[zeveb]

> you should not assume the format of a non-200 status code

You should never assume format based on status code at all! You should detect it based on the Content-Type header.

> You'll get a 500 with a plaintext body. Don't parse that as JSON.

Any intermediary which returns plain text with an application/json Content-Type is badly, badly broken.

[shadowgovt]

> You should detect it based on the Content-Type header

In approximately a decade of working on JavaScript and TypeScript UI code, I can count on one hand the number of RPC handlers I've seen that inspect Content-Type in the success codepath.

... for that matter, I can count on one hand the number of RPC handlers I've see nthat inspect Content-Type in the failure codepath also. The common behavior is to dump body to text, log error, and recover.