diff --git a/packages/server/src/utils/federationPeerActivation.test.ts b/packages/server/src/utils/federationPeerActivation.test.ts
index d3e154d1..d1dbc5da 100644
--- a/packages/server/src/utils/federationPeerActivation.test.ts
+++ b/packages/server/src/utils/federationPeerActivation.test.ts
@@ -34,6 +34,15 @@ vi.mock('../routes/federation.js', () => ({
   processRelayEvents: vi.fn().mockResolvedValue({ accepted: [], rejected: [] }),
 }));
 
+vi.mock('../ws/handler.js', () => ({
+  connectionManager: {
+    sendToAdmins: vi.fn(),
+    getAllOnlineUserIds: () => [],
+    sendToUser: vi.fn(),
+    sendToDmMembers: 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();
@@ -237,3 +246,51 @@ describe('syncPeerMutationLog', () => {
     expect(bodies[3]?.contextType).toBe('profile');
   });
 });
+
+describe('onPeerActivated', () => {
+  beforeEach(() => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+    vi.restoreAllMocks();
+  });
+
+  it('runs resetOutboxBackoff and syncPeerMutationLog once, even under concurrent calls', async () => {
+    const { onPeerActivated } = await import('./federationPeerActivation.js');
+
+    testDb.insert(schema.federationPeers).values({
+      id: 'peer-x', origin: 'https://peer-x.example', hmacSecret: 'secret',
+      status: 'active', lastSyncedAt: 0, createdAt: Date.now(),
+    }).run();
+
+    let fetchCount = 0;
+    vi.spyOn(globalThis, 'fetch').mockImplementation(async () => {
+      fetchCount++;
+      // Deliberately slow to let the second concurrent call share the in-flight promise.
+      await new Promise(r => setTimeout(r, 20));
+      return new Response(JSON.stringify({ events: [], hasMore: false, checkpoint: 0 }), { status: 200 });
+    });
+
+    const p1 = onPeerActivated('peer-x', 'health_check_recovery');
+    const p2 = onPeerActivated('peer-x', 'accept_new');
+    await Promise.all([p1, p2]);
+
+    // Three fetch calls for the three sync passes (dm, friend, profile) — not six.
+    expect(fetchCount).toBe(3);
+  });
+
+  it('swallows errors from syncPeerMutationLog so the handler does not throw', async () => {
+    const { onPeerActivated } = await import('./federationPeerActivation.js');
+
+    testDb.insert(schema.federationPeers).values({
+      id: 'peer-err', origin: 'https://peer-err.example', hmacSecret: 'secret',
+      status: 'active', lastSyncedAt: 0, createdAt: Date.now(),
+    }).run();
+
+    vi.spyOn(globalThis, 'fetch').mockImplementation(async () => {
+      throw new Error('network down');
+    });
+
+    await expect(onPeerActivated('peer-err', 'ensure_peered')).resolves.toBeUndefined();
+  });
+});
diff --git a/packages/server/src/utils/federationPeerActivation.ts b/packages/server/src/utils/federationPeerActivation.ts
index 8a5da288..67d5bf2f 100644
--- a/packages/server/src/utils/federationPeerActivation.ts
+++ b/packages/server/src/utils/federationPeerActivation.ts
@@ -42,9 +42,26 @@ export async function onPeerActivated(
   peerId: string,
   reason: PeerActivationReason,
 ): Promise<void> {
-  // Stub — implemented in Task 4.
-  void peerId;
-  void reason;
+  const existing = inFlightActivation.get(peerId);
+  if (existing) return existing;
+
+  const promise = (async () => {
+    try {
+      resetOutboxBackoff(peerId);
+      await syncPeerMutationLog(peerId, reason);
+      const { connectionManager } = await import('../ws/handler.js');
+      connectionManager.sendToAdmins({ type: 'federation_peers_changed' as const });
+    } catch (err) {
+      console.error(`[federation] onPeerActivated(${peerId}, ${reason}) failed:`, err);
+    }
+  })();
+
+  inFlightActivation.set(peerId, promise);
+  try {
+    await promise;
+  } finally {
+    inFlightActivation.delete(peerId);
+  }
 }
 
 /**