diff --git a/packages/server/src/routes/auth.test.ts b/packages/server/src/routes/auth.test.ts
new file mode 100644
index 00000000..05f31bd5
--- /dev/null
+++ b/packages/server/src/routes/auth.test.ts
@@ -0,0 +1,212 @@
+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 fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import * as schema from '../db/schema.js';
+import { setWorkerId } from '../utils/snowflake.js';
+
+setWorkerId(2);
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+// Module-level mutable state — see invites.test.ts for the rationale on why
+// the `getDb` mock closes over a getter rather than the binding directly.
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let sqlite: Database.Database;
+let testDb: TestDb;
+let app: FastifyInstance;
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  getRawDb: () => sqlite,
+  schema,
+}));
+
+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);
+    }
+  }
+}
+
+async function buildApp(): Promise<FastifyInstance> {
+  const { authRoutes } = await import('./auth.js');
+  const f = Fastify();
+  await f.register(authRoutes);
+  return f;
+}
+
+const ADMIN_ID = 'admin-1';
+
+beforeEach(async () => {
+  sqlite = new Database(':memory:');
+  sqlite.pragma('foreign_keys = ON');
+  applyMigrations(sqlite);
+  testDb = drizzle(sqlite, { schema });
+
+  // The invite_links rows we'll create reference users(id) via createdBy. Seed
+  // a single admin to act as creator across all tests.
+  testDb.insert(schema.users).values({
+    id: ADMIN_ID,
+    username: 'admin',
+    passwordHash: 'x',
+    isAdmin: 1,
+    createdAt: Date.now(),
+  }).run();
+
+  app = await buildApp();
+});
+
+describe('GET /api/auth/check-invite', () => {
+  it('returns valid: true with name for active token', async () => {
+    const token = 'a'.repeat(22);
+    testDb.insert(schema.inviteLinks).values({
+      id: 'inv-1',
+      token,
+      name: 'Friends batch 1',
+      createdBy: ADMIN_ID,
+      createdAt: Date.now(),
+      maxUses: 10,
+      usedCount: 0,
+      expiresAt: null,
+      revokedAt: null,
+    }).run();
+
+    const res = await app.inject({
+      method: 'GET',
+      url: `/api/auth/check-invite?token=${token}`,
+    });
+    expect(res.statusCode).toBe(200);
+    const body = res.json();
+    expect(body.valid).toBe(true);
+    expect(body.name).toBe('Friends batch 1');
+    expect(body.reason).toBeUndefined();
+  });
+
+  it("returns valid: false, reason: 'expired' for past expiresAt", async () => {
+    const token = 'b'.repeat(22);
+    testDb.insert(schema.inviteLinks).values({
+      id: 'inv-2',
+      token,
+      name: 'Old link',
+      createdBy: ADMIN_ID,
+      createdAt: Date.now() - 10_000,
+      maxUses: 10,
+      usedCount: 0,
+      expiresAt: Date.now() - 1_000,
+      revokedAt: null,
+    }).run();
+
+    const res = await app.inject({
+      method: 'GET',
+      url: `/api/auth/check-invite?token=${token}`,
+    });
+    expect(res.statusCode).toBe(200);
+    const body = res.json();
+    expect(body.valid).toBe(false);
+    expect(body.reason).toBe('expired');
+    expect(body.name).toBeUndefined();
+  });
+
+  it("returns valid: false, reason: 'exhausted' for used-up invite", async () => {
+    const token = 'c'.repeat(22);
+    testDb.insert(schema.inviteLinks).values({
+      id: 'inv-3',
+      token,
+      name: 'Burned',
+      createdBy: ADMIN_ID,
+      createdAt: Date.now(),
+      maxUses: 1,
+      usedCount: 1,
+      expiresAt: null,
+      revokedAt: null,
+    }).run();
+
+    const res = await app.inject({
+      method: 'GET',
+      url: `/api/auth/check-invite?token=${token}`,
+    });
+    expect(res.statusCode).toBe(200);
+    const body = res.json();
+    expect(body.valid).toBe(false);
+    expect(body.reason).toBe('exhausted');
+    expect(body.name).toBeUndefined();
+  });
+
+  it("returns valid: false, reason: 'invalid' for revoked invite (collapsed shield)", async () => {
+    const token = 'd'.repeat(22);
+    testDb.insert(schema.inviteLinks).values({
+      id: 'inv-4',
+      token,
+      name: 'Revoked',
+      createdBy: ADMIN_ID,
+      createdAt: Date.now(),
+      maxUses: 10,
+      usedCount: 0,
+      expiresAt: null,
+      revokedAt: Date.now(),
+    }).run();
+
+    const res = await app.inject({
+      method: 'GET',
+      url: `/api/auth/check-invite?token=${token}`,
+    });
+    expect(res.statusCode).toBe(200);
+    const body = res.json();
+    expect(body.valid).toBe(false);
+    expect(body.reason).toBe('invalid');
+    expect(body.name).toBeUndefined();
+  });
+
+  it("returns valid: false, reason: 'invalid' for unknown token", async () => {
+    // 22-char base64url string that is not in the DB.
+    const token = 'Z'.repeat(22);
+    const res = await app.inject({
+      method: 'GET',
+      url: `/api/auth/check-invite?token=${token}`,
+    });
+    expect(res.statusCode).toBe(200);
+    const body = res.json();
+    expect(body.valid).toBe(false);
+    expect(body.reason).toBe('invalid');
+    expect(body.name).toBeUndefined();
+  });
+
+  it("returns valid: false, reason: 'invalid' for malformed token", async () => {
+    const res = await app.inject({
+      method: 'GET',
+      url: '/api/auth/check-invite?token=tooshort',
+    });
+    expect(res.statusCode).toBe(200);
+    const body = res.json();
+    expect(body.valid).toBe(false);
+    expect(body.reason).toBe('invalid');
+    expect(body.name).toBeUndefined();
+  });
+
+  it('does not include name field on invalid responses', async () => {
+    // Sweep across all invalid permutations to assert the absence-of-leak
+    // contract once for the whole endpoint, not just per-status.
+    const cases = [
+      { url: '/api/auth/check-invite' },
+      { url: '/api/auth/check-invite?token=' },
+      { url: '/api/auth/check-invite?token=tooshort' },
+      { url: `/api/auth/check-invite?token=${'Z'.repeat(22)}` },
+    ];
+    for (const c of cases) {
+      const res = await app.inject({ method: 'GET', url: c.url });
+      expect(res.statusCode).toBe(200);
+      const body = res.json();
+      expect(body.valid).toBe(false);
+      expect(body).not.toHaveProperty('name');
+    }
+  });
+});
diff --git a/packages/server/src/routes/auth.ts b/packages/server/src/routes/auth.ts
index be717e38..9f42517d 100644
--- a/packages/server/src/routes/auth.ts
+++ b/packages/server/src/routes/auth.ts
@@ -8,6 +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';
 
 export async function authRoutes(app: FastifyInstance): Promise<void> {
   app.post<{ Body: RegisterRequest }>('/api/auth/register', {
@@ -239,6 +240,48 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
     return reply.code(200).send({ available: !existing });
   });
 
+  // Public — used by RegisterPage to debounce-validate invite tokens during
+  // typing. The status -> response mapping enforces a "collapsed enumeration
+  // shield": revoked, not-found, and malformed tokens all collapse to
+  // `'invalid'` so this endpoint can't be used to distinguish them. Only
+  // `expired` and `exhausted` surface as themselves because those are
+  // legitimate UX hints ("ask the admin to extend it") rather than
+  // existence/state leaks. The `name` field is returned ONLY in the valid
+  // case — invalid responses must not leak any invite metadata.
+  // Status code is always 200; the response body discriminates.
+  app.get<{ Querystring: { token?: string } }>('/api/auth/check-invite', {
+    config: {
+      rateLimit: {
+        max: 30,
+        timeWindow: '1 minute',
+        keyGenerator: (request: any) => request.ip,
+      },
+    },
+  }, async (request, reply) => {
+    const token = request.query.token;
+    if (!token || typeof token !== 'string') {
+      return reply.code(200).send({ valid: false, reason: 'invalid' });
+    }
+
+    // getInviteByToken pre-validates the 22-char base64url shape before
+    // hitting the DB; malformed inputs return null here, so the same branch
+    // covers both "wrong shape" and "shape ok, not in DB".
+    const row = getInviteByToken(token);
+    if (!row) {
+      return reply.code(200).send({ valid: false, reason: 'invalid' });
+    }
+
+    const status = inviteStatus(row);
+    if (status === 'active') {
+      return reply.code(200).send({ valid: true, name: row.name });
+    }
+    if (status === 'expired' || status === 'exhausted') {
+      return reply.code(200).send({ valid: false, reason: status });
+    }
+    // status === 'revoked' — collapsed to 'invalid' (no enumeration leak)
+    return reply.code(200).send({ valid: false, reason: 'invalid' });
+  });
+
   app.post<{ Body: LoginRequest }>('/api/auth/login', {
     config: {
       rateLimit: {