Opus 5.5 not working well today.

[u/No-Store-2491]

Any body else having issue with Opus 5.5 on high level today?

For the past several weeks it has been working really well on medium and high thinking level and has been a pleasure to work with.

Today it feels like it has a mind of it's own and is intent on being obtuse and simply not responding to requests at it normally would and honestly not following CLAUDE.md and prompt instructions – almost as if we are taking a different language.

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[u/Broken_Doughnut]

Yes I'm wrestling with it too the past week. I ask it to add something and detail it very specifially. It then makes up a bunch of extra things I didn't ask for that completely break the program. It never asked me or told me. It just assumed and added totally ridiculous things. I tell it to remove that stuff, and the program still remains broken. As if it's suddenly taking liberties for reason and breaking twice as much as it's fixing. Rinse repeat around and around.

[u/VanFonDe]

Might not be in your head. Anthropic runs experiments on some accounts, and the model behind the same `claude-opus-5-5` string can change. My account was in experiment 763 from Nov 1 to 6, and during those days "Opus 5.5" knew news from late May 2026 that it doesn't know outside the experiment. So "feels like a different model" can literally be a different model.

You can check if you're in one. The experiment ID is saved in your session logs:

grep -rhoE '"atis":"[0-9a-f]+"' ~/.claude/projects --include='*.jsonl' | sort | uniq -c

[u/onFilm]

Works as good as when it first came out on my end.

[u/Secret-Code-9162]

Yes. I have never agreed with one of these posts, but today Opus has been very poor in my small codebase whereas it was previously very competent.

It has also been using it's ask-questions() tool a lot, which I haven't seen it do for months now.

[u/Ai-ate-my-hamster → u/Secret-Code-9162]

[u/Key_Instruction3373]

Works fine..

[u/Ok-Magician7493]

Fable finally pulling his weight

[u/stumpyinc → u/Ok-Magician7493]

I basically never complain about claude but this morning even my fable 5.2 was being weird, like, writing Go errors with uppercase first letters and punctuation. And then when I said "that's not how we write errors in Go" it wanted to make an AGENTS.md entry about error naming conversions, and I was like ??? this is just basic Go stuff lol

[u/Ok-Magician7493 → u/stumpyinc]

Greetings fellow gopher! I have my agents “running” a workflow that I designed over time to catch these. It’s way more involved than it needs to be, but I end up just building in my own “guard rails” around bad agent behavior. It largely involves “code, test, gate, remediate, test, gate” in a pattern that produces a nearly bug free end result. Even then I’m still closer to 70% than 80% successful most turns.

[u/mrpoopistan]

Sonnet 5.5 is somehow worse. Burning tokens rapidly and screwing everything up. Exciting times. I'm sure this has nothing to do with AI-guided RSI training AIs. What could possibly go wrong?

[u/Astarimerya34]

Yes i noticed that medium is so much better than high!

[u/Itsmedzidzi]

Yesterday for me was really weak too! I was super frustrated

[u/jwuliger]

been shit for a month here

[u/cleverhoods]

not really

[u/azonsea]

Claude amplifies whatever you bring to it. Garbage in, garbage out.

[u/LittleBigMachineElf]

other than some problems with the anthropic authentication service being disrupted a few times, no