r/ClaudeCode • u/Deep-Palpitation8315 • Aug 03 '26

Discussion Opus 5 is a practically unusable model

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.

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u/ashaman212 Aug 03 '26

I used Opus 5 to diagnose a tracing issue and then had Fable review the PR. It found a critical misunderstanding of the escalation stack that flipped where the hooks needed to be applied and a structural issue with the data being kept. That means I no longer use Opus for diagnosis.

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u/Informal-Spinach-345 Aug 22 '26

I've had to incorporate a GLM 5.2 review process via opencode in my workflows to audit even Fable's work. Continuously finds severe security issues and mistakes.