diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 50f11cc..cb4237c 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -310,7 +310,7 @@ Validate: `go test ./internal/engine -run 'Secret|Sensitivity|Inheritance|Histor
 relevant root facade secret/inheritance tests, engine race tests, and the everyday
 check. Record collection and propagation outcomes separately.
 
-## Stage 12: Index CLI overlaps and validation reads
+## Stage 12: Index CLI overlaps and validation reads — Complete
 
 Roadmap candidate 4. Owner: engine `override.go`.
 
diff --git a/internal/engine/override.go b/internal/engine/override.go
index ae620eb..f41981c 100644
--- a/internal/engine/override.go
+++ b/internal/engine/override.go
@@ -50,12 +50,16 @@ func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []
 	}
 
 	seen := make(map[string]int, len(overrides))
-	pathAccepted := make([]bool, len(overrides))
+	var acceptedPaths pathPrefixIndex
+	var baseLookup *documentLookup
+	if len(overrides) != 0 {
+		baseLookup = buildDocumentLookup(base.root)
+	}
 	var usedBytes int64
 	usedNodes := 0
 	for index, override := range overrides {
 		location := &Location{SourceID: prepared[index].source.ID}
-		if issue := validateOverridePath(override.Path, base.root); issue != nil {
+		if issue := validateOverridePath(override.Path, base.root, baseLookup); issue != nil {
 			issue.Primary = location
 			diagnostics = append(diagnostics, *issue)
 			continue
@@ -70,7 +74,7 @@ func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []
 			continue
 		}
 		seen[pointer] = index
-		if conflicting := overlappingOverride(overrides[:index], pathAccepted[:index], override.Path); conflicting >= 0 {
+		if conflicting, overlaps := acceptedPaths.earliestIntersectingID(pointer); overlaps {
 			diagnostics = append(diagnostics, Diagnostic{
 				Code: CodeOverrideOverlap, Severity: SeverityError, Path: override.Path.clone(),
 				Message: "configuration override paths overlap", Primary: location,
@@ -92,7 +96,7 @@ func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []
 			})
 			continue
 		}
-		pathAccepted[index] = true
+		acceptedPaths.insert(pointer, index)
 
 		builder := valueBuilder{sourceID: prepared[index].source.ID, active: make(map[valueVisit]bool)}
 		value, issue := builder.build(override.Value, override.Path, len(override.Path.segments))
@@ -150,7 +154,7 @@ func validOverrideLabel(label string) bool {
 	return true
 }
 
-func validateOverridePath(path Path, root *document.Node) *Diagnostic {
+func validateOverridePath(path Path, root *document.Node, lookup *documentLookup) *Diagnostic {
 	if !path.valid() || len(path.segments) == 0 {
 		return &Diagnostic{Code: CodeOverrideInvalidPath, Severity: SeverityError, Path: path.clone(), Message: "configuration override path must be a valid non-root mapping path"}
 	}
@@ -165,7 +169,7 @@ func validateOverridePath(path Path, root *document.Node) *Diagnostic {
 		if current.Kind != document.Mapping {
 			return &Diagnostic{Code: CodeOverrideInvalidPath, Severity: SeverityError, Path: path.clone(), Message: "configuration override path traverses a non-mapping value"}
 		}
-		position := documentEntryIndex(current.Mapping, segment.key)
+		position := lookup.find(current, segment.key)
 		if index == len(path.segments)-1 {
 			continue
 		}
@@ -178,15 +182,6 @@ func validateOverridePath(path Path, root *document.Node) *Diagnostic {
 	return nil
 }
 
-func overlappingOverride(previous []Override, valid []bool, path Path) int {
-	for index, override := range previous {
-		if valid[index] && (pathContains(override.Path, path) || pathContains(path, override.Path)) {
-			return index
-		}
-	}
-	return -1
-}
-
 func pathContainsDirective(path Path) bool {
 	for _, segment := range path.segments {
 		if segment.key == directiveKey {
diff --git a/internal/engine/override_test.go b/internal/engine/override_test.go
index ea3b8f1..d3c7153 100644
--- a/internal/engine/override_test.go
+++ b/internal/engine/override_test.go
@@ -206,6 +206,132 @@ func TestOverrideValidationRejectsPathsOverlapValuesAndProtectedTargets(t *testi
 	}
 }
 
+func TestOverrideOverlapIndexPreservesFirstAcceptedSource(t *testing.T) {
+	type config struct {
+		Nested map[string]any `yaml:"nested"`
+	}
+	base := Bytes("override-overlap.yml", []byte("nested: {}\n"))
+	_, err := testLoad(base, Options[config]{Overrides: []Override{
+		{Path: Keys("nested"), Value: map[string]any{}, Flag: "--nested"},
+		{Path: Keys("nested", "first"), Value: "one", Flag: "--first"},
+		{Path: Keys("nested", "second"), Value: "two", Flag: "--second"},
+	}})
+	if err == nil {
+		t.Fatal("load succeeded with overlapping overrides")
+	}
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("error = %T %v, want *LoadError", err, err)
+	}
+	diagnostics := loadErr.Diagnostics()
+	var conflicts []Diagnostic
+	for _, diagnostic := range diagnostics {
+		if diagnostic.Code == CodeOverrideOverlap {
+			conflicts = append(conflicts, diagnostic)
+		}
+	}
+	if len(conflicts) != 2 {
+		t.Fatalf("overlap diagnostics = %#v, want two conflicts", diagnostics)
+	}
+	for index, path := range []string{"/nested/first", "/nested/second"} {
+		if conflicts[index].Path.String() != path || len(conflicts[index].Related) != 1 || conflicts[index].Related[0].SourceID != "source-2" {
+			t.Fatalf("conflict[%d] = %#v, want path %q related to first CLI source", index, conflicts[index], path)
+		}
+	}
+}
+
+func TestOverrideOverlapIndexIncludesAcceptedPathWithInvalidValue(t *testing.T) {
+	type config struct {
+		Nested map[string]any `yaml:"nested"`
+	}
+	base := Bytes("override-value-overlap.yml", []byte("nested: {}\n"))
+	_, err := testLoad(base, Options[config]{Overrides: []Override{
+		{Path: Keys("nested"), Value: time.Second, Flag: "--nested"},
+		{Path: Keys("nested", "child"), Value: "value", Flag: "--child"},
+	}})
+	if err == nil {
+		t.Fatal("load succeeded with an invalid value and overlapping path")
+	}
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("error = %T %v, want *LoadError", err, err)
+	}
+	var invalidValue, overlap bool
+	for _, diagnostic := range loadErr.Diagnostics() {
+		switch diagnostic.Code {
+		case CodeOverrideInvalidValue:
+			invalidValue = diagnostic.Path.String() == "/nested"
+		case CodeOverrideOverlap:
+			overlap = diagnostic.Path.String() == "/nested/child" && len(diagnostic.Related) == 1 && diagnostic.Related[0].SourceID == "source-2"
+		}
+	}
+	if !invalidValue || !overlap {
+		t.Fatalf("diagnostics = %#v, want invalid parent value and child overlap related to its source", loadErr.Diagnostics())
+	}
+}
+
+func TestRejectedOverridePathDoesNotCauseOverlap(t *testing.T) {
+	type config struct {
+		Scalar string `yaml:"scalar"`
+	}
+	base := Bytes("override-rejected-overlap.yml", []byte("scalar: value\n"))
+	_, err := testLoad(base, Options[config]{Overrides: []Override{
+		{Path: Keys("scalar", "child"), Value: "invalid", Flag: "--child"},
+		{Path: Keys("scalar"), Value: "valid", Flag: "--scalar"},
+	}})
+	if err == nil {
+		t.Fatal("load succeeded with a path traversing a scalar")
+	}
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("error = %T %v, want *LoadError", err, err)
+	}
+	diagnostics := loadErr.Diagnostics()
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeOverrideInvalidPath || diagnostics[0].Path.String() != "/scalar/child" {
+		t.Fatalf("diagnostics = %#v, want only the invalid descendant path", diagnostics)
+	}
+}
+
+func TestVersionRejectedOverridePathDoesNotCauseOverlap(t *testing.T) {
+	type config struct {
+		Nested map[string]any `yaml:"nested"`
+	}
+	base := Bytes("override-version-overlap.yml", []byte("nested: {}\n"))
+	version := &VersionRule{Path: Keys("nested", "locked"), Check: func(Version, *Validation) error { return nil }}
+	_, err := testLoad(base, Options[config]{
+		Definition: Definition[config]{Version: version},
+		Overrides: []Override{
+			{Path: Keys("nested"), Value: map[string]any{}, Flag: "--nested"},
+			{Path: Keys("nested", "other"), Value: "allowed", Flag: "--other"},
+		},
+	})
+	if err == nil {
+		t.Fatal("load succeeded despite an override of a schema-version ancestor")
+	}
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("error = %T %v, want *LoadError", err, err)
+	}
+	diagnostics := loadErr.Diagnostics()
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeOverrideVersion || diagnostics[0].Path.String() != "/nested" {
+		t.Fatalf("diagnostics = %#v, want only the protected ancestor path", diagnostics)
+	}
+}
+
+func TestEscapedOverrideKeysStayDistinctFromMultipleSegments(t *testing.T) {
+	type config struct {
+		Nested map[string]any `yaml:"nested"`
+	}
+	base := Bytes("override-escaped-keys.yml", []byte("nested: {}\n"))
+	_, err := testLoad(base, Options[config]{Overrides: []Override{
+		{Path: Keys("nested", "a/b"), Value: "single key", Flag: "--single"},
+		{Path: Keys("nested", "a", "b"), Value: "nested key", Flag: "--nested"},
+	}})
+	if err != nil {
+		t.Fatalf("load rejected distinct escaped path tokens: %v", err)
+	}
+}
+
 func TestOverrideCannotReplaceSchemaVersionAncestor(t *testing.T) {
 	type schemaConfig struct {
 		Version string `yaml:"version"`