I tried to control LLM output quality by different means, including fuzzing. Had several cases when LLM "cheated" on that too. So, I have my own shades and grades of being sure the code is not BS.

Well, that’s obviously bad.

But once you told it to stop cheating, did it eventually figure it out? I mean, correctly implementing fuzzer support for a project is entirely within the wheelhouse of current models. It’s not rocket science.