Are you sure you didn't run out credits and set effort to low? This exact thing happened to me when I did that. It just became, kinda lazy.

3.7 "I'm tired" it was just direct API "chat", no CC that I could use at the time.

Current 4.7 Opus with claude code, with effort pinned to max, because I'm on an API only plan, with a personal daily limit you would probably be jealous of. ;)