diff --git a/docs/systems/federation.md b/docs/systems/federation.md
index d7d1127a..ec9121d2 100644
--- a/docs/systems/federation.md
+++ b/docs/systems/federation.md
@@ -57,17 +57,19 @@ Backspace federation is peer-to-peer with no central authority. Each instance ma
 - Validates `sourceOrigin`, `challenge`, `hmacSecret`, and (optional) `instanceName` from body
 - Handles existing peers: active -> return 200 (idempotent), revoked -> return 403, pending -> update with new secret and activate
 - New peer: creates record with provided `hmacSecret`, sets `status='active'`
-- Returns `{ accepted: true, instanceName: <ourName | null> }` on success — see "Instance name exchange" below
+- Returns `{ accepted: true, instanceName: <ourName | null>, instanceId: <ourEpoch> }` on success — see "Instance name & epoch exchange" below
 
-### Instance name exchange
+### Instance name & epoch exchange
 
-The handshake is bidirectional for the `instance_name` label rendered in the federation panel and in DM-call toasts (`peerLabel`):
+The handshake is bidirectional for two pieces of metadata: the `instance_name` label rendered in the federation panel and in DM-call toasts (`peerLabel`), and the **instance epoch** (`instance_id`, this instance's persistent incarnation UUID minted by `ensureDefaults`, accessed via `getInstanceId()`). The epoch is the authenticated baseline used by the instance-epoch self-healing feature to detect a wipe-and-reinstall on the same domain (design: `docs/superpowers/specs/2026-07-01-federation-instance-epoch-self-healing-design.md`).
 
-- **Initiator → responder:** the request body to `/peer/accept` carries `{ sourceOrigin, hmacSecret, instanceName }`. The responder reads `instanceName` and persists it to `federation_peers.instance_name` on every state-mutating activation path: `pending → active`, `awaiting_approval → active`, `rejected → active` (override), and new-peer create. The idempotent early-return path for already-`active` and `needs_attention` peers does NOT overwrite — same security posture that already refuses to overwrite `hmac_secret` on these paths from an unauthenticated request.
+- **Initiator → responder:** the request body to `/peer/accept` carries `{ sourceOrigin, hmacSecret, instanceName, instanceId }`. The responder reads `instanceName` → `federation_peers.instance_name` and `instanceId` → `federation_peers.peer_instance_id` on every state-mutating activation path: `pending → active`, `awaiting_approval → active` (token-valid and autoAccept-fallback), `rejected → active` (override), and new-peer create. The idempotent early-return path for already-`active` and `needs_attention` peers does NOT overwrite — same security posture that already refuses to overwrite `hmac_secret` on these paths from an unauthenticated request. (The idempotent guard's *detection* of a changed epoch on that path is a later part of the self-healing feature; the handshake itself only writes the epoch on true activation.)
 
-- **Responder → initiator:** the `/peer/accept` response body is `{ accepted: true, instanceName: <ourName | null> }`. The initiator (`performHandshake` in `utils/federationPeering.ts` and `/peer/initiate` in `routes/federation.ts`) parses it and persists alongside the `status='active'` write. Older peers that omit the field are tolerated — the column stays `null`. Non-JSON bodies are tolerated defensively.
+- **Responder → initiator:** the `/peer/accept` response body is `{ accepted: true, instanceName: <ourName | null>, instanceId: <ourEpoch> }`. The initiator (`performHandshake` in `utils/federationPeering.ts`, `/peer/initiate`, and both `/approval-requests/:id/approve` handlers in `routes/federation.ts`) parses `instanceName` and `instanceId` and persists them alongside the `status='active'` write (`peer_instance_id`). Older peers that omit either field are tolerated — the respective column stays `null` (backstopped later by the deterministic epoch-refresh and relay-envelope population). Non-JSON bodies are tolerated defensively.
 
-`instance_name` is cosmetic metadata, eventually-consistent. Anywhere `peerLabel` is rendered falls back to origin hostname when `instance_name IS NULL`. Instance renames do not currently re-broadcast — that's a separate, unimplemented feature.
+All four outbound `/peer/accept` senders (`performHandshake`, `/peer/initiate`, and the inbound + outbound `/approve` handlers) include `instanceId: getInstanceId()` in the request body, so a peer learns our epoch regardless of which path activated the relationship.
+
+`instance_name` is cosmetic metadata, eventually-consistent. Anywhere `peerLabel` is rendered falls back to origin hostname when `instance_name IS NULL`. Instance renames do not currently re-broadcast — that's a separate, unimplemented feature. `peer_instance_id` is trust-consequential (only ever written from authenticated channels) — see the self-healing design spec for detection/heal semantics.
 
 ### Secret Storage & Rotation
 
diff --git a/packages/server/src/routes/federation.approveOutbound.test.ts b/packages/server/src/routes/federation.approveOutbound.test.ts
index 5683fb57..28e9c46e 100644
--- a/packages/server/src/routes/federation.approveOutbound.test.ts
+++ b/packages/server/src/routes/federation.approveOutbound.test.ts
@@ -81,6 +81,7 @@ function seedInstanceSettings(): void {
   testDb.insert(schema.instanceSettings).values({
     id: 1,
     instanceName: 'Local Backspace',
+    instanceId: 'test-epoch-local',
     autoAcceptPeering: 0,
     registrationOpen: 1,
     updatedAt: Date.now(),
diff --git a/packages/server/src/routes/federation.outboundApprove.test.ts b/packages/server/src/routes/federation.outboundApprove.test.ts
index 80e38b63..868e8434 100644
--- a/packages/server/src/routes/federation.outboundApprove.test.ts
+++ b/packages/server/src/routes/federation.outboundApprove.test.ts
@@ -90,6 +90,7 @@ function seedInstanceSettings(): void {
   testDb.insert(schema.instanceSettings).values({
     id: 1,
     instanceName: 'Local Backspace',
+    instanceId: 'test-epoch-local',
     autoAcceptPeering: 0,
     registrationOpen: 1,
     updatedAt: Date.now(),
diff --git a/packages/server/src/routes/federation.outboundDeny.test.ts b/packages/server/src/routes/federation.outboundDeny.test.ts
index 27f66bc1..6d566f7c 100644
--- a/packages/server/src/routes/federation.outboundDeny.test.ts
+++ b/packages/server/src/routes/federation.outboundDeny.test.ts
@@ -85,6 +85,7 @@ function seedInstanceSettings(): void {
   testDb.insert(schema.instanceSettings).values({
     id: 1,
     instanceName: 'Local Backspace',
+    instanceId: 'test-epoch-local',
     autoAcceptPeering: 0,
     registrationOpen: 1,
     updatedAt: Date.now(),
diff --git a/packages/server/src/routes/federation.peerAccept.test.ts b/packages/server/src/routes/federation.peerAccept.test.ts
index 6ce1e1c3..0e9bad04 100644
--- a/packages/server/src/routes/federation.peerAccept.test.ts
+++ b/packages/server/src/routes/federation.peerAccept.test.ts
@@ -62,6 +62,7 @@ function seedInstanceSettings(name: string): void {
   testDb.insert(schema.instanceSettings).values({
     id: 1,
     instanceName: name,
+    instanceId: 'test-epoch-local',
     autoAcceptPeering: 1,
     registrationOpen: 1,
     updatedAt: Date.now(),
diff --git a/packages/server/src/routes/federation.peerAcceptApprovalToken.test.ts b/packages/server/src/routes/federation.peerAcceptApprovalToken.test.ts
index 933741f4..e2a4262c 100644
--- a/packages/server/src/routes/federation.peerAcceptApprovalToken.test.ts
+++ b/packages/server/src/routes/federation.peerAcceptApprovalToken.test.ts
@@ -61,6 +61,7 @@ function seedInstanceSettings(autoAccept: 0 | 1): void {
   testDb.insert(schema.instanceSettings).values({
     id: 1,
     instanceName: 'Local Backspace',
+    instanceId: 'test-epoch-local',
     autoAcceptPeering: autoAccept,
     registrationOpen: 1,
     updatedAt: Date.now(),
diff --git a/packages/server/src/routes/federation.peerInitiateOutbound.test.ts b/packages/server/src/routes/federation.peerInitiateOutbound.test.ts
index daf5e9a7..328caa23 100644
--- a/packages/server/src/routes/federation.peerInitiateOutbound.test.ts
+++ b/packages/server/src/routes/federation.peerInitiateOutbound.test.ts
@@ -81,6 +81,7 @@ function seedInstanceSettings(): void {
   testDb.insert(schema.instanceSettings).values({
     id: 1,
     instanceName: 'Local Backspace',
+    instanceId: 'test-epoch-local',
     autoAcceptPeering: 0,
     registrationOpen: 1,
     updatedAt: Date.now(),
diff --git a/packages/server/src/routes/federation.ts b/packages/server/src/routes/federation.ts
index 8f912f1c..378537a1 100644
--- a/packages/server/src/routes/federation.ts
+++ b/packages/server/src/routes/federation.ts
@@ -19,6 +19,7 @@ import { deleteAttachmentFiles, deleteUploadFile } from '../utils/fileCleanup.js
 import { tombstoneUser, collectDeletionBroadcastTargets, collectProfileBroadcastTargetIds } from '../utils/userDeletion.js';
 import { computeFederatedId, getDmParticipants, sendCallRelay } from '../utils/federationOutbox.js';
 import { onPeerActivated, onPeerDeactivated } from '../utils/federationPeerActivation.js';
+import { getInstanceId } from '../utils/federationEpoch.js';
 import { probePeerReachable, markPeerRecovered } from '../utils/federationRecovery.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';
@@ -379,6 +380,7 @@ async function handleInboundApprove(
         sourceOrigin: localOrigin,
         hmacSecret,
         instanceName,
+        instanceId: getInstanceId(),
         // Forward the stored token (issued in our 202 response when the
         // remote first sent /peer/accept). Lets the remote verify mutual
         // admin approval. Spec §3.7.
@@ -430,21 +432,26 @@ async function handleInboundApprove(
       return reply.code(502).send({ error: errorMessage, statusCode: 502 });
     }
 
-    // Parse the remote's instanceName from the response body so the
-    // federation panel renders a friendly label. Tolerate omission and
-    // non-JSON bodies — same pattern as performHandshake and /peer/initiate.
+    // Parse the remote's instanceName and instanceId (epoch) from the response
+    // body so the federation panel renders a friendly label and we record the
+    // peer's authenticated epoch baseline. Tolerate omission and non-JSON
+    // bodies — same pattern as performHandshake and /peer/initiate.
     let remoteInstanceName: string | null = null;
+    let remoteInstanceId: string | null = null;
     try {
-      const body = (await response.json()) as { instanceName?: string | null };
+      const body = (await response.json()) as { instanceName?: string | null; instanceId?: string | null };
       if (typeof body?.instanceName === 'string' && body.instanceName.length > 0) {
         remoteInstanceName = body.instanceName;
       }
+      if (typeof body?.instanceId === 'string' && body.instanceId.length > 0) {
+        remoteInstanceId = body.instanceId;
+      }
     } catch {
       // Non-JSON body — leave null.
     }
 
     db.update(schema.federationPeers)
-      .set({ status: 'active', lastSeenAt: now, instanceName: remoteInstanceName, approvalToken: null })
+      .set({ status: 'active', lastSeenAt: now, instanceName: remoteInstanceName, peerInstanceId: remoteInstanceId, approvalToken: null })
       .where(eq(schema.federationPeers.id, peerId))
       .run();
 
@@ -532,6 +539,7 @@ async function handleOutboundApprove(
         sourceOrigin: localOrigin,
         hmacSecret,
         instanceName,
+        instanceId: getInstanceId(),
         // No approvalToken — outbound rows are admin-initiated locally; we
         // hold no prior token from the remote and rely on the remote's own
         // autoAcceptPeering setting to decide 200 vs 202.
@@ -612,13 +620,18 @@ async function handleOutboundApprove(
     });
   }
 
-  // 200 — peer activated. Capture remote's instanceName for the friendly label.
+  // 200 — peer activated. Capture remote's instanceName for the friendly label
+  // and instanceId (epoch) for the authenticated baseline.
   let remoteInstanceName: string | null = approvalReq.instanceName;
+  let remoteInstanceId: string | null = null;
   try {
-    const body = (await response.json()) as { instanceName?: string | null };
+    const body = (await response.json()) as { instanceName?: string | null; instanceId?: string | null };
     if (typeof body?.instanceName === 'string' && body.instanceName.length > 0) {
       remoteInstanceName = body.instanceName;
     }
+    if (typeof body?.instanceId === 'string' && body.instanceId.length > 0) {
+      remoteInstanceId = body.instanceId;
+    }
   } catch {
     // Non-JSON body — keep approvalReq.instanceName (may be null).
   }
@@ -628,6 +641,7 @@ async function handleOutboundApprove(
       status: 'active',
       lastSeenAt: now,
       instanceName: remoteInstanceName,
+      peerInstanceId: remoteInstanceId,
       approvalToken: null,
     })
     .where(eq(schema.federationPeers.id, peerId))
@@ -883,6 +897,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
               .from(schema.instanceSettings)
               .where(eq(schema.instanceSettings.id, 1))
               .get()?.name ?? undefined,
+            instanceId: getInstanceId(),
           }),
           signal: AbortSignal.timeout(10_000),
         });
@@ -940,20 +955,25 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
         }
 
         // Remote accepted — activate the peer. Parse the remote's instanceName
-        // from the response body so the federation panel renders a friendly
-        // label. Tolerate omission and non-JSON bodies.
+        // and instanceId (epoch) from the response body so the federation panel
+        // renders a friendly label and we record the peer's authenticated
+        // epoch baseline. Tolerate omission and non-JSON bodies.
         let remoteInstanceName: string | null = null;
+        let remoteInstanceId: string | null = null;
         try {
-          const body = (await response.json()) as { instanceName?: string | null };
+          const body = (await response.json()) as { instanceName?: string | null; instanceId?: string | null };
           if (typeof body?.instanceName === 'string' && body.instanceName.length > 0) {
             remoteInstanceName = body.instanceName;
           }
+          if (typeof body?.instanceId === 'string' && body.instanceId.length > 0) {
+            remoteInstanceId = body.instanceId;
+          }
         } catch {
           // Non-JSON body — leave null.
         }
 
         db.update(schema.federationPeers)
-          .set({ status: 'active', lastSeenAt: Date.now(), instanceName: remoteInstanceName, approvalToken: null })
+          .set({ status: 'active', lastSeenAt: Date.now(), instanceName: remoteInstanceName, peerInstanceId: remoteInstanceId, approvalToken: null })
           .where(eq(schema.federationPeers.id, peerId))
           .run();
         connectionManager.sendToAdmins({ type: 'federation_peers_changed' as const });
@@ -994,7 +1014,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
   // ─── POST /api/federation/peer/accept ──────────────────────────────────────
   // Server-to-server: accept a peering request from a remote instance.
   // No JWT auth — this is first contact. Rate-limited by IP.
-  app.post<{ Body: { sourceOrigin: string; challenge?: string; hmacSecret: string; instanceName?: string; approvalToken?: string } }>(
+  app.post<{ Body: { sourceOrigin: string; challenge?: string; hmacSecret: string; instanceName?: string; instanceId?: string; approvalToken?: string } }>(
     '/api/federation/peer/accept',
     async (request, reply) => {
       const clientIp = request.ip;
@@ -1005,7 +1025,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
         });
       }
 
-      const { sourceOrigin: rawOrigin, hmacSecret, instanceName: reqInstanceName, approvalToken: inboundToken } = request.body ?? {};
+      const { sourceOrigin: rawOrigin, hmacSecret, instanceName: reqInstanceName, instanceId: reqInstanceId, approvalToken: inboundToken } = request.body ?? {};
 
       if (!rawOrigin || typeof rawOrigin !== 'string') {
         return reply.code(400).send({ error: 'sourceOrigin is required', statusCode: 400 });
@@ -1031,6 +1051,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
         .get();
 
       const ourInstanceName = settings?.instanceName ?? null;
+      const ourInstanceId = getInstanceId();
       const autoAccept = settings?.autoAcceptPeering ?? 1;
 
       // ── autoAcceptPeering gate ──────────────────────────────────────────
@@ -1099,7 +1120,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
           // Legitimate recovery path: local admin clicks "Reset peering" →
           // row is deleted → remote's /peer/accept then lands on a
           // non-existent row and the normal handshake path runs.
-          return reply.code(200).send({ accepted: true, instanceName: ourInstanceName });
+          return reply.code(200).send({ accepted: true, instanceName: ourInstanceName, instanceId: ourInstanceId });
         }
         if (existing.status === 'revoked') {
           return reply.code(403).send({
@@ -1114,6 +1135,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
             .set({
               hmacSecret,
               instanceName: reqInstanceName ?? null,
+              peerInstanceId: reqInstanceId ?? null,
               status: 'active',
               lastSeenAt: Date.now(),
             })
@@ -1133,7 +1155,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
             console.error('[federation] onPeerActivated from /peer/accept (rejected override) failed:', err)
           );
 
-          return reply.code(200).send({ accepted: true, instanceName: ourInstanceName });
+          return reply.code(200).send({ accepted: true, instanceName: ourInstanceName, instanceId: ourInstanceId });
         }
         if (existing.status === 'awaiting_approval') {
           // Spec §3.5: token verification gates the awaiting_approval → active
@@ -1153,6 +1175,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
               .set({
                 hmacSecret,
                 instanceName: reqInstanceName ?? null,
+                peerInstanceId: reqInstanceId ?? null,
                 status: 'active',
                 lastSeenAt: Date.now(),
                 approvalToken: null,
@@ -1176,7 +1199,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
             onPeerActivated(existing.id, 'accept_awaiting_approval').catch(err =>
               console.error('[federation] onPeerActivated from /peer/accept (awaiting_approval) failed:', err)
             );
-            return reply.code(200).send({ accepted: true, instanceName: ourInstanceName });
+            return reply.code(200).send({ accepted: true, instanceName: ourInstanceName, instanceId: ourInstanceId });
           }
 
           // Token absent or mismatched. Cannot prove mutual approval.
@@ -1188,6 +1211,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
               .set({
                 hmacSecret,
                 instanceName: reqInstanceName ?? null,
+                peerInstanceId: reqInstanceId ?? null,
                 status: 'active',
                 lastSeenAt: Date.now(),
                 approvalToken: null,
@@ -1205,7 +1229,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
             onPeerActivated(existing.id, 'accept_awaiting_approval_fallback').catch(err =>
               console.error('[federation] onPeerActivated from /peer/accept (awaiting_approval fallback) failed:', err)
             );
-            return reply.code(200).send({ accepted: true, instanceName: ourInstanceName });
+            return reply.code(200).send({ accepted: true, instanceName: ourInstanceName, instanceId: ourInstanceId });
           }
 
           // autoAccept=0 + unverifiable inbound → queue as new approval-request.
@@ -1218,6 +1242,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
           .set({
             hmacSecret,
             instanceName: reqInstanceName ?? null,
+            peerInstanceId: reqInstanceId ?? null,
             status: 'active',
             lastSeenAt: Date.now(),
           })
@@ -1229,7 +1254,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
           console.error('[federation] onPeerActivated from /peer/accept (pending) failed:', err)
         );
 
-        return reply.code(200).send({ accepted: true, instanceName: ourInstanceName });
+        return reply.code(200).send({ accepted: true, instanceName: ourInstanceName, instanceId: ourInstanceId });
       }
 
       // New peer — create and activate
@@ -1239,6 +1264,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
         origin: sourceOrigin,
         hmacSecret,
         instanceName: reqInstanceName ?? null,
+        peerInstanceId: reqInstanceId ?? null,
         status: 'active',
         lastSeenAt: Date.now(),
         createdAt: Date.now(),
@@ -1249,7 +1275,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
         console.error('[federation] onPeerActivated from /peer/accept (new) failed:', err)
       );
 
-      return reply.code(200).send({ accepted: true, instanceName: ourInstanceName });
+      return reply.code(200).send({ accepted: true, instanceName: ourInstanceName, instanceId: ourInstanceId });
     },
   );
 
diff --git a/packages/server/src/utils/federationEpochHandshake.test.ts b/packages/server/src/utils/federationEpochHandshake.test.ts
new file mode 100644
index 00000000..d51054af
--- /dev/null
+++ b/packages/server/src/utils/federationEpochHandshake.test.ts
@@ -0,0 +1,301 @@
+import { describe, it, expect, beforeEach, afterEach, 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 './snowflake.js';
+
+setWorkerId(1);
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let sqlite: Database.Database;
+let testDb: TestDb;
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  getRawDb: () => sqlite,
+  schema,
+}));
+
+vi.mock('../utils/auth.js', () => ({
+  authenticate: async (req: { userId?: string }) => {
+    req.userId = 'admin-user';
+  },
+  requireAdmin: async () => {
+    // peer/accept is unauthenticated anyway
+  },
+}));
+
+vi.mock('../ws/handler.js', () => ({
+  connectionManager: {
+    sendToAdmins: vi.fn(),
+    getAllOnlineUserIds: () => [],
+    sendToUser: vi.fn(),
+    sendToDmMembers: vi.fn(),
+  },
+}));
+
+vi.mock('../utils/federationPeerActivation.js', () => ({
+  onPeerActivated: vi.fn(async () => undefined),
+  onPeerDeactivated: vi.fn(async () => undefined),
+}));
+
+const LOCAL_EPOCH = 'local-epoch-0000';
+
+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 sqlText = fs.readFileSync(path.join(migrationsDir, f), 'utf8');
+    const statements = sqlText.split(/-->\s*statement-breakpoint/);
+    for (const stmt of statements) {
+      const clean = stmt.trim();
+      if (clean) db.exec(clean);
+    }
+  }
+}
+
+function seedInstanceSettings(): void {
+  testDb.insert(schema.instanceSettings).values({
+    id: 1,
+    instanceName: 'Local Backspace',
+    instanceId: LOCAL_EPOCH,
+    autoAcceptPeering: 1,
+    registrationOpen: 1,
+    updatedAt: Date.now(),
+  }).run();
+}
+
+async function buildApp(): Promise<FastifyInstance> {
+  const app = Fastify({ logger: false });
+  const { federationRoutes } = await import('../routes/federation.js');
+  await app.register(federationRoutes);
+  await app.ready();
+  return app;
+}
+
+describe('POST /api/federation/peer/accept — peer_instance_id (epoch) persistence', () => {
+  let app: FastifyInstance;
+
+  beforeEach(async () => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+    seedInstanceSettings();
+    const { __resetInstanceIdCacheForTest } = await import('./federationEpoch.js');
+    __resetInstanceIdCacheForTest();
+    app = await buildApp();
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+    sqlite.close();
+  });
+
+  it('writes peer_instance_id when activating an existing pending peer', async () => {
+    testDb.insert(schema.federationPeers).values({
+      id: 'peer-pending',
+      origin: 'https://remote.example',
+      hmacSecret: 'old-secret',
+      status: 'pending',
+      createdAt: Date.now(),
+    }).run();
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/federation/peer/accept',
+      payload: {
+        sourceOrigin: 'https://remote.example',
+        hmacSecret: 'new-secret',
+        instanceName: 'Remote Backspace',
+        instanceId: 'epoch-A',
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const row = testDb.select().from(schema.federationPeers)
+      .where(eq(schema.federationPeers.id, 'peer-pending')).get();
+    expect(row?.status).toBe('active');
+    expect(row?.peerInstanceId).toBe('epoch-A');
+  });
+
+  it('writes peer_instance_id when creating a brand-new peer', async () => {
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/federation/peer/accept',
+      payload: {
+        sourceOrigin: 'https://remote.example',
+        hmacSecret: 'remote-secret',
+        instanceName: 'Remote Backspace',
+        instanceId: 'epoch-B',
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const row = testDb.select().from(schema.federationPeers)
+      .where(eq(schema.federationPeers.origin, 'https://remote.example')).get();
+    expect(row?.status).toBe('active');
+    expect(row?.peerInstanceId).toBe('epoch-B');
+  });
+
+  it('writes peer_instance_id when overriding rejected → active', async () => {
+    testDb.insert(schema.federationPeers).values({
+      id: 'peer-rejected',
+      origin: 'https://remote.example',
+      hmacSecret: 'old-secret',
+      status: 'rejected',
+      createdAt: Date.now(),
+    }).run();
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/federation/peer/accept',
+      payload: {
+        sourceOrigin: 'https://remote.example',
+        hmacSecret: 'new-secret',
+        instanceId: 'epoch-C',
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const row = testDb.select().from(schema.federationPeers)
+      .where(eq(schema.federationPeers.id, 'peer-rejected')).get();
+    expect(row?.status).toBe('active');
+    expect(row?.peerInstanceId).toBe('epoch-C');
+  });
+
+  it('writes peer_instance_id on the awaiting_approval autoAccept fallback path', async () => {
+    testDb.insert(schema.federationPeers).values({
+      id: 'peer-await',
+      origin: 'https://remote.example',
+      hmacSecret: 'old-secret',
+      status: 'awaiting_approval',
+      createdAt: Date.now(),
+    }).run();
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/federation/peer/accept',
+      payload: {
+        sourceOrigin: 'https://remote.example',
+        hmacSecret: 'new-secret',
+        instanceId: 'epoch-D',
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const row = testDb.select().from(schema.federationPeers)
+      .where(eq(schema.federationPeers.id, 'peer-await')).get();
+    expect(row?.status).toBe('active');
+    expect(row?.peerInstanceId).toBe('epoch-D');
+  });
+
+  it('writes null peer_instance_id when body omits instanceId (legacy peer)', async () => {
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/federation/peer/accept',
+      payload: {
+        sourceOrigin: 'https://remote.example',
+        hmacSecret: 'remote-secret',
+        instanceName: 'Remote Backspace',
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const row = testDb.select().from(schema.federationPeers)
+      .where(eq(schema.federationPeers.origin, 'https://remote.example')).get();
+    expect(row?.status).toBe('active');
+    expect(row?.peerInstanceId).toBeNull();
+  });
+
+  it('returns our own instanceId in the response body', async () => {
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/federation/peer/accept',
+      payload: {
+        sourceOrigin: 'https://remote.example',
+        hmacSecret: 'remote-secret',
+        instanceId: 'epoch-E',
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const body = response.json() as { accepted: boolean; instanceName?: string | null; instanceId?: string };
+    expect(body.accepted).toBe(true);
+    expect(body.instanceId).toBe(LOCAL_EPOCH);
+  });
+});
+
+describe('POST /api/federation/peer/initiate — persists remote epoch from handshake response', () => {
+  let app: FastifyInstance;
+
+  beforeEach(async () => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+    seedInstanceSettings();
+    const { __resetInstanceIdCacheForTest } = await import('./federationEpoch.js');
+    __resetInstanceIdCacheForTest();
+    app = await buildApp();
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    vi.restoreAllMocks();
+    sqlite.close();
+  });
+
+  it('writes peer_instance_id from the remote /peer/accept response body', async () => {
+    const fetchMock = vi.fn(async () =>
+      new Response(JSON.stringify({ accepted: true, instanceName: 'Remote', instanceId: 'remote-epoch-1' }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      }),
+    );
+    vi.stubGlobal('fetch', fetchMock);
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/federation/peer/initiate',
+      payload: { remoteOrigin: 'https://remote.example' },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const row = testDb.select().from(schema.federationPeers)
+      .where(eq(schema.federationPeers.origin, 'https://remote.example')).get();
+    expect(row?.status).toBe('active');
+    expect(row?.peerInstanceId).toBe('remote-epoch-1');
+
+    // Our epoch must be sent in the outbound handshake body.
+    const call = fetchMock.mock.calls[0] as unknown as [string, RequestInit];
+    const sentBody = JSON.parse(call[1].body as string) as { instanceId?: string };
+    expect(sentBody.instanceId).toBe(LOCAL_EPOCH);
+  });
+
+  it('writes null peer_instance_id when the remote response omits instanceId', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () =>
+      new Response(JSON.stringify({ accepted: true, instanceName: 'Remote' }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      }),
+    ));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/federation/peer/initiate',
+      payload: { remoteOrigin: 'https://remote.example' },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const row = testDb.select().from(schema.federationPeers)
+      .where(eq(schema.federationPeers.origin, 'https://remote.example')).get();
+    expect(row?.status).toBe('active');
+    expect(row?.peerInstanceId).toBeNull();
+  });
+});
diff --git a/packages/server/src/utils/federationPeering.approvalToken.test.ts b/packages/server/src/utils/federationPeering.approvalToken.test.ts
index 385fe076..666a8150 100644
--- a/packages/server/src/utils/federationPeering.approvalToken.test.ts
+++ b/packages/server/src/utils/federationPeering.approvalToken.test.ts
@@ -70,6 +70,7 @@ function seedInstanceSettings(): void {
   testDb.insert(schema.instanceSettings).values({
     id: 1,
     instanceName: 'Local Backspace',
+    instanceId: 'test-epoch-local',
     autoAcceptPeering: 1,
     registrationOpen: 1,
     updatedAt: Date.now(),
diff --git a/packages/server/src/utils/federationPeering.instanceName.test.ts b/packages/server/src/utils/federationPeering.instanceName.test.ts
index ca48bd33..46ac1e0c 100644
--- a/packages/server/src/utils/federationPeering.instanceName.test.ts
+++ b/packages/server/src/utils/federationPeering.instanceName.test.ts
@@ -69,6 +69,7 @@ function seedInstanceSettings(): void {
   testDb.insert(schema.instanceSettings).values({
     id: 1,
     instanceName: 'Local Backspace',
+    instanceId: 'test-epoch-local',
     autoAcceptPeering: 1,
     registrationOpen: 1,
     updatedAt: Date.now(),
diff --git a/packages/server/src/utils/federationPeering.outboundGate.test.ts b/packages/server/src/utils/federationPeering.outboundGate.test.ts
index a189b190..7de0a61a 100644
--- a/packages/server/src/utils/federationPeering.outboundGate.test.ts
+++ b/packages/server/src/utils/federationPeering.outboundGate.test.ts
@@ -70,6 +70,7 @@ function seedInstanceSettings(autoAcceptPeering: 0 | 1): void {
   testDb.insert(schema.instanceSettings).values({
     id: 1,
     instanceName: 'Local Backspace',
+    instanceId: 'test-epoch-local',
     autoAcceptPeering,
     registrationOpen: 1,
     updatedAt: Date.now(),
diff --git a/packages/server/src/utils/federationPeering.ts b/packages/server/src/utils/federationPeering.ts
index 4fdc8527..04da65cb 100644
--- a/packages/server/src/utils/federationPeering.ts
+++ b/packages/server/src/utils/federationPeering.ts
@@ -5,6 +5,7 @@ import { generateSnowflake } from './snowflake.js';
 import { getOurOrigin, generateHmacSecret } from './federationAuth.js';
 import { validateOrigin } from '../routes/federation.js';
 import { onPeerActivated, onPeerDeactivated } from './federationPeerActivation.js';
+import { getInstanceId } from './federationEpoch.js';
 import type { EnsurePeeredCallerIntent } from '@backspace/shared';
 
 // ─── Types ───────────────────────────────────────────────────────────────────
@@ -304,6 +305,7 @@ async function performHandshake(
         sourceOrigin: ourOrigin,
         hmacSecret,
         instanceName: getInstanceName(),
+        instanceId: getInstanceId(),
       }),
       signal: AbortSignal.timeout(10_000),
     });
@@ -333,21 +335,26 @@ async function performHandshake(
     }
 
     if (response.ok) {
-      // 200 = peer accepted and activated. Parse remote's instanceName from
-      // the response body so we can render a friendly label for the peer.
+      // 200 = peer accepted and activated. Parse remote's instanceName and
+      // instanceId (epoch) from the response body so we can render a friendly
+      // label for the peer and record its authenticated epoch baseline.
       // Tolerate omission (older peers) and non-JSON bodies (defensive).
       let remoteInstanceName: string | null = null;
+      let remoteInstanceId: string | null = null;
       try {
-        const body = (await response.json()) as { instanceName?: string | null };
+        const body = (await response.json()) as { instanceName?: string | null; instanceId?: string | null };
         if (typeof body?.instanceName === 'string' && body.instanceName.length > 0) {
           remoteInstanceName = body.instanceName;
         }
+        if (typeof body?.instanceId === 'string' && body.instanceId.length > 0) {
+          remoteInstanceId = body.instanceId;
+        }
       } catch {
-        // Non-JSON or empty body — leave remoteInstanceName as null.
+        // Non-JSON or empty body — leave remoteInstanceName/Id as null.
       }
 
       db.update(schema.federationPeers)
-        .set({ status: 'active', lastSeenAt: Date.now(), instanceName: remoteInstanceName, approvalToken: null })
+        .set({ status: 'active', lastSeenAt: Date.now(), instanceName: remoteInstanceName, peerInstanceId: remoteInstanceId, approvalToken: null })
         .where(eq(schema.federationPeers.id, peerId))
         .run();
       const { connectionManager } = await import('../ws/handler.js');