diff --git a/.env.example b/.env.example
index 7b1715c..94d5527 100644
--- a/.env.example
+++ b/.env.example
@@ -11,6 +11,11 @@ PORT=8000
 FRONTEND_URL=http://localhost:3000
 API_URL=http://localhost:8000
 
+# Google OAuth (optional; required only for "Sign in with Google")
+GOOGLE_CLIENT_ID=
+GOOGLE_CLIENT_SECRET=
+GOOGLE_REDIRECT_URL=http://localhost:8000/api/v1/auth/google/callback
+
 # Frontend
 NEXT_PUBLIC_API_URL=http://localhost:8000/api/v1
 
diff --git a/backend/api/src/api/handlers/auth_handler.rs b/backend/api/src/api/handlers/auth_handler.rs
index 1fb7e65..e9f4ad1 100644
--- a/backend/api/src/api/handlers/auth_handler.rs
+++ b/backend/api/src/api/handlers/auth_handler.rs
@@ -203,6 +203,11 @@ pub struct OAuthCallbackQuery {
 
 pub async fn google_login() -> Result<axum::response::Redirect, AppError> {
     let client_id = std::env::var("GOOGLE_CLIENT_ID").unwrap_or_default();
+    if client_id.trim().is_empty() {
+        return Err(AppError::BadRequest(
+            "Google OAuth is not configured. Set GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET.".into(),
+        ));
+    }
     let redirect_uri = std::env::var("GOOGLE_REDIRECT_URL")
         .unwrap_or_else(|_| "http://localhost:8000/api/v1/auth/google/callback".to_string());
 
@@ -219,6 +224,11 @@ pub async fn google_callback(
 ) -> Result<axum::response::Redirect, AppError> {
     let client_id = std::env::var("GOOGLE_CLIENT_ID").unwrap_or_default();
     let client_secret = std::env::var("GOOGLE_CLIENT_SECRET").unwrap_or_default();
+    if client_id.trim().is_empty() || client_secret.trim().is_empty() {
+        return Err(AppError::BadRequest(
+            "Google OAuth is not configured. Set GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET.".into(),
+        ));
+    }
     let redirect_uri = std::env::var("GOOGLE_REDIRECT_URL")
         .unwrap_or_else(|_| "http://localhost:8000/api/v1/auth/google/callback".to_string());
     let frontend_url =