diff --git a/docs/systems/auth.md b/docs/systems/auth.md
index 79dad638..f3642804 100644
--- a/docs/systems/auth.md
+++ b/docs/systems/auth.md
@@ -118,11 +118,54 @@ There is **no token blocklist**. The only revocation mechanism is the `passwordC
 
 ### Registration Gate
 
-Registration open/closed is determined by:
+The `/api/auth/register` route splits its gate by request shape (spec §1.2). There are **two independent toggles** plus an **invite-token bypass** for the local path.
+
+**Local anonymous signup** (no `homeInstance` in body):
+
 1. `instanceSettings.registrationOpen` (DB, id=1) -- if not null, this takes priority
 2. `config.registrationOpen` (env `REGISTRATION_OPEN`, default `true`) -- fallback
 
-Both are checked. DB value overrides env when explicitly set by admin.
+DB value overrides env when explicitly set by admin. When closed, a valid `inviteToken` bypasses the gate and is **atomically consumed** alongside the user insert (see "Invite Tokens" below). When open, an `inviteToken` field is **silently ignored** (no validation, no consumption).
+
+**Federated identity replication** (request body has `homeInstance`):
+
+- Gated solely by `instanceSettings.federatedRegistrationOpen` (NOT NULL DEFAULT 1).
+- `inviteToken` is **ignored entirely** on this path -- tokens never unlock federated creation, even when supplied.
+- Closed → 403 `"Federated registration is closed on this instance"`.
+
+**Invariants** (spec §1.3):
+
+- **Login is never gated** by either toggle. Both gates affect *creation only*. Existing accounts remain loginable regardless of policy.
+- The federated stub upgrade flow (below) is gated by `federatedRegistrationOpen`, never by an invite token.
+
+**Toggle matrix** (spec §5.6):
+
+| `registrationOpen` | `federatedRegistrationOpen` | Local register | Federated register |
+|---|---|---|---|
+| true | true | open | allowed |
+| true | false | open | 403 |
+| false | true | invite-required | allowed |
+| false | false | invite-required | 403 |
+
+### Invite Tokens
+
+When `registrationOpen` is false, the local-signup path accepts an `inviteToken` field on the register body. Token format: 22-char base64url (16 random bytes). Lifecycle and admin CRUD live in `inviteService.ts` and `routes/invites.ts` -- see `docs/systems/admin.md` for the panel UX and the full status state machine.
+
+Atomic redemption (spec §2.4):
+
+```
+db.transaction(() => {
+  // 1. Re-fetch invite by token under txn (closes TOCTOU vs /check-invite)
+  // 2. Reject if status !== 'active' → throw InviteUnavailableError → 403
+  // 3. INSERT user row
+  // 4. UPDATE invite_links SET usedCount = usedCount + 1
+  // 5. INSERT invite_redemptions row (forensic audit, snapshots username)
+})
+```
+
+If any step throws (concurrent revoke, last-slot race, username collision against the unique index), the entire transaction rolls back -- `usedCount` is never incremented on a failed registration. The route catches `InviteUnavailableError` from `redeemInvite()` and surfaces it as 403 `"Invalid or expired invite"`.
+
+The `/api/auth/check-invite` debounced UX endpoint pre-validates a token from the register page; the in-txn re-derive inside `redeemInvite()` is the authoritative enforcement point.
 
 ### First-User Admin Promotion
 
@@ -144,18 +187,21 @@ If `requestedAvatarColor` is provided and is in `AVATAR_COLORS`, use it. Otherwi
 ### Registration Steps
 
 1. Validate inputs (username format, password length)
-2. Check registration is open
-3. **Federated stub upgrade check** (if `homeInstance` is set): call `findFederatedUser` to look for an existing relay-created stub. If found and upgradeable, upgrade it instead of creating a new record (see below).
-4. Check username uniqueness (exact match on lowercased username)
-5. Hash password (bcrypt, 12 rounds)
-6. Generate Snowflake ID
-7. Insert user row (status defaults to `'offline'` at the schema level; it is set to `'online'` only when the client establishes a WebSocket via the WS auth path in `ws/handler.ts`). Admin flag set if first user.
-8. Sign JWT with `{ userId, username }`
-9. Return `{ token, user }` (user sanitized via `sanitizeUser(user, true)`)
+2. Read both registration gates from `instance_settings`
+3. **Branch by request shape** (spec §1.2):
+   - If `homeInstance` set → reject with 403 unless `federatedRegistrationOpen === true`. `inviteToken` ignored on this path.
+   - Else (local) → if `registrationOpen` is false, require a valid `inviteToken`; otherwise reject with 403. Pre-flight token check rejects obvious-invalid tokens before bcrypt.
+4. **Federated stub upgrade check** (if `homeInstance` is set): call `findFederatedUser` to look for an existing relay-created stub. If found and upgradeable, upgrade it instead of creating a new record (see below).
+5. Check username uniqueness (exact match on lowercased username)
+6. Hash password (bcrypt, 12 rounds)
+7. Generate Snowflake ID
+8. Insert user row (status defaults to `'offline'` at the schema level; it is set to `'online'` only when the client establishes a WebSocket via the WS auth path in `ws/handler.ts`). Admin flag set if first user. **When the local-closed-with-token path is in play**, the insert runs inside `redeemInvite()`'s transaction so the user row, the `usedCount` bump, and the `invite_redemptions` row commit atomically (or all roll back).
+9. Sign JWT with `{ userId, username }`
+10. Return `{ token, user }` (user sanitized via `sanitizeUser(user, true)`)
 
 ### Federated Stub Upgrade
 
-When a user registers with `homeInstance` set (federated registration via friend-connect), the registration path checks for an existing relay-created stub using `findFederatedUser`. If found and the stub has `passwordHash = '!federation-replicated'` (not a real account), the stub is upgraded:
+When a user registers with `homeInstance` set (federated registration via friend-connect), the registration path checks for an existing relay-created stub using `findFederatedUser`. The stub-upgrade flow is **always gated by `federatedRegistrationOpen`**, never by an invite token (spec §1.3). If found and the stub has `passwordHash = '!federation-replicated'` (not a real account), the stub is upgraded:
 
 - `passwordHash` is set to the new bcrypt hash (enables login)
 - `username` is updated to the registration's chosen username (replaces placeholder like `291255103060533248@nova.ddns.net` with `nova@nova.ddns.net`)
diff --git a/packages/server/src/routes/auth.test.ts b/packages/server/src/routes/auth.test.ts
index 99adbdab..57f41432 100644
--- a/packages/server/src/routes/auth.test.ts
+++ b/packages/server/src/routes/auth.test.ts
@@ -2,6 +2,7 @@ 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';
@@ -203,3 +204,254 @@ describe('GET /api/auth/check-invite', () => {
     }
   });
 });
+
+describe('POST /api/auth/register — federation gate split', () => {
+  beforeEach(() => {
+    // Ensure a fresh instance_settings singleton row with both gates default-true.
+    // The test harness's applyMigrations creates the table but does not seed the
+    // id=1 row (production does so via migrate.ts:ensureDefaults on first boot).
+    // Each test then mutates the toggles it cares about.
+    testDb.delete(schema.instanceSettings).run();
+    testDb.insert(schema.instanceSettings).values({
+      id: 1,
+      registrationOpen: 1,
+      federatedRegistrationOpen: 1,
+      updatedAt: Date.now(),
+    }).run();
+  });
+
+  it('open registration: register without token succeeds; token field ignored if present', async () => {
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: { username: 'alice', password: 'password123' },
+    });
+    expect(res.statusCode).toBe(201);
+
+    // Try with bogus token — still succeeds, token ignored
+    const res2 = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: { username: 'bob', password: 'password123', inviteToken: 'fakefakefakefakefakeXX' },
+    });
+    expect(res2.statusCode).toBe(201);
+  });
+
+  it('closed registration without token: 403 "An invite is required"', async () => {
+    testDb.update(schema.instanceSettings)
+      .set({ registrationOpen: 0 })
+      .where(eq(schema.instanceSettings.id, 1))
+      .run();
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: { username: 'newalice', password: 'password123' },
+    });
+    expect(res.statusCode).toBe(403);
+    expect(res.json().error).toContain('invite is required');
+  });
+
+  it('closed registration with valid token: succeeds, usedCount incremented, redemption written', async () => {
+    testDb.update(schema.instanceSettings)
+      .set({ registrationOpen: 0 })
+      .where(eq(schema.instanceSettings.id, 1))
+      .run();
+    const token = 'abcdefghijklmnopqrstuv';
+    testDb.insert(schema.inviteLinks).values({
+      id: 'inv-redeem-1',
+      token,
+      name: 'F',
+      createdBy: ADMIN_ID,
+      createdAt: Date.now(),
+      maxUses: 5,
+      usedCount: 0,
+      expiresAt: null,
+      revokedAt: null,
+    }).run();
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: { username: 'newalice', password: 'password123', inviteToken: token },
+    });
+    expect(res.statusCode).toBe(201);
+
+    const inv = testDb.select().from(schema.inviteLinks)
+      .where(eq(schema.inviteLinks.id, 'inv-redeem-1')).get();
+    expect(inv?.usedCount).toBe(1);
+    const redemptions = testDb.select().from(schema.inviteRedemptions)
+      .where(eq(schema.inviteRedemptions.inviteId, 'inv-redeem-1')).all();
+    expect(redemptions).toHaveLength(1);
+    expect(redemptions[0]?.registrantUsername).toBe('newalice');
+  });
+
+  it('closed registration with invalid token: 403 "Invalid or expired invite"', async () => {
+    testDb.update(schema.instanceSettings)
+      .set({ registrationOpen: 0 })
+      .where(eq(schema.instanceSettings.id, 1))
+      .run();
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: { username: 'newalice', password: 'password123', inviteToken: 'fakefakefakefakefakeXX' },
+    });
+    expect(res.statusCode).toBe(403);
+    expect(res.json().error).toContain('Invalid or expired');
+  });
+
+  it('open registration with token: usedCount NOT incremented', async () => {
+    const token = 'abcdefghijklmnopqrstuv';
+    testDb.insert(schema.inviteLinks).values({
+      id: 'inv-ignore-1',
+      token,
+      name: 'F',
+      createdBy: ADMIN_ID,
+      createdAt: Date.now(),
+      maxUses: 5,
+      usedCount: 0,
+      expiresAt: null,
+      revokedAt: null,
+    }).run();
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: { username: 'newalice', password: 'password123', inviteToken: token },
+    });
+    expect(res.statusCode).toBe(201);
+
+    const inv = testDb.select().from(schema.inviteLinks)
+      .where(eq(schema.inviteLinks.id, 'inv-ignore-1')).get();
+    expect(inv?.usedCount).toBe(0);
+
+    const redemptions = testDb.select().from(schema.inviteRedemptions)
+      .where(eq(schema.inviteRedemptions.inviteId, 'inv-ignore-1')).all();
+    expect(redemptions).toHaveLength(0);
+  });
+
+  it('federated registration: blocked when federatedRegistrationOpen=false', async () => {
+    testDb.update(schema.instanceSettings)
+      .set({ federatedRegistrationOpen: 0 })
+      .where(eq(schema.instanceSettings.id, 1))
+      .run();
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: {
+        username: 'alice@otherhost',
+        password: 'password123',
+        homeInstance: 'otherhost',
+        homeUserId: 'remote-id',
+      },
+    });
+    expect(res.statusCode).toBe(403);
+    expect(res.json().error).toContain('Federated registration');
+  });
+
+  it('federated registration: blocked even with valid token when federatedRegistrationOpen=false', async () => {
+    testDb.update(schema.instanceSettings)
+      .set({ registrationOpen: 0, federatedRegistrationOpen: 0 })
+      .where(eq(schema.instanceSettings.id, 1))
+      .run();
+    const token = 'abcdefghijklmnopqrstuv';
+    testDb.insert(schema.inviteLinks).values({
+      id: 'inv-fed-blocked',
+      token,
+      name: 'F',
+      createdBy: ADMIN_ID,
+      createdAt: Date.now(),
+      maxUses: 5,
+      usedCount: 0,
+      expiresAt: null,
+      revokedAt: null,
+    }).run();
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: {
+        username: 'alice@otherhost',
+        password: 'password123',
+        homeInstance: 'otherhost',
+        homeUserId: 'remote-id',
+        inviteToken: token,
+      },
+    });
+    expect(res.statusCode).toBe(403);
+    expect(res.json().error).toContain('Federated registration');
+
+    // Token MUST NOT be consumed
+    const inv = testDb.select().from(schema.inviteLinks)
+      .where(eq(schema.inviteLinks.id, 'inv-fed-blocked')).get();
+    expect(inv?.usedCount).toBe(0);
+  });
+
+  it('federated registration: token IGNORED even if provided (no usedCount increment)', async () => {
+    testDb.update(schema.instanceSettings)
+      .set({ registrationOpen: 0, federatedRegistrationOpen: 1 })
+      .where(eq(schema.instanceSettings.id, 1))
+      .run();
+    const token = 'abcdefghijklmnopqrstuv';
+    testDb.insert(schema.inviteLinks).values({
+      id: 'inv-fed-ignore',
+      token,
+      name: 'F',
+      createdBy: ADMIN_ID,
+      createdAt: Date.now(),
+      maxUses: 5,
+      usedCount: 0,
+      expiresAt: null,
+      revokedAt: null,
+    }).run();
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: {
+        username: 'alice@otherhost',
+        password: 'password123',
+        homeInstance: 'otherhost',
+        homeUserId: 'remote-id',
+        inviteToken: token,
+      },
+    });
+    expect(res.statusCode).toBe(201);
+
+    const inv = testDb.select().from(schema.inviteLinks)
+      .where(eq(schema.inviteLinks.id, 'inv-fed-ignore')).get();
+    expect(inv?.usedCount).toBe(0);
+
+    const redemptions = testDb.select().from(schema.inviteRedemptions)
+      .where(eq(schema.inviteRedemptions.inviteId, 'inv-fed-ignore')).all();
+    expect(redemptions).toHaveLength(0);
+  });
+
+  it('closed registration: token last-slot race → 403 (in-txn re-derive)', async () => {
+    testDb.update(schema.instanceSettings)
+      .set({ registrationOpen: 0 })
+      .where(eq(schema.instanceSettings.id, 1))
+      .run();
+    const token = 'abcdefghijklmnopqrstuv';
+    testDb.insert(schema.inviteLinks).values({
+      id: 'inv-exhausted',
+      token,
+      name: 'F',
+      createdBy: ADMIN_ID,
+      createdAt: Date.now(),
+      maxUses: 1,
+      usedCount: 1, // already at the cap
+      expiresAt: null,
+      revokedAt: null,
+    }).run();
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/auth/register',
+      payload: { username: 'newalice', password: 'password123', inviteToken: token },
+    });
+    expect(res.statusCode).toBe(403);
+  });
+});
diff --git a/packages/server/src/routes/auth.ts b/packages/server/src/routes/auth.ts
index 9f42517d..157bbb4e 100644
--- a/packages/server/src/routes/auth.ts
+++ b/packages/server/src/routes/auth.ts
@@ -8,7 +8,7 @@ import type { RegisterRequest, LoginRequest, AuthResponse } from '@backspace/sha
 import { AVATAR_COLORS } from '@backspace/shared';
 import { sanitizeUser } from '../utils/sanitize.js';
 import { findFederatedUser } from './federation.js';
-import { getInviteByToken, inviteStatus } from '../utils/inviteService.js';
+import { getInviteByToken, inviteStatus, redeemInvite, InviteUnavailableError } from '../utils/inviteService.js';
 
 export async function authRoutes(app: FastifyInstance): Promise<void> {
   app.post<{ Body: RegisterRequest }>('/api/auth/register', {
@@ -76,13 +76,47 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
 
     const db = getDb();
 
-    // Check registration: DB setting overrides env var if explicitly set by admin
+    // Read both gates from instance_settings.
+    // - registrationOpen: nullable column; null falls back to env var (config.registrationOpen).
+    //   Admin-explicit 0/1 overrides env. Gates LOCAL anonymous signup.
+    // - federatedRegistrationOpen: NOT NULL DEFAULT 1 column. Gates FEDERATED identity
+    //   replication (homeInstance set). Independent of registrationOpen by spec §1.2.
     const instanceRow = db.select().from(schema.instanceSettings).where(eq(schema.instanceSettings.id, 1)).get();
     const registrationOpen = instanceRow?.registrationOpen !== null && instanceRow?.registrationOpen !== undefined
       ? instanceRow.registrationOpen === 1
       : config.registrationOpen;
-    if (!registrationOpen) {
-      return reply.code(403).send({ error: 'Registration is currently closed', statusCode: 403 });
+    const federatedRegistrationOpen = instanceRow?.federatedRegistrationOpen === 1;
+
+    // Optional invite token. Only meaningful for the local-closed path; ignored
+    // entirely on the federated path (spec §1.3, §5.6) and on the local-open path
+    // (spec §5.7).
+    const inviteToken = typeof request.body.inviteToken === 'string'
+      ? request.body.inviteToken
+      : undefined;
+
+    if (homeInstance) {
+      // Federated path: token IGNORED entirely. Gate is federatedRegistrationOpen.
+      if (!federatedRegistrationOpen) {
+        return reply.code(403).send({ error: 'Federated registration is closed on this instance', statusCode: 403 });
+      }
+      // Fall through to existing federated stub upgrade / new federated user logic below.
+    } else {
+      // Local path: registrationOpen is the primary gate. A valid invite token
+      // bypasses it when closed. When open, the token is silently ignored.
+      if (!registrationOpen) {
+        if (!inviteToken) {
+          return reply.code(403).send({ error: 'Registration is closed. An invite is required.', statusCode: 403 });
+        }
+        // Pre-flight check: reject obviously-invalid tokens before any expensive
+        // work (bcrypt). The final enforcement still happens inside the redemption
+        // transaction below — this only short-circuits the easy reject path.
+        const inviteRow = getInviteByToken(inviteToken);
+        if (!inviteRow || inviteStatus(inviteRow) !== 'active') {
+          return reply.code(403).send({ error: 'Invalid or expired invite', statusCode: 403 });
+        }
+      }
+      // If registrationOpen is true: inviteToken is silently ignored — no validation,
+      // no consumption (spec §5.7).
     }
 
     const passwordHash = await hashPassword(password);
@@ -175,7 +209,7 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
     // which would otherwise produce a permanently stuck-online row that no
     // disconnect timer can clean up. The WS handshake will flip it to
     // 'online' once a real socket attaches.
-    db.insert(schema.users).values({
+    const userRow = {
       id: userId,
       username: trimmedUsername,
       displayName: displayName?.trim() || null,
@@ -185,7 +219,35 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
       homeUserId: (homeInstance && homeUserId && typeof homeUserId === 'string') ? homeUserId : null,
       avatarColor,
       createdAt: now,
-    }).run();
+    };
+
+    // Only the LOCAL-CLOSED-WITH-VALID-TOKEN path consumes an invite. The federated
+    // paths (handled above and in the stub-upgrade block) and the local-open path
+    // never touch the invite_links table.
+    const consumesInvite = !homeInstance && !registrationOpen && !!inviteToken;
+
+    if (consumesInvite) {
+      // Atomic redemption: the user INSERT, the usedCount bump, and the
+      // invite_redemptions row all run inside one SQLite transaction. If any
+      // step throws (token consumed by a concurrent request, username collision
+      // bumping into the unique index, etc.) the entire transaction rolls back —
+      // we never burn a redemption on a failed registration.
+      try {
+        redeemInvite(inviteToken!, () => {
+          db.insert(schema.users).values(userRow).run();
+          return { id: userId, username: trimmedUsername };
+        });
+      } catch (err) {
+        if (err instanceof InviteUnavailableError) {
+          // Concurrent revoke / last-slot race / expiry-while-typing all surface here.
+          return reply.code(403).send({ error: 'Invalid or expired invite', statusCode: 403 });
+        }
+        throw err;
+      }
+    } else {
+      // Standard local-open or federated-new-user path: plain user insert.
+      db.insert(schema.users).values(userRow).run();
+    }
 
     const user = db.select().from(schema.users).where(eq(schema.users.id, userId)).get();
     if (!user) {