diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index be646f7..0c6bef3 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -1,6 +1,6 @@
 # Sequence Identity And Combination Implementation Plan
 
-Status: implementation in progress; Stages 1–2 complete.
+Status: implementation in progress; Stages 1–6 complete.
 
 Audience: a Luna coding agent implementing one numbered stage per prompt, in
 order. This plan starts at commit
@@ -312,6 +312,9 @@ for overlays and inheritance.
 Validation: focused combination tests and the everyday check. Do not change the
 ordinary recursive mapping rule or add unkeyed append.
 
+**Completion:** Implemented and verified with focused sequence-combination tests
+and `./scripts/check.sh`.
+
 ## Stage 7 — Extend Sequence History Representation And Replay
 
 Scope: represent and replay sequence operations without yet changing all metadata
diff --git a/internal/engine/sequence_combine.go b/internal/engine/sequence_combine.go
new file mode 100644
index 0000000..a63f66d
--- /dev/null
+++ b/internal/engine/sequence_combine.go
@@ -0,0 +1,256 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+type sequenceCombinationContext struct {
+	destination Path
+}
+
+type sequenceItemProjection struct {
+	inputIndex  int
+	outputIndex int
+}
+
+type sequenceItemReplacement struct {
+	lowerIndex  int
+	higherIndex int
+	outputIndex int
+}
+
+type sequenceCombinationResult struct {
+	value          *document.Node
+	lowerItems     []sequenceItemProjection
+	higherItems    []sequenceItemProjection
+	replacements   []sequenceItemReplacement
+	appendedHigher []int
+	discardedLower []int
+}
+
+func combineSequence(
+	lower, higher *document.Node,
+	policy collectionPolicyResolution,
+	context sequenceCombinationContext,
+) (sequenceCombinationResult, []Diagnostic) {
+	if higher == nil {
+		return unchangedSequenceValue(lower), nil
+	}
+	switch policy.operation {
+	case sequenceModeReplace:
+		return replaceSequence(lower, higher, policy.effectiveIdentityFields, context)
+	case sequenceModeAppend, sequenceModeUpsert:
+		return combineIdentifiedSequences(lower, higher, policy.operation, policy.effectiveIdentityFields, context)
+	default:
+		return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+			context.destination, "sequence operation is invalid", higher,
+		)}
+	}
+}
+
+func unchangedSequenceValue(lower *document.Node) sequenceCombinationResult {
+	result := sequenceCombinationResult{value: document.Clone(lower)}
+	if lower != nil && lower.Kind == document.Sequence {
+		result.lowerItems = sequenceItemProjections(len(lower.Sequence), 0)
+	}
+	return result
+}
+
+func replaceSequence(
+	lower, higher *document.Node,
+	identityFields []string,
+	context sequenceCombinationContext,
+) (sequenceCombinationResult, []Diagnostic) {
+	if higher.Kind == document.Sequence && len(identityFields) != 0 {
+		if _, diagnostics := validateSequenceIdentity(higher, context.destination, identityFields, sequenceCombinationIdentityCodes); len(diagnostics) != 0 {
+			return sequenceCombinationResult{}, diagnostics
+		}
+	}
+	result := sequenceCombinationResult{value: document.Clone(higher)}
+	if higher.Kind == document.Sequence {
+		result.higherItems = sequenceItemProjections(len(higher.Sequence), 0)
+	}
+	if lower != nil && lower.Kind == document.Sequence {
+		result.discardedLower = sequenceIndexes(len(lower.Sequence))
+	}
+	return result, nil
+}
+
+func combineIdentifiedSequences(
+	lower, higher *document.Node,
+	operation sequenceMode,
+	identityFields []string,
+	context sequenceCombinationContext,
+) (sequenceCombinationResult, []Diagnostic) {
+	if higher.Kind != document.Sequence {
+		return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+			context.destination, "append and upsert require a sequence contribution", higher,
+		)}
+	}
+	if lower != nil && lower.Kind != document.Sequence {
+		return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+			context.destination, "append and upsert require a lower sequence or an absent lower value", lower,
+		)}
+	}
+	if len(identityFields) == 0 {
+		return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+			context.destination, "append and upsert require an identity definition", higher,
+		)}
+	}
+
+	higherIndex, diagnostics := validateSequenceIdentity(higher, context.destination, identityFields, sequenceCombinationIdentityCodes)
+	if len(diagnostics) != 0 {
+		return sequenceCombinationResult{}, diagnostics
+	}
+	var lowerIndex map[string]sequenceIdentityRecord
+	if lower != nil {
+		lowerIndex, diagnostics = validateSequenceIdentity(lower, context.destination, identityFields, sequenceCombinationIdentityCodes)
+		if len(diagnostics) != 0 {
+			return sequenceCombinationResult{}, diagnostics
+		}
+	}
+	if operation == sequenceModeAppend {
+		if duplicate := duplicateAcrossSequenceContributions(lowerIndex, higherIndex, context.destination); duplicate != nil {
+			return sequenceCombinationResult{}, []Diagnostic{*duplicate}
+		}
+	}
+
+	if operation == sequenceModeAppend {
+		return appendSequenceItems(lower, higher), nil
+	}
+	if operation == sequenceModeUpsert {
+		return upsertSequenceItems(lower, higher, lowerIndex, higherIndex), nil
+	}
+	return sequenceCombinationResult{}, []Diagnostic{sequenceCombinationDiagnostic(
+		context.destination, "sequence operation is invalid", higher,
+	)}
+}
+
+func appendSequenceItems(lower, higher *document.Node) sequenceCombinationResult {
+	lowerCount := 0
+	if lower != nil {
+		lowerCount = len(lower.Sequence)
+	}
+	items := make([]*document.Node, 0, lowerCount+len(higher.Sequence))
+	if lower != nil {
+		items = append(items, lower.Sequence...)
+	}
+	items = append(items, higher.Sequence...)
+	return sequenceCombinationResult{
+		value:          cloneSequenceWithItems(higher, items),
+		lowerItems:     sequenceItemProjections(lowerCount, 0),
+		higherItems:    sequenceItemProjections(len(higher.Sequence), lowerCount),
+		appendedHigher: sequenceIndexes(len(higher.Sequence)),
+	}
+}
+
+func upsertSequenceItems(
+	lower, higher *document.Node,
+	lowerIndex, higherIndex map[string]sequenceIdentityRecord,
+) sequenceCombinationResult {
+	lowerCount := 0
+	var lowerItems []*document.Node
+	if lower != nil {
+		lowerCount = len(lower.Sequence)
+		lowerItems = lower.Sequence
+	}
+	matchedLower := make(map[int]sequenceIdentityRecord, len(higherIndex))
+	matchedLowerByHigher := make([]int, len(higher.Sequence))
+	for index := range matchedLowerByHigher {
+		matchedLowerByHigher[index] = -1
+	}
+	for tuple, incoming := range higherIndex {
+		if retained, exists := lowerIndex[tuple]; exists {
+			matchedLower[retained.index] = incoming
+			matchedLowerByHigher[incoming.index] = retained.index
+		}
+	}
+	items := make([]*document.Node, lowerCount, lowerCount+len(higher.Sequence))
+	result := sequenceCombinationResult{
+		lowerItems:  make([]sequenceItemProjection, 0, lowerCount-len(matchedLower)),
+		higherItems: sequenceItemProjections(len(higher.Sequence), 0),
+	}
+	for lowerPosition, item := range lowerItems {
+		if incoming, replaced := matchedLower[lowerPosition]; replaced {
+			items[lowerPosition] = higher.Sequence[incoming.index]
+			result.higherItems[incoming.index].outputIndex = lowerPosition
+			result.replacements = append(result.replacements, sequenceItemReplacement{
+				lowerIndex: lowerPosition, higherIndex: incoming.index, outputIndex: lowerPosition,
+			})
+			continue
+		}
+		items[lowerPosition] = item
+		result.lowerItems = append(result.lowerItems, sequenceItemProjection{inputIndex: lowerPosition, outputIndex: lowerPosition})
+	}
+	for higherPosition := range higher.Sequence {
+		if matchedLowerByHigher[higherPosition] >= 0 {
+			continue
+		}
+		outputIndex := len(items)
+		items = append(items, higher.Sequence[higherPosition])
+		result.appendedHigher = append(result.appendedHigher, higherPosition)
+		result.higherItems[higherPosition].outputIndex = outputIndex
+	}
+	result.value = cloneSequenceWithItems(higher, items)
+	return result
+}
+
+func duplicateAcrossSequenceContributions(
+	lowerIndex, higherIndex map[string]sequenceIdentityRecord,
+	path Path,
+) *Diagnostic {
+	higherTuples := make([]string, len(higherIndex))
+	for tuple, record := range higherIndex {
+		higherTuples[record.index] = tuple
+	}
+	for _, tuple := range higherTuples {
+		if lower, exists := lowerIndex[tuple]; exists {
+			higher := higherIndex[tuple]
+			primary := publicLocation(lower.location)
+			related := publicLocation(higher.location)
+			return &Diagnostic{
+				Code: CodeSequenceConflict, Severity: SeverityError, Path: path.clone(),
+				Message: "sequence contributions contain a duplicate identity",
+				Primary: &primary, Related: []Location{related},
+			}
+		}
+	}
+	return nil
+}
+
+func cloneSequenceWithItems(template *document.Node, items []*document.Node) *document.Node {
+	cloned := *template
+	cloned.Mapping = nil
+	cloned.Sequence = make([]*document.Node, len(items))
+	for index, item := range items {
+		cloned.Sequence[index] = document.Clone(item)
+	}
+	return &cloned
+}
+
+func sequenceItemProjections(length, outputOffset int) []sequenceItemProjection {
+	projections := make([]sequenceItemProjection, length)
+	for index := range projections {
+		projections[index] = sequenceItemProjection{inputIndex: index, outputIndex: index + outputOffset}
+	}
+	return projections
+}
+
+func sequenceIndexes(length int) []int {
+	indexes := make([]int, length)
+	for index := range indexes {
+		indexes[index] = index
+	}
+	return indexes
+}
+
+func sequenceCombinationDiagnostic(path Path, message string, node *document.Node) Diagnostic {
+	var primary *Location
+	if node != nil {
+		location := publicLocation(node.Location)
+		primary = &location
+	}
+	return Diagnostic{Code: CodeSequenceInvalid, Severity: SeverityError, Path: path.clone(), Message: message, Primary: primary}
+}
+
+var sequenceCombinationIdentityCodes = sequenceIdentityDiagnosticCodes{
+	invalid: CodeSequenceInvalid, conflict: CodeSequenceConflict,
+}
diff --git a/internal/engine/sequence_combine_test.go b/internal/engine/sequence_combine_test.go
new file mode 100644
index 0000000..eaea064
--- /dev/null
+++ b/internal/engine/sequence_combine_test.go
@@ -0,0 +1,302 @@
+package engine
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestCombineSequenceOperationsAndItemEvidence(t *testing.T) {
+	tests := []struct {
+		name             string
+		lower            string
+		higher           string
+		operation        sequenceMode
+		want             []string
+		wantLower        []sequenceItemProjection
+		wantHigher       []sequenceItemProjection
+		wantReplacements []sequenceItemReplacement
+		wantAppended     []int
+		wantDiscarded    []int
+	}{
+		{
+			name:  "replace uses only higher records",
+			lower: "items: [{id: a}, {id: b}]\n", higher: "items: [{id: b, text: revised}]\n",
+			operation: sequenceModeReplace, want: []string{"b"},
+			wantHigher: []sequenceItemProjection{{inputIndex: 0, outputIndex: 0}}, wantDiscarded: []int{0, 1},
+		},
+		{
+			name:  "append retains then appends",
+			lower: "items: [{id: a}, {id: b}]\n", higher: "items: [{id: c}]\n",
+			operation: sequenceModeAppend, want: []string{"a", "b", "c"},
+			wantLower:  []sequenceItemProjection{{inputIndex: 0, outputIndex: 0}, {inputIndex: 1, outputIndex: 1}},
+			wantHigher: []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}}, wantAppended: []int{0},
+		},
+		{
+			name:      "upsert replaces whole records in place and appends new records",
+			lower:     "items: [{id: a, text: old}, {id: b}, {id: c, remove: true}]\n",
+			higher:    "items: [{id: c, text: revised}, {id: a, text: revised}, {id: d}]\n",
+			operation: sequenceModeUpsert, want: []string{"a", "b", "c", "d"},
+			wantLower:        []sequenceItemProjection{{inputIndex: 1, outputIndex: 1}},
+			wantHigher:       []sequenceItemProjection{{inputIndex: 0, outputIndex: 2}, {inputIndex: 1, outputIndex: 0}, {inputIndex: 2, outputIndex: 3}},
+			wantReplacements: []sequenceItemReplacement{{lowerIndex: 0, higherIndex: 1, outputIndex: 0}, {lowerIndex: 2, higherIndex: 0, outputIndex: 2}},
+			wantAppended:     []int{2},
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			lower := sequenceFromYAML(t, test.lower)
+			higher := sequenceFromYAML(t, test.higher)
+			policy := sequenceCombinationPolicy(test.operation, "id")
+			result, diagnostics := combineSequence(lower, higher, policy, sequenceCombinationContext{destination: Keys("items")})
+			if len(diagnostics) != 0 {
+				t.Fatalf("combination diagnostics = %#v", diagnostics)
+			}
+			if got := sequenceIdentityValues(result.value, "id"); !reflect.DeepEqual(got, test.want) {
+				t.Fatalf("combined IDs = %#v, want %#v", got, test.want)
+			}
+			if !reflect.DeepEqual(result.lowerItems, test.wantLower) || !reflect.DeepEqual(result.higherItems, test.wantHigher) || !reflect.DeepEqual(result.replacements, test.wantReplacements) || !reflect.DeepEqual(result.appendedHigher, test.wantAppended) || !reflect.DeepEqual(result.discardedLower, test.wantDiscarded) {
+				t.Fatalf("item evidence = %#v", result)
+			}
+		})
+	}
+}
+
+func TestCombineSequenceDistinguishesAbsenceFromNullAndKindMismatch(t *testing.T) {
+	policy := sequenceCombinationPolicy(sequenceModeAppend, "id")
+	retained := sequenceFromYAML(t, "items: [{id: lower}]\n")
+	unchanged, diagnostics := combineSequence(retained, nil, policy, sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(unchanged.value, "id"), []string{"lower"}) || len(unchanged.lowerItems) != 1 {
+		t.Fatalf("absent higher value changed lower sequence: %#v, %#v", unchanged, diagnostics)
+	}
+
+	tests := []struct {
+		name   string
+		lower  string
+		higher string
+	}{
+		{name: "null higher value", lower: "items: [{id: lower}]\n", higher: "items: null\n"},
+		{name: "mapping higher value", lower: "items: [{id: lower}]\n", higher: "items: {id: higher}\n"},
+		{name: "null lower value", lower: "items: null\n", higher: "items: []\n"},
+		{name: "scalar lower value", lower: "items: blocked\n", higher: "items: []\n"},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			result, issues := combineSequence(sequenceFromYAML(t, test.lower), sequenceFromYAML(t, test.higher), policy, sequenceCombinationContext{destination: Keys("items")})
+			if result.value != nil || len(issues) != 1 || issues[0].Code != CodeSequenceInvalid || issues[0].Path.String() != "/items" {
+				t.Fatalf("kind mismatch result = %#v, %#v", result, issues)
+			}
+		})
+	}
+
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		result, issues := combineSequence(nil, sequenceFromYAML(t, "items: null\n"), sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if result.value != nil || len(issues) != 1 || issues[0].Code != CodeSequenceInvalid {
+			t.Fatalf("null incoming value under operation %d = %#v, %#v", operation, result, issues)
+		}
+	}
+}
+
+func TestReplaceSequenceAllowsNullAndNonSequenceValues(t *testing.T) {
+	for _, test := range []struct {
+		name     string
+		higher   string
+		wantKind document.Kind
+	}{
+		{name: "null", higher: "items: null\n", wantKind: document.Null},
+		{name: "mapping", higher: "items: {value: replacement}\n", wantKind: document.Mapping},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			lower := sequenceFromYAML(t, "items: [{id: lower}]\n")
+			higher := sequenceFromYAML(t, test.higher)
+			result, diagnostics := combineSequence(lower, higher, sequenceCombinationPolicy(sequenceModeReplace), sequenceCombinationContext{destination: Keys("items")})
+			if len(diagnostics) != 0 || result.value == nil || result.value.Kind != test.wantKind || !reflect.DeepEqual(result.discardedLower, []int{0}) {
+				t.Fatalf("replacement result = %#v, %#v", result, diagnostics)
+			}
+		})
+	}
+	result, diagnostics := combineSequence(sequenceFromYAML(t, "items: null\n"), sequenceFromYAML(t, "items: [{id: new}]\n"), sequenceCombinationPolicy(sequenceModeReplace), sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "id"), []string{"new"}) {
+		t.Fatalf("replacement over explicit lower null = %#v, %#v", result, diagnostics)
+	}
+}
+
+func TestCombineSequenceRequiresIdentityForAppendAndUpsertEvenWhenEmpty(t *testing.T) {
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		result, diagnostics := combineSequence(nil, sequenceFromYAML(t, "items: []\n"), sequenceCombinationPolicy(operation), sequenceCombinationContext{destination: Keys("items")})
+		if result.value != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid {
+			t.Fatalf("empty input bypassed identity requirement for operation %d: %#v, %#v", operation, result, diagnostics)
+		}
+	}
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		result, diagnostics := combineSequence(nil, sequenceFromYAML(t, "items: [{name: no-id}]\n"), sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if result.value != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceInvalid || diagnostics[0].Path.String() != "/items/0/id" {
+			t.Fatalf("missing identity for operation %d = %#v, %#v", operation, result, diagnostics)
+		}
+	}
+}
+
+func TestCombineSequenceValidatesDuplicatesWithinEachContributionInEveryMode(t *testing.T) {
+	duplicate := "items: [{id: same}, {id: same}]\n"
+	for _, operation := range []sequenceMode{sequenceModeReplace, sequenceModeAppend, sequenceModeUpsert} {
+		_, diagnostics := combineSequence(nil, sequenceFromYAML(t, duplicate), sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict {
+			t.Fatalf("higher duplicate under operation %d = %#v", operation, diagnostics)
+		}
+	}
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		_, diagnostics := combineSequence(sequenceFromYAML(t, duplicate), sequenceFromYAML(t, "items: [{id: other}]\n"), sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict {
+			t.Fatalf("lower duplicate under operation %d = %#v", operation, diagnostics)
+		}
+	}
+}
+
+func TestAppendSequenceRejectsDuplicatesAcrossContributions(t *testing.T) {
+	const identityCanary = "identity-canary-64"
+	lower := sequenceFromYAML(t, "items: [{id: "+identityCanary+"}]\n")
+	higher := sequenceFromYAML(t, "items: [{id: "+identityCanary+"}]\n")
+	_, diagnostics := combineSequence(lower, higher, sequenceCombinationPolicy(sequenceModeAppend, "id"), sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict || diagnostics[0].Path.String() != "/items" || diagnostics[0].Primary == nil || len(diagnostics[0].Related) != 1 {
+		t.Fatalf("cross-contribution duplicate = %#v", diagnostics)
+	}
+	if strings.Contains(diagnostics[0].Message, identityCanary) {
+		t.Fatalf("identity value leaked in diagnostic: %#v", diagnostics[0])
+	}
+}
+
+func TestCombineSequenceReplacementResetsLowerIdentity(t *testing.T) {
+	lower := sequenceFromYAML(t, "items: [{id: old}]\n")
+	higher := sequenceFromYAML(t, "items: [{name: new}]\n")
+	policy := sequenceCombinationPolicy(sequenceModeReplace, "name")
+	result, diagnostics := combineSequence(lower, higher, policy, sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "name"), []string{"new"}) || !reflect.DeepEqual(result.discardedLower, []int{0}) {
+		t.Fatalf("replacement applied obsolete identity: %#v, %#v", result, diagnostics)
+	}
+}
+
+func TestCombineSequenceDifferentIdentityDeclarationsConflict(t *testing.T) {
+	declarations := extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [items]
+      key: id
+    - destination: [items]
+      key: name
+`, rootDirectiveContextSource).sequences
+	if _, diagnostics := resolveCollectionPolicy(Keys("items"), declarations, nil, nil); len(diagnostics) != 1 || diagnostics[0].Code != CodeSequenceConflict {
+		t.Fatalf("different collection key declarations = %#v", diagnostics)
+	}
+}
+
+func TestCombineSequenceEmptyInputsAndStableOrder(t *testing.T) {
+	for _, operation := range []sequenceMode{sequenceModeAppend, sequenceModeUpsert} {
+		lower := sequenceFromYAML(t, "items: [{id: a}, {id: b}]\n")
+		empty := sequenceFromYAML(t, "items: []\n")
+		result, diagnostics := combineSequence(lower, empty, sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "id"), []string{"a", "b"}) {
+			t.Fatalf("empty higher sequence under operation %d = %#v, %#v", operation, result, diagnostics)
+		}
+	}
+	result, diagnostics := combineSequence(nil, sequenceFromYAML(t, "items: []\n"), sequenceCombinationPolicy(sequenceModeReplace, "id"), sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || result.value == nil || result.value.Kind != document.Sequence || len(result.value.Sequence) != 0 {
+		t.Fatalf("explicit replacement with empty input = %#v, %#v", result, diagnostics)
+	}
+}
+
+func TestCombineSequenceInitializesAbsentLowerCollection(t *testing.T) {
+	higher := sequenceFromYAML(t, "items: [{id: first}, {id: second}]\n")
+	for _, operation := range []sequenceMode{sequenceModeReplace, sequenceModeAppend, sequenceModeUpsert} {
+		result, diagnostics := combineSequence(nil, higher, sequenceCombinationPolicy(operation, "id"), sequenceCombinationContext{destination: Keys("items")})
+		if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "id"), []string{"first", "second"}) {
+			t.Fatalf("absent lower collection under operation %d = %#v, %#v", operation, result, diagnostics)
+		}
+	}
+}
+
+func TestCombineSequenceClonesInputsAndDoesNotMutatePolicy(t *testing.T) {
+	for _, test := range []struct {
+		name      string
+		operation sequenceMode
+	}{
+		{name: "replace", operation: sequenceModeReplace},
+		{name: "append", operation: sequenceModeAppend},
+		{name: "upsert", operation: sequenceModeUpsert},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			lower := sequenceFromYAML(t, "items: [{id: a, text: original}]\n")
+			higherID := "b"
+			if test.operation == sequenceModeUpsert {
+				higherID = "a"
+			}
+			higher := sequenceFromYAML(t, "items: [{id: "+higherID+", text: incoming}]\n")
+			policy := sequenceCombinationPolicy(test.operation, "id")
+			policyCopy := append([]string(nil), policy.effectiveIdentityFields...)
+			result, diagnostics := combineSequence(lower, higher, policy, sequenceCombinationContext{destination: Keys("items")})
+			if len(diagnostics) != 0 {
+				t.Fatal(diagnostics)
+			}
+			for _, item := range result.value.Sequence {
+				for entryIndex := range item.Mapping {
+					if item.Mapping[entryIndex].Key == "text" {
+						item.Mapping[entryIndex].Value.Text = "changed"
+					}
+				}
+			}
+			if lower.Sequence[0].Mapping[1].Value.Text != "original" || higher.Sequence[0].Mapping[1].Value.Text != "incoming" {
+				t.Fatal("combined result aliases a mutable input node")
+			}
+			if !reflect.DeepEqual(policy.effectiveIdentityFields, policyCopy) {
+				t.Fatal("combination mutated the resolved policy")
+			}
+		})
+	}
+}
+
+func TestCombineSequenceEquivalentIdentityFieldSets(t *testing.T) {
+	lower := sequenceFromYAML(t, "items: [{id: a, namespace: one}]\n")
+	higher := sequenceFromYAML(t, "items: [{namespace: one, id: b}]\n")
+	policy, diagnostics := resolveCollectionPolicy(Keys("items"), extractSequenceDeclarationsYAML(t, `configkit:
+  sequences:
+    - destination: [items]
+      key: [namespace, id, id]
+`, rootDirectiveContextSource).sequences, nil, nil)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	result, diagnostics := combineSequence(lower, higher, policy, sequenceCombinationContext{destination: Keys("items")})
+	if len(diagnostics) != 0 || !reflect.DeepEqual(sequenceIdentityValues(result.value, "id"), []string{"a", "b"}) {
+		t.Fatalf("equivalent identity policy failed: %#v, %#v", result, diagnostics)
+	}
+}
+
+func sequenceCombinationPolicy(operation sequenceMode, fields ...string) collectionPolicyResolution {
+	normalized, _ := normalizeSequenceIdentityFields(fields)
+	return collectionPolicyResolution{operation: operation, effectiveIdentityFields: normalized}
+}
+
+func sequenceFromYAML(t *testing.T, input string) *document.Node {
+	t.Helper()
+	parsed := parsePolicyNode(t, input)
+	if len(parsed.Mapping) != 1 || parsed.Mapping[0].Key != "items" {
+		t.Fatalf("sequence fixture root = %#v", parsed.Mapping)
+	}
+	return parsed.Mapping[0].Value
+}
+
+func sequenceIdentityValues(sequence *document.Node, field string) []string {
+	if sequence == nil || sequence.Kind != document.Sequence {
+		return nil
+	}
+	values := make([]string, len(sequence.Sequence))
+	for index, item := range sequence.Sequence {
+		for _, entry := range item.Mapping {
+			if entry.Key == field {
+				values[index] = entry.Value.Text
+				break
+			}
+		}
+	}
+	return values
+}