diff --git a/packages/server/src/routes/federation.groupMetadataUpdate.test.ts b/packages/server/src/routes/federation.groupMetadataUpdate.test.ts
new file mode 100644
index 00000000..9b576a92
--- /dev/null
+++ b/packages/server/src/routes/federation.groupMetadataUpdate.test.ts
@@ -0,0 +1,473 @@
+import { describe, it, expect, beforeEach, afterEach, vi } 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 os from 'node:os';
+import crypto from 'node:crypto';
+import { fileURLToPath } from 'node:url';
+import { eq, and } from 'drizzle-orm';
+import * as schema from '../db/schema.js';
+import { setWorkerId } from '../utils/snowflake.js';
+import type { FederationRelayEvent, FederationRelayParticipant } from '@backspace/shared';
+
+setWorkerId(3);
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let sqlite: Database.Database;
+let testDb: TestDb;
+let tmpUploadDir: string;
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  getRawDb: () => sqlite,
+  schema,
+}));
+
+vi.mock('../config.js', () => ({
+  config: {
+    domain: 'local.test',
+    port: 3000,
+    host: '0.0.0.0',
+    jwtSecret: 'test-secret-12345678901234567890123456789012',
+    maxUploadSize: 100 * 1024 * 1024,
+    registrationOpen: true,
+    // Set in beforeEach via reassignment trick — vi.mock factory runs once.
+    get uploadDir(): string { return tmpUploadDir; },
+  },
+}));
+
+vi.mock('../ws/handler.js', () => ({
+  connectionManager: {
+    sendToUser: vi.fn(),
+    sendToSpace: vi.fn(),
+    sendToDmMembers: vi.fn(),
+    sendToAdmins: vi.fn(),
+    getAllOnlineUserIds: () => [],
+    evictFederatedCallsForHost: vi.fn(),
+    federatedCalls: new Map(),
+    isUserOnline: vi.fn(),
+    lateBindFederatedCall: vi.fn(),
+  },
+}));
+
+vi.mock('../utils/fileCleanup.js', async () => {
+  const actual = await vi.importActual<typeof import('../utils/fileCleanup.js')>('../utils/fileCleanup.js');
+  return {
+    ...actual,
+    deleteUploadFile: vi.fn(),
+    deleteAttachmentByFilename: vi.fn(),
+    deleteAttachmentFiles: vi.fn(),
+  };
+});
+
+import { connectionManager } from '../ws/handler.js';
+import { deleteUploadFile } from '../utils/fileCleanup.js';
+
+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');
+    for (const stmt of sql.split(/-->\s*statement-breakpoint/)) {
+      const clean = stmt.trim();
+      if (clean) db.exec(clean);
+    }
+  }
+}
+
+const OWNER_INSTANCE = 'orbit.ddns.net';
+const OWNER_ORIGIN = `https://${OWNER_INSTANCE}`;
+const FEDERATED_ID = 'fed-channel-1';
+const CHANNEL_ID = 'ch-local-1';
+const OWNER_ID = 'owner-stub-1';
+const MEMBER_ID = 'member-local-1';
+
+function seedChannelAndOwner(opts: {
+  channelName: string | null;
+  channelIcon: string | null;
+  metadataUpdatedAt: number;
+}): void {
+  testDb.insert(schema.users).values({
+    id: OWNER_ID,
+    username: `owner@${OWNER_INSTANCE}`,
+    displayName: 'owner',
+    passwordHash: '!federation-replicated',
+    status: 'offline',
+    isAdmin: 0,
+    homeInstance: OWNER_INSTANCE,
+    homeUserId: 'home-owner-1',
+    createdAt: Date.now(),
+  }).run();
+
+  testDb.insert(schema.users).values({
+    id: MEMBER_ID,
+    username: 'localmember',
+    displayName: 'localmember',
+    passwordHash: 'hash',
+    status: 'online',
+    isAdmin: 0,
+    createdAt: Date.now(),
+  }).run();
+
+  testDb.insert(schema.dmChannels).values({
+    id: CHANNEL_ID,
+    federatedId: FEDERATED_ID,
+    ownerId: OWNER_ID,
+    ownerHomeUserId: 'home-owner-1',
+    ownerHomeInstance: OWNER_INSTANCE,
+    name: opts.channelName,
+    icon: opts.channelIcon,
+    metadataUpdatedAt: opts.metadataUpdatedAt,
+    createdAt: Date.now(),
+  }).run();
+
+  testDb.insert(schema.dmMembers).values({
+    dmChannelId: CHANNEL_ID,
+    userId: OWNER_ID,
+    closed: 0,
+  }).run();
+  testDb.insert(schema.dmMembers).values({
+    dmChannelId: CHANNEL_ID,
+    userId: MEMBER_ID,
+    closed: 0,
+  }).run();
+}
+
+function ownerActor(): FederationRelayParticipant {
+  return {
+    homeUserId: 'home-owner-1',
+    homeInstance: OWNER_INSTANCE,
+    profile: { username: 'owner', displayName: 'owner' },
+  };
+}
+
+function buildEvent(opts: {
+  messageId?: string;
+  name: string | null;
+  icon: string | null;
+  metadataUpdatedAt: number;
+  actor?: FederationRelayParticipant;
+}): FederationRelayEvent {
+  return {
+    eventType: 'group_metadata_update',
+    contextType: 'dm',
+    messageId: opts.messageId ?? 'evt-1',
+    federatedId: FEDERATED_ID,
+    encryptionVersion: 0,
+    timestamp: Date.now(),
+    metadata: {
+      name: opts.name,
+      icon: opts.icon,
+      metadataUpdatedAt: opts.metadataUpdatedAt,
+      actor: opts.actor ?? ownerActor(),
+    },
+  };
+}
+
+// Build a tiny PNG byte stream (1x1 transparent) so the disk write in
+// downloadProfileAsset succeeds with a valid `image/*` content-type.
+const TINY_PNG = Buffer.from(
+  '89504e470d0a1a0a0000000d49484452000000010000000108060000001f15c4890000000d4944415478da63600000000200015d3a87' +
+    '6f0000000049454e44ae426082',
+  'hex',
+);
+
+function makeImageResponse(): Response {
+  return new Response(TINY_PNG, {
+    status: 200,
+    headers: { 'content-type': 'image/png' },
+  });
+}
+
+function makeFailureResponse(): Response {
+  return new Response('boom', { status: 500 });
+}
+
+beforeEach(() => {
+  sqlite = new Database(':memory:');
+  testDb = drizzle(sqlite, { schema });
+  applyMigrations(sqlite);
+  tmpUploadDir = path.join(os.tmpdir(), `bs-fed-meta-${crypto.randomBytes(6).toString('hex')}`);
+  fs.mkdirSync(tmpUploadDir, { recursive: true });
+  vi.mocked(connectionManager.sendToDmMembers).mockReset();
+  vi.mocked(deleteUploadFile).mockReset();
+});
+
+afterEach(() => {
+  try { fs.rmSync(tmpUploadDir, { recursive: true, force: true }); } catch { /* ignore */ }
+  vi.unstubAllGlobals();
+});
+
+describe('processGroupMetadataUpdateEvent — authority', () => {
+  it('accepts and mutates the channel when sourceInstance matches ownerHomeInstance', async () => {
+    seedChannelAndOwner({ channelName: 'old', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    const event = buildEvent({ name: 'new name', icon: null, metadataUpdatedAt: 2000 });
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_INSTANCE, testDb, accepted, rejected);
+
+    expect(rejected).toEqual([]);
+    expect(accepted).toEqual(['evt-1']);
+    const row = testDb.select().from(schema.dmChannels).where(eq(schema.dmChannels.id, CHANNEL_ID)).get();
+    expect(row!.name).toBe('new name');
+    expect(row!.metadataUpdatedAt).toBe(2000);
+  });
+
+  it('rejects with attribution_mismatch when sourceInstance differs from ownerHomeInstance', async () => {
+    seedChannelAndOwner({ channelName: 'old', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    const event = buildEvent({ name: 'evil', icon: null, metadataUpdatedAt: 2000 });
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, 'evil.example.com', testDb, accepted, rejected);
+
+    expect(accepted).toEqual([]);
+    expect(rejected).toEqual([{ messageId: 'evt-1', reason: 'attribution_mismatch' }]);
+    const row = testDb.select().from(schema.dmChannels).where(eq(schema.dmChannels.id, CHANNEL_ID)).get();
+    expect(row!.name).toBe('old');
+    expect(row!.metadataUpdatedAt).toBe(1000);
+  });
+});
+
+describe('processGroupMetadataUpdateEvent — version check', () => {
+  it('silently accepts a stale metadataUpdatedAt without mutating the channel', async () => {
+    seedChannelAndOwner({ channelName: 'current', channelIcon: null, metadataUpdatedAt: 5000 });
+    const fed = await import('./federation.js');
+    const event = buildEvent({ name: 'older', icon: null, metadataUpdatedAt: 3000 });
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_INSTANCE, testDb, accepted, rejected);
+
+    expect(rejected).toEqual([]);
+    expect(accepted).toEqual(['evt-1']);
+    const row = testDb.select().from(schema.dmChannels).where(eq(schema.dmChannels.id, CHANNEL_ID)).get();
+    expect(row!.name).toBe('current');
+    expect(row!.metadataUpdatedAt).toBe(5000);
+    const sysRows = testDb.select().from(schema.dmMessages).where(eq(schema.dmMessages.dmChannelId, CHANNEL_ID)).all();
+    expect(sysRows).toHaveLength(0);
+  });
+});
+
+describe('processGroupMetadataUpdateEvent — receiver hardening', () => {
+  it('rejects an oversized name with invalid_payload', async () => {
+    seedChannelAndOwner({ channelName: 'old', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    const event = buildEvent({ name: 'a'.repeat(10_000), icon: null, metadataUpdatedAt: 2000 });
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_INSTANCE, testDb, accepted, rejected);
+
+    expect(accepted).toEqual([]);
+    expect(rejected).toEqual([{ messageId: 'evt-1', reason: 'invalid_payload' }]);
+    const row = testDb.select().from(schema.dmChannels).where(eq(schema.dmChannels.id, CHANNEL_ID)).get();
+    expect(row!.name).toBe('old');
+  });
+
+  it('rejects a non-http(s) icon URL with invalid_payload', async () => {
+    seedChannelAndOwner({ channelName: 'old', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    const event = buildEvent({
+      name: null,
+      icon: '/api/uploads/foo.png',
+      metadataUpdatedAt: 2000,
+    });
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_INSTANCE, testDb, accepted, rejected);
+
+    expect(accepted).toEqual([]);
+    expect(rejected).toEqual([{ messageId: 'evt-1', reason: 'invalid_payload' }]);
+  });
+
+  it('rejects a missing metadata payload with missing_metadata_payload', async () => {
+    seedChannelAndOwner({ channelName: 'old', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    const event: FederationRelayEvent = {
+      eventType: 'group_metadata_update',
+      contextType: 'dm',
+      messageId: 'evt-1',
+      federatedId: FEDERATED_ID,
+      encryptionVersion: 0,
+      timestamp: Date.now(),
+      // metadata intentionally omitted
+    };
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_INSTANCE, testDb, accepted, rejected);
+
+    expect(accepted).toEqual([]);
+    expect(rejected).toEqual([{ messageId: 'evt-1', reason: 'missing_metadata_payload' }]);
+  });
+});
+
+describe('processGroupMetadataUpdateEvent — icon download', () => {
+  it('caches the downloaded icon as a local filename on success', async () => {
+    seedChannelAndOwner({ channelName: 'g', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    vi.stubGlobal('fetch', vi.fn(async () => makeImageResponse()));
+
+    const remoteIcon = `${OWNER_ORIGIN}/api/uploads/icon-remote.png`;
+    const event = buildEvent({ name: null, icon: remoteIcon, metadataUpdatedAt: 2000 });
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_ORIGIN, testDb, accepted, rejected);
+
+    expect(rejected).toEqual([]);
+    const row = testDb.select().from(schema.dmChannels).where(eq(schema.dmChannels.id, CHANNEL_ID)).get();
+    expect(row!.icon).not.toBeNull();
+    expect(row!.icon!.startsWith('http')).toBe(false); // local filename, not URL
+    expect(row!.icon).toMatch(/\.png$/);
+    // File was actually written to the temp upload dir
+    expect(fs.existsSync(path.join(tmpUploadDir, row!.icon!))).toBe(true);
+  });
+
+  it('falls back to the absolute URL when the download fails', async () => {
+    seedChannelAndOwner({ channelName: 'g', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    vi.stubGlobal('fetch', vi.fn(async () => makeFailureResponse()));
+
+    const remoteIcon = `${OWNER_ORIGIN}/api/uploads/icon-remote.png`;
+    const event = buildEvent({ name: null, icon: remoteIcon, metadataUpdatedAt: 2000 });
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_ORIGIN, testDb, accepted, rejected);
+
+    expect(rejected).toEqual([]);
+    const row = testDb.select().from(schema.dmChannels).where(eq(schema.dmChannels.id, CHANNEL_ID)).get();
+    expect(row!.icon).toBe(remoteIcon);
+  });
+});
+
+describe('processGroupMetadataUpdateEvent — system messages', () => {
+  it('inserts two system messages with :name and :icon dedup suffixes when both change', async () => {
+    seedChannelAndOwner({ channelName: 'old', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    vi.stubGlobal('fetch', vi.fn(async () => makeImageResponse()));
+
+    const remoteIcon = `${OWNER_ORIGIN}/api/uploads/icon.png`;
+    const event = buildEvent({
+      messageId: 'evt-multi',
+      name: 'fresh',
+      icon: remoteIcon,
+      metadataUpdatedAt: 2000,
+    });
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_ORIGIN, testDb, accepted, rejected);
+
+    expect(rejected).toEqual([]);
+    expect(accepted).toEqual(['evt-multi']);
+
+    const sysRows = testDb.select().from(schema.dmMessages).where(eq(schema.dmMessages.dmChannelId, CHANNEL_ID)).all();
+    expect(sysRows).toHaveLength(2);
+    const sourceMessageIds = sysRows.map(r => r.sourceMessageId).sort();
+    expect(sourceMessageIds).toEqual(['evt-multi:icon', 'evt-multi:name']);
+    for (const r of sysRows) {
+      expect(r.sourceInstance).toBe(OWNER_ORIGIN);
+      expect(r.type).toBe('system');
+    }
+    const nameRow = sysRows.find(r => r.sourceMessageId === 'evt-multi:name')!;
+    expect(JSON.parse(nameRow.content!)).toEqual({ event: 'name_changed', oldName: 'old', newName: 'fresh' });
+    const iconRow = sysRows.find(r => r.sourceMessageId === 'evt-multi:icon')!;
+    expect(JSON.parse(iconRow.content!)).toEqual({ event: 'icon_changed' });
+  });
+
+  it('is idempotent — a second delivery of the same event creates no additional rows', async () => {
+    seedChannelAndOwner({ channelName: 'old', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    const event = buildEvent({ messageId: 'evt-retry', name: 'fresh', icon: null, metadataUpdatedAt: 2000 });
+
+    const accepted1: string[] = [];
+    const rejected1: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_INSTANCE, testDb, accepted1, rejected1);
+    expect(rejected1).toEqual([]);
+    expect(accepted1).toEqual(['evt-retry']);
+
+    const sendSpy = vi.mocked(connectionManager.sendToDmMembers);
+    const callsAfterFirst = sendSpy.mock.calls.length;
+
+    // Reset metadataUpdatedAt back so the version-check doesn't short-circuit
+    // before the dedup check. (Forcing the redelivery to actually exercise the
+    // dedup logic, not the cheaper version short-circuit.)
+    testDb.update(schema.dmChannels)
+      .set({ metadataUpdatedAt: 1000, name: 'old' })
+      .where(eq(schema.dmChannels.id, CHANNEL_ID))
+      .run();
+
+    const accepted2: string[] = [];
+    const rejected2: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_INSTANCE, testDb, accepted2, rejected2);
+    expect(rejected2).toEqual([]);
+    expect(accepted2).toEqual(['evt-retry']);
+
+    const sysRows = testDb.select().from(schema.dmMessages)
+      .where(and(
+        eq(schema.dmMessages.dmChannelId, CHANNEL_ID),
+        eq(schema.dmMessages.sourceMessageId, 'evt-retry:name'),
+      )).all();
+    expect(sysRows).toHaveLength(1);
+    // No additional broadcasts on retry.
+    expect(sendSpy.mock.calls.length).toBe(callsAfterFirst);
+  });
+
+  it('treats a no-op (received metadata identical to local) as a silent accept', async () => {
+    seedChannelAndOwner({ channelName: 'same', channelIcon: null, metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    const event = buildEvent({
+      name: 'same',
+      icon: null,
+      // Bump timestamp so the version-check passes — no-op detection must
+      // happen on the diff, not just on the timestamp.
+      metadataUpdatedAt: 2000,
+    });
+
+    const sendSpy = vi.mocked(connectionManager.sendToDmMembers);
+    sendSpy.mockClear();
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_INSTANCE, testDb, accepted, rejected);
+
+    expect(rejected).toEqual([]);
+    expect(accepted).toEqual(['evt-1']);
+    const sysRows = testDb.select().from(schema.dmMessages).where(eq(schema.dmMessages.dmChannelId, CHANNEL_ID)).all();
+    expect(sysRows).toHaveLength(0);
+    expect(sendSpy).not.toHaveBeenCalled();
+  });
+});
+
+describe('processGroupMetadataUpdateEvent — icon clear', () => {
+  it('clears the icon and unlinks the previous local file', async () => {
+    seedChannelAndOwner({ channelName: null, channelIcon: 'old-local.png', metadataUpdatedAt: 1000 });
+    const fed = await import('./federation.js');
+    const event = buildEvent({ name: null, icon: null, metadataUpdatedAt: 2000 });
+
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processGroupMetadataUpdateEvent(event, OWNER_INSTANCE, testDb, accepted, rejected);
+
+    expect(rejected).toEqual([]);
+    expect(accepted).toEqual(['evt-1']);
+    const row = testDb.select().from(schema.dmChannels).where(eq(schema.dmChannels.id, CHANNEL_ID)).get();
+    expect(row!.icon).toBeNull();
+
+    const sysRows = testDb.select().from(schema.dmMessages).where(eq(schema.dmMessages.dmChannelId, CHANNEL_ID)).all();
+    expect(sysRows).toHaveLength(1);
+    expect(JSON.parse(sysRows[0]!.content!)).toEqual({ event: 'icon_changed' });
+
+    expect(deleteUploadFile).toHaveBeenCalledWith('old-local.png');
+  });
+});
diff --git a/packages/server/src/routes/federation.ts b/packages/server/src/routes/federation.ts
index 60357276..dbb5afca 100644
--- a/packages/server/src/routes/federation.ts
+++ b/packages/server/src/routes/federation.ts
@@ -21,6 +21,7 @@ import { computeFederatedId, getDmParticipants, sendCallRelay } from '../utils/f
 import { onPeerActivated, onPeerDeactivated } from '../utils/federationPeerActivation.js';
 import { getDmMessageWithUser } from './dm.js';
 import type { FederationRelayRequest, FederationRelayResponse, FederationRelayEvent, FederationRelayAttachment, FederationSyncRequest, FederationSyncResponse, DmMessageWithUser, DmChannel, FederationRelayProfileSnapshot, FederationIdentityDeleteS2SRequest, FederationProfileUpdatePayload, ServerEvent, ApprovalRequestSubscriberSummary, PeeringTriggerReason } from '@backspace/shared';
+import { GROUP_DM_NAME_MIN_LENGTH, GROUP_DM_NAME_MAX_LENGTH } from '@backspace/shared/src/constants.js';
 
 /** Fields safe to expose to admin callers (everything except hmacSecret). */
 interface SanitizedPeer {
@@ -3006,6 +3007,9 @@ export async function processRelayEvents(
         case 'profile_update':
           await processProfileUpdateEvent(event, sourceInstance, db, accepted, rejected);
           break;
+        case 'group_metadata_update':
+          await processGroupMetadataUpdateEvent(event, sourceInstance, db, accepted, rejected);
+          break;
         case 'presence_update':
           processPresenceUpdateEvent(event, sourceInstance, db, accepted, rejected);
           break;
@@ -5665,7 +5669,7 @@ function processDmTypingStopEvent(
  * Returns the local filename on success, or null on failure.
  * On failure, the caller stores the absolute URL as a display fallback.
  */
-async function downloadProfileAsset(
+export async function downloadProfileAsset(
   url: string,
   sourceInstance: string,
 ): Promise<string | null> {
@@ -5852,6 +5856,268 @@ export async function processProfileUpdateEvent(
   accepted.push(event.messageId);
 }
 
+/**
+ * Inbound group_metadata_update relay handler.
+ *
+ * Authority: only the group's owner instance may mutate name/icon. We compare
+ * `extractDomain(sourceInstance)` with `extractDomain(channel.ownerHomeInstance)`;
+ * any other peer relaying this event is treated as an attribution mismatch
+ * (mirrors the strict check in processProfileUpdateEvent).
+ *
+ * Receiver hardening: never trust the wire payload's bounds — re-validate name
+ * length and icon URL scheme. A malicious or buggy peer cannot push us past
+ * the same constraints we enforce in PATCH /api/dm/:id.
+ *
+ * Side-effects on success:
+ *   1. dm_channels.{name,icon,metadataUpdatedAt} updated in a single tx.
+ *   2. One or two `dm_messages` system rows inserted (name_changed / icon_changed),
+ *      each tagged with `(sourceInstance, sourceMessageId)` using the dedup-suffix
+ *      scheme `${event.messageId}:name` / `${event.messageId}:icon`. This mirrors
+ *      processMemberAddEvent's idempotency contract — a retry of the same wire
+ *      event must not insert a second row.
+ *   3. dm_channel_updated WS broadcast to local members.
+ *   4. dm_message_created WS broadcast for each new system message.
+ *   5. Old local icon file is unlinked from disk if it changed away from a local
+ *      filename (same precedent as the local PATCH endpoint).
+ */
+export async function processGroupMetadataUpdateEvent(
+  event: FederationRelayEvent,
+  sourceInstance: string,
+  db: ReturnType<typeof getDb>,
+  accepted: string[],
+  rejected: Array<{ messageId: string; reason: string }>,
+): Promise<void> {
+  if (!event.federatedId) {
+    rejected.push({ messageId: event.messageId, reason: 'missing_federated_id' });
+    return;
+  }
+
+  // Lookup channel by federated_id. Missing → idempotent accept (this peer has
+  // no replica of the channel, nothing to update).
+  const channel = db
+    .select()
+    .from(schema.dmChannels)
+    .where(eq(schema.dmChannels.federatedId, event.federatedId))
+    .get();
+
+  if (!channel) {
+    accepted.push(event.messageId);
+    return;
+  }
+
+  // Authority: only the owner's home instance can mutate group metadata.
+  if (extractDomain(sourceInstance) !== extractDomain(channel.ownerHomeInstance ?? '')) {
+    rejected.push({ messageId: event.messageId, reason: 'attribution_mismatch' });
+    return;
+  }
+
+  // Receiver hardening — payload validation. Don't trust remote peers to
+  // respect our bounds; a peer bug or malicious actor must not be able to
+  // push values our local PATCH endpoint would have rejected.
+  const metadata = event.metadata;
+  if (!metadata) {
+    rejected.push({ messageId: event.messageId, reason: 'missing_metadata_payload' });
+    return;
+  }
+
+  if (metadata.name !== null) {
+    const trimmedLength = metadata.name.trim().length;
+    if (trimmedLength < GROUP_DM_NAME_MIN_LENGTH || trimmedLength > GROUP_DM_NAME_MAX_LENGTH) {
+      rejected.push({ messageId: event.messageId, reason: 'invalid_payload' });
+      return;
+    }
+  }
+
+  if (metadata.icon !== null && !(metadata.icon.startsWith('http://') || metadata.icon.startsWith('https://'))) {
+    rejected.push({ messageId: event.messageId, reason: 'invalid_payload' });
+    return;
+  }
+
+  // Version check: stale or duplicate timestamp → silent accept.
+  if (metadata.metadataUpdatedAt <= (channel.metadataUpdatedAt ?? 0)) {
+    accepted.push(event.messageId);
+    return;
+  }
+
+  // Diff against stored row — if neither field actually changed, no-op.
+  const nameChanged = metadata.name !== channel.name;
+  const iconChanged = metadata.icon !== channel.icon;
+  if (!nameChanged && !iconChanged) {
+    accepted.push(event.messageId);
+    return;
+  }
+
+  // Idempotency pre-check: if either system message already exists under
+  // `(sourceInstance, sourceMessageId)`, this event was already processed.
+  // Mirrors processMemberAddEvent's dedup contract — outbox retries and
+  // initial-sync replay must not double-insert.
+  const nameMessageId = `${event.messageId}:name`;
+  const iconMessageId = `${event.messageId}:icon`;
+  const existingNameRow = nameChanged
+    ? db
+      .select({ id: schema.dmMessages.id })
+      .from(schema.dmMessages)
+      .where(and(
+        eq(schema.dmMessages.sourceInstance, sourceInstance),
+        eq(schema.dmMessages.sourceMessageId, nameMessageId),
+      ))
+      .get()
+    : null;
+  const existingIconRow = iconChanged
+    ? db
+      .select({ id: schema.dmMessages.id })
+      .from(schema.dmMessages)
+      .where(and(
+        eq(schema.dmMessages.sourceInstance, sourceInstance),
+        eq(schema.dmMessages.sourceMessageId, iconMessageId),
+      ))
+      .get()
+    : null;
+  // If every changed field already has its corresponding system row, the
+  // entire event has been applied — accept silently.
+  if (
+    (!nameChanged || existingNameRow)
+    && (!iconChanged || existingIconRow)
+  ) {
+    accepted.push(event.messageId);
+    return;
+  }
+
+  // ── Resolve icon: download to local upload dir, fall back to absolute URL ──
+  let resolvedIcon: string | null = metadata.icon;
+  if (iconChanged && metadata.icon !== null) {
+    const localFile = await downloadProfileAsset(metadata.icon, sourceInstance);
+    resolvedIcon = localFile ?? metadata.icon;
+  }
+
+  // Resolve actor → local user id for the system-message foreign key.
+  // Falls back to channel.ownerId if the actor stub can't be created (e.g.
+  // tombstoned identity); the system message still has to render somewhere.
+  const actorParticipant = metadata.actor;
+  let actorUserId: string | null = null;
+  if (actorParticipant) {
+    const actorUser = resolveOrCreateReplicatedUser(
+      actorParticipant.homeUserId,
+      actorParticipant.homeInstance,
+      db,
+      { username: actorParticipant.profile?.username, status: actorParticipant.profile?.status },
+    );
+    actorUserId = actorUser?.id ?? null;
+  }
+  if (!actorUserId) {
+    actorUserId = channel.ownerId;
+  }
+  if (!actorUserId) {
+    // No owner user row to attach a system message to — extremely unusual,
+    // bail out cleanly without persisting anything.
+    rejected.push({ messageId: event.messageId, reason: 'actor_not_found' });
+    return;
+  }
+
+  const oldName = channel.name;
+  const oldIcon = channel.icon;
+
+  type SystemMessageRow = { id: string; sourceMessageId: string; content: string; createdAt: number };
+  const sysMessageRows: SystemMessageRow[] = [];
+
+  // Single transaction: channel update + system message insert(s).
+  db.transaction((tx) => {
+    tx.update(schema.dmChannels)
+      .set({
+        name: metadata.name,
+        icon: resolvedIcon,
+        metadataUpdatedAt: metadata.metadataUpdatedAt,
+      })
+      .where(eq(schema.dmChannels.id, channel.id))
+      .run();
+
+    if (nameChanged && !existingNameRow) {
+      const sysId = generateSnowflake();
+      const content = JSON.stringify({ event: 'name_changed', oldName, newName: metadata.name });
+      tx.insert(schema.dmMessages).values({
+        id: sysId,
+        dmChannelId: channel.id,
+        userId: actorUserId,
+        content,
+        type: 'system',
+        sourceInstance,
+        sourceMessageId: nameMessageId,
+        createdAt: metadata.metadataUpdatedAt,
+      }).run();
+      sysMessageRows.push({
+        id: sysId,
+        sourceMessageId: nameMessageId,
+        content,
+        createdAt: metadata.metadataUpdatedAt,
+      });
+    }
+
+    if (iconChanged && !existingIconRow) {
+      const sysId = generateSnowflake();
+      const content = JSON.stringify({ event: 'icon_changed' });
+      tx.insert(schema.dmMessages).values({
+        id: sysId,
+        dmChannelId: channel.id,
+        userId: actorUserId,
+        content,
+        type: 'system',
+        sourceInstance,
+        sourceMessageId: iconMessageId,
+        createdAt: metadata.metadataUpdatedAt,
+      }).run();
+      sysMessageRows.push({
+        id: sysId,
+        sourceMessageId: iconMessageId,
+        content,
+        createdAt: metadata.metadataUpdatedAt,
+      });
+    }
+  });
+
+  // ── Broadcast channel update to local members ──
+  connectionManager.sendToDmMembers(channel.id, {
+    type: 'dm_channel_updated',
+    dmChannelId: channel.id,
+    name: metadata.name,
+    icon: resolvedIcon,
+  });
+
+  // ── Broadcast each new system message ──
+  const actorRow = db.select().from(schema.users).where(eq(schema.users.id, actorUserId)).get();
+  const sanitizedActor = actorRow ? sanitizeUser(actorRow) : undefined;
+  for (const sys of sysMessageRows) {
+    connectionManager.sendToDmMembers(channel.id, {
+      type: 'dm_message_created',
+      message: {
+        id: sys.id,
+        dmChannelId: channel.id,
+        userId: actorUserId,
+        content: sys.content,
+        type: 'system',
+        createdAt: sys.createdAt,
+        sourceInstance,
+        sourceMessageId: sys.sourceMessageId,
+        editedAt: null,
+        replyToId: null,
+        user: sanitizedActor,
+        attachments: [],
+        embeds: [],
+        reactions: [],
+      } as DmMessageWithUser,
+    });
+  }
+
+  // ── Cleanup old local icon file ──
+  // Mirrors the local PATCH precedent (dm.ts:1595): only unlink when the
+  // previous icon was a bare local filename (i.e. we own the file on disk).
+  // Absolute URLs point at remote files we never owned.
+  if (iconChanged && oldIcon && !oldIcon.startsWith('http://') && !oldIcon.startsWith('https://') && oldIcon !== resolvedIcon) {
+    deleteUploadFile(oldIcon);
+  }
+
+  accepted.push(event.messageId);
+}
+
 /**
  * Inbound presence_update relay handler.
  *