diff --git a/src/constants.ts b/src/constants.ts
new file mode 100644
index 0000000..bea0d34
--- /dev/null
+++ b/src/constants.ts
@@ -0,0 +1,10 @@
+// Family-wide session policy. Used by every bchen.dev app's session cookie.
+// DO NOT diverge per-app — coherence across apps is a feature.
+
+export const SESSION_TTL_DAYS = 30;
+export const SESSION_TTL_SECONDS = SESSION_TTL_DAYS * 24 * 60 * 60;
+export const SESSION_RENEW_THRESHOLD_SECONDS = 60 * 60;
+// Phase 1: keep cookie name as 'token' so existing tests stay green.
+// Phase 2 flips this to 'nanodrop_session'.
+export const SESSION_COOKIE_NAME = 'token';
+export const LOGOUT_PATHS = new Set<string>(['/logout', '/api/v1/auth/logout']);
diff --git a/src/middleware/auth.ts b/src/middleware/auth.ts
index 77050e5..812a637 100644
--- a/src/middleware/auth.ts
+++ b/src/middleware/auth.ts
@@ -1,24 +1,45 @@
-import type { FastifyRequest, FastifyReply } from 'fastify';
+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';
 
-export async function requireAuth(request: FastifyRequest, reply: FastifyReply): Promise<void> {
-  try {
-    await request.jwtVerify();
-  } catch {
-    // API routes get 401, page routes get redirect
-    const isApi = request.url.startsWith('/api/');
-    if (isApi) {
-      reply.status(401).send({ error: 'Unauthorized' });
-    } else {
-      reply.redirect('/');
-    }
-  }
-}
-
-export function tokenCookieOptions(secure: boolean): {
+export function sessionCookieOptions(secure: boolean): {
   httpOnly: boolean;
   sameSite: 'strict';
   secure: boolean;
   path: string;
+  maxAge: number;
 } {
-  return { httpOnly: true, sameSite: 'strict', secure, path: '/' };
+  return {
+    httpOnly: true,
+    sameSite: 'strict',
+    secure,
+    path: '/',
+    maxAge: SESSION_TTL_SECONDS,
+  };
+}
+
+export function issueSessionCookie(
+  reply: FastifyReply,
+  server: FastifyInstance,
+  claims: JwtPayload,
+  cookieSecure: boolean,
+): void {
+  const token = server.jwt.sign(claims, { expiresIn: SESSION_TTL_SECONDS });
+  reply.setCookie(SESSION_COOKIE_NAME, token, sessionCookieOptions(cookieSecure));
+}
+
+export function makeRequireAuth(_config: Config) {
+  return async function requireAuth(request: FastifyRequest, reply: FastifyReply): Promise<void> {
+    try {
+      await request.jwtVerify<JwtPayload & { iat?: number }>();
+    } catch {
+      const isApi = request.url.startsWith('/api/');
+      if (isApi) {
+        reply.status(401).send({ error: 'Unauthorized' });
+      } else {
+        reply.redirect('/');
+      }
+    }
+  };
 }
diff --git a/src/routes/api/v1/auth.ts b/src/routes/api/v1/auth.ts
index 7b11200..fe3632b 100644
--- a/src/routes/api/v1/auth.ts
+++ b/src/routes/api/v1/auth.ts
@@ -4,7 +4,8 @@ import type { Config } from '../../../config.ts';
 import type { Logger } from '../../../middleware/logging.ts';
 import type { LockoutService } from '../../../services/lockout.ts';
 import { attemptLogin } from '../../../services/login-handler.ts';
-import { requireAuth, tokenCookieOptions } from '../../../middleware/auth.ts';
+import { makeRequireAuth, issueSessionCookie } from '../../../middleware/auth.ts';
+import { SESSION_COOKIE_NAME } from '../../../constants.ts';
 
 interface Deps {
   db: Database.Database;
@@ -20,6 +21,7 @@ interface LoginBody {
 
 export const authApiRoutes: FastifyPluginAsync<{ deps: Deps }> = async (app, { deps }) => {
   const { config } = deps;
+  const requireAuth = makeRequireAuth(config);
 
   app.post<{ Body: LoginBody }>(
     '/login',
@@ -56,15 +58,17 @@ export const authApiRoutes: FastifyPluginAsync<{ deps: Deps }> = async (app, { d
         return reply.status(401).send({ error: 'Invalid credentials' });
       }
 
-      const token = app.jwt.sign(
+      issueSessionCookie(
+        reply,
+        app,
         { sub: result.user.id, username: result.user.username },
-        { expiresIn: config.jwtExpiry },
+        config.cookieSecure,
       );
-      reply.setCookie('token', token, tokenCookieOptions(config.cookieSecure)).send({ ok: true });
+      reply.send({ ok: true });
     },
   );
 
   app.post('/logout', { preHandler: requireAuth }, async (_request, reply) => {
-    reply.clearCookie('token', { path: '/' }).send({ ok: true });
+    reply.clearCookie(SESSION_COOKIE_NAME, { path: '/' }).send({ ok: true });
   });
 };
diff --git a/src/routes/api/v1/files.ts b/src/routes/api/v1/files.ts
index 4da58ce..fd582df 100644
--- a/src/routes/api/v1/files.ts
+++ b/src/routes/api/v1/files.ts
@@ -7,7 +7,7 @@ import type { Logger } from '../../../middleware/logging.ts';
 import type { JwtPayload } from '../../../types.ts';
 import { createFile, getFilesByUserId, getFileById, deleteFile } from '../../../db/files.ts';
 import { saveFile, deleteStoredFile } from '../../../services/storage.ts';
-import { requireAuth } from '../../../middleware/auth.ts';
+import { makeRequireAuth } from '../../../middleware/auth.ts';
 
 interface Deps {
   db: Database.Database;
@@ -17,6 +17,7 @@ interface Deps {
 
 export const filesApiRoutes: FastifyPluginAsync<{ deps: Deps }> = async (app, { deps }) => {
   const { db, config } = deps;
+  const requireAuth = makeRequireAuth(config);
 
   app.get('/', { preHandler: requireAuth }, async (request, reply) => {
     const { sub: userId } = request.user as JwtPayload;
diff --git a/src/routes/pages.ts b/src/routes/pages.ts
index 0800896..6b16a52 100644
--- a/src/routes/pages.ts
+++ b/src/routes/pages.ts
@@ -10,7 +10,8 @@ import type { LockoutService } from '../services/lockout.ts';
 import { createFile, getFileById, getFilesByUserId, deleteFile } from '../db/files.ts';
 import { saveFile, deleteStoredFile, getFilePath } from '../services/storage.ts';
 import { attemptLogin } from '../services/login-handler.ts';
-import { requireAuth, tokenCookieOptions } from '../middleware/auth.ts';
+import { makeRequireAuth, issueSessionCookie } from '../middleware/auth.ts';
+import { SESSION_COOKIE_NAME } from '../constants.ts';
 import { loginPage } from '../views/login.ts';
 import { uploadPage, uploadResultPage } from '../views/upload.ts';
 import { fileListPage } from '../views/file-list.ts';
@@ -49,6 +50,7 @@ function parseRangeHeader(header: string, fileSize: number): { start: number; en
 
 export const pageRoutes: FastifyPluginAsync<{ deps: Deps }> = async (app, { deps }) => {
   const { db, config, logger } = deps;
+  const requireAuth = makeRequireAuth(config);
   const loginRateLimit = {
     rateLimit: {
       max: config.loginRateLimitMax,
@@ -57,6 +59,7 @@ export const pageRoutes: FastifyPluginAsync<{ deps: Deps }> = async (app, { deps
   };
 
   // GET / — login page or redirect if authed
+  // opportunistic auth — does not slide the session; see middleware/session-renewal.ts
   app.get('/', async (request, reply) => {
     try {
       await request.jwtVerify();
@@ -91,17 +94,19 @@ export const pageRoutes: FastifyPluginAsync<{ deps: Deps }> = async (app, { deps
         return reply.type('text/html').send(loginPage({ error: 'Invalid username or password' }));
       }
 
-      const token = app.jwt.sign(
+      issueSessionCookie(
+        reply,
+        app,
         { sub: result.user.id, username: result.user.username },
-        { expiresIn: config.jwtExpiry },
+        config.cookieSecure,
       );
-      reply.setCookie('token', token, tokenCookieOptions(config.cookieSecure)).redirect('/upload');
+      reply.redirect('/upload');
     },
   );
 
   // POST /logout
   app.post('/logout', { preHandler: requireAuth }, async (_request, reply) => {
-    reply.clearCookie('token', { path: '/' }).redirect('/');
+    reply.clearCookie(SESSION_COOKIE_NAME, { path: '/' }).redirect('/');
   });
 
   // GET /upload
@@ -163,6 +168,7 @@ export const pageRoutes: FastifyPluginAsync<{ deps: Deps }> = async (app, { deps
   });
 
   // GET /f/:id — public file view (owner-aware)
+  // opportunistic auth — does not slide the session; see middleware/session-renewal.ts
   app.get<{ Params: { id: string } }>('/f/:id', async (request, reply) => {
     const { id } = request.params;
 
diff --git a/src/types.ts b/src/types.ts
index 7408ccf..5c126e2 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -1,4 +1,5 @@
 export interface JwtPayload {
   sub: number;
   username: string;
+  iat?: number;
 }