diff --git a/apps/backend/src/app.ts b/apps/backend/src/app.ts
index 6116b91b..44842088 100644
--- a/apps/backend/src/app.ts
+++ b/apps/backend/src/app.ts
@@ -7,7 +7,7 @@ import helmet from '@fastify/helmet';
 import jwt from '@fastify/jwt';
 import multipart from '@fastify/multipart';
 import rateLimit from '@fastify/rate-limit';
-import Fastify, {type FastifyInstance} from 'fastify';
+import Fastify, {type FastifyInstance, type FastifyReply, type FastifyRequest} from 'fastify';
 
 import { prismaPlugin } from './plugins/prisma.js';
 import { redisPlugin } from './plugins/redis.js';
@@ -24,6 +24,8 @@ import { teamRoutes } from './routes/team.js';
 import { extractRawJwt, blocklistKey } from './utils/jwt.js';
 import { validateEnv } from './utils/validateEnv.js';
 
+import type { AuthenticatedUser } from './types/fastify.js';
+
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
 export async function buildApp():Promise<FastifyInstance> {
@@ -104,7 +106,7 @@ export async function buildApp():Promise<FastifyInstance> {
   // Checks the Redis blocklist before calling jwtVerify so that a logged-out
   // token is rejected immediately even if it has not yet expired.
   // The blocklist check is skipped when Redis is not registered (test env).
-  app.decorate('authenticate', async function (request: any, reply: any) {
+  app.decorate('authenticate', async function (request: FastifyRequest, reply: FastifyReply) {
     try {
       if (app.hasDecorator('redis')) {
         const raw = extractRawJwt(request);
@@ -122,7 +124,7 @@ export async function buildApp():Promise<FastifyInstance> {
         }
       }
       // Assign verified payload to request.user (upstream addition).
-      const payload = await request.jwtVerify();
+      const payload = await request.jwtVerify<AuthenticatedUser>();
       if (payload) { request.user = payload; }
     } catch (_err) {
       return reply.status(401).send({ error: 'Unauthorized' });
diff --git a/apps/backend/src/routes/auth.ts b/apps/backend/src/routes/auth.ts
index 11351267..3bc39ad4 100644
--- a/apps/backend/src/routes/auth.ts
+++ b/apps/backend/src/routes/auth.ts
@@ -613,7 +613,7 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
     // eslint-disable-next-line @typescript-eslint/unbound-method
     preHandler: [app.authenticate],
   }, async (request: FastifyRequest, reply: FastifyReply) => {
-    const userId = (request.user as any).id;
+    const userId = request.user.id;
     const user = await app.prisma.user.findUnique({
       where: { id: userId },
       select: {
@@ -676,7 +676,7 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
             await app.redis.set(blocklistKey(raw), '1', 'EX', ttl);
           } catch (err) {
             // Non-fatal: log and continue. The token will expire on its own.
-            app.log.warn({ err, userId: (request.user as any)?.id }, 'Redis blocklist write failed during logout — token will expire naturally');
+            app.log.warn({ err, userId: request.user?.id }, 'Redis blocklist write failed during logout — token will expire naturally');
           }
         }
       } else {
diff --git a/apps/backend/src/types/fastify.d.ts b/apps/backend/src/types/fastify.d.ts
index 8e7aee95..faeddd2a 100644
--- a/apps/backend/src/types/fastify.d.ts
+++ b/apps/backend/src/types/fastify.d.ts
@@ -1,8 +1,24 @@
 import '@fastify/cookie';
-import { FastifyRequest } from 'fastify';
+import '@fastify/jwt';
+import { FastifyReply, FastifyRequest } from 'fastify';
+
+export interface AuthenticatedUser {
+  id: string;
+  username: string;
+}
+
+declare module '@fastify/jwt' {
+  interface FastifyJWT {
+    user: AuthenticatedUser;
+  }
+}
 
 declare module 'fastify' {
   interface FastifyRequest {
     cookies: Record<string, string | undefined>;
   }
+
+  interface FastifyInstance {
+    authenticate: (request: FastifyRequest, reply: FastifyReply) => Promise<void>;
+  }
 }