diff --git a/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts b/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts
index f9c5fc3..2cb058a 100644
--- a/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts
+++ b/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts
@@ -22,6 +22,7 @@ interface BrokerPaths {
 const frameworkRoot = new URL('../../framework/', import.meta.url).pathname;
 const daemonPath = join(frameworkRoot, 'tools/lease-broker/daemon.py');
 const gatePath = join(frameworkRoot, 'tools/lease-broker/mutator-gate.py');
+const launcherPath = join(frameworkRoot, 'tools/lease-broker/launch-runtime.py');
 const claudeSettingsPath = join(frameworkRoot, 'runtime/claude/settings.json');
 const piExtensionPath = join(frameworkRoot, 'runtime/pi/mosaic-extension.ts');
 const children: ChildProcess[] = [];
@@ -288,6 +289,51 @@ describe('whole mutator-class lease gate', () => {
     });
   });
 
+  test('runtime launcher anchors broker identity before exec and fails closed without broker', async () => {
+    const { socket } = await startBroker();
+    const probe = [
+      'import json,os,socket',
+      's=socket.socket(socket.AF_UNIX,socket.SOCK_STREAM)',
+      "s.connect(os.environ['MOSAIC_LEASE_BROKER_SOCKET'])",
+      "request={'action':'authorize_tool','session_id':os.environ['MOSAIC_LEASE_SESSION_ID'],'runtime_generation':int(os.environ['MOSAIC_RUNTIME_GENERATION']),'runtime':'claude','tool_name':'Read'}",
+      "s.sendall((json.dumps(request)+'\\n').encode())",
+      's.shutdown(socket.SHUT_WR)',
+      "print(json.dumps({'session_id':os.environ['MOSAIC_LEASE_SESSION_ID'],'reply':json.loads(s.recv(65536))}))",
+    ].join(';');
+    const launched = spawnSync(
+      'python3',
+      [launcherPath, '--runtime', 'claude', '--', 'python3', '-c', probe],
+      {
+        encoding: 'utf8',
+        env: {
+          ...process.env,
+          MOSAIC_LEASE_BROKER_SOCKET: socket,
+          MOSAIC_RUNTIME_GENERATION: '1',
+        },
+      },
+    );
+    expect(launched.status, launched.stderr).toBe(0);
+    expect(JSON.parse(launched.stdout)).toMatchObject({
+      session_id: expect.stringMatching(/^[a-f0-9]{64}$/),
+      reply: { ok: true, decision: 'allow' },
+    });
+
+    const unavailable = spawnSync(
+      'python3',
+      [launcherPath, '--runtime', 'claude', '--', 'python3', '-c', "print('EXECUTED')"],
+      {
+        encoding: 'utf8',
+        env: {
+          ...process.env,
+          MOSAIC_LEASE_BROKER_SOCKET: join(tmpdir(), 'missing-mosaic-broker.sock'),
+          MOSAIC_RUNTIME_GENERATION: '1',
+        },
+      },
+    );
+    expect(unavailable.status).not.toBe(0);
+    expect(unavailable.stdout).not.toContain('EXECUTED');
+  });
+
   test('Claude and Pi runtime adapters consult the broker for every tool class', async () => {
     const { socket } = await startBroker();
     const sessionId = await register(socket);