“Claude Opus 5 Is Highly Capable, But Is No Mythos” by Zvi

Claude Opus 5 is a weirder than usual release to evaluate, for two reasons. The most obvious is that Fable 5 already exists. Opus 5 is pitched not as the world's most advanced AI model, but as a way to mostly match Fable performance, while being half the price of Fable per token at the API and a lot cheaper than that via subscriptions, and with far more permissive classifiers. Opus 5 often costs more than half of Fable to run on benchmarks, which I think is because they use effort settings that are too high and offer only marginal returns. If you put Opus 5 on higher effort levels it can spin around in circles, and for tasks where Opus 5 is the best tool I suspect you usually are fine with Medium effort. Opus 5 is in many ways and for the bulk of real world tasks about as capable as Fable. In some cases it is modestly better. It is still not Mythos class. Fable is your only Mythos-class option. Opus 5 does not have The Juice, the ability to autonomously string together a bunch of seemingly unrelated exploits, which extends to other domains, or as much [...]

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Outline:

(03:54) The Official Pitch

(06:25) Official Benchmarks

(15:33) Other People's Benchmarks

(20:28) The System Prompt

(20:50) Every Gets Frustrated

(21:54) Positive Reactions

(25:14) Keep It Classy

(26:22) It's Not Mythos Class

(30:03) Other Reactions

(31:02) Claude Codes

(37:03) Subagent Opus

(39:23) Toys Are Fun

(41:37) Too Many Models

(42:10) Wrong On The Internet

(44:40) Claude Slop

(46:27) Negative Reactions

(50:09) And Then There Were Three

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First published:
July 28th, 2026

Source:
https://www.lesswrong.com/posts/Pj4Eewb4KXvXFCcGv/claude-opus-5-is-highly-capable-but-is-no-mythos

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Narrated by TYPE III AUDIO.

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