It depends on how you look at the degree to which you are interacting with the runtime.  This is a fairly low-level hook into the runtime.  So arguably if you are using this API then you should specify being a "core" extension.  That said, getting that clever about it is a bit too much.  The authors or PEP 523 can correct me if I'm wrong, but it seems like there isn't a good reason to restrict access.

So basically, I agree with you. :)

How about one of the following?

* _PyInterpreterState_SetEvalFrame(_PyFrameEvalFunction eval_frame)
* _PyInterpreterState_SetFrameEval(_PyFrameEvalFunction eval_frame)

The underscore basically says "don't use this unless you know what you are doing".  Or perhaps that is overkill too?  "_PyFrameEvalFunction" has an underscore, so perhaps not.

Also, it would make sense to have a matching getter.