If this is in Claude Code, check whether your session got switched to Opus 4.8. After a cyber safeguards refusal it continues on claude-opus-4-8 with a one-line notice. I saw this on ordinary auth-token fixes, in 5 of 60 runs last week.
I would also agree that 5.5 feels dumber the past week. More weird logical problems. it's not as dumb as 5.1 or 4.8 but it feels like it regressed.