diff --git a/packages/server/src/routes/federation.ts b/packages/server/src/routes/federation.ts
index 54591cea..176e4304 100644
--- a/packages/server/src/routes/federation.ts
+++ b/packages/server/src/routes/federation.ts
@@ -2753,6 +2753,116 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
     },
   );
 
+  // ─── POST /api/federation/verify-attach-proof ───────────────────────────────
+  // Server-to-server: verify a one-time attach-proof token minted by
+  // /api/auth/attach-proof (re-attach spec §3.1). The token is single-use (an
+  // atomic claim guarantees only one concurrent verification can win) and is
+  // bound to the CALLING peer's domain — the binding is checked against the
+  // authenticated peer row (extractDomain(peer.origin)), NEVER trusted from the
+  // request body. This is the anti-replay control: a compromised requester
+  // cannot redeem a token minted for a different peer. The response is HMAC-
+  // signed (epoch pattern) so the caller can trust the identity it carries; all
+  // failure modes fail closed to a signed { valid: false }.
+  app.post<{ Body: { token?: unknown } }>(
+    '/api/federation/verify-attach-proof',
+    { bodyLimit: 4 * 1024 },
+    async (request, reply) => {
+      const db = getDb();
+      const rawDb = getRawDb();
+
+      // 1. Verify HMAC headers (mirror by-home-id / users-lookup).
+      const fedHeaders = parseFederationHeaders(request.headers as Record<string, string | string[] | undefined>);
+      if (!fedHeaders) {
+        return reply.code(401).send({ error: 'Missing or malformed federation headers', statusCode: 401 });
+      }
+
+      const peer = db
+        .select()
+        .from(schema.federationPeers)
+        .where(eq(schema.federationPeers.origin, fedHeaders.origin))
+        .get();
+
+      if (!peer || peer.status !== 'active') {
+        return reply.code(403).send({ error: 'Unknown or inactive peer', statusCode: 403 });
+      }
+
+      if (isLookupRateLimited(peer.origin)) {
+        return reply.code(429).header('Retry-After', '60').send({ error: 'Rate limit exceeded', statusCode: 429 });
+      }
+
+      const bodyString = JSON.stringify(request.body);
+      if (!verifyPeerSignature(bodyString, fedHeaders.signature, fedHeaders.timestamp, fedHeaders.nonce, peer)) {
+        return reply.code(401).send({ error: 'Invalid signature', statusCode: 401 });
+      }
+
+      // Replay protection
+      if (fedHeaders.nonce) {
+        if (isNonceDuplicate(peer.origin, fedHeaders.nonce)) {
+          return reply.code(409).send({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
+        }
+      } else if (peer.nonceSupported) {
+        return reply.code(401).send({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
+      }
+
+      // 2. Sign every downstream response with the peer's shared secret so the
+      // caller can trust the identity (or the fail-closed verdict) it carries.
+      const sendSigned = (payload: { valid: false } | { valid: true; homeUserId: string; username: string }): FastifyReply => {
+        const responseBody = JSON.stringify(payload);
+        const sigHeaders = buildFederationHeaders(responseBody, peer.hmacSecret, getOurOrigin());
+        reply.headers({
+          'X-Federation-Signature': sigHeaders['X-Federation-Signature'],
+          'X-Federation-Timestamp': sigHeaders['X-Federation-Timestamp'],
+          'X-Federation-Nonce': sigHeaders['X-Federation-Nonce'],
+          'Content-Type': 'application/json',
+        });
+        return reply.code(200).send(responseBody);
+      };
+
+      // 3. Validate the token shape (64 hex chars, as minted by attach-proof).
+      const rawToken = (request.body as { token?: unknown } | null)?.token;
+      if (typeof rawToken !== 'string' || !/^[0-9a-f]{64}$/i.test(rawToken)) {
+        return sendSigned({ valid: false });
+      }
+
+      // 4. Atomic single-use claim. The domain binding is server-side: the
+      // token's target_domain must equal the AUTHENTICATED peer's domain, never
+      // a value from the request body. Concurrent verifications cannot both win
+      // because only the first UPDATE that flips used_at from NULL matches.
+      const peerDomain = extractDomain(peer.origin).toLowerCase();
+      const now = Date.now();
+      const claimed = rawDb.prepare(`
+        UPDATE federation_attach_proofs
+        SET used_at = ?
+        WHERE token = ? AND used_at IS NULL AND expires_at > ? AND lower(target_domain) = ?
+        RETURNING home_user_id
+      `).get(now, rawToken, now, peerDomain) as { home_user_id: string } | undefined;
+
+      if (!claimed) {
+        return sendSigned({ valid: false });
+      }
+
+      // 5. Re-confirm the home user is still native (not tombstoned, not turned
+      // into a replicated stub) since the token was minted.
+      const homeUser = db
+        .select()
+        .from(schema.users)
+        .where(
+          and(
+            eq(schema.users.id, claimed.home_user_id),
+            eq(schema.users.isDeleted, 0),
+            isNull(schema.users.homeInstance),
+          ),
+        )
+        .get();
+
+      if (!homeUser) {
+        return sendSigned({ valid: false });
+      }
+
+      return sendSigned({ valid: true, homeUserId: homeUser.id, username: homeUser.username });
+    },
+  );
+
   // ─── POST /api/federation/sync ──────────────────────────────────────────────
   // Server-to-server: checkpoint catch-up sync. A peer calls this after downtime
   // to retrieve missed DM mutations from the mutation log.
diff --git a/packages/server/src/routes/federation.verifyAttachProof.test.ts b/packages/server/src/routes/federation.verifyAttachProof.test.ts
new file mode 100644
index 00000000..8963ef69
--- /dev/null
+++ b/packages/server/src/routes/federation.verifyAttachProof.test.ts
@@ -0,0 +1,205 @@
+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 { eq } from 'drizzle-orm';
+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';
+import { signRequest, verifySignature } from '../utils/federationAuth.js';
+import { randomUUID } from 'node:crypto';
+
+setWorkerId(1);
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+// Module-level mutable state. Each beforeEach reassigns sqlite/testDb;
+// the getDb getter in the mock closes over the current binding.
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let sqlite: Database.Database;
+let testDb: TestDb;
+
+const PEER_ORIGIN = 'https://orbit.test';
+const PEER_SECRET = 'a'.repeat(64);
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  getRawDb: () => sqlite,
+  schema,
+}));
+
+vi.mock('../utils/federationAuth.js', async (importActual) => {
+  const actual = await importActual<typeof import('../utils/federationAuth.js')>();
+  return { ...actual, getOurOrigin: () => 'https://home.test' };
+});
+
+function applyMigrations(db: Database.Database): void {
+  const dir = path.resolve(__dirname, '../../drizzle');
+  for (const f of fs.readdirSync(dir).filter(f => f.endsWith('.sql')).sort()) {
+    const sqlText = fs.readFileSync(path.join(dir, f), 'utf8');
+    for (const stmt of sqlText.split(/-->\s*statement-breakpoint/)) {
+      const clean = stmt.trim();
+      if (clean) db.exec(clean);
+    }
+  }
+}
+
+function seedActivePeer(): void {
+  testDb.insert(schema.federationPeers).values({
+    id: 'peer-1',
+    origin: PEER_ORIGIN,
+    hmacSecret: PEER_SECRET,
+    status: 'active',
+    nonceSupported: 1,
+    createdAt: Date.now(),
+    lastSeenAt: Date.now(),
+    consecutiveFailures: 0,
+    consecutiveAuthFailures: 0,
+  } as typeof schema.federationPeers.$inferInsert).run();
+}
+
+function seedNativeUser(): void {
+  testDb.insert(schema.users).values({
+    id: 'native-1',
+    username: 'youruser',
+    displayName: null,
+    passwordHash: 'x',
+    status: 'offline',
+    isAdmin: 0,
+    isDeleted: 0,
+    discoverable: 1,
+    homeInstance: null,
+    homeUserId: null,
+    createdAt: 1,
+  } as typeof schema.users.$inferInsert).run();
+}
+
+// Tokens are minted by /api/auth/attach-proof as randomBytes(32).toString('hex')
+// — always 64 lowercase hex chars — so the fixture token must be valid hex too.
+function seedProof(overrides: Partial<typeof schema.federationAttachProofs.$inferInsert> = {}): string {
+  const token = 'a1'.repeat(32);
+  testDb.insert(schema.federationAttachProofs).values({
+    token,
+    homeUserId: 'native-1',
+    targetDomain: 'orbit.test',
+    createdAt: Date.now(),
+    expiresAt: Date.now() + 60_000,
+    usedAt: null,
+    ...overrides,
+  }).run();
+  return overrides.token ?? token;
+}
+
+async function buildApp(): Promise<FastifyInstance> {
+  const app = Fastify({ logger: false });
+  const { _resetLookupRateBuckets, federationRoutes } = await import('./federation.js');
+  _resetLookupRateBuckets();
+  await app.register(federationRoutes);
+  await app.ready();
+  return app;
+}
+
+function signedHeaders(body: string): Record<string, string> {
+  const timestamp = Date.now();
+  const nonce = randomUUID();
+  const sig = signRequest(body, PEER_SECRET, timestamp, nonce);
+  return {
+    'X-Federation-Origin': PEER_ORIGIN,
+    'X-Federation-Timestamp': String(timestamp),
+    'X-Federation-Nonce': nonce,
+    'X-Federation-Signature': `sha256=${sig}`,
+    'Content-Type': 'application/json',
+  };
+}
+
+async function verify(app: FastifyInstance, body: object) {
+  const bodyStr = JSON.stringify(body);
+  return app.inject({
+    method: 'POST',
+    url: '/api/federation/verify-attach-proof',
+    headers: signedHeaders(bodyStr),
+    payload: bodyStr,
+  });
+}
+
+describe('POST /api/federation/verify-attach-proof', () => {
+  let app: FastifyInstance;
+
+  beforeEach(async () => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+    seedActivePeer();
+    seedNativeUser();
+    app = await buildApp();
+  });
+
+  it('valid token → identity returned, marked used', async () => {
+    const token = seedProof();
+    const res = await verify(app, { token });
+    expect(res.statusCode).toBe(200);
+    const body = JSON.parse(res.body);
+    expect(body).toEqual({ valid: true, homeUserId: 'native-1', username: 'youruser' });
+    const row = testDb.select().from(schema.federationAttachProofs).all()[0]!;
+    expect(row.usedAt).not.toBeNull();
+  });
+
+  it('second verification of the same token → valid:false (single-use)', async () => {
+    const token = seedProof();
+    await verify(app, { token });
+    const res = await verify(app, { token });
+    expect(JSON.parse(res.body)).toEqual({ valid: false });
+  });
+
+  it('expired token → valid:false', async () => {
+    const token = seedProof({ expiresAt: Date.now() - 1 });
+    const res = await verify(app, { token });
+    expect(JSON.parse(res.body)).toEqual({ valid: false });
+  });
+
+  it('token bound to a DIFFERENT target domain → valid:false (peer-domain binding)', async () => {
+    const token = seedProof({ targetDomain: 'someone-else.test' });
+    const res = await verify(app, { token });
+    expect(JSON.parse(res.body)).toEqual({ valid: false });
+  });
+
+  it('unknown token → valid:false', async () => {
+    const res = await verify(app, { token: 'f'.repeat(64) });
+    expect(JSON.parse(res.body)).toEqual({ valid: false });
+  });
+
+  it('home user deleted after mint → valid:false', async () => {
+    const token = seedProof();
+    testDb.update(schema.users).set({ isDeleted: 1 }).where(eq(schema.users.id, 'native-1')).run();
+    const res = await verify(app, { token });
+    expect(JSON.parse(res.body)).toEqual({ valid: false });
+  });
+
+  it('home user no longer native (homeInstance set) after mint → valid:false', async () => {
+    const token = seedProof();
+    testDb.update(schema.users).set({ homeInstance: 'orbit.test' }).where(eq(schema.users.id, 'native-1')).run();
+    const res = await verify(app, { token });
+    expect(JSON.parse(res.body)).toEqual({ valid: false });
+  });
+
+  it('unsigned request → 401', async () => {
+    const res = await app.inject({
+      method: 'POST', url: '/api/federation/verify-attach-proof',
+      headers: { 'Content-Type': 'application/json' }, payload: JSON.stringify({ token: 'x' }),
+    });
+    expect(res.statusCode).toBe(401);
+  });
+
+  it('response is HMAC-signed (epoch pattern)', async () => {
+    const token = seedProof();
+    const res = await verify(app, { token });
+    const sig = res.headers['x-federation-signature'] as string;
+    expect(sig).toMatch(/^sha256=/);
+    const ts = res.headers['x-federation-timestamp'] as string;
+    expect(ts).toBeDefined();
+    const nonce = res.headers['x-federation-nonce'] as string;
+    // Signature must verify against the response body with the shared secret.
+    expect(verifySignature(res.body, sig.slice('sha256='.length), PEER_SECRET, Number(ts), nonce)).toBe(true);
+  });
+});