Please see my earlier posts above (#3 and #4).
I end up in a redirect loop where App Engine just issues a series of 302 redirects back to the same page over and over until the browser steps in and errors with “The page isn’t redirecting properly”.
…
just don’t expect to be able to access /_ah/warmup from your browser to check to see if it works.
Instead, depend on the GCP Logs. You can use
protoPayload.wasLoadingRequest="true"as a log filter to see your cold start requests, as long as /_ah/warmup is returning 200 responses on there it kind of doesn’t matter if you can reach it from your browser or not.