diff --git a/src/lib/api/auth-fetch.ts b/src/lib/api/auth-fetch.ts
index 766e511..53b1cfb 100644
--- a/src/lib/api/auth-fetch.ts
+++ b/src/lib/api/auth-fetch.ts
@@ -7,10 +7,9 @@
 import { refreshSession } from './client'
 import type { AuthSession } from './types'
 
+/** Client: relative URLs (empty string). Server: internal Docker network URL. */
 const API_URL =
-  typeof window !== 'undefined'
-    ? (process.env.NEXT_PUBLIC_API_URL ?? '')
-    : (process.env.API_INTERNAL_URL ?? 'http://localhost:3000')
+  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.ts b/src/lib/api/client.ts
index 112a8fd..48f2ac6 100644
--- a/src/lib/api/client.ts
+++ b/src/lib/api/client.ts
@@ -60,10 +60,9 @@ import type {
   BehavioralFlag,
 } from './types'
 
+/** Client: relative URLs (empty string). Server: internal Docker network URL. */
 const API_URL =
-  typeof window !== 'undefined'
-    ? (process.env.NEXT_PUBLIC_API_URL ?? '')
-    : (process.env.API_INTERNAL_URL ?? 'http://localhost:3000')
+  typeof window === 'undefined' ? (process.env.API_INTERNAL_URL ?? 'http://localhost:3000') : ''
 
 interface FetchOptions {
   headers?: Record<string, string>