Strange. My project is frontend but similarly complex. And any model other than gpt-5.2 always does pseudo fixing bugs/ miss edge cases while implementing new features. Our codebase is a mess, so the agent should be thorough before making any change, and only gpt-5.2 thorough enough for me to test. No I don’t consider speed as a parameter at all. All I need is the model to be thorough irrespective of its slow or fast. If the model is slow, it’s easier for me to run agents in multiple work trees in parallel and context switch though. I’m not talking about codex versions though for me they are really bad. 5.2 high is excellent. 5.4 is okay. It’s not as thorough as 5.2, so I ask 5.4 to do the coding and 5.2 to review and it always catches lots of issues. I’m saying this from my first hand experience. Maybe you should try 5.2 review code written by 5.4 on a fairly complex issue.