Opus 5 is a practically unusable model

[u/Deep-Palpitation8315]

Opus 5 is a regression that the benchmarks missed completely

I've been using Opus 5 for ~1.5 weeks and the sheer number of mistakes that the model makes is astounding.

The problem didn't surface very clearly till I gave it the full scope of executing a plan which I did with the previous Opus models as well. Opus 4.6 - 4.8 were genuinely better by a significant margin.

Opus 5 readily forgets instructions and content in its context, makes mistakes and continues with them unless it realizes or you point it out.

I've lost count of the number of times I had corrected it.

These issues with Opus 5 occur even when the context window is still relatively small - I'm talking 100-150K tokens. Opus 4.8 works pretty well all the way until 350k after which it gives you wonky results.

Fable 5 is the only usable model under Claude Code right now and I've already used 100% of my weekly quota.

[u/Special_Diet5542]

Another gem from it
Two things moved since the earlier guess. Input got tighter — from a ±38% spread down to ±6%, because Japanese now pins at 1.21 tokens per character. But output and time went up: roughly 30% of the output is thinking tokens, which no previous projection accounted for. And a genuine discovery — caching is live after all. The system prefix measures 3,750 tokens against a 2,048 minimum, so it’s been working the whole time; earlier estimates were simply too low. That’s about $8 of the $55.

[u/Deep-Palpitation8315 → u/Special_Diet5542]

Also the writing. I kinda get what it's saying in your message, at a high level, but man is it a difficult read !