> This is all handled in an extension

And why should I need to load an extension to handle a 400-series ? That's just nuts.

The kludge is loading the following JS snippet on your page:

   if(evt.detail.xhr.status >= 400 && evt.detail.xhr.status <= 499 ) {
      evt.detail.shouldSwap = true;
   }

Every web framework has error handling features. These are HTMX’s. It’s two HTML attributes and you’re done, if you don’t want any of the additional functionality.