I use an agent to author my nix config - personally I don't have the determination to have picked Nix up entirely on my own so I find it to be a godsend. I can code review config changes and keep it all in git - also makes it easy to pick up the entire config and deploy it on a different machine.

Deterministic systems like tracing GCs, JIT compilers, PGO data, machine learning optimising passes that vary their output, depending on program behaviours?

LLMs can be deterministic too! People just don't bother because the applications of this aren't widely known yet. See https://lukechampine.com/repligraphs

The configuration can be sloppy and vibed, as long as it behaves the same every time nix retains its power :)