diff --git a/continuation_contract_test.go b/continuation_contract_test.go
new file mode 100644
index 0000000..e837fbd
--- /dev/null
+++ b/continuation_contract_test.go
@@ -0,0 +1,141 @@
+package promptkit_test
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"reflect"
+	"sync"
+	"testing"
+
+	"go.maximumdirect.net/promptkit"
+)
+
+func TestContinuationBranchesAcrossEngines(t *testing.T) {
+	ctx := context.Background()
+	firstClient := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "first answer"}}
+	firstEngine := newSimpleEngine(t, "first question", promptkit.WithLLMClient(firstClient))
+	initial, err := firstEngine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true, APIKey: "synthetic-first-key"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	original := initial.Exchange.Details()
+	secondClient := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "follow-up answer"}}
+	secondEngine := newNamespacedContractEngine(t, secondClient)
+	scoped, err := secondEngine.WithNamespaces(promptkit.Namespaces{Prompt: "alpha", Profile: "alpha"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	request := promptkit.RunRequest{PromptID: "shared", ProfileID: "shared", ContinueFrom: initial.Exchange, SessionID: "new-session", Validation: &promptkit.OutputContract{Format: promptkit.FormatJSON, ValidationMode: promptkit.ValidationJSON}, AppendedMessages: []promptkit.RenderedMessage{{Role: promptkit.RoleUser, Content: "branch one"}}}
+	branch, err := scoped.Run(ctx, request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	request.AppendedMessages[0].Content = "branch two"
+	sibling, err := scoped.Run(ctx, request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	next, err := scoped.Run(ctx, promptkit.RunRequest{PromptID: "shared", ProfileID: "shared", ContinueFrom: branch.Exchange})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if branch.ParentRunID != initial.RunID || sibling.ParentRunID != initial.RunID || next.ParentRunID != branch.RunID || branch.PromptID != "alpha/shared" || branch.SelectedProfileID != "alpha/shared" || branch.SessionID != "new-session" {
+		t.Fatalf("ancestry or selections incorrect: %#v %#v %#v", branch, sibling, next)
+	}
+	if !reflect.DeepEqual(initial.Exchange.Details(), original) {
+		t.Fatal("branching changed source")
+	}
+	if branch.Validation.Status != promptkit.ValidationFailed || branch.ModelName != "alpha-model" {
+		t.Fatalf("follow-up settings: %#v", branch)
+	}
+	branchMessages := branch.Exchange.Details().Messages
+	siblingMessages := sibling.Exchange.Details().Messages
+	if len(branchMessages) != 5 || len(siblingMessages) != 5 || branchMessages[3].Content != "branch one" || siblingMessages[3].Content != "branch two" || len(next.Exchange.Details().Messages) != 7 {
+		t.Fatalf("branch histories = %#v / %#v", branchMessages, siblingMessages)
+	}
+	encoded, err := json.Marshal(branch.Exchange.Details())
+	if err != nil || !json.Valid(encoded) {
+		t.Fatalf("details JSON = %s, %v", encoded, err)
+	}
+	branchMessages[0].Content = "changed details"
+	branch.RawOutput = "changed result"
+	if initial.Exchange.Details().Messages[0].Content != "first question" || branch.Exchange.Details().Messages[4].Content != "follow-up answer" {
+		t.Fatal("caller mutation changed captured history")
+	}
+	for _, request := range secondClient.requests {
+		if request.APIKey == "synthetic-first-key" || request.Prompt.Messages[0].Content != "first question" || request.Prompt.Messages[1].Content != "first answer" {
+			t.Fatalf("unexpected follow-up request: %#v", request)
+		}
+	}
+}
+
+func TestContinuationDoesNotReloadCapturedInputs(t *testing.T) {
+	ctx := context.Background()
+	path := filepath.Join(t.TempDir(), "input.txt")
+	if err := os.WriteFile(path, []byte("original input"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: ""}}
+	engine := newSimpleEngine(t, `{{input "source"}}`, promptkit.WithLLMClient(client))
+	initial, err := engine.Run(ctx, promptkit.RunRequest{PromptID: "prompt", Inputs: map[string]promptkit.ArtifactRef{"source": promptkit.File(path)}, CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte("new input"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	request := promptkit.RunRequest{PromptID: "prompt", Inputs: map[string]promptkit.ArtifactRef{"source": promptkit.File(path)}, ContinueFrom: initial.Exchange}
+	prepared, err := engine.PrepareExecution(ctx, request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Remove(path); err != nil {
+		t.Fatal(err)
+	}
+	result, err := engine.RunPrepared(ctx, prepared)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []promptkit.RenderedMessage{{Role: promptkit.RoleUser, Content: "original input"}, {Role: promptkit.RoleAssistant, Content: ""}, {Role: promptkit.RoleUser, Content: "new input"}, {Role: promptkit.RoleAssistant, Content: ""}}
+	if !reflect.DeepEqual(result.Exchange.Details().Messages, want) {
+		t.Fatalf("history = %#v", result.Exchange.Details().Messages)
+	}
+	if result, err := engine.Run(ctx, request); err == nil || result != nil {
+		t.Fatalf("missing new input = %#v, %v", result, err)
+	}
+}
+
+func TestContinuationSourceSupportsConcurrentRuns(t *testing.T) {
+	client := &preparedRecordingClient{response: &promptkit.GenerateResponse{Content: "answer"}}
+	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(client))
+	initial, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", CaptureExchange: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	const count = 8
+	var wg sync.WaitGroup
+	errors := make(chan error, count)
+	for i := 0; i < count; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			result, err := engine.Run(context.Background(), promptkit.RunRequest{PromptID: "prompt", ContinueFrom: initial.Exchange})
+			if err == nil && (result.Exchange == nil || result.ParentRunID != initial.RunID) {
+				t.Error("missing continuation ancestry")
+			}
+			errors <- err
+		}()
+	}
+	wg.Wait()
+	close(errors)
+	for err := range errors {
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+	if len(initial.Exchange.Details().Messages) != 2 {
+		t.Fatal("source changed")
+	}
+}
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index f66f5e7..92b6336 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -413,6 +413,8 @@ Do not add a separate preparation or validation algorithm for continuation.
 
 ## Stage 5 — Verify Independent Continuation At The Public Boundary
 
+**Status:** Complete.
+
 **Scope:** assembled continuation behavior before adding coordination.
 
 1. Add an external-package contract test that captures an initial exchange and