Just the other day I needed to code some interlocked indices. It wasn't particularly hard but I didn't want to context switch and think so instead I asked gpt 4o. After a back and worth for 4 or 5 times, where it gave wrong answers I finally decided to just take a pen and paper and do it by hand. I have a hard time believing that these models are reasoning, because if they are they are very poor at it.

GPT-4o isn't classified as a "reasoning" model (by common 2025 terminology at least) - I suggest trying again with o3 or Claude 4 or Gemini 2.5.