Introducing Claude Fable 5

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Introducing Claude Fable 5: a Mythos-class model that we've made
safe for general use. Its capabilities exceed those of any model we've
ever made generally available.

Fable 5 is state of the art on nearly all tested benchmarks, with
exceptional performance in software engineering, knowledge work,
scientific research, and vision. It can run for days, and the longer the
task, the larger its lead over our other models.

Fable 5 launches today alongside Claude Mythos 5. The two share
the same underlying model, but Mythos 5, so far deployed only through Project Glasswing,
has the safeguards lifted in some areas. The safeguards are what
distinguish the two, and why we've given them different names.

Releasing a model this capable comes with risks. Without
safeguards, Fable 5's capabilities in areas like cybersecurity could be
misused to cause serious damage. So when Fable's classifiers detect a
request related to cybersecurity, biology and chemistry, or
distillation, the response is handled by Claude Opus 4.8, our
next-most-capable model. Users are informed whenever this occurs, more
than 95% of sessions involve no fallback at all, and performance
everywhere else is unaffected. We'll keep refining the safeguards to
reduce false positives.

Claude Fable 5 is available today on paid plans, in Claude Code,
on the Claude API, and all major cloud platforms. Through June 22, it's
included in paid Claude plans at no additional cost.

Claude Mythos 5 is available to Glasswing partners, with a broader trusted access program to follow.