diff --git a/configuration_inspection_contract_test.go b/configuration_inspection_contract_test.go
index 552933c..41e6761 100644
--- a/configuration_inspection_contract_test.go
+++ b/configuration_inspection_contract_test.go
@@ -144,9 +144,17 @@ func TestConfigurationInspectionUsesCapturedSourcesWithoutProviderCalls(t *testi
 	if err != nil {
 		t.Fatalf("select inputs: %v", err)
 	}
+	otherSource := &unreadableAfterConstructionFS{FS: fstest.MapFS{
+		"config/prompt.yaml": &fstest.MapFile{Data: []byte(capturedPromptDocument)},
+		"config/body.tmpl":   &fstest.MapFile{Data: []byte("hello")},
+	}}
+	otherInputs, err := promptkit.FilesystemInputs("comparison source", otherSource, "config")
+	if err != nil {
+		t.Fatalf("select comparison inputs: %v", err)
+	}
 	var modelCalls atomic.Int64
 	client := countingLLMClient{calls: &modelCalls}
-	endpoint, model := "https://example.invalid/v1", "offline-model"
+	endpoint, model, otherModel := "https://example.invalid/v1", "offline-model", "comparison-model"
 	engine, err := promptkit.NewEngine(promptkit.Config{Sources: promptkit.ConfigurationSources{
 		Underlays: []configkit.Underlay{{Inputs: inputs}, {Inputs: []configkit.Input{{Source: promptkit.ProfileSource("offline profile", map[string]promptkit.ProfileConfig{
 			"offline": {Endpoint: &endpoint, Model: &model},
@@ -155,11 +163,42 @@ func TestConfigurationInspectionUsesCapturedSourcesWithoutProviderCalls(t *testi
 	if err != nil {
 		t.Fatalf("construct engine: %v", err)
 	}
+	otherEngine, err := promptkit.NewEngine(promptkit.Config{Sources: promptkit.ConfigurationSources{
+		Underlays: []configkit.Underlay{{Inputs: otherInputs}, {Inputs: []configkit.Input{{Source: promptkit.ProfileSource("comparison profile", map[string]promptkit.ProfileConfig{
+			"offline": {Endpoint: &endpoint, Model: &otherModel},
+		})}}}},
+	}}, promptkit.WithLLMClient(client))
+	if err != nil {
+		t.Fatalf("construct comparison engine: %v", err)
+	}
 	source.closed.Store(true)
+	otherSource.closed.Store(true)
 	view, err := engine.InspectConfiguration()
 	if err != nil {
 		t.Fatalf("inspect configuration: %v", err)
 	}
+	otherView, err := otherEngine.InspectConfiguration()
+	if err != nil {
+		t.Fatalf("inspect comparison configuration: %v", err)
+	}
+	var expected configkit.Difference
+	for index := 0; index < 3; index++ {
+		differences, err := view.Diff(otherView)
+		if err != nil || len(differences) != 1 {
+			t.Fatalf("compare captured configurations = (%+v, %v), want one difference", differences, err)
+		}
+		if differences[0].Path.String() != "/profiles/offline/model" ||
+			!strings.Contains(differences[0].Left.YAML, model) ||
+			!strings.Contains(differences[0].Right.YAML, otherModel) {
+			t.Fatalf("captured difference = %+v", differences[0])
+		}
+		if index == 0 {
+			expected = differences[0]
+		} else if differences[0].Path.String() != expected.Path.String() ||
+			differences[0].Left.YAML != expected.Left.YAML || differences[0].Right.YAML != expected.Right.YAML {
+			t.Fatalf("captured difference changed between reads: first=%+v next=%+v", expected, differences[0])
+		}
+	}
 	if _, found := view.Explain(configkit.Keys("prompts", "captured", "1", "messages").Index(0).Key("content_file")); !found {
 		t.Fatal("captured prompt content path was not inspectable")
 	}
@@ -167,24 +206,32 @@ func TestConfigurationInspectionUsesCapturedSourcesWithoutProviderCalls(t *testi
 	if err := view.WriteEffective(&report); err != nil || !strings.Contains(report.String(), "captured") {
 		t.Fatalf("write captured report = (%q, %v)", report.String(), err)
 	}
-	if source.reads.Load() != 0 || modelCalls.Load() != 0 {
-		t.Fatalf("inspection touched source or provider: reads=%d requests=%d", source.reads.Load(), modelCalls.Load())
+	if source.reads.Load() != 0 || otherSource.reads.Load() != 0 || modelCalls.Load() != 0 {
+		t.Fatalf("inspection or comparison touched source or provider: reads=(%d,%d) requests=%d", source.reads.Load(), otherSource.reads.Load(), modelCalls.Load())
 	}
 
-	const calls = 16
+	const calls = 24
 	var wait sync.WaitGroup
 	errs := make(chan error, calls)
 	for index := 0; index < calls; index++ {
 		wait.Add(1)
 		go func(index int) {
 			defer wait.Done()
-			if index%2 == 0 {
+			switch index % 3 {
+			case 0:
 				_, err := engine.InspectConfiguration()
 				errs <- err
-				return
+			case 1:
+				_, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "captured", SessionID: "direct"})
+				errs <- err
+			case 2:
+				differences, err := view.Diff(otherView)
+				if err == nil && (len(differences) != 1 || differences[0].Path.String() != expected.Path.String() ||
+					differences[0].Left.YAML != expected.Left.YAML || differences[0].Right.YAML != expected.Right.YAML) {
+					err = errors.New("concurrent comparison returned an unexpected difference")
+				}
+				errs <- err
 			}
-			_, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "captured", SessionID: "direct"})
-			errs <- err
 		}(index)
 	}
 	wait.Wait()
@@ -194,7 +241,7 @@ func TestConfigurationInspectionUsesCapturedSourcesWithoutProviderCalls(t *testi
 			t.Fatal(err)
 		}
 	}
-	if source.reads.Load() != 0 || modelCalls.Load() != calls/2 {
-		t.Fatalf("concurrent inspection/execution = source reads %d, model calls %d", source.reads.Load(), modelCalls.Load())
+	if source.reads.Load() != 0 || otherSource.reads.Load() != 0 || modelCalls.Load() != calls/3 {
+		t.Fatalf("concurrent inspection/comparison/execution = source reads (%d,%d), model calls %d", source.reads.Load(), otherSource.reads.Load(), modelCalls.Load())
 	}
 }
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 7c2632f..36fef47 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -159,6 +159,13 @@ sequence, sensitivity, and ordering cases.
 
 ## Stage 4: Verify Frozen State And Concurrent Inspection
 
+**Status: Complete.** The existing captured-source fixture now compares two
+views repeatedly after both sources become unreadable, and concurrently runs
+inspection, comparison, and offline execution against those captured engines.
+It confirms stable differences without source reads or comparison-triggered
+model calls. `GOTOOLCHAIN=local go test -race . ./internal/configuration
+./internal/engine` and `git diff --check` passed.
+
 Scope: lifecycle and concurrency integration tests.
 
 1. Reuse or extend the existing captured-source inspection fixture and counting