diff --git a/apps/backend/src/__tests__/cards.test.ts b/apps/backend/src/__tests__/cards.test.ts
index 813883e8..f110c0da 100644
--- a/apps/backend/src/__tests__/cards.test.ts
+++ b/apps/backend/src/__tests__/cards.test.ts
@@ -1,7 +1,10 @@
+import Fastify, { type FastifyInstance } from 'fastify';
 import { describe, it, expect, beforeEach, vi } from 'vitest';
-import Fastify from 'fastify';
+
 import { cardRoutes } from '../routes/cards.js';
 
+import type { PrismaClient } from '@prisma/client';
+
 const USER_ID = 'user-123';
 const CARD_ID = 'card-abc';
 // Must be valid UUIDs — createCardSchema and updateCardSchema use z.string().uuid()
@@ -43,15 +46,15 @@ const mockPrisma = {
 
 // Re-wire $transaction before every test so that it executes the callback
 // against the same mock client, preserving existing per-operation mocks.
-function wireTransaction() {
+function wireTransaction(): void {
   mockPrisma.$transaction.mockImplementation(
     async (callback: (tx: typeof mockPrisma) => Promise<unknown>) => callback(mockPrisma),
   );
 }
 
-async function buildApp() {
+async function buildApp():Promise<FastifyInstance> {
   const app = Fastify({ logger: false });
-  app.decorate('prisma', mockPrisma);
+  app.decorate('prisma', mockPrisma as unknown as PrismaClient);
   app.decorate('authenticate', async (request: any) => {
     request.user = { id: USER_ID };
   });
diff --git a/apps/backend/src/__tests__/profiles.test.ts b/apps/backend/src/__tests__/profiles.test.ts
index 07d10f98..0633b841 100644
--- a/apps/backend/src/__tests__/profiles.test.ts
+++ b/apps/backend/src/__tests__/profiles.test.ts
@@ -1,6 +1,8 @@
+import Fastify, { type FastifyInstance } from 'fastify';
 import { describe, it, expect, beforeEach, vi } from 'vitest';
-import Fastify from 'fastify';
+
 import { profileRoutes } from '../routes/profiles.js';
+
 import type { PrismaClient } from '@prisma/client';
 
 const mockUser = {
@@ -20,15 +22,15 @@ const mockUser = {
   providerId: 'gh-123',
 };
 
-const mockPrisma: Pick<PrismaClient, 'user'> = {
+const mockPrisma = {
   user: {
     findUnique: vi.fn(),
     findFirst: vi.fn(),
     update: vi.fn(),
-  } as unknown as PrismaClient['user'],
+  },
 };
 
-async function buildApp() {
+async function buildApp():Promise<FastifyInstance> {
   const app = Fastify();
   app.decorate('prisma', mockPrisma as unknown as PrismaClient);
   app.decorate('authenticate', async (request: any) => {
diff --git a/apps/backend/src/__tests__/validateEnv.test.ts b/apps/backend/src/__tests__/validateEnv.test.ts
index eb0574bd..34fce500 100644
--- a/apps/backend/src/__tests__/validateEnv.test.ts
+++ b/apps/backend/src/__tests__/validateEnv.test.ts
@@ -1,4 +1,5 @@
 import { describe, it, expect, vi, afterEach } from 'vitest';
+
 import { validateEnv } from '../utils/validateEnv.js';
 
 // ── helpers ──────────────────────────────────────────────────────────────────
@@ -8,8 +9,8 @@ import { validateEnv } from '../utils/validateEnv.js';
  * that a failing validateEnv() call does not terminate the test process.
  * Returns the spy so callers can assert the exit code.
  */
-function stubExit() {
-  return vi.spyOn(process, 'exit').mockImplementation((code?: number | string) => {
+function stubExit(): ReturnType<typeof vi.spyOn> {
+  return vi.spyOn(process, 'exit').mockImplementation((code?: number | string | null) => {
     throw new Error(`process.exit(${code})`);
   }) as unknown as ReturnType<typeof vi.spyOn>;
 }