You've found a kind "trap street". This disambiguates the error message generated from Fastly vs the error message generated from an origin which happens to use

You've found a kind "trap street". This disambiguates the error message generated from Fastly vs the error message generated from an origin which happens to use varnish as well. Edit: As to why Fastly wants to expose the relatively stock error message instead of custom text, I dont know. My guess would be Faslty (or their customers) have tooling built around parsing the http response and they're preserving compatibility.