diff --git a/.env.example b/.env.example
index 6f0d34b..0b659a3 100644
--- a/.env.example
+++ b/.env.example
@@ -2,7 +2,10 @@
 # Copy this file to .env.local and fill in your values
 
 # API Configuration
-NEXT_PUBLIC_API_URL=http://localhost:3100
+# Browser-side requests use relative URLs (empty string) automatically.
+# API_INTERNAL_URL is only needed for server-side rendering in Docker.
+# For local dev, the default (http://localhost:3000) works out of the box.
+# API_INTERNAL_URL=http://localhost:3100
 
 # Application
 NEXT_PUBLIC_APP_NAME=Barazo
diff --git a/src/__tests__/auth/auth-context.test.tsx b/src/__tests__/auth/auth-context.test.tsx
index dc3f4f7..6c955ca 100644
--- a/src/__tests__/auth/auth-context.test.tsx
+++ b/src/__tests__/auth/auth-context.test.tsx
@@ -9,7 +9,7 @@ import { useAuth } from '@/hooks/use-auth'
 import { http, HttpResponse } from 'msw'
 import { server } from '@/mocks/server'
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL ?? 'http://localhost:3000'
+const API_URL = ''
 
 // Test component that exposes auth context values
 function AuthDisplay() {
diff --git a/src/__tests__/auth/auth-fetch.test.ts b/src/__tests__/auth/auth-fetch.test.ts
index 4ac6ad2..34ca993 100644
--- a/src/__tests__/auth/auth-fetch.test.ts
+++ b/src/__tests__/auth/auth-fetch.test.ts
@@ -8,7 +8,7 @@ import { server } from '@/mocks/server'
 import { createAuthFetch } from '@/lib/api/auth-fetch'
 import { mockAuthSession } from '@/mocks/data'
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL ?? 'http://localhost:3000'
+const API_URL = ''
 
 describe('createAuthFetch', () => {
   let getToken: ReturnType<typeof vi.fn>
diff --git a/src/__tests__/auth/callback-page.test.tsx b/src/__tests__/auth/callback-page.test.tsx
index 1be754e..80cf2b6 100644
--- a/src/__tests__/auth/callback-page.test.tsx
+++ b/src/__tests__/auth/callback-page.test.tsx
@@ -8,7 +8,7 @@ import { http, HttpResponse } from 'msw'
 import { server } from '@/mocks/server'
 import CallbackPage from '@/app/auth/callback/page'
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL ?? 'http://localhost:3000'
+const API_URL = ''
 
 const mockSetSessionFromCallback = vi.fn()
 
diff --git a/src/hooks/use-onboarding.test.ts b/src/hooks/use-onboarding.test.ts
index 33d89d7..bd9a97c 100644
--- a/src/hooks/use-onboarding.test.ts
+++ b/src/hooks/use-onboarding.test.ts
@@ -28,7 +28,7 @@ vi.mock('@/hooks/use-auth', () => ({
   }),
 }))
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL ?? 'http://localhost:3000'
+const API_URL = ''
 
 const mockStorage: Record<string, string> = {}
 
diff --git a/src/lib/api/auth-fetch.ts b/src/lib/api/auth-fetch.ts
index 2a1f11f..ab8c35a 100644
--- a/src/lib/api/auth-fetch.ts
+++ b/src/lib/api/auth-fetch.ts
@@ -7,7 +7,8 @@
 import { refreshSession } from './client'
 import type { AuthSession } from './types'
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL ?? 'http://localhost:3000'
+const API_URL =
+  typeof window !== 'undefined' ? '' : (process.env.API_INTERNAL_URL ?? 'http://localhost:3000')
 
 interface AuthFetchOptions {
   method?: 'GET' | 'POST' | 'PUT' | 'DELETE'
diff --git a/src/lib/api/client.test.ts b/src/lib/api/client.test.ts
index 69459df..72b937f 100644
--- a/src/lib/api/client.test.ts
+++ b/src/lib/api/client.test.ts
@@ -4,7 +4,7 @@ import { http, HttpResponse } from 'msw'
 import { server } from '@/mocks/server'
 import { mockCategories } from '@/mocks/data'
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL ?? 'http://localhost:3000'
+const API_URL = ''
 
 describe('API client', () => {
   describe('getCategories', () => {
diff --git a/src/lib/api/client.ts b/src/lib/api/client.ts
index 9865da0..735d9e0 100644
--- a/src/lib/api/client.ts
+++ b/src/lib/api/client.ts
@@ -45,7 +45,8 @@ import type {
   MyReportsResponse,
 } from './types'
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL ?? 'http://localhost:3000'
+const API_URL =
+  typeof window !== 'undefined' ? '' : (process.env.API_INTERNAL_URL ?? 'http://localhost:3000')
 
 interface FetchOptions {
   headers?: Record<string, string>
diff --git a/src/mocks/handlers.ts b/src/mocks/handlers.ts
index e2c24dc..0133c4a 100644
--- a/src/mocks/handlers.ts
+++ b/src/mocks/handlers.ts
@@ -29,7 +29,7 @@ import {
   mockMyReports,
 } from './data'
 
-const API_URL = process.env.NEXT_PUBLIC_API_URL ?? 'http://localhost:3000'
+const API_URL = ''
 
 export const handlers = [
   // --- Auth endpoints ---