diff --git a/configuration_diff_contract_test.go b/configuration_diff_contract_test.go
index b873ee5..8b1ad82 100644
--- a/configuration_diff_contract_test.go
+++ b/configuration_diff_contract_test.go
@@ -68,13 +68,147 @@ func TestConfigurationInspectionDiffRejectsZeroViews(t *testing.T) {
 	}
 }
 
+func TestConfigurationInspectionDiffIgnoresSourceHistory(t *testing.T) {
+	left := comparisonInspectionFrom(t, "earlier source", "same-model", nil)
+	right := comparisonInspectionFrom(t, "later source", "same-model", nil)
+	if !hasSourceLabel(left, "earlier source") || !hasSourceLabel(right, "later source") {
+		t.Fatal("comparison fixtures did not retain their distinct source labels")
+	}
+
+	differences, err := left.Diff(right)
+	if err != nil || len(differences) != 0 {
+		t.Fatalf("compare equal effective values with different sources = (%+v, %v), want no differences", differences, err)
+	}
+}
+
+func TestConfigurationInspectionDiffReportsPresenceAndSequences(t *testing.T) {
+	left := comparisonInspectionFrom(t, "before", "same-model", map[string]any{
+		"sequence": []any{"earlier", "unchanged"},
+	})
+	right := comparisonInspectionFrom(t, "after", "same-model", map[string]any{
+		"nullable": nil,
+		"sequence": []any{"later", "unchanged"},
+	})
+
+	differences, err := left.Diff(right)
+	if err != nil {
+		t.Fatalf("compare presence and sequence values: %v", err)
+	}
+	if len(differences) != 2 {
+		t.Fatalf("difference count = %d, want 2: %+v", len(differences), differences)
+	}
+	byPath := differencesByPath(differences)
+	nullable, ok := byPath["/backends/local/extra_params/nullable"]
+	if !ok || nullable.Left.Present || !nullable.Right.Present || strings.TrimSpace(nullable.Right.YAML) != "null" {
+		t.Fatalf("absent-to-null difference = (%+v, %t)", nullable, ok)
+	}
+	sequence, ok := byPath["/backends/local/extra_params/sequence"]
+	if !ok || !sequence.Left.Present || !sequence.Right.Present ||
+		!strings.Contains(sequence.Left.YAML, "earlier") || !strings.Contains(sequence.Right.YAML, "later") {
+		t.Fatalf("atomic sequence difference = (%+v, %t)", sequence, ok)
+	}
+}
+
+func TestConfigurationInspectionDiffProtectsSensitiveValuesAndReturnedRecords(t *testing.T) {
+	left := comparisonInspectionFrom(t, "before", "same-model", map[string]any{
+		"token":   "left-secret-sentinel",
+		"visible": "before-value",
+	})
+	right := comparisonInspectionFrom(t, "after", "same-model", map[string]any{
+		"token":   "right-secret-sentinel",
+		"visible": "after-value",
+	})
+
+	differences, err := left.Diff(right)
+	if err != nil {
+		t.Fatalf("compare sensitive values: %v", err)
+	}
+	if len(differences) != 2 {
+		t.Fatalf("difference count = %d, want 2: %+v", len(differences), differences)
+	}
+	byPath := differencesByPath(differences)
+	secret, ok := byPath["/backends/local/extra_params/token"]
+	if !ok || !secret.Left.Redacted || !secret.Right.Redacted || !secret.HiddenChanged {
+		t.Fatalf("sensitive difference = (%+v, %t)", secret, ok)
+	}
+	if strings.Contains(secret.Left.YAML, "left-secret-sentinel") || strings.Contains(secret.Left.YAML, "right-secret-sentinel") ||
+		strings.Contains(secret.Right.YAML, "left-secret-sentinel") || strings.Contains(secret.Right.YAML, "right-secret-sentinel") {
+		t.Fatalf("sensitive snapshots disclosed a value: %+v", secret)
+	}
+	visible, ok := byPath["/backends/local/extra_params/visible"]
+	if !ok || visible.Left.Redacted || visible.Right.Redacted || visible.HiddenChanged ||
+		!strings.Contains(visible.Left.YAML, "before-value") || !strings.Contains(visible.Right.YAML, "after-value") {
+		t.Fatalf("public sibling difference = (%+v, %t)", visible, ok)
+	}
+
+	differences[0].Path = configkit.Keys("caller", "mutation")
+	differences[0].Left.YAML = "caller mutation"
+	differences[0].Right.YAML = "caller mutation"
+	again, err := left.Diff(right)
+	if err != nil {
+		t.Fatalf("repeat comparison after mutating returned records: %v", err)
+	}
+	if len(again) != 2 {
+		t.Fatalf("repeat difference count = %d, want 2: %+v", len(again), again)
+	}
+	againByPath := differencesByPath(again)
+	if _, exists := againByPath["/caller/mutation"]; exists {
+		t.Fatal("mutating returned path reached retained inspection")
+	}
+	if _, exists := againByPath["/backends/local/extra_params/token"]; !exists {
+		t.Fatalf("original sensitive path was lost after mutation: %+v", again)
+	}
+	if _, exists := againByPath["/backends/local/extra_params/visible"]; !exists {
+		t.Fatalf("original public path was lost after mutation: %+v", again)
+	}
+	for _, difference := range again {
+		if difference.Left.YAML == "caller mutation" || difference.Right.YAML == "caller mutation" {
+			t.Fatalf("mutating returned snapshots reached retained inspection: %+v", difference)
+		}
+	}
+}
+
+func TestConfigurationInspectionDiffMasksSensitiveShapeReplacement(t *testing.T) {
+	left := comparisonInspectionFrom(t, "before", "same-model", map[string]any{
+		"details": map[string]any{
+			"token": "nested-secret-sentinel",
+			"label": "public detail",
+		},
+	})
+	right := comparisonInspectionFrom(t, "after", "same-model", map[string]any{
+		"details": "replacement",
+	})
+
+	differences, err := left.Diff(right)
+	if err != nil {
+		t.Fatalf("compare sensitive shape replacement: %v", err)
+	}
+	if len(differences) != 1 || differences[0].Path.String() != "/backends/local/extra_params/details" {
+		t.Fatalf("shape replacement differences = %+v", differences)
+	}
+	difference := differences[0]
+	if !difference.Left.Redacted || !difference.Right.Redacted || !difference.HiddenChanged {
+		t.Fatalf("shape replacement masking = %+v", difference)
+	}
+	if strings.Contains(difference.Left.YAML, "nested-secret-sentinel") || strings.Contains(difference.Right.YAML, "nested-secret-sentinel") {
+		t.Fatalf("shape replacement disclosed a sensitive value: %+v", difference)
+	}
+}
+
 func comparisonInspection(t *testing.T, model string) promptkit.ConfigurationInspection {
 	t.Helper()
+	return comparisonInspectionFrom(t, "comparison fixture", model, nil)
+}
+
+func comparisonInspectionFrom(t *testing.T, source, model string, extraParams map[string]any) promptkit.ConfigurationInspection {
+	t.Helper()
+	backend := map[string]any{"endpoint": "https://example.invalid/v1"}
+	if extraParams != nil {
+		backend["extra_params"] = extraParams
+	}
 	engine, err := promptkit.NewEngine(promptkit.Config{Sources: promptkit.ConfigurationSources{
-		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("comparison fixture", map[string]any{
-			"backends": map[string]any{"local": map[string]any{
-				"endpoint": "https://example.invalid/v1",
-			}},
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping(source, map[string]any{
+			"backends": map[string]any{"local": backend},
 			"profiles": map[string]any{"offline": map[string]any{
 				"backend": "local",
 				"model":   model,
@@ -90,3 +224,20 @@ func comparisonInspection(t *testing.T, model string) promptkit.ConfigurationIns
 	}
 	return inspection
 }
+
+func differencesByPath(differences []configkit.Difference) map[string]configkit.Difference {
+	byPath := make(map[string]configkit.Difference, len(differences))
+	for _, difference := range differences {
+		byPath[difference.Path.String()] = difference
+	}
+	return byPath
+}
+
+func hasSourceLabel(view promptkit.ConfigurationInspection, label string) bool {
+	for _, source := range view.Sources() {
+		if source.Label == label {
+			return true
+		}
+	}
+	return false
+}
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 7007176..7c2632f 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -121,6 +121,12 @@ still behave as before. No runner or domain-package integration is needed.
 
 ## Stage 3: Verify Result Semantics And Sensitive Changes
 
+**Status: Complete.** Facade tests cover distinct source labels with equal
+effective values, absent versus null, atomic sequence changes, paired secret
+masking, sensitive shape replacement, and mutation of returned records.
+`GOTOOLCHAIN=local go test . ./internal/configuration` and
+`git diff --check` passed.
+
 Scope: focused facade integration tests, not a second comparator test suite.
 
 1. Extend the comparison tests with two valid engine configurations whose