Anthropic launched Claude Fable 5, the first Mythos-class model made generally available, alongside Claude Mythos 5 — the same underlying model with cyber safeguards lifted for vetted Project Glasswing partners working with the US government. Anthropic describes Fable 5 as state-of-the-art on nearly every tested capability benchmark, with the margin over prior Claude models widening as tasks get longer and more complex. Latent Space reports the model is at least twice the size of Opus 4.8, which itself was barely two weeks old and already near the top of the field.
The capability claims are concrete. In early testing Stripe ran a codebase-wide migration across a fifty-million-line Ruby codebase in a single day, work the company estimated would have taken a team more than two months by hand. On Cognition's FrontierCode evaluation Fable 5 posts the highest score of any frontier model even at medium effort, and it is markedly more token-efficient than past Claude models. It is the new state-of-the-art on vision, able to rebuild a web app's source from screenshots alone and to beat Pokemon FireRed with a minimal vision-only harness where earlier models needed elaborate scaffolding. On long-horizon work it stays coherent across millions of tokens and uses file-based memory to improve its own outputs. The Mythos variant accelerated internal protein design by roughly ten times, produced novel molecular-biology hypotheses that human scientists preferred about eighty percent of the time in blind comparisons — one later corroborated by an independent lab — and, in a week of largely autonomous genomics work, trained a cross-species cell-type model a hundred times smaller that beat a recent Science publication.
What makes this release unusual is the safety architecture wrapped around it. Anthropic shipped new classifiers that detect requests touching cybersecurity, biology and chemistry, or model distillation, and silently route those queries to a fallback on Opus 4.8 rather than answering with Fable. The company says fewer than five percent of sessions trigger a fallback, an external bug bounty found no universal jailbreak in over a thousand hours, and the UK AI Safety Institute made only partial progress toward one. Anthropic also now requires thirty-day data retention on all Mythos-class traffic, first and third party, for safety monitoring. Pricing is ten dollars per million input tokens and fifty per million output, less than half the cost of the earlier Mythos Preview, and Fable 5 is free on paid consumer and team plans only through June 22 before moving to usage credits.
Reaction split along predictable lines. Simon Willison, after five and a half hours of hands-on testing, called it “something of a beast” — slow and expensive but able to chew through nearly anything he gave it. Ethan Mollick, with early access, said it outperformed every public model he had used by a considerable margin and would run up to a dozen hours executing multi-page specifications. Nathan Lambert was sharply critical of the unevenly applied, sometimes-undisclosed safety measures, arguing they read less as protection than as an attempt to entrench Anthropic's lead, and predicting the policy will become a cautionary fable in its own right. Artificial Analysis independently placed Fable 5 at number one on its Intelligence Index, while several testers flagged that the headline benchmark wins come with asterisks around the fallback behavior and the conservative guardrails.
- Anthropic frames the dual release as releasing Mythos-level capability safely: identical model, with Mythos 5's cyber safeguards lifted only for vetted Glasswing partners.
- Simon Willison: a “beast” — slow and expensive, but the hard part is finding tasks it cannot do; notes new API messaging when guardrails fire.
- Ethan Mollick: a “very real leap,” running up to a dozen hours on multi-page specs; says our relationship with AI is shifting.
- Nathan Lambert (Interconnects): critical of unevenly applied, partly-undisclosed safety measures; reads them as entrenching Anthropic's lead.
- Artificial Analysis ranked it #1 on its Intelligence Index; Latent Space pegs it at ~2x Opus 4.8 size and flags benchmark wins “with asterisks.”
- TechCrunch's angle: it is a gift to vibe-coders, one-shotting playable browser games.