diff --git a/frontend/vite.config.js b/frontend/vite.config.js
index 41c0723..0ab79c1 100644
--- a/frontend/vite.config.js
+++ b/frontend/vite.config.js
@@ -25,7 +25,14 @@ export default defineConfig({
   },
   ssgOptions: {
     dirStyle: 'nested',
-    script: 'async',
+    // 'defer' (not 'async'): the entry module reads window.__VITE_REACT_SSG_HASH__,
+    // which is set by a plain inline <script> placed later in the document body.
+    // An async module can start executing as soon as it's fetched, racing ahead
+    // of the parser reaching that later inline script — intermittently reading
+    // it as undefined depending on network/cache timing. defer guarantees the
+    // module only runs after the full document (including that inline script)
+    // has been parsed.
+    script: 'defer',
     onFinished: writeSitemap,
   },
 });