diff --git a/frontend/.env.example b/frontend/.env.example
new file mode 100644
index 0000000..5317fce
--- /dev/null
+++ b/frontend/.env.example
@@ -0,0 +1 @@
+VITE_API_URL=http://localhost:3000
diff --git a/frontend/src/services/api.ts b/frontend/src/services/api.ts
index d97db38..55f4791 100644
--- a/frontend/src/services/api.ts
+++ b/frontend/src/services/api.ts
@@ -4,7 +4,11 @@
 
 import type { EncryptedNote, LoginCredentials, RegisterCredentials, User } from '../models/types';
 
-const API_BASE_URL = import.meta.env.VITE_API_URL || 'http://localhost:3000';
+const API_BASE_URL = import.meta.env.VITE_API_URL;
+
+if (!API_BASE_URL) {
+  throw new Error('VITE_API_URL environment variable is not defined');
+}
 
 class ApiClient {
   private baseUrl: string;
diff --git a/frontend/src/test/setup.ts b/frontend/src/test/setup.ts
index 28dc603..7dcd43c 100644
--- a/frontend/src/test/setup.ts
+++ b/frontend/src/test/setup.ts
@@ -2,7 +2,10 @@
  * Test setup file for Vitest
  */
 
-import { expect, afterEach } from 'vitest';
+import { afterEach, vi } from 'vitest';
+
+// Mock environment variables
+vi.stubEnv('VITE_API_URL', 'http://localhost:3000');
 import { cleanup } from '@testing-library/react';
 import '@testing-library/jest-dom';