Add safe historical configuration explanations

README.md

@@ -12,8 +12,8 @@ then report domain errors and warnings through read-only semantic validation. Se
 tags and a path predicate drive deterministic redacted effective YAML. Typed secret
 references resolve environment names before validation, and explicit
 secrets-directory bootstrap is available as a separate startup operation. Explicit
-CLI override records apply above profiles. Value explanation is still under
+CLI override records apply above profiles. Value explanation is available through
-development.
+detached structured records and deterministic redacted output.
 
 ```go
 type Config struct {

compose.go

@@ -6,12 +6,13 @@ import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
 // precedence layers. Claims remain ordered by source and declaration traversal;
 // origins retain every source that contributes to a composite mapping.
 type composedDocument struct {
-	root       *document.Node
+	root         *document.Node
-	claims     []compositionClaim
+	claims       []compositionClaim
-	origins    map[string]Origin
+	origins      map[string]Origin
-	overlays   []overlayRecord
+	overlays     []overlayRecord
-	inputBytes int64
+	applications []historyApplication
-	inputNodes int
+	inputBytes   int64
+	inputNodes   int
 }
 
 type compositionClaim struct {
@@ -58,10 +59,15 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 	for _, input := range inputs[1:] {
 		mergeDisjointMappings(root, input)
 	}
+	applications := make([]historyApplication, 0, len(inputs))
+	for _, input := range inputs {
+		applications = append(applications, historyApplication{layer: LayerBase, value: input})
+	}
 	return &composedDocument{
-		root:    root,
+		root:         root,
-		claims:  cloneCompositionClaims(collected.ordered),
+		claims:       cloneCompositionClaims(collected.ordered),
-		origins: compositionOrigins(collected.ordered),
+		origins:      compositionOrigins(collected.ordered),
+		applications: applications,
 	}, nil
 }
 

docs/config.md

@@ -141,10 +141,60 @@ translated to a fixed safe error. There is no unredacted rendering option and
 ConfigKit performs no implicit logging.
 
 `Result.Config` is the explicit trusted boundary and contains ordinary typed values,
-including secrets. Metadata, histories, diagnostics, and load errors contain no raw
+including secrets. Metadata records, diagnostics, and load errors contain no
-configuration values. Applications must still keep source labels, resolution
+configuration values. Explanation snapshots deliberately expose non-secret
-reasons, and diagnostic messages value-free because ConfigKit cannot infer and
+configuration history after applying the redaction contract below. Applications
-sanitize secrets embedded in arbitrary application prose.
+must still keep source labels, resolution reasons, and diagnostic messages value-free
+because ConfigKit cannot infer and sanitize secrets embedded in arbitrary application
+prose.
+
+### Historical-value explanation
+
+`Result.Explain(path)` returns a detached `Explanation` for any known effective or
+historical path. Its effective value and each chronological before/after value are
+`ValueSnapshot` records containing deterministic YAML text rather than mutable Go or
+parser values. `Present=false` with empty YAML means absence or deletion; explicit
+null is present and renders as `null`. The history starts with the canonical typed
+application-default or Go-zero baseline, then includes only relevant base sources in
+traversal order, the selected profile, supplied CLI records, and audited net
+resolution operations. Explicit equal writes remain visible.
+
+Base, profile, and CLI history preserves the policy-validated input representation,
+including a superseded value that could not have decoded into the final Go field.
+Baseline, effective, and resolution values use canonical typed projection. Mapping
+queries reconstruct each source or override contribution, while sequences remain
+atomic: a descendant history identifies its presence before and after the containing
+sequence replacement without implying element-wise merging. Removed paths remain
+explainable; unknown and malformed paths return false.
+
+Every snapshot applies the same historical tag, predicate, and recorded-dependency
+sensitivity closure as effective rendering before YAML is exposed. A redacted scalar
+is the quoted marker `'[REDACTED]'`, and a container snapshot has `Redacted=true` if
+it contains a masked descendant. Typed `Secret` snapshots contain only the configured
+environment name. `SecretReference` reports that name only for a successfully
+resolved effective reference, and is omitted when its path is classified. Credential
+values are never retained in explanation history.
+
+`Result.WriteExplanation(output, path)` renders the safe structured result with the
+canonical JSON-pointer path, canonical-versus-input representation labels, effective
+source, composite locations, chronological values, resolution reasons and
+dependencies, and successful secret-reference lookup attribution. Control characters
+in paths, source identifiers and labels, reasons, dependencies, and reference names
+are escaped. An unknown path returns a fixed safe error before writing anything;
+writer errors and short writes are returned and may leave partial output. There is no
+unredacted mode.
+
+Retained history consists of immutable source applications and audited subtree
+deltas, rather than a full tree copy per field. Shared nodes count once toward a
+4,000,000-node retained-history limit; exceeding it fails loading with
+`explain.limit_exceeded`. Replaying one requested path is performed on demand, and
+the returned YAML strings and nested records cannot mutate later explanations.
+
+Application commands should render the separate `SecretsReport` immediately after
+an explicit `LoadSecretsDir` bootstrap, then use `WriteDiagnostics`,
+`WriteEffective`, or `WriteExplanation` on the later load result as appropriate.
+ConfigKit does not infer bootstrap outcomes from a secret reference and does not
+bundle these independent reports into a command framework.
 
 ## Typed secret references
 

docs/development.md

@@ -7,7 +7,7 @@ composition, selected profile overlays, typed defaults, schema-version checks,
 explicit CLI overrides, recorded application resolution, semantic validation,
 layered metadata, secret
 classification, typed environment references, explicit secrets bootstrap, and
-redacted effective rendering. The
+redacted effective rendering and historical value explanation. The
 [component overview](internal/overview.md) owns the implementation inventory.
 
 ## Initial orientation

docs/internal/overview.md

@@ -39,14 +39,16 @@ module and package documentation boundary. Its implemented public primitives are
 - explicit bounded startup environment bootstrap and safe partial reporting in
   `secrets.go`;
 - fixed-point tag/predicate/dependency sensitivity classification in `secret.go`; and
-- frozen-snapshot redaction and canonical effective output in `render.go`, with the
+- frozen-snapshot redaction and canonical effective output in `render.go`;
+- bounded immutable input applications, safe historical replay, detached explanation
+  records, and deterministic human rendering in `explain.go`, with the
   only upstream dumper integration isolated in `internal/yaml/write.go`.
 
 The [configuration contract](../config.md) owns implemented input, schema, decoding,
 loading lifecycle, path imports, composition, profile/CLI overlay, version checking,
 recorded resolution, typed secret references/bootstrap, semantic validation,
-metadata, secret classification, redacted effective rendering, and ownership
+metadata, secret classification, redacted effective rendering, historical
-behavior.
+explanation, and ownership behavior.
 
 The source-extraction inventory records the application foundations assigned to
 future implementation work:

docs/roadmap/implementation.md

@@ -1365,7 +1365,43 @@ Acceptance:
 
 ## Stage 19 — Expose safe historical-value explanation
 
-Status: pending.
+Status: complete.
+
+Completion evidence (2026-09-06):
+
+- Added detached `ValueSnapshot`, `ValueChange`, `SecretReferenceInfo`, and
+  `Explanation` records plus `Result.Explain` and `Result.WriteExplanation` in
+  [`explain.go`](../../explain.go). Effective/default/domain values use canonical
+  typed projection; base/profile/CLI events preserve policy-validated input, with
+  explicit absent, null, equal-write, replacement, and deletion states.
+- Extended composition, profile, and CLI handling to retain immutable applications
+  in traversal/precedence order. Explanation replays those applications on demand
+  with mapping recursion and atomic sequence replacement, then applies the existing
+  audited resolution subtree deltas. This reconstructs container and descendant
+  histories without a full retained tree per field, including superseded input that
+  could not decode into the final destination type.
+- Applied tag, predicate, and dependency-cycle sensitivity across final and
+  historical application paths before snapshot projection. Snapshots recursively
+  redact classified descendants, resolved `Secret` metadata retains only successful
+  effective environment names, and classified references omit that annotation.
+  Shared immutable nodes count once toward a fixed 4,000,000-node retained-history
+  budget; excess fails safely with `explain.limit_exceeded`.
+- Added deterministic human rendering with canonical quoted pointer paths, effective
+  and composite source locations/labels, representation labels, chronological
+  before/after YAML, reasons, dependencies, and secret lookup attribution. Unknown
+  paths write nothing; writer errors and short writes propagate. Added executable
+  `ExampleResult_Explain` and moved normative behavior to the
+  [current explanation contract](../config.md#historical-value-explanation), keeping
+  bootstrap reporting explicitly separate.
+- Added focused coverage in [`explain_test.go`](../../explain_test.go) for the
+  imported-default/profile sequence 4 → 8 → 1, CLI and normalization, equal writes,
+  removed paths, parent replacement, composite mappings, atomic sequences,
+  superseded invalid input, null/empty values, successful and classified references,
+  historical/dependency-cycle canaries, returned-record mutation, deterministic and
+  failing writers, structural-sharing budgets, and an end-to-end secrets-bootstrap/
+  load/explain flow. Verified `go test ./...`, `go test -race ./...`, `go vet ./...`,
+  `go build ./...`, `go mod tidy -diff`, and `git diff --check` with Go 1.26.7 on
+  linux/amd64. No deviations were required; consumer ergonomics remain later work.
 
 Read: [explanation contract](initial.md#historical-value-explanation), redaction
 rules, and history bounds above.

docs/roadmap/initial.md

@@ -916,7 +916,9 @@ the [current redaction contract](../config.md#secret-classification-and-effectiv
 `Result.Config`, `Metadata`, `Sources`, `Diagnostics`, and `WriteEffective`, together
 with `LoadError` and `WriteDiagnostics`, are implemented at the current loading boundary.
 The [current loading contract](../config.md#single-source-loading) and Go reference
-comments own their behavior. Historical-value explanation below remains target work.
+comments own their behavior. Historical-value explanation is also implemented and
+owned by the
+[current explanation contract](../config.md#historical-value-explanation).
 
 ```go
 type Result[T any] struct {
@@ -976,74 +978,14 @@ func InspectPipeline(path, profile string, output io.Writer) error {
 
 Applications can use `Metadata().Lookup`, `Metadata().History`, and `Sources()`
 for structured inspection without implementing value rendering or importing a
-parser. The explanation API below supplies safe values and formatting for application-owned
+parser. The implemented explanation API supplies safe values and formatting for
-`config explain` commands.
+application-owned `config explain` commands.
 
 ### Historical-value explanation
 
-```go
+Historical explanation is implemented. The
-func (r *Result[T]) Explain(path Path) (Explanation, bool)
+[current explanation contract](../config.md#historical-value-explanation) owns its
-func (r *Result[T]) WriteExplanation(output io.Writer, path Path) error
+public records, replay ordering, redaction, rendering, and allocation bounds.
-
-type Explanation struct {
-    Path Path
-    Effective ValueSnapshot
-    Origin Origin
-    History []ValueChange
-    SecretReference *SecretReferenceInfo
-}
-type ValueSnapshot struct {
-    Present bool
-    Redacted bool
-    YAML string
-}
-type ValueChange struct {
-    Before, After ValueSnapshot
-    Origin Origin
-    Reasons []string
-    Dependencies []Path
-}
-type SecretReferenceInfo struct { Name string } // successful resolved lookup
-```
-
-Snapshots contain detached deterministic YAML text, never `any`, parser objects,
-or mutable internal trees. `Present=false` is absence/deletion and has empty YAML;
-null is present with `null` YAML. Redacted snapshots use the quoted marker
-`[REDACTED]`; `Redacted` also marks containers containing redacted descendants.
-Apply the same historical sensitivity/dependency closure as effective rendering
-before exposing any snapshots. A `Secret` projects only its environment name;
-its resolved value is never captured, even privately as history. For classified
-reference paths omit `SecretReference` as well as masking the projected name.
-
-Retain private persistent node snapshots or operation deltas with structural
-sharing; do not copy the entire tree for every changed leaf. Explain a known final
-or historical path, including removed paths; unknown paths return false, and the
-writer returns a safe unknown-path error without output. Malformed query paths
-also fail safely. Render the canonical JSON-pointer path, effective value/source,
-and chronological history: initial application-default (or Go-zero baseline),
-base contributions in traversal order, profile, CLI, and recorded domain changes.
-Include before/after values for changed or explicitly redefined paths, equal-value
-writes, removals, reasons/dependencies, and composite origins. Inputs overridden
-before decoding retain policy-validated node values even when they would not fit
-the final Go type. Defaults/final/domain snapshots use canonical typed projection;
-label those representations rather than claiming every historical input was typed.
-
-Container queries reconstruct the value at each relevant operation; descendant
-queries include ancestor replacement/removal and whole-sequence replacements,
-without implying element-wise sequence merging. Omitted fields/default retention
-must agree with the presence/default contract; do not fabricate a write for every
-import that leaves a path alone. Resolution history records the audited net change
-per path, not every intermediate Go assignment. Lookup of a secret reference
-annotates its effective name as resolved from the process environment; bootstrap
-file outcomes come from the separate report and are never guessed.
-
-`WriteExplanation` is a deterministic human renderer over this safe structured
-result. Escape control characters in paths, labels, names, and reasons, propagate
-writer errors/short writes, and permit partial output only on writer failure.
-No unredacted switch or bundled command is provided. For example an application
-can call it for `/transcription/concurrency` to show default `4`, an explicitly
-imported file's `8`, then a testing profile's `1`, with file line numbers and the
-profile as effective source. This does not require automatic `conf.d` discovery.
 
 ### Module-specific options
 

example_test.go

@@ -134,3 +134,17 @@ func ExampleOverride() {
 	fmt.Println(result.Config.Workers, result.Sources()[0].Label, err)
 	// Output: 0 --workers <nil>
 }
+
+func ExampleResult_Explain() {
+	type Config struct {
+		Workers int `yaml:"workers"`
+	}
+	result, err := configkit.Load(configkit.Bytes("config.yml", []byte("workers: 8\n")), configkit.Options[Config]{
+		Definition: configkit.Definition[Config]{Defaults: func() Config { return Config{Workers: 4} }},
+	})
+	explanation, ok := result.Explain(configkit.Keys("workers"))
+	fmt.Printf("%s -> %s known=%t error=%v\n",
+		strings.TrimSpace(explanation.History[0].After.YAML),
+		strings.TrimSpace(explanation.Effective.YAML), ok, err)
+	// Output: 4 -> 8 known=true error=<nil>
+}

explain.go

@@ -0,0 +1,507 @@
+package configkit
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"slices"
+	"strconv"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+const historyNodeLimit = 4_000_000
+
+var (
+	errExplanationUnavailable = errors.New("configkit: configuration explanation is unavailable")
+	errExplanationUnknownPath = errors.New("configkit: configuration explanation path is unknown")
+)
+
+// ValueSnapshot is a detached, deterministic YAML representation of one value.
+// Present distinguishes absence or deletion from an explicit null value.
+type ValueSnapshot struct {
+	Present  bool
+	Redacted bool
+	YAML     string
+}
+
+// ValueChange describes one explicit definition or audited domain operation.
+type ValueChange struct {
+	Before       ValueSnapshot
+	After        ValueSnapshot
+	Origin       Origin
+	Reasons      []string
+	Dependencies []Path
+}
+
+// SecretReferenceInfo describes a successfully resolved effective reference.
+// Name is the environment-variable name, never its value.
+type SecretReferenceInfo struct {
+	Name string
+}
+
+// Explanation is a detached safe account of one effective or historical path.
+type Explanation struct {
+	Path            Path
+	Effective       ValueSnapshot
+	Origin          Origin
+	History         []ValueChange
+	SecretReference *SecretReferenceInfo
+}
+
+type historyApplication struct {
+	layer  Layer
+	path   Path
+	value  *document.Node
+	origin Origin
+}
+
+func cloneHistoryApplications(values []historyApplication) []historyApplication {
+	if values == nil {
+		return nil
+	}
+	cloned := make([]historyApplication, len(values))
+	for index, value := range values {
+		cloned[index] = historyApplication{
+			layer: value.layer, path: value.path.clone(), value: value.value,
+			origin: cloneOrigin(value.origin),
+		}
+	}
+	return cloned
+}
+
+func cloneSecretReferences(values map[string]string) map[string]string {
+	if values == nil {
+		return nil
+	}
+	cloned := make(map[string]string, len(values))
+	for path, name := range values {
+		cloned[path] = name
+	}
+	return cloned
+}
+
+// Explain returns safe effective and chronological values for a known final or
+// historical path. Unknown and malformed paths return false.
+func (r *Result[T]) Explain(path Path) (Explanation, bool) {
+	if r == nil || r.final == nil || r.baseline == nil || !path.valid() {
+		return Explanation{}, false
+	}
+	explanation := Explanation{Path: path.clone(), Effective: valueSnapshot(r.final, path, r.secrets)}
+	if info, ok := r.metadata.Lookup(path); ok {
+		explanation.Origin = cloneOrigin(info.Origin)
+	}
+
+	working := document.Clone(r.baseline)
+	if _, present := documentAtPath(working, path); present {
+		explanation.History = append(explanation.History, ValueChange{
+			After: valueSnapshot(working, path, r.secrets), Origin: Origin{Layer: LayerDefault},
+		})
+	}
+	for _, application := range r.applications {
+		operationPath, relevant := applicationOperationPath(application, path)
+		var before ValueSnapshot
+		if relevant {
+			before = valueSnapshot(working, path, r.secrets)
+		}
+		applyHistoryApplication(&working, application)
+		if !relevant {
+			continue
+		}
+		explanation.History = append(explanation.History, ValueChange{
+			Before: before,
+			After:  valueSnapshot(working, path, r.secrets),
+			Origin: historyApplicationOrigin(application, operationPath),
+		})
+	}
+
+	working = document.Clone(r.input)
+	for _, snapshot := range r.resolution {
+		relevant := pathsIntersect(snapshot.change.Path, path)
+		var before ValueSnapshot
+		if relevant {
+			before = valueSnapshot(working, path, r.secrets)
+		}
+		replaceHistoryPath(&working, snapshot.change.Path, snapshot.after)
+		if !relevant {
+			continue
+		}
+		explanation.History = append(explanation.History, ValueChange{
+			Before:       before,
+			After:        valueSnapshot(working, path, r.secrets),
+			Origin:       cloneOrigin(snapshot.change.Origin),
+			Reasons:      append([]string(nil), snapshot.change.Reasons...),
+			Dependencies: clonePaths(snapshot.change.Dependencies),
+		})
+	}
+
+	if !explanation.Effective.Present && len(explanation.History) == 0 {
+		return Explanation{}, false
+	}
+	if name, ok := r.secretReferences[path.String()]; ok && !pathIsSecret(r.secrets, path) {
+		explanation.SecretReference = &SecretReferenceInfo{Name: name}
+	}
+	return explanation, true
+}
+
+func applicationOperationPath(application historyApplication, query Path) (Path, bool) {
+	if !application.path.valid() || application.value == nil {
+		return Path{}, false
+	}
+	if application.path.String() != "" {
+		if pathsIntersect(application.path, query) {
+			return application.path.clone(), true
+		}
+		return Path{}, false
+	}
+	if len(query.segments) == 0 {
+		return Path{}, true
+	}
+	for length := 1; length <= len(query.segments); length++ {
+		prefix := Path{segments: clonePathSegments(query.segments[:length], 0)}
+		node, present := documentAtPath(application.value, prefix)
+		if !present {
+			return Path{}, false
+		}
+		if length == len(query.segments) || node.Kind != document.Mapping {
+			return prefix, true
+		}
+	}
+	return Path{}, false
+}
+
+func historyApplicationOrigin(application historyApplication, operationPath Path) Origin {
+	if application.origin.Layer != "" {
+		return cloneOrigin(application.origin)
+	}
+	origin := Origin{Layer: application.layer}
+	relative, ok := relativeHistoryPath(application.path, operationPath)
+	if !ok {
+		return origin
+	}
+	if node, present := documentAtPath(application.value, relative); present {
+		origin.Locations = []Location{publicLocation(node.Location)}
+	}
+	return origin
+}
+
+func relativeHistoryPath(root, path Path) (Path, bool) {
+	if !pathContains(root, path) {
+		return Path{}, false
+	}
+	return Path{segments: clonePathSegments(path.segments[len(root.segments):], 0)}, true
+}
+
+func pathsIntersect(left, right Path) bool {
+	return pathContains(left, right) || pathContains(right, left)
+}
+
+func applyHistoryApplication(root **document.Node, application historyApplication) {
+	if application.path.String() == "" {
+		mergeHistoryDocument(root, application.value)
+		return
+	}
+	mergeHistoryAtPath(root, application.path, application.value)
+}
+
+func mergeHistoryDocument(target **document.Node, incoming *document.Node) {
+	if incoming == nil {
+		return
+	}
+	if *target == nil || (*target).Kind != document.Mapping || incoming.Kind != document.Mapping {
+		*target = document.Clone(incoming)
+		return
+	}
+	for _, entry := range incoming.Mapping {
+		position := documentEntryIndex((*target).Mapping, entry.Key)
+		if position < 0 {
+			cloned := entry
+			cloned.Value = document.Clone(entry.Value)
+			(*target).Mapping = append((*target).Mapping, cloned)
+			continue
+		}
+		mergeHistoryDocument(&(*target).Mapping[position].Value, entry.Value)
+	}
+}
+
+func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node) {
+	if root == nil || *root == nil || len(path.segments) == 0 {
+		mergeHistoryDocument(root, incoming)
+		return
+	}
+	current := *root
+	for index, segment := range path.segments {
+		if current.Kind != document.Mapping || segment.kind == pathSegmentIndex {
+			return
+		}
+		position := documentEntryIndex(current.Mapping, segment.key)
+		if index == len(path.segments)-1 {
+			if position < 0 {
+				current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: document.Clone(incoming)})
+			} else {
+				mergeHistoryDocument(&current.Mapping[position].Value, incoming)
+			}
+			return
+		}
+		if position < 0 {
+			child := &document.Node{Kind: document.Mapping}
+			current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: child})
+			current = child
+		} else {
+			current = current.Mapping[position].Value
+		}
+	}
+}
+
+func replaceHistoryPath(root **document.Node, path Path, replacement *document.Node) {
+	if root == nil || *root == nil {
+		return
+	}
+	if len(path.segments) == 0 {
+		*root = document.Clone(replacement)
+		return
+	}
+	current := *root
+	for index, segment := range path.segments {
+		last := index == len(path.segments)-1
+		switch current.Kind {
+		case document.Mapping:
+			if segment.kind == pathSegmentIndex {
+				return
+			}
+			position := documentEntryIndex(current.Mapping, segment.key)
+			if position < 0 {
+				return
+			}
+			if last {
+				if replacement == nil {
+					current.Mapping = append(current.Mapping[:position], current.Mapping[position+1:]...)
+				} else {
+					current.Mapping[position].Value = document.Clone(replacement)
+				}
+				return
+			}
+			current = current.Mapping[position].Value
+		case document.Sequence:
+			position, ok := schemaIndex(segment)
+			if !ok || position >= len(current.Sequence) {
+				return
+			}
+			if last {
+				if replacement != nil {
+					current.Sequence[position] = document.Clone(replacement)
+				}
+				return
+			}
+			current = current.Sequence[position]
+		default:
+			return
+		}
+	}
+}
+
+func valueSnapshot(root *document.Node, path Path, secrets map[string]bool) ValueSnapshot {
+	node, present := documentAtPath(root, path)
+	if !present {
+		return ValueSnapshot{}
+	}
+	safe, redacted := safeSnapshotDocument(node, secrets, path)
+	var output bytes.Buffer
+	if err := yamladapter.Write(&output, safe); err != nil {
+		return ValueSnapshot{Present: true, Redacted: true, YAML: "'[REDACTED]'\n"}
+	}
+	return ValueSnapshot{Present: true, Redacted: redacted, YAML: output.String()}
+}
+
+func safeSnapshotDocument(node *document.Node, secrets map[string]bool, path Path) (*document.Node, bool) {
+	if node == nil {
+		return nil, false
+	}
+	if pathIsSecret(secrets, path) {
+		return &document.Node{Kind: document.String, Text: "[REDACTED]"}, true
+	}
+	result := *node
+	redacted := false
+	switch node.Kind {
+	case document.Mapping:
+		result.Mapping = make([]document.Entry, len(node.Mapping))
+		copy(result.Mapping, node.Mapping)
+		sortDocumentMapping(result.Mapping)
+		for index := range result.Mapping {
+			entry := &result.Mapping[index]
+			entry.Value, redacted = safeSnapshotChild(entry.Value, secrets, path.Key(entry.Key), redacted)
+		}
+	case document.Sequence:
+		result.Sequence = make([]*document.Node, len(node.Sequence))
+		for index, child := range node.Sequence {
+			result.Sequence[index], redacted = safeSnapshotChild(child, secrets, path.Index(index), redacted)
+		}
+	}
+	return &result, redacted
+}
+
+func safeSnapshotChild(node *document.Node, secrets map[string]bool, path Path, prior bool) (*document.Node, bool) {
+	safe, redacted := safeSnapshotDocument(node, secrets, path)
+	return safe, prior || redacted
+}
+
+func sortDocumentMapping(entries []document.Entry) {
+	slices.SortFunc(entries, func(left, right document.Entry) int {
+		return strings.Compare(left.Key, right.Key)
+	})
+}
+
+func historyNodesWithinLimit(baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
+	return historyNodesWithin(historyNodeLimit, baseline, input, final, applications, resolutions)
+}
+
+func historyNodesWithin(limit int, baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
+	seen := make(map[*document.Node]bool)
+	count := 0
+	var visit func(*document.Node) bool
+	visit = func(node *document.Node) bool {
+		if node == nil || seen[node] {
+			return true
+		}
+		seen[node] = true
+		count++
+		if count > limit {
+			return false
+		}
+		for _, entry := range node.Mapping {
+			if !visit(entry.Value) {
+				return false
+			}
+		}
+		for _, child := range node.Sequence {
+			if !visit(child) {
+				return false
+			}
+		}
+		return true
+	}
+	if !visit(baseline) || !visit(input) || !visit(final) {
+		return false
+	}
+	for _, application := range applications {
+		if !visit(application.value) {
+			return false
+		}
+	}
+	for _, resolution := range resolutions {
+		if !visit(resolution.before) || !visit(resolution.after) {
+			return false
+		}
+	}
+	return true
+}
+
+// WriteExplanation writes a deterministic human-readable rendering of Explain.
+// Unknown paths produce no output. Writer failures may leave partial output.
+func (r *Result[T]) WriteExplanation(output io.Writer, path Path) error {
+	if r == nil || output == nil {
+		return errExplanationUnavailable
+	}
+	explanation, ok := r.Explain(path)
+	if !ok {
+		return errExplanationUnknownPath
+	}
+	var rendered strings.Builder
+	rendered.WriteString("path: ")
+	rendered.WriteString(strconv.Quote(explanation.Path.String()))
+	rendered.WriteByte('\n')
+	writeExplanationOrigin(&rendered, "effective source", explanation.Origin, r.sources)
+	writeExplanationSnapshot(&rendered, "effective (canonical typed)", explanation.Effective, 0)
+	if explanation.SecretReference != nil {
+		rendered.WriteString("secret reference: ")
+		rendered.WriteString(strconv.Quote(explanation.SecretReference.Name))
+		rendered.WriteString(" (resolved from process environment)\n")
+	}
+	rendered.WriteString("history:\n")
+	for _, change := range explanation.History {
+		rendered.WriteString("  - representation: ")
+		if change.Origin.Layer == LayerBase || change.Origin.Layer == LayerProfile || change.Origin.Layer == LayerCLI {
+			rendered.WriteString("policy-validated input\n")
+		} else {
+			rendered.WriteString("canonical typed\n")
+		}
+		writeExplanationOrigin(&rendered, "    source", change.Origin, r.sources)
+		writeExplanationSnapshot(&rendered, "    before", change.Before, 4)
+		writeExplanationSnapshot(&rendered, "    after", change.After, 4)
+		for _, reason := range change.Reasons {
+			rendered.WriteString("    reason: ")
+			rendered.WriteString(strconv.Quote(reason))
+			rendered.WriteByte('\n')
+		}
+		for _, dependency := range change.Dependencies {
+			rendered.WriteString("    dependency: ")
+			rendered.WriteString(strconv.Quote(dependency.String()))
+			rendered.WriteByte('\n')
+		}
+	}
+	data := []byte(rendered.String())
+	written, err := output.Write(data)
+	if err == nil && written != len(data) {
+		return io.ErrShortWrite
+	}
+	return err
+}
+
+func writeExplanationOrigin(output *strings.Builder, label string, origin Origin, sources []SourceInfo) {
+	output.WriteString(label)
+	output.WriteString(": layer=")
+	output.WriteString(strconv.Quote(string(origin.Layer)))
+	if origin.Operation != "" {
+		output.WriteString(" operation=")
+		output.WriteString(strconv.Quote(string(origin.Operation)))
+	}
+	output.WriteByte('\n')
+	for _, location := range origin.Locations {
+		output.WriteString("      location: source=")
+		output.WriteString(strconv.Quote(location.SourceID))
+		if label := sourceLabel(sources, location.SourceID); label != "" {
+			output.WriteString(" label=")
+			output.WriteString(strconv.Quote(label))
+		}
+		output.WriteString(" line=")
+		output.WriteString(strconv.Itoa(location.Line))
+		output.WriteString(" column=")
+		output.WriteString(strconv.Itoa(location.Column))
+		output.WriteByte('\n')
+	}
+}
+
+func sourceLabel(sources []SourceInfo, sourceID string) string {
+	for _, source := range sources {
+		if source.ID == sourceID {
+			return source.Label
+		}
+	}
+	return ""
+}
+
+func writeExplanationSnapshot(output *strings.Builder, label string, snapshot ValueSnapshot, indent int) {
+	if !snapshot.Present {
+		output.WriteString(label)
+		output.WriteString(": absent\n")
+		return
+	}
+	output.WriteString(label)
+	if snapshot.Redacted {
+		output.WriteString(" (redacted)")
+	}
+	output.WriteString(":\n")
+	prefix := strings.Repeat(" ", indent+2)
+	for _, line := range strings.SplitAfter(snapshot.YAML, "\n") {
+		if line == "" {
+			continue
+		}
+		output.WriteString(prefix)
+		output.WriteString(line)
+		if !strings.HasSuffix(line, "\n") {
+			output.WriteByte('\n')
+		}
+	}
+}

explain_test.go

@@ -0,0 +1,328 @@
+package configkit
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type explanationConcurrency struct {
+	Transcription struct {
+		Concurrency int `yaml:"concurrency"`
+	} `yaml:"transcription"`
+}
+
+func TestExplainShowsDefaultBaseAndEffectiveProfileValue(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "config.yml", "_configkit:\n  imports: [{path: transcription.yml}]\n")
+	writeImportFile(t, directory, "transcription.yml", "transcription:\n  concurrency: 8\n")
+	writeImportFile(t, directory, filepath.Join("profiles", "testing.yml"), "transcription:\n  concurrency: 1\n")
+	result, err := Load[explanationConcurrency](File(mainPath), Options[explanationConcurrency]{
+		Imports: true, Profile: "testing",
+		Definition: Definition[explanationConcurrency]{Defaults: func() explanationConcurrency {
+			value := explanationConcurrency{}
+			value.Transcription.Concurrency = 4
+			return value
+		}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	explanation, ok := result.Explain(Keys("transcription", "concurrency"))
+	if !ok {
+		t.Fatal("known path was not explained")
+	}
+	if got := explanationValues(explanation); !reflect.DeepEqual(got, []string{"4\n", "8\n", "1\n"}) {
+		t.Fatalf("history values = %#v", got)
+	}
+	if explanation.Effective.YAML != "1\n" || explanation.Origin.Layer != LayerProfile ||
+		len(explanation.Origin.Locations) != 1 || explanation.Origin.Locations[0].SourceID != "source-3" ||
+		explanation.Origin.Locations[0].Line != 2 {
+		t.Fatalf("effective explanation = %#v", explanation)
+	}
+}
+
+type explanationConfig struct {
+	Concurrency int               `yaml:"concurrency"`
+	Equal       string            `yaml:"equal"`
+	Limit       int               `yaml:"limit"`
+	Items       []string          `yaml:"items"`
+	Labels      map[string]string `yaml:"labels"`
+	Dynamic     any               `yaml:"dynamic"`
+	Maybe       *string           `yaml:"maybe"`
+	Empty       []string          `yaml:"empty"`
+	Credential  Secret            `yaml:"credential"`
+	Classified  Secret            `yaml:"classified"`
+	DerivedA    string            `yaml:"derived_a"`
+	DerivedB    string            `yaml:"derived_b"`
+}
+
+func TestExplainReplaysCLIResolutionContainersAndSafeHistoricalValues(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "config.yml", `concurrency: 8
+equal: same
+limit: wrong-before-profile
+items: [base, removed]
+labels: {base: retained}
+dynamic: {retired: HISTORICAL_EXPLAIN_CANARY, public: visible}
+credential: APP_SECRET
+classified: APP_SECRET
+`)
+	writeImportFile(t, directory, filepath.Join("profiles", "testing.yml"), `concurrency: 1
+equal: same
+limit: 7
+items: [profile]
+labels: {profile: retained}
+dynamic: selected
+`)
+	result, err := loadWithSecretLookup(File(mainPath), func(name string) (string, bool) {
+		if name != "APP_SECRET" {
+			t.Fatalf("secret lookup = %q", name)
+		}
+		return "RESOLVED_EXPLAIN_CANARY", true
+	}, Options[explanationConfig]{
+		Profile: "testing",
+		Overrides: []Override{
+			{Path: Keys("concurrency"), Value: 2, Flag: "--concurrency"},
+			{Path: Keys("equal"), Value: "same", Flag: "--equal"},
+			{Path: Keys("items"), Value: []any{"cli", "restored"}, Flag: "--items"},
+			{Path: Keys("labels"), Value: map[string]any{"cli": "retained"}, Flag: "--label"},
+		},
+		Definition: Definition[explanationConfig]{
+			Defaults: func() explanationConfig {
+				return explanationConfig{
+					Concurrency: 4, Equal: "same", Labels: map[string]string{"default": "retained"},
+					Maybe: nil, Empty: []string{}, Credential: SecretEnv("APP_SECRET"), Classified: SecretEnv("APP_SECRET"),
+				}
+			},
+			Secret: func(path Path) bool {
+				return path.String() == "/dynamic/retired" || path.String() == "/classified"
+			},
+			Resolve: func(configuration *explanationConfig, resolution *Resolution) error {
+				Normalize(resolution, Keys("concurrency"), &configuration.Concurrency, 3,
+					"normalized\nconcurrency", Keys("labels"))
+				Normalize(resolution, Keys("equal"), &configuration.Equal, "same", "record equal value")
+				Normalize(resolution, Keys("derived_a"), &configuration.DerivedA, "DERIVED_EXPLAIN_A",
+					"derived cycle a", Keys("derived_b"))
+				Normalize(resolution, Keys("derived_b"), &configuration.DerivedB, "DERIVED_EXPLAIN_B",
+					"derived cycle b", Keys("derived_a"), Keys("dynamic"))
+				return nil
+			},
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	concurrency, ok := result.Explain(Keys("concurrency"))
+	if !ok {
+		t.Fatal("concurrency was not explained")
+	}
+	if got := explanationValues(concurrency); !reflect.DeepEqual(got, []string{"4\n", "8\n", "1\n", "2\n", "3\n"}) {
+		t.Fatalf("concurrency history = %#v", got)
+	}
+	if got := historyValueLayers(concurrency); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerBase, LayerProfile, LayerCLI, LayerResolution}) {
+		t.Fatalf("concurrency layers = %#v", got)
+	}
+	if concurrency.History[4].Before.YAML != "2\n" || concurrency.History[4].Reasons[0] != "normalized\nconcurrency" ||
+		concurrency.History[4].Dependencies[0].String() != "/labels" {
+		t.Fatalf("resolution history = %#v", concurrency.History[4])
+	}
+
+	equal, _ := result.Explain(Keys("equal"))
+	if len(equal.History) != 5 {
+		t.Fatalf("equal writes were lost: %#v", equal.History)
+	}
+	for _, change := range equal.History[1:] {
+		if change.Before.YAML != "same\n" || change.After.YAML != "same\n" {
+			t.Fatalf("equal replacement = %#v", change)
+		}
+	}
+
+	limit, _ := result.Explain(Keys("limit"))
+	if got := explanationValues(limit); !reflect.DeepEqual(got, []string{"0\n", "wrong-before-profile\n", "7\n"}) {
+		t.Fatalf("overridden invalid input was not retained: %#v", got)
+	}
+	removed, ok := result.Explain(Keys("dynamic", "retired"))
+	if !ok || removed.Effective.Present || len(removed.History) < 2 {
+		t.Fatalf("removed path = %#v, %v", removed, ok)
+	}
+	for _, change := range removed.History {
+		if strings.Contains(change.Before.YAML+change.After.YAML, "HISTORICAL_EXPLAIN_CANARY") {
+			t.Fatalf("historical secret escaped redaction: %#v", change)
+		}
+	}
+	if !removed.History[0].After.Redacted || removed.History[len(removed.History)-1].After.Present {
+		t.Fatalf("removed secret history = %#v", removed.History)
+	}
+	dynamic, _ := result.Explain(Keys("dynamic"))
+	if len(dynamic.History) < 3 || !dynamic.History[1].After.Redacted ||
+		strings.Contains(dynamic.History[1].After.YAML, "HISTORICAL_EXPLAIN_CANARY") {
+		t.Fatalf("container redaction history = %#v", dynamic.History)
+	}
+
+	items, _ := result.Explain(Keys("items"))
+	if len(items.History) != 4 || !strings.Contains(items.History[1].After.YAML, "removed") ||
+		strings.Contains(items.History[2].After.YAML, "removed") || !strings.Contains(items.History[3].After.YAML, "restored") {
+		t.Fatalf("atomic sequence history = %#v", items.History)
+	}
+	removedIndex, ok := result.Explain(Keys("items").Index(1))
+	if !ok || !removedIndex.History[1].Before.Present || removedIndex.History[1].After.Present ||
+		!removedIndex.History[2].After.Present {
+		t.Fatalf("sequence descendant history = %#v", removedIndex.History)
+	}
+	labels, _ := result.Explain(Keys("labels"))
+	if len(labels.History) != 4 || !strings.Contains(labels.Effective.YAML, "default: retained") ||
+		!strings.Contains(labels.Effective.YAML, "profile: retained") || !strings.Contains(labels.Effective.YAML, "cli: retained") {
+		t.Fatalf("composite mapping history = %#v", labels)
+	}
+	if labels.Origin.Layer != LayerCLI || len(labels.Origin.Locations) != 3 {
+		t.Fatalf("composite effective origin = %#v", labels.Origin)
+	}
+
+	maybe, _ := result.Explain(Keys("maybe"))
+	empty, _ := result.Explain(Keys("empty"))
+	if !maybe.Effective.Present || maybe.Effective.YAML != "null\n" || !empty.Effective.Present || empty.Effective.YAML != "[]\n" {
+		t.Fatalf("null/empty snapshots = maybe %#v empty %#v", maybe.Effective, empty.Effective)
+	}
+	credential, _ := result.Explain(Keys("credential"))
+	if credential.SecretReference == nil || credential.SecretReference.Name != "APP_SECRET" || credential.Effective.YAML != "APP_SECRET\n" {
+		t.Fatalf("secret reference = %#v", credential)
+	}
+	classified, _ := result.Explain(Keys("classified"))
+	if classified.SecretReference != nil || !classified.Effective.Redacted ||
+		strings.Contains(classified.Effective.YAML, "APP_SECRET") {
+		t.Fatalf("classified secret reference = %#v", classified)
+	}
+	var derivedExplanations []Explanation
+	for _, path := range []Path{Keys("derived_a"), Keys("derived_b")} {
+		derived, _ := result.Explain(path)
+		if !derived.Effective.Redacted || strings.Contains(derived.Effective.YAML, "DERIVED_EXPLAIN") {
+			t.Fatalf("dependency closure at %s = %#v", path.String(), derived)
+		}
+		derivedExplanations = append(derivedExplanations, derived)
+	}
+
+	formatted := fmt.Sprintf("%v %#v %v", removed, credential, derivedExplanations)
+	encoded, marshalErr := json.Marshal(append([]Explanation{removed, credential, classified}, derivedExplanations...))
+	if marshalErr != nil || strings.Contains(formatted+string(encoded), "RESOLVED_EXPLAIN_CANARY") ||
+		strings.Contains(formatted+string(encoded), "HISTORICAL_EXPLAIN_CANARY") ||
+		strings.Contains(formatted+string(encoded), "DERIVED_EXPLAIN") {
+		t.Fatalf("public records disclosed a canary: %s %s", formatted, encoded)
+	}
+
+	concurrency.Path.segments[0].key = "mutated"
+	concurrency.History[4].Reasons[0] = "mutated"
+	concurrency.History[4].Dependencies[0].segments[0].key = "mutated"
+	result.Config.Concurrency = 99
+	again, _ := result.Explain(Keys("concurrency"))
+	if again.Path.String() != "/concurrency" || again.Effective.YAML != "3\n" ||
+		again.History[4].Reasons[0] != "normalized\nconcurrency" ||
+		again.History[4].Dependencies[0].String() != "/labels" {
+		t.Fatalf("returned explanation aliased internal history: %#v", again)
+	}
+
+	var first, second bytes.Buffer
+	if err := result.WriteExplanation(&first, Keys("concurrency")); err != nil {
+		t.Fatal(err)
+	}
+	if err := result.WriteExplanation(&second, Keys("concurrency")); err != nil || first.String() != second.String() {
+		t.Fatalf("nondeterministic rendering: %v\n%s\n%s", err, first.String(), second.String())
+	}
+	if strings.Contains(first.String(), "\nconcurrency\"") || !strings.Contains(first.String(), `"normalized\nconcurrency"`) {
+		t.Fatalf("reason controls were not escaped: %q", first.String())
+	}
+	unknownOutput := bytes.NewBufferString("unchanged")
+	if err := result.WriteExplanation(unknownOutput, Keys("unknown")); !errors.Is(err, errExplanationUnknownPath) || unknownOutput.String() != "unchanged" {
+		t.Fatalf("unknown writer = %q, %v", unknownOutput.String(), err)
+	}
+	if _, ok := result.Explain(Keys("items").Index(-1)); ok {
+		t.Fatal("malformed path was explained")
+	}
+	wantWriteError := errors.New("write failed")
+	if err := result.WriteExplanation(errorWriter{err: wantWriteError}, Keys("concurrency")); !errors.Is(err, wantWriteError) {
+		t.Fatalf("writer error = %v", err)
+	}
+	if err := result.WriteExplanation(shortWriter{}, Keys("concurrency")); !errors.Is(err, io.ErrShortWrite) {
+		t.Fatalf("short writer error = %v", err)
+	}
+	var invalidInput bytes.Buffer
+	if err := result.WriteExplanation(&invalidInput, Keys("limit")); err != nil ||
+		!strings.Contains(invalidInput.String(), "policy-validated input") ||
+		strings.Contains(invalidInput.String(), "wrong-before-profile\n    source: layer=\"base\"\n    before (canonical typed)") {
+		t.Fatalf("historical representation rendering = %v\n%s", err, invalidInput.String())
+	}
+}
+
+func TestSecretsBootstrapLoadAndExplanationRemainSeparateAndSafe(t *testing.T) {
+	const name = "CONFIGKIT_EXPLAIN_BOOTSTRAP_TEST"
+	previous, present := os.LookupEnv(name)
+	if err := os.Unsetenv(name); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if present {
+			_ = os.Setenv(name, previous)
+		} else {
+			_ = os.Unsetenv(name)
+		}
+	})
+	directory := t.TempDir()
+	writeSecretTestFile(t, directory, name, "BOOTSTRAP_EXPLAIN_CANARY\n")
+	report, err := LoadSecretsDir(directory)
+	if err != nil || len(report.Entries()) != 1 || report.Entries()[0].Status != SecretFileLoaded {
+		t.Fatalf("bootstrap = %#v, %v", report, err)
+	}
+	type config struct {
+		Credential Secret `yaml:"credential"`
+	}
+	result, err := Load[config](Bytes("config.yml", []byte("credential: "+name+"\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var output bytes.Buffer
+	if err := result.WriteExplanation(&output, Keys("credential")); err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(output.String(), name) || strings.Contains(output.String(), "BOOTSTRAP_EXPLAIN_CANARY") {
+		t.Fatalf("explanation mixed bootstrap value into reference report: %s", output.String())
+	}
+}
+
+func TestHistoryNodeBudgetCountsSharedTreesOnce(t *testing.T) {
+	leaf := &document.Node{Kind: document.String, Text: "value"}
+	root := &document.Node{Kind: document.Mapping, Mapping: []document.Entry{{Key: "value", Value: leaf}}}
+	applications := []historyApplication{{layer: LayerBase, value: root}}
+	if !historyNodesWithin(2, root, root, root, applications, nil) {
+		t.Fatal("structurally shared history was counted more than once")
+	}
+	other := document.Clone(root)
+	if historyNodesWithin(3, root, other, root, applications, nil) {
+		t.Fatal("distinct retained nodes did not consume the budget")
+	}
+}
+
+func explanationValues(explanation Explanation) []string {
+	values := make([]string, len(explanation.History))
+	for index, change := range explanation.History {
+		values[index] = change.After.YAML
+	}
+	return values
+}
+
+func historyValueLayers(explanation Explanation) []Layer {
+	layers := make([]Layer, len(explanation.History))
+	for index, change := range explanation.History {
+		layers[index] = change.Origin.Layer
+	}
+	return layers
+}

load.go

@@ -67,13 +67,16 @@ type Version struct {
 type Result[T any] struct {
 	Config T
 
-	metadata    Metadata
+	metadata         Metadata
-	sources     []SourceInfo
+	sources          []SourceInfo
-	diagnostics []Diagnostic
+	diagnostics      []Diagnostic
-	final       *document.Node
+	baseline         *document.Node
-	resolution  []resolutionSnapshot
+	input            *document.Node
-	overlays    []overlayRecord
+	final            *document.Node
-	secrets     map[string]bool
+	resolution       []resolutionSnapshot
+	applications     []historyApplication
+	secrets          map[string]bool
+	secretReferences map[string]string
 }
 
 // Metadata returns detached effective presence, origin, and history records.
@@ -239,6 +242,16 @@ func loadWithSecretLookup[T any](source Source, lookup secretLookup, supplied ..
 		resolutionSnapshots = resolved.snapshots
 		applyResolutionMetadata(&metadata, resolved)
 	}
+	applications := []historyApplication(nil)
+	if composition != nil {
+		applications = cloneHistoryApplications(composition.applications)
+	} else if inputDocument != nil {
+		applications = []historyApplication{{layer: LayerBase, value: inputDocument}}
+	}
+	if !historyNodesWithinLimit(decoded.baseline, decoded.final, final, applications, resolutionSnapshots) {
+		accumulator.add(Diagnostic{Code: CodeExplainLimitExceeded, Severity: SeverityError, Message: "retained configuration history exceeds the node limit"})
+		return nil, accumulator.loadError()
+	}
 	secretResolution := resolveSecretReferences(reflect.ValueOf(&configuration).Elem(), schema, inputDocument, lookup)
 	accumulator.add(secretResolution.diagnostics...)
 	if hasErrors(secretResolution.diagnostics) {
@@ -251,20 +264,19 @@ func loadWithSecretLookup[T any](source Source, lookup secretLookup, supplied ..
 			return nil, accumulator.loadError()
 		}
 	}
-	secrets := classifySecretPaths(schema, options.Definition.Secret, metadata, final)
+	secrets := classifySecretPaths(schema, options.Definition.Secret, metadata, final, applications)
-	var overlaySnapshots []overlayRecord
-	if composition != nil {
-		overlaySnapshots = cloneOverlayRecords(composition.overlays)
-	}
 	return &Result[T]{
-		Config:      configuration,
+		Config:           configuration,
-		metadata:    metadata,
+		metadata:         metadata,
-		sources:     cloneSourceInfos(accumulator.sources),
+		sources:          cloneSourceInfos(accumulator.sources),
-		diagnostics: canonicalDiagnostics(accumulator.diagnostics),
+		diagnostics:      canonicalDiagnostics(accumulator.diagnostics),
-		final:       document.Clone(final),
+		baseline:         decoded.baseline,
-		resolution:  cloneResolutionSnapshots(resolutionSnapshots),
+		input:            decoded.final,
-		overlays:    overlaySnapshots,
+		final:            final,
-		secrets:     secrets,
+		resolution:       resolutionSnapshots,
+		applications:     applications,
+		secrets:          secrets,
+		secretReferences: cloneSecretReferences(secretResolution.references),
 	}, nil
 }
 

overlay.go

@@ -27,15 +27,17 @@ func overlayDocuments(base *composedDocument, overlay *document.Node) (*composed
 		}}
 	}
 	result := &composedDocument{
-		root:       document.Clone(base.root),
+		root:         document.Clone(base.root),
-		claims:     cloneCompositionClaims(base.claims),
+		claims:       cloneCompositionClaims(base.claims),
-		origins:    cloneOriginIndex(base.origins),
+		origins:      cloneOriginIndex(base.origins),
-		overlays:   cloneOverlayRecords(base.overlays),
+		overlays:     cloneOverlayRecords(base.overlays),
-		inputBytes: base.inputBytes,
+		applications: cloneHistoryApplications(base.applications),
-		inputNodes: base.inputNodes,
+		inputBytes:   base.inputBytes,
+		inputNodes:   base.inputNodes,
 	}
 	mergeOverlayOrigin(result.origins, Path{}, overlay.Location, LayerProfile)
 	mergeOverlayMappings(result, result.root, overlay, Path{}, LayerProfile, true)
+	result.applications = append(result.applications, historyApplication{layer: LayerProfile, value: overlay})
 	return result, nil
 }
 

override.go

@@ -378,7 +378,8 @@ func cloneComposedDocument(base *composedDocument) *composedDocument {
 	return &composedDocument{
 		root: document.Clone(base.root), claims: cloneCompositionClaims(base.claims),
 		origins: cloneOriginIndex(base.origins), overlays: cloneOverlayRecords(base.overlays),
-		inputBytes: base.inputBytes, inputNodes: base.inputNodes,
+		applications: cloneHistoryApplications(base.applications),
+		inputBytes:   base.inputBytes, inputNodes: base.inputNodes,
 	}
 }
 
@@ -428,6 +429,10 @@ func applyPreparedOverride(result *composedDocument, override preparedOverride)
 		result.overlays = append(result.overlays, overlayRecord{change: Change{
 			Path: path.clone(), Origin: overlayOrigin(LayerCLI, location), Previous: previousOrigins(previous),
 		}, displaced: displaced, before: before, after: document.Clone(current.Mapping[position].Value)})
+		result.applications = append(result.applications, historyApplication{
+			layer: LayerCLI, path: path.clone(), value: override.value,
+			origin: overlayOrigin(LayerCLI, location),
+		})
 	}
 }
 

records.go

@@ -141,6 +141,8 @@ const (
 	CodeOverrideDirective     Code = "override.directive"
 	CodeOverrideVersion       Code = "override.version"
 	CodeOverrideLimitExceeded Code = "override.limit_exceeded"
+
+	CodeExplainLimitExceeded Code = "explain.limit_exceeded"
 )
 
 // Location identifies a position in a configuration source. Zero line and column

render.go

@@ -3,7 +3,6 @@ package configkit
 import (
 	"errors"
 	"io"
-	"slices"
 	"strings"
 
 	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
@@ -24,31 +23,8 @@ func (r *Result[T]) WriteEffective(output io.Writer) error {
 }
 
 func redactedDocument(node *document.Node, secret map[string]bool, path Path) *document.Node {
-	if node == nil {
+	result, _ := safeSnapshotDocument(node, secret, path)
-		return nil
+	return result
-	}
-	if pathIsSecret(secret, path) {
-		return &document.Node{Kind: document.String, Text: "[REDACTED]"}
-	}
-	result := *node
-	switch node.Kind {
-	case document.Mapping:
-		result.Mapping = make([]document.Entry, len(node.Mapping))
-		copy(result.Mapping, node.Mapping)
-		slices.SortFunc(result.Mapping, func(left, right document.Entry) int {
-			return strings.Compare(left.Key, right.Key)
-		})
-		for index := range result.Mapping {
-			entry := &result.Mapping[index]
-			entry.Value = redactedDocument(entry.Value, secret, path.Key(entry.Key))
-		}
-	case document.Sequence:
-		result.Sequence = make([]*document.Node, len(node.Sequence))
-		for index, child := range node.Sequence {
-			result.Sequence[index] = redactedDocument(child, secret, path.Index(index))
-		}
-	}
-	return &result
 }
 
 func pathIsSecret(secret map[string]bool, path Path) bool {

secret.go

@@ -7,13 +7,20 @@ type secretDependency struct {
 	dependency Path
 }
 
-func classifySecretPaths(schema *schemaDescriptor, predicate func(Path) bool, metadata Metadata, final *document.Node) map[string]bool {
+func classifySecretPaths(schema *schemaDescriptor, predicate func(Path) bool, metadata Metadata, final *document.Node, applications []historyApplication) map[string]bool {
 	paths := make(map[string]Path)
 	finalPaths := make(map[string]*document.Node)
 	indexDocument(final, Path{}, finalPaths)
 	for pointer := range finalPaths {
 		addSecretPath(paths, pointer)
 	}
+	for _, application := range applications {
+		applicationPaths := make(map[string]*document.Node)
+		indexDocument(application.value, application.path, applicationPaths)
+		for pointer := range applicationPaths {
+			addSecretPath(paths, pointer)
+		}
+	}
 	var dependencies []secretDependency
 	for pointer, history := range metadata.history {
 		addSecretPath(paths, pointer)

secret_reference.go

@@ -64,12 +64,13 @@ type secretLookupResult struct {
 
 type secretResolutionResult struct {
 	lookups     map[string]secretLookupResult
+	references  map[string]string
 	diagnostics []Diagnostic
 }
 
 func resolveSecretReferences(root reflect.Value, schema *schemaDescriptor, input *document.Node, lookup secretLookup) secretResolutionResult {
 	result := secretResolutionResult{
-		lookups: make(map[string]secretLookupResult),
+		lookups: make(map[string]secretLookupResult), references: make(map[string]string),
 	}
 	if schema == nil || schema.root == nil || !root.IsValid() || root.Type() != schema.root.typeOf {
 		result.diagnostics = []Diagnostic{{Code: CodeSecretRequired, Severity: SeverityError, Message: "configuration value does not match its secret-reference schema"}}
@@ -124,6 +125,7 @@ func resolveSecretValue(value reflect.Value, descriptor *typeDescriptor, path Pa
 			return
 		}
 		value.Set(reflect.ValueOf(Secret{name: name, value: found.value, specified: true, resolved: true}))
+		result.references[path.String()] = name
 	case descriptorStruct:
 		for _, member := range descriptor.members {
 			memberPath := path