fix(v3): resolve 'Unexpected token <' error from missing custom.js

v3/UNRELEASED_CHANGELOG.md

@@ -23,6 +23,7 @@ After processing, the content will be moved to the main changelog and this file
 
 ## Fixed
 <!-- Bug fixes -->
+- Fix `Unexpected token '<'` error when `/wails/custom.js` is missing in desktop dev mode. Added explicit 404 handler for `/wails/custom.js` and case-insensitive `Content-Type` validation in `loadOptionalScript` to prevent HTML SPA fallbacks from being injected as JavaScript. ([#5068](https://github.com/wailsapp/wails/issues/5068))
 
 ## Deprecated
 <!-- Soon-to-be removed features -->

v3/internal/runtime/desktop/@wailsio/runtime/src/index.ts

@@ -89,9 +89,14 @@ export function loadOptionalScript(url: string): Promise<void> {
     return fetch(url, { method: 'HEAD' })
         .then(response => {
             if (response.ok) {
-                const script = document.createElement('script');
-                script.src = url;
-                document.head.appendChild(script);
+                // Verify the response is actually JavaScript and not an HTML fallback
+                // (e.g. Vite dev server returns index.html for unknown routes)
+                const contentType = (response.headers.get('content-type') || '').toLowerCase();
+                if (contentType.includes('javascript')) {
+                    const script = document.createElement('script');
+                    script.src = url;
+                    document.head.appendChild(script);
+                }
             }
         })
         .catch(() => {}); // Silently ignore - script is optional

v3/pkg/application/application.go

@@ -116,6 +116,10 @@ func New(appOptions Options) *App {
 					if err != nil {
 						result.fatal("unable to serve transport.js: %w", err)
 					}
+				case "/wails/custom.js":
+					// custom.js is only served in server mode.
+					// Return 404 so the runtime's loadOptionalScript skips it.
+					http.NotFound(rw, req)
 				default:
 					next.ServeHTTP(rw, req)
 				}