>It’s not just about non-determinism

I'm very specifically addressing prompt reproducibility mentioned above, because it's a notorious red herring in these discussions. What you want is correctness, not determinism/reproducibility which is relatively trivial. (although thinking of it more, maybe not that trivial... if you want usable repro in the long run, you'll have to store the model snapshot, the inference code, and make it deterministic too)

>A one word difference in a spec can and frequently does produce unrecognizably different output.

This is well out of scope for the reproducibility and doesn't affect it in the slightest. And for practical software development this is also a red herring, the real issue is correctness and spec gaming. As long as the output is correct and doesn't circumvent the intention of the spec, prompt instability is unimportant, it's just the ambiguous nature of the domain LLMs and humans operate in.