diff --git a/src/api/config.ts b/src/api/config.ts
index 63789f1..6631311 100644
--- a/src/api/config.ts
+++ b/src/api/config.ts
@@ -13,9 +13,16 @@ export const APP_ID = '434';
 declare global {
   interface Window { __RUNTIME_CONFIG__?: Record<string, string> }
 }
+const runtimeApiURL =
+  typeof window !== 'undefined' ? window.__RUNTIME_CONFIG__?.VITE_ZINO_API_URL : undefined;
+
+// SAME_ORIGIN means "call the host that served this page": on a custom domain the
+// same Ingress serves /usr and /app/, so the API is this origin. It is a sentinel
+// rather than '' because the || below treats '' as absent and falls through.
 export const BASE_URL =
-  (typeof window !== 'undefined' && window.__RUNTIME_CONFIG__?.VITE_ZINO_API_URL) ||
-  'https://dev.getzino.in';
+  runtimeApiURL === 'SAME_ORIGIN'
+    ? window.location.origin.replace(/\/$/, '')
+    : runtimeApiURL || 'https://dev.getzino.in';
 
 // --- Order Booking (src/docs/api/order_booking.md) ---
 export const ORDER_BOOKING = {