>You have two options: fail open and let the request through, or fail closed and block the request with a 429.

If the metaphor of a software circuit breaker is meant to emulate an electrical circuit breaker, then it seems to me that these two are inverted. Whenever a physical circuit breaker is open, it is not dangerous and not passing current.

Agreed, and I don’t know why you’re being downvoted. If someone told me their virtual circuit breaker “fails open,” I would assume that it stops processing data upon failure.