diff --git a/packages/server/src/utils/federationOutbox.ts b/packages/server/src/utils/federationOutbox.ts
index 57aad54d..ea5ca260 100644
--- a/packages/server/src/utils/federationOutbox.ts
+++ b/packages/server/src/utils/federationOutbox.ts
@@ -524,7 +524,15 @@ export type CallRelayFailureReason =
   | 'post_failed';
 
 export type CallRelayResult =
-  | { ok: true }
+  | {
+      ok: true;
+      /**
+       * messageIds the remote reported as undeliverable (remote processed the
+       * event cleanly but had no reachable recipient). Empty array for old
+       * peers that don't set `undeliverable` on FederationRelayResponse.
+       */
+      undeliverable: string[];
+    }
   | { ok: false; reason: CallRelayFailureReason; error: string };
 
 /** Per-peer failure record returned by Path-1 call fan-out helpers. */
@@ -631,7 +639,18 @@ export async function sendCallRelay(
       signal: AbortSignal.timeout(10_000),
     });
 
-    if (res.ok) return { ok: true };
+    if (res.ok) {
+      // Parse response body to surface the undeliverable bucket. Old peers
+      // omit the field; treat as empty. Body shape: FederationRelayResponse.
+      let undeliverable: string[] = [];
+      try {
+        const body = (await res.json()) as { undeliverable?: Array<{ messageId: string }> };
+        undeliverable = body.undeliverable?.map(u => u.messageId) ?? [];
+      } catch {
+        // Body missing or unparseable — assume old-format response.
+      }
+      return { ok: true, undeliverable };
+    }
 
     const text = await res.text().catch(() => '');
     if (res.status >= 400 && res.status < 500) {
diff --git a/packages/server/src/utils/federationOutbox.undeliverable.test.ts b/packages/server/src/utils/federationOutbox.undeliverable.test.ts
new file mode 100644
index 00000000..8a3cdb4b
--- /dev/null
+++ b/packages/server/src/utils/federationOutbox.undeliverable.test.ts
@@ -0,0 +1,133 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+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';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let testDb: TestDb;
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  schema,
+}));
+
+vi.mock('./federationAuth.js', async () => {
+  const actual = await vi.importActual<typeof import('./federationAuth.js')>('./federationAuth.js');
+  return {
+    ...actual,
+    getOurOrigin: () => 'https://local.example',
+    buildFederationHeaders: () => ({}),
+  };
+});
+
+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 sql = fs.readFileSync(path.join(migrationsDir, f), 'utf8');
+    const statements = sql.split(/-->\s*statement-breakpoint/);
+    for (const stmt of statements) {
+      const clean = stmt.trim();
+      if (clean) db.exec(clean);
+    }
+  }
+}
+
+function seedActivePeer(origin: string): void {
+  testDb.insert(schema.federationPeers).values({
+    id: `peer-${origin}`,
+    origin,
+    hmacSecret: 'secret',
+    status: 'active',
+    instanceName: 'Peer',
+    lastSyncedAt: 0,
+    createdAt: Date.now(),
+  }).run();
+}
+
+let sqlite: Database.Database;
+
+describe('sendCallRelay response shape', () => {
+  beforeEach(() => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+    seedActivePeer('https://peer.example');
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+    sqlite.close();
+  });
+
+  const baseEvent = {
+    eventType: 'dm_call_start' as const,
+    messageId: 'msg-X',
+    encryptionVersion: 0 as const,
+    timestamp: Date.now(),
+    federatedId: 'fed-X',
+    call: {
+      livekitUrl: 'wss://lk.example',
+      tokens: {},
+      caller: { homeUserId: 'c', homeInstance: 'https://local.example', displayName: 'C' },
+      participants: [],
+    },
+  };
+
+  it('returns {ok:true, undeliverable:[]} when remote omits the field', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () =>
+      new Response(JSON.stringify({ accepted: ['msg-X'], rejected: [], maxUploadSize: 1000 }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      }),
+    ));
+
+    const { sendCallRelay } = await import('./federationOutbox.js');
+    const result = await sendCallRelay('https://peer.example', [baseEvent]);
+
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      expect(result.undeliverable).toEqual([]);
+    }
+  });
+
+  it('returns {ok:true, undeliverable:["msg-X"]} when remote lists it', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () =>
+      new Response(JSON.stringify({
+        accepted: [],
+        rejected: [],
+        undeliverable: [{ messageId: 'msg-X', reason: 'no_recipient' }],
+        maxUploadSize: 1000,
+      }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      }),
+    ));
+
+    const { sendCallRelay } = await import('./federationOutbox.js');
+    const result = await sendCallRelay('https://peer.example', [baseEvent]);
+
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      expect(result.undeliverable).toEqual(['msg-X']);
+    }
+  });
+
+  it('failure shape unchanged on HTTP 5xx', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () =>
+      new Response('server down', { status: 503 }),
+    ));
+
+    const { sendCallRelay } = await import('./federationOutbox.js');
+    const result = await sendCallRelay('https://peer.example', [baseEvent]);
+
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.reason).toBe('peer_transient_failure');
+    }
+  });
+});
diff --git a/packages/server/src/ws/events.dmCallRelay.test.ts b/packages/server/src/ws/events.dmCallRelay.test.ts
index 26547070..aae10a6e 100644
--- a/packages/server/src/ws/events.dmCallRelay.test.ts
+++ b/packages/server/src/ws/events.dmCallRelay.test.ts
@@ -144,7 +144,7 @@ describe('handleDmCallEnd Path-2 relay failure', () => {
     const fedCall = makeFedCall({ state: 'active' });
     connectionManager.createFederatedCall(fedCall);
     const sendToUserSpy = vi.spyOn(connectionManager, 'sendToUser');
-    sendCallRelayMock.mockResolvedValue({ ok: true });
+    sendCallRelayMock.mockResolvedValue({ ok: true, undeliverable: [] });
 
     await handleDmCallEndForTest(
       { federatedCallId: fedCall.federatedId },
@@ -191,7 +191,7 @@ describe('handleDmCallReject Path-2 relay failure', () => {
     const fedCall = makeFedCall();
     connectionManager.createFederatedCall(fedCall);
     const sendToUserSpy = vi.spyOn(connectionManager, 'sendToUser');
-    sendCallRelayMock.mockResolvedValue({ ok: true });
+    sendCallRelayMock.mockResolvedValue({ ok: true, undeliverable: [] });
 
     await handleDmCallRejectForTest(
       { federatedCallId: fedCall.federatedId },
@@ -282,7 +282,7 @@ describe('handleDmCallAccept Path-2 relay failure', () => {
     const fedCall = makeFedCall();
     connectionManager.createFederatedCall(fedCall);
     const sendToUserSpy = vi.spyOn(connectionManager, 'sendToUser');
-    sendCallRelayMock.mockResolvedValue({ ok: true });
+    sendCallRelayMock.mockResolvedValue({ ok: true, undeliverable: [] });
 
     await handleDmCallAcceptForTest(
       { federatedCallId: fedCall.federatedId },