Claude Opus 5: The System Card

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 improvement on everyday ordinary tasks. There is a reason so many tokens get routed to smaller models.

This doesn’t prevent Opus 5 from improving a lot on Opus 4.8 on such dangerous tasks. Opus 5 is very clearly closer to Mythos 5’s level of capability than to Opus 4.8’s on these tasks. In general, capability looks modestly below Fable 5, but closer to Fable than Opus 4.8.

There are several suggestions, and I’ve seen it echoed elsewhere, that you may want to usually use less effort than you might expect, that this can even actively help.

Staying Opus-sized and not training on cyber won’t work for long. You can buy some time, and allow defenders to have more access to a relatively superior tool, and of course hope the government is calmer. Thus, you can get classifiers that trigger 85% less often than Fable’s, while still getting high levels of pract…