+    - path: custom.yml

examples/consumers/testdata/collection-bundle/collections/weather/prompts/welcome.txt

@@ -0,0 +1 @@
+Welcome to the weather collection.

examples/consumers/testdata/collection-bundle/collections/weather/version.yml

@@ -0,0 +1 @@
+version: "2.4.0"

examples/consumers/testdata/collection-bundle/operators/weather-prompt-version.yml

@@ -0,0 +1,2 @@
+brief:
+  version: "2"

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

examples/consumers/testdata/ordinary-imports/config.yml

@@ -0,0 +1,6 @@
+configkit:
+  imports:
+    - path: metadata.yml
+    - path: prompts/brief.yml
+      destination: [prompts]
+name: Root import example

examples/consumers/testdata/ordinary-imports/metadata.yml

@@ -0,0 +1,2 @@
+version: "1.0"
+description: Metadata imported into the root document.

examples/consumers/testdata/ordinary-imports/prompts/brief.yml

@@ -0,0 +1,2 @@
+brief:
+  summary: A root-loaded prompt.

examples/consumers/testdata/source-version/main.yml

@@ -0,0 +1,4 @@
+version: service-v2
+name: example service
+metadata:
+  release: application-2026.10

examples/consumers/testdata/source-version/underlay.yml

@@ -0,0 +1,2 @@
+version: catalog-v1
+endpoint: https://service.example.invalid

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_destination_test.go

@@ -0,0 +1,705 @@
+package configkit_test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestFacadeInputDestinationsAcrossSourceKinds(t *testing.T) {
+	directory := t.TempDir()
+	hostPath := filepath.Join(directory, "host.yml")
+	if err := os.WriteFile(hostPath, []byte("value: host\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	native := configkit.NewFilesystem("native", fstest.MapFS{
+		"main.yml": &fstest.MapFile{Data: []byte("value: native\n")},
+	})
+	const literalValuePath = "/collections/weather.reporter/profiles/a~1b/~0/0"
+	destination := configkit.Keys("collections", "weather.reporter", "profiles", "a/b", "~", "0")
+	for _, test := range []struct {
+		name   string
+		source configkit.Source
+		want   string
+		kind   configkit.SourceKind
+	}{
+		{name: "host file", source: configkit.File(hostPath), want: "host", kind: configkit.SourceKindFile},
+		{name: "native filesystem", source: native.File("main.yml"), want: "native", kind: configkit.SourceKindFilesystem},
+		{name: "bytes", source: configkit.Bytes("bytes", []byte("value: bytes\n")), want: "bytes", kind: configkit.SourceKindBytes},
+		{name: "reader", source: configkit.Reader("reader", strings.NewReader("value: reader\n")), want: "reader", kind: configkit.SourceKindReader},
+		{name: "mapping", source: configkit.Mapping("mapping", map[string]any{"value": "mapping"}), want: "mapping", kind: configkit.SourceKindMapping},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: test.source, Destination: destination})
+			if err != nil {
+				t.Fatalf("LoadLayered: %v", err)
+			}
+			if got := facadeDestinationValue(t, result.Config, "collections", "weather.reporter", "profiles", "a/b", "~", "0", "value"); got != test.want {
+				t.Fatalf("placed value = %#v, want %q", got, test.want)
+			}
+			sources := result.Sources()
+			if len(sources) != 1 || sources[0].Kind != test.kind || sources[0].Destination.String() != destination.String() || destination.String() != literalValuePath {
+				t.Fatalf("source placement = %#v, destination = %q", sources, destination.String())
+			}
+		})
+	}
+}
+
+func TestFacadeDestinationRootEmptyMappingNoSourceAndInvalidPaths(t *testing.T) {
+	for _, test := range []struct {
+		name        string
+		destination configkit.Path
+	}{
+		{name: "omitted destination"},
+		{name: "explicit empty destination", destination: configkit.Path{}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Bytes("root", []byte("value: root\n")), Destination: test.destination})
+			if err != nil {
+				t.Fatalf("root load: %v", err)
+			}
+			if result.Config["value"] != "root" || len(result.Sources()) != 1 || result.Sources()[0].Destination.String() != "" {
+				t.Fatalf("root placement = %#v, sources %#v", result.Config, result.Sources())
+			}
+		})
+	}
+
+	emptyPath := configkit.Keys("collections", "empty")
+	empty, err := configkit.LoadLayered[map[string]any](configkit.Input{
+		Source: configkit.Mapping("empty", map[string]any{}), Destination: emptyPath,
+	})
+	if err != nil {
+		t.Fatalf("empty mapping load: %v", err)
+	}
+	if value := facadeDestinationMap(t, empty.Config, "collections")["empty"]; value == nil {
+		t.Fatal("empty mapping did not materialize its destination")
+	}
+	if info, ok := empty.Metadata().Lookup(emptyPath); !ok || !info.Explicit {
+		t.Fatalf("empty destination metadata = %#v, %v", info, ok)
+	}
+
+	absentPath := configkit.Keys("collections", "absent")
+	absent, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource(), Destination: absentPath})
+	if err != nil {
+		t.Fatalf("NoSource with a destination: %v", err)
+	}
+	if len(absent.Sources()) != 0 {
+		t.Fatalf("NoSource created source records: %#v", absent.Sources())
+	}
+	if _, ok := absent.Metadata().Lookup(absentPath); ok {
+		t.Fatal("NoSource materialized its destination")
+	}
+
+	reader := &destinationCountingReader{}
+	for _, test := range []struct {
+		name        string
+		source      configkit.Source
+		destination configkit.Path
+		wantReads   int
+	}{
+		{name: "index segment", source: configkit.Reader("unread", reader), destination: configkit.Keys("catalog").Index(0)},
+		{name: "reserved configkit key", source: configkit.NoSource(), destination: configkit.Keys("configkit")},
+		{name: "reserved extends key", source: configkit.NoSource(), destination: configkit.Keys("catalog", "extends")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			_, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: test.source, Destination: test.destination})
+			loadError, ok := err.(*configkit.LoadError)
+			if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeOptionsInvalid {
+				t.Fatalf("invalid destination error = %#v", err)
+			}
+			if reader.reads != test.wantReads {
+				t.Fatalf("reader calls = %d, want %d", reader.reads, test.wantReads)
+			}
+		})
+	}
+}
+
+func TestFacadeImportedSourcesRetainInputDestination(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := filepath.Join(directory, "main.yml")
+	fragmentPath := filepath.Join(directory, "fragment.yml")
+	if err := os.WriteFile(mainPath, []byte("configkit: {imports: [{path: fragment.yml}]}\nmain: present\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(fragmentPath, []byte("imported: present\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	destination := configkit.Keys("collections", "weatherreporter")
+	result, err := configkit.LoadLayered[map[string]any](configkit.Input{
+		Source: configkit.File(mainPath), Imports: true, Destination: destination,
+	})
+	if err != nil {
+		t.Fatalf("mounted import load: %v", err)
+	}
+	collection := facadeDestinationMap(t, result.Config, "collections", "weatherreporter")
+	if collection["main"] != "present" || collection["imported"] != "present" {
+		t.Fatalf("mounted imported values = %#v", collection)
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Destination.String() != destination.String() || sources[1].Destination.String() != destination.String() {
+		t.Fatalf("mounted import source destinations = %#v", sources)
+	}
+	if sources[1].ImportedFrom == nil {
+		t.Fatalf("imported source lacks its authored directive: %#v", sources[1])
+	}
+}
+
+func TestFacadeRelativeImportDestinationsForRootAndMountedLoads(t *testing.T) {
+	directory := t.TempDir()
+	write := func(relative, contents string) string {
+		t.Helper()
+		filename := filepath.Join(directory, filepath.FromSlash(relative))
+		if err := os.MkdirAll(filepath.Dir(filename), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(filename, []byte(contents), 0o600); err != nil {
+			t.Fatal(err)
+		}
+		return filename
+	}
+
+	mainPath := write("main.yml", `configkit:
+  imports:
+    - path: metadata.yml
+      destination: [metadata]
+    - glob: prompts/*.yml
+      destination: [prompts]
+    - path: entries/main.yml
+      destination: [profiles]
+    - path: sibling.yml
+      destination: [sibling]
+root: main
+`)
+	write("metadata.yml", "name: imported\n")
+	write("prompts/a.yml", "a: {name: alpha}\n")
+	write("prompts/b.yml", "b: {name: beta}\n")
+	write("entries/main.yml", `configkit:
+  imports:
+    - path: nested/details.yml
+      destination: [details]
+name: profile
+`)
+	write("entries/nested/details.yml", "summary: nested\n")
+	write("sibling.yml", "name: separate\n")
+
+	root, err := configkit.Load(
+		configkit.File(mainPath), configkit.Options[map[string]any]{Imports: true},
+	)
+	if err != nil {
+		if loadError, ok := err.(*configkit.LoadError); ok {
+			t.Fatalf("ordinary root Load: %v, diagnostics %#v", err, loadError.Diagnostics())
+		}
+		t.Fatalf("ordinary root Load: %v", err)
+	}
+	mountedDestination := configkit.Keys("collections", "weatherreporter")
+	mounted, err := configkit.LoadLayered[map[string]any](configkit.Input{
+		Source: configkit.File(mainPath), Imports: true, Destination: mountedDestination,
+	})
+	if err != nil {
+		t.Fatalf("mounted layered Load: %v", err)
+	}
+
+	for _, test := range []struct {
+		name   string
+		result *configkit.Result[map[string]any]
+		prefix []string
+	}{
+		{name: "ordinary root", result: root},
+		{name: "mounted entry", result: mounted, prefix: []string{"collections", "weatherreporter"}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			at := func(keys ...string) []string {
+				return append(append([]string(nil), test.prefix...), keys...)
+			}
+			if got := facadeDestinationValue(t, test.result.Config, at("root")...); got != "main" {
+				t.Fatalf("root value = %#v", got)
+			}
+			if got := facadeDestinationValue(t, test.result.Config, at("metadata", "name")...); got != "imported" {
+				t.Fatalf("metadata value = %#v", got)
+			}
+			if got := facadeDestinationValue(t, test.result.Config, at("prompts", "a", "name")...); got != "alpha" {
+				t.Fatalf("first glob value = %#v", got)
+			}
+			if got := facadeDestinationValue(t, test.result.Config, at("prompts", "b", "name")...); got != "beta" {
+				t.Fatalf("second glob value = %#v", got)
+			}
+			if got := facadeDestinationValue(t, test.result.Config, at("profiles", "name")...); got != "profile" {
+				t.Fatalf("entry value = %#v", got)
+			}
+			if got := facadeDestinationValue(t, test.result.Config, at("profiles", "details", "summary")...); got != "nested" {
+				t.Fatalf("nested import value = %#v", got)
+			}
+			if got := facadeDestinationValue(t, test.result.Config, at("sibling", "name")...); got != "separate" {
+				t.Fatalf("subsequent sibling value = %#v", got)
+			}
+
+			wantPaths := []string{
+				"main.yml", "metadata.yml", "a.yml", "b.yml", "main.yml", "details.yml", "sibling.yml",
+			}
+			wantPhysicalPaths := []string{
+				mainPath,
+				filepath.Join(directory, "metadata.yml"),
+				filepath.Join(directory, "prompts", "a.yml"),
+				filepath.Join(directory, "prompts", "b.yml"),
+				filepath.Join(directory, "entries", "main.yml"),
+				filepath.Join(directory, "entries", "nested", "details.yml"),
+				filepath.Join(directory, "sibling.yml"),
+			}
+			wantImportParents := []string{"", "source-1", "source-1", "source-1", "source-1", "source-5", "source-1"}
+			wantDestinations := []configkit.Path{configkit.Keys(test.prefix...)}
+			for _, suffix := range [][]string{{"metadata"}, {"prompts"}, {"prompts"}, {"profiles"}, {"profiles", "details"}, {"sibling"}} {
+				destination := append([]string(nil), test.prefix...)
+				for _, segment := range suffix {
+					destination = append(destination, segment)
+				}
+				wantDestinations = append(wantDestinations, configkit.Keys(destination...))
+			}
+			sources := test.result.Sources()
+			if len(sources) != len(wantPaths) {
+				t.Fatalf("sources = %#v, want %d", sources, len(wantPaths))
+			}
+			for index, source := range sources {
+				if filepath.Base(source.Path) != wantPaths[index] || source.Path != wantPhysicalPaths[index] || source.Destination.String() != wantDestinations[index].String() {
+					t.Errorf("source %d path/destination = (%q, %q), want physical path %q and destination %q", index, source.Path, source.Destination.String(), wantPhysicalPaths[index], wantDestinations[index].String())
+				}
+				if source.ID != fmt.Sprintf("source-%d", index+1) || source.Kind != configkit.SourceKindFile {
+					t.Errorf("source %d identity/kind = (%q, %q)", index, source.ID, source.Kind)
+				}
+				if wantImportParents[index] == "" {
+					if source.ImportedFrom != nil {
+						t.Errorf("entry source unexpectedly has an import location: %#v", source.ImportedFrom)
+					}
+				} else if source.ImportedFrom == nil || source.ImportedFrom.SourceID != wantImportParents[index] || source.ImportedFrom.Line == 0 || source.ImportedFrom.Column == 0 {
+					t.Errorf("source %d import location = %#v, want declaration in %s", index, source.ImportedFrom, wantImportParents[index])
+				}
+			}
+		})
+	}
+}
+
+func TestFacadeSameFragmentMountsAtSeparateCollectionDestinations(t *testing.T) {
+	weatherDestination := configkit.Keys("collections", "weather", "prompts")
+	homeDestination := configkit.Keys("collections", "homewatch", "prompts")
+	result, err := configkit.LoadLayered(
+		configkit.Input{Source: configkit.NoSource()},
+		configkit.LayeredOptions[map[string]any]{Underlays: []configkit.Underlay{{Inputs: []configkit.Input{
+			{Source: configkit.Bytes("weather", []byte("summarize.alerts: {title: weather}\n")), Destination: weatherDestination},
+			{Source: configkit.Bytes("homewatch", []byte("summarize.alerts: {title: home}\n")), Destination: homeDestination},
+		}}}},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	for _, test := range []struct {
+		path []string
+		want string
+	}{
+		{path: []string{"collections", "weather", "prompts", "summarize.alerts", "title"}, want: "weather"},
+		{path: []string{"collections", "homewatch", "prompts", "summarize.alerts", "title"}, want: "home"},
+	} {
+		if got := facadeDestinationValue(t, result.Config, test.path...); got != test.want {
+			t.Errorf("value at %v = %#v, want %q", test.path, got, test.want)
+		}
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Destination.String() != weatherDestination.String() || sources[1].Destination.String() != homeDestination.String() {
+		t.Fatalf("independent collection source placements = %#v", sources)
+	}
+}
+
+type destinationCountingReader struct{ reads int }
+
+func (reader *destinationCountingReader) Read([]byte) (int, error) {
+	reader.reads++
+	return 0, io.EOF
+}
+
+func TestFacadeMountedMainAndUnderlayInspectionResourcesAndDiff(t *testing.T) {
+	t.Parallel()
+
+	filesystem := configkit.NewFilesystem("resources", fstest.MapFS{
+		"assets/summary.md": &fstest.MapFile{Data: []byte("source-relative summary")},
+	})
+	context := &configkit.FilesystemContext{Root: ".", BaseDir: "assets", FS: &filesystem}
+	destination := configkit.Keys("catalog", "entries")
+	tokenPath := configkit.Keys("catalog", "entries", "item", "token")
+	titlePath := configkit.Keys("catalog", "entries", "item", "title")
+	var callbacks struct{ resolve, validate int }
+	load := func(token string) *configkit.Result[map[string]any] {
+		t.Helper()
+		main := configkit.Input{
+			Source:      configkit.Bytes("main", []byte("item:\n  title: main\n  resource: summary.md\n  token: "+token+"\n")),
+			Destination: destination, Filesystem: context,
+		}
+		underlay := configkit.Input{
+			Source:      configkit.Bytes("underlay", []byte("item:\n  title: lower\n  retained: lower-value\n  token: LOWER-TOKEN-CANARY\n")),
+			Destination: destination,
+		}
+		result, err := configkit.LoadLayered(main, configkit.LayeredOptions[map[string]any]{
+			Underlays: []configkit.Underlay{{Inputs: []configkit.Input{underlay}}},
+			Definition: configkit.Definition[map[string]any]{
+				Resolve: func(configuration *map[string]any, resolution *configkit.Resolution) error {
+					callbacks.resolve++
+					resourcePath := configkit.Keys("catalog", "entries", "item", "resource")
+					input, ok := resolution.InputInfo(resourcePath)
+					if !ok || len(input.Origin.Locations) != 1 {
+						return fmt.Errorf("mounted resource input has no single source origin")
+					}
+					source, ok := resolution.Source(input.Origin.Locations[0].SourceID)
+					if !ok || source.Destination.String() != destination.String() {
+						return fmt.Errorf("resolution source did not retain its destination")
+					}
+					resourceContext, ok := resolution.SourceContext(source.ID)
+					if !ok {
+						return fmt.Errorf("resolution source context is unavailable")
+					}
+					data, err := fs.ReadFile(resourceContext.FS, path.Join(resourceContext.BaseDir, "summary.md"))
+					if err != nil {
+						return fmt.Errorf("read source-relative resource: %w", err)
+					}
+					entry := facadeDestinationMap(t, facadeDestinationMap(t, facadeDestinationMap(t, *configuration, "catalog"), "entries"), "item")
+					entry["resource_text"] = string(data)
+					resolution.Record(configkit.Keys("catalog", "entries", "item", "resource_text"), configkit.ChangeNormalization, "read source-relative summary", resourcePath)
+					return nil
+				},
+				Validate: func(*map[string]any, *configkit.Validation) { callbacks.validate++ },
+				Secret:   func(candidate configkit.Path) bool { return candidate.String() == tokenPath.String() },
+			},
+		})
+		if err != nil {
+			t.Fatalf("mounted load: %v", err)
+		}
+		return result
+	}
+
+	first := load("FIRST-TOKEN-CANARY")
+	second := load("SECOND-TOKEN-CANARY")
+	if callbacks.resolve != 2 || callbacks.validate != 2 {
+		t.Fatalf("lifecycle callbacks = %#v", callbacks)
+	}
+	for _, result := range []*configkit.Result[map[string]any]{first, second} {
+		entry := facadeDestinationMap(t, facadeDestinationMap(t, facadeDestinationMap(t, result.Config, "catalog"), "entries"), "item")
+		if entry["title"] != "main" || entry["retained"] != "lower-value" || entry["resource_text"] != "source-relative summary" {
+			t.Fatalf("mounted main/underlay configuration = %#v", entry)
+		}
+		sources := result.Sources()
+		if len(sources) != 2 || sources[0].Destination.String() != destination.String() || sources[1].Destination.String() != destination.String() {
+			t.Fatalf("public source destinations = %#v", sources)
+		}
+		resource, ok := result.SourceContext(sources[0].ID)
+		if !ok || resource.BaseDir != "assets" {
+			t.Fatalf("completed source context = %#v, %v", resource, ok)
+		}
+		data, err := fs.ReadFile(resource.FS, path.Join(resource.BaseDir, "summary.md"))
+		if err != nil || string(data) != "source-relative summary" {
+			t.Fatalf("completed relative resource = %q, %v", data, err)
+		}
+		if info, ok := result.Metadata().Lookup(titlePath); !ok || info.Origin.Layer != configkit.LayerBase {
+			t.Fatalf("mounted title metadata = %#v, %v", info, ok)
+		}
+		if history := result.Metadata().History(titlePath); len(history) < 2 {
+			t.Fatalf("mounted title history = %#v", history)
+		}
+		if explanation, ok := result.Explain(tokenPath); !ok || !explanation.Effective.Redacted {
+			t.Fatalf("mounted secret explanation = %#v, %v", explanation, ok)
+		}
+		var effective, explanation bytes.Buffer
+		if err := result.WriteEffective(&effective); err != nil {
+			t.Fatal(err)
+		}
+		if err := result.WriteExplanation(&explanation, tokenPath); err != nil {
+			t.Fatal(err)
+		}
+		output := effective.String() + explanation.String()
+		for _, canary := range []string{"FIRST-TOKEN-CANARY", "SECOND-TOKEN-CANARY", "LOWER-TOKEN-CANARY"} {
+			if strings.Contains(output, canary) {
+				t.Fatalf("public inspection leaked %q: %s", canary, output)
+			}
+		}
+	}
+
+	// Mutating published records and query copies must not alter the retained result.
+	returnedSources := first.Sources()
+	returnedSources[0].Destination = configkit.Keys("mutated")
+	if got := first.Sources()[0].Destination.String(); got != destination.String() {
+		t.Fatalf("source destination changed after caller mutation: %q", got)
+	}
+	info, ok := first.Metadata().Lookup(titlePath)
+	if !ok {
+		t.Fatal("mounted metadata lookup failed")
+	}
+	info.Origin.Locations[0].SourceID = "mutated"
+	if fresh, ok := first.Metadata().Lookup(titlePath); !ok || fresh.Origin.Locations[0].SourceID == "mutated" {
+		t.Fatalf("metadata copy mutation escaped: %#v, %v", fresh, ok)
+	}
+	history := first.Metadata().History(titlePath)
+	if len(history) == 0 {
+		t.Fatal("mounted history is empty")
+	}
+	history[0].Path = configkit.Keys("mutated")
+	if fresh := first.Metadata().History(titlePath); len(fresh) == 0 || fresh[0].Path.String() != titlePath.String() {
+		t.Fatalf("history copy mutation escaped: %#v", fresh)
+	}
+
+	differences, err := configkit.Diff(first, second)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 1 || differences[0].Path.String() != tokenPath.String() ||
+		!differences[0].Left.Redacted || !differences[0].Right.Redacted ||
+		strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "TOKEN-CANARY") {
+		t.Fatalf("public mounted secret diff = %#v", differences)
+	}
+	if callbacks.resolve != 2 || callbacks.validate != 2 {
+		t.Fatalf("inspection reran lifecycle callbacks: %#v", callbacks)
+	}
+}
+
+func TestFacadeFailedLoadExposesDetachedSourceDestination(t *testing.T) {
+	type configuration struct {
+		Tenant struct {
+			Count int `yaml:"count"`
+		} `yaml:"tenant"`
+	}
+	destination := configkit.Keys("tenant")
+	_, err := configkit.LoadLayered[configuration](configkit.Input{
+		Source: configkit.Bytes("invalid", []byte("count: not-an-integer\n")), Destination: destination,
+	})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok {
+		t.Fatalf("failed load error = %T %v", err, err)
+	}
+	sources := loadError.Sources()
+	if len(sources) != 1 || sources[0].Destination.String() != destination.String() {
+		t.Fatalf("failed source destinations = %#v", sources)
+	}
+	sources[0].Destination = configkit.Keys("caller-change")
+	if fresh := loadError.Sources(); len(fresh) != 1 || fresh[0].Destination.String() != destination.String() {
+		t.Fatalf("failed source destination was not detached: %#v", fresh)
+	}
+}
+
+func TestFacadeRelativeImportFailuresRetainSourceRecordsAndCoordinates(t *testing.T) {
+	directory := t.TempDir()
+	write := func(name, contents string) string {
+		t.Helper()
+		filename := filepath.Join(directory, name)
+		if err := os.MkdirAll(filepath.Dir(filename), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(filename, []byte(contents), 0o600); err != nil {
+			t.Fatal(err)
+		}
+		return filename
+	}
+
+	t.Run("imported parse failure retains effective source destination", func(t *testing.T) {
+		mainPath := write("parse/main.yml", "configkit:\n  imports:\n    - path: child.yml\n      destination: [tenant]\n")
+		childPath := write("parse/child.yml", "count: [unterminated\n")
+		_, err := configkit.LoadLayered[map[string]any](configkit.Input{
+			Source: configkit.File(mainPath), Imports: true, Destination: configkit.Keys("mount"),
+		})
+		var loadError *configkit.LoadError
+		if !errors.As(err, &loadError) {
+			t.Fatalf("load error = %T %v", err, err)
+		}
+		diagnostics := loadError.Diagnostics()
+		if len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeYAMLInvalid || diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-2" {
+			t.Fatalf("imported parse diagnostic = %#v", diagnostics)
+		}
+		sources := loadError.Sources()
+		if len(sources) != 2 || sources[0].Destination.String() != "/mount" || sources[1].Destination.String() != "/mount/tenant" || sources[1].Path != childPath {
+			t.Fatalf("imported parse failure sources = %#v", sources)
+		}
+	})
+
+	t.Run("directive failure stays source local", func(t *testing.T) {
+		mainPath := write("directive/main.yml", "configkit:\n  imports:\n    - path: parent.yml\n      destination: [tenant]\n")
+		parentPath := write("directive/parent.yml", "configkit:\n  imports:\n    - path: details.yml\n      destination: [3]\n")
+		_, err := configkit.LoadLayered[map[string]any](configkit.Input{
+			Source: configkit.File(mainPath), Imports: true, Destination: configkit.Keys("mount"),
+		})
+		var loadError *configkit.LoadError
+		if !errors.As(err, &loadError) {
+			t.Fatalf("load error = %T %v", err, err)
+		}
+		diagnostics := loadError.Diagnostics()
+		wantPath := configkit.Keys("configkit", "imports").Index(0).Key("destination").Index(0)
+		if len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeImportInvalid || !reflect.DeepEqual(diagnostics[0].Path, wantPath) || diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-2" {
+			t.Fatalf("import directive diagnostic = %#v", diagnostics)
+		}
+		sources := loadError.Sources()
+		if len(sources) != 2 || sources[0].Destination.String() != "/mount" || sources[1].Destination.String() != "/mount/tenant" || sources[1].Path != parentPath {
+			t.Fatalf("directive failure sources = %#v", sources)
+		}
+	})
+
+	t.Run("composition failure reports effective path and all sources", func(t *testing.T) {
+		mainPath := write("compose/main.yml", "configkit:\n  imports:\n    - path: left.yml\n      destination: [tenant]\n    - path: right.yml\n      destination: [tenant]\n")
+		write("compose/left.yml", "value: same\n")
+		write("compose/right.yml", "value: same\n")
+		_, err := configkit.LoadLayered[map[string]any](configkit.Input{
+			Source: configkit.File(mainPath), Imports: true, Destination: configkit.Keys("mount"),
+		})
+		var loadError *configkit.LoadError
+		if !errors.As(err, &loadError) {
+			t.Fatalf("load error = %T %v", err, err)
+		}
+		diagnostics := loadError.Diagnostics()
+		if len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeComposeConflict || diagnostics[0].Path.String() != "/mount/tenant/value" {
+			t.Fatalf("composition diagnostic = %#v", diagnostics)
+		}
+		sources := loadError.Sources()
+		if len(sources) != 3 || sources[0].Destination.String() != "/mount" || sources[1].Destination.String() != "/mount/tenant" || sources[2].Destination.String() != "/mount/tenant" {
+			t.Fatalf("composition failure sources = %#v", sources)
+		}
+	})
+
+	t.Run("version failure reports effective path and imported source", func(t *testing.T) {
+		mainPath := write("version/main.yml", "configkit:\n  imports:\n    - path: child.yml\n      destination: [tenant]\n")
+		childPath := write("version/child.yml", "version: forbidden\n")
+		_, err := configkit.LoadLayered[map[string]any](configkit.Input{
+			Source: configkit.File(mainPath), Imports: true, Destination: configkit.Keys("mount"),
+		}, configkit.LayeredOptions[map[string]any]{
+			Definition: configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+				Path:  configkit.Keys("mount", "tenant", "version"),
+				Check: func(configkit.Version, *configkit.Validation) error { return nil },
+			}},
+		})
+		var loadError *configkit.LoadError
+		if !errors.As(err, &loadError) {
+			t.Fatalf("load error = %T %v", err, err)
+		}
+		diagnostics := loadError.Diagnostics()
+		if len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeImportVersion || diagnostics[0].Path.String() != "/mount/tenant/version" || diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-2" {
+			t.Fatalf("version diagnostic = %#v", diagnostics)
+		}
+		sources := loadError.Sources()
+		if len(sources) != 2 || sources[0].Destination.String() != "/mount" || sources[1].Destination.String() != "/mount/tenant" || sources[1].Path != childPath {
+			t.Fatalf("version failure sources = %#v", sources)
+		}
+	})
+
+	t.Run("typed validation reports effective path and authored location", func(t *testing.T) {
+		type configuration struct {
+			Mount struct {
+				Tenant struct {
+					Count int `yaml:"count"`
+				} `yaml:"tenant"`
+			} `yaml:"mount"`
+		}
+		mainPath := write("typed/main.yml", "configkit:\n  imports:\n    - path: child.yml\n      destination: [tenant]\n")
+		childPath := write("typed/child.yml", "count: invalid\n")
+		_, err := configkit.LoadLayered[configuration](configkit.Input{
+			Source: configkit.File(mainPath), Imports: true, Destination: configkit.Keys("mount"),
+		})
+		var loadError *configkit.LoadError
+		if !errors.As(err, &loadError) {
+			t.Fatalf("load error = %T %v", err, err)
+		}
+		diagnostics := loadError.Diagnostics()
+		if len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeDecodeType || diagnostics[0].Path.String() != "/mount/tenant/count" || diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-2" || diagnostics[0].Primary.Line != 1 {
+			t.Fatalf("typed validation diagnostic = %#v", diagnostics)
+		}
+		sources := loadError.Sources()
+		if len(sources) != 2 || sources[0].Destination.String() != "/mount" || sources[1].Destination.String() != "/mount/tenant" || sources[1].Path != childPath {
+			t.Fatalf("typed validation sources = %#v", sources)
+		}
+	})
+}
+
+func TestFacadeDestinationMappingFailurePaths(t *testing.T) {
+	t.Parallel()
+
+	cycle := map[string]any{}
+	cycle["self"] = cycle
+	for _, fixture := range []struct {
+		name   string
+		values map[string]any
+		path   func(configkit.Path) configkit.Path
+	}{
+		{
+			name:   "unsupported nested sequence value",
+			values: map[string]any{"a/b": []any{map[string]any{"~": func() {}}}},
+			path:   func(base configkit.Path) configkit.Path { return base.Key("a/b").Index(0).Key("~") },
+		},
+		{
+			name:   "cyclic value",
+			values: map[string]any{"item": cycle},
+			path:   func(base configkit.Path) configkit.Path { return base.Key("item").Key("self") },
+		},
+		{
+			name: "constructor failure keeps source coordinates",
+			path: func(configkit.Path) configkit.Path { return configkit.Path{} },
+		},
+	} {
+		t.Run(fixture.name, func(t *testing.T) {
+			for _, role := range []string{"root", "mounted main", "mounted underlay"} {
+				t.Run(role, func(t *testing.T) {
+					var destination configkit.Path
+					if role != "root" {
+						destination = configkit.Keys("catalog", "entries")
+					}
+					wantPath := fixture.path(destination)
+					input := configkit.Input{Source: configkit.Mapping("invalid mapping", fixture.values), Destination: destination}
+					var result *configkit.Result[map[string]any]
+					var err error
+					if role == "mounted underlay" {
+						result, err = configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{
+							Underlays: []configkit.Underlay{{Inputs: []configkit.Input{input}}},
+						})
+					} else {
+						result, err = configkit.LoadLayered[map[string]any](input)
+					}
+					var loadError *configkit.LoadError
+					if result != nil || !errors.As(err, &loadError) {
+						t.Fatalf("failed load = %#v, %v", result, err)
+					}
+					diagnostics := loadError.Diagnostics()
+					if len(diagnostics) != 1 {
+						t.Fatalf("diagnostics = %#v", diagnostics)
+					}
+					diagnostic := diagnostics[0]
+					if diagnostic.Code != configkit.CodeSourceInvalid || !reflect.DeepEqual(diagnostic.Path, wantPath) {
+						t.Fatalf("diagnostic = %#v; want source.invalid at %q", diagnostic, wantPath.String())
+					}
+					if diagnostic.Primary == nil || *diagnostic.Primary != (configkit.Location{SourceID: "source-1"}) || len(diagnostic.Related) != 0 {
+						t.Fatalf("mapping source locations = %#v, %#v", diagnostic.Primary, diagnostic.Related)
+					}
+					sources := loadError.Sources()
+					if len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Destination.String() != destination.String() {
+						t.Fatalf("error sources = %#v", sources)
+					}
+				})
+			}
+		})
+	}
+}
+
+func facadeDestinationMap(t *testing.T, value map[string]any, keys ...string) map[string]any {
+	t.Helper()
+	for _, key := range keys {
+		child, ok := value[key].(map[string]any)
+		if !ok {
+			t.Fatalf("%q = %#v, want mapping", key, value[key])
+		}
+		value = child
+	}
+	return value
+}
+
+func facadeDestinationValue(t *testing.T, value map[string]any, keys ...string) any {
+	t.Helper()
+	return facadeDestinationMap(t, value, keys[:len(keys)-1]...)[keys[len(keys)-1]]
+}

facade_diff_test.go

@@ -0,0 +1,327 @@
+package configkit_test
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type facadeDiffConfig struct {
+	Value   string         `yaml:"value"`
+	Count   int            `yaml:"count"`
+	Items   []string       `yaml:"items"`
+	Dynamic map[string]any `yaml:"dynamic"`
+}
+
+type facadeDiffSecretConfig struct {
+	Secret configkit.Secret `yaml:"secret"`
+}
+
+func TestFacadeDiffKeepsPublicSequenceChangesWithHistoricalDescendants(t *testing.T) {
+	type configuration struct {
+		Items []any `yaml:"items"`
+	}
+	load := func(public string) *configkit.Result[configuration] {
+		t.Helper()
+		result, err := configkit.Load(configkit.Bytes("historical-sequence.yml", []byte("items: [{secret: disposable-canary}, "+public+"]\n")),
+			configkit.Options[configuration]{Definition: configkit.Definition[configuration]{
+				Secret: func(path configkit.Path) bool { return path.String() == "/items/0/secret" },
+				Resolve: func(current *configuration, resolution *configkit.Resolution) error {
+					current.Items[0] = "public scalar"
+					resolution.Record(configkit.Keys("items"), configkit.ChangeNormalization, "normalize first item")
+					return nil
+				},
+			}})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+	left, right := load("1"), load("2")
+	history, ok := left.Explain(configkit.Keys("items").Index(0).Key("secret"))
+	if !ok || history.Effective.Present || len(history.History) == 0 {
+		t.Fatalf("removed historical secret = %#v, known=%v", history, ok)
+	}
+	maskedHistory := false
+	for _, change := range history.History {
+		maskedHistory = maskedHistory || change.Before.Redacted || change.After.Redacted
+	}
+	if !maskedHistory {
+		t.Fatal("historical sensitivity was not retained")
+	}
+	differences, err := configkit.Diff(left, right)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/items" {
+		t.Fatalf("sequence difference = %#v, %v", differences, err)
+	}
+	difference := differences[0]
+	if !difference.Left.Present || !difference.Right.Present || difference.Left.Redacted || difference.Right.Redacted || difference.HiddenChanged {
+		t.Fatalf("public sequence change was concealed: %#v", difference)
+	}
+	if difference.Left.YAML != "- public scalar\n- 1\n" || difference.Right.YAML != "- public scalar\n- 2\n" {
+		t.Fatalf("public sequence snapshots = %#v", difference)
+	}
+}
+
+func TestFacadeDiffKeepsPublicScalarAcrossHistoricalAbsence(t *testing.T) {
+	type configuration struct {
+		Dynamic map[string]any `yaml:"dynamic"`
+	}
+	options := configkit.Options[configuration]{Definition: configkit.Definition[configuration]{
+		Secret: func(path configkit.Path) bool { return path.String() == "/dynamic/entry/token" },
+		Resolve: func(current *configuration, resolution *configkit.Resolution) error {
+			delete(current.Dynamic, "entry")
+			resolution.Record(configkit.Keys("dynamic", "entry"), configkit.ChangeNormalization, "remove entry")
+			return nil
+		},
+	}}
+	absent, err := configkit.Load(configkit.Bytes("removed-entry.yml", []byte("dynamic: {entry: {token: disposable-canary}}\n")), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	options.Definition.Resolve = nil
+	present, err := configkit.Load(configkit.Bytes("public-entry.yml", []byte("dynamic: {entry: public}\n")), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, test := range []struct {
+		name        string
+		left, right *configkit.Result[configuration]
+		leftPresent bool
+	}{
+		{name: "addition", left: absent, right: present},
+		{name: "removal", left: present, right: absent, leftPresent: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			differences, err := configkit.Diff(test.left, test.right)
+			if err != nil || len(differences) != 1 || differences[0].Path.String() != "/dynamic/entry" {
+				t.Fatalf("entry difference = %#v, %v", differences, err)
+			}
+			difference := differences[0]
+			if difference.Left.Present != test.leftPresent || difference.Right.Present == test.leftPresent ||
+				difference.Left.Redacted || difference.Right.Redacted || difference.HiddenChanged {
+				t.Fatalf("public presence change was concealed: %#v", difference)
+			}
+			wantLeft, wantRight := "", "public\n"
+			if test.leftPresent {
+				wantLeft, wantRight = wantRight, wantLeft
+			}
+			if difference.Left.YAML != wantLeft || difference.Right.YAML != wantRight {
+				t.Fatalf("entry snapshots = %#v", difference)
+			}
+		})
+	}
+}
+
+func TestFacadeDiffReturnsDetachedCanonicalRecords(t *testing.T) {
+	left, err := configkit.Load[facadeDiffConfig](configkit.Bytes("left.yml", []byte("value: before\ncount: 0\nitems: [one, two]\ndynamic: {}\n")))
+	if err != nil {
+		t.Fatalf("load left: %v", err)
+	}
+	right, err := configkit.Load[facadeDiffConfig](configkit.Bytes("right.yml", []byte("value: after\ncount: 0\nitems: [one, three]\ndynamic:\n  added: null\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	differences, err := configkit.Diff(left, right)
+	if err != nil {
+		t.Fatal(err)
+	}
+	wantPaths := []string{"/dynamic/added", "/items", "/value"}
+	if len(differences) != len(wantPaths) {
+		t.Fatalf("differences = %#v", differences)
+	}
+	for index, difference := range differences {
+		if got := difference.Path.String(); got != wantPaths[index] {
+			t.Errorf("path[%d] = %q, want %q", index, got, wantPaths[index])
+		}
+	}
+	if differences[0].Left.Present || !differences[0].Right.Present || differences[0].Right.YAML != "null\n" {
+		t.Errorf("null/absence snapshots = %#v / %#v", differences[0].Left, differences[0].Right)
+	}
+	if differences[1].Left.YAML != "- one\n- two\n" || differences[1].Right.YAML != "- one\n- three\n" {
+		t.Errorf("sequence snapshots = %#v / %#v", differences[1].Left, differences[1].Right)
+	}
+
+	left.Config.Value = "caller mutation"
+	left.Config.Items[0] = "caller mutation"
+	right.Config.Dynamic["added"] = "caller mutation"
+	differences[2].Left.YAML = "record mutation"
+	differences[2].Path = configkit.Keys("rewritten")
+
+	again, err := configkit.Diff(left, right)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if again[2].Path.String() != "/value" || again[2].Left.YAML != "before\n" || again[1].Left.YAML != "- one\n- two\n" {
+		t.Fatalf("repeat diff changed = %#v", again)
+	}
+	explanation, ok := left.Explain(configkit.Keys("value"))
+	if !ok || explanation.Effective.YAML != "before\n" {
+		t.Fatalf("frozen explanation = %#v, %v", explanation, ok)
+	}
+}
+
+func TestFacadeDiffRejectsUnavailableResults(t *testing.T) {
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	loaded, err := configkit.Load[config](configkit.Bytes("valid.yml", []byte("value: valid\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var nilResult *configkit.Result[config]
+	zeroResult := &configkit.Result[config]{Config: config{Value: "must not be inspected"}}
+	for name, left := range map[string]*configkit.Result[config]{"nil": nilResult, "zero": zeroResult} {
+		t.Run(name, func(t *testing.T) {
+			if records, err := configkit.Diff(left, loaded); err == nil || records != nil {
+				t.Fatalf("Diff = %#v, %v; want nil records and error", records, err)
+			}
+		})
+	}
+	if records, err := configkit.Diff(loaded, nilResult); err == nil || records != nil {
+		t.Fatalf("right nil Diff = %#v, %v", records, err)
+	}
+}
+
+func TestFacadeDiffIgnoresSourceAndResolutionHistory(t *testing.T) {
+	type config struct {
+		Count int `yaml:"count"`
+	}
+	defaults := configkit.Options[config]{Definition: configkit.Definition[config]{
+		Defaults: func() config { return config{Count: 7} },
+	}}
+	defaulted, err := configkit.Load[config](configkit.NoSource(), defaults)
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicit, err := configkit.Load[config](configkit.Bytes("explicit.yml", []byte("count: 7\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(defaulted, explicit); err != nil || len(differences) != 0 {
+		t.Fatalf("default/source difference = %#v, %v", differences, err)
+	}
+
+	zeroDefault, err := configkit.Load[config](configkit.NoSource(), configkit.Options[config]{Definition: configkit.Definition[config]{Defaults: func() config { return config{} }}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicitZero, err := configkit.Load[config](configkit.Bytes("explicit-zero.yml", []byte("count: 0\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(zeroDefault, explicitZero); err != nil || len(differences) != 0 {
+		t.Fatalf("explicit/defaulted zero difference = %#v, %v", differences, err)
+	}
+
+	resolve := func(value string) configkit.Options[config] {
+		return configkit.Options[config]{Definition: configkit.Definition[config]{Resolve: func(current *config, resolution *configkit.Resolution) error {
+			current.Count = len(value) + 8
+			resolution.Record(configkit.Keys("count"), configkit.ChangeNormalization, "set effective count")
+			return nil
+		}}}
+	}
+	resolvedA, err := configkit.Load[config](configkit.Bytes("a.yml", []byte("{}\n")), resolve("a"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	resolvedB, err := configkit.Load[config](configkit.Bytes("b.yml", []byte("{}\n")), resolve("b"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(resolvedA, resolvedB); err != nil || len(differences) != 0 {
+		t.Fatalf("equal resolved values = %#v, %v", differences, err)
+	}
+}
+
+func TestFacadeDiffUsesFrozenFilesAndRunsCallbacksOnlyDuringLoad(t *testing.T) {
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	calls := 0
+	options := configkit.Options[config]{Definition: configkit.Definition[config]{Defaults: func() config {
+		calls++
+		return config{}
+	}}}
+	dir := t.TempDir()
+	paths := []string{filepath.Join(dir, "left.yml"), filepath.Join(dir, "right.yml")}
+	for index, contents := range []string{"value: left\n", "value: right\n"} {
+		if err := os.WriteFile(paths[index], []byte(contents), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	left, err := configkit.Load[config](configkit.File(paths[0]), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	right, err := configkit.Load[config](configkit.File(paths[1]), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if calls != 2 {
+		t.Fatalf("defaults calls after loads = %d", calls)
+	}
+	for _, path := range paths {
+		if err := os.Remove(path); err != nil {
+			t.Fatal(err)
+		}
+	}
+	differences, err := configkit.Diff(left, right)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/value" {
+		t.Fatalf("Diff after removing files = %#v, %v", differences, err)
+	}
+	if calls != 2 {
+		t.Fatalf("defaults reran during Diff: %d calls", calls)
+	}
+}
+
+func TestFacadeDiffCombinesSecretSensitivityAndLoadErrors(t *testing.T) {
+	t.Setenv("FACADE_DIFF_SECRET", "credential-one")
+	secretOptions := configkit.Options[facadeDiffSecretConfig]{Definition: configkit.Definition[facadeDiffSecretConfig]{Secret: func(path configkit.Path) bool {
+		return path.String() == "/secret"
+	}}}
+	left, err := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("left.yml", []byte("secret: FACADE_DIFF_SECRET\n")), secretOptions)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("FACADE_DIFF_SECRET", "credential-two")
+	right, err := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("right.yml", []byte("secret: FACADE_DIFF_SECRET\n")), secretOptions)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(left, right); err != nil || len(differences) != 0 {
+		t.Fatalf("credential rotation = %#v, %v", differences, err)
+	}
+	t.Setenv("OTHER_FACADE_DIFF_SECRET", "other-credential")
+	otherReference, err := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("other.yml", []byte("secret: OTHER_FACADE_DIFF_SECRET\n")), secretOptions)
+	if err != nil {
+		t.Fatal(err)
+	}
+	differences, err := configkit.Diff(left, otherReference)
+	if err != nil || len(differences) != 1 || !differences[0].HiddenChanged || !differences[0].Left.Redacted || !differences[0].Right.Redacted {
+		t.Fatalf("reference change = %#v, %v", differences, err)
+	}
+	if strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "credential") {
+		t.Fatalf("credential leaked: %#v", differences)
+	}
+
+	invalidLeft, leftErr := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("invalid-left.yml", []byte("unknown: true\n")))
+	if invalidLeft != nil || leftErr == nil {
+		t.Fatalf("invalid left load = %#v, %v", invalidLeft, leftErr)
+	}
+	var loadError *configkit.LoadError
+	if !errors.As(leftErr, &loadError) {
+		t.Fatalf("left load error = %T, want *configkit.LoadError", leftErr)
+	}
+	invalidRight, rightErr := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("invalid-right.yml", []byte("unknown: true\n")))
+	if invalidRight != nil || rightErr == nil {
+		t.Fatalf("invalid right load = %#v, %v", invalidRight, rightErr)
+	}
+	if !errors.As(rightErr, &loadError) {
+		t.Fatalf("right load error = %T, want *configkit.LoadError", rightErr)
+	}
+}

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 +1,297 @@
+package configkit_test
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"reflect"
+	"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")
+	}
+}
+
+func TestFacadeRelativeImportDestinationsPreserveNativeResourceContexts(t *testing.T) {
+	borrowed := &observedResourceFS{files: fstest.MapFS{
+		"bundle/config/main.yml":                   &fstest.MapFile{Data: []byte("configkit:\n  imports:\n    - path: entries/parent.yml\n      destination: [logical]\n")},
+		"bundle/config/entries/parent.yml":         &fstest.MapFile{Data: []byte("configkit:\n  imports:\n    - path: nested/details.yml\n      destination: [details]\nresource: icon.txt\n")},
+		"bundle/config/entries/nested/details.yml": &fstest.MapFile{Data: []byte("resource: detail.txt\n")},
+		"bundle/config/entries/icon.txt":           &fstest.MapFile{Data: []byte("parent-resource")},
+		"bundle/config/entries/nested/detail.txt":  &fstest.MapFile{Data: []byte("nested-resource")},
+	}}
+	namespace := configkit.NewFilesystem("bundle", borrowed)
+	input := configkit.Input{
+		Source: namespace.File("bundle/config/main.yml"), Imports: true,
+		Filesystem: &configkit.FilesystemContext{Root: "bundle", FS: &namespace},
+	}
+	load := func() *configkit.Result[map[string]any] {
+		t.Helper()
+		result, err := configkit.LoadLayered[map[string]any](input)
+		if err != nil {
+			t.Fatalf("load relative native imports: %v", err)
+		}
+		return result
+	}
+	result := load()
+	wantDestinations := []string{configkit.Keys().String(), "/logical", "/logical/details"}
+	wantPaths := []string{"bundle/config/main.yml", "bundle/config/entries/parent.yml", "bundle/config/entries/nested/details.yml"}
+	sources := result.Sources()
+	if len(sources) != len(wantPaths) {
+		t.Fatalf("sources = %#v", sources)
+	}
+	for index, source := range sources {
+		if source.ID != fmt.Sprintf("source-%d", index+1) || source.Kind != configkit.SourceKindFilesystem || source.Path != wantPaths[index] || source.Destination.String() != wantDestinations[index] {
+			t.Errorf("source %d = %#v", index, source)
+		}
+		if index == 0 {
+			if source.ImportedFrom != nil {
+				t.Errorf("entry source has import location %#v", source.ImportedFrom)
+			}
+		} else if source.ImportedFrom == nil || source.ImportedFrom.SourceID != fmt.Sprintf("source-%d", index) || source.ImportedFrom.Line == 0 || source.ImportedFrom.Column == 0 {
+			t.Errorf("source %d import location = %#v", index, source.ImportedFrom)
+		}
+	}
+
+	for _, test := range []struct {
+		field, effectivePath, wantBase, wantResource, wantContents string
+	}{
+		{"parent resource", "/logical/resource", "config/entries", "icon.txt", "parent-resource"},
+		{"nested resource", "/logical/details/resource", "config/entries/nested", "detail.txt", "nested-resource"},
+	} {
+		info, ok := result.Metadata().Lookup(configkit.Keys(strings.Split(strings.Trim(test.effectivePath, "/"), "/")...))
+		if !ok || len(info.Origin.Locations) != 1 {
+			t.Fatalf("%s origin = %#v, %v", test.field, info, ok)
+		}
+		context, ok := result.SourceContext(info.Origin.Locations[0].SourceID)
+		if !ok || context.BaseDir != test.wantBase || context.FS == nil {
+			t.Fatalf("%s context = %#v, %v", test.field, context, ok)
+		}
+		contents, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, test.wantResource))
+		if err != nil || string(contents) != test.wantContents {
+			t.Fatalf("%s opened resource = %q, %v", test.field, contents, err)
+		}
+	}
+
+	sources[1].Destination = configkit.Keys("caller", "mutation")
+	for query := 0; query < 2; query++ {
+		fresh := result.Sources()
+		if fresh[1].Destination.String() != "/logical" {
+			t.Fatalf("query %d source destination = %q", query, fresh[1].Destination.String())
+		}
+	}
+	second := load()
+	if !reflect.DeepEqual(second.Sources(), result.Sources()) {
+		t.Fatalf("independent source records differ: %#v vs %#v", second.Sources(), result.Sources())
+	}
+}

facade_source_version_mode_test.go

@@ -0,0 +1,748 @@
+package configkit_test
+
+import (
+	"bytes"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type sourceVersionModeReader struct {
+	*strings.Reader
+	reads int
+}
+
+func TestFacadeSourceVersionEvidenceRedactsCollidingShapes(t *testing.T) {
+	t.Parallel()
+	type header struct {
+		Version string `yaml:"version"`
+	}
+	type secretHeader struct {
+		Version string `yaml:"version" configkit:"secret"`
+	}
+	type configuration struct {
+		Meta map[string]header `yaml:"meta"`
+		Data struct {
+			Meta map[string]secretHeader `yaml:"meta"`
+		} `yaml:"data"`
+	}
+	path, err := configkit.ParsePath("/meta/0/version")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, reverse := range []bool{false, true} {
+		groups := []configkit.Underlay{
+			{Inputs: []configkit.Input{{
+				Source:      configkit.Bytes("mapping", []byte("meta: {\"0\": {version: PRIVATE-HEADER-CANARY}}\n")),
+				Destination: configkit.Keys("data"),
+			}}},
+			{Inputs: []configkit.Input{{
+				Source:      configkit.Bytes("sequence", []byte("meta: [{version: another-header}]\n")),
+				Destination: configkit.Keys("data"),
+			}}},
+		}
+		if reverse {
+			groups[0], groups[1] = groups[1], groups[0]
+		}
+		loaded, err := configkit.LoadLayered(configkit.Input{
+			Source: configkit.Bytes("main", []byte("meta: {\"0\": {version: main}}\ndata: {meta: null}\n")),
+		}, configkit.LayeredOptions[configuration]{
+			Definition: configkit.Definition[configuration]{Version: &configkit.VersionRule{
+				Path: path, Check: func(configkit.Version, *configkit.Validation) error { return nil },
+			}},
+			SourceVersionMode: configkit.SourceVersionModeRequired, Underlays: groups,
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if loaded.Config.Meta["0"].Version != "main" || loaded.Config.Data.Meta != nil {
+			t.Fatalf("reverse=%v: final payload changed: %#v", reverse, loaded.Config)
+		}
+		foundMapping := false
+		for _, source := range loaded.Sources() {
+			if source.Label != "mapping" {
+				continue
+			}
+			foundMapping = true
+			if check := source.VersionCheck; check == nil || !check.Version.Present || !check.Redacted || check.Version.Text != "" {
+				t.Errorf("reverse=%v: schema-tagged header was not redacted", reverse)
+			}
+		}
+		if !foundMapping {
+			t.Fatal("missing mapping source record")
+		}
+	}
+}
+
+func TestFacadeSourceVersionModesPreserveImportOptIn(t *testing.T) {
+	t.Parallel()
+	for _, mode := range []configkit.SourceVersionMode{configkit.SourceVersionModeIfPresent, configkit.SourceVersionModeRequired} {
+		for _, underlay := range []bool{false, true} {
+			bad := configkit.Bytes("disabled-imports", []byte("version: accepted\nconfigkit: {imports: [{path: missing.yml}]}\nvalue: visible\n"))
+			definition := configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+				Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil },
+			}}
+			var err error
+			var resultPresent bool
+			wantSource := "source-1"
+			if underlay {
+				loaded, loadErr := configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte("version: main\n"))}, configkit.LayeredOptions[map[string]any]{
+					Definition: definition, SourceVersionMode: mode,
+					Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: bad}}}},
+				})
+				err, resultPresent, wantSource = loadErr, loaded != nil, "source-2"
+			} else {
+				loaded, loadErr := configkit.Load(bad, configkit.Options[map[string]any]{Definition: definition, SourceVersionMode: mode})
+				err, resultPresent = loadErr, loaded != nil
+			}
+			var failure *configkit.LoadError
+			if resultPresent || !errors.As(err, &failure) {
+				t.Errorf("mode=%v underlay=%v: import opt-in did not fail safely", mode, underlay)
+				continue
+			}
+			diagnostics := failure.Diagnostics()
+			if len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeImportDisabled || diagnostics[0].Path.String() != "/configkit" ||
+				diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != wantSource {
+				t.Errorf("mode=%v underlay=%v: diagnostics=%#v", mode, underlay, diagnostics)
+			}
+		}
+	}
+}
+
+func TestFacadeSourceVersionAbsenceKeepsCompleteEvidencePath(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name, main, underlay string
+		path                 configkit.Path
+	}{
+		{"missing member", "meta: {version: main}\n", "{}\n", configkit.Keys("meta", "version")},
+		{"scalar obstruction", "meta: {version: main}\n", "meta: false\n", configkit.Keys("meta", "version")},
+		{"unavailable index", "meta: [null, null, null, {version: main}]\n", "meta: []\n", configkit.Keys("meta").Index(3).Key("version")},
+	} {
+		for _, mode := range []configkit.SourceVersionMode{configkit.SourceVersionModeIfPresent, configkit.SourceVersionModeRequired} {
+			var callbackEvidence *configkit.SourceVersionCheck
+			loaded, err := configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte(test.main))}, configkit.LayeredOptions[map[string]any]{
+				SourceVersionMode: mode,
+				Definition: configkit.Definition[map[string]any]{
+					Version: &configkit.VersionRule{Path: test.path, Check: func(configkit.Version, *configkit.Validation) error { return nil }},
+					Resolve: func(_ *map[string]any, resolution *configkit.Resolution) error {
+						source, found := resolution.Source("source-2")
+						if !found {
+							t.Fatal("missing callback source")
+						}
+						callbackEvidence = source.VersionCheck
+						return nil
+					},
+				},
+				Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+					Source: configkit.Bytes("underlay", []byte(test.underlay)), Destination: configkit.Keys("under"),
+				}}}},
+			})
+			var evidence *configkit.SourceVersionCheck
+			if mode == configkit.SourceVersionModeRequired {
+				var failure *configkit.LoadError
+				if loaded != nil || !errors.As(err, &failure) {
+					t.Fatalf("%s: required omission did not fail", test.name)
+				}
+				diagnostics := failure.Diagnostics()
+				if len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeVersionError || diagnostics[0].Path.String() != test.path.String() {
+					t.Fatalf("%s: required diagnostics=%#v", test.name, diagnostics)
+				}
+				evidence = failure.Sources()[1].VersionCheck
+			} else {
+				if err != nil {
+					t.Fatalf("%s: %v", test.name, err)
+				}
+				evidence = loaded.Sources()[1].VersionCheck
+				if callbackEvidence == nil || callbackEvidence.Path.String() != test.path.String() {
+					t.Errorf("%s: callback path is incomplete", test.name)
+				}
+			}
+			if evidence == nil || evidence.Version.Present || evidence.Path.String() != test.path.String() ||
+				evidence.Location == nil || evidence.Location.SourceID != "source-2" || evidence.Location.Line != 1 {
+				t.Errorf("%s mode=%v: absence evidence=%#v", test.name, mode, evidence)
+			}
+		}
+	}
+}
+
+func TestFacadeSourceVersionMissingHeaderDoesNotOverlapControl(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name     string
+		mode     configkit.SourceVersionMode
+		control  string
+		wantCode configkit.Code
+	}{
+		{"optional omission", configkit.SourceVersionModeIfPresent, "base", ""},
+		{"required omission", configkit.SourceVersionModeRequired, "base", configkit.CodeVersionError},
+		{"invalid control", configkit.SourceVersionModeIfPresent, "true", configkit.CodeInheritanceInvalid},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			main := configkit.Bytes("main", []byte("{}\n"))
+			if test.mode == configkit.SourceVersionModeRequired {
+				main = configkit.NoSource()
+			}
+			checks := 0
+			path := configkit.Keys("entries", "child", "extends", "version")
+			loaded, err := configkit.LoadLayered(configkit.Input{Source: main, Destination: configkit.Keys("main")}, configkit.LayeredOptions[map[string]any]{
+				SourceVersionMode: test.mode,
+				Definition: configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+					Path: path, Check: func(version configkit.Version, _ *configkit.Validation) error {
+						checks++
+						if version.Present {
+							t.Error("checker received a nonexistent declaration")
+						}
+						return nil
+					},
+				}},
+				Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+					Source: configkit.Bytes("underlay", []byte("entries:\n  base: {value: good}\n  child: {extends: "+test.control+"}\n")), Destination: configkit.Keys("under"),
+				}}}},
+			})
+			if checks != 1 {
+				t.Errorf("checker ran %d times, want absent main only", checks)
+			}
+			if test.wantCode != "" {
+				var failure *configkit.LoadError
+				if loaded != nil || !errors.As(err, &failure) {
+					t.Fatal("invalid input did not fail safely")
+				}
+				if diagnostics := failure.Diagnostics(); len(diagnostics) != 1 || diagnostics[0].Code != test.wantCode {
+					t.Fatalf("diagnostics=%#v", diagnostics)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			child := loaded.Config["under"].(map[string]any)["entries"].(map[string]any)["child"].(map[string]any)
+			if child["value"] != "good" {
+				t.Fatal("valid sibling inheritance was not applied")
+			}
+			check := loaded.Sources()[1].VersionCheck
+			if check == nil || check.Version.Present || check.Path.String() != path.String() {
+				t.Fatal("optional omission evidence was not retained")
+			}
+		})
+	}
+}
+
+func (reader *sourceVersionModeReader) Read(data []byte) (int, error) {
+	reader.reads++
+	return reader.Reader.Read(data)
+}
+
+func TestFacadeSourceVersionModesAcrossOrdinaryAndLayeredLoads(t *testing.T) {
+	t.Parallel()
+
+	var checked []configkit.Version
+	definition := configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+		Path: configkit.Keys("meta", "version"),
+		Check: func(version configkit.Version, _ *configkit.Validation) error {
+			checked = append(checked, version)
+			return nil
+		},
+	}}
+	ordinary, err := configkit.Load(configkit.Bytes("ordinary", []byte("meta: {version: ordinary-v}\n")), configkit.Options[map[string]any]{
+		Definition: definition, SourceVersionMode: configkit.SourceVersionModeIfPresent,
+	})
+	if err != nil || ordinary == nil {
+		t.Fatalf("ordinary enabled load = %#v, %v", ordinary, err)
+	}
+	if got := ordinary.Config["meta"].(map[string]any)["version"]; got != "ordinary-v" {
+		t.Fatalf("main declaration was not retained: %#v", ordinary.Config)
+	}
+
+	directory := t.TempDir()
+	mainPath := filepath.Join(directory, "main.yml")
+	if err := os.WriteFile(mainPath, []byte("meta: {version: main-v}\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	layered, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(mainPath)}, configkit.LayeredOptions[map[string]any]{
+		Definition: definition, SourceVersionMode: configkit.SourceVersionModeRequired,
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+			Source: configkit.Mapping("underlay", map[string]any{
+				"meta": map[string]any{"version": "underlay-v"}, "fallback": "present",
+			}),
+			Destination: configkit.Keys("defaults"),
+		}}}},
+	})
+	if err != nil || layered == nil {
+		t.Fatalf("layered enabled load = %#v, %v", layered, err)
+	}
+	defaults, ok := layered.Config["defaults"].(map[string]any)
+	if !ok || defaults["fallback"] != "present" {
+		t.Fatalf("layered underlay payload = %#v", layered.Config["defaults"])
+	}
+	if meta, ok := defaults["meta"].(map[string]any); !ok || len(meta) != 0 {
+		t.Fatalf("accepted underlay declaration was not consumed: %#v", defaults["meta"])
+	}
+	if len(checked) != 3 {
+		t.Fatalf("checker calls across separate mode uses = %#v", checked)
+	}
+	for _, want := range []string{"ordinary-v", "main-v", "underlay-v"} {
+		found := false
+		for _, version := range checked {
+			found = found || version.Present && version.Text == want
+		}
+		if !found {
+			t.Errorf("checked versions %#v do not include %q", checked, want)
+		}
+	}
+}
+
+func TestFacadeSourceVersionModeZeroPreservesDefaultBehavior(t *testing.T) {
+	t.Parallel()
+
+	checks := 0
+	result, err := configkit.Load(configkit.Bytes("default", []byte("version: v1\n")), configkit.Options[map[string]any]{
+		Definition: configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+			Path: configkit.Keys("version"),
+			Check: func(version configkit.Version, _ *configkit.Validation) error {
+				checks++
+				if version != (configkit.Version{Present: true, Text: "v1"}) {
+					t.Errorf("default version = %#v", version)
+				}
+				return nil
+			},
+		}},
+	})
+	if err != nil || result == nil || result.Config["version"] != "v1" || checks != 1 {
+		t.Fatalf("zero-mode load = %#v, %v; checks = %d", result, err, checks)
+	}
+}
+
+func TestFacadeSourceVersionModeValidationPrecedesSourceReads(t *testing.T) {
+	t.Parallel()
+
+	validDefinition := configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+		Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil },
+	}}
+	for _, test := range []struct {
+		name    string
+		mode    configkit.SourceVersionMode
+		def     configkit.Definition[map[string]any]
+		layered bool
+	}{
+		{name: "unknown ordinary mode", mode: configkit.SourceVersionMode(255), def: validDefinition},
+		{name: "ordinary mode requires checker", mode: configkit.SourceVersionModeIfPresent},
+		{name: "unknown layered mode", mode: configkit.SourceVersionMode(255), def: validDefinition, layered: true},
+		{name: "layered mode requires checker", mode: configkit.SourceVersionModeRequired, layered: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			reader := &sourceVersionModeReader{Reader: strings.NewReader("version: v1\n")}
+			source := configkit.Reader("version.yml", reader)
+			var err error
+			if test.layered {
+				_, err = configkit.LoadLayered(configkit.Input{Source: source}, configkit.LayeredOptions[map[string]any]{
+					Definition: test.def, SourceVersionMode: test.mode,
+				})
+			} else {
+				_, err = configkit.Load(source, configkit.Options[map[string]any]{
+					Definition: test.def, SourceVersionMode: test.mode,
+				})
+			}
+			var loadError *configkit.LoadError
+			if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != configkit.CodeOptionsInvalid {
+				t.Fatalf("invalid mode error = %#v", err)
+			}
+			if reader.reads != 0 {
+				t.Fatalf("invalid options read source %d times", reader.reads)
+			}
+		})
+	}
+}
+
+func TestFacadeSourceVersionDefinitionCanBeReusedWithDifferentModes(t *testing.T) {
+	t.Parallel()
+
+	checks := 0
+	definition := configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+		Path: configkit.Keys("version"),
+		Check: func(version configkit.Version, _ *configkit.Validation) error {
+			checks++
+			if version.Present {
+				t.Errorf("missing declaration reached checker as %#v", version)
+			}
+			return nil
+		},
+	}}
+	source := configkit.Bytes("reused", []byte("value: retained\n"))
+	ifPresent, err := configkit.Load(source, configkit.Options[map[string]any]{
+		Definition: definition, SourceVersionMode: configkit.SourceVersionModeIfPresent,
+	})
+	if err != nil || ifPresent == nil || checks != 1 {
+		t.Fatalf("if-present load = %#v, %v; checks = %d", ifPresent, err, checks)
+	}
+	required, err := configkit.Load(source, configkit.Options[map[string]any]{
+		Definition: definition, SourceVersionMode: configkit.SourceVersionModeRequired,
+	})
+	var loadError *configkit.LoadError
+	if required != nil || !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != configkit.CodeVersionError {
+		t.Fatalf("required load = %#v, %v", required, err)
+	}
+	if checks != 1 {
+		t.Fatalf("required absence reached checker: calls = %d", checks)
+	}
+}
+
+func TestFacadeSourceVersionEvidenceIsSanitizedAndDetached(t *testing.T) {
+	t.Parallel()
+
+	const canary = "PRIVATE-SOURCE-VERSION-CANARY"
+	directory := t.TempDir()
+	path := filepath.Join(directory, "main.yml")
+	if err := os.WriteFile(path, []byte("version: "+canary+"\nvalue: visible\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	var callbackSource configkit.SourceInfo
+	var callbackContext configkit.ResourceContext
+	definition := configkit.Definition[map[string]any]{
+		Version: &configkit.VersionRule{Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil }},
+		Secret:  func(candidate configkit.Path) bool { return candidate.String() == "/service/version" },
+		Resolve: func(_ *map[string]any, resolution *configkit.Resolution) error {
+			var found bool
+			callbackSource, found = resolution.Source("source-1")
+			if !found {
+				t.Fatal("resolution callback could not query the acquired source")
+			}
+			if callbackSource.VersionCheck == nil || !callbackSource.VersionCheck.Redacted || callbackSource.VersionCheck.Version.Text != "" {
+				t.Fatalf("callback source did not conservatively redact evidence: %#v", callbackSource.VersionCheck)
+			}
+			callbackContext, found = resolution.SourceContext("source-1")
+			if !found || callbackContext.FS == nil || callbackContext.BaseDir != "." {
+				t.Fatalf("callback source context = %#v, found=%v", callbackContext, found)
+			}
+			return nil
+		},
+	}
+	loaded, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(path), Destination: configkit.Keys("service")}, configkit.LayeredOptions[map[string]any]{
+		Definition: definition, SourceVersionMode: configkit.SourceVersionModeIfPresent,
+	})
+	if err != nil || loaded == nil {
+		t.Fatalf("LoadLayered = %#v, %v", loaded, err)
+	}
+	if callbackSource.VersionCheck == nil || !callbackSource.VersionCheck.Redacted || callbackSource.VersionCheck.Version.Text != "" {
+		t.Fatalf("callback evidence lost its conservative snapshot: %#v", callbackSource.VersionCheck)
+	}
+	callbackSource.VersionCheck.Path = configkit.Keys("mutated-callback")
+	callbackSource.VersionCheck.Version.Text = canary
+	callbackSource.VersionCheck.Location.Line = 98
+	finalSources := loaded.Sources()
+	if len(finalSources) != 1 || finalSources[0].Path != path || finalSources[0].VersionCheck == nil {
+		t.Fatalf("final sources = %#v", finalSources)
+	}
+	check := finalSources[0].VersionCheck
+	if check.Path.String() != "/version" || check.Version != (configkit.Version{Present: true}) || !check.Redacted ||
+		check.Location == nil || check.Location.SourceID != finalSources[0].ID || check.Location.Line != 1 {
+		t.Fatalf("final classified evidence = %#v", check)
+	}
+	if err := os.Remove(path); err != nil {
+		t.Fatal(err)
+	}
+	if callbackContext.FS == nil || callbackContext.BaseDir != "." {
+		t.Fatalf("resource metadata changed after removing the source: %#v", callbackContext)
+	}
+	check.Path = configkit.Keys("mutated")
+	check.Location.Line = 99
+	check.Redacted = false
+	check.Version.Text = "mutated"
+	repeated := loaded.Sources()
+	if len(repeated) != 1 || repeated[0].VersionCheck.Path.String() != "/version" ||
+		repeated[0].VersionCheck.Location.Line != 1 || !repeated[0].VersionCheck.Redacted ||
+		repeated[0].VersionCheck.Version.Text != "" {
+		t.Fatalf("source evidence was aliased or re-read after source removal: %#v", repeated)
+	}
+
+	publicText := "PUBLIC-SOURCE-VERSION"
+	var publicCallback configkit.SourceInfo
+	public, err := configkit.Load(configkit.Bytes("public", []byte("version: "+publicText+"\n")), configkit.Options[map[string]any]{
+		SourceVersionMode: configkit.SourceVersionModeIfPresent,
+		Definition: configkit.Definition[map[string]any]{
+			Version: definition.Version,
+			Resolve: func(_ *map[string]any, resolution *configkit.Resolution) error {
+				publicCallback, _ = resolution.Source("source-1")
+				return nil
+			},
+		},
+	})
+	if err != nil || public == nil || len(public.Sources()) != 1 || public.Sources()[0].VersionCheck == nil {
+		t.Fatalf("unclassified source evidence = %#v, %v", public, err)
+	}
+	if got := public.Sources()[0].VersionCheck; got.Redacted || got.Version.Text != publicText || !got.Version.Present {
+		t.Fatalf("unclassified evidence = %#v", got)
+	}
+	if publicCallback.VersionCheck == nil || !publicCallback.VersionCheck.Redacted || publicCallback.VersionCheck.Version.Text != "" {
+		t.Fatalf("unclassified callback evidence was not initially conservative: %#v", publicCallback.VersionCheck)
+	}
+}
+
+func TestFacadeSourceVersionEvidenceOnFailedLoadsAndSyntheticSources(t *testing.T) {
+	t.Parallel()
+
+	definition := configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+		Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil },
+	}}
+	t.Run("early import acquisition failure", func(t *testing.T) {
+		const canary = "EARLY-ACQUISITION-SOURCE-VERSION-CANARY"
+		checked := 0
+		earlyDefinition := configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+			Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { checked++; return nil },
+		}}
+		path := filepath.Join(t.TempDir(), "main.yml")
+		contents := "version: " + canary + "\nconfigkit:\n  imports:\n    - path: missing.yml\n"
+		if err := os.WriteFile(path, []byte(contents), 0o600); err != nil {
+			t.Fatal(err)
+		}
+		_, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(path), Imports: true}, configkit.LayeredOptions[map[string]any]{
+			Definition: earlyDefinition, SourceVersionMode: configkit.SourceVersionModeIfPresent,
+		})
+		var failure *configkit.LoadError
+		if !errors.As(err, &failure) || len(failure.Sources()) != 1 {
+			t.Fatalf("early acquisition failure sources = %#v, error %v", failure, err)
+		}
+		if checked != 1 {
+			t.Fatalf("main declaration checker calls = %d; error %v", checked, err)
+		}
+		check := failure.Sources()[0].VersionCheck
+		if check == nil || !check.Redacted || !check.Version.Present || check.Version.Text != "" {
+			t.Fatalf("early acquisition evidence was not conservatively redacted: %#v; sources %#v", check, failure.Sources())
+		}
+		if strings.Contains(err.Error(), canary) {
+			t.Fatalf("early acquisition failure disclosed source-version canary: %v", err)
+		}
+	})
+	_, err := configkit.Load(configkit.Bytes("malformed", []byte("version: true\n")), configkit.Options[map[string]any]{
+		Definition: definition, SourceVersionMode: configkit.SourceVersionModeIfPresent,
+	})
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) {
+		t.Fatalf("malformed version error = %v", err)
+	}
+	sources := loadError.Sources()
+	if len(sources) != 1 || sources[0].VersionCheck == nil || !sources[0].VersionCheck.Redacted ||
+		!sources[0].VersionCheck.Version.Present || sources[0].VersionCheck.Version.Text != "" || sources[0].VersionCheck.Path.String() != "/version" {
+		t.Fatalf("failed-load source evidence = %#v", sources)
+	}
+	sources[0].VersionCheck.Path = configkit.Keys("mutated-error")
+	sources[0].VersionCheck.Version.Text = "mutated"
+	sources[0].VersionCheck.Location.Line = 99
+	if again := loadError.Sources()[0].VersionCheck; again.Path.String() != "/version" || again.Version.Text != "" || again.Location.Line == 99 {
+		t.Fatalf("LoadError source evidence was aliased: %#v", again)
+	}
+	for _, test := range []struct {
+		name   string
+		def    configkit.Definition[map[string]any]
+		canary string
+	}{
+		{
+			name:   "resolver failure",
+			canary: "RESOLVER-SOURCE-VERSION-CANARY",
+			def: configkit.Definition[map[string]any]{Version: definition.Version, Resolve: func(*map[string]any, *configkit.Resolution) error {
+				return errors.New("resolver failed")
+			}},
+		},
+		{
+			name:   "validator failure",
+			canary: "VALIDATOR-SOURCE-VERSION-CANARY",
+			def: configkit.Definition[map[string]any]{Version: definition.Version, Validate: func(_ *map[string]any, validation *configkit.Validation) {
+				validation.Error(configkit.Keys("value"), "invalid value")
+			}},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			_, err := configkit.Load(configkit.Bytes("late-failure", []byte("version: "+test.canary+"\nvalue: configured\n")), configkit.Options[map[string]any]{
+				Definition: test.def, SourceVersionMode: configkit.SourceVersionModeIfPresent,
+			})
+			var failure *configkit.LoadError
+			if !errors.As(err, &failure) || len(failure.Sources()) != 1 {
+				t.Fatalf("late failure sources = %#v, error %v", failure, err)
+			}
+			if strings.Contains(err.Error(), test.canary) {
+				t.Fatalf("late failure disclosed source-version canary: %v", err)
+			}
+			check := failure.Sources()[0].VersionCheck
+			if check == nil || !check.Redacted || !check.Version.Present || check.Version.Text != "" {
+				t.Fatalf("late-failure evidence was not conservatively redacted: %#v", check)
+			}
+		})
+	}
+
+	defaultResult, err := configkit.Load(configkit.Bytes("default", []byte("version: 7\n")), configkit.Options[map[string]any]{})
+	if err != nil || defaultResult == nil || len(defaultResult.Sources()) != 1 || defaultResult.Sources()[0].VersionCheck != nil {
+		t.Fatalf("default source version check = %#v, %v", defaultResult, err)
+	}
+
+	withCLI, err := configkit.Load(configkit.Bytes("main", []byte("version: 8\nvalue: old\n")), configkit.Options[map[string]any]{
+		Definition: definition, SourceVersionMode: configkit.SourceVersionModeIfPresent,
+		Overrides: []configkit.Override{{Path: configkit.Keys("value"), Value: "new", Flag: "--value"}},
+	})
+	if err != nil || withCLI == nil {
+		t.Fatalf("enabled load with CLI source = %#v, %v", withCLI, err)
+	}
+	sawCLI := false
+	for _, source := range withCLI.Sources() {
+		if source.Kind == configkit.SourceKindCLI && source.VersionCheck != nil {
+			t.Fatalf("synthetic CLI source acquired declaration evidence: %#v", source)
+		}
+		sawCLI = sawCLI || source.Kind == configkit.SourceKindCLI
+	}
+	if !sawCLI {
+		t.Fatalf("CLI source record missing: %#v", withCLI.Sources())
+	}
+}
+
+func TestFacadeSourceVersionEvidencePreservesAbsenceAndNull(t *testing.T) {
+	t.Parallel()
+
+	var checked []configkit.Version
+	loaded, err := configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte("version: 17\n"))}, configkit.LayeredOptions[map[string]any]{
+		SourceVersionMode: configkit.SourceVersionModeIfPresent,
+		Definition: configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+			Path: configkit.Keys("version"),
+			Check: func(version configkit.Version, _ *configkit.Validation) error {
+				checked = append(checked, version)
+				return nil
+			},
+		}},
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{
+			{Source: configkit.Bytes("absent", []byte("value: visible\n")), Destination: configkit.Keys("absent")},
+			{Source: configkit.Bytes("null", []byte("version: null\nvalue: visible\n")), Destination: configkit.Keys("nullable")},
+		}}},
+	})
+	if err != nil || loaded == nil {
+		t.Fatalf("LoadLayered = %#v, %v", loaded, err)
+	}
+	if len(checked) != 2 || !checked[0].Present || checked[0].Text != "17" ||
+		!checked[1].Present || !checked[1].Null {
+		t.Fatalf("checker calls = %#v; absent contribution must skip while null is checked", checked)
+	}
+	sources := loaded.Sources()
+	if len(sources) != 3 || sources[1].VersionCheck == nil || sources[2].VersionCheck == nil {
+		t.Fatalf("source version evidence = %#v", sources)
+	}
+	absent := sources[1].VersionCheck
+	if absent.Version.Present || absent.Path.String() != "/version" || absent.Location == nil || absent.Redacted {
+		t.Fatalf("optional omission evidence = %#v", absent)
+	}
+	null := sources[2].VersionCheck
+	if !null.Version.Present || !null.Version.Null || null.Version.Text != "" || null.Path.String() != "/version" {
+		t.Fatalf("null declaration evidence = %#v", null)
+	}
+}
+
+func TestFacadeSourceVersionLifecycleKeepsConsumedHeadersOutOfPayloadHistory(t *testing.T) {
+	type serviceConfig struct {
+		Version    uint64           `yaml:"version"`
+		Count      int              `yaml:"count"`
+		Enabled    bool             `yaml:"enabled"`
+		Nullable   *string          `yaml:"nullable"`
+		Revision   string           `yaml:"revision"`
+		Defaulted  string           `yaml:"defaulted"`
+		Resolved   string           `yaml:"resolved"`
+		Credential configkit.Secret `yaml:"credential"`
+	}
+	type configuration struct {
+		Service serviceConfig `yaml:"service"`
+	}
+	t.Setenv("CONFIGKIT_LIFECYCLE_SECRET", "lifecycle-secret-value")
+	var checked []configkit.Version
+	var validationCalls int
+	definition := configkit.Definition[configuration]{
+		Defaults: func() configuration {
+			return configuration{Service: serviceConfig{Defaulted: "from defaults"}}
+		},
+		Version: &configkit.VersionRule{
+			Path: configkit.Keys("version"),
+			Check: func(version configkit.Version, _ *configkit.Validation) error {
+				checked = append(checked, version)
+				return nil
+			},
+		},
+		Resolve: func(current *configuration, resolution *configkit.Resolution) error {
+			current.Service.Resolved = "resolved once"
+			resolution.Record(configkit.Keys("service", "resolved"), configkit.ChangeNormalization, "derive resolved value")
+			return nil
+		},
+		Validate: func(current *configuration, validation *configkit.Validation) {
+			validationCalls++
+			if current.Service.Version != 9007199254740993 || current.Service.Count != 0 || current.Service.Enabled ||
+				current.Service.Nullable != nil || current.Service.Revision != "ordinary-payload" || current.Service.Defaulted != "from defaults" ||
+				current.Service.Resolved != "resolved once" || current.Service.Credential.Value() != "lifecycle-secret-value" {
+				validation.Error(configkit.Keys("service"), "assembled values did not survive source declaration processing")
+			}
+		},
+	}
+
+	load := func(header string) *configkit.Result[configuration] {
+		t.Helper()
+		result, err := configkit.LoadLayered(configkit.Input{
+			Source: configkit.Bytes("main", []byte("version: 9007199254740993\n")), Destination: configkit.Keys("service"),
+		}, configkit.LayeredOptions[configuration]{
+			Definition: definition, SourceVersionMode: configkit.SourceVersionModeIfPresent,
+			Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+				Source:      configkit.Bytes("underlay", []byte("version: "+header+"\ncount: 0\nenabled: false\nnullable: null\nrevision: ordinary-payload\ncredential: CONFIGKIT_LIFECYCLE_SECRET\n")),
+				Destination: configkit.Keys("service"),
+			}}}},
+		})
+		if err != nil || result == nil {
+			var failure *configkit.LoadError
+			var diagnostics []configkit.Diagnostic
+			if errors.As(err, &failure) {
+				diagnostics = failure.Diagnostics()
+			}
+			t.Fatalf("LoadLayered(%q) = %#v, %v; diagnostics %#v", header, result, err, diagnostics)
+		}
+		return result
+	}
+
+	first := load("underlay-release-a")
+	second := load("underlay-release-b")
+	if len(checked) != 4 || checked[0].Text != "9007199254740993" || checked[1].Text != "underlay-release-a" ||
+		checked[2].Text != "9007199254740993" || checked[3].Text != "underlay-release-b" {
+		t.Fatalf("source declarations checked = %#v", checked)
+	}
+	if validationCalls != 2 || first.Config.Service != second.Config.Service {
+		t.Fatalf("final configs or validation count = %#v, %#v, %d", first.Config, second.Config, validationCalls)
+	}
+	if differences, err := configkit.Diff(first, second); err != nil || len(differences) != 0 {
+		t.Fatalf("different consumed declarations created effective differences: %#v, %v", differences, err)
+	}
+	var firstEffective, secondEffective bytes.Buffer
+	if err := first.WriteEffective(&firstEffective); err != nil {
+		t.Fatal(err)
+	}
+	if err := second.WriteEffective(&secondEffective); err != nil {
+		t.Fatal(err)
+	}
+	if firstEffective.String() != secondEffective.String() || strings.Contains(firstEffective.String(), "underlay-release") {
+		t.Fatalf("consumed declarations affected effective rendering:\n%s\n%s", firstEffective.String(), secondEffective.String())
+	}
+	sourcesA, sourcesB := first.Sources(), second.Sources()
+	if len(sourcesA) != 2 || len(sourcesB) != 2 || sourcesA[1].VersionCheck == nil || sourcesB[1].VersionCheck == nil ||
+		sourcesA[1].VersionCheck.Version.Text != "underlay-release-a" || sourcesB[1].VersionCheck.Version.Text != "underlay-release-b" {
+		t.Fatalf("sanitized evidence did not retain distinct accepted declarations: %#v / %#v", sourcesA, sourcesB)
+	}
+	versionExplanation, found := first.Explain(configkit.Keys("service", "version"))
+	if !found {
+		t.Fatal("retained main version missing from effective explanation")
+	}
+	underlayID := sourcesA[1].ID
+	for _, location := range versionExplanation.Origin.Locations {
+		if location.SourceID == underlayID {
+			t.Fatalf("consumed underlay declaration entered effective origin: %#v", versionExplanation.Origin)
+		}
+	}
+	for _, change := range versionExplanation.History {
+		for _, location := range change.Origin.Locations {
+			if location.SourceID == underlayID {
+				t.Fatalf("consumed underlay declaration entered effective history: %#v", versionExplanation.History)
+			}
+		}
+	}
+}

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)
+	}
+}

go.mod

@@ -1,5 +1,13 @@
 module gitea.maximumdirect.net/eric/ConfigKit
 
-go 1.25.0
+go 1.26.0
 
 require go.yaml.in/yaml/v4 v4.0.0-rc.6
+
+require (
+	gitea.maximumdirect.net/eric/ReleaseKit v0.4.1 // indirect
+	github.com/yuin/goldmark/v2 v2.1.6 // indirect
+	golang.org/x/mod v0.40.0 // indirect
+)
+
+tool gitea.maximumdirect.net/eric/ReleaseKit/cmd/releasekit

go.sum

@@ -1,2 +1,8 @@
+gitea.maximumdirect.net/eric/ReleaseKit v0.4.1 h1:Qn+mdebYOzCVCEXgCbuGSO/WHmJtItwaQ8e3XwY3qtI=
+gitea.maximumdirect.net/eric/ReleaseKit v0.4.1/go.mod h1:LVodqNQ3OIKv9NBq+fFeyMByd1lHY25YOmmpah/NaNI=
+github.com/yuin/goldmark/v2 v2.1.6 h1:igy0q9JcqovoFXFx/XjtU8sHVhcUHavPj2sA3EoKJUE=
+github.com/yuin/goldmark/v2 v2.1.6/go.mod h1:G6M4/qOFtfn01/o14BU1UR2Lo5N3S9Qo7xCuz/sHjGQ=
 go.yaml.in/yaml/v4 v4.0.0-rc.6 h1:1h7H1ohdUh93/FyE4YaDa1Zh64K6VVbjF4K6WUxMtH4=
 go.yaml.in/yaml/v4 v4.0.0-rc.6/go.mod h1:aZqd9kCMsGL7AuUv/m/PvWLdg5sjJsZ4oHDEnfPPfY0=
+golang.org/x/mod v0.40.0 h1:hUv+3cXcdRHz08UmSiOob7sadHig73uo5bkXxQ/tvUs=
+golang.org/x/mod v0.40.0/go.mod h1:0/weTWkPWGBikyTWAX3dkjVztMmBA5hM0DH6BElSupE=

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/destination.go

@@ -0,0 +1,128 @@
+package engine
+
+import (
+	"errors"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+var (
+	errInvalidDestination = errors.New("configkit: destination must contain only non-reserved mapping keys")
+	errDestinationLimit   = errors.New("configkit: destination exceeds document limits")
+)
+
+func normalizeDestination(path Path) (Path, error) {
+	if !path.valid() {
+		return Path{}, errInvalidDestination
+	}
+
+	segments := make([]pathSegment, 0, len(path.segments))
+	for _, segment := range path.segments {
+		if segment.kind != pathSegmentKey && segment.kind != pathSegmentParsed {
+			return Path{}, errInvalidDestination
+		}
+		if segment.key == "configkit" || segment.key == "extends" {
+			return Path{}, errInvalidDestination
+		}
+		segments = append(segments, pathSegment{kind: pathSegmentKey, key: segment.key})
+	}
+	return Path{segments: segments}, nil
+}
+
+// appendDestination combines validated mapping-key paths with owned segments.
+func appendDestination(parent, relative Path) Path {
+	segments := clonePathSegments(parent.segments, len(relative.segments))
+	segments = append(segments, relative.segments...)
+	return Path{segments: segments}
+}
+
+func invalidDestinationDiagnostic(destination Path) Diagnostic {
+	return Diagnostic{
+		Code: CodeOptionsInvalid, Severity: SeverityError,
+		Path:    destination.clone(),
+		Message: "input destination must contain only non-reserved mapping keys",
+	}
+}
+
+// placeDocument checks all per-document bounds before allocating destination
+// wrappers. Aggregate accounting remains the caller's responsibility.
+func placeDocument(body *document.Node, destination Path, sourceRoot document.Location, maxDepth, maxNodes int) (Path, *document.Node, error) {
+	normalized, err := normalizeDestination(destination)
+	if err != nil {
+		return Path{}, nil, err
+	}
+	if body == nil {
+		return normalized, nil, nil
+	}
+	if !destinationWithinLimits(body, normalized, maxDepth, maxNodes) {
+		return normalized, nil, errDestinationLimit
+	}
+
+	placed := body
+	for index := len(normalized.segments) - 1; index >= 0; index-- {
+		key := normalized.segments[index].key
+		placed = &document.Node{
+			Kind:     document.Mapping,
+			Location: sourceRoot,
+			Mapping: []document.Entry{{
+				Key:           key,
+				KeyLocation:   sourceRoot,
+				ValueLocation: sourceRoot,
+				Value:         placed,
+			}},
+		}
+	}
+	return normalized, placed, nil
+}
+
+// destinationWithinLimits expects a normalized mapping-key destination.
+func destinationWithinLimits(body *document.Node, destination Path, maxDepth, maxNodes int) bool {
+	if body == nil {
+		return true
+	}
+	if maxDepth < 0 || maxNodes < 0 || !destination.valid() {
+		return false
+	}
+	segments := len(destination.segments)
+	if segments > maxDepth || segments > maxNodes/2 {
+		return false
+	}
+	wrapperNodes := destinationWrapperNodeCount(destination)
+	for _, segment := range destination.segments {
+		if segment.kind != pathSegmentKey {
+			return false
+		}
+	}
+	if documentDepth(body) > maxDepth-segments {
+		return false
+	}
+	return countInputNodes(body) <= maxNodes-wrapperNodes
+}
+
+func destinationWrapperNodeCount(destination Path) int {
+	return 2 * len(destination.segments)
+}
+
+func documentDepth(node *document.Node) int {
+	if node == nil {
+		return -1
+	}
+	depth := 0
+	var visit func(*document.Node, int)
+	visit = func(current *document.Node, currentDepth int) {
+		if current == nil {
+			return
+		}
+		if currentDepth > depth {
+			depth = currentDepth
+		}
+		for _, entry := range current.Mapping {
+			visit(entry.Value, currentDepth+1)
+		}
+		for _, child := range current.Sequence {
+			visit(child, currentDepth+1)
+		}
+	}
+	visit(node, 0)
+	return depth
+}

internal/engine/destination_accounting_test.go

@@ -0,0 +1,379 @@
+package engine
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"testing/fstest"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestDestinationPlacementCountsAgainstSharedNodeBudget(t *testing.T) {
+	t.Parallel()
+
+	limits := defaultImportLimits()
+	destination := Keys("collections", "entry")
+	empty := Mapping("empty", map[string]any{})
+	parsed, diagnostics := parseSource(empty, "fixture", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	perInput := countInputNodes(parsed.document) + destinationWrapperNodeCount(destination)
+
+	for _, test := range []struct {
+		name       string
+		delta      int
+		wantError  bool
+		wantCharge int
+	}{
+		{name: "below aggregate bound", delta: -1, wantCharge: perInput},
+		{name: "at aggregate bound", delta: 0, wantCharge: perInput},
+		{name: "above aggregate bound", delta: 1, wantError: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			accounting := &importAccounting{nodes: limits.nodes - perInput + test.delta}
+			composition, _, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{
+				source: empty, destination: destination,
+			}}}, 1, nil, accounting)
+			if test.wantError {
+				if composition != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSourceLimitExceeded {
+					t.Fatalf("over-budget placement = %#v, %#v", composition, diagnostics)
+				}
+				return
+			}
+			if composition == nil || len(diagnostics) != 0 || composition.inputNodes != test.wantCharge {
+				t.Fatalf("bounded placement = %#v, %#v", composition, diagnostics)
+			}
+		})
+	}
+}
+
+func TestMultipleMountedInputsSharePlacementCharges(t *testing.T) {
+	t.Parallel()
+
+	limits := defaultImportLimits()
+	inputs := []loadInput{
+		{source: Mapping("one", map[string]any{}), destination: Keys("collection", "one")},
+		{source: Mapping("two", map[string]any{}), destination: Keys("collection", "two")},
+		{source: Mapping("three", map[string]any{}), destination: Keys("collection", "three")},
+	}
+	perInput := countInputNodes(&document.Node{Kind: document.Mapping}) + destinationWrapperNodeCount(Keys("collection", "one"))
+	charge := len(inputs) * perInput
+	accounting := &importAccounting{nodes: limits.nodes - charge}
+	composition, _, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: inputs}, 1, nil, accounting)
+	if len(diagnostics) != 0 || composition == nil || accounting.nodes != limits.nodes || composition.inputNodes != charge {
+		t.Fatalf("shared placement accounting = %#v, %#v, accounting %#v; want charge %d", composition, diagnostics, accounting, charge)
+	}
+}
+
+func TestImportedEmptyBodiesChargeEachDestinationWrapper(t *testing.T) {
+	t.Parallel()
+
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":     &fstest.MapFile{Data: []byte("configkit: {imports: [{path: fragment.yml}]}\n")},
+		"fragment.yml": &fstest.MapFile{Data: []byte("{}\n")},
+	})
+	mainSource := filesystem.File("main.yml")
+	mainParsed, diagnostics := parseSource(mainSource, "fixture", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	fragmentParsed, diagnostics := parseSource(Bytes("fragment", []byte("{}\n")), "fixture", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	rawNodes := countInputNodes(mainParsed.document) + countInputNodes(fragmentParsed.document)
+	destination := Keys("collection", "empty")
+	placementNodes := 2 * destinationWrapperNodeCount(destination)
+	limits := defaultImportLimits()
+	for _, test := range []struct {
+		name      string
+		delta     int
+		wantError bool
+	}{
+		{name: "below aggregate bound", delta: -1},
+		{name: "at aggregate bound", delta: 0},
+		{name: "above aggregate bound", delta: 1, wantError: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			accounting := &importAccounting{nodes: limits.nodes - rawNodes - placementNodes + test.delta}
+			composition, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{
+				source: mainSource, imports: true, destination: destination,
+			}}}, 1, nil, accounting)
+			if test.wantError {
+				if composition != nil || len(diagnostics) == 0 || diagnostics[0].Code != CodeImportLimitExceeded {
+					t.Fatalf("over-budget imports = %#v, %#v", composition, diagnostics)
+				}
+				return
+			}
+			if len(diagnostics) != 0 || composition == nil || len(sources) != 2 {
+				t.Fatalf("bounded imports = %#v, sources %#v, diagnostics %#v", composition, sources, diagnostics)
+			}
+			if composition.inputNodes != rawNodes+placementNodes || accounting.nodes != limits.nodes+test.delta {
+				t.Fatalf("import node charge = %d, accounting %d; want %d and %d", composition.inputNodes, accounting.nodes, rawNodes+placementNodes, limits.nodes+test.delta)
+			}
+			if node, present := documentAtPath(composition.root, Keys("collection", "empty")); !present || node.Kind != document.Mapping || len(node.Mapping) != 0 {
+				t.Fatalf("placed empty import body = %#v, present %v", node, present)
+			}
+		})
+	}
+}
+
+func TestImportedEmptyBodiesChargeFullRelativeDestinations(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: empty.yml
+      destination: [groups, empty]
+`)
+	fragmentPath := writeImportFile(t, directory, "empty.yml", "{}\n")
+	main := parseImportMain(t, mainPath)
+	fragment, diagnostics := parseSource(File(fragmentPath), "source-2", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("empty fragment diagnostics = %#v", diagnostics)
+	}
+	entryDestination := Keys("collections", "entry")
+	importDestination := appendDestination(entryDestination, Keys("groups", "empty"))
+	wantNodes := countInputNodes(main.document) + countInputNodes(fragment.document) +
+		destinationWrapperNodeCount(entryDestination) + destinationWrapperNodeCount(importDestination)
+	limits := defaultImportLimits()
+
+	for _, test := range []struct {
+		name      string
+		delta     int
+		wantError bool
+	}{
+		{name: "below aggregate bound", delta: -1, wantError: true},
+		{name: "at aggregate bound"},
+		{name: "above aggregate bound", delta: 1},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			currentLimits := limits
+			currentLimits.nodes = wantNodes + test.delta
+			accounting := &importAccounting{}
+			composed, sources, diagnostics := loadImportedDocumentsTracked(
+				File(mainPath), nil, main, entryDestination, nil, currentLimits, accounting, nil, nil,
+			)
+			if test.wantError {
+				if composed != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeImportLimitExceeded {
+					t.Fatalf("below-bound relative placement = %#v, %#v", composed, diagnostics)
+				}
+				return
+			}
+			if len(diagnostics) != 0 || composed == nil || len(sources) != 2 {
+				t.Fatalf("bounded relative placement = %#v, %#v, %#v", composed, sources, diagnostics)
+			}
+			if composed.inputNodes != wantNodes || accounting.nodes != wantNodes {
+				t.Fatalf("relative placement node charge = %d/%d, want %d", composed.inputNodes, accounting.nodes, wantNodes)
+			}
+			if sources[1].Destination.String() != importDestination.String() {
+				t.Fatalf("empty import destination = %q, want %q", sources[1].Destination.String(), importDestination.String())
+			}
+			if node, present := documentAtPath(composed.root, importDestination); !present || node.Kind != document.Mapping || len(node.Mapping) != 0 {
+				t.Fatalf("empty relative import body = %#v, present %v", node, present)
+			}
+		})
+	}
+}
+
+func TestGlobMatchesChargeTheirSharedRelativeDestination(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - glob: fragments/*.yml
+      destination: [groups, empty]
+`)
+	fragmentPaths := []string{
+		writeImportFile(t, directory, "fragments/a.yml", "{}\n"),
+		writeImportFile(t, directory, "fragments/b.yml", "{}\n"),
+		writeImportFile(t, directory, "fragments/c.yml", "{}\n"),
+	}
+	main := parseImportMain(t, mainPath)
+	fragment, diagnostics := parseSource(File(fragmentPaths[0]), "source-fragment", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("glob fragment diagnostics = %#v", diagnostics)
+	}
+	entryDestination := Keys("collections", "entry")
+	importDestination := appendDestination(entryDestination, Keys("groups", "empty"))
+	wantNodes := countInputNodes(main.document) + len(fragmentPaths)*countInputNodes(fragment.document) +
+		destinationWrapperNodeCount(entryDestination) + len(fragmentPaths)*destinationWrapperNodeCount(importDestination)
+	limits := defaultImportLimits()
+
+	for _, test := range []struct {
+		name      string
+		delta     int
+		wantError bool
+	}{
+		{name: "below aggregate bound", delta: -1, wantError: true},
+		{name: "at aggregate bound"},
+		{name: "above aggregate bound", delta: 1},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			currentLimits := limits
+			currentLimits.nodes = wantNodes + test.delta
+			accounting := &importAccounting{}
+			composed, sources, diagnostics := loadImportedDocumentsTracked(
+				File(mainPath), nil, main, entryDestination, nil, currentLimits, accounting, nil, nil,
+			)
+			if test.wantError {
+				if composed != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeImportLimitExceeded {
+					t.Fatalf("below-bound glob placement = %#v, %#v", composed, diagnostics)
+				}
+				return
+			}
+			if len(diagnostics) != 0 || composed == nil || len(sources) != len(fragmentPaths)+1 {
+				t.Fatalf("bounded glob placement = %#v, %#v, %#v", composed, sources, diagnostics)
+			}
+			if composed.inputNodes != wantNodes || accounting.nodes != wantNodes {
+				t.Fatalf("glob relative placement node charge = %d/%d, want %d", composed.inputNodes, accounting.nodes, wantNodes)
+			}
+			for index, source := range sources[1:] {
+				if source.Destination.String() != importDestination.String() {
+					t.Errorf("glob source %d destination = %q, want %q", index, source.Destination.String(), importDestination.String())
+				}
+			}
+			if node, present := documentAtPath(composed.root, importDestination); !present || node.Kind != document.Mapping || len(node.Mapping) != 0 {
+				t.Fatalf("empty glob contributions = %#v, present %v", node, present)
+			}
+		})
+	}
+}
+
+func TestRelativeImportDestinationUsesAccumulatedDocumentDepth(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: fragment.yml, destination: [extra]}]}\n")
+	writeImportFile(t, directory, "fragment.yml", "value: nested\n")
+	destination := Path{segments: make([]pathSegment, maxDocumentDepth)}
+	for index := range destination.segments {
+		destination.segments[index] = pathSegment{kind: pathSegmentKey, key: "entry"}
+	}
+	_, err := LoadLayered(
+		Input{Source: File(mainPath), Imports: true, Destination: destination}, testSecretBinding(),
+		LayeredOptions[map[string]any]{},
+	)
+	loadError, ok := err.(*LoadError)
+	if !ok || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != CodeImportLimitExceeded {
+		t.Fatalf("accumulated depth result = %v", err)
+	}
+	sources := loadError.Sources()
+	if len(sources) != 2 || len(sources[0].Destination.segments) != maxDocumentDepth || len(sources[1].Destination.segments) != maxDocumentDepth+1 {
+		t.Fatalf("depth failure source destinations = %#v", sources)
+	}
+}
+
+func TestAbsentOptionalImportsDoNotChargeUnusedDeepDestinations(t *testing.T) {
+	t.Parallel()
+
+	destination := Path{segments: make([]pathSegment, maxDocumentDepth)}
+	for index := range destination.segments {
+		destination.segments[index] = pathSegment{kind: pathSegmentKey, key: "entry"}
+	}
+
+	for _, test := range []struct {
+		name  string
+		body  string
+		setup func(*testing.T, string)
+	}{
+		{
+			name: "optional missing path",
+			body: "configkit: {imports: [{path: missing.yml, optional: true, destination: [unused]}]}\n",
+		},
+		{
+			name: "optional unmatched glob",
+			body: "configkit: {imports: [{glob: empty/*.yml, optional: true, destination: [unused]}]}\n",
+			setup: func(t *testing.T, directory string) {
+				if err := os.Mkdir(filepath.Join(directory, "empty"), 0o755); err != nil {
+					t.Fatal(err)
+				}
+			},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			if test.setup != nil {
+				test.setup(t, directory)
+			}
+			mainPath := writeImportFile(t, directory, "main.yml", test.body)
+			main := parseImportMain(t, mainPath)
+			limits := defaultImportLimits()
+			accounting := &importAccounting{}
+			composed, sources, diagnostics := loadImportedDocumentsTracked(
+				File(mainPath), nil, main, destination, nil, limits, accounting, nil, nil,
+			)
+			if len(diagnostics) != 0 || composed == nil || len(sources) != 1 {
+				t.Fatalf("absent optional import = %#v, %#v, %#v", composed, sources, diagnostics)
+			}
+			wantNodes := countInputNodes(main.document) + destinationWrapperNodeCount(destination)
+			if composed.inputNodes != wantNodes || accounting.nodes != wantNodes {
+				t.Fatalf("absent optional node charge = %d/%d, want %d", composed.inputNodes, accounting.nodes, wantNodes)
+			}
+			if node, present := documentAtPath(composed.root, destination); !present || node.Kind != document.Mapping || len(node.Mapping) != 0 {
+				t.Fatalf("empty entry body = %#v, present %v", node, present)
+			}
+		})
+	}
+}
+
+func TestNoSourceAndRootPlacementAddNoWrapperCharges(t *testing.T) {
+	t.Parallel()
+
+	accounting := &importAccounting{}
+	composition, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{
+		{source: NoSource(), destination: Keys("absent")},
+		{source: Mapping("root", map[string]any{})},
+	}}, 1, nil, accounting)
+	if len(diagnostics) != 0 || composition == nil || len(sources) != 1 || accounting.nodes != 1 || composition.inputNodes != 1 {
+		t.Fatalf("root and absent placement = %#v, sources %#v, diagnostics %#v, accounting %#v", composition, sources, diagnostics, accounting)
+	}
+}
+
+func TestSelectedOverlayAndCLIUsePlacementNodeCharges(t *testing.T) {
+	t.Parallel()
+
+	limits := defaultImportLimits()
+	baseNodes := countInputNodes(&document.Node{Kind: document.Mapping}) + destinationWrapperNodeCount(Keys("mounted"))
+
+	t.Run("selected overlay", func(t *testing.T) {
+		t.Parallel()
+		filesystem := NewFilesystem("bundle", fstest.MapFS{
+			"overlays/test.yml": &fstest.MapFile{Data: []byte("{}\n")},
+		})
+		request := loadRequest[map[string]any]{
+			underlays: []loadGroup{{inputs: []loadInput{{source: Mapping("empty", map[string]any{}), destination: Keys("mounted")}}}},
+			overlay:   "test", overlayFilesystem: &FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem},
+		}
+		accounting := &importAccounting{nodes: limits.nodes - baseNodes}
+		_, diagnostics := assembleLayeredInput(request, assembledInput{}, accounting)
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeOverlayLimitExceeded {
+			t.Fatalf("overlay diagnostics = %#v", diagnostics)
+		}
+	})
+
+	t.Run("CLI override", func(t *testing.T) {
+		t.Parallel()
+		override := Override{Path: Keys("mounted", "value"), Value: "x", Flag: "--value"}
+		builder := valueBuilder{sourceID: "fixture", active: make(map[valueVisit]bool)}
+		if _, issue := builder.build(override.Value, override.Path, len(override.Path.segments)); issue != nil {
+			t.Fatal(issue)
+		}
+		overrideNodes := builder.nodes + destinationWrapperNodeCount(override.Path)
+		request := loadRequest[map[string]any]{
+			underlays: []loadGroup{{inputs: []loadInput{{source: Mapping("empty", map[string]any{}), destination: Keys("mounted")}}}},
+			overrides: []Override{override},
+		}
+		accounting := &importAccounting{nodes: limits.nodes - baseNodes - overrideNodes + 1}
+		_, diagnostics := assembleLayeredInput(request, assembledInput{}, accounting)
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeOverrideLimitExceeded {
+			t.Fatalf("override diagnostics = %#v", diagnostics)
+		}
+	})
+}

internal/engine/destination_diagnostic_test.go

@@ -0,0 +1,142 @@
+package engine
+
+import (
+	"path/filepath"
+	"testing"
+	"testing/fstest"
+)
+
+func TestLayeredDestinationDiagnosticsUseEffectivePathsAndAuthoredLocations(t *testing.T) {
+	t.Parallel()
+
+	t.Run("strict conflict retains both source locations", func(t *testing.T) {
+		t.Parallel()
+		loaded, err := LoadLayered(
+			Input{Source: NoSource()},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{
+				{Source: Bytes("left", []byte("section:\n  value: left\n")), Destination: Keys("application")},
+				{Source: Bytes("right", []byte("section:\n  value: right\n")), Destination: Keys("application")},
+			}}}},
+		)
+		if loaded != nil {
+			t.Fatalf("conflicting load returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeComposeConflict || diagnostic.Path.String() != "/application/section/value" {
+			t.Fatalf("conflict diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || diagnostic.Primary.Line != 2 {
+			t.Fatalf("primary location = %#v", diagnostic.Primary)
+		}
+		if len(diagnostic.Related) != 1 || diagnostic.Related[0].SourceID != "source-2" || diagnostic.Related[0].Line != 2 {
+			t.Fatalf("related locations = %#v", diagnostic.Related)
+		}
+	})
+
+	t.Run("decode failure keeps effective path and content line", func(t *testing.T) {
+		t.Parallel()
+		type section struct {
+			Count int `yaml:"count"`
+		}
+		type config struct {
+			Application struct {
+				Section section `yaml:"section"`
+			} `yaml:"application"`
+		}
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("section:\n  count: invalid\n")), Destination: Keys("application")},
+			testSecretBinding(),
+			LayeredOptions[config]{},
+		)
+		if loaded != nil {
+			t.Fatalf("invalid load returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeDecodeType || diagnostic.Path.String() != "/application/section/count" {
+			t.Fatalf("decode diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || diagnostic.Primary.Line != 2 {
+			t.Fatalf("decode location = %#v", diagnostic.Primary)
+		}
+	})
+
+	t.Run("imported version failure uses effective path and import source", func(t *testing.T) {
+		t.Parallel()
+		directory := t.TempDir()
+		writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: fragment.yml}]}\n")
+		writeImportFile(t, directory, "fragment.yml", "version: forbidden\n")
+		rule := &VersionRule{Path: Keys("application", "version"), Check: func(Version, *Validation) error { return nil }}
+		loaded, err := LoadLayered(
+			Input{Source: NoSource()},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{
+				Definition: Definition[map[string]any]{Version: rule},
+				Underlays: []Underlay{{Inputs: []Input{{
+					Source: File(filepath.Join(directory, "main.yml")), Imports: true, Destination: Keys("application"),
+				}}}},
+			},
+		)
+		if loaded != nil {
+			t.Fatalf("invalid import returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeImportVersion || diagnostic.Path.String() != "/application/version" {
+			t.Fatalf("import version diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-2" || diagnostic.Primary.Line != 1 {
+			t.Fatalf("import version location = %#v", diagnostic.Primary)
+		}
+	})
+
+	t.Run("invalid destination has no fictional source location", func(t *testing.T) {
+		t.Parallel()
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("value: present\n")), Destination: Keys("configkit")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{},
+		)
+		if loaded != nil {
+			t.Fatalf("invalid destination returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeOptionsInvalid || diagnostic.Path.String() != "/configkit" || diagnostic.Primary != nil {
+			t.Fatalf("invalid destination diagnostic = %#v", diagnostic)
+		}
+	})
+
+	t.Run("malformed source directive keeps its local path and location", func(t *testing.T) {
+		t.Parallel()
+		filesystem := NewFilesystem("bundle", fstest.MapFS{
+			"main.yml": &fstest.MapFile{Data: []byte("configkit: {imports: [true]}\nvalue: present\n")},
+		})
+		loaded, err := LoadLayered(
+			Input{Source: filesystem.File("main.yml"), Imports: true, Destination: Keys("application")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{},
+		)
+		if loaded != nil {
+			t.Fatalf("malformed directive returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeImportInvalid || diagnostic.Path.String() != "/configkit/imports/0" {
+			t.Fatalf("malformed directive diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || diagnostic.Primary.Line != 1 {
+			t.Fatalf("malformed directive location = %#v", diagnostic.Primary)
+		}
+	})
+}
+
+func firstDestinationDiagnostic(t *testing.T, err error) Diagnostic {
+	t.Helper()
+	loadError, ok := err.(*LoadError)
+	if !ok {
+		t.Fatalf("load error = %#v", err)
+	}
+	diagnostics := loadError.Diagnostics()
+	if len(diagnostics) == 0 {
+		t.Fatal("load error has no diagnostics")
+	}
+	return diagnostics[0]
+}

internal/engine/destination_import_lifecycle_test.go

@@ -0,0 +1,211 @@
+package engine
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestRelativeImportsFeedFinalInheritanceAndInspectionLifecycle(t *testing.T) {
+	t.Parallel()
+
+	type entry struct {
+		Extends string  `yaml:"extends"`
+		Label   string  `yaml:"label"`
+		Token   *Secret `yaml:"token" configkit:"secret"`
+		Dynamic string  `yaml:"dynamic"`
+		Derived string  `yaml:"derived"`
+		Height  int     `yaml:"height"`
+	}
+	type configuration struct {
+		Catalog struct {
+			Entries map[string]entry `yaml:"entries"`
+		} `yaml:"catalog"`
+	}
+
+	load := func(dynamic string) *testResult[configuration] {
+		t.Helper()
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "entry.yml", `configkit:
+  imports:
+    - path: parent.yml
+      destination: [entries]
+    - path: child.yml
+      destination: [entries]
+`)
+		writeImportFile(t, directory, "parent.yml", `parent:
+  label: lower
+  token: PARENT_TOKEN
+  height: 1
+`)
+		writeImportFile(t, directory, "child.yml", fmt.Sprintf(`child:
+  extends: parent
+  label: child
+  dynamic: %s
+  height: 9
+`, dynamic))
+
+		var defaultsCalls, resolveCalls, validateCalls int
+		secretPaths := make(map[string]bool)
+		lookupCalls := make(map[string]int)
+		request := loadRequest[configuration]{
+			main: loadInput{
+				source:      Bytes("main", []byte("parent: {label: final, height: 4}\n")),
+				destination: Keys("catalog", "entries"),
+			},
+			underlays: []loadGroup{{inputs: []loadInput{{
+				source: File(mainPath), imports: true, destination: Keys("catalog"),
+			}}}},
+			definition: Definition[configuration]{
+				Defaults: func() configuration {
+					defaultsCalls++
+					var defaults configuration
+					defaults.Catalog.Entries = map[string]entry{"fallback": {Label: "default"}}
+					return defaults
+				},
+				Resolve: func(config *configuration, resolution *Resolution) error {
+					resolveCalls++
+					child := config.Catalog.Entries["child"]
+					child.Derived = child.Label + ":" + child.Dynamic + ":" + fmt.Sprint(child.Height)
+					config.Catalog.Entries["child"] = child
+					resolution.Record(Keys("catalog", "entries", "child", "derived"), ChangeNormalization, "derive final child summary", Keys("catalog", "entries", "child", "label"), Keys("catalog", "entries", "child", "dynamic"), Keys("catalog", "entries", "child", "height"))
+					return nil
+				},
+				Validate: func(config *configuration, validation *Validation) {
+					validateCalls++
+					parent := config.Catalog.Entries["parent"]
+					child := config.Catalog.Entries["child"]
+					if parent.Label != "final" || parent.Height != 4 || child.Label != "child" || child.Height != 9 || child.Derived != child.Label+":"+child.Dynamic+":9" || child.Token == nil || !child.Token.IsResolved() || child.Token.Value() != "parent-secret-canary" {
+						validation.Error(Keys("catalog", "entries", "child"), "final imported configuration was not available to validation")
+					}
+				},
+				Secret: func(path Path) bool {
+					secretPaths[path.String()] = true
+					return path.String() == "/catalog/entries/child/dynamic"
+				},
+			},
+		}
+		loaded, err := loadGroupedWithSecretLookup(request, testSecretBinding(), func(name string) (string, bool) {
+			lookupCalls[name]++
+			if name == "PARENT_TOKEN" {
+				return "parent-secret-canary", true
+			}
+			return "", false
+		})
+		if err != nil {
+			if loadError, ok := err.(*LoadError); ok {
+				t.Fatalf("relative import lifecycle diagnostics: %#v", loadError.Diagnostics())
+			}
+			t.Fatalf("relative import lifecycle: %v", err)
+		}
+		if defaultsCalls != 1 || resolveCalls != 1 || validateCalls != 1 || lookupCalls["PARENT_TOKEN"] != 1 {
+			t.Fatalf("lifecycle calls defaults=%d resolve=%d validate=%d lookups=%#v", defaultsCalls, resolveCalls, validateCalls, lookupCalls)
+		}
+		if !secretPaths["/catalog/entries/child/dynamic"] {
+			t.Fatalf("secret classifier did not see effective paths: %#v", secretPaths)
+		}
+		return testLoaded(loaded)
+	}
+
+	left := load("LEFT-DYNAMIC-CANARY")
+	child := left.Config.Catalog.Entries["child"]
+	if child.Extends != "" || child.Label != "child" || child.Height != 9 || child.Derived != "child:LEFT-DYNAMIC-CANARY:9" || child.Token == nil || child.Token.Value() != "parent-secret-canary" {
+		t.Fatalf("resolved imported child = %#v", child)
+	}
+	if left.Config.Catalog.Entries["parent"].Label != "final" || left.Config.Catalog.Entries["parent"].Height != 4 || left.Config.Catalog.Entries["fallback"].Label != "default" {
+		t.Fatalf("final parent/default shape = %#v", left.Config.Catalog.Entries)
+	}
+
+	childTokenPath := Keys("catalog", "entries", "child", "token")
+	childToken, found := left.Inspection.Explain(childTokenPath)
+	childTokenInfo, tokenInfoFound := left.Inspection.Metadata().Lookup(childTokenPath)
+	if !found || !tokenInfoFound || !childToken.Effective.Redacted || strings.Contains(childToken.Effective.YAML, "parent-secret-canary") || len(childTokenInfo.Origin.Inheritance) != 1 {
+		t.Fatalf("inherited field explanation = %#v, %v", childToken, found)
+	}
+	var explanation bytes.Buffer
+	if err := left.Inspection.WriteExplanation(&explanation, childTokenPath); err != nil || strings.Contains(explanation.String(), "parent-secret-canary") || strings.Contains(explanation.String(), "LEFT-DYNAMIC-CANARY") {
+		t.Fatalf("safe explanation = %q, %v", explanation.String(), err)
+	}
+	labelHistory := left.Inspection.Metadata().History(Keys("catalog", "entries", "parent", "label"))
+	if len(labelHistory) < 2 {
+		t.Fatalf("parent label history omitted replaced contribution: %#v", labelHistory)
+	}
+	derived, found := left.Inspection.Explain(Keys("catalog", "entries", "child", "derived"))
+	if !found || !derived.Effective.Redacted || strings.Contains(derived.Effective.YAML, "LEFT-DYNAMIC-CANARY") || len(derived.History) == 0 || derived.History[len(derived.History)-1].Origin.Operation != ChangeNormalization {
+		t.Fatalf("recorded derived field explanation = %#v, %v", derived, found)
+	}
+
+	right := load("RIGHT-DYNAMIC-CANARY")
+	differences, err := Diff(left.Inspection, right.Inspection)
+	if err != nil {
+		t.Fatal(err)
+	}
+	seen := make(map[string]bool)
+	for _, difference := range differences {
+		path := difference.Path.String()
+		seen[path] = true
+		if !difference.Left.Redacted || !difference.Right.Redacted || !difference.HiddenChanged || strings.Contains(difference.Left.YAML+difference.Right.YAML, "DYNAMIC-CANARY") || strings.Contains(difference.Left.YAML+difference.Right.YAML, "parent-secret-canary") {
+			t.Errorf("sensitive difference at %s = %#v", path, difference)
+		}
+	}
+	for _, path := range []string{"/catalog/entries/child/dynamic", "/catalog/entries/child/derived"} {
+		if !seen[path] {
+			t.Errorf("missing inherited/derived difference at %s: %#v", path, differences)
+		}
+	}
+}
+
+func TestRelativeImportMatchesWrappedPlacementAndSafeDiff(t *testing.T) {
+	t.Parallel()
+
+	load := func(relativeImport bool, value string) *testResult[map[string]any] {
+		t.Helper()
+		input := Input{Source: NoSource()}
+		options := LayeredOptions[map[string]any]{}
+		if relativeImport {
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: fragment.yml
+      destination: [application]
+`)
+			writeImportFile(t, directory, "fragment.yml", "field: "+value+"\n")
+			options.Underlays = []Underlay{{Inputs: []Input{{Source: File(mainPath), Imports: true}}}}
+		} else {
+			options.Underlays = []Underlay{{Inputs: []Input{{Source: Bytes("wrapped", []byte("application: {field: "+value+"}\n"))}}}}
+		}
+		loaded, err := LoadLayered(input, testSecretBinding(), options)
+		if err != nil {
+			t.Fatalf("LoadLayered: %v", err)
+		}
+		return testLoaded(loaded)
+	}
+
+	relativeBefore := load(true, "before")
+	relativeAfter := load(true, "after")
+	wrappedBefore := load(false, "before")
+	wrappedAfter := load(false, "after")
+	for _, pair := range [][2]*testResult[map[string]any]{{relativeBefore, wrappedBefore}, {relativeAfter, wrappedAfter}} {
+		if !reflect.DeepEqual(pair[0].Config, pair[1].Config) {
+			t.Fatalf("relative and wrapped configurations differ: %#v vs %#v", pair[0].Config, pair[1].Config)
+		}
+		for _, result := range pair {
+			if info, ok := result.Metadata().Lookup(Keys("application", "field")); !ok || !info.Explicit {
+				t.Fatalf("effective field presence = %#v, %v", info, ok)
+			}
+		}
+	}
+	relativeDiff, err := Diff(relativeBefore.Inspection, relativeAfter.Inspection)
+	if err != nil {
+		t.Fatal(err)
+	}
+	wrappedDiff, err := Diff(wrappedBefore.Inspection, wrappedAfter.Inspection)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(relativeDiff, wrappedDiff) {
+		t.Fatalf("relative safe diff = %#v, wrapped safe diff = %#v", relativeDiff, wrappedDiff)
+	}
+}

internal/engine/destination_import_test.go

@@ -0,0 +1,568 @@
+package engine
+
+import (
+	"path/filepath"
+	"testing"
+	"testing/fstest"
+)
+
+type importDestinationConfig struct {
+	Mounted     map[string]string            `yaml:"mounted"`
+	Collections map[string]map[string]string `yaml:"collections"`
+}
+
+func TestLayeredHostImportsInheritInputDestinationTransitively(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: first.yml
+    - path: second.yml
+root: main
+`)
+	firstPath := writeImportFile(t, directory, "first.yml", `configkit:
+  imports:
+    - path: nested/third.yml
+first: one
+`)
+	thirdPath := writeImportFile(t, directory, "nested/third.yml", "nested: three\n")
+	secondPath := writeImportFile(t, directory, "second.yml", "second: two\n")
+
+	loaded, err := LoadLayered(
+		Input{Source: File(mainPath), Imports: true, Destination: Keys("mounted")},
+		testSecretBinding(),
+		LayeredOptions[importDestinationConfig]{},
+	)
+	if err != nil {
+		if loadError, ok := err.(*LoadError); ok {
+			t.Fatalf("LoadLayered diagnostics: %#v", loadError.Diagnostics())
+		}
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	want := map[string]string{"root": "main", "first": "one", "nested": "three", "second": "two"}
+	if len(result.Config.Mounted) != len(want) {
+		t.Fatalf("mounted values = %#v", result.Config.Mounted)
+	}
+	for key, value := range want {
+		if result.Config.Mounted[key] != value {
+			t.Errorf("mounted[%q] = %q, want %q", key, result.Config.Mounted[key], value)
+		}
+	}
+	if _, ok := result.Metadata().Lookup(Keys("mounted", "configkit")); ok {
+		t.Fatal("source-root import directive leaked into application content")
+	}
+	sources := result.Sources()
+	wantPaths := []string{canonicalTestPath(t, mainPath), canonicalTestPath(t, firstPath), canonicalTestPath(t, thirdPath), canonicalTestPath(t, secondPath)}
+	if len(sources) != len(wantPaths) {
+		t.Fatalf("sources = %#v", sources)
+	}
+	for index, source := range sources {
+		if source.Destination.String() != "/mounted" {
+			t.Errorf("source %d destination = %q", index, source.Destination.String())
+		}
+		if source.Path != wantPaths[index] {
+			t.Errorf("source %d path = %q, want %q", index, source.Path, wantPaths[index])
+		}
+		if index > 0 && source.ImportedFrom == nil {
+			t.Errorf("imported source %d has no physical import location", index)
+		}
+	}
+}
+
+func TestImportTraversalUsesPerDeclarationDestinations(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: parent.yml
+    - path: sibling.yml
+    - glob: fragments/*.yml
+`)
+	parentPath := writeImportFile(t, directory, "parent.yml", `configkit:
+  imports:
+    - path: nested/child.yml
+parent: true
+`)
+	childPath := writeImportFile(t, directory, "nested/child.yml", "child: true\n")
+	siblingPath := writeImportFile(t, directory, "sibling.yml", "sibling: true\n")
+	alphaPath := writeImportFile(t, directory, "fragments/a.yml", "alpha: true\n")
+	betaPath := writeImportFile(t, directory, "fragments/b.yml", "beta: true\n")
+
+	main := parseImportMain(t, mainPath)
+	_, declarations, diagnostics := splitImportDirectives(main.document)
+	if len(diagnostics) != 0 {
+		t.Fatalf("main directive diagnostics = %#v", diagnostics)
+	}
+	declarations[0].destination = Keys("parent")
+	declarations[1].destination = Keys("sibling")
+	declarations[2].destination = Keys("collection")
+
+	access, err := openHostAccess(directory)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() { _ = access.close() }()
+	state := importTraversal{
+		access: access, accounting: &importAccounting{}, limits: defaultImportLimits(),
+		active: make(map[resourceIdentity]int), visited: make(map[resourceIdentity]importRoute),
+	}
+	if diagnostics := state.walkDeclarations(directory, declarations, 0, Keys("entry")); len(diagnostics) != 0 {
+		t.Fatalf("walk diagnostics = %#v", diagnostics)
+	}
+
+	wantSources := []struct {
+		path        string
+		destination string
+	}{
+		{path: parentPath, destination: "/entry/parent"},
+		{path: childPath, destination: "/entry/parent"},
+		{path: siblingPath, destination: "/entry/sibling"},
+		{path: alphaPath, destination: "/entry/collection"},
+		{path: betaPath, destination: "/entry/collection"},
+	}
+	if len(state.sources) != len(wantSources) {
+		t.Fatalf("import sources = %#v, want %d", state.sources, len(wantSources))
+	}
+	for index, want := range wantSources {
+		got := state.sources[index]
+		if got.Path != canonicalTestPath(t, want.path) || got.Destination.String() != want.destination {
+			t.Errorf("source %d = (%q, %q), want (%q, %q)", index, got.Path, got.Destination.String(), canonicalTestPath(t, want.path), want.destination)
+		}
+	}
+
+	composed, diagnostics := composeDocuments(state.documents...)
+	if len(diagnostics) != 0 {
+		t.Fatalf("composition diagnostics = %#v", diagnostics)
+	}
+	for _, path := range []Path{
+		Keys("entry", "parent", "parent"), Keys("entry", "parent", "child"),
+		Keys("entry", "sibling", "sibling"), Keys("entry", "collection", "alpha"),
+		Keys("entry", "collection", "beta"),
+	} {
+		if _, present := documentAtPath(composed.root, path); !present {
+			t.Errorf("no composed value at %q", path.String())
+		}
+	}
+}
+
+func TestEntryDestinationSurvivesImportSetupAndParseFailures(t *testing.T) {
+	t.Parallel()
+
+	t.Run("import context failure", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: child.yml}]}\n")
+		input, diagnostics := assembleBaseInputTracked(
+			File(mainPath),
+			Options[map[string]any]{Imports: true, Filesystem: &FilesystemContext{Root: t.TempDir()}},
+			Keys("mounted"), &importAccounting{}, nil, nil,
+		)
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeImportOutsideRoot {
+			t.Fatalf("context diagnostics = %#v", diagnostics)
+		}
+		if len(input.sources) != 1 || input.sources[0].Destination.String() != "/mounted" {
+			t.Fatalf("entry source fallback = %#v", input.sources)
+		}
+	})
+
+	t.Run("imported parse failure", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: broken.yml}]}\n")
+		writeImportFile(t, directory, "broken.yml", "broken: [\n")
+		input, diagnostics := assembleBaseInputTracked(
+			File(mainPath), Options[map[string]any]{Imports: true},
+			Keys("mounted"), &importAccounting{}, nil, nil,
+		)
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeYAMLInvalid {
+			t.Fatalf("parse diagnostics = %#v", diagnostics)
+		}
+		if len(input.sources) != 2 {
+			t.Fatalf("parse-failure sources = %#v", input.sources)
+		}
+		for index, source := range input.sources {
+			if source.Destination.String() != "/mounted" {
+				t.Errorf("source %d destination = %q, want /mounted", index, source.Destination.String())
+			}
+		}
+	})
+}
+
+func TestRelativeImportDestinationsKeepStrictCompositionBoundaries(t *testing.T) {
+	t.Parallel()
+
+	for _, test := range []struct {
+		name     string
+		main     string
+		files    map[string]string
+		want     map[string]any
+		conflict bool
+		wantPath string
+	}{
+		{
+			name:  "disjoint peers at one destination",
+			main:  "configkit:\n  imports:\n    - path: left.yml\n      destination: [application]\n    - path: right.yml\n      destination: [application]\n",
+			files: map[string]string{"left.yml": "left: one\n", "right.yml": "right: two\n"},
+			want:  map[string]any{"/application/left": "one", "/application/right": "two"},
+		},
+		{
+			name:  "different destinations",
+			main:  "configkit:\n  imports:\n    - path: left.yml\n      destination: [left]\n    - path: right.yml\n      destination: [right]\n",
+			files: map[string]string{"left.yml": "value: shared\n", "right.yml": "value: shared\n"},
+			want:  map[string]any{"/left/value": "shared", "/right/value": "shared"},
+		},
+		{
+			name:  "ancestor and descendant overlap",
+			main:  "configkit:\n  imports:\n    - path: parent.yml\n      destination: [application]\n    - path: child.yml\n      destination: [application, branch]\n",
+			files: map[string]string{"parent.yml": "branch: {base: one}\n", "child.yml": "leaf: two\n"},
+			want:  map[string]any{"/application/branch/base": "one", "/application/branch/leaf": "two"},
+		},
+		{
+			name:     "collision with explicitly wrapped content",
+			main:     "configkit: {imports: [{path: fragment.yml, destination: [application]}]}\napplication: {value: root}\n",
+			files:    map[string]string{"fragment.yml": "value: imported\n"},
+			conflict: true,
+			wantPath: "/application/value",
+		},
+		{
+			name:     "equal peer values conflict",
+			main:     "configkit:\n  imports:\n    - path: left.yml\n      destination: [application]\n    - path: right.yml\n      destination: [application]\n",
+			files:    map[string]string{"left.yml": "value: same\n", "right.yml": "value: same\n"},
+			conflict: true,
+			wantPath: "/application/value",
+		},
+		{
+			name:     "unequal peer values conflict",
+			main:     "configkit:\n  imports:\n    - path: left.yml\n      destination: [application]\n    - path: right.yml\n      destination: [application]\n",
+			files:    map[string]string{"left.yml": "value: left\n", "right.yml": "value: right\n"},
+			conflict: true,
+			wantPath: "/application/value",
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", test.main)
+			for name, body := range test.files {
+				writeImportFile(t, directory, name, body)
+			}
+			loaded, err := LoadLayered(
+				Input{Source: File(mainPath), Imports: true}, testSecretBinding(),
+				LayeredOptions[map[string]any]{},
+			)
+			if test.conflict {
+				if loaded != nil || groupedErrorCode(t, err) != CodeComposeConflict {
+					t.Fatalf("relative import composition = %#v, %v", loaded, err)
+				}
+				loadError := err.(*LoadError)
+				if diagnostics := loadError.Diagnostics(); diagnostics[0].Path.String() != test.wantPath {
+					t.Fatalf("conflict path = %q, want %q", diagnostics[0].Path.String(), test.wantPath)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("relative import composition: %v", err)
+			}
+			configuration := testLoaded(loaded).Config
+			for path, want := range test.want {
+				parsed, err := ParsePath(path)
+				if err != nil {
+					t.Fatal(err)
+				}
+				keys := make([]string, len(parsed.segments))
+				for index, segment := range parsed.segments {
+					keys[index] = segment.key
+				}
+				if got := layeredMapValue(t, configuration, keys...); got != want {
+					t.Errorf("value at %s = %#v, want %#v", path, got, want)
+				}
+			}
+		})
+	}
+
+	t.Run("upper tier cannot repair a lower import conflict", func(t *testing.T) {
+		directory := t.TempDir()
+		lowerPath := writeImportFile(t, directory, "lower.yml", `configkit:
+  imports:
+    - path: left.yml
+      destination: [application]
+    - path: right.yml
+      destination: [application]
+`)
+		writeImportFile(t, directory, "left.yml", "value: left\n")
+		writeImportFile(t, directory, "right.yml", "value: right\n")
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("upper", []byte("application: {value: upper}\n"))},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{{Source: File(lowerPath), Imports: true}}}}},
+		)
+		if loaded != nil || groupedErrorCode(t, err) != CodeComposeConflict {
+			t.Fatalf("upper tier repaired a lower conflict: %#v, %v", loaded, err)
+		}
+	})
+}
+
+func TestRelativeImportDestinationsUseEffectiveVersionOwnership(t *testing.T) {
+	t.Parallel()
+
+	for _, test := range []struct {
+		name     string
+		rulePath Path
+		fragment string
+	}{
+		{name: "direct claim", rulePath: Keys("collection", "group", "nested", "version"), fragment: "version: v2\n"},
+		{name: "ancestor claim", rulePath: Keys("collection", "group", "nested", "schema", "version"), fragment: "schema: null\n"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: parent.yml
+      destination: [group]
+`)
+			writeImportFile(t, directory, "parent.yml", `configkit:
+  imports:
+    - path: fragment.yml
+      destination: [nested]
+`)
+			writeImportFile(t, directory, "fragment.yml", test.fragment)
+			_, err := LoadLayered(
+				Input{Source: File(mainPath), Imports: true, Destination: Keys("collection")},
+				testSecretBinding(),
+				LayeredOptions[map[string]any]{Definition: Definition[map[string]any]{Version: &VersionRule{
+					Path: test.rulePath, Check: func(Version, *Validation) error { return nil },
+				}}},
+			)
+			if groupedErrorCode(t, err) != CodeImportVersion {
+				t.Fatalf("effective imported claim error = %v", err)
+			}
+			loadError := err.(*LoadError)
+			diagnostic := loadError.Diagnostics()[0]
+			if diagnostic.Path.String() != test.rulePath.String() || diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-3" {
+				t.Fatalf("effective version diagnostic = %#v", diagnostic)
+			}
+			sources := loadError.Sources()
+			wantDestinations := []string{"/collection", "/collection/group", "/collection/group/nested"}
+			if len(sources) != len(wantDestinations) {
+				t.Fatalf("version failure sources = %#v", sources)
+			}
+			for index, source := range sources {
+				if source.Destination.String() != wantDestinations[index] {
+					t.Errorf("source %d destination = %q, want %q", index, source.Destination.String(), wantDestinations[index])
+				}
+			}
+		})
+	}
+}
+
+func TestRelativeImportDestinationsKeepMainVersionCheckScoped(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Collection struct {
+			SchemaVersion string         `yaml:"schema_version"`
+			Group         map[string]any `yaml:"group"`
+		} `yaml:"collection"`
+	}
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: parent.yml
+      destination: [group]
+schema_version: 6
+`)
+	writeImportFile(t, directory, "parent.yml", `configkit:
+  imports:
+    - path: fragment.yml
+      destination: [nested]
+`)
+	writeImportFile(t, directory, "fragment.yml", "version: 7\n")
+	checks := 0
+	loaded, err := LoadLayered(
+		Input{Source: File(mainPath), Imports: true, Destination: Keys("collection")},
+		testSecretBinding(),
+		LayeredOptions[config]{Definition: Definition[config]{Version: &VersionRule{
+			Path: Keys("collection", "schema_version"),
+			Check: func(version Version, _ *Validation) error {
+				checks++
+				if !version.Present || version.Text != "6" {
+					t.Errorf("main-owned version = %#v", version)
+				}
+				return nil
+			},
+		}}},
+	)
+	if err != nil {
+		t.Fatalf("ordinary nested imported version was rejected: %v", err)
+	}
+	result := testLoaded(loaded)
+	if checks != 1 || result.Config.Collection.SchemaVersion != "6" {
+		t.Fatalf("main version callback/value = %d/%q", checks, result.Config.Collection.SchemaVersion)
+	}
+	nested, ok := result.Config.Collection.Group["nested"].(map[string]any)
+	if !ok {
+		t.Fatalf("nested imported payload = %#v", result.Config.Collection.Group["nested"])
+	}
+	if got, ok := nested["version"].(int64); !ok || got != 7 {
+		t.Fatalf("ordinary nested version = %#v, want integer payload 7", nested["version"])
+	}
+}
+
+func TestLayeredNativeFilesystemImportsInheritInputDestination(t *testing.T) {
+	t.Parallel()
+
+	native := fstest.MapFS{
+		"main.yml":   &fstest.MapFile{Data: []byte("configkit:\n  imports:\n    - path: first.yml\nroot: main\n")},
+		"first.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: nested.yml}]}\nfirst: imported\n")},
+		"nested.yml": &fstest.MapFile{Data: []byte("nested: imported\n")},
+	}
+	namespace := newNativeNamespace("bundle", native)
+	loaded, err := LoadLayered(
+		Input{Source: filesystemFile(namespace, "main.yml"), Imports: true, Destination: Keys("mounted")},
+		testSecretBinding(),
+		LayeredOptions[importDestinationConfig]{},
+	)
+	if err != nil {
+		if loadError, ok := err.(*LoadError); ok {
+			t.Fatalf("LoadLayered diagnostics: %#v", loadError.Diagnostics())
+		}
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	if result.Config.Mounted["root"] != "main" || result.Config.Mounted["first"] != "imported" || result.Config.Mounted["nested"] != "imported" {
+		t.Fatalf("mounted native filesystem values = %#v", result.Config.Mounted)
+	}
+	sources := result.Sources()
+	if len(sources) != 3 || sources[0].Destination.String() != "/mounted" || sources[1].Destination.String() != "/mounted" || sources[2].Destination.String() != "/mounted" {
+		t.Fatalf("native source destinations = %#v", sources)
+	}
+}
+
+func TestLayeredImportOnlyInputMaterializesDestination(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: optional.yml
+      optional: true
+`)
+	loaded, err := LoadLayered(
+		Input{Source: File(mainPath), Imports: true, Destination: Keys("collections", "empty")},
+		testSecretBinding(),
+		LayeredOptions[importDestinationConfig]{},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	info, ok := result.Metadata().Lookup(Keys("collections", "empty"))
+	if !ok || !info.Explicit {
+		t.Fatalf("import-only body destination = %#v, %v", info, ok)
+	}
+	sources := result.Sources()
+	if len(sources) != 1 || sources[0].Destination.String() != "/collections/empty" {
+		t.Fatalf("sources = %#v", sources)
+	}
+}
+
+func TestLayeredImportedVersionClaimsUseEffectiveDestination(t *testing.T) {
+	t.Parallel()
+
+	for _, test := range []struct {
+		name        string
+		rulePath    Path
+		destination Path
+		wantCode    Code
+	}{
+		{name: "protected effective path", rulePath: Keys("mounted", "version"), destination: Keys("mounted"), wantCode: CodeImportVersion},
+		{name: "ordinary nested domain version", rulePath: Keys("version"), destination: Keys("collections", "entry")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			directory := t.TempDir()
+			writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: fragment.yml}]}\n")
+			writeImportFile(t, directory, "fragment.yml", "version: v2\n")
+			rule := &VersionRule{Path: test.rulePath, Check: func(Version, *Validation) error { return nil }}
+			loaded, err := LoadLayered(
+				Input{Source: NoSource()},
+				testSecretBinding(),
+				LayeredOptions[importDestinationConfig]{
+					Definition: Definition[importDestinationConfig]{Version: rule},
+					Underlays: []Underlay{{Inputs: []Input{{
+						Source: File(filepath.Join(directory, "main.yml")), Imports: true, Destination: test.destination,
+					}}}},
+				},
+			)
+			if test.wantCode != "" {
+				if err == nil {
+					t.Fatal("protected imported version claim was accepted")
+				}
+				loadError, ok := err.(*LoadError)
+				if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != test.wantCode {
+					t.Fatalf("load error = %#v", err)
+				}
+				if test.name == "protected effective path" {
+					sources := loadError.Sources()
+					if len(sources) != 2 || sources[0].Destination.String() != "/mounted" || sources[1].Destination.String() != "/mounted" {
+						t.Fatalf("error source destinations = %#v", sources)
+					}
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("ordinary nested version was rejected: %v", err)
+			}
+			result := testLoaded(loaded)
+			if got := result.Config.Collections["entry"]["version"]; got != "v2" {
+				t.Fatalf("nested version = %q", got)
+			}
+		})
+	}
+}
+
+func TestLayeredDestinationsDoNotChangeRepeatedSourceIdentity(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	writeImportFile(t, directory, "shared.yml", "value: shared\n")
+	importerPath := writeImportFile(t, directory, "importer.yml", "configkit: {imports: [{path: shared.yml, destination: [nested]}]}\n")
+	_, err := LoadLayered(
+		Input{Source: NoSource()},
+		testSecretBinding(),
+		LayeredOptions[importDestinationConfig]{Underlays: []Underlay{{Inputs: []Input{
+			{Source: File(importerPath), Imports: true, Destination: Keys("imported")},
+			{Source: File(filepath.Join(directory, "shared.yml")), Destination: Keys("direct")},
+		}}}},
+	)
+	if err == nil {
+		t.Fatal("the same physical source was accepted at two destinations in one group")
+	}
+	loadError, ok := err.(*LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != CodeImportRepeated {
+		t.Fatalf("load error = %#v", err)
+	}
+	sources := loadError.Sources()
+	if len(sources) != 3 || sources[1].Destination.String() != "/imported/nested" || sources[2].Destination.String() != "/direct" {
+		t.Fatalf("repeated import sources = %#v", sources)
+	}
+
+	t.Run("placement does not break a physical cycle", func(t *testing.T) {
+		t.Parallel()
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: parent.yml, destination: [one]}]}\n")
+		writeImportFile(t, directory, "parent.yml", "configkit: {imports: [{path: main.yml, destination: [two]}]}\n")
+		_, err := LoadLayered(Input{Source: File(mainPath), Imports: true}, testSecretBinding(), LayeredOptions[map[string]any]{})
+		loadError, ok := err.(*LoadError)
+		if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != CodeImportCycle {
+			t.Fatalf("cycle load error = %#v", err)
+		}
+		diagnostic := loadError.Diagnostics()[0]
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-2" {
+			t.Fatalf("cycle primary location = %#v", diagnostic.Primary)
+		}
+		if sources := loadError.Sources(); len(sources) != 2 || sources[1].Destination.String() != "/one" {
+			t.Fatalf("cycle sources = %#v", sources)
+		}
+	})
+}

internal/engine/destination_integration_test.go

@@ -0,0 +1,463 @@
+package engine
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+func TestLayeredDestinationInheritanceUsesFinalSiblingScopes(t *testing.T) {
+	t.Parallel()
+
+	loaded, err := LoadLayered(
+		Input{Source: Bytes("main", []byte("parent:\n  label: final\n  settings:\n    primary: {height: 4}\ncancelled:\n  extends: null\n  own: retained\n")), Destination: Keys("catalog", "entries")},
+		testSecretBinding(),
+		LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{
+			{Source: Bytes("parent", []byte("parent:\n  label: lower\n  settings:\n    primary: {width: '10'}\n    secondary: {extends: primary}\ncancelled:\n  extends: parent\n  own: retained\n")), Destination: Keys("catalog", "entries")},
+			{Source: Bytes("child", []byte("child:\n  extends: parent\n  settings:\n    primary: {width: '20'}\n    secondary: {extends: primary}\n")), Destination: Keys("catalog", "entries")},
+		}}}},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	entries := layeredMap(t, layeredMap(t, result.Config, "catalog"), "entries")
+	parent := layeredMap(t, entries, "parent")
+	child := layeredMap(t, entries, "child")
+	parentSettings := layeredMap(t, parent, "settings")
+	childSettings := layeredMap(t, child, "settings")
+	if parent["label"] != "final" || child["label"] != "final" {
+		t.Fatalf("higher-tier parent was not inherited after precedence: parent=%#v child=%#v", parent, child)
+	}
+	if layeredMapValue(t, parentSettings, "secondary", "width") != "10" ||
+		fmt.Sprint(layeredMapValue(t, parentSettings, "secondary", "height")) != "4" ||
+		layeredMapValue(t, childSettings, "secondary", "width") != "20" ||
+		fmt.Sprint(layeredMapValue(t, childSettings, "secondary", "height")) != "4" {
+		t.Fatalf("nested sibling scopes were not resolved at their destinations: parent=%#v child=%#v", parentSettings, childSettings)
+	}
+	cancelled := layeredMap(t, entries, "cancelled")
+	if len(cancelled) != 1 || cancelled["own"] != "retained" {
+		t.Fatalf("higher-tier cancellation did not suppress inheritance: %#v", cancelled)
+	}
+	if _, ok := result.Metadata().Lookup(Keys("catalog", "entries", "child", inheritanceKey)); ok {
+		t.Fatal("effective child retained its extends directive")
+	}
+}
+
+func TestLayeredDestinationInheritanceDoesNotSearchOtherCollections(t *testing.T) {
+	t.Parallel()
+
+	_, err := LoadLayered(
+		Input{Source: Bytes("main", []byte("parent: {value: foreign}\n")), Destination: Keys("catalog", "other")},
+		testSecretBinding(),
+		LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{{
+			Source:      Bytes("child", []byte("child: {extends: parent}\n")),
+			Destination: Keys("catalog", "entries"),
+		}}}}},
+	)
+	if got := groupedErrorCode(t, err); got != CodeInheritanceMissing {
+		t.Fatalf("cross-collection parent lookup error = %s, want %s", got, CodeInheritanceMissing)
+	}
+}
+
+func TestLayeredMountedVersionProtectionCoversInheritanceAndUpperTiers(t *testing.T) {
+	t.Parallel()
+
+	for _, test := range []struct {
+		name     string
+		body     string
+		rulePath Path
+	}{
+		{name: "inherited protected value", body: "parent: {version: v2}\nchild: {extends: parent}\n", rulePath: Keys("catalog", "entries", "child", "version")},
+		{name: "inherited protected ancestor", body: "parent: {schema: null}\nchild: {extends: parent}\n", rulePath: Keys("catalog", "entries", "child", "schema", "version")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			_, err := LoadLayered(
+				Input{Source: NoSource()}, testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: &VersionRule{Path: test.rulePath, Check: func(Version, *Validation) error { return nil }}},
+					Underlays:  []Underlay{{Inputs: []Input{{Source: Bytes("underlay", []byte(test.body)), Destination: Keys("catalog", "entries")}}}},
+				},
+			)
+			if got := groupedErrorCode(t, err); got != CodeInheritanceVersion {
+				t.Fatalf("inheritance version protection code = %s", got)
+			}
+		})
+	}
+	t.Run("inheritance consumes a protected directive", func(t *testing.T) {
+		_, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("parent: {value: inherited}\nchild: {extends: parent}\n")), Destination: Keys("catalog", "entries")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Definition: Definition[map[string]any]{Version: &VersionRule{
+				Path: Keys("catalog", "entries", "child", "extends"),
+				Check: func(version Version, _ *Validation) error {
+					if !version.Present || version.Text != "parent" {
+						t.Errorf("mounted protected directive value = %#v", version)
+					}
+					return nil
+				},
+			}}},
+		)
+		if got := groupedErrorCode(t, err); got != CodeInheritanceVersion {
+			t.Fatalf("consumed directive protection code = %s", got)
+		}
+	})
+	t.Run("inherited mapping ancestor without a version claim", func(t *testing.T) {
+		loaded, err := LoadLayered(
+			Input{Source: NoSource()}, testSecretBinding(),
+			LayeredOptions[map[string]any]{
+				Definition: Definition[map[string]any]{Version: &VersionRule{Path: Keys("catalog", "entries", "child", "schema", "version"), Check: func(Version, *Validation) error { return nil }}},
+				Underlays: []Underlay{{Inputs: []Input{{
+					Source: Bytes("underlay", []byte("parent: {schema: {name: ordinary}}\nchild: {extends: parent}\n")), Destination: Keys("catalog", "entries"),
+				}}}},
+			},
+		)
+		if err != nil {
+			t.Fatalf("nonclaiming inherited mapping ancestor was rejected: %v", err)
+		}
+		if got := layeredMapValue(t, testLoaded(loaded).Config, "catalog", "entries", "child", "schema", "name"); got != "ordinary" {
+			t.Fatalf("inherited non-version value = %#v", got)
+		}
+	})
+
+	for _, test := range []struct {
+		name string
+		body string
+	}{
+		{name: "overlay protected value", body: "catalog: {schema: {version: v2}}\n"},
+		{name: "overlay protected ancestor", body: "catalog: {schema: null}\n"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			filesystem := NewFilesystem("overlay", fstest.MapFS{"overlays/test.yml": &fstest.MapFile{Data: []byte(test.body)}})
+			_, err := LoadLayered(
+				Input{Source: Bytes("main", []byte("schema: {version: v1}\n")), Destination: Keys("catalog")},
+				testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: &VersionRule{Path: Keys("catalog", "schema", "version"), Check: func(version Version, _ *Validation) error {
+						if !version.Present || version.Text != "v1" {
+							t.Errorf("mounted main version = %#v", version)
+						}
+						return nil
+					}}},
+					Overlay: "test", OverlayFilesystem: &FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem},
+				},
+			)
+			if got := groupedErrorCode(t, err); got != CodeOverlayVersion {
+				t.Fatalf("overlay version protection code = %s", got)
+			}
+		})
+	}
+
+	for _, test := range []struct {
+		name  string
+		path  Path
+		value any
+	}{
+		{name: "override protected value", path: Keys("catalog", "schema", "version"), value: "v2"},
+		{name: "override protected ancestor", path: Keys("catalog", "schema"), value: map[string]any{"version": "v2"}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			_, err := LoadLayered(
+				Input{Source: Bytes("main", []byte("schema: {version: v1}\n")), Destination: Keys("catalog")},
+				testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: &VersionRule{Path: Keys("catalog", "schema", "version"), Check: func(Version, *Validation) error { return nil }}},
+					Overrides:  []Override{{Path: test.path, Value: test.value, Flag: "--config"}},
+				},
+			)
+			if got := groupedErrorCode(t, err); got != CodeOverrideVersion {
+				t.Fatalf("override version protection code = %s", got)
+			}
+		})
+	}
+
+	t.Run("unrelated inheritance under the protected mapping ancestor", func(t *testing.T) {
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("schema: {version: v1}\nentries:\n  parent: {value: inherited}\n  child: {extends: parent}\n")), Destination: Keys("catalog")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Definition: Definition[map[string]any]{Version: &VersionRule{Path: Keys("catalog", "schema", "version"), Check: func(Version, *Validation) error { return nil }}}},
+		)
+		if err != nil {
+			t.Fatalf("unrelated mounted inheritance was rejected: %v", err)
+		}
+		child := layeredMapValue(t, testLoaded(loaded).Config, "catalog", "entries", "child", "value")
+		if child != "inherited" {
+			t.Fatalf("unrelated inheritance value = %#v", child)
+		}
+	})
+}
+
+type destinationDecodeProbe string
+
+var destinationDecodeProbeValues []string
+
+func (value *destinationDecodeProbe) UnmarshalText(text []byte) error {
+	destinationDecodeProbeValues = append(destinationDecodeProbeValues, string(text))
+	*value = destinationDecodeProbe(text)
+	return nil
+}
+
+func (value destinationDecodeProbe) MarshalText() ([]byte, error) {
+	return []byte(value), nil
+}
+
+type destinationLifecycleConfig struct {
+	Defaulted string `yaml:"defaulted"`
+	Tenant    struct {
+		Tagged     string                 `yaml:"tagged" configkit:"secret"`
+		Dynamic    string                 `yaml:"dynamic"`
+		Credential Secret                 `yaml:"credential"`
+		Derived    string                 `yaml:"derived"`
+		Decoded    destinationDecodeProbe `yaml:"decoded"`
+	} `yaml:"tenant"`
+}
+
+func TestLayeredDestinationLifecycleAndSensitiveInspectionUseFinalShape(t *testing.T) {
+	const (
+		oldTagged   = "SUPERSEDED-TAGGED-CANARY"
+		newTagged   = "CURRENT-TAGGED-CANARY"
+		oldDynamic  = "SUPERSEDED-DYNAMIC-CANARY"
+		newDynamic  = "CURRENT-DYNAMIC-CANARY"
+		credential  = "RESOLVED-CREDENTIAL-CANARY"
+		secretName  = "DESTINATION_SECRET_NAME"
+		decodeValue = "decoded-once"
+	)
+	type calls struct{ defaults, resolve, lookup, validate int }
+	newLoad := func(mounted bool, observed *calls) (*testResult[destinationLifecycleConfig], error) {
+		mainBody := "tagged: " + newTagged + "\ndynamic: " + newDynamic + "\ncredential: " + secretName + "\ndecoded: " + decodeValue + "\n"
+		underlayBody := "tagged: " + oldTagged + "\ndynamic: " + oldDynamic + "\ncredential: OLD_DESTINATION_SECRET_NAME\ndecoded: old\n"
+		if !mounted {
+			mainBody = "tenant:\n" + indentDestinationBody(mainBody)
+			underlayBody = "tenant:\n" + indentDestinationBody(underlayBody)
+		}
+		main := loadInput{source: Bytes("main", []byte(mainBody))}
+		underlay := loadInput{source: Bytes("underlay", []byte(underlayBody))}
+		if mounted {
+			main.destination = Keys("tenant")
+			underlay.destination = Keys("tenant")
+		}
+		definition := Definition[destinationLifecycleConfig]{
+			Defaults: func() destinationLifecycleConfig {
+				observed.defaults++
+				return destinationLifecycleConfig{Defaulted: "from-default"}
+			},
+			Resolve: func(configuration *destinationLifecycleConfig, resolution *Resolution) error {
+				observed.resolve++
+				configuration.Tenant.Derived = configuration.Tenant.Dynamic
+				resolution.Record(Keys("tenant", "derived"), ChangeNormalization, "derive from mounted input", Keys("tenant", "dynamic"))
+				return nil
+			},
+			Validate: func(configuration *destinationLifecycleConfig, _ *Validation) {
+				observed.validate++
+				if configuration.Tenant.Decoded != destinationDecodeProbe(decodeValue) || configuration.Tenant.Dynamic != newDynamic {
+					t.Errorf("validation saw a non-final mounted configuration: %#v", configuration.Tenant)
+				}
+			},
+			Secret: func(path Path) bool { return path.String() == "/tenant/dynamic" },
+		}
+		loaded, err := loadGroupedWithSecretLookup(loadRequest[destinationLifecycleConfig]{
+			main: main, underlays: []loadGroup{{inputs: []loadInput{underlay}}}, definition: definition,
+		}, testSecretBinding(), func(name string) (string, bool) {
+			observed.lookup++
+			if name != secretName {
+				return "", false
+			}
+			return credential, true
+		})
+		return testLoaded(loaded), err
+	}
+
+	var observed calls
+	destinationDecodeProbeValues = nil
+	mounted, err := newLoad(true, &observed)
+	if err != nil {
+		if loadError, ok := err.(*LoadError); ok {
+			t.Fatalf("mounted load: %#v", loadError.Diagnostics())
+		}
+		t.Fatalf("mounted load: %v", err)
+	}
+	if observed != (calls{defaults: 1, resolve: 1, lookup: 1, validate: 1}) {
+		t.Fatalf("mounted lifecycle counts = %#v", observed)
+	}
+	sawFinalDecode := false
+	for _, value := range destinationDecodeProbeValues {
+		if value == decodeValue {
+			sawFinalDecode = true
+		} else if value != "" {
+			t.Fatalf("decoder observed a non-final mounted value %q: %#v", value, destinationDecodeProbeValues)
+		}
+	}
+	if !sawFinalDecode {
+		t.Fatal("final mounted value was not decoded")
+	}
+	if mounted.Config.Defaulted != "from-default" || mounted.Config.Tenant.Tagged != newTagged || mounted.Config.Tenant.Dynamic != newDynamic ||
+		mounted.Config.Tenant.Derived != newDynamic || mounted.Config.Tenant.Decoded != destinationDecodeProbe(decodeValue) ||
+		mounted.Config.Tenant.Credential.Value() != credential {
+		t.Fatalf("mounted final configuration = %#v", mounted.Config)
+	}
+	for _, path := range []Path{Keys("tenant", "tagged"), Keys("tenant", "dynamic"), Keys("tenant", "derived")} {
+		if !mounted.Inspection.secrets[path.String()] {
+			t.Errorf("effective sensitivity missing at %s: %#v", path, mounted.Inspection.secrets)
+		}
+	}
+	for _, path := range []Path{Keys("tenant", "tagged"), Keys("tenant", "dynamic"), Keys("tenant", "derived")} {
+		explanation, ok := mounted.Explain(path)
+		if !ok || !explanation.Effective.Redacted {
+			t.Errorf("effective explanation at %s = %#v, %v", path, explanation, ok)
+			continue
+		}
+		for _, change := range explanation.History {
+			if strings.Contains(change.Before.YAML+change.After.YAML, oldTagged) ||
+				strings.Contains(change.Before.YAML+change.After.YAML, newTagged) ||
+				strings.Contains(change.Before.YAML+change.After.YAML, oldDynamic) ||
+				strings.Contains(change.Before.YAML+change.After.YAML, newDynamic) {
+				t.Errorf("classified history leaked a canary at %s: %#v", path, change)
+			}
+		}
+		var rendered bytes.Buffer
+		if err := mounted.WriteExplanation(&rendered, path); err != nil || strings.Contains(rendered.String(), "CANARY") {
+			t.Errorf("safe explanation at %s = %q, %v", path, rendered.String(), err)
+		}
+	}
+	var effective bytes.Buffer
+	if err := mounted.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	for _, canary := range []string{oldTagged, newTagged, oldDynamic, newDynamic, credential} {
+		if strings.Contains(effective.String(), canary) {
+			t.Fatalf("effective rendering exposed %q: %s", canary, effective.String())
+		}
+	}
+	if serializedConfig := fmt.Sprintf("%#v", mounted.Config); strings.Contains(serializedConfig, credential) {
+		t.Fatalf("configuration formatting exposed the resolved credential: %s", serializedConfig)
+	}
+	for _, path := range []Path{Keys("tenant", "tagged"), Keys("tenant", "dynamic")} {
+		if history := fmt.Sprintf("%#v", mounted.Metadata().History(path)); strings.Contains(history, "CANARY") {
+			t.Fatalf("metadata history at %s exposed a canary: %s", path, history)
+		}
+	}
+
+	var authoredCalls calls
+	destinationDecodeProbeValues = nil
+	authored, err := newLoad(false, &authoredCalls)
+	if err != nil {
+		t.Fatalf("explicitly wrapped load: %v", err)
+	}
+	if authoredCalls != (calls{defaults: 1, resolve: 1, lookup: 1, validate: 1}) {
+		t.Fatalf("authored lifecycle counts = %#v", authoredCalls)
+	}
+	sawFinalDecode = false
+	for _, value := range destinationDecodeProbeValues {
+		if value == decodeValue {
+			sawFinalDecode = true
+		} else if value != "" {
+			t.Fatalf("authored decoder observed a non-final value %q: %#v", value, destinationDecodeProbeValues)
+		}
+	}
+	if !sawFinalDecode {
+		t.Fatal("final authored value was not decoded")
+	}
+	if !reflect.DeepEqual(mounted.Config, authored.Config) {
+		t.Fatalf("mounted config %#v differs from explicitly wrapped config %#v", mounted.Config, authored.Config)
+	}
+	differences, err := Diff(mounted.Inspection, authored.Inspection)
+	if err != nil || len(differences) != 0 {
+		t.Fatalf("mounted versus authored diff = %#v, %v", differences, err)
+	}
+}
+
+func TestLayeredDestinationDiffUsesBilateralSensitivity(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Tenant struct {
+			Dynamic string `yaml:"dynamic"`
+			Derived string `yaml:"derived"`
+		} `yaml:"tenant"`
+	}
+	load := func(value string, classify bool) *testResult[config] {
+		t.Helper()
+		loaded, err := loadGroupedWithSecretLookup(loadRequest[config]{
+			main: loadInput{source: Bytes("input", []byte("dynamic: "+value+"\n")), destination: Keys("tenant")},
+			definition: Definition[config]{
+				Resolve: func(configuration *config, resolution *Resolution) error {
+					configuration.Tenant.Derived = configuration.Tenant.Dynamic
+					resolution.Record(Keys("tenant", "derived"), ChangeNormalization, "derive from dynamic input", Keys("tenant", "dynamic"))
+					return nil
+				},
+				Secret: func(path Path) bool { return classify && path.String() == "/tenant/dynamic" },
+			},
+		}, testSecretBinding(), func(string) (string, bool) { return "", false })
+		if err != nil {
+			t.Fatalf("LoadLayered: %v", err)
+		}
+		return testLoaded(loaded)
+	}
+	left := load("LEFT-DIFF-CANARY", true)
+	right := load("RIGHT-DIFF-CANARY", false)
+	differences, err := Diff(left.Inspection, right.Inspection)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 2 {
+		t.Fatalf("differences = %#v", differences)
+	}
+	seen := make(map[string]bool)
+	for _, difference := range differences {
+		path := difference.Path.String()
+		seen[path] = true
+		if !difference.Left.Redacted || !difference.Right.Redacted || !difference.HiddenChanged ||
+			strings.Contains(difference.Left.YAML+difference.Right.YAML, "DIFF-CANARY") {
+			t.Errorf("bilaterally classified difference at %s = %#v", path, difference)
+		}
+	}
+	for _, path := range []string{"/tenant/dynamic", "/tenant/derived"} {
+		if !seen[path] || !left.Inspection.secrets[path] || right.Inspection.secrets[path] {
+			t.Errorf("classification/difference at %s: seen=%v left=%v right=%v", path, seen[path], left.Inspection.secrets[path], right.Inspection.secrets[path])
+		}
+	}
+}
+
+func TestLayeredDestinationDecodeFailureReturnsNoResultOrLaterCallbacks(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Tenant struct {
+			Count int `yaml:"count"`
+		} `yaml:"tenant"`
+	}
+	calls := struct{ defaults, resolve, lookup, validate int }{}
+	loaded, err := loadGroupedWithSecretLookup(loadRequest[config]{
+		main: loadInput{source: Bytes("invalid", []byte("count: not-an-integer\n")), destination: Keys("tenant")},
+		definition: Definition[config]{
+			Defaults: func() config { calls.defaults++; return config{} },
+			Resolve:  func(*config, *Resolution) error { calls.resolve++; return nil },
+			Validate: func(*config, *Validation) { calls.validate++ },
+		},
+	}, testSecretBinding(), func(string) (string, bool) { calls.lookup++; return "", false })
+	if loaded != nil || err == nil {
+		t.Fatalf("mounted decode failure returned a usable result: %#v, %v", loaded, err)
+	}
+	if calls.defaults != 1 || calls.resolve != 0 || calls.lookup != 0 || calls.validate != 0 {
+		t.Fatalf("callbacks after mounted decode failure = %#v", calls)
+	}
+	if got := groupedErrorCode(t, err); got != CodeDecodeType {
+		t.Fatalf("mounted decode failure code = %s", got)
+	}
+}
+
+func indentDestinationBody(body string) string {
+	var result strings.Builder
+	for _, line := range strings.Split(strings.TrimSuffix(body, "\n"), "\n") {
+		result.WriteString("  ")
+		result.WriteString(line)
+		result.WriteByte('\n')
+	}
+	return result.String()
+}

internal/engine/destination_load_test.go

@@ -0,0 +1,118 @@
+package engine
+
+import (
+	"bytes"
+	"testing"
+)
+
+type destinationLoadConfig struct {
+	Main        map[string]string            `yaml:"main"`
+	Defaults    map[string]string            `yaml:"defaults"`
+	Collections map[string]map[string]string `yaml:"collections"`
+	Value       string                       `yaml:"value"`
+}
+
+func TestLayeredLoadPlacesNonImportMainAndUnderlayInputs(t *testing.T) {
+	t.Parallel()
+
+	destination := Keys("main")
+	main := Input{Source: Bytes("main", []byte("value: primary\n")), Destination: destination}
+	underlay := Input{Source: Bytes("defaults", []byte("value: fallback\n")), Destination: Keys("defaults")}
+	loaded, err := LoadLayered(main, testSecretBinding(), LayeredOptions[destinationLoadConfig]{
+		Underlays: []Underlay{{Inputs: []Input{underlay}}},
+	})
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	if result.Config.Main["value"] != "primary" || result.Config.Defaults["value"] != "fallback" {
+		t.Fatalf("mounted configuration = %#v", result.Config)
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Destination.String() != "/main" || sources[1].Destination.String() != "/defaults" {
+		t.Fatalf("source destinations = %#v", sources)
+	}
+
+	destination.segments[0].key = "caller mutation"
+	sources[0].Destination.segments[0].key = "record mutation"
+	if got := result.Sources()[0].Destination.String(); got != "/main" {
+		t.Fatalf("source destination was not independently owned: %q", got)
+	}
+}
+
+func TestLayeredLoadOmittedAndEmptyDestinationsKeepRootPlacement(t *testing.T) {
+	t.Parallel()
+
+	inputs := []Input{
+		{Source: Bytes("omitted", []byte("value: same\n"))},
+		{Source: Bytes("empty", []byte("value: same\n")), Destination: Keys()},
+	}
+	for _, input := range inputs {
+		loaded, err := LoadLayered(input, testSecretBinding(), LayeredOptions[destinationLoadConfig]{})
+		if err != nil {
+			t.Fatalf("LoadLayered: %v", err)
+		}
+		result := testLoaded(loaded)
+		if result.Config.Value != "same" {
+			t.Fatalf("root value = %q", result.Config.Value)
+		}
+		sources := result.Sources()
+		if len(sources) != 1 || sources[0].Destination.String() != "" {
+			t.Fatalf("root source destination = %#v", sources)
+		}
+	}
+}
+
+func TestLayeredLoadDistinguishesEmptyBodyFromNoSource(t *testing.T) {
+	t.Parallel()
+
+	loaded, err := LoadLayered(
+		Input{Source: NoSource(), Destination: Keys("absent")},
+		testSecretBinding(),
+		LayeredOptions[destinationLoadConfig]{Underlays: []Underlay{{Inputs: []Input{{
+			Source: Bytes("empty", []byte("{}\n")), Destination: Keys("collections", "one"),
+		}}}}},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	info, ok := result.Metadata().Lookup(Keys("collections", "one"))
+	if !ok || !info.Explicit || info.Origin.Layer != LayerUnderlay {
+		t.Fatalf("empty mounted mapping = %#v, %v", info, ok)
+	}
+	if _, ok := result.Metadata().Lookup(Keys("absent")); ok {
+		t.Fatal("NoSource materialized its destination")
+	}
+	sources := result.Sources()
+	if len(sources) != 1 || sources[0].Label != "empty" || sources[0].Destination.String() != "/collections/one" {
+		t.Fatalf("sources = %#v", sources)
+	}
+}
+
+func TestLayeredLoadPreflightsDestinationsBeforeReadingSources(t *testing.T) {
+	t.Parallel()
+
+	reader := bytes.NewBufferString("value: unused\n")
+	_, err := LoadLayered(
+		Input{Source: NoSource()},
+		testSecretBinding(),
+		LayeredOptions[destinationLoadConfig]{Underlays: []Underlay{{Inputs: []Input{{
+			Source: Reader("reader", reader), Destination: Keys("configkit"),
+		}}}}},
+	)
+	if err == nil {
+		t.Fatal("reserved destination was accepted")
+	}
+	if reader.Len() == 0 {
+		t.Fatal("source reader was consumed before destination preflight")
+	}
+	loadError, ok := err.(*LoadError)
+	if !ok || len(loadError.Diagnostics()) != 1 {
+		t.Fatalf("error = %#v", err)
+	}
+	diagnostic := loadError.Diagnostics()[0]
+	if diagnostic.Code != CodeOptionsInvalid || diagnostic.Path.String() != "/configkit" || diagnostic.Primary != nil {
+		t.Fatalf("destination diagnostic = %#v", diagnostic)
+	}
+}

internal/engine/destination_provenance_test.go

@@ -0,0 +1,181 @@
+package engine
+
+import (
+	"errors"
+	"io/fs"
+	"path"
+	"reflect"
+	"testing"
+)
+
+func TestLayeredDestinationsRetainOriginsHistoryAndDetachedRecords(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	sharedPath := writeImportFile(t, directory, "shared.yml", "section:\n  shared: lower\n  branch:\n    child: old\n    kept: retained\n")
+	shared := File(sharedPath)
+	load := func() (*Loaded[map[string]any], error) {
+		return LoadLayered(
+			Input{Source: NoSource()}, testSecretBinding(),
+			LayeredOptions[map[string]any]{Underlays: []Underlay{
+				{Inputs: []Input{
+					{Source: shared, Destination: Keys("catalog")},
+					{Source: Bytes("peer", []byte("section:\n  peer: present\n")), Destination: Keys("catalog")},
+				}},
+				{Inputs: []Input{{Source: shared, Destination: Keys("backup")}}},
+				{Inputs: []Input{{Source: Bytes("replacement", []byte("section:\n  branch: replaced\n")), Destination: Keys("catalog")}}},
+			}},
+		)
+	}
+	loaded, err := load()
+	if err != nil {
+		t.Fatalf("LoadLayered: %v, diagnostics %#v", err, err.(*LoadError).Diagnostics())
+	}
+	result := testLoaded(loaded)
+	metadata := result.Metadata()
+
+	container, ok := metadata.Lookup(Keys("catalog"))
+	if !ok || !container.Explicit || len(container.Origin.Locations) < 3 {
+		t.Fatalf("mounted container origin = %#v, %v", container, ok)
+	}
+	containerSources := make(map[string]bool)
+	for _, location := range container.Origin.Locations {
+		containerSources[location.SourceID] = true
+		if location.Line < 1 {
+			t.Fatalf("introduced container lacks its source-root fallback: %#v", location)
+		}
+	}
+	for _, sourceID := range []string{"source-1", "source-2", "source-4"} {
+		if !containerSources[sourceID] {
+			t.Fatalf("container contributors = %#v, missing %s", container.Origin.Locations, sourceID)
+		}
+	}
+
+	sharedField, ok := metadata.Lookup(Keys("catalog", "section", "shared"))
+	if !ok || len(sharedField.Origin.Locations) != 1 || sharedField.Origin.Locations[0].SourceID != "source-1" || sharedField.Origin.Locations[0].Line != 2 {
+		t.Fatalf("authored leaf origin = %#v, %v", sharedField, ok)
+	}
+	peerField, ok := metadata.Lookup(Keys("catalog", "section", "peer"))
+	if !ok || len(peerField.Origin.Locations) != 1 || peerField.Origin.Locations[0].SourceID != "source-2" {
+		t.Fatalf("peer leaf origin = %#v, %v", peerField, ok)
+	}
+	if _, ok := metadata.Lookup(Keys("catalog", "section", "branch", "child")); ok {
+		t.Fatal("replaced descendant remains effective")
+	}
+	removed := metadata.History(Keys("catalog", "section", "branch", "child"))
+	if len(removed) < 2 || removed[0].Origin.Locations[0].SourceID != "source-1" {
+		t.Fatalf("removed descendant history = %#v", removed)
+	}
+
+	backup, ok := metadata.Lookup(Keys("backup", "section", "shared"))
+	if !ok || len(backup.Origin.Locations) != 1 || backup.Origin.Locations[0].SourceID != "source-3" {
+		t.Fatalf("second-tier leaf origin = %#v, %v", backup, ok)
+	}
+	sources := result.Sources()
+	if len(sources) != 4 || sources[0].Path != sources[2].Path || sources[0].Destination.String() != "/catalog" || sources[2].Destination.String() != "/backup" {
+		t.Fatalf("same source in separate tiers = %#v", sources)
+	}
+	if sources[0].Layer != LayerUnderlay || sources[0].UnderlayIndex != 1 || sources[2].Layer != LayerUnderlay || sources[2].UnderlayIndex != 2 {
+		t.Fatalf("tier source records = %#v", sources)
+	}
+
+	other, err := load()
+	if err != nil {
+		t.Fatalf("independent load: %v", err)
+	}
+	differences, err := Diff(loaded.Inspection, other.Inspection)
+	if err != nil || len(differences) != 0 {
+		t.Fatalf("equivalent inspection diff = %#v, %v", differences, err)
+	}
+
+	sources[0].Destination.segments[0].key = "mutated"
+	sharedField.Origin.Locations[0].SourceID = "mutated"
+	removed[0].Path.segments[0].key = "mutated"
+	if got := result.Sources()[0].Destination.String(); got != "/catalog" {
+		t.Fatalf("source record copy changed after inspection: %q", got)
+	}
+	fresh, ok := result.Metadata().Lookup(Keys("catalog", "section", "shared"))
+	if !ok || fresh.Origin.Locations[0].SourceID != "source-1" {
+		t.Fatalf("effective origin changed after mutating returned records: %#v, %v", fresh, ok)
+	}
+	if got := result.Metadata().History(Keys("catalog", "section", "branch", "child")); len(got) < 2 || got[0].Path.String() != "/catalog/section/branch/child" {
+		t.Fatalf("historical path changed after mutating returned records: %#v", got)
+	}
+	if got := other.Inspection.sources[0].Destination.String(); got != "/catalog" {
+		t.Fatalf("independent load destination changed: %q", got)
+	}
+}
+
+func TestLayeredImportedInheritedValuesRetainResourceContexts(t *testing.T) {
+	t.Parallel()
+
+	type entry struct {
+		Extends  string `yaml:"extends"`
+		Resource string `yaml:"resource"`
+		Count    int    `yaml:"count"`
+	}
+	type config struct {
+		Catalog struct {
+			Entries map[string]entry `yaml:"entries"`
+		} `yaml:"catalog"`
+	}
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "config/main.yml", "child:\n  extends: parent\n")
+	inputPath := writeImportFile(t, directory, "shared/input.yml", "configkit: {imports: [{path: parent.yml}]}\n")
+	writeImportFile(t, directory, "shared/parent.yml", "parent:\n  resource: icon.txt\n  count: 1\n")
+	writeImportFile(t, directory, "shared/icon.txt", "parent-resource")
+	var resolutionContext ResourceContext
+	loaded, err := LoadLayered(
+		Input{Source: File(mainPath), Destination: Keys("catalog", "entries")},
+		testSecretBinding(),
+		LayeredOptions[config]{
+			Underlays: []Underlay{{Inputs: []Input{{
+				Source: File(inputPath), Imports: true, Destination: Keys("catalog", "entries"),
+			}}}},
+			Definition: Definition[config]{Resolve: func(configuration *config, resolution *Resolution) error {
+				info, ok := resolution.InputInfo(Keys("catalog", "entries", "child", "resource"))
+				if !ok || len(info.Origin.Locations) != 1 || len(info.Origin.Inheritance) != 1 {
+					return errors.New("inherited resource lacks its source origin")
+				}
+				var found bool
+				resolutionContext, found = resolution.state.input.sourceContext(info.Origin.Locations[0].SourceID)
+				if !found || resolutionContext.BaseDir != "." || resolutionContext.FS == nil {
+					return errors.New("resolver could not find the declaring resource context")
+				}
+				data, err := fs.ReadFile(resolutionContext.FS, path.Join(resolutionContext.BaseDir, configuration.Catalog.Entries["child"].Resource))
+				if err != nil || string(data) != "parent-resource" {
+					return errors.New("resolver could not read the inherited relative resource")
+				}
+				return nil
+			}},
+		},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v, diagnostics %#v", err, err.(*LoadError).Diagnostics())
+	}
+	result := testLoaded(loaded)
+	child := result.Config.Catalog.Entries["child"]
+	if child.Resource != "icon.txt" || child.Count != 1 || child.Extends != "" {
+		t.Fatalf("inherited child = %#v", child)
+	}
+	field, ok := result.Metadata().Lookup(Keys("catalog", "entries", "child", "resource"))
+	if !ok || len(field.Origin.Locations) != 1 || len(field.Origin.Inheritance) != 1 {
+		t.Fatalf("inherited resource origin = %#v, %v", field, ok)
+	}
+	context, ok := result.Inspection.SourceContext(field.Origin.Locations[0].SourceID)
+	if !ok || context.BaseDir != "." || context.FS == nil || resolutionContext.BaseDir != context.BaseDir {
+		t.Fatalf("completed resource contexts = %#v, %#v, %v", resolutionContext, context, ok)
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, child.Resource))
+	if err != nil || string(data) != "parent-resource" {
+		t.Fatalf("completed source-relative resource = %q, %v", data, err)
+	}
+	if !reflect.DeepEqual(result.Config.Catalog.Entries["parent"], entry{Resource: "icon.txt", Count: 1}) {
+		t.Fatalf("surviving lower-tier parent = %#v", result.Config.Catalog.Entries["parent"])
+	}
+	for _, source := range result.Sources() {
+		if source.Layer == LayerUnderlay && source.Destination.String() != "/catalog/entries" {
+			t.Errorf("underlay destination = %#v", source)
+		}
+	}
+}

internal/engine/destination_test.go

@@ -0,0 +1,206 @@
+package engine
+
+import (
+	"errors"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestNormalizeDestinationUsesLiteralMappingKeys(t *testing.T) {
+	t.Parallel()
+
+	parsed, err := ParsePath("/a~1b/0/~0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	normalized, err := normalizeDestination(parsed)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := normalized.String(), "/a~1b/0/~0"; got != want {
+		t.Fatalf("normalized path = %q, want %q", got, want)
+	}
+	for index, want := range []string{"a/b", "0", "~"} {
+		segment := normalized.segments[index]
+		if segment.kind != pathSegmentKey || segment.key != want {
+			t.Fatalf("segment %d = %#v, want key %q", index, segment, want)
+		}
+	}
+
+	original := Keys("owned", "path")
+	owned, err := normalizeDestination(original)
+	if err != nil {
+		t.Fatal(err)
+	}
+	owned.segments[0].key = "changed"
+	if got := original.String(); got != "/owned/path" {
+		t.Fatalf("normalizing shared path storage mutated its input: %q", got)
+	}
+}
+
+func TestAppendDestinationCopiesLiteralSegments(t *testing.T) {
+	t.Parallel()
+
+	parent := Keys("collection", "entry")
+	relative := Keys("a/b", "0", "~")
+	combined := appendDestination(parent, relative)
+	if got, want := combined.String(), "/collection/entry/a~1b/0/~0"; got != want {
+		t.Fatalf("combined destination = %q, want %q", got, want)
+	}
+	if len(combined.segments) != 5 {
+		t.Fatalf("combined segments = %#v", combined.segments)
+	}
+	for index, segment := range combined.segments {
+		if segment.kind != pathSegmentKey {
+			t.Errorf("segment %d kind = %v, want mapping key", index, segment.kind)
+		}
+	}
+
+	combined.segments[0].key = "changed parent"
+	combined.segments[2].key = "changed relative"
+	if got := parent.String(); got != "/collection/entry" {
+		t.Fatalf("combined path shares parent segment storage: %q", got)
+	}
+	if got := relative.String(); got != "/a~1b/0/~0" {
+		t.Fatalf("combined path shares relative segment storage: %q", got)
+	}
+}
+
+func TestNormalizeDestinationRejectsIndicesAndReservedKeys(t *testing.T) {
+	t.Parallel()
+
+	for name, path := range map[string]Path{
+		"explicit index":   Keys("items").Index(0),
+		"negative index":   Keys("items").Index(-1),
+		"invalid path":     Path{invalid: true},
+		"configkit":        Keys("configkit"),
+		"extends":          Keys("extends"),
+		"nested configkit": Keys("outer", "configkit"),
+		"nested extends":   Keys("outer", "extends"),
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			if _, err := normalizeDestination(path); !errors.Is(err, errInvalidDestination) {
+				t.Fatalf("normalizeDestination(%q) error = %v, want invalid destination", path.String(), err)
+			}
+		})
+	}
+
+	for _, key := range []string{"myextends", "configkit2", "", ".", "..", "../x", "a.b", "a/b", "~"} {
+		t.Run("ordinary key "+key, func(t *testing.T) {
+			t.Parallel()
+			if _, err := normalizeDestination(Keys(key)); err != nil {
+				t.Fatalf("normalizeDestination(%q): %v", key, err)
+			}
+		})
+	}
+}
+
+func TestPlaceDocumentWrapsEmptyMappingAndPreservesSourceCoordinates(t *testing.T) {
+	t.Parallel()
+
+	rootLocation := document.Location{SourceID: "source-a", Line: 1, Column: 1}
+	body := &document.Node{
+		Kind:     document.Mapping,
+		Location: document.Location{SourceID: "source-a", Line: 4, Column: 1},
+	}
+	path, placed, err := placeDocument(body, Keys("a", "b"), rootLocation, 2, 5)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := path.String(), "/a/b"; got != want {
+		t.Fatalf("normalized destination = %q, want %q", got, want)
+	}
+	if placed == body || len(placed.Mapping) != 1 || placed.Mapping[0].Key != "a" {
+		t.Fatalf("outer destination mapping was not created: %#v", placed)
+	}
+	inner := placed.Mapping[0].Value
+	if inner.Kind != document.Mapping || len(inner.Mapping) != 1 || inner.Mapping[0].Key != "b" {
+		t.Fatalf("inner destination mapping was not created: %#v", inner)
+	}
+	if inner.Mapping[0].Value != body || len(body.Mapping) != 0 {
+		t.Fatal("placement mutated or copied the authored body")
+	}
+	if placed.Location != rootLocation || inner.Location != rootLocation || placed.Mapping[0].KeyLocation != rootLocation || placed.Mapping[0].ValueLocation != rootLocation {
+		t.Fatal("introduced structure did not use the source-root coordinate fallback")
+	}
+	if body.Location != (document.Location{SourceID: "source-a", Line: 4, Column: 1}) {
+		t.Fatalf("authored body location changed: %#v", body.Location)
+	}
+	if got, want := countInputNodes(placed), countInputNodes(body)+4; got != want {
+		t.Fatalf("placed node count = %d, want body count plus four synthetic nodes (%d)", got, want)
+	}
+}
+
+func TestPlaceDocumentPreservesAuthoredEntryCoordinates(t *testing.T) {
+	t.Parallel()
+
+	rootLocation := document.Location{SourceID: "source-b", Line: 1, Column: 1}
+	bodyLocation := document.Location{SourceID: "source-b", Line: 3, Column: 1}
+	keyLocation := document.Location{SourceID: "source-b", Line: 3, Column: 3}
+	valueLocation := document.Location{SourceID: "source-b", Line: 4, Column: 5}
+	value := &document.Node{Kind: document.String, Location: valueLocation, Text: "value"}
+	body := &document.Node{
+		Kind:     document.Mapping,
+		Location: bodyLocation,
+		Mapping: []document.Entry{{
+			Key:           "leaf",
+			KeyLocation:   keyLocation,
+			ValueLocation: valueLocation,
+			Value:         value,
+		}},
+	}
+	_, placed, err := placeDocument(body, Keys("mounted"), rootLocation, 2, 5)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if placed.Mapping[0].KeyLocation != rootLocation || placed.Mapping[0].ValueLocation != rootLocation {
+		t.Fatal("introduced destination entry did not use the source-root fallback")
+	}
+	if placed.Mapping[0].Value != body || body.Location != bodyLocation {
+		t.Fatal("placement changed the authored body node or location")
+	}
+	entry := body.Mapping[0]
+	if entry.KeyLocation != keyLocation || entry.ValueLocation != valueLocation || entry.Value != value || value.Location != valueLocation {
+		t.Fatal("placement changed authored entry or value coordinates")
+	}
+}
+
+func TestPlaceDocumentRootAndBounds(t *testing.T) {
+	t.Parallel()
+
+	body := &document.Node{
+		Kind: document.Mapping,
+		Mapping: []document.Entry{{
+			Key:   "leaf",
+			Value: &document.Node{Kind: document.String, Text: "value"},
+		}},
+	}
+	path, placed, err := placeDocument(body, Path{}, body.Location, 1, 3)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if path.String() != "" || placed != body {
+		t.Fatal("root placement must preserve the existing body")
+	}
+
+	destination := Keys("a", "b")
+	if !destinationWithinLimits(body, destination, 3, 7) {
+		t.Fatal("exact depth and node limits should admit the placed document")
+	}
+	if destinationWithinLimits(body, destination, 2, 7) {
+		t.Fatal("placement exceeding the depth limit was accepted")
+	}
+	if destinationWithinLimits(body, destination, 3, 6) {
+		t.Fatal("placement exceeding the node limit was accepted")
+	}
+	_, placed, err = placeDocument(body, destination, body.Location, 3, 6)
+	if !errors.Is(err, errDestinationLimit) || placed != nil {
+		t.Fatalf("bounded placement returned node %p and error %v", placed, err)
+	}
+	_, absent, err := placeDocument(nil, destination, document.Location{}, 0, 0)
+	if err != nil || absent != nil {
+		t.Fatalf("absent body should not consume placement limits: node %p, error %v", absent, err)
+	}
+}

internal/engine/destination_tiers_test.go

@@ -0,0 +1,347 @@
+package engine
+
+import (
+	"path/filepath"
+	"reflect"
+	"testing"
+	"testing/fstest"
+)
+
+func TestLayeredDestinationStrictCompositionBoundaries(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		inputs   []Input
+		wantPath []string
+		want     any
+		alsoPath []string
+		alsoWant any
+	}{
+		{
+			name: "disjoint peers at one destination",
+			inputs: []Input{
+				{Source: Bytes("left", []byte("left: one\n")), Destination: Keys("application")},
+				{Source: Bytes("right", []byte("right: two\n")), Destination: Keys("application")},
+			},
+			wantPath: []string{"application", "right"},
+			want:     "two",
+			alsoPath: []string{"application", "left"},
+			alsoWant: "one",
+		},
+		{
+			name: "different destinations",
+			inputs: []Input{
+				{Source: Bytes("left", []byte("value: shared\n")), Destination: Keys("left")},
+				{Source: Bytes("right", []byte("value: shared\n")), Destination: Keys("right")},
+			},
+			wantPath: []string{"right", "value"},
+			want:     "shared",
+		},
+		{
+			name: "ancestor and descendant destinations",
+			inputs: []Input{
+				{Source: Bytes("parent", []byte("branch: {base: one}\n")), Destination: Keys("application")},
+				{Source: Bytes("child", []byte("leaf: two\n")), Destination: Keys("application", "branch")},
+			},
+			wantPath: []string{"application", "branch", "leaf"},
+			want:     "two",
+			alsoPath: []string{"application", "branch", "base"},
+			alsoWant: "one",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			loaded, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), LayeredOptions[map[string]any]{
+				Underlays: []Underlay{{Inputs: test.inputs}},
+			})
+			if err != nil {
+				t.Fatalf("LoadLayered: %v", err)
+			}
+			result := testLoaded(loaded)
+			if got := layeredMapValue(t, result.Config, test.wantPath...); got != test.want {
+				t.Fatalf("value at %v = %#v, want %#v", test.wantPath, got, test.want)
+			}
+			if len(test.alsoPath) != 0 {
+				if got := layeredMapValue(t, result.Config, test.alsoPath...); got != test.alsoWant {
+					t.Fatalf("value at %v = %#v, want %#v", test.alsoPath, got, test.alsoWant)
+				}
+			}
+		})
+	}
+
+	for _, test := range []struct {
+		name  string
+		left  string
+		right string
+	}{
+		{name: "equal values conflict", left: "value: same\n", right: "value: same\n"},
+		{name: "unequal values conflict", left: "value: left\n", right: "value: right\n"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			inputs := []Input{
+				{Source: Bytes("left", []byte(test.left)), Destination: Keys("application")},
+				{Source: Bytes("right", []byte(test.right)), Destination: Keys("application")},
+			}
+			loaded, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), LayeredOptions[map[string]any]{
+				Underlays: []Underlay{{Inputs: inputs}},
+			})
+			if loaded != nil || groupedErrorCode(t, err) != CodeComposeConflict {
+				t.Fatalf("strict composition result = %#v, %v", loaded, err)
+			}
+		})
+	}
+
+	t.Run("strict conflict is not hidden by an upper override", func(t *testing.T) {
+		inputs := []Input{
+			{Source: Bytes("left", []byte("value: left\n")), Destination: Keys("application")},
+			{Source: Bytes("right", []byte("value: right\n")), Destination: Keys("application")},
+		}
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("value: upper\n")), Destination: Keys("application")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: inputs}}},
+		)
+		if loaded != nil || groupedErrorCode(t, err) != CodeComposeConflict {
+			t.Fatalf("strict composition result = %#v, %v", loaded, err)
+		}
+	})
+}
+
+func TestLayeredDestinationMatchesExplicitlyWrappedInput(t *testing.T) {
+	t.Parallel()
+
+	load := func(input Input) map[string]any {
+		t.Helper()
+		loaded, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), LayeredOptions[map[string]any]{
+			Underlays: []Underlay{{Inputs: []Input{input}}},
+		})
+		if err != nil {
+			t.Fatalf("LoadLayered: %v", err)
+		}
+		return testLoaded(loaded).Config
+	}
+
+	placed := load(Input{Source: Bytes("placed", []byte("field: value\n")), Destination: Keys("application")})
+	authored := load(Input{Source: Bytes("authored", []byte("application: {field: value}\n"))})
+	if !reflect.DeepEqual(placed, authored) {
+		t.Fatalf("placed configuration %#v differs from explicitly wrapped %#v", placed, authored)
+	}
+}
+
+func TestEmptyMountedMappingRetainsLowerChildren(t *testing.T) {
+	t.Parallel()
+
+	destination := Keys("collections", "weather")
+	loaded, err := LoadLayered(
+		Input{Source: Bytes("empty-main", []byte("{}\n")), Destination: destination},
+		testSecretBinding(),
+		LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{{
+			Source: Bytes("lower", []byte("prompt: {title: retained}\n")), Destination: destination,
+		}}}}},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	if got := layeredMapValue(t, testLoaded(loaded).Config, "collections", "weather", "prompt", "title"); got != "retained" {
+		t.Fatalf("lower child beneath empty mounted mapping = %#v, want retained", got)
+	}
+}
+
+func TestLayeredDestinationOverlayRetainsOrdinaryValueSemantics(t *testing.T) {
+	t.Parallel()
+
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"overlays/test.yml": &fstest.MapFile{Data: []byte("settings:\n  metadata:\n    version: overlay\n")},
+	})
+	loaded, err := LoadLayered(
+		Input{Source: Bytes("main", []byte("mapping: {shared: main}\nsequence: [9]\ncleared: null\nenabled: false\nmetadata: {version: main}\n")), Destination: Keys("settings")},
+		testSecretBinding(),
+		LayeredOptions[map[string]any]{
+			Underlays: []Underlay{{Inputs: []Input{{
+				Source:      Bytes("underlay", []byte("mapping: {retained: lower, shared: lower}\nsequence: [1, 2]\ncleared: {old: true}\nenabled: true\ncount: 7\nmetadata: {version: lower, label: retained}\n")),
+				Destination: Keys("settings"),
+			}}}},
+			Overlay:           "test",
+			OverlayFilesystem: &FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem},
+			Overrides:         []Override{{Path: Keys("settings", "count"), Value: 0, Flag: "--count"}},
+		},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	settings := layeredMap(t, result.Config, "settings")
+	mapping := layeredMap(t, settings, "mapping")
+	if mapping["retained"] != "lower" || mapping["shared"] != "main" {
+		t.Fatalf("recursive mapping overlay = %#v", mapping)
+	}
+	sequence, ok := settings["sequence"].([]any)
+	if !ok || len(sequence) != 1 || sequence[0] != int64(9) {
+		t.Fatalf("sequence overlay = %#v", settings["sequence"])
+	}
+	if value, present := settings["cleared"]; !present || value != nil {
+		t.Fatalf("explicit null = %#v, present %v", value, present)
+	}
+	if value, ok := settings["enabled"].(bool); !ok || value {
+		t.Fatalf("explicit false = %#v", settings["enabled"])
+	}
+	if value, ok := settings["count"].(int64); !ok || value != 0 {
+		t.Fatalf("explicit zero override = %#v", settings["count"])
+	}
+	metadata := layeredMap(t, settings, "metadata")
+	if metadata["version"] != "overlay" || metadata["label"] != "retained" {
+		t.Fatalf("version-only overlay = %#v", metadata)
+	}
+	if _, present := settings["settings"]; present {
+		t.Fatalf("upper tiers were incorrectly nested under the main destination: %#v", result.Config)
+	}
+}
+
+func TestLayeredDestinationChecksVersionOnceOnComposedMain(t *testing.T) {
+	t.Parallel()
+
+	type appConfig struct {
+		Version  int8   `yaml:"version"`
+		Count    int    `yaml:"count"`
+		Main     string `yaml:"main"`
+		Imported string `yaml:"imported"`
+		Lower    string `yaml:"lower"`
+	}
+	type config struct {
+		App appConfig `yaml:"app"`
+	}
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":     &fstest.MapFile{Data: []byte("configkit: {imports: [{path: fragment.yml}]}\nversion: 6\ncount: 4\nmain: main\n")},
+		"fragment.yml": &fstest.MapFile{Data: []byte("imported: included\n")},
+	})
+	checks := 0
+	loaded, err := LoadLayered(
+		Input{Source: filesystem.File("main.yml"), Imports: true, Destination: Keys("app")},
+		testSecretBinding(),
+		LayeredOptions[config]{
+			Definition: Definition[config]{Version: &VersionRule{Path: Keys("app", "version"), Check: func(version Version, _ *Validation) error {
+				checks++
+				if !version.Present || version.Text != "6" {
+					t.Errorf("nested main version = %#v", version)
+				}
+				return nil
+			}}},
+			Underlays: []Underlay{{Inputs: []Input{{
+				Source: Bytes("lower", []byte("lower: underlay\n")), Destination: Keys("app"),
+			}}}},
+		},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	if checks != 1 {
+		t.Fatalf("version hook calls = %d, want 1", checks)
+	}
+	if result.Config.App.Version != 6 || result.Config.App.Count != 4 || result.Config.App.Main != "main" || result.Config.App.Imported != "included" || result.Config.App.Lower != "underlay" {
+		t.Fatalf("nested app = %#v", result.Config.App)
+	}
+}
+
+func TestLayeredDestinationVersionClaimsUsePlacedApplicationPaths(t *testing.T) {
+	t.Parallel()
+
+	rule := &VersionRule{Path: Keys("app", "schema", "version"), Check: func(Version, *Validation) error { return nil }}
+	for _, test := range []struct {
+		name      string
+		body      string
+		wantCode  Code
+		wantValue any
+	}{
+		{name: "underlay direct claim", body: "schema: {version: v2}\n", wantCode: CodeUnderlayVersion},
+		{name: "underlay ancestor claim", body: "schema: null\n", wantCode: CodeUnderlayVersion},
+		{name: "underlay mapping ancestor without claim", body: "schema: {name: ordinary}\n", wantValue: "ordinary"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			loaded, err := LoadLayered(
+				Input{Source: NoSource()},
+				testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: rule},
+					Underlays: []Underlay{{Inputs: []Input{{
+						Source: Bytes("underlay", []byte(test.body)), Destination: Keys("app"),
+					}}}},
+				},
+			)
+			if test.wantCode != "" {
+				if loaded != nil || groupedErrorCode(t, err) != test.wantCode {
+					t.Fatalf("version claim result = %#v, %v", loaded, err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("nonclaiming mapping ancestor was rejected: %v", err)
+			}
+			result := testLoaded(loaded)
+			if got := layeredMapValue(t, result.Config, "app", "schema", "name"); got != test.wantValue {
+				t.Fatalf("nested ordinary value = %#v", got)
+			}
+		})
+	}
+
+	for _, test := range []struct {
+		name     string
+		body     string
+		wantCode Code
+	}{
+		{name: "imported direct claim", body: "schema: {version: v2}\n", wantCode: CodeImportVersion},
+		{name: "imported ancestor claim", body: "schema: null\n", wantCode: CodeImportVersion},
+		{name: "imported mapping ancestor without claim", body: "schema: {name: ordinary}\n"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			directory := t.TempDir()
+			writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: fragment.yml}]}\n")
+			writeImportFile(t, directory, "fragment.yml", test.body)
+			loaded, err := LoadLayered(
+				Input{Source: NoSource()},
+				testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: rule},
+					Underlays: []Underlay{{Inputs: []Input{{
+						Source: File(filepath.Join(directory, "main.yml")), Imports: true, Destination: Keys("app"),
+					}}}},
+				},
+			)
+			if test.wantCode != "" {
+				if loaded != nil || groupedErrorCode(t, err) != test.wantCode {
+					t.Fatalf("version claim result = %#v, %v", loaded, err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("nonclaiming imported mapping ancestor was rejected: %v", err)
+			}
+			result := testLoaded(loaded)
+			if got := layeredMapValue(t, result.Config, "app", "schema", "name"); got != "ordinary" {
+				t.Fatalf("nested ordinary value = %#v", got)
+			}
+		})
+	}
+}
+
+func layeredMap(t *testing.T, value map[string]any, key string) map[string]any {
+	t.Helper()
+	result, ok := value[key].(map[string]any)
+	if !ok {
+		t.Fatalf("%q = %#v, want mapping", key, value[key])
+	}
+	return result
+}
+
+func layeredMapValue(t *testing.T, value map[string]any, keys ...string) any {
+	t.Helper()
+	for _, key := range keys[:len(keys)-1] {
+		value = layeredMap(t, value, key)
+	}
+	return value[keys[len(keys)-1]]
+}

internal/engine/diagnostic.go

@@ -63,8 +63,9 @@ func WriteDiagnostics(output io.Writer, diagnostics []Diagnostic) error {
 }
 
 type diagnosticAccumulator struct {
-	diagnostics []Diagnostic
-	sources     []SourceInfo
+	diagnostics  []Diagnostic
+	sources      []SourceInfo
+	versionState *sourceVersionState
 }
 
 func (a *diagnosticAccumulator) add(diagnostics ...Diagnostic) {
@@ -74,9 +75,13 @@ func (a *diagnosticAccumulator) add(diagnostics ...Diagnostic) {
 }
 
 func (a *diagnosticAccumulator) loadError() *LoadError {
+	sources := a.sources
+	if a.versionState != nil {
+		sources = sourceInfosWithVersionEvidence(sources, a.versionState.evidence, nil, true)
+	}
 	return &LoadError{
 		diagnostics: canonicalDiagnostics(a.diagnostics),
-		sources:     cloneSourceInfos(a.sources),
+		sources:     cloneSourceInfos(sources),
 	}
 }
 
@@ -85,15 +90,28 @@ func canonicalDiagnostics(diagnostics []Diagnostic) []Diagnostic {
 	for i := range canonical {
 		slices.SortFunc(canonical[i].Related, compareLocation)
 	}
-	slices.SortFunc(canonical, compareDiagnostic)
+	type orderedDiagnostic struct {
+		diagnostic Diagnostic
+		pointer    string
+	}
+	ordered := make([]orderedDiagnostic, len(canonical))
+	for i, diagnostic := range canonical {
+		ordered[i] = orderedDiagnostic{diagnostic: diagnostic, pointer: diagnostic.Path.String()}
+	}
+	slices.SortFunc(ordered, func(left, right orderedDiagnostic) int {
+		return compareDiagnosticWithPaths(left.diagnostic, right.diagnostic, left.pointer, right.pointer)
+	})
+	for i, item := range ordered {
+		canonical[i] = item.diagnostic
+	}
 	return canonical
 }
 
-func compareDiagnostic(left, right Diagnostic) int {
+func compareDiagnosticWithPaths(left, right Diagnostic, leftPath, rightPath string) int {
 	if order := compareOptionalLocation(left.Primary, right.Primary); order != 0 {
 		return order
 	}
-	if order := cmp.Compare(left.Path.String(), right.Path.String()); order != 0 {
+	if order := cmp.Compare(leftPath, rightPath); order != 0 {
 		return order
 	}
 	if order := cmp.Compare(left.Code, right.Code); order != 0 {

internal/engine/diagnostic_test.go

@@ -90,6 +90,26 @@ func TestDiagnosticsAreCanonicalAndDetached(t *testing.T) {
 	}
 }
 
+func TestCanonicalDiagnosticsSortEscapedPaths(t *testing.T) {
+	primary := &Location{SourceID: "source-1", Line: 1, Column: 1}
+	diagnostics := []Diagnostic{
+		{Code: "same", Severity: SeverityError, Path: Keys("a/b"), Message: "same", Primary: primary},
+		{Code: "same", Severity: SeverityError, Path: Keys("a~b"), Message: "same", Primary: primary},
+		{Code: "z", Severity: SeverityError, Path: Keys("same"), Message: "same", Primary: primary},
+		{Code: "a", Severity: SeverityError, Path: Keys("same"), Message: "same", Primary: primary},
+	}
+	canonical := canonicalDiagnostics(diagnostics)
+	want := []string{"/a~0b", "/a~1b", "/same", "/same"}
+	for i, diagnostic := range canonical {
+		if got := diagnostic.Path.String(); got != want[i] {
+			t.Fatalf("diagnostic[%d] path = %q, want %q", i, got, want[i])
+		}
+	}
+	if canonical[2].Code != "a" || canonical[3].Code != "z" {
+		t.Fatalf("same-path code order = %q, %q", canonical[2].Code, canonical[3].Code)
+	}
+}
+
 func TestWriteDiagnosticsEscapesControls(t *testing.T) {
 	t.Parallel()
 

internal/engine/diff.go

@@ -0,0 +1,98 @@
+package engine
+
+import (
+	"bytes"
+	"errors"
+	"io"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+var errDiffInspectionUnavailable = errors.New("configkit: diff inspection unavailable")
+var errDiffSnapshotUnavailable = errors.New("configkit: diff snapshot unavailable")
+
+// Difference describes one canonical changed region with safe snapshots.
+type Difference struct {
+	Path          Path
+	Left          ValueSnapshot
+	Right         ValueSnapshot
+	HiddenChanged bool
+}
+
+type diffRegion struct {
+	path  Path
+	left  *document.Node
+	right *document.Node
+}
+
+func prepareDiffRegions(left, right *Inspection) ([]diffRegion, error) {
+	if left == nil || left.final == nil || right == nil || right.final == nil {
+		return nil, errDiffInspectionUnavailable
+	}
+
+	deltas := compareConfigurationValues(left.final, right.final)
+	regions := make([]diffRegion, 0, len(deltas))
+	for _, delta := range deltas {
+		leftNode, leftPresent := documentAtPathWithLookup(left.final, delta.path, left.immutableLookup)
+		if !leftPresent {
+			leftNode = nil
+		}
+		rightNode, rightPresent := documentAtPathWithLookup(right.final, delta.path, right.immutableLookup)
+		if !rightPresent {
+			rightNode = nil
+		}
+		regions = append(regions, diffRegion{path: delta.path, left: leftNode, right: rightNode})
+	}
+	return regions, nil
+}
+
+// Diff compares the final frozen values retained by two successful inspections.
+// Values are serialized only after applying the combined sensitivity of both loads.
+func Diff(left, right *Inspection) ([]Difference, error) {
+	regions, err := prepareDiffRegions(left, right)
+	if err != nil {
+		return nil, err
+	}
+	sensitivity := newDiffSensitivity(left.secrets, right.secrets)
+	differences := make([]Difference, 0, len(regions))
+	for _, region := range regions {
+		paired, supported := preparePairedDiffSnapshot(region.left, region.right, region.path, sensitivity)
+		if !supported {
+			return nil, errDiffSnapshotUnavailable
+		}
+		leftSnapshot, err := serializeDiffSnapshot(paired.left, paired.leftRedacted)
+		if err != nil {
+			return nil, errDiffSnapshotUnavailable
+		}
+		rightSnapshot, err := serializeDiffSnapshot(paired.right, paired.rightRedacted)
+		if err != nil {
+			return nil, errDiffSnapshotUnavailable
+		}
+		differences = append(differences, Difference{
+			Path:          region.path,
+			Left:          leftSnapshot,
+			Right:         rightSnapshot,
+			HiddenChanged: paired.hiddenChanged,
+		})
+	}
+	return differences, nil
+}
+
+func serializeDiffSnapshot(node *document.Node, redacted bool) (ValueSnapshot, error) {
+	if node == nil {
+		return ValueSnapshot{}, nil
+	}
+	var output bytes.Buffer
+	if err := writeDiffDocument(&output, node); err != nil {
+		return ValueSnapshot{}, errDiffSnapshotUnavailable
+	}
+	return ValueSnapshot{Present: true, Redacted: redacted, YAML: output.String()}, nil
+}
+
+func writeDiffDocument(output io.Writer, node *document.Node) error {
+	if err := yamladapter.Write(output, node); err != nil {
+		return errDiffSnapshotUnavailable
+	}
+	return nil
+}

internal/engine/diff_hidden_test.go

@@ -0,0 +1,130 @@
+package engine
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestPreparePairedDiffSnapshotReportsWhollyMaskedChanges(t *testing.T) {
+	left := &document.Node{Kind: document.String, Text: "private-before"}
+	right := &document.Node{Kind: document.String, Text: "private-after"}
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Keys("credential"), newDiffSensitivity(nil, map[string]bool{"/credential": true}),
+	)
+	if !supported || !paired.hiddenChanged || !paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("masked change = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, paired.left, "[REDACTED]")
+	assertDiffStringNode(t, paired.right, "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotDetectsRawChangesBehindIdenticalMarkers(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "token: old-canary\n")
+	right := parseCompositionDocument(t, "right", "token: new-canary\n")
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "token"), mappingValue(t, right, "token"), Keys("token"),
+		newDiffSensitivity(map[string]bool{"/token": true}, nil),
+	)
+	if !supported || !paired.hiddenChanged || paired.left.Text != paired.right.Text {
+		t.Fatalf("identically rendered masked values lost raw change: %#v, supported=%v", paired, supported)
+	}
+}
+
+func TestPreparePairedDiffSnapshotCombinesPublicAndHiddenChanges(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "public: before\ntoken: old\n")
+	right := parseCompositionDocument(t, "right", "public: after\ntoken: new\n")
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Path{}, newDiffSensitivity(nil, map[string]bool{"/token": true}),
+	)
+	if !supported || !paired.hiddenChanged {
+		t.Fatalf("mixed public and hidden change = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, mappingValue(t, paired.left, "public"), "before")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "public"), "after")
+	assertDiffStringNode(t, mappingValue(t, paired.left, "token"), "[REDACTED]")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "token"), "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotDoesNotFlagUnchangedMaskedValues(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "public: before\ntoken: unchanged\n")
+	right := parseCompositionDocument(t, "right", "public: after\ntoken: unchanged\n")
+	regions, err := prepareDiffRegions(diffTestInspection(left), diffTestInspection(right))
+	if err != nil || len(regions) != 1 || regions[0].path.String() != "/public" {
+		t.Fatalf("changed regions = %#v, err = %v; want only /public", regions, err)
+	}
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Path{}, newDiffSensitivity(map[string]bool{"/token": true}, nil),
+	)
+	if !supported || !paired.leftRedacted || !paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("unchanged masked values affected hidden-change state: %#v, supported=%v", paired, supported)
+	}
+
+	leftSequence := parseCompositionDocument(t, "left-sequence", "items: [unchanged, before]\n")
+	rightSequence := parseCompositionDocument(t, "right-sequence", "items: [unchanged, after]\n")
+	sequence, supported := preparePairedDiffSnapshot(
+		mappingValue(t, leftSequence, "items"), mappingValue(t, rightSequence, "items"), Keys("items"),
+		newDiffSensitivity(nil, map[string]bool{"/items/0": true}),
+	)
+	if !supported || !sequence.leftRedacted || !sequence.rightRedacted || sequence.hiddenChanged {
+		t.Fatalf("unchanged masked sequence member affected hidden-change state: %#v, supported=%v", sequence, supported)
+	}
+}
+
+func TestPreparePairedDiffSnapshotReportsMaskedAdditionAndRemoval(t *testing.T) {
+	value := &document.Node{Kind: document.String, Text: "private"}
+	tests := []struct {
+		name        string
+		left, right *document.Node
+		leftHidden  bool
+		rightHidden bool
+	}{
+		{name: "addition", right: value, rightHidden: true},
+		{name: "removal", left: value, leftHidden: true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			paired, supported := preparePairedDiffSnapshot(
+				test.left, test.right, Keys("token"), newDiffSensitivity(nil, map[string]bool{"/token": true}),
+			)
+			if !supported || !paired.hiddenChanged || paired.leftRedacted != test.leftHidden || paired.rightRedacted != test.rightHidden {
+				t.Fatalf("masked presence change = %#v, supported=%v", paired, supported)
+			}
+			if test.left == nil && paired.left != nil || test.right == nil && paired.right != nil {
+				t.Fatalf("absence became a value: %#v", paired)
+			}
+		})
+	}
+}
+
+func TestPreparePairedDiffSnapshotReportsHiddenSequenceLengthChange(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "items: [public]\n")
+	right := parseCompositionDocument(t, "right", "items: [public, private]\n")
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "items"), mappingValue(t, right, "items"), Keys("items"),
+		newDiffSensitivity(nil, map[string]bool{"/items/1": true}),
+	)
+	if !supported || !paired.hiddenChanged || paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("masked sequence length change = %#v, supported=%v", paired, supported)
+	}
+	if len(paired.left.Sequence) != 1 || len(paired.right.Sequence) != 2 {
+		t.Fatalf("safe sequence lengths = %d, %d; want 1, 2", len(paired.left.Sequence), len(paired.right.Sequence))
+	}
+	assertDiffStringNode(t, paired.right.Sequence[1], "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotClassificationAloneDoesNotCreateHiddenChange(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "token: same\n")
+	right := parseCompositionDocument(t, "right", "token: same\n")
+	regions, err := prepareDiffRegions(diffTestInspection(left), diffTestInspection(right))
+	if err != nil || len(regions) != 0 {
+		t.Fatalf("classification-independent changed regions = %#v, err = %v", regions, err)
+	}
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "token"), mappingValue(t, right, "token"), Keys("token"),
+		newDiffSensitivity(map[string]bool{"/token": true}, nil),
+	)
+	if !supported || !paired.leftRedacted || !paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("classification alone reported hidden change: %#v, supported=%v", paired, supported)
+	}
+}

internal/engine/diff_operation_test.go

@@ -0,0 +1,282 @@
+package engine
+
+import (
+	"bytes"
+	"errors"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type diffIntegrationConfig struct {
+	Defaulted  string         `yaml:"defaulted"`
+	Dynamic    map[string]any `yaml:"dynamic"`
+	Credential Secret         `yaml:"credential" configkit:"secret"`
+	Derived    string         `yaml:"derived"`
+}
+
+type diffOptionalSecretConfig struct {
+	Optional *Secret `yaml:"optional" configkit:"secret"`
+}
+
+func TestDiffUsesFinalValuesAndDependencyDerivedSensitivity(t *testing.T) {
+	var resolveCalls int
+	options := Options[diffIntegrationConfig]{Definition: Definition[diffIntegrationConfig]{
+		Defaults: func() diffIntegrationConfig {
+			return diffIntegrationConfig{Defaulted: "from-default", Dynamic: map[string]any{"defaulted": "retained"}}
+		},
+		Resolve: func(configuration *diffIntegrationConfig, resolution *Resolution) error {
+			resolveCalls++
+			configuration.Derived = configuration.Dynamic["visible"].(string)
+			resolution.Record(Keys("derived"), ChangeNormalization, "derive from effective values", Keys("credential"))
+			return nil
+		},
+	}}
+	input := func(value string) Source {
+		return Bytes("effective.yml", []byte("dynamic: {visible: "+value+"}\ncredential: DIFF_ENGINE_SECRET\n"))
+	}
+	t.Setenv("DIFF_ENGINE_SECRET", "credential-left-canary")
+	left, err := testLoad(input("before"), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("DIFF_ENGINE_SECRET", "credential-right-canary")
+	right, err := testLoad(input("after"), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if left.Config.Defaulted != "from-default" || right.Config.Defaulted != "from-default" {
+		t.Fatalf("defaulted values = %q, %q", left.Config.Defaulted, right.Config.Defaulted)
+	}
+	if !left.Inspection.secrets["/credential"] || !left.Inspection.secrets["/derived"] ||
+		!right.Inspection.secrets["/credential"] || !right.Inspection.secrets["/derived"] {
+		t.Fatalf("typed or dependency-derived sensitivity was not retained: left=%#v right=%#v", left.Inspection.secrets, right.Inspection.secrets)
+	}
+	if node, present := documentAtPathWithLookup(left.Inspection.final, Keys("derived"), left.Inspection.immutableLookup); !present || node.Text != "before" {
+		t.Fatalf("left frozen derived value = %#v, present=%v", node, present)
+	}
+
+	differences, err := Diff(left.Inspection, right.Inspection)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 2 || differences[0].Path.String() != "/derived" || differences[1].Path.String() != "/dynamic/visible" {
+		t.Fatalf("differences = %#v", differences)
+	}
+	if !differences[0].Left.Redacted || !differences[0].Right.Redacted || !differences[0].HiddenChanged ||
+		differences[0].Left.YAML != "'[REDACTED]'\n" || differences[0].Right.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("dependency-derived difference = %#v", differences[0])
+	}
+	if differences[1].Left.Redacted || differences[1].Right.Redacted || differences[1].HiddenChanged ||
+		differences[1].Left.YAML != "before\n" || differences[1].Right.YAML != "after\n" {
+		t.Fatalf("public dynamic difference = %#v", differences[1])
+	}
+	for _, value := range differences {
+		if strings.Contains(value.Left.YAML+value.Right.YAML, "credential-") {
+			t.Fatal("diff snapshot exposed a resolved credential")
+		}
+	}
+
+	var effective bytes.Buffer
+	if err := left.Inspection.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String(), "credential-left-canary") {
+		t.Fatal("standalone effective rendering exposed a credential after diff")
+	}
+	explanation, ok := left.Inspection.Explain(Keys("derived"))
+	if !ok || !explanation.Effective.Redacted || explanation.Effective.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("explanation changed after diff: %#v, ok=%v", explanation, ok)
+	}
+	if _, ok := left.Inspection.Metadata().Lookup(Keys("dynamic", "visible")); !ok {
+		t.Fatal("metadata lookup failed after diff")
+	}
+	if resolveCalls != 2 {
+		t.Fatalf("resolution callbacks ran during diff: %d calls", resolveCalls)
+	}
+
+	// Returned records can be modified without affecting a later comparison.
+	differences[0].Path.segments[0].key = "changed-by-caller"
+	differences[0].Left.YAML = "changed-by-caller"
+	t.Setenv("DIFF_ENGINE_SECRET", "credential-after-load-canary")
+	repeated, err := Diff(left.Inspection, right.Inspection)
+	if err != nil || len(repeated) != 2 || repeated[0].Path.String() != "/derived" || repeated[0].Left.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("repeated diff = %#v, err = %v", repeated, err)
+	}
+	if resolveCalls != 2 {
+		t.Fatalf("repeated diff reran resolution: %d calls", resolveCalls)
+	}
+
+	independent, err := testLoad(input("before"), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resolveCalls != 3 {
+		t.Fatalf("independent load did not retain normal resolution behavior: %d calls", resolveCalls)
+	}
+	if _, err := Diff(left.Inspection, independent.Inspection); err != nil {
+		t.Fatalf("independent load comparison = %v", err)
+	}
+}
+
+func TestDiffUsesReferenceNamesAndFrozenSecretState(t *testing.T) {
+	loadReference := func(name string) *testResult[diffOptionalSecretConfig] {
+		t.Helper()
+		result, err := testLoad[diffOptionalSecretConfig](Bytes("secret-reference.yml", []byte("optional: "+name+"\n")))
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+	t.Setenv("DIFF_ROTATING_SECRET", "first-credential-canary")
+	left := loadReference("DIFF_ROTATING_SECRET")
+	t.Setenv("DIFF_ROTATING_SECRET", "second-credential-canary")
+	right := loadReference("DIFF_ROTATING_SECRET")
+	t.Setenv("DIFF_ROTATING_SECRET", "changed-after-load-canary")
+	differences, err := Diff(left.Inspection, right.Inspection)
+	if err != nil || len(differences) != 0 {
+		t.Fatalf("credential rotation behind one reference = %#v, err = %v", differences, err)
+	}
+	if node, present := documentAtPathWithLookup(left.Inspection.final, Keys("optional"), left.Inspection.immutableLookup); !present || node.Text != "DIFF_ROTATING_SECRET" {
+		t.Fatalf("frozen reference = %#v, present=%v", node, present)
+	}
+	if node, present := documentAtPathWithLookup(right.Inspection.final, Keys("optional"), right.Inspection.immutableLookup); !present || node.Text != "DIFF_ROTATING_SECRET" {
+		t.Fatalf("right frozen reference = %#v, present=%v", node, present)
+	}
+
+	t.Setenv("DIFF_OTHER_SECRET", "other-credential-canary")
+	other := loadReference("DIFF_OTHER_SECRET")
+	differences, err = Diff(left.Inspection, other.Inspection)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/optional" {
+		t.Fatalf("reference-name change = %#v, err = %v", differences, err)
+	}
+	if !differences[0].Left.Present || !differences[0].Right.Present || !differences[0].Left.Redacted ||
+		!differences[0].Right.Redacted || !differences[0].HiddenChanged ||
+		differences[0].Left.YAML != "'[REDACTED]'\n" || differences[0].Right.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("classified reference-name diff = %#v", differences[0])
+	}
+
+	absent, err := testLoad[diffOptionalSecretConfig](Bytes("absent-secret.yml", []byte("{}\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if absent.Config.Optional != nil {
+		t.Fatalf("missing optional reference decoded as %#v", absent.Config.Optional)
+	}
+	if node, present := documentAtPathWithLookup(absent.Inspection.final, Keys("optional"), absent.Inspection.immutableLookup); !present || node.Kind != document.Null {
+		t.Fatalf("optional nil projection = %#v, present=%v", node, present)
+	}
+	differences, err = Diff(absent.Inspection, left.Inspection)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/optional" {
+		t.Fatalf("optional absence diff = %#v, err = %v", differences, err)
+	}
+	if !differences[0].Left.Present || !differences[0].Left.Redacted ||
+		!differences[0].Right.Present || !differences[0].Right.Redacted || !differences[0].HiddenChanged {
+		t.Fatalf("optional reference change snapshots = %#v", differences[0])
+	}
+	for _, value := range differences {
+		if strings.Contains(value.Left.YAML+value.Right.YAML, "credential-canary") {
+			t.Fatal("reference diff exposed a resolved credential")
+		}
+	}
+}
+
+func TestDiffProtectsHistoricalSensitiveRemovalAndReplacement(t *testing.T) {
+	type configuration struct {
+		Dynamic map[string]any `yaml:"dynamic"`
+	}
+	secretPath := Keys("dynamic", "token")
+	secretSelector := func(path Path) bool { return path.String() == secretPath.String() }
+	load := func(input string, resolve func(*configuration, *Resolution) error) *testResult[configuration] {
+		t.Helper()
+		result, err := testLoad(Bytes("historical-secret.yml", []byte(input)), Options[configuration]{Definition: Definition[configuration]{
+			Secret:  secretSelector,
+			Resolve: resolve,
+		}})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+
+	t.Run("removed historical value", func(t *testing.T) {
+		left := load("dynamic: {token: removed-canary}\n", func(configuration *configuration, resolution *Resolution) error {
+			delete(configuration.Dynamic, "token")
+			resolution.Record(secretPath, ChangeNormalization, "remove token")
+			return nil
+		})
+		right := load("dynamic: {token: current-canary}\n", nil)
+		if !left.Inspection.secrets[secretPath.String()] {
+			t.Fatal("removed historical selector was not retained")
+		}
+		if _, present := documentAtPathWithLookup(left.Inspection.final, secretPath, left.Inspection.immutableLookup); present {
+			t.Fatal("removed token remains in the effective left tree")
+		}
+		differences, err := Diff(left.Inspection, right.Inspection)
+		if err != nil || len(differences) != 1 || differences[0].Path.String() != secretPath.String() {
+			t.Fatalf("historical removal diff = %#v, err = %v", differences, err)
+		}
+		if differences[0].Left.Present || !differences[0].Right.Present || !differences[0].Right.Redacted || !differences[0].HiddenChanged {
+			t.Fatalf("historical removal snapshots = %#v", differences[0])
+		}
+		if strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "canary") {
+			t.Fatal("historical removal diff exposed a sensitive value")
+		}
+	})
+
+	t.Run("replaced historical value", func(t *testing.T) {
+		left := load("dynamic: {token: original-canary}\n", func(configuration *configuration, resolution *Resolution) error {
+			configuration.Dynamic["token"] = "replacement-canary"
+			resolution.Record(secretPath, ChangeNormalization, "replace token")
+			return nil
+		})
+		right := load("dynamic: {token: other-canary}\n", nil)
+		if !left.Inspection.secrets[secretPath.String()] {
+			t.Fatal("replaced historical selector was not retained")
+		}
+		differences, err := Diff(left.Inspection, right.Inspection)
+		if err != nil || len(differences) != 1 || differences[0].Path.String() != secretPath.String() {
+			t.Fatalf("historical replacement diff = %#v, err = %v", differences, err)
+		}
+		if !differences[0].Left.Present || !differences[0].Right.Present || !differences[0].Left.Redacted ||
+			!differences[0].Right.Redacted || !differences[0].HiddenChanged {
+			t.Fatalf("historical replacement snapshots = %#v", differences[0])
+		}
+		if strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "canary") {
+			t.Fatal("historical replacement diff exposed a sensitive value")
+		}
+	})
+}
+
+func TestDiffReturnsSafeErrorsWithoutRecords(t *testing.T) {
+	valid := diffTestInspection(&document.Node{Kind: document.String, Text: "safe"})
+	for _, test := range []struct {
+		name        string
+		left, right *Inspection
+	}{
+		{name: "nil left", right: valid},
+		{name: "nil right", left: valid},
+		{name: "missing final tree", left: &Inspection{}, right: valid},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			differences, err := Diff(test.left, test.right)
+			if differences != nil || err == nil || err != errDiffInspectionUnavailable || strings.Contains(err.Error(), "safe") {
+				t.Fatalf("differences, err = %#v, %v", differences, err)
+			}
+		})
+	}
+}
+
+func TestDiffSerializationErrorDoesNotExposeWriterDetails(t *testing.T) {
+	err := writeDiffDocument(diffErrorWriter{}, &document.Node{Kind: document.String, Text: "safe"})
+	if err != errDiffSnapshotUnavailable || strings.Contains(err.Error(), "CANARY") {
+		t.Fatalf("serialization error = %v", err)
+	}
+}
+
+type diffErrorWriter struct{}
+
+func (diffErrorWriter) Write([]byte) (int, error) {
+	return 0, errors.New("CANARY_WRITER_ERROR")
+}

internal/engine/diff_sensitivity.go

@@ -0,0 +1,48 @@
+package engine
+
+type diffSensitivity struct {
+	prefixes pathPrefixIndex
+	pointers []string
+}
+
+func newDiffSensitivity(left, right map[string]bool) *diffSensitivity {
+	sensitivity := &diffSensitivity{}
+	seen := make(map[string]struct{}, len(left)+len(right))
+	for _, classifications := range []map[string]bool{left, right} {
+		for pointer, classified := range classifications {
+			if !classified {
+				continue
+			}
+			if _, found := seen[pointer]; found {
+				continue
+			}
+			seen[pointer] = struct{}{}
+			id := len(sensitivity.pointers)
+			sensitivity.pointers = append(sensitivity.pointers, pointer)
+			sensitivity.prefixes.insert(pointer, id)
+		}
+	}
+	return sensitivity
+}
+
+func (sensitivity *diffSensitivity) isSensitiveAtOrAbove(pointer string) bool {
+	if sensitivity == nil {
+		return false
+	}
+	found := false
+	sensitivity.prefixes.visitAncestors(pointer, func(int) { found = true })
+	return found
+}
+
+func (sensitivity *diffSensitivity) hasStrictDescendant(pointer string) bool {
+	if sensitivity == nil {
+		return false
+	}
+	found := false
+	sensitivity.prefixes.visitDescendants(pointer, func(id int) {
+		if sensitivity.pointers[id] != pointer {
+			found = true
+		}
+	})
+	return found
+}

internal/engine/diff_sensitivity_test.go

@@ -0,0 +1,90 @@
+package engine
+
+import (
+	"maps"
+	"testing"
+)
+
+func TestDiffSensitivityCombinesClassificationsWithoutMutation(t *testing.T) {
+	left := map[string]bool{
+		"/a":                  true,
+		"/a/b":                true,
+		"/historical/removed": true,
+		"/cross~1key":         true,
+		"/literal~01slash":    true,
+		"/items/01":           true,
+		"/items/+1":           false,
+		"/false-left":         false,
+	}
+	right := map[string]bool{
+		"/a/b":         true,
+		"/right-only":  true,
+		"/false-right": false,
+	}
+	leftBefore, rightBefore := maps.Clone(left), maps.Clone(right)
+	sensitivity := newDiffSensitivity(left, right)
+
+	for _, test := range []struct {
+		pointer string
+		want    bool
+	}{
+		{pointer: "/a", want: true},
+		{pointer: "/a/b", want: true},
+		{pointer: "/a/b/c", want: true},
+		{pointer: "/ab", want: false},
+		{pointer: "/ab/c", want: false},
+		{pointer: "/historical/removed", want: true},
+		{pointer: "/historical/removed/child", want: true},
+		{pointer: "/historical", want: false},
+		{pointer: "/right-only", want: true},
+		{pointer: "/false-left", want: false},
+		{pointer: "/false-right", want: false},
+		{pointer: "/cross~1key", want: true},
+		{pointer: "/cross/key", want: false},
+		{pointer: "/literal~01slash", want: true},
+		{pointer: "/literal~1slash", want: false},
+		{pointer: "/items/01", want: true},
+		{pointer: "/items/1", want: false},
+		{pointer: "/items/+1", want: false},
+	} {
+		if got := sensitivity.isSensitiveAtOrAbove(test.pointer); got != test.want {
+			t.Errorf("isSensitiveAtOrAbove(%q) = %v, want %v", test.pointer, got, test.want)
+		}
+	}
+
+	for _, test := range []struct {
+		pointer string
+		want    bool
+	}{
+		{pointer: "", want: true},
+		{pointer: "/a", want: true},
+		{pointer: "/a/b", want: false},
+		{pointer: "/ab", want: false},
+		{pointer: "/historical", want: true},
+		{pointer: "/right-only", want: false},
+		{pointer: "/cross~1key", want: false},
+	} {
+		if got := sensitivity.hasStrictDescendant(test.pointer); got != test.want {
+			t.Errorf("hasStrictDescendant(%q) = %v, want %v", test.pointer, got, test.want)
+		}
+	}
+
+	if !maps.Equal(left, leftBefore) || !maps.Equal(right, rightBefore) {
+		t.Fatal("classification maps were modified")
+	}
+}
+
+func TestDiffSensitivityDistinguishesRootAndEmptyKey(t *testing.T) {
+	root := newDiffSensitivity(map[string]bool{"": true}, nil)
+	if !root.isSensitiveAtOrAbove("") || !root.isSensitiveAtOrAbove("/empty") ||
+		root.hasStrictDescendant("") {
+		t.Fatal("root classification did not remain distinct from strict descendants")
+	}
+
+	emptyKey := newDiffSensitivity(map[string]bool{"/": true}, nil)
+	if emptyKey.isSensitiveAtOrAbove("") || !emptyKey.isSensitiveAtOrAbove("/") ||
+		emptyKey.isSensitiveAtOrAbove("/other") || !emptyKey.hasStrictDescendant("") ||
+		emptyKey.hasStrictDescendant("/") {
+		t.Fatal("empty-key classification was treated as the root or an unrelated path")
+	}
+}

internal/engine/diff_snapshot.go

@@ -0,0 +1,195 @@
+package engine
+
+import (
+	"sort"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type pairedDiffSnapshot struct {
+	left             *document.Node
+	right            *document.Node
+	leftRedacted     bool
+	rightRedacted    bool
+	requiresFallback bool
+	hiddenChanged    bool
+}
+
+func preparePairedDiffSnapshot(left, right *document.Node, path Path, sensitivity *diffSensitivity) (pairedDiffSnapshot, bool) {
+	paired, supported := preparePairedDiffNodes(left, right, path, sensitivity)
+	if !supported || !paired.requiresFallback {
+		return paired, supported
+	}
+	return pairedDiffSnapshot{
+		left:          redactedDiffNode(left),
+		right:         redactedDiffNode(right),
+		leftRedacted:  left != nil,
+		rightRedacted: right != nil,
+		hiddenChanged: true,
+	}, true
+}
+
+func preparePairedDiffNodes(left, right *document.Node, path Path, sensitivity *diffSensitivity) (pairedDiffSnapshot, bool) {
+	if left == nil && right == nil {
+		return pairedDiffSnapshot{}, true
+	}
+	if sensitivity.isSensitiveAtOrAbove(path.String()) {
+		return pairedDiffSnapshot{
+			left:          redactedDiffNode(left),
+			right:         redactedDiffNode(right),
+			leftRedacted:  left != nil,
+			rightRedacted: right != nil,
+			hiddenChanged: !configurationValuesEqual(left, right),
+		}, true
+	}
+	if left != nil && right != nil && left.Kind != right.Kind {
+		if sensitivity.hasStrictDescendant(path.String()) {
+			return pairedDiffSnapshot{requiresFallback: true}, true
+		}
+		return pairedDiffSnapshot{left: left, right: right}, true
+	}
+
+	kind := document.Null
+	if left != nil {
+		kind = left.Kind
+	} else if right != nil {
+		kind = right.Kind
+	}
+	switch kind {
+	case document.Mapping:
+		return preparePairedDiffMapping(left, right, path, sensitivity)
+	case document.Sequence:
+		return preparePairedDiffSequence(left, right, path, sensitivity)
+	default:
+		return pairedDiffSnapshot{left: left, right: right}, true
+	}
+}
+
+func preparePairedDiffMapping(left, right *document.Node, path Path, sensitivity *diffSensitivity) (pairedDiffSnapshot, bool) {
+	leftEntries := diffMappingEntries(left)
+	rightEntries := diffMappingEntries(right)
+	keys := make([]string, 0, len(leftEntries)+len(rightEntries))
+	seen := make(map[string]struct{}, len(leftEntries)+len(rightEntries))
+	for key := range leftEntries {
+		seen[key] = struct{}{}
+		keys = append(keys, key)
+	}
+	for key := range rightEntries {
+		if _, found := seen[key]; found {
+			continue
+		}
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+
+	var leftCopy, rightCopy *document.Node
+	if left != nil {
+		copy := *left
+		copy.Mapping = make([]document.Entry, 0, len(left.Mapping))
+		leftCopy = &copy
+	}
+	if right != nil {
+		copy := *right
+		copy.Mapping = make([]document.Entry, 0, len(right.Mapping))
+		rightCopy = &copy
+	}
+	redacted := pairedDiffSnapshot{}
+	for _, key := range keys {
+		leftEntry, leftPresent := leftEntries[key]
+		rightEntry, rightPresent := rightEntries[key]
+		var leftChild, rightChild *document.Node
+		if leftPresent {
+			leftChild = leftEntry.Value
+		}
+		if rightPresent {
+			rightChild = rightEntry.Value
+		}
+		child, supported := preparePairedDiffNodes(leftChild, rightChild, path.Key(key), sensitivity)
+		if !supported {
+			return pairedDiffSnapshot{}, false
+		}
+		if child.requiresFallback {
+			return pairedDiffSnapshot{requiresFallback: true}, true
+		}
+		if leftPresent {
+			leftEntry.Value = child.left
+			leftCopy.Mapping = append(leftCopy.Mapping, leftEntry)
+		}
+		if rightPresent {
+			rightEntry.Value = child.right
+			rightCopy.Mapping = append(rightCopy.Mapping, rightEntry)
+		}
+		redacted.leftRedacted = redacted.leftRedacted || child.leftRedacted
+		redacted.rightRedacted = redacted.rightRedacted || child.rightRedacted
+		redacted.hiddenChanged = redacted.hiddenChanged || child.hiddenChanged
+	}
+	redacted.left = leftCopy
+	redacted.right = rightCopy
+	return redacted, true
+}
+
+func preparePairedDiffSequence(left, right *document.Node, path Path, sensitivity *diffSensitivity) (pairedDiffSnapshot, bool) {
+	var leftCopy, rightCopy *document.Node
+	if left != nil {
+		copy := *left
+		copy.Sequence = make([]*document.Node, len(left.Sequence))
+		leftCopy = &copy
+	}
+	if right != nil {
+		copy := *right
+		copy.Sequence = make([]*document.Node, len(right.Sequence))
+		rightCopy = &copy
+	}
+	length := 0
+	if left != nil {
+		length = len(left.Sequence)
+	}
+	if right != nil && len(right.Sequence) > length {
+		length = len(right.Sequence)
+	}
+	result := pairedDiffSnapshot{left: leftCopy, right: rightCopy}
+	for index := 0; index < length; index++ {
+		var leftChild, rightChild *document.Node
+		if left != nil && index < len(left.Sequence) {
+			leftChild = left.Sequence[index]
+		}
+		if right != nil && index < len(right.Sequence) {
+			rightChild = right.Sequence[index]
+		}
+		child, supported := preparePairedDiffNodes(leftChild, rightChild, path.Index(index), sensitivity)
+		if !supported {
+			return pairedDiffSnapshot{}, false
+		}
+		if child.requiresFallback {
+			return pairedDiffSnapshot{requiresFallback: true}, true
+		}
+		if left != nil && index < len(left.Sequence) {
+			leftCopy.Sequence[index] = child.left
+		}
+		if right != nil && index < len(right.Sequence) {
+			rightCopy.Sequence[index] = child.right
+		}
+		result.leftRedacted = result.leftRedacted || child.leftRedacted
+		result.rightRedacted = result.rightRedacted || child.rightRedacted
+		result.hiddenChanged = result.hiddenChanged || child.hiddenChanged
+	}
+	return result, true
+}
+
+func diffMappingEntries(node *document.Node) map[string]document.Entry {
+	entries := make(map[string]document.Entry)
+	if node == nil {
+		return entries
+	}
+	for _, entry := range node.Mapping {
+		entries[entry.Key] = entry
+	}
+	return entries
+}
+
+func redactedDiffNode(node *document.Node) *document.Node {
+	if node == nil {
+		return nil
+	}
+	return &document.Node{Kind: document.String, Text: "[REDACTED]"}
+}

internal/engine/diff_snapshot_test.go

@@ -0,0 +1,351 @@
+package engine
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestPreparePairedDiffSnapshotRedactsAlignedMappingFromEitherSide(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "credential: alpha\npublic: before\n")
+	right := parseCompositionDocument(t, "right", "credential: bravo\npublic: after\n")
+	sensitivity := newDiffSensitivity(map[string]bool{"/credential": true}, nil)
+
+	paired, supported := preparePairedDiffSnapshot(left, right, Path{}, sensitivity)
+	if !supported {
+		t.Fatal("compatible mappings were not supported")
+	}
+	if !paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("redacted flags = %v, %v; want true on both sides", paired.leftRedacted, paired.rightRedacted)
+	}
+	assertDiffStringNode(t, mappingValue(t, paired.left, "credential"), "[REDACTED]")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "credential"), "[REDACTED]")
+	assertDiffStringNode(t, mappingValue(t, paired.left, "public"), "before")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "public"), "after")
+	assertDiffStringNode(t, mappingValue(t, left, "credential"), "alpha")
+	assertDiffStringNode(t, mappingValue(t, right, "credential"), "bravo")
+	if paired.left == left || paired.right == right || &paired.left.Mapping[0] == &left.Mapping[0] {
+		t.Fatal("safe mapping containers or entries share mutable storage with retained trees")
+	}
+}
+
+func TestPreparePairedDiffSnapshotPreservesSequencePositionsAndLengths(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "items: [left-public, hidden, old-tail]\n")
+	right := parseCompositionDocument(t, "right", "items: [right-public, hidden, old-tail, extra]\n")
+	sensitivity := newDiffSensitivity(nil, map[string]bool{"/items/1": true})
+
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "items"), mappingValue(t, right, "items"), Keys("items"), sensitivity,
+	)
+	if !supported {
+		t.Fatal("compatible sequences were not supported")
+	}
+	if !paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("redacted flags = %v, %v; want true on both sides", paired.leftRedacted, paired.rightRedacted)
+	}
+	if len(paired.left.Sequence) != 3 || len(paired.right.Sequence) != 4 {
+		t.Fatalf("sequence lengths = %d, %d; want 3, 4", len(paired.left.Sequence), len(paired.right.Sequence))
+	}
+	assertDiffStringNode(t, paired.left.Sequence[0], "left-public")
+	assertDiffStringNode(t, paired.right.Sequence[0], "right-public")
+	assertDiffStringNode(t, paired.left.Sequence[1], "[REDACTED]")
+	assertDiffStringNode(t, paired.right.Sequence[1], "[REDACTED]")
+	assertDiffStringNode(t, paired.right.Sequence[3], "extra")
+	assertDiffStringNode(t, mappingValue(t, left, "items").Sequence[1], "hidden")
+	if &paired.left.Sequence[0] == &mappingValue(t, left, "items").Sequence[0] {
+		t.Fatal("safe sequence shares mutable storage with retained tree")
+	}
+}
+
+func TestPreparePairedDiffSnapshotMasksSensitiveDescendantsOfAbsentSubtrees(t *testing.T) {
+	tests := []struct {
+		name        string
+		left, right string
+		absentSide  string
+	}{
+		{
+			name:       "removed subtree",
+			left:       "removed: {token: old-secret, public: retained}\n",
+			right:      "other: value\n",
+			absentSide: "right",
+		},
+		{
+			name:       "added subtree",
+			left:       "other: value\n",
+			right:      "added: {token: new-secret, public: retained}\n",
+			absentSide: "left",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			left := parseCompositionDocument(t, "left", test.left)
+			right := parseCompositionDocument(t, "right", test.right)
+			paired, supported := preparePairedDiffSnapshot(
+				mappingValueIfPresent(left, "removed", "added"),
+				mappingValueIfPresent(right, "removed", "added"),
+				Keys("subtree"),
+				newDiffSensitivity(map[string]bool{"/subtree/token": true}, nil),
+			)
+			if !supported {
+				t.Fatal("compatible subtree and absence were not supported")
+			}
+			if test.absentSide == "left" {
+				if paired.left != nil || paired.leftRedacted || !paired.rightRedacted {
+					t.Fatalf("paired absence/redaction = %#v", paired)
+				}
+				assertDiffStringNode(t, mappingValue(t, paired.right, "token"), "[REDACTED]")
+				assertDiffStringNode(t, mappingValue(t, paired.right, "public"), "retained")
+			} else {
+				if paired.right != nil || paired.rightRedacted || !paired.leftRedacted {
+					t.Fatalf("paired absence/redaction = %#v", paired)
+				}
+				assertDiffStringNode(t, mappingValue(t, paired.left, "token"), "[REDACTED]")
+				assertDiffStringNode(t, mappingValue(t, paired.left, "public"), "retained")
+			}
+		})
+	}
+}
+
+func TestPreparePairedDiffSnapshotMasksSelectedRootAndContainer(t *testing.T) {
+	tests := []struct {
+		name        string
+		left, right *document.Node
+		path        Path
+		selector    string
+	}{
+		{
+			name:     "root",
+			left:     parseCompositionDocument(t, "left", "token: old\n"),
+			right:    parseCompositionDocument(t, "right", "token: new\n"),
+			path:     Path{},
+			selector: "",
+		},
+		{
+			name:     "container",
+			left:     parseCompositionDocument(t, "left", "group: {public: old}\n"),
+			right:    parseCompositionDocument(t, "right", "group: {public: new}\n"),
+			path:     Keys("group"),
+			selector: "/group",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			paired, supported := preparePairedDiffSnapshot(
+				test.left, test.right, test.path,
+				newDiffSensitivity(map[string]bool{test.selector: true}, nil),
+			)
+			if !supported {
+				t.Fatal("selected nodes were not supported")
+			}
+			if !paired.leftRedacted || !paired.rightRedacted {
+				t.Fatalf("redacted flags = %v, %v; want true on both sides", paired.leftRedacted, paired.rightRedacted)
+			}
+			assertDiffStringNode(t, paired.left, "[REDACTED]")
+			assertDiffStringNode(t, paired.right, "[REDACTED]")
+		})
+	}
+}
+
+func TestPreparePairedDiffSnapshotDoesNotInterpretMarkerLookingInput(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "label: '[REDACTED]'\n")
+	right := parseCompositionDocument(t, "right", "label: '[REDACTED]'\n")
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "label"), mappingValue(t, right, "label"), Keys("label"),
+		newDiffSensitivity(nil, nil),
+	)
+	if !supported {
+		t.Fatal("ordinary scalar values were not supported")
+	}
+	if paired.leftRedacted || paired.rightRedacted {
+		t.Fatalf("redacted flags = %v, %v; want false", paired.leftRedacted, paired.rightRedacted)
+	}
+	if paired.left != mappingValue(t, left, "label") || paired.right != mappingValue(t, right, "label") {
+		t.Fatal("unselected scalar nodes should remain borrowed immutable nodes")
+	}
+	assertDiffStringNode(t, paired.left, "[REDACTED]")
+	assertDiffStringNode(t, paired.right, "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotMasksSensitiveStructuralReplacements(t *testing.T) {
+	tests := []struct {
+		name        string
+		left, right string
+		path        Path
+		selector    string
+	}{
+		{
+			name:     "mapping to scalar",
+			left:     "changed: {token: old-secret, public: old}\n",
+			right:    "changed: new-scalar\n",
+			path:     Keys("changed"),
+			selector: "/changed/token",
+		},
+		{
+			name:     "mapping to scalar with historical-only descendant",
+			left:     "changed: {public: old}\n",
+			right:    "changed: new-scalar\n",
+			path:     Keys("changed"),
+			selector: "/changed/removed/secret",
+		},
+		{
+			name:     "scalar to mapping",
+			left:     "changed: old-scalar\n",
+			right:    "changed: {token: new-secret, public: new}\n",
+			path:     Keys("changed"),
+			selector: "/changed/token",
+		},
+		{
+			name:     "mapping to sequence with numeric paths",
+			left:     "changed: {'0': {token: old-secret}}\n",
+			right:    "changed: [{token: new-secret}]\n",
+			path:     Keys("changed"),
+			selector: "/changed/0/token",
+		},
+		{
+			name:     "sequence to mapping with numeric paths",
+			left:     "changed: [{token: old-secret}]\n",
+			right:    "changed: {'0': {token: new-secret}}\n",
+			path:     Keys("changed"),
+			selector: "/changed/0/token",
+		},
+		{
+			name:     "null to container",
+			left:     "changed: null\n",
+			right:    "changed: {token: new-secret}\n",
+			path:     Keys("changed"),
+			selector: "/changed/token",
+		},
+		{
+			name:     "container to null",
+			left:     "changed: {token: old-secret}\n",
+			right:    "changed: null\n",
+			path:     Keys("changed"),
+			selector: "/changed/token",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			leftRoot := parseCompositionDocument(t, "left", test.left)
+			rightRoot := parseCompositionDocument(t, "right", test.right)
+			left := mappingValue(t, leftRoot, test.path.segments[0].key)
+			right := mappingValue(t, rightRoot, test.path.segments[0].key)
+			paired, supported := preparePairedDiffSnapshot(
+				left, right, test.path, newDiffSensitivity(nil, map[string]bool{test.selector: true}),
+			)
+			assertWholeRegionFallback(t, paired, supported, left, right)
+		})
+	}
+}
+
+func TestPreparePairedDiffSnapshotMasksOriginalRegionForNestedSequenceMismatch(t *testing.T) {
+	leftRoot := parseCompositionDocument(t, "left", "items: [{branch: {token: old-secret, public: old}}]\n")
+	rightRoot := parseCompositionDocument(t, "right", "items: [{branch: new-scalar}]\n")
+	left := mappingValue(t, leftRoot, "items")
+	right := mappingValue(t, rightRoot, "items")
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Keys("items"), newDiffSensitivity(nil, map[string]bool{"/items/0/branch/token": true}),
+	)
+	assertWholeRegionFallback(t, paired, supported, left, right)
+}
+
+func TestPreparePairedDiffSnapshotKeepsPublicScalarsWithHistoricalDescendants(t *testing.T) {
+	leftRoot := parseCompositionDocument(t, "left", "changed: before\n")
+	rightRoot := parseCompositionDocument(t, "right", "changed: after\n")
+	left := mappingValue(t, leftRoot, "changed")
+	right := mappingValue(t, rightRoot, "changed")
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Keys("changed"), newDiffSensitivity(map[string]bool{"/changed/removed/secret": true}, nil),
+	)
+	if !supported || paired.leftRedacted || paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("public scalar replacement = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, paired.left, "before")
+	assertDiffStringNode(t, paired.right, "after")
+}
+
+func TestPreparePairedDiffSnapshotKeepsPublicContentAcrossAbsence(t *testing.T) {
+	right := parseCompositionDocument(t, "right", "branch: scalar\n")
+	paired, supported := preparePairedDiffSnapshot(
+		nil, right, Keys("added"), newDiffSensitivity(nil, map[string]bool{"/added/branch/secret": true}),
+	)
+	if !supported || paired.left != nil || paired.leftRedacted || paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("public addition presence/redaction = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, mappingValue(t, paired.right, "branch"), "scalar")
+}
+
+func TestPreparePairedDiffSnapshotPreservesPublicReplacementBesideSensitiveSibling(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "child: before\n")
+	right := &document.Node{Kind: document.String, Text: "after"}
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "child"), right, Keys("public"),
+		newDiffSensitivity(nil, map[string]bool{"/private/token": true}),
+	)
+	if !supported || paired.left != mappingValue(t, left, "child") || paired.right != right || paired.leftRedacted || paired.rightRedacted {
+		t.Fatalf("unrelated classification changed public replacement: %#v, supported=%v", paired, supported)
+	}
+}
+
+func TestPreparePairedDiffSnapshotTargetsRedactionAcrossAbsence(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "items: []\n")
+	right := parseCompositionDocument(t, "right", "items: [{token: new-secret, public: visible}]\n")
+	leftItems := mappingValue(t, left, "items")
+	rightItems := mappingValue(t, right, "items")
+	paired, supported := preparePairedDiffSnapshot(
+		leftItems, rightItems, Keys("items"), newDiffSensitivity(nil, map[string]bool{"/items/0/token": true}),
+	)
+	if !supported || paired.left == nil || paired.right == nil || paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("targeted sequence redaction = %#v, supported=%v", paired, supported)
+	}
+	if len(paired.left.Sequence) != 0 || len(paired.right.Sequence) != 1 {
+		t.Fatalf("sequence lengths = %d, %d; want 0, 1", len(paired.left.Sequence), len(paired.right.Sequence))
+	}
+	assertDiffStringNode(t, mappingValue(t, paired.right.Sequence[0], "token"), "[REDACTED]")
+	assertDiffStringNode(t, mappingValue(t, paired.right.Sequence[0], "public"), "visible")
+}
+
+func assertWholeRegionFallback(t *testing.T, paired pairedDiffSnapshot, supported bool, left, right *document.Node) {
+	t.Helper()
+	if !supported {
+		t.Fatal("sensitive incompatible shape was not handled")
+	}
+	if !paired.hiddenChanged {
+		t.Fatal("fallback for a changed reported region did not mark hidden change")
+	}
+	if (left == nil) != (paired.left == nil) || (right == nil) != (paired.right == nil) {
+		t.Fatalf("fallback changed root presence: %#v", paired)
+	}
+	if left != nil {
+		assertDiffStringNode(t, paired.left, "[REDACTED]")
+		if !paired.leftRedacted {
+			t.Fatal("present left region was not marked redacted")
+		}
+	}
+	if right != nil {
+		assertDiffStringNode(t, paired.right, "[REDACTED]")
+		if !paired.rightRedacted {
+			t.Fatal("present right region was not marked redacted")
+		}
+	}
+}
+
+func mappingValueIfPresent(node *document.Node, keys ...string) *document.Node {
+	if node == nil {
+		return nil
+	}
+	for _, key := range keys {
+		for _, entry := range node.Mapping {
+			if entry.Key == key {
+				return entry.Value
+			}
+		}
+	}
+	return nil
+}
+
+func assertDiffStringNode(t *testing.T, node *document.Node, want string) {
+	t.Helper()
+	if node == nil || node.Kind != document.String || node.Text != want {
+		t.Fatalf("node = %#v, want string %q", node, want)
+	}
+}

internal/engine/diff_test.go

@@ -0,0 +1,144 @@
+package engine
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestPrepareDiffRegionsUsesCanonicalConfigurationComparison(t *testing.T) {
+	tests := []struct {
+		name  string
+		left  string
+		right string
+		want  []expectedDiffRegion
+	}{
+		{
+			name:  "mapping order is ignored for equal values",
+			left:  "first: 1\nsecond: true\n",
+			right: "second: true\nfirst: 1\n",
+		},
+		{
+			name:  "addition removal null and empty values",
+			left:  "removed: null\n",
+			right: "added: null\nempty_map: {}\nempty_sequence: []\n",
+			want: []expectedDiffRegion{
+				{path: "/added", rightKind: document.Null, rightPresent: true},
+				{path: "/empty_map", rightKind: document.Mapping, rightPresent: true},
+				{path: "/empty_sequence", rightKind: document.Sequence, rightPresent: true},
+				{path: "/removed", leftKind: document.Null, leftPresent: true},
+			},
+		},
+		{
+			name:  "scalar kinds and atomic sequences",
+			left:  "scalar: 1\nitems: [one, two]\n",
+			right: "scalar: '1'\nitems: [one, changed]\n",
+			want: []expectedDiffRegion{
+				{path: "/items", leftKind: document.Sequence, rightKind: document.Sequence, leftPresent: true, rightPresent: true},
+				{path: "/scalar", leftKind: document.Number, rightKind: document.String, leftPresent: true, rightPresent: true},
+			},
+		},
+		{
+			name:  "escaped empty and numeric mapping keys sort by pointer",
+			left:  `"": old` + "\n'0': old\n'01': old\n'a/b': old\n'a~b': old\n",
+			right: `"": new` + "\n'0': new\n'01': new\n'a/b': new\n'a~b': new\n",
+			want: []expectedDiffRegion{
+				{path: "/", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+				{path: "/0", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+				{path: "/01", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+				{path: "/a~0b", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+				{path: "/a~1b", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			leftRoot := parseCompositionDocument(t, "left", test.left)
+			rightRoot := parseCompositionDocument(t, "right", test.right)
+			regions, err := prepareDiffRegions(diffTestInspection(leftRoot), diffTestInspection(rightRoot))
+			if err != nil {
+				t.Fatal(err)
+			}
+			if len(regions) != len(test.want) {
+				t.Fatalf("regions = %#v, want %#v", regions, test.want)
+			}
+			for index, want := range test.want {
+				region := regions[index]
+				if region.path.String() != want.path {
+					t.Fatalf("region[%d].path = %q, want %q", index, region.path.String(), want.path)
+				}
+				assertDiffRegionNode(t, "left", region.left, want.leftPresent, want.leftKind)
+				assertDiffRegionNode(t, "right", region.right, want.rightPresent, want.rightKind)
+			}
+		})
+	}
+}
+
+func TestPrepareDiffRegionsHandlesRootReplacementAndBorrowsNodes(t *testing.T) {
+	leftRoot := &document.Node{Kind: document.String, Text: "before"}
+	rightRoot := &document.Node{Kind: document.Mapping, Mapping: []document.Entry{{
+		Key: "value", Value: &document.Node{Kind: document.Number, Text: "1"},
+	}}}
+	regions, err := prepareDiffRegions(diffTestInspection(leftRoot), diffTestInspection(rightRoot))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(regions) != 1 || regions[0].path.String() != "" || regions[0].left != leftRoot || regions[0].right != rightRoot {
+		t.Fatalf("root region = %#v", regions)
+	}
+
+	leftRoot = parseCompositionDocument(t, "left-child", "value: before\n")
+	rightRoot = parseCompositionDocument(t, "right-child", "value: after\n")
+	regions, err = prepareDiffRegions(diffTestInspection(leftRoot), diffTestInspection(rightRoot))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(regions) != 1 || regions[0].left != mappingValue(t, leftRoot, "value") || regions[0].right != mappingValue(t, rightRoot, "value") {
+		t.Fatalf("child nodes were not borrowed from the frozen trees: %#v", regions)
+	}
+}
+
+func TestPrepareDiffRegionsRejectsUnavailableInspections(t *testing.T) {
+	valid := diffTestInspection(&document.Node{Kind: document.String, Text: "safe"})
+	tests := []struct {
+		name        string
+		left, right *Inspection
+	}{
+		{name: "nil left", left: nil, right: valid},
+		{name: "nil right", left: valid, right: nil},
+		{name: "missing left tree", left: &Inspection{}, right: valid},
+		{name: "missing right tree", left: valid, right: &Inspection{}},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			regions, err := prepareDiffRegions(test.left, test.right)
+			if err == nil || regions != nil {
+				t.Fatalf("regions, err = %#v, %v", regions, err)
+			}
+		})
+	}
+}
+
+type expectedDiffRegion struct {
+	path                      string
+	leftKind, rightKind       document.Kind
+	leftPresent, rightPresent bool
+}
+
+func diffTestInspection(root *document.Node) *Inspection {
+	return &Inspection{final: root, immutableLookup: buildDocumentLookup(root)}
+}
+
+func assertDiffRegionNode(t *testing.T, side string, node *document.Node, wantPresent bool, wantKind document.Kind) {
+	t.Helper()
+	if !wantPresent {
+		if node != nil {
+			t.Fatalf("%s node = %#v, want absence", side, node)
+		}
+		return
+	}
+	if node == nil || node.Kind != wantKind {
+		t.Fatalf("%s node = %#v, want kind %v", side, node, wantKind)
+	}
+}

internal/engine/directives.go

@@ -10,10 +10,11 @@ import (
 const directiveKey = "configkit"
 
 type importDeclaration struct {
-	path     string
-	glob     string
-	optional bool
-	location Location
+	path        string
+	glob        string
+	destination Path
+	optional    bool
+	location    Location
 }
 
 func splitImportDirectives(root *document.Node) (*document.Node, []importDeclaration, []Diagnostic) {
@@ -90,6 +91,27 @@ func parseImportDeclaration(node *document.Node, declarationPath Path) (importDe
 				return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "optional must be a boolean", entry.Value)}
 			}
 			declaration.optional = entry.Value.Text == "true"
+		case "destination":
+			if entry.Value.Kind != document.Sequence {
+				return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "destination must be a sequence of strings", entry.Value)}
+			}
+			relative := Path{segments: make([]pathSegment, 0, len(entry.Value.Sequence))}
+			for index, item := range entry.Value.Sequence {
+				memberPath := fieldPath.Index(index)
+				if item.Kind != document.String {
+					return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, memberPath, "destination members must be strings", item)}
+				}
+				member := Path{segments: []pathSegment{{kind: pathSegmentKey, key: item.Text}}}
+				if _, err := normalizeDestination(member); err != nil {
+					return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, memberPath, "destination members must be non-reserved mapping keys", item)}
+				}
+				relative.segments = append(relative.segments, pathSegment{kind: pathSegmentKey, key: item.Text})
+			}
+			normalized, err := normalizeDestination(relative)
+			if err != nil {
+				return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "destination must contain only non-reserved mapping keys", entry.Value)}
+			}
+			declaration.destination = normalized
 		default:
 			return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "import declaration field is not recognized", entry.Value)}
 		}

internal/engine/document_compare.go

@@ -24,12 +24,23 @@ type documentDelta struct {
 func compareConfigurationValues(before, after *document.Node) []documentDelta {
 	var deltas []documentDelta
 	compareConfigurationValue(before, after, Path{}, &deltas)
-	slices.SortFunc(deltas, func(left, right documentDelta) int {
-		if order := cmp.Compare(left.path.String(), right.path.String()); order != 0 {
+	type orderedDelta struct {
+		delta   documentDelta
+		pointer string
+	}
+	ordered := make([]orderedDelta, len(deltas))
+	for i, delta := range deltas {
+		ordered[i] = orderedDelta{delta: delta, pointer: delta.path.String()}
+	}
+	slices.SortFunc(ordered, func(left, right orderedDelta) int {
+		if order := cmp.Compare(left.pointer, right.pointer); order != 0 {
 			return order
 		}
-		return int(left.kind) - int(right.kind)
+		return int(left.delta.kind) - int(right.delta.kind)
 	})
+	for i, item := range ordered {
+		deltas[i] = item.delta
+	}
 	return deltas
 }
 

internal/engine/document_lookup.go

@@ -0,0 +1,54 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// documentLookup contains read-only mapping positions for a completed document.
+type documentLookup struct {
+	byNode map[*document.Node]map[string]int
+}
+
+func buildDocumentLookup(roots ...*document.Node) *documentLookup {
+	lookup := &documentLookup{byNode: make(map[*document.Node]map[string]int)}
+	visited := make(map[*document.Node]struct{})
+	var visit func(*document.Node)
+	visit = func(node *document.Node) {
+		if node == nil {
+			return
+		}
+		if _, ok := visited[node]; ok {
+			return
+		}
+		visited[node] = struct{}{}
+		switch node.Kind {
+		case document.Mapping:
+			entries := make(map[string]int, len(node.Mapping))
+			for index, entry := range node.Mapping {
+				if _, exists := entries[entry.Key]; !exists {
+					entries[entry.Key] = index
+				}
+				visit(entry.Value)
+			}
+			lookup.byNode[node] = entries
+		case document.Sequence:
+			for _, child := range node.Sequence {
+				visit(child)
+			}
+		}
+	}
+	for _, root := range roots {
+		visit(root)
+	}
+	return lookup
+}
+
+func (lookup *documentLookup) find(node *document.Node, key string) int {
+	if lookup == nil {
+		return -1
+	}
+	if entries := lookup.byNode[node]; entries != nil {
+		if index, ok := entries[key]; ok {
+			return index
+		}
+	}
+	return -1
+}

internal/engine/document_walk.go

@@ -31,6 +31,10 @@ func indexDocument(node *document.Node, path Path, index map[string]*document.No
 }
 
 func documentAtPath(root *document.Node, path Path) (*document.Node, bool) {
+	return documentAtPathWithLookup(root, path, nil)
+}
+
+func documentAtPathWithLookup(root *document.Node, path Path, lookup *documentLookup) (*document.Node, bool) {
 	if root == nil || !path.valid() {
 		return nil, false
 	}
@@ -41,17 +45,21 @@ func documentAtPath(root *document.Node, path Path) (*document.Node, bool) {
 			if segment.kind == pathSegmentIndex {
 				return nil, false
 			}
-			found := false
-			for _, entry := range current.Mapping {
-				if entry.Key == segment.key {
-					current = entry.Value
-					found = true
-					break
+			entryIndex := -1
+			if lookup != nil {
+				entryIndex = lookup.find(current, segment.key)
+			} else {
+				for index, entry := range current.Mapping {
+					if entry.Key == segment.key {
+						entryIndex = index
+						break
+					}
 				}
 			}
-			if !found {
+			if entryIndex < 0 || entryIndex >= len(current.Mapping) {
 				return nil, false
 			}
+			current = current.Mapping[entryIndex].Value
 		case document.Sequence:
 			index, ok := schemaIndex(segment)
 			if !ok || index >= len(current.Sequence) {