diff --git a/packages/server/src/routes/dm.federatedId.test.ts b/packages/server/src/routes/dm.federatedId.test.ts
new file mode 100644
index 00000000..67669804
--- /dev/null
+++ b/packages/server/src/routes/dm.federatedId.test.ts
@@ -0,0 +1,144 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import Fastify, { type FastifyInstance } from 'fastify';
+import Database from 'better-sqlite3';
+import { drizzle } from 'drizzle-orm/better-sqlite3';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import * as schema from '../db/schema.js';
+import { setWorkerId } from '../utils/snowflake.js';
+
+setWorkerId(1);
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let sqlite: Database.Database;
+let testDb: TestDb;
+let currentUserId = 'user-A';
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  schema,
+}));
+
+vi.mock('../utils/auth.js', () => ({
+  authenticate: async (req: { userId?: string }) => {
+    req.userId = currentUserId;
+  },
+}));
+
+vi.mock('../ws/handler.js', () => ({
+  connectionManager: {
+    sendToUser: vi.fn(),
+    sendToDmMembers: vi.fn(),
+    sendToAdmins: vi.fn(),
+    getAllOnlineUserIds: () => [],
+  },
+}));
+
+vi.mock('../utils/federationOutbox.js', async () => {
+  const actual = await vi.importActual<typeof import('../utils/federationOutbox.js')>('../utils/federationOutbox.js');
+  return {
+    ...actual,
+    isFederationRelayEnabled: () => true,
+    queueDmCloseRelay: vi.fn(),
+    sendTypingRelay: vi.fn(),
+    queueDmRelay: vi.fn(),
+    queueOutboxEvent: vi.fn(),
+    appendMutationLog: vi.fn(),
+  };
+});
+
+function applyMigrations(db: Database.Database): void {
+  const migrationsDir = path.resolve(__dirname, '../../drizzle');
+  const files = fs.readdirSync(migrationsDir).filter(f => f.endsWith('.sql')).sort();
+  for (const f of files) {
+    const sqlText = fs.readFileSync(path.join(migrationsDir, f), 'utf8');
+    const statements = sqlText.split(/-->\s*statement-breakpoint/);
+    for (const stmt of statements) {
+      const clean = stmt.trim();
+      if (clean) db.exec(clean);
+    }
+  }
+}
+
+function seedTwoUsers(): void {
+  testDb.insert(schema.users).values({
+    id: 'user-A',
+    username: 'alice',
+    displayName: 'Alice',
+    passwordHash: 'x',
+    homeUserId: 'user-A',
+    homeInstance: 'https://local.example',
+    createdAt: Date.now(),
+  }).run();
+
+  testDb.insert(schema.users).values({
+    id: 'user-B',
+    username: 'bob',
+    displayName: 'Bob',
+    passwordHash: 'x',
+    homeUserId: 'remote-bob',
+    homeInstance: 'https://remote.example',
+    createdAt: Date.now(),
+  }).run();
+}
+
+async function buildApp(): Promise<FastifyInstance> {
+  const app = Fastify({ logger: false });
+  const { dmRoutes } = await import('./dm.js');
+  await app.register(dmRoutes);
+  await app.ready();
+  return app;
+}
+
+describe('POST /api/dm — idempotent existing DM response includes federatedId', () => {
+  let app: FastifyInstance;
+
+  beforeEach(async () => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+    seedTwoUsers();
+    currentUserId = 'user-A';
+    app = await buildApp();
+  });
+
+  it('fresh-create returns a federatedId for federated 1-on-1 DM', async () => {
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/dm',
+      payload: { userId: 'user-B' },
+    });
+
+    expect(response.statusCode).toBe(201);
+    const body = response.json() as { id: string; federatedId: string | null };
+    expect(body.federatedId).toMatch(/^[a-f0-9]{32}$/);
+  });
+
+  it('idempotent existing-DM path returns the same federatedId field', async () => {
+    // First call creates the DM
+    const first = await app.inject({
+      method: 'POST',
+      url: '/api/dm',
+      payload: { userId: 'user-B' },
+    });
+    expect(first.statusCode).toBe(201);
+    const firstBody = first.json() as { id: string; federatedId: string | null };
+    const expectedFederatedId = firstBody.federatedId;
+    expect(expectedFederatedId).toMatch(/^[a-f0-9]{32}$/);
+
+    // Second call returns the existing DM idempotently — must carry federatedId
+    const second = await app.inject({
+      method: 'POST',
+      url: '/api/dm',
+      payload: { userId: 'user-B' },
+    });
+    expect(second.statusCode).toBe(200);
+    const secondBody = second.json() as { id: string; federatedId: string | null };
+
+    expect(secondBody.id).toBe(firstBody.id);
+    expect(secondBody.federatedId).toBe(expectedFederatedId);
+  });
+});
diff --git a/packages/server/src/routes/dm.ts b/packages/server/src/routes/dm.ts
index 532e8c5f..d35ba09b 100644
--- a/packages/server/src/routes/dm.ts
+++ b/packages/server/src/routes/dm.ts
@@ -471,6 +471,7 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
         const result: DmChannel = {
           id: dmChannel.id,
           ownerId: dmChannel.ownerId ?? null,
+          federatedId: dmChannel.federatedId ?? null,
           createdAt: dmChannel.createdAt,
           members: users.map(u => sanitizeUser(u)),
           lastMessage: lastMsg ? {