Codex for me behaves very junior engineer-ish. Claude is smarter and tries to think long term.

A great example of their behaviours for a problem that isn't 100% specified in detail (because detail would need iterations) is available at https://gist.github.com/hashhar/b1215035c19a31bbe4b58f44dbb4....

I gave both Codex (GPT5-ExHi) and Claude (Opus 4.5 Thinking) the exact same prompts and the end results were very different.

The most interesting bit was asking both of them to try to justify why there were differences and then critiquing each other's code. Claude was so good at this - took the best parts of GPTs code, fixed a bug there and ended up with a pretty nice implementation.

The Claude generated code was much more well-organised too (less script-like, more program like).