diff --git a/client/nginx/default.conf b/client/nginx/default.conf
index 7651bae..1067eee 100644
--- a/client/nginx/default.conf
+++ b/client/nginx/default.conf
@@ -13,7 +13,11 @@ server {
     }
 
     location /api/ {
-        set $backend http://server:3000;
+        # Use a globally-unique hostname across attached Docker networks.
+        # `client` is connected to both `toir-light` and external `proxy` networks,
+        # so a generic name like `server` can resolve to multiple containers and
+        # cause intermittent routing (200/404 flapping).
+        set $backend http://toir-light-server:3000;
         proxy_pass $backend;
         proxy_http_version 1.1;
         proxy_set_header Host $host;