diff --git a/docs/systems/federation.md b/docs/systems/federation.md
index c278144d..c6b85ffd 100644
--- a/docs/systems/federation.md
+++ b/docs/systems/federation.md
@@ -406,6 +406,7 @@ Allows a home instance to remove a user's replicated identity from a remote inst
 **Behavior:**
 
 - **Attribution guard:** Rejects with `403` if the user's `homeInstance` doesn't match the `X-Federation-Origin` of the signing peer. Prevents one instance from deleting another instance's users.
+- **Detached-account guard:** After the attribution guard, if the resolved user has `federation_home_orphaned = 1` (detached — its home domain was reset and it is now a sovereign local account, see §4.2/§4.3 of the orphaned-account-detach design), returns an idempotent `200 { success: true }` **without deleting**. A new incarnation on the reset domain must never delete an established account by replaying its old `homeUserId`.
 - **Idempotent:** Returns `{ success: true }` for already-deleted or nonexistent users (no error).
 - **Owned spaces check:** Returns `409` with `{ ownedSpaces: string[] }` if the user owns any spaces on the remote. The user must transfer or delete those spaces before identity removal proceeds.
 - **Mode `"soft"`:** Calls `tombstoneUser(uid, { purgeContent: false })` — anonymizes the user row and removes the user from spaces, friends, DM membership (`dm_members`), and read-states. The `purgeContent: false` flag skips only `reactions`, `dm_reactions`, and the user's space `messages` (with attachments + embeds); DM membership cleanup and orphaned-DM purge always run in both modes (per `userDeletion.ts:121-126, 169-202`) because zero-member DM channels are unreachable garbage regardless of authorship retention.
@@ -503,6 +504,7 @@ Two layers of replay protection:
 - Three-tier lookup: homeUserId match → domain + username hint match → not found
 - Tier 1: delegates to `resolveLocalUser` (fast path)
 - Tier 2: uses `extractDomain(homeInstance)` + `hints.username` to match stubs created by the auth registration path (which may have a different homeUserId)
+- **Tier 2 excludes detached accounts** (`federation_home_orphaned = 1`): a detached account is sovereign and must never be re-bound to the reset domain's new incarnation via username heuristics — that is exactly how a new same-name user would capture the established account. **Tier 1 (`homeUserId` match) is deliberately NOT excluded:** the new incarnation mints fresh `homeUserId`s, so a tier-1 hit on a detached row is a legitimate historical reference (e.g. an old group-DM attribution relayed by a third instance), not the new incarnation. Mutations are blocked at their own sites (profile_update handler, S2S identity delete).
 - Side-effect-free — does not modify any records
 - When multiple candidates match in tier 2, prefers real accounts over stubs, then most profile data
 - **Use when:** Read-only lookup that needs to find users created by either auth or relay path
@@ -1311,6 +1313,8 @@ Profile data is synced server-to-server. The home instance is authoritative —
 
 **Processing:** Remote overwrites all 6 mutable fields unconditionally; `displayName` falls back to `payload.displayName ?? payload.username`. Rejects if incoming `profileUpdatedAt ≤ stored`. Broadcasts `user_updated` to local WS clients.
 
+**Detached-account guard:** After the domain-collision check and before the version check, if the resolved `localUser` has `federation_home_orphaned = 1` (detached — home domain was reset, now a sovereign local account), the event is **acked (messageId pushed to `accepted`) and skipped without applying**. The reset domain's new incarnation must never overwrite an established account's profile by replaying its old `homeUserId`. Ack rather than reject because the sender legitimately considers the identity theirs to update; from this side the update simply no-ops.
+
 #### Profile Image File Replication
 
 When a `profile_update` relay carries avatar or banner absolute URLs, the receiving instance downloads the image files locally rather than storing remote URLs. This eliminates cross-origin dependencies — avatars render from the local `/api/uploads/` endpoint.
diff --git a/packages/server/src/routes/federation.detachedGuards.test.ts b/packages/server/src/routes/federation.detachedGuards.test.ts
new file mode 100644
index 00000000..d0f2c0c8
--- /dev/null
+++ b/packages/server/src/routes/federation.detachedGuards.test.ts
@@ -0,0 +1,183 @@
+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 { randomUUID } from 'node:crypto';
+import * as schema from '../db/schema.js';
+import { setWorkerId } from '../utils/snowflake.js';
+import { signRequest } from '../utils/federationAuth.js';
+import type { FederationRelayEvent } from '@backspace/shared';
+
+setWorkerId(9);
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let sqlite: Database.Database;
+let testDb: TestDb;
+
+const PEER_ORIGIN = 'https://orbit.test';
+const PEER_DOMAIN = 'orbit.test';
+const PEER_SECRET = 'a'.repeat(64);
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  getRawDb: () => sqlite,
+  schema,
+}));
+
+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(),
+    forceDisconnectUser: vi.fn(),
+  },
+}));
+
+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();
+}
+
+const DETACHED_ID = 'detached-1';
+const DETACHED_HOME_UID = 'old-home-uid';
+
+// A REAL federated account whose home domain (orbit.test) was reset. It has been
+// detached (federationHomeOrphaned = 1): sovereign local account, never re-bindable
+// to the reset domain's new incarnation.
+function seedDetachedAccount(): void {
+  testDb.insert(schema.users).values({
+    id: DETACHED_ID,
+    username: 'alice@orbit.test',
+    displayName: 'Alice',
+    passwordHash: '$2b$10$abcdefghijklmnopqrstuv', // real bcrypt-like hash
+    status: 'offline',
+    isAdmin: 0,
+    isDeleted: 0,
+    homeInstance: PEER_DOMAIN,
+    homeUserId: DETACHED_HOME_UID,
+    federationHomeOrphaned: 1,
+    profileUpdatedAt: 1000,
+    createdAt: Date.now(),
+  }).run();
+}
+
+async function buildApp(): Promise<FastifyInstance> {
+  const app = Fastify({ logger: false });
+  const { federationRoutes } = await import('./federation.js');
+  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',
+  };
+}
+
+beforeEach(() => {
+  sqlite = new Database(':memory:');
+  testDb = drizzle(sqlite, { schema });
+  applyMigrations(sqlite);
+  seedActivePeer();
+  seedDetachedAccount();
+});
+
+describe('S2S identity delete — detached account guard', () => {
+  it('skips a detached account (idempotent 200, row intact)', async () => {
+    const app = await buildApp();
+    const body = JSON.stringify({
+      homeUserId: DETACHED_HOME_UID,
+      homeInstance: PEER_DOMAIN,
+      mode: 'full',
+    });
+    const res = await app.inject({
+      method: 'DELETE',
+      url: '/api/federation/identity',
+      headers: signedHeaders(body),
+      payload: body,
+    });
+
+    expect(res.statusCode).toBe(200);
+    expect(JSON.parse(res.body)).toEqual({ success: true });
+
+    const row = testDb.select().from(schema.users).where(eq(schema.users.id, DETACHED_ID)).get();
+    expect(row?.isDeleted).toBe(0); // NOT deleted — detached account is sovereign
+    await app.close();
+  });
+});
+
+describe('S2S profile_update — detached account guard', () => {
+  it('skips a detached account (acked, profile unchanged)', async () => {
+    const fed = await import('./federation.js');
+    const event: FederationRelayEvent = {
+      eventType: 'profile_update',
+      contextType: 'profile',
+      messageId: 'm-hijack',
+      encryptionVersion: 0,
+      timestamp: Date.now(),
+      profileUpdate: {
+        homeUserId: DETACHED_HOME_UID,
+        homeInstance: PEER_DOMAIN,
+        profileUpdatedAt: 999999, // newer than stored 1000 — would apply if not guarded
+        username: 'alice',
+        displayName: 'Hijacked',
+        avatar: null,
+        banner: null,
+        accentColor: null,
+        avatarColor: null,
+        bio: null,
+      },
+    };
+    const accepted: string[] = [];
+    const rejected: Array<{ messageId: string; reason: string }> = [];
+    await fed.processProfileUpdateEvent(event, PEER_DOMAIN, testDb, accepted, rejected);
+
+    // Acked (not rejected) — the sender considers this identity theirs to update.
+    expect(rejected).toEqual([]);
+    expect(accepted).toEqual(['m-hijack']);
+
+    // But the detached row's profile is untouched.
+    const row = testDb.select().from(schema.users).where(eq(schema.users.id, DETACHED_ID)).get();
+    expect(row?.displayName).toBe('Alice');
+    expect(row?.profileUpdatedAt).toBe(1000);
+  });
+});
diff --git a/packages/server/src/routes/federation.ts b/packages/server/src/routes/federation.ts
index 49fb7ec0..d65cba19 100644
--- a/packages/server/src/routes/federation.ts
+++ b/packages/server/src/routes/federation.ts
@@ -2322,6 +2322,15 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
         return reply.code(403).send({ error: 'Attribution mismatch: you can only delete users from your own instance', statusCode: 403 });
       }
 
+      // Detached (home-orphaned) accounts are sovereign local accounts. The
+      // domain's new incarnation must not delete them by replaying old
+      // homeUserIds. Idempotent 200: from the caller's perspective this
+      // identity does not exist here.
+      if (user.federationHomeOrphaned === 1) {
+        console.log(`[federation] Ignoring S2S identity delete for detached account ${user.id} from ${fedHeaders.origin}`);
+        return reply.code(200).send({ success: true });
+      }
+
       // 5. Check for owned spaces
       const ownedSpaces = db.select({ id: schema.spaces.id, name: schema.spaces.name })
         .from(schema.spaces)
@@ -3478,6 +3487,10 @@ export function findFederatedUser(
       and(
         eq(schema.users.homeInstance, domain),
         eq(schema.users.isDeleted, 0),
+        // Detached (home-orphaned) accounts are sovereign: never re-bindable to
+        // the domain's new incarnation via username heuristics — that is exactly
+        // how a new same-name user would capture the established account.
+        eq(schema.users.federationHomeOrphaned, 0),
         or(
           sql`lower(substr(${schema.users.username}, 1, instr(${schema.users.username}, '@') - 1)) = ${hintLower}`,
           and(
@@ -6110,6 +6123,16 @@ export async function processProfileUpdateEvent(
     return;
   }
 
+  // Detached accounts are sovereign: the domain now belongs to a different
+  // incarnation, which must never overwrite the established account's profile
+  // by replaying its old homeUserId. Ack (not reject) — the sender considers
+  // this identity theirs to update; from our side the update simply no-ops.
+  if (localUser.federationHomeOrphaned === 1) {
+    console.log(`[federation] Skipping profile_update for detached account ${localUser.id} (home-orphaned)`);
+    accepted.push(event.messageId);
+    return;
+  }
+
   // Version check: reject stale/duplicate events
   const storedTs = localUser.profileUpdatedAt ?? 0;
   const incomingTs = payload.profileUpdatedAt ?? 0;
diff --git a/packages/server/src/routes/federation.userLookup.test.ts b/packages/server/src/routes/federation.userLookup.test.ts
index 2826af4a..e3872452 100644
--- a/packages/server/src/routes/federation.userLookup.test.ts
+++ b/packages/server/src/routes/federation.userLookup.test.ts
@@ -71,12 +71,14 @@ function seedUser(opts: {
   avatarColor?: string | null;
   banner?: string | null;
   bio?: string | null;
+  passwordHash?: string;
+  federationHomeOrphaned?: 0 | 1;
 }): void {
   testDb.insert(schema.users).values({
     id: opts.id,
     username: opts.username,
     displayName: opts.displayName ?? null,
-    passwordHash: 'x',
+    passwordHash: opts.passwordHash ?? 'x',
     status: 'offline',
     isAdmin: 0,
     isDeleted: opts.isDeleted ?? 0,
@@ -87,6 +89,7 @@ function seedUser(opts: {
     avatarColor: opts.avatarColor ?? null,
     banner: opts.banner ?? null,
     bio: opts.bio ?? null,
+    federationHomeOrphaned: opts.federationHomeOrphaned ?? 0,
     createdAt: Date.now(),
   }).run();
 }
@@ -249,3 +252,37 @@ describe('POST /api/federation/users/lookup', () => {
     expect(blocked.headers['retry-after']).toBe('60');
   });
 });
+
+describe('findFederatedUser — detached (home-orphaned) accounts', () => {
+  beforeEach(() => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+    // A REAL federated account whose home domain was reset. It has been detached
+    // (federationHomeOrphaned = 1): it now owns its identity locally and must
+    // never be re-captured by the reset domain's new incarnation.
+    seedUser({
+      id: 'detached-1',
+      username: 'alice@peer.example',
+      homeInstance: 'peer.example',
+      homeUserId: 'old-home-uid',
+      passwordHash: '$2b$10$abcdefghijklmnopqrstuv', // real bcrypt-like hash, not a stub
+      federationHomeOrphaned: 1,
+    });
+  });
+
+  it('tier-2 never matches a detached (home-orphaned) account', async () => {
+    const { findFederatedUser } = await import('./federation.js');
+    // Fresh homeUserId → tier-1 miss; the reset domain replays 'alice' as a hint.
+    const found = findFederatedUser('new-home-uid', 'peer.example', testDb, { username: 'alice' });
+    expect(found).toBeUndefined();
+  });
+
+  it('tier-1 (homeUserId) still resolves a detached account for historical references', async () => {
+    const { findFederatedUser } = await import('./federation.js');
+    // The original homeUserId is a legitimate historical reference (e.g. an old
+    // group-DM attribution relayed by a third instance) — tier-1 must still resolve it.
+    const found = findFederatedUser('old-home-uid', 'peer.example', testDb, { username: 'alice' });
+    expect(found?.federationHomeOrphaned).toBe(1);
+  });
+});