I'm not sure why you disbelieve agents can do "guaranteed to never fail". Proving correctness is one of their strengths. Proofs in general, in fact, much much better than the typical human coder.
Because I must warn you - it's extremely easy to ask a coding agent to prove code correctness before committing his work.
But that doesn't mean they don't have >0% failure modes, often in very non-human, unexpected, even dangerous ways.
Whether that's "sloppy"?
Eh, it's just the nature of the tool, like a perfect screwdriver that randomly strips 1% of screws.
> it's extremely easy to ask a coding agent to prove code correctness
That statement is categorically wrong.
If you're talking about a formal proof for non-trivial, real-world code, no.
If you're talking about a comprehensive set of tests... well, I'd challenge you to elucidate how you see creating a comprehensive set of tests as trivial.
And if the answer is more LLM... then I think we're at a fundamental disagreement about what satisfies "guarantee" or "provable".
I invite you to examine the test suite on the project I just published. Was it trivial? No, I'd say the whole project did take me a couple days.
https://github.com/qarl/arcade-js
You have a nice day.