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this post was submitted on 28 Jul 2026<br>82 points (85% upvoted)<br>shortlink:

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Opus 5: extremely RL-fried and mistake-prone for anyone else?Question about Claude models (self.ClaudeAI)<br>submitted 4 hours ago by erebueius

normally when people say a new model is bad i roll my eyes a bit, but Opus 5 is truly not good for any task, imo. i have thoroughly tried it in every possible role in a large, complicated project. it is bad for all tasks. i keep hearing "ok, but it's good as a subagent though" but it's no good as a subagent - even when just reading code for recon, it misinterprets the code reliably.

it has serious problems with "just doing things" and immediately forgetting it did them. to give you an example: during some reverse engineering it randomly decided that an extremely import native function was pointless, so it commented it out, breaking the engine and then forgetting that it even did so. i have had Fable and Opus 4.8 working on the same engine doing very similar work for months and that class of mistake has never happened before.

my smell test is that this is mostly just Opus 4.8's base model, except RL'd with Fable 5 logits to the point where it thinks it is a model with 10x more parameters, when it isn't, leading to extreme overconfidence and amnesia.

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[–]ClaudeAI-mod-botWilson, lead ClaudeAI modbot[M] [score hidden] 2 hours ago stickied comment (0 children)<br>TL;DR of the discussion generated automatically after 40 comments.

Looks like we've got a classic r/ClaudeAI civil war on our hands. The thread is heavily split on whether Opus 5 is a step forward or a disastrous step back.

The camp agreeing with OP is large and loud. The consensus among them is that Opus 5 is overconfident, forgetful, and prone to making baffling mistakes. Common gripes include it ignoring direct instructions, introducing new bugs while "fixing" old ones, hallucinating details instead of checking provided files, and generally needing way too much hand-holding. Many in this thread are reporting they've retreated to the perceived safety of Opus 4.x or even Sonnet 5.

However, there's an equally strong, highly-upvoted counter-argument from several users, including some veteran devs, who insist it's working perfectly fine for them on large, complex projects. They've had no issues and find it to be a capable improvement. The implication from this side is that the problems might be due to user-specific setups...