Claude Fable 5, a Mythos-class AI model that Anthropic had pledged to make public after limiting its most potent system to a small group of cyber defenders in April, was released on Tuesday.
Claude Mythos 5, which is still only available to companies authorized by Anthropic’s Project Glasswing program, shares the model’s basic design. The distinction is in the guardrails: the algorithm automatically directs the response to Claude Opus 4.8 when Fable 5’s classifiers identify queries of cybersecurity, biology and chemistry, or model distillation. According to Anthropic, there is no such fallback in over 95% of sessions.
The Potential Of Fable 5
Fable 5, according to Anthropic, is better than any model it has previously made widely accessible, with special advantages in software engineering, knowledge work, and vision. Early testing partners have claimed impressive results: according to Stripe, the model completed a codebase-wide conversion spanning 50 million lines of Ruby code in a single day, saving months of engineering effort that would have taken a team more than two months.
Additionally, the model defeated Pokémon FireRed with just a basic vision-based harness, a job that had proven difficult for earlier Claude models to complete even with extra scaffolding tools. Fable 5 received the greatest score of any evaluated model on financial reasoning criteria.
Data Retention And Safety Trade-offs
An obligatory 30-day data retention policy on all traffic for Mythos-class models is included with the release, and it applies to both first-party and third-party surfaces. According to Anthropic, the saved data will often be erased after 30 days and won’t be utilized for training.
Anthropic noted that the security measures are “stricter than would be ideal,” pointing out that the classifiers are occasionally activated by innocent queries. Although the UK AI Safety Institute made headway toward a universal jailbreak during preliminary evaluations, an external bug bounty failed to provide one in more than 1,000 hours of testing.
Cost And Availability
The cost of Fable 5 and Mythos 5 is $10 per million input tokens and $50 per million output tokens, respectively. Fable 5 is free for subscribers on Pro, Max, Team, and Enterprise plans till June 22. After that, usage credits will be needed.
Existing Glasswing partners can immediately use Claude Mythos 5, the same model with cyber protections removed. Anthropic is preparing a more comprehensive trusted access program in conjunction with the US government. Researchers who require the model’s chemistry and biology skills will be able to enroll in an unrestricted, separate biology-focused trusted access track.
On Sunday, tech journalist Alex Heath revealed that Anthropic was “planning to release a public version of Mythos” with “substantial guardrails.”