Interestingly I'm experiencing the opposite as you. Was mostly using Claude Sonnet 4 and GPT 4.1 through copilot for a few months and was overall fairly satisfied with it. First task I threw at GPT 5, it excelled in a fraction of the time Sonnet 4 normally takes, but after a few iterations, it all went downhill. GPT 5 almost systematically does things I didn't ask it to do. After failing to solve an issue for almost an hour, I switched back to Claude which fixed it in the first try. YMMV