Anthropic (finally!) released Claude Fable 5 yesterday, marking the first time a Mythos-class model is available to the general public. Fable 5 shares the same underlying architecture as Mythos, the model that spooked government officials when it was revealed in April and kept behind closed doors through Project Glasswing. Interestingly, rather than a stripped down variant, Fable 5 is effectively Mythos with a routing layer on top. Queries touching cybersecurity, biology, chemistry, and a few other high-risk areas get transparently redirected to Opus 4.8.
These are guardrails that are deliberately conservative. Anthropic acknowledged upfront that the classifiers will sometimes catch harmless requests, they trigger in under 5% of sessions on average, but the tuning errs on the side of over-blocking while the team works to reduce false positives. Meanwhile, the same launch quietly introduced Claude Mythos 5 for the Glasswing cohort: same model, safeguards partially lifted in specific domains, now showing capabilities in drug design and autonomous genomics research that go well beyond anything Fable 5 is permitted to do.
On many laderboards, Fable 5 leads the field by a clear margin, sitting at the top in Artificial Analysis and among the top in Live Bench. The numbers hold across software engineering, knowledge work, vision, and long-context tasks. Stripe reportedly used an early build to compress a two-month codebase migration into a single day; IMC found it "aced" their trading-analysis evaluations nearly across the board; Cursor named it state of the art on CursorBench. Apparently, the longer and more complex the task, the wider the gap over previous models.
But the access situation is messier. Fable 5 is included for Pro, Max, Team, and Enterprise users until June 22... after which it moves to usage credit billing, with Anthropic citing capacity constraints and leaving open the possibility of restoring broader subscription access later (read more about the end of the subsidised tokens). The immediate reaction on social media was predictable confusion: users assumed "included" meant permanent, and Anthropic staff ended up doing clarification rounds after the announcement. Rate limits were reset entirely within hours due to demand.
Nevertheless, users seem happy with the release, and the benchmark lead is real enough that the enthusiasm is earned. Still, the model costs twice as much as the Opus series and draws down plan usage twice as fast, which, paired with the June 22 cutoff, makes capacity once again the most important constraint on who gets to use this.
*At the time of the posting of this Story, the Arena's score for Fable 5 hasn't been released; it's very likely that Fable 5's score in our AIW Leaderboard chart will go down when this happens.