diff --git a/.github/workflows/fleet-e2e.yml b/.github/workflows/fleet-e2e.yml
index 21e7221a36..4fc229d2ec 100644
--- a/.github/workflows/fleet-e2e.yml
+++ b/.github/workflows/fleet-e2e.yml
@@ -73,7 +73,7 @@ jobs:
         with:
           persist-credentials: false
           repository: AgentWorkforce/relaycast
-          ref: a5c7ac66f64a650274fbe2c60e3cd5d3c9b87dcd # relaycast#387: verified served-node spawn completion
+          ref: 716baa88dcafb91481b4bbd0e1d640352e416834 # relaycast#452: agent@machine addressing on POST /v1/dm
           path: relaycast-engine
 
       - name: Setup Node.js
diff --git a/tests/e2e/fleet/address-e2e.test.ts b/tests/e2e/fleet/address-e2e.test.ts
new file mode 100644
index 0000000000..5233f255b7
--- /dev/null
+++ b/tests/e2e/fleet/address-e2e.test.ts
@@ -0,0 +1,234 @@
+import { readFileSync } from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { DatabaseSync } from 'node:sqlite';
+import { afterAll, beforeAll, describe, expect, it } from 'vitest';
+import {
+  cleanupTmp,
+  createWorkspace,
+  FleetNode,
+  getInvocation,
+  getNodes,
+  invokeAction,
+  makeTmpRoot,
+  preflight,
+  registerAgent,
+  startCloudEnrollmentEndpoint,
+  startEngine,
+  waitFor,
+  type EngineHandle,
+} from './harness.js';
+
+/**
+ * `agent@machine` addressing (relaycast `POST /v1/dm` with `address`) against a real
+ * engine and a real `relay node up` (Rust broker + sidecar) enrolled the way a
+ * Cloud sandbox is: through `relay cloud enroll`, named `fleet-sandbox-<uuid>`,
+ * carrying server-owned `cloud:*` tags, with no machine_id.
+ *
+ * Skips cleanly when prerequisites are absent (see harness preflight).
+ */
+const pre = preflight();
+
+const SANDBOX_UUID = '0b7c2f4e-5d1a-4c3b-9e8f-7a6b5c4d3e2f';
+const SANDBOX_NODE_NAME = `fleet-sandbox-${SANDBOX_UUID}`; // matches nodes/sandbox.ts
+const SANDBOX_NODE_ID = 'node_fleet_sandbox';
+const SANDBOX_NODE_FILE = fileURLToPath(new URL('./nodes/sandbox.ts', import.meta.url));
+const AGENT = 'sbx-worker';
+const ADDRESS = `${AGENT}@${SANDBOX_NODE_NAME}`;
+
+describe.skipIf(!pre.ok)('agent@machine addressing to a Cloud-shaped sandbox node', () => {
+  let tmpRoot: string;
+  let engine: EngineHandle;
+  let workspaceKey: string;
+  let driverToken: string;
+  let sandbox: FleetNode;
+
+  const auth = (token: string) => ({ authorization: `Bearer ${token}` });
+
+  const sendTo = (address: string, text: string, headers: Record<string, string> = {}) =>
+    engine.fetchJson('/v1/dm', {
+      method: 'POST',
+      headers: { 'content-type': 'application/json', ...auth(driverToken), ...headers },
+      body: JSON.stringify({ address, text }),
+    });
+
+  const agentAddress = async (name: string) => {
+    const { status, body } = await engine.fetchJson(`/v1/agents/${name}`, { headers: auth(workspaceKey) });
+    expect(status).toBe(200);
+    return body.data.address as string | null;
+  };
+
+  /** The stub PTY agent writes one file per nonce it reads from its terminal. */
+  const waitForNonce = (nonce: string, label: string) =>
+    waitFor(
+      async () => {
+        try {
+          return JSON.parse(
+            readFileSync(
+              path.join(sandbox.projectDir, '.agentworkforce', 'relay', 'e2e-brief-actions', `${nonce}.json`),
+              'utf8'
+            )
+          ) as { nonce: string; agent: string; node: string };
+        } catch {
+          return null;
+        }
+      },
+      { label, timeoutMs: 40_000 }
+    );
+
+  beforeAll(async () => {
+    tmpRoot = makeTmpRoot();
+    engine = await startEngine(pre.engineServe!, tmpRoot);
+    workspaceKey = await createWorkspace(engine, 'address-e2e');
+    driverToken = await registerAgent(engine, workspaceKey, 'driver');
+
+    // Cloud's register route enrolls the node on the sandbox's behalf with its
+    // provider/sandbox-id tags; model that exact POST /v1/nodes body.
+    const enrolled = await engine.fetchJson('/v1/nodes', {
+      method: 'POST',
+      headers: { 'content-type': 'application/json', ...auth(workspaceKey) },
+      body: JSON.stringify({
+        node_id: SANDBOX_NODE_ID,
+        name: SANDBOX_NODE_NAME,
+        capabilities: ['spawn:claude', 'sandbox:ping'],
+        max_agents: 1,
+        tags: ['cloud:sandbox-provider:daytona', `cloud:sandbox-id:sbx_${SANDBOX_UUID}`],
+      }),
+    });
+    expect(enrolled.status).toBe(201);
+
+    const enrollmentToken = 'ocl_node_enr_address_e2e';
+    const endpoint = await startCloudEnrollmentEndpoint({
+      enrollmentToken,
+      nodeId: SANDBOX_NODE_ID,
+      nodeName: SANDBOX_NODE_NAME,
+      nodeToken: enrolled.body.data.token,
+      relayWorkspaceId: 'fleet-e2e',
+      relaycastUrl: engine.baseUrl,
+    });
+    sandbox = new FleetNode({
+      name: SANDBOX_NODE_NAME,
+      nodeId: SANDBOX_NODE_ID,
+      nodeFile: SANDBOX_NODE_FILE,
+      nodeToken: enrolled.body.data.token,
+      workspaceKey,
+      engineBaseUrl: engine.baseUrl,
+      brokerBinary: pre.brokerBinary!,
+      tmpRoot,
+      capacityHarnesses: 'claude',
+      usePersistedEnrollment: true,
+    });
+    try {
+      await sandbox.cloudEnroll(endpoint.url, enrollmentToken);
+    } finally {
+      await endpoint.stop();
+    }
+    sandbox.start();
+
+    await waitFor(
+      async () => {
+        const [node] = await getNodes(engine, workspaceKey, { name: SANDBOX_NODE_NAME });
+        return node?.live &&
+          node.handlers_live &&
+          node.capabilities.some((capability) => capability.name === 'sandbox:ping')
+          ? node
+          : null;
+      },
+      { timeoutMs: 45_000, label: 'sandbox node online with its sidecar provider' }
+    );
+
+    // Spawn a real PTY agent onto the sandbox and wait until it has provably
+    // acted on its brief, so later injections land after harness readiness.
+    const briefNonce = `address-brief-${Date.now().toString(36)}`;
+    const spawn = await invokeAction(engine, driverToken, 'spawn', {
+      cli: 'claude',
+      name: AGENT,
+      target_node: SANDBOX_NODE_NAME,
+      task: `Record RELAY_E2E_BRIEF_NONCE=${briefNonce} `,
+    });
+    expect(spawn.status).toBe(201);
+    const done = await waitFor(
+      async () => {
+        const invocation = await getInvocation(engine, driverToken, 'spawn', spawn.invocationId!);
+        return invocation.status === 'completed' || invocation.status === 'failed' ? invocation : null;
+      },
+      { label: 'sandbox spawn settled', timeoutMs: 35_000 }
+    );
+    expect(done.status).toBe('completed');
+    await waitForNonce(briefNonce, 'spawned agent acted on its brief');
+  }, 150_000);
+
+  afterAll(async () => {
+    await sandbox?.stop();
+    await engine?.stop();
+    if (tmpRoot && !process.env.CI) cleanupTmp(tmpRoot);
+  });
+
+  it("reports the sandboxed agent's address from its sandbox node name", async () => {
+    expect(await agentAddress(AGENT)).toBe(ADDRESS);
+    expect(await agentAddress('driver')).toBe('driver@direct');
+  });
+
+  it('delivers a DM sent to agent@machine into the agent process on that sandbox', async () => {
+    const nonce = `address-dm-${Date.now().toString(36)}`;
+    const res = await sendTo(ADDRESS, `Record RELAY_E2E_BRIEF_NONCE=${nonce} `);
+    expect(res.status).toBe(201);
+    // The recipient is handed the sender's address to reply on.
+    expect(res.body.data.message.agent_address).toBe('driver@direct');
+
+    const observed = await waitForNonce(nonce, 'addressed DM reached the sandboxed PTY agent');
+    expect(observed).toMatchObject({ nonce, agent: AGENT, node: SANDBOX_NODE_NAME });
+  }, 60_000);
+
+  it('replays an idempotent retry instead of injecting twice', async () => {
+    const key = { 'Idempotency-Key': `address-e2e-${Date.now()}` };
+    const first = await sendTo(ADDRESS, 'once', key);
+    const retry = await sendTo(ADDRESS, 'once', key);
+    expect(first.status).toBe(201);
+    expect(retry.status).toBe(201);
+    expect(first.body.data.message.id).toEqual(expect.any(String));
+    expect(retry.body.data.message.id).toBe(first.body.data.message.id);
+  });
+
+  it('rejects addresses that do not name where the agent runs', async () => {
+    for (const address of [
+      `${AGENT}@direct`,
+      `${AGENT}@sbx_${SANDBOX_UUID}`, // cloud:* tags are not machine names
+      `${AGENT}@fleet-sandbox-00000000-0000-4000-8000-000000000000`,
+      `nobody@${SANDBOX_NODE_NAME}`,
+    ]) {
+      const res = await sendTo(address, 'x');
+      expect(res.status, address).toBe(404);
+      expect(res.body.error.code).toBe('address_not_found');
+    }
+    expect((await sendTo(AGENT, 'x')).status).toBe(400);
+  });
+
+  it('leaves the agent unaddressable once the sandbox is torn down', async () => {
+    // Cloud's teardown: the sandbox goes away, then its node row is deleted
+    // outright (relaycast-cloud fleet/routes.ts handleDeleteNode), which nulls
+    // the agent's location through the foreign key.
+    await sandbox.stop();
+    await waitFor(
+      async () => {
+        const [node] = await getNodes(engine, workspaceKey, { name: SANDBOX_NODE_NAME });
+        return node && !node.live ? node : null;
+      },
+      { timeoutMs: 30_000, label: 'sandbox node offline' }
+    );
+    const db = new DatabaseSync(path.join(tmpRoot, 'relaycast.db'));
+    try {
+      db.exec('PRAGMA foreign_keys = ON');
+      db.prepare('DELETE FROM nodes WHERE id = ?').run(SANDBOX_NODE_ID);
+    } finally {
+      db.close();
+    }
+
+    expect(await agentAddress(AGENT)).toBeNull();
+    for (const address of [ADDRESS, `${AGENT}@direct`]) {
+      const res = await sendTo(address, 'into the void');
+      expect(res.status, address).toBe(404);
+      expect(res.body.error.code).toBe('address_not_found');
+    }
+  }, 60_000);
+});
diff --git a/tests/e2e/fleet/nodes/sandbox.ts b/tests/e2e/fleet/nodes/sandbox.ts
new file mode 100644
index 0000000000..0681268853
--- /dev/null
+++ b/tests/e2e/fleet/nodes/sandbox.ts
@@ -0,0 +1,30 @@
+import { fileURLToPath } from 'node:url';
+import { z } from 'zod';
+import { definePtyHarness } from '@agent-relay/harnesses';
+import { action, defineNode, spawn } from '@agent-relay/fleet';
+
+/**
+ * A node shaped like a Cloud fleet sandbox: `relay fleet spawn --sandbox`
+ * names it `fleet-sandbox-<uuid>` (the uuid of its `sbx_<uuid>` id).
+ * Keep in sync with SANDBOX_NODE_NAME in address-e2e.test.ts.
+ */
+const SANDBOX_NODE_NAME = 'fleet-sandbox-0b7c2f4e-5d1a-4c3b-9e8f-7a6b5c4d3e2f';
+
+const stub = definePtyHarness({
+  runtime: 'pty',
+  command: process.execPath,
+  args: [fileURLToPath(new URL('./stub-agent.cjs', import.meta.url))],
+  env: { RELAY_E2E_NODE_NAME: SANDBOX_NODE_NAME, RELAY_INJECT_RATE_MS: '0' },
+});
+
+export default defineNode({
+  name: SANDBOX_NODE_NAME,
+  maxAgents: 1,
+  capabilities: {
+    'spawn:claude': spawn(stub),
+    // Served only by this node's sidecar provider. The broker's native provider
+    // also advertises spawn:claude, so the roster showing this action is the
+    // signal that the stub shadow is registered and will win the spawn.
+    'sandbox:ping': action({ input: z.object({}) }, async () => ({ pong: true })),
+  },
+});