diff --git a/cockpit/langgraph/client-tools/angular/e2e/checkpoint-protocol.spec.ts b/cockpit/langgraph/client-tools/angular/e2e/checkpoint-protocol.spec.ts
new file mode 100644
index 000000000..9c53e836c
--- /dev/null
+++ b/cockpit/langgraph/client-tools/angular/e2e/checkpoint-protocol.spec.ts
@@ -0,0 +1,244 @@
+import { createServer } from 'node:http';
+import { test, expect } from '@playwright/test';
+import { Client, type Checkpoint, type ThreadState } from '@langchain/langgraph-sdk';
+
+// This lane uses the published middleware from python/uv.lock (0.1.0), not a
+// PYTHONPATH override to packages/threadplane-middleware. It proves the actual
+// client-tools graph/API protocol, not browser tool execution or store claims.
+const prompt = 'Checkpoint protocol weather in Paris';
+const input = {
+  messages: [{ type: 'human', content: prompt }],
+  client_tools: [{
+    name: 'get_weather',
+    description: 'Get weather for a location',
+    parameters: { type: 'object', properties: { location: { type: 'string' } } },
+  }],
+};
+type ToolCall = { id: string; name: string; args: Record<string, unknown> };
+type State = { messages: { type: string; content: unknown; tool_calls?: ToolCall[]; tool_call_id?: string }[] };
+type CheckpointEvent = {
+  id?: string;
+  event: string;
+  data: { config: { configurable?: Record<string, unknown> }; values: State; next: string[]; metadata: Record<string, unknown> };
+};
+
+function backendUrl(): string {
+  const url = process.env['CLIENT_TOOLS_API_URL'];
+  if (!url) throw new Error('Global setup must expose the local client-tools API URL');
+  return url;
+}
+
+function client() {
+  return new Client<State>({ apiUrl: backendUrl(), apiKey: null, callerOptions: { maxRetries: 0 }, timeoutMs: 20_000 });
+}
+
+// Copy only supported routing fields, never the complete event configurable
+// object (which may also contain run identity or request-specific metadata).
+function position(config: Record<string, unknown> | undefined): Checkpoint & { checkpoint_id: string } {
+  if (!config) throw new Error('Checkpoint event must include routing configuration');
+  expect(config['thread_id']).toEqual(expect.any(String));
+  expect(config['checkpoint_ns']).toEqual(expect.any(String));
+  expect(config['checkpoint_id']).toEqual(expect.any(String));
+  return {
+    thread_id: config['thread_id'] as string,
+    checkpoint_ns: config['checkpoint_ns'] as string,
+    checkpoint_id: config['checkpoint_id'] as string,
+    checkpoint_map: undefined,
+  };
+}
+
+async function run(api: Client<State>, threadId: string, checkpoint?: Checkpoint, values: Record<string, unknown> | null = null) {
+  let final: CheckpointEvent | undefined;
+  for await (const event of api.runs.stream(threadId, 'client-tools', {
+    input: values, checkpoint, streamMode: ['values', 'checkpoints'], signal: AbortSignal.timeout(20_000),
+  })) {
+    expect(event.event).not.toBe('error');
+    if (event.event === 'checkpoints') final = event as CheckpointEvent;
+  }
+  if (!final) throw new Error('Run must emit a checkpoint event');
+  const saved = await api.threads.getState(threadId, position(final.data.config.configurable));
+  expect(saved.values).toEqual(final.data.values);
+  expect(saved.next).toEqual(final.data.next);
+  return saved;
+}
+
+function calls(state: ThreadState<State>) {
+  return state.values.messages.flatMap((message) => message.tool_calls ?? []);
+}
+
+async function modelRequests() {
+  const url = process.env['CLIENT_TOOLS_AIMOCK_URL'];
+  if (!url) throw new Error('Global setup must expose the local aimock journal URL');
+  const response = await fetch(`${url}/__aimock/journal`, { signal: AbortSignal.timeout(5_000) });
+  expect(response.ok).toBe(true);
+  const entries = await response.json() as { body?: { messages?: { role: string; content: unknown }[] } }[];
+  return entries.filter((entry) => entry.body?.messages?.some((message) => message.role === 'user' && message.content === prompt)).length;
+}
+
+test('checkpoint protocol: terminal replay preserves a pending call; pre-agent replay creates a new call', async () => {
+  const api = client();
+  const { thread_id: threadId } = await api.threads.create();
+  try {
+    const pending = await run(api, threadId, undefined, input);
+    expect(pending.next).toEqual([]);
+    expect(calls(pending)).toHaveLength(1);
+    const original = calls(pending)[0];
+    expect(original).toMatchObject({ id: expect.stringMatching(/\S/), name: 'get_weather', args: { location: 'Paris' } });
+    const beforeAgent = (await api.threads.getHistory(threadId)).find((state) => state.next.includes('agent'));
+    if (!beforeAgent) throw new Error('History must include a checkpoint before the agent runs');
+    expect(calls(beforeAgent)).toEqual([]);
+
+    const count = await modelRequests();
+    const replayed = await run(api, threadId, pending.checkpoint);
+    expect(replayed.next).toEqual([]);
+    expect(replayed.values).toEqual(pending.values);
+    expect(calls(replayed)).toEqual([original]);
+    expect(await modelRequests()).toBe(count);
+
+    const regenerated = await run(api, threadId, beforeAgent.checkpoint);
+    expect(calls(regenerated)).toHaveLength(1);
+    expect(calls(regenerated)[0]).toMatchObject({ id: expect.stringMatching(/\S/), name: original.name, args: original.args });
+    expect(calls(regenerated)[0].id).not.toBe(original.id);
+    expect(await modelRequests()).toBe(count + 1);
+
+    const completed = await run(api, threadId, pending.checkpoint, {
+      messages: [{ type: 'tool', tool_call_id: original.id, content: '68°F' }],
+    });
+    expect(completed.next).toEqual([]);
+    expect(completed.values.messages.filter((message) => message.type === 'tool')).toMatchObject([{ tool_call_id: original.id }]);
+    expect(completed.values.messages.at(-1)).toMatchObject({ type: 'ai', content: 'Protocol weather complete: Paris is 68°F.' });
+    expect(await modelRequests()).toBe(count + 2);
+    const completedReplay = await run(api, threadId, completed.checkpoint);
+    expect(completedReplay.next).toEqual([]);
+    expect(completedReplay.values).toEqual(completed.values);
+    expect(await modelRequests()).toBe(count + 2);
+  } finally {
+    await api.threads.delete(threadId);
+  }
+});
+
+test('checkpoint protocol: reconnect retains the run and exact saved checkpoint identity', async () => {
+  const requests: { path: string; method: string }[] = [];
+  const api = new Client<State>({
+    apiUrl: backendUrl(), apiKey: null, callerOptions: { maxRetries: 0 }, timeoutMs: 20_000,
+    onRequest: (url, init) => {
+      requests.push({ path: url.pathname, method: init.method ?? 'GET' });
+      return init;
+    },
+  });
+  const { thread_id: threadId } = await api.threads.create();
+  const controller = new AbortController();
+  try {
+    let runId: string | undefined;
+    let cursor: string | undefined;
+    let disconnectedCheckpoint: string | undefined;
+    try {
+      for await (const event of api.runs.stream(threadId, 'client-tools', {
+        input, streamMode: ['values', 'checkpoints'], streamResumable: true,
+        onDisconnect: 'continue', signal: AbortSignal.any([controller.signal, AbortSignal.timeout(20_000)]),
+      })) {
+        expect(event.event).not.toBe('error');
+        if (event.event === 'metadata') runId = event.data.run_id;
+        if (event.event === 'checkpoints' && event.id && event.data.next.includes('agent')) {
+          cursor = event.id;
+          disconnectedCheckpoint = position(event.data.config.configurable).checkpoint_id;
+          controller.abort();
+          break;
+        }
+      }
+    } catch (error) {
+      if (!controller.signal.aborted) throw error;
+    }
+    if (!runId || !cursor) throw new Error('Disconnected stream must provide a run ID and checkpoint cursor');
+    const checkpoints: CheckpointEvent[] = [];
+    for await (const event of api.runs.joinStream(threadId, runId, {
+      lastEventId: cursor, cancelOnDisconnect: false, streamMode: ['values', 'checkpoints'], signal: AbortSignal.timeout(20_000),
+    })) {
+      expect(event.event).not.toBe('error');
+      expect(event.id).not.toBe(cursor);
+      if (event.event === 'checkpoints') checkpoints.push(event as CheckpointEvent);
+    }
+    const final = checkpoints.at(-1);
+    if (!final) throw new Error('Reconnected stream must emit a checkpoint event');
+    const config = final.data.config.configurable;
+    if (!config) throw new Error('Reconnected checkpoint must include routing configuration');
+    expect(config['run_id']).toBe(runId);
+    // Event metadata does not carry run_id; the retained config and the saved
+    // checkpoint metadata are the two independently checked run identities.
+    expect(final.data.metadata['run_id']).toBeUndefined();
+    const exactPosition = position(config);
+    expect(exactPosition.checkpoint_id).not.toBe(disconnectedCheckpoint);
+    const saved = await api.threads.getState(threadId, exactPosition);
+    expect(saved.checkpoint.checkpoint_id).toBe(exactPosition.checkpoint_id);
+    expect(saved.metadata?.['run_id']).toBe(runId);
+    expect(saved.values).toEqual(final.data.values);
+    expect(saved.next).toEqual([]);
+    expect((await api.runs.get(threadId, runId)).status).toBe('success');
+    expect(requests.filter((request) => request.method === 'POST' && /^\/threads\/[^/]+\/runs(?:\/stream|\/wait)?$/.test(request.path)))
+      .toEqual([{ method: 'POST', path: `/threads/${threadId}/runs/stream` }]);
+    expect(requests).toContainEqual({ method: 'GET', path: `/threads/${threadId}/runs/${runId}/stream` });
+  } finally {
+    controller.abort();
+    await api.threads.delete(threadId);
+  }
+});
+
+test('checkpoint protocol: a lost write response is ambiguous even with retries disabled', async () => {
+  const api = client();
+  const { thread_id: threadId } = await api.threads.create();
+  const accepted: Checkpoint[] = [];
+  const proxyErrors: unknown[] = [];
+  let writeRequests = 0;
+  const proxy = createServer(async (request, response) => {
+    try {
+      expect(request.method).toBe('POST');
+      expect(request.url).toBe(`/threads/${threadId}/state`);
+      writeRequests++;
+      const chunks: Buffer[] = [];
+      for await (const chunk of request) chunks.push(Buffer.from(chunk));
+      const upstream = await fetch(`${backendUrl()}${request.url}`, {
+        method: 'POST', headers: { 'content-type': 'application/json' }, body: Buffer.concat(chunks), signal: AbortSignal.timeout(10_000),
+      });
+      expect(upstream.status).toBe(200);
+      const result = await upstream.json() as { configurable: Record<string, unknown> };
+      accepted.push(position(result.configurable));
+      // Upstream has accepted and returned the child position. Only this test
+      // proxy knows it: the caller receives no successful response to adopt.
+      response.destroy();
+    } catch (error) {
+      proxyErrors.push(error);
+      response.writeHead(500).end();
+    }
+  });
+  try {
+    const parent = await run(api, threadId, undefined, input);
+    await new Promise<void>((resolve, reject) => {
+      proxy.once('error', reject);
+      proxy.listen(0, '127.0.0.1', resolve);
+    });
+    const address = proxy.address();
+    if (!address || typeof address === 'string') throw new Error('Proxy did not bind a TCP port');
+    const caller = new Client({
+      apiUrl: `http://127.0.0.1:${address.port}`, apiKey: null,
+      // Mirrors the owned session default; standalone SDK defaults differ.
+      callerOptions: { maxRetries: 0 },
+    });
+    await expect(caller.threads.updateState(threadId, {
+      checkpoint: position(parent.checkpoint), values: { messages: [] }, asNode: 'agent', signal: AbortSignal.timeout(15_000),
+    })).rejects.toThrow();
+    expect(proxyErrors).toEqual([]);
+    expect(writeRequests).toBe(1);
+    expect(accepted).toHaveLength(1);
+    expect(accepted[0].checkpoint_id).not.toBe(parent.checkpoint.checkpoint_id);
+    // Read the proxy-observed child, never the thread's global latest tip.
+    const child = await api.threads.getState(threadId, accepted[0]);
+    expect(child.checkpoint.checkpoint_id).toBe(accepted[0].checkpoint_id);
+    expect(child.parent_checkpoint?.checkpoint_id).toBe(parent.checkpoint.checkpoint_id);
+    expect(child.values).toEqual(parent.values);
+    expect(child.metadata?.['source']).toBe('update');
+  } finally {
+    proxy.closeAllConnections();
+    if (proxy.listening) await new Promise<void>((resolve) => proxy.close(() => resolve()));
+    await api.threads.delete(threadId);
+  }
+});
diff --git a/cockpit/langgraph/client-tools/angular/e2e/fixtures/checkpoint-protocol.json b/cockpit/langgraph/client-tools/angular/e2e/fixtures/checkpoint-protocol.json
new file mode 100644
index 000000000..2164aa5c3
--- /dev/null
+++ b/cockpit/langgraph/client-tools/angular/e2e/fixtures/checkpoint-protocol.json
@@ -0,0 +1,13 @@
+{
+  "fixtures": [
+    {
+      "match": { "userMessage": "Checkpoint protocol weather in Paris", "hasToolResult": true },
+      "response": { "content": "Protocol weather complete: Paris is 68°F." }
+    },
+    {
+      "match": { "userMessage": "Checkpoint protocol weather in Paris" },
+      "response": { "toolCalls": [{ "name": "get_weather", "arguments": { "location": "Paris" } }] },
+      "latency": 250
+    }
+  ]
+}
diff --git a/cockpit/langgraph/client-tools/angular/e2e/global-setup-impl.ts b/cockpit/langgraph/client-tools/angular/e2e/global-setup-impl.ts
index 26c29df26..194c29c1f 100644
--- a/cockpit/langgraph/client-tools/angular/e2e/global-setup-impl.ts
+++ b/cockpit/langgraph/client-tools/angular/e2e/global-setup-impl.ts
@@ -4,10 +4,20 @@ import { createGlobalSetup } from '@threadplane-internal/e2e-harness';
 
 const ports = portsFor('cockpit-langgraph-client-tools-angular');
 
-export default createGlobalSetup({
+const setup = createGlobalSetup({
   langgraphCwd: 'cockpit/langgraph/client-tools/python',
   langgraphPort: ports.langgraph,
   angularProject: 'cockpit-langgraph-client-tools-angular',
   angularPort: ports.angular,
   fixturesDir: resolve(__dirname, 'fixtures'),
 });
+
+export default async function globalSetup(): Promise<void> {
+  await setup();
+  const harness = globalThis.__AIMOCK_HARNESS_STATE__?.get('cockpit-langgraph-client-tools-angular');
+  if (!harness) throw new Error('Client-tools harness did not start');
+  // Playwright forwards setup environment changes to workers. Expose the local
+  // API and fixture journal URLs for protocol tests and model request counts.
+  process.env['CLIENT_TOOLS_API_URL'] = `http://127.0.0.1:${ports.langgraph}`;
+  process.env['CLIENT_TOOLS_AIMOCK_URL'] = `http://127.0.0.1:${harness.aimock.port}`;
+}