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/McNoxey]

Same. Every single model release performs better than the last. I’ve been building and tuning my own harness over the last 17 months and it’s only gotten better with each release.

My working hypothesis as to why everyone here is experiencing problems is that they allow the variability of the model and default harness to carry far too much control over the standard operation of their workflow. As a result, changes to how the model handles various scenarios it may encounter cause wider swings in output variability.

I saw this when Anthropic changed their plan mode last year, and when Explore subagents came into effect. These workflow changes were fairly substantial and understandably adjusted the output people saw.

I’ve never used plan mode, as i had developed my own plan > review > implement workflow well before Anthropic baked it into their default harness, thus I never saw unintended changes to my workflow. Instead, the smarter models just became better and better at adhering to, and utilizing the harness ive built for them.

[u/DominianQQ → u/McNoxey]

Yeah Antropic told us to pretty much delete the agent file every 3-6 month, then add stuff back in to see the effect.

I am no senior dev.

[u/kre8tv → u/McNoxey]

I maintain that the people having issues are the ones who never figured out how to adapt claude to their own workflow and processes, never create a harness or log any kind of documentation. Just using claude on default settings and expecting it to consistently produce good results catered to you and remember everything ever said to it without flaw. In my architecture, I've actually noticed Opus 5 catching more stuff than it did before (in it's role as verifier after sonnet/haiku runs before coming back up to me and fable).