diff --git a/apps/backend/src/__tests__/refresh.test.ts b/apps/backend/src/__tests__/refresh.test.ts
new file mode 100644
index 00000000..c230ad92
--- /dev/null
+++ b/apps/backend/src/__tests__/refresh.test.ts
@@ -0,0 +1,327 @@
+import cookiePlugin from '@fastify/cookie';
+import jwtPlugin from '@fastify/jwt';
+import Fastify, { type FastifyInstance } from 'fastify';
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+import { authRoutes } from '../routes/auth.js';
+import { hashRefreshToken } from '../utils/refreshToken.js';
+
+// ─── Constants ────────────────────────────────────────────────────────────────
+
+const TEST_JWT_SECRET = 'test-secret-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
+const USER_ID = 'user-abc';
+const USERNAME = 'testuser';
+const FAMILY_ID = 'family-uuid-1234';
+
+// A raw token value that the mock will recognise via its hash.
+// The route hashes the cookie value with hashRefreshToken() before querying,
+// so we store the hash in the mock and present the raw value in cookies.
+const RAW_TOKEN_A = 'a'.repeat(128); // 128 hex chars = 64 random bytes
+const RAW_TOKEN_B = 'b'.repeat(128);
+const HASH_A = hashRefreshToken(RAW_TOKEN_A);
+const HASH_B = hashRefreshToken(RAW_TOKEN_B);
+
+const mockUser = {
+  id: USER_ID,
+  username: USERNAME,
+  email: 'test@example.com',
+};
+
+// ─── Prisma mock factory ──────────────────────────────────────────────────────
+
+function createMockPrisma(): {
+  user: { findUnique: ReturnType<typeof vi.fn> };
+  refreshToken: {
+    findUnique: ReturnType<typeof vi.fn>;
+    create: ReturnType<typeof vi.fn>;
+    update: ReturnType<typeof vi.fn>;
+    updateMany: ReturnType<typeof vi.fn>;
+  };
+} {
+  return {
+    user: { findUnique: vi.fn() },
+    refreshToken: {
+      findUnique: vi.fn(),
+      create: vi.fn(),
+      update: vi.fn(),
+      updateMany: vi.fn(),
+    },
+  };
+}
+
+type MockPrisma = ReturnType<typeof createMockPrisma>;
+
+// ─── App factory ─────────────────────────────────────────────────────────────
+
+async function buildApp(mockPrisma: MockPrisma): Promise<FastifyInstance> {
+  const app = Fastify({ logger: false });
+
+  await app.register(cookiePlugin as any);
+  await app.register(jwtPlugin as any, {
+    secret: TEST_JWT_SECRET,
+    cookie: { cookieName: 'access_Token', signed: false },
+  });
+
+  app.decorate('prisma', mockPrisma as any);
+  app.decorate('redis', { set: vi.fn(), get: vi.fn(), getdel: vi.fn() } as any);
+  app.decorate('authenticate', async (request: any, reply: any) => {
+    try {
+      await request.jwtVerify();
+    } catch {
+      return reply.status(401).send({ error: 'Unauthorized' });
+    }
+  });
+
+  app.register(authRoutes, { prefix: '/auth' });
+  await app.ready();
+  return app;
+}
+
+// ─── Helper — build a stored-token record ────────────────────────────────────
+
+function makeStoredToken(overrides: Partial<{
+  tokenHash: string;
+  revokedAt: Date | null;
+  expiresAt: Date;
+  family: string;
+}> = {}): {
+  id: string;
+  tokenHash: string;
+  family: string;
+  userId: string;
+  revokedAt: Date | null;
+  expiresAt: Date;
+  ip: string;
+  userAgent: string;
+  user: typeof mockUser;
+} {
+  return {
+    id: 'token-id-1',
+    tokenHash: HASH_A,
+    family: FAMILY_ID,
+    userId: USER_ID,
+    revokedAt: null,
+    expiresAt: new Date(Date.now() + 90 * 24 * 60 * 60 * 1000),
+    ip: 'hashed-ip',
+    userAgent: 'vitest',
+    user: mockUser,
+    ...overrides,
+  };
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// POST /auth/refresh — normal rotation
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe('POST /auth/refresh — normal token rotation', () => {
+  let mockPrisma: MockPrisma;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockPrisma = createMockPrisma();
+  });
+
+  it('rotates a valid token: revokes old, issues new, returns 200', async () => {
+    mockPrisma.refreshToken.findUnique.mockResolvedValue(makeStoredToken());
+    mockPrisma.refreshToken.update.mockResolvedValue({});
+    mockPrisma.refreshToken.create.mockResolvedValue({});
+
+    const app = await buildApp(mockPrisma);
+    const res = await app.inject({
+      method: 'POST',
+      url: '/auth/refresh',
+      cookies: { refresh_token: RAW_TOKEN_A },
+    });
+
+    expect(res.statusCode).toBe(200);
+
+    // Old token must be revoked
+    expect(mockPrisma.refreshToken.update).toHaveBeenCalledWith(
+      expect.objectContaining({ data: { revokedAt: expect.any(Date) } }),
+    );
+
+    // New token must be created in the same family
+    expect(mockPrisma.refreshToken.create).toHaveBeenCalledWith(
+      expect.objectContaining({
+        data: expect.objectContaining({ family: FAMILY_ID }),
+      }),
+    );
+
+    // Family-wide revocation must NOT have been called on a clean rotation
+    expect(mockPrisma.refreshToken.updateMany).not.toHaveBeenCalled();
+  });
+
+  it('returns 401 for a missing refresh token cookie', async () => {
+    const app = await buildApp(mockPrisma);
+    const res = await app.inject({ method: 'POST', url: '/auth/refresh' });
+
+    expect(res.statusCode).toBe(401);
+    expect(res.json().error).toBe('Refresh token missing');
+  });
+
+  it('returns 401 for an unrecognised token hash', async () => {
+    mockPrisma.refreshToken.findUnique.mockResolvedValue(null);
+
+    const app = await buildApp(mockPrisma);
+    const res = await app.inject({
+      method: 'POST',
+      url: '/auth/refresh',
+      cookies: { refresh_token: RAW_TOKEN_A },
+    });
+
+    expect(res.statusCode).toBe(401);
+    expect(res.json().error).toBe('Invalid refresh token');
+  });
+
+  it('returns 401 for an expired token without rotating', async () => {
+    mockPrisma.refreshToken.findUnique.mockResolvedValue(
+      makeStoredToken({ expiresAt: new Date(Date.now() - 1000) }),
+    );
+
+    const app = await buildApp(mockPrisma);
+    const res = await app.inject({
+      method: 'POST',
+      url: '/auth/refresh',
+      cookies: { refresh_token: RAW_TOKEN_A },
+    });
+
+    expect(res.statusCode).toBe(401);
+    expect(res.json().error).toBe('Refresh token expired');
+    expect(mockPrisma.refreshToken.update).not.toHaveBeenCalled();
+    expect(mockPrisma.refreshToken.create).not.toHaveBeenCalled();
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// POST /auth/refresh — reuse-detection & family revocation
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe('POST /auth/refresh — reuse-detection triggers family-wide revocation', () => {
+  let mockPrisma: MockPrisma;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockPrisma = createMockPrisma();
+  });
+
+  it('revokes entire family when a previously-revoked token is presented', async () => {
+    // Token A was already rotated (revokedAt is set).
+    // Presenting it again is the theft signal.
+    mockPrisma.refreshToken.findUnique.mockResolvedValue(
+      makeStoredToken({ revokedAt: new Date(Date.now() - 5000) }),
+    );
+    mockPrisma.refreshToken.updateMany.mockResolvedValue({ count: 2 });
+
+    const app = await buildApp(mockPrisma);
+    const res = await app.inject({
+      method: 'POST',
+      url: '/auth/refresh',
+      cookies: { refresh_token: RAW_TOKEN_A },
+    });
+
+    expect(res.statusCode).toBe(401);
+    expect(res.json().error).toBe('Refresh token revoked');
+
+    // Family-wide revocation must have fired
+    expect(mockPrisma.refreshToken.updateMany).toHaveBeenCalledOnce();
+    expect(mockPrisma.refreshToken.updateMany).toHaveBeenCalledWith({
+      where: { family: FAMILY_ID, revokedAt: null },
+      data: { revokedAt: expect.any(Date) },
+    });
+
+    // No new token must have been issued
+    expect(mockPrisma.refreshToken.create).not.toHaveBeenCalled();
+  });
+
+  it('does not issue a new token after family revocation', async () => {
+    mockPrisma.refreshToken.findUnique.mockResolvedValue(
+      makeStoredToken({ revokedAt: new Date() }),
+    );
+    mockPrisma.refreshToken.updateMany.mockResolvedValue({ count: 1 });
+
+    const app = await buildApp(mockPrisma);
+    await app.inject({
+      method: 'POST',
+      url: '/auth/refresh',
+      cookies: { refresh_token: RAW_TOKEN_A },
+    });
+
+    expect(mockPrisma.refreshToken.update).not.toHaveBeenCalled();
+    expect(mockPrisma.refreshToken.create).not.toHaveBeenCalled();
+  });
+
+  it('rotation chain: A→B→C succeeds normally; presenting stale A kills B and C', async () => {
+    // ── Step 1: legitimate client rotates A → B ──
+    mockPrisma.refreshToken.findUnique.mockResolvedValueOnce(makeStoredToken()); // A is live
+    mockPrisma.refreshToken.update.mockResolvedValue({});
+    mockPrisma.refreshToken.create.mockResolvedValue({});
+
+    const app = await buildApp(mockPrisma);
+    const step1 = await app.inject({
+      method: 'POST',
+      url: '/auth/refresh',
+      cookies: { refresh_token: RAW_TOKEN_A },
+    });
+    expect(step1.statusCode).toBe(200);
+
+    // ── Step 2: attacker presents stale token A ──
+    // A is now revoked (stored with revokedAt); B is the live descendant.
+    mockPrisma.refreshToken.findUnique.mockResolvedValueOnce(
+      makeStoredToken({ revokedAt: new Date(Date.now() - 1000) }),
+    );
+    mockPrisma.refreshToken.updateMany.mockResolvedValue({ count: 1 }); // kills B
+
+    const step2 = await app.inject({
+      method: 'POST',
+      url: '/auth/refresh',
+      cookies: { refresh_token: RAW_TOKEN_A },
+    });
+    expect(step2.statusCode).toBe(401);
+    expect(step2.json().error).toBe('Refresh token revoked');
+
+    // Family kill must target only the still-live tokens in the family
+    expect(mockPrisma.refreshToken.updateMany).toHaveBeenCalledWith({
+      where: { family: FAMILY_ID, revokedAt: null },
+      data: { revokedAt: expect.any(Date) },
+    });
+
+    // ── Step 3: legitimate client now tries with B (now killed) → 401 ──
+    mockPrisma.refreshToken.findUnique.mockResolvedValueOnce(
+      makeStoredToken({
+        tokenHash: HASH_B,
+        revokedAt: new Date(), // killed by step 2's updateMany
+      }),
+    );
+    // On this third presentation the family is already all-revoked,
+    // so updateMany returns count: 0 — still must be called.
+    mockPrisma.refreshToken.updateMany.mockResolvedValue({ count: 0 });
+
+    const step3 = await app.inject({
+      method: 'POST',
+      url: '/auth/refresh',
+      cookies: { refresh_token: RAW_TOKEN_B },
+    });
+    expect(step3.statusCode).toBe(401);
+
+    // No new token issued at any point after the theft was detected
+    // (create was called once in step 1, never again)
+    expect(mockPrisma.refreshToken.create).toHaveBeenCalledTimes(1);
+  });
+
+  it('returns 500 and does not issue a token if the family-revocation updateMany throws', async () => {
+    mockPrisma.refreshToken.findUnique.mockResolvedValue(
+      makeStoredToken({ revokedAt: new Date() }),
+    );
+    mockPrisma.refreshToken.updateMany.mockRejectedValue(new Error('DB timeout'));
+
+    const app = await buildApp(mockPrisma);
+    const res = await app.inject({
+      method: 'POST',
+      url: '/auth/refresh',
+      cookies: { refresh_token: RAW_TOKEN_A },
+    });
+
+    expect(res.statusCode).toBe(500);
+    expect(mockPrisma.refreshToken.create).not.toHaveBeenCalled();
+  });
+});
\ No newline at end of file
diff --git a/apps/backend/src/routes/auth.ts b/apps/backend/src/routes/auth.ts
index 3bc39ad4..56263edf 100644
--- a/apps/backend/src/routes/auth.ts
+++ b/apps/backend/src/routes/auth.ts
@@ -522,6 +522,16 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
     }
 
     if (storedToken.revokedAt) {
+      await app.prisma.refreshToken.updateMany({
+        where: { family: storedToken.family, revokedAt: null },
+        data: { revokedAt: new Date() },
+      });
+
+      app.log.warn(
+        { family: storedToken.family, userId: storedToken.userId },
+        'Refresh token reuse detected — entire family revoked (possible theft)',
+      );
+
       return reply.status(401).send({
         error: 'Refresh token revoked',
       });