“Claude Opus 5: The System Card” by Zvi

Claude Opus 5 is trying to be the best of both worlds. On many practical tasks, Opus 5 is pitched as straight up as good or better than Fable 5, while being faster, at half the price. Most tasks do not require Mythos-level big model smell.

Claude Opus 5 is substantially stronger than Claude Opus 4.8 across the board, with the largest gains in agentic coding, computer use, and long-horizon knowledge work. It sets a new state-of-the-art on several third-party benchmarks, and on many evaluations it is comparable to—and in some cases ahead of—Claude Fable 5 and Claude Mythos 5.

On the particular tasks we are most worried about, as in cyber offense (and bio threats), in part by avoiding relevant training, Opus 5 lacks a full version of ‘The Juice’ that makes something functionally Mythos-class. Opus 5 cannot string together lots of exploits on the fly the way that Mythos 5 can. Part of this is that they deliberately avoided training on cyber-related tasks.

I suspect model size is key as well. It makes sense that a model getting bigger makes it more capable of the most dangerous, scary and complex tasks, relative to the [...]

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Outline:

(03:23) RSP Evaluations (2)

(05:59) Cyber (3)

(11:02) Safeguards and Harmlessness (4)

(12:37) Agentic Safety (5)

(16:04) Alignment (6)

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First published:
July 25th, 2026

Source:
https://www.lesswrong.com/posts/ywGX6FhgbZEkHRfQR/claude-opus-5-the-system-card

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Narrated by TYPE III AUDIO.

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