@joerdav, it looks like this PR doesn't fix the issue.

The 404 page https://xcfile.dev/404.html exists, but when I go to an unknown page like https://xcfile.dev/unknown, a standard Not Found message appears:

Could you take a look?

Yeah I've tried a couple of things with no luck. I think the problem is that the catch-all route is being hit:

- url: /(.*)/
  static_files: public/\1/index.html
  upload: public/(.*)

The app engine routes work top to bottom, and I think if they match even if there is no file corresponding it doesn't propagate onto the next rule.

There isn't great documentation around it either.

I think the answer might be to have a custom python handler?

Try now https://xcfile.dev/unknown :)

Works! Thank you.