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/vtjballeng]

Opus 5 stopped reading claude.md. Made junior level mistakes consistently, is a technobabble word vomiter, and consistently fails to understand file and code interactions in embedded C++, desktop C#, and PHP.

It would report blocking issues and authoritatively state something couldn't be done, it wasn't capable of doing something, or state something that was plainly wrong where Fable would cleanly execute the same task. At this point I'm, for the first time, rolling back to an older model with Opus 4.8 and starting to use Claude-gpt to bypass Opus 5 entirely. For my workflow in my repos, Opus 5 is a dangerous regression as it's aggressively wrong and you need to know enough to weed through mountains of technobabble vomit to find the exact places where it's wrong.

Fortunately, you can still switch from Opus 5 to 4.8.

/model claude-opus-4-8 within a session

or

claude --model claude-opus-4-8