You should be using two (or even three) different LLMs. For example: Claude to do the coding, and Codex to do an adversarial review. This can go back and forth for several iterations before converging on an acceptably "correct" version of the code...

Occasionally, Claude may not be able to fix the issue – and it may turn out that Codex can, because it has a different approach.

Either way, at least two LLMs is the only way to get (mostly) reliable code.