diff --git a/scripts/llm-stub/server.mjs b/scripts/llm-stub/server.mjs
new file mode 100644
index 0000000000..23eea046ab
--- /dev/null
+++ b/scripts/llm-stub/server.mjs
@@ -0,0 +1,63 @@
+#!/usr/bin/env node
+// Deterministic OpenAI-compatible chat stub for ./dev.sh up --ephemeral (D177, TASK-620).
+// Native agents call LITELLM_BASE_URL/chat/completions; pointing that at this server lets a
+// local walk see "agent replies" with no provider key and no quota. The reply text is fixed,
+// so a walk can assert on it.
+//
+//   LLM_STUB_PORT=5102 node scripts/llm-stub/server.mjs
+
+import http from 'node:http';
+import { pathToFileURL } from 'node:url';
+
+export const STUB_REPLY = 'Scout stub reply: the local stack answered.';
+
+function completion(model) {
+  const now = Math.floor(Date.now() / 1000);
+  return {
+    id: 'chatcmpl-stub',
+    object: 'chat.completion',
+    created: now,
+    model: model || 'stub',
+    choices: [{
+      index: 0,
+      message: { role: 'assistant', content: STUB_REPLY },
+      finish_reason: 'stop',
+    }],
+    usage: { prompt_tokens: 1, completion_tokens: 1, total_tokens: 2 },
+  };
+}
+
+export function createStubServer() {
+  return http.createServer((req, res) => {
+    const send = (status, body) => {
+      res.writeHead(status, { 'Content-Type': 'application/json' });
+      res.end(JSON.stringify(body));
+    };
+    if (req.method === 'GET' && req.url === '/health') {
+      send(200, { ok: true });
+      return;
+    }
+    if (req.method === 'GET' && (req.url === '/v1/models' || req.url === '/models')) {
+      send(200, { object: 'list', data: [{ id: 'stub', object: 'model' }] });
+      return;
+    }
+    if (req.method === 'POST' && (req.url === '/v1/chat/completions' || req.url === '/chat/completions')) {
+      let raw = '';
+      req.on('data', (chunk) => { raw += chunk; });
+      req.on('end', () => {
+        let model;
+        try { model = JSON.parse(raw || '{}').model; } catch { /* a malformed body still gets a reply */ }
+        send(200, completion(model));
+      });
+      return;
+    }
+    send(404, { error: { message: `no stub route for ${req.method} ${req.url}` } });
+  });
+}
+
+if (process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href) {
+  const port = Number(process.env.LLM_STUB_PORT || 5102);
+  createStubServer().listen(port, '127.0.0.1', () => {
+    console.log(`llm stub on 127.0.0.1:${port}`);
+  });
+}
diff --git a/scripts/llm-stub/server.test.mjs b/scripts/llm-stub/server.test.mjs
new file mode 100644
index 0000000000..aae7ef991a
--- /dev/null
+++ b/scripts/llm-stub/server.test.mjs
@@ -0,0 +1,90 @@
+// Run: node --test scripts/llm-stub/server.test.mjs
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { createStubServer, STUB_REPLY } from './server.mjs';
+
+function listen(server) {
+  return new Promise((resolve) => server.listen(0, '127.0.0.1', () => resolve(server.address().port)));
+}
+
+test('chat completions return the fixed reply in OpenAI shape, every time', async () => {
+  const server = createStubServer();
+  const port = await listen(server);
+  try {
+    const body = JSON.stringify({ model: 'scout', messages: [{ role: 'user', content: 'hi' }] });
+    const call = () => fetch(`http://127.0.0.1:${port}/v1/chat/completions`, {
+      method: 'POST', headers: { 'Content-Type': 'application/json' }, body,
+    }).then((r) => r.json());
+    const first = await call();
+    const second = await call();
+    assert.equal(first.object, 'chat.completion');
+    assert.equal(first.choices[0].message.role, 'assistant');
+    assert.equal(first.choices[0].message.content, STUB_REPLY);
+    assert.equal(first.choices[0].finish_reason, 'stop');
+    assert.equal(second.choices[0].message.content, first.choices[0].message.content);
+    assert.equal(first.model, 'scout');
+  } finally {
+    server.close();
+  }
+});
+
+test('health, unversioned routes and the 404 body', async () => {
+  const server = createStubServer();
+  const port = await listen(server);
+  const base = `http://127.0.0.1:${port}`;
+  try {
+    const health = await fetch(`${base}/health`);
+    assert.equal(health.status, 200);
+    assert.deepEqual(await health.json(), { ok: true });
+
+    const models = await fetch(`${base}/models`);
+    assert.equal(models.status, 200);
+    assert.equal((await models.json()).data[0].id, 'stub');
+
+    const chat = await fetch(`${base}/chat/completions`, {
+      method: 'POST', headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ model: 'scout', messages: [] }),
+    });
+    assert.equal(chat.status, 200);
+    assert.equal((await chat.json()).choices[0].message.content, STUB_REPLY);
+
+    const missing = await fetch(`${base}/v1/nope`);
+    assert.equal(missing.status, 404);
+    const body = await missing.json();
+    assert.equal(body.error.message, 'no stub route for GET /v1/nope');
+  } finally {
+    server.close();
+  }
+});
+
+test('empty and malformed completion bodies still get the fixed reply with the default model', async () => {
+  const server = createStubServer();
+  const port = await listen(server);
+  const url = `http://127.0.0.1:${port}/v1/chat/completions`;
+  try {
+    for (const body of ['', '{not json']) {
+      const res = await fetch(url, {
+        method: 'POST', headers: { 'Content-Type': 'application/json' }, body,
+      });
+      assert.equal(res.status, 200, `status for body ${JSON.stringify(body)}`);
+      const json = await res.json();
+      assert.equal(json.choices[0].message.content, STUB_REPLY);
+      assert.equal(json.model, 'stub');
+    }
+  } finally {
+    server.close();
+  }
+});
+
+test('models list and unknown routes', async () => {
+  const server = createStubServer();
+  const port = await listen(server);
+  try {
+    const models = await fetch(`http://127.0.0.1:${port}/v1/models`).then((r) => r.json());
+    assert.equal(models.data[0].id, 'stub');
+    const missing = await fetch(`http://127.0.0.1:${port}/v1/nope`);
+    assert.equal(missing.status, 404);
+  } finally {
+    server.close();
+  }
+});