Anthropic on Tuesday, June 9, launched Claude Fable 5, its most capable AI model yet made available for general use, alongside Claude Mythos 5, a restricted version of the same underlying model deployed exclusively to vetted cybersecurity partners through its Project Glasswing initiative.
The company described Fable 5 as a "Mythos-class" model, its highest capability tier, that has been fitted with new safeguards to make it safe for broad release. The model is described as state-of-the-art on nearly all tested benchmarks, with exceptional performance in software engineering, knowledge work, vision, and scientific research. Anthropic said the performance gap over its previous models widens on longer, more complex tasks.
To manage dual-use risks inherent to Mythos-class capabilities, particularly in cybersecurity and biology, Anthropic has deployed classifier systems that automatically route sensitive queries to Claude Opus 4.8, its next-most-capable model, rather than returning a refusal. The company said this fallback is triggered in fewer than 5% of sessions on average.
Mythos 5 carries the same capabilities as Fable 5 but operates without the cybersecurity safeguards for its designated partners. It is being deployed as an upgrade to Claude Mythos Preview, the first Mythos-class model Anthropic released in April. Both models are priced at $10 per million input tokens and $50 per million output tokens, less than half the cost of Mythos Preview. Anthropic said it plans to expand access to Mythos 5 through a broader trusted access program over time, including a separate programme for biomedical researchers.
Early testing by Stripe found Fable 5 completed a codebase-wide migration across a 50-million-line Ruby repository in a day, a task the company estimated would have required a full team more than two months. Internally, Anthropic reported that Mythos 5 accelerated certain stages of the drug design process by approximately ten times, with nine of fourteen protein targets yielding viable drug design candidates.
Fable 5 is available immediately on the Claude API under the model string claude-fable-5. For subscription plans, including Pro, Max, Team, and Enterprise, the model will be available at no additional cost through June 22, after which usage credits will be required until capacity allows its restoration as a standard plan feature.
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