> Write them by hand or generate them and check them in?

Yes, written by hand. I think that ultimately you should know what valid inputs and outputs are and as such the tests should be written by a human in accordance with the spec.

> Less flawed than checking in vibe-coded feature changes, but still flawed.

This is what I'm trying to get at. I agree it's not perfect, but I'm arguing it's less evil than what is currently happening.