diff --git a/src/middleware/auth.ts b/src/middleware/auth.ts
index 812a637..a72cb1c 100644
--- a/src/middleware/auth.ts
+++ b/src/middleware/auth.ts
@@ -2,6 +2,7 @@ import type { FastifyRequest, FastifyReply, FastifyInstance } from 'fastify';
 import type { Config } from '../config.ts';
 import type { JwtPayload } from '../types.ts';
 import { SESSION_COOKIE_NAME, SESSION_TTL_SECONDS } from '../constants.ts';
+import { slideSessionIfNeeded } from './session-renewal.ts';
 
 export function sessionCookieOptions(secure: boolean): {
   httpOnly: boolean;
@@ -29,10 +30,11 @@ export function issueSessionCookie(
   reply.setCookie(SESSION_COOKIE_NAME, token, sessionCookieOptions(cookieSecure));
 }
 
-export function makeRequireAuth(_config: Config) {
+export function makeRequireAuth(config: Config) {
   return async function requireAuth(request: FastifyRequest, reply: FastifyReply): Promise<void> {
     try {
-      await request.jwtVerify<JwtPayload & { iat?: number }>();
+      const payload = await request.jwtVerify<JwtPayload & { iat?: number }>();
+      slideSessionIfNeeded(request, reply, payload, request.server, config.cookieSecure);
     } catch {
       const isApi = request.url.startsWith('/api/');
       if (isApi) {
diff --git a/src/middleware/session-renewal.ts b/src/middleware/session-renewal.ts
new file mode 100644
index 0000000..b6878b9
--- /dev/null
+++ b/src/middleware/session-renewal.ts
@@ -0,0 +1,27 @@
+import type { FastifyRequest, FastifyReply, FastifyInstance } from 'fastify';
+import type { JwtPayload } from '../types.ts';
+import { LOGOUT_PATHS, SESSION_RENEW_THRESHOLD_SECONDS } from '../constants.ts';
+import { issueSessionCookie } from './auth.ts';
+
+// Family pattern includes an mfa-pending early-return; nanodrop has no MFA, so this is N/A.
+export function slideSessionIfNeeded(
+  request: FastifyRequest,
+  reply: FastifyReply,
+  payload: JwtPayload & { iat?: number },
+  server: FastifyInstance,
+  cookieSecure: boolean,
+): void {
+  const path = request.url.split('?', 1)[0];
+  if (LOGOUT_PATHS.has(path)) return;
+
+  const nowSec = Math.floor(Date.now() / 1000);
+  const iat = payload.iat ?? 0;
+  if (nowSec - iat < SESSION_RENEW_THRESHOLD_SECONDS) return;
+
+  issueSessionCookie(
+    reply,
+    server,
+    { sub: payload.sub, username: payload.username },
+    cookieSecure,
+  );
+}
diff --git a/tests/integration/session-persistence.test.ts b/tests/integration/session-persistence.test.ts
new file mode 100644
index 0000000..ae54bf7
--- /dev/null
+++ b/tests/integration/session-persistence.test.ts
@@ -0,0 +1,100 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import { createTestApp, type TestContext, loginAs } from '../helpers/setup.ts';
+import { createUser } from '../../src/db/users.ts';
+import { hashPassword } from '../../src/services/auth.ts';
+
+describe('session persistence (sliding renewal)', () => {
+  let ctx: TestContext;
+
+  beforeEach(async () => {
+    vi.useFakeTimers({ toFake: ['Date'] });
+    vi.setSystemTime(new Date('2026-01-01T00:00:00Z'));
+
+    ctx = createTestApp();
+    const hash = await hashPassword('secret');
+    createUser(ctx.db, { username: 'alice', passwordHash: hash });
+  });
+
+  afterEach(async () => {
+    vi.useRealTimers();
+    await ctx.app.close();
+    ctx.cleanup();
+  });
+
+  it('login response includes Set-Cookie with Max-Age=2592000', async () => {
+    const res = await ctx.app.inject({
+      method: 'POST',
+      url: '/api/v1/auth/login',
+      headers: { 'content-type': 'application/json' },
+      body: JSON.stringify({ username: 'alice', password: 'secret' }),
+    });
+    expect(res.statusCode).toBe(200);
+    expect(res.headers['set-cookie']).toMatch(/nanodrop_session=/);
+    expect(res.headers['set-cookie']).toMatch(/Max-Age=2592000/);
+  });
+
+  it('request 25 days after login triggers a fresh Set-Cookie', async () => {
+    const token = await loginAs(ctx.app, 'alice', 'secret');
+    vi.setSystemTime(new Date('2026-01-26T00:00:00Z')); // +25 days
+
+    const res = await ctx.app.inject({
+      method: 'GET',
+      url: '/api/v1/files',
+      cookies: { nanodrop_session: token },
+    });
+    expect(res.statusCode).toBe(200);
+    expect(res.headers['set-cookie']).toBeTruthy();
+    expect(res.headers['set-cookie']).toMatch(/nanodrop_session=/);
+    expect(res.headers['set-cookie']).toMatch(/Max-Age=2592000/);
+  });
+
+  it('request within renewal threshold does NOT include a fresh Set-Cookie', async () => {
+    const token = await loginAs(ctx.app, 'alice', 'secret');
+    vi.setSystemTime(new Date('2026-01-01T00:30:00Z')); // +30 minutes (under 1 hour)
+
+    const res = await ctx.app.inject({
+      method: 'GET',
+      url: '/api/v1/files',
+      cookies: { nanodrop_session: token },
+    });
+    expect(res.statusCode).toBe(200);
+    expect(res.headers['set-cookie']).toBeFalsy();
+  });
+
+  it('logout still clears cookie even after renewal threshold has passed', async () => {
+    const token = await loginAs(ctx.app, 'alice', 'secret');
+    vi.setSystemTime(new Date('2026-01-01T02:00:00Z')); // +2 hours (past threshold)
+
+    const res = await ctx.app.inject({
+      method: 'POST',
+      url: '/api/v1/auth/logout',
+      cookies: { nanodrop_session: token },
+    });
+    expect(res.statusCode).toBe(200);
+    const setCookie = res.headers['set-cookie'] as string | string[];
+    const setCookieStr = Array.isArray(setCookie) ? setCookie.join('\n') : setCookie;
+    // Cookie cleared (Max-Age=0 or empty value)
+    expect(setCookieStr).toMatch(/nanodrop_session=;/);
+    // No fresh session cookie issued by the renewer
+    const freshIssue = /nanodrop_session=eyJ/.test(setCookieStr);
+    expect(freshIssue).toBe(false);
+  });
+
+  it('request 31 days after login is bounced (idle lapse)', async () => {
+    const token = await loginAs(ctx.app, 'alice', 'secret');
+    vi.setSystemTime(new Date('2026-02-01T01:00:00Z')); // +31 days, past 30-day TTL
+
+    const res = await ctx.app.inject({
+      method: 'GET',
+      url: '/api/v1/files',
+      cookies: { nanodrop_session: token },
+    });
+    expect(res.statusCode).toBe(401);
+  });
+
+  it('anonymous request to /api/v1/files returns 401 with no Set-Cookie', async () => {
+    const res = await ctx.app.inject({ method: 'GET', url: '/api/v1/files' });
+    expect(res.statusCode).toBe(401);
+    expect(res.headers['set-cookie']).toBeFalsy();
+  });
+});
diff --git a/tests/unit/session-renewal.test.ts b/tests/unit/session-renewal.test.ts
new file mode 100644
index 0000000..abd789a
--- /dev/null
+++ b/tests/unit/session-renewal.test.ts
@@ -0,0 +1,122 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import Fastify, { type FastifyInstance } from 'fastify';
+import fastifyCookie from '@fastify/cookie';
+import fastifyJwt from '@fastify/jwt';
+import { slideSessionIfNeeded } from '../../src/middleware/session-renewal.ts';
+import {
+  SESSION_COOKIE_NAME,
+  SESSION_RENEW_THRESHOLD_SECONDS,
+  SESSION_TTL_SECONDS,
+} from '../../src/constants.ts';
+
+interface TestPayload {
+  sub: number;
+  username: string;
+  iat?: number;
+}
+
+async function buildTestServer(): Promise<FastifyInstance> {
+  const app = Fastify();
+  await app.register(fastifyCookie);
+  await app.register(fastifyJwt, {
+    secret: 'test-secret',
+    cookie: { cookieName: SESSION_COOKIE_NAME, signed: false },
+  });
+  await app.ready();
+  return app;
+}
+
+const NOW_ISO = '2026-01-01T12:00:00Z';
+const NOW_SEC = Math.floor(new Date(NOW_ISO).getTime() / 1000);
+
+describe('slideSessionIfNeeded', () => {
+  let app: FastifyInstance;
+
+  beforeEach(async () => {
+    vi.useFakeTimers({ toFake: ['Date'] });
+    vi.setSystemTime(new Date(NOW_ISO));
+    app = await buildTestServer();
+  });
+
+  afterEach(async () => {
+    vi.useRealTimers();
+    await app.close();
+  });
+
+  function fakeReply(): { setCookieCalls: Array<{ name: string; value: string; opts: Record<string, unknown> }> } & Record<string, unknown> {
+    const setCookieCalls: Array<{ name: string; value: string; opts: Record<string, unknown> }> = [];
+    return {
+      setCookieCalls,
+      setCookie(name: string, value: string, opts: Record<string, unknown>) {
+        setCookieCalls.push({ name, value, opts });
+        return this;
+      },
+    };
+  }
+
+  function fakeRequest(url: string): { url: string } {
+    return { url };
+  }
+
+  it('re-issues cookie when now - iat >= SESSION_RENEW_THRESHOLD_SECONDS', () => {
+    const reply = fakeReply();
+    const payload: TestPayload = {
+      sub: 1,
+      username: 'alice',
+      iat: NOW_SEC - SESSION_RENEW_THRESHOLD_SECONDS,
+    };
+    slideSessionIfNeeded(fakeRequest('/upload') as never, reply as never, payload, app, false);
+    expect(reply.setCookieCalls.length).toBe(1);
+    expect(reply.setCookieCalls[0].name).toBe(SESSION_COOKIE_NAME);
+    expect(reply.setCookieCalls[0].opts.maxAge).toBe(SESSION_TTL_SECONDS);
+  });
+
+  it('skips re-issue when now - iat < SESSION_RENEW_THRESHOLD_SECONDS', () => {
+    const reply = fakeReply();
+    const payload: TestPayload = {
+      sub: 1,
+      username: 'alice',
+      iat: NOW_SEC - (SESSION_RENEW_THRESHOLD_SECONDS - 1),
+    };
+    slideSessionIfNeeded(fakeRequest('/upload') as never, reply as never, payload, app, false);
+    expect(reply.setCookieCalls.length).toBe(0);
+  });
+
+  it('skips re-issue on /logout path', () => {
+    const reply = fakeReply();
+    const payload: TestPayload = { sub: 1, username: 'alice', iat: 0 }; // ancient
+    slideSessionIfNeeded(fakeRequest('/logout') as never, reply as never, payload, app, false);
+    expect(reply.setCookieCalls.length).toBe(0);
+  });
+
+  it('skips re-issue on /api/v1/auth/logout path', () => {
+    const reply = fakeReply();
+    const payload: TestPayload = { sub: 1, username: 'alice', iat: 0 };
+    slideSessionIfNeeded(fakeRequest('/api/v1/auth/logout') as never, reply as never, payload, app, false);
+    expect(reply.setCookieCalls.length).toBe(0);
+  });
+
+  it('skips re-issue when /logout has a query string', () => {
+    const reply = fakeReply();
+    const payload: TestPayload = { sub: 1, username: 'alice', iat: 0 };
+    slideSessionIfNeeded(fakeRequest('/logout?redirect=foo') as never, reply as never, payload, app, false);
+    expect(reply.setCookieCalls.length).toBe(0);
+  });
+
+  it('treats missing iat as 0 (forces refresh)', () => {
+    const reply = fakeReply();
+    const payload: TestPayload = { sub: 1, username: 'alice' };
+    slideSessionIfNeeded(fakeRequest('/upload') as never, reply as never, payload, app, false);
+    expect(reply.setCookieCalls.length).toBe(1);
+  });
+
+  it('produces a cookie carrying Max-Age=SESSION_TTL_SECONDS', () => {
+    const reply = fakeReply();
+    const payload: TestPayload = { sub: 1, username: 'alice', iat: 0 };
+    slideSessionIfNeeded(fakeRequest('/upload') as never, reply as never, payload, app, true);
+    expect(reply.setCookieCalls[0].opts.maxAge).toBe(SESSION_TTL_SECONDS);
+    expect(reply.setCookieCalls[0].opts.secure).toBe(true);
+    expect(reply.setCookieCalls[0].opts.httpOnly).toBe(true);
+    expect(reply.setCookieCalls[0].opts.sameSite).toBe('strict');
+  });
+});