>> I am on the opposite camp. Open models are starting to perform better. GPT 5.5 keeps on messing things up.

I'm working in a 600k+ LoC codebase that has complex domain-specific logic and lots of moving parts. I find that Codex 5.5 is pretty good at surgical fixes, but does not go out of its way to explore and figure out what those surgical fixes might break. So I only use it to work on parts of the system that are pretty isolated from everything else so that risk of regression is small.

I'm trying not to be the "you're holding it wrong" guy, but ... have you just tried telling it to explore the codebase for things it might break?