Anthropic Launches Claude Fable 5 and Claude Mythos 5 AI Models

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Anthropic today launched two new AI models — Claude Fable 5 and Claude Mythos 5 — marking the company’s first broad release of the powerful “Mythos-class” AI capabilities it previously made available only to participating organizations in its restricted cybersecurity program, Project Glasswing, which it announced two months ago. The company says Fable 5, which is the version most users and developers will get starting today, exceeds every Claude model it has previously made generally available — featuring stronger performance across software engineering, knowledge work, vision, scientific research and long-running tasks. It smashes the existing benchmarks and comes atop on nearly all of them, though the prior Claude Mythos Preview version of the model still takes the top spots on computer use and multidisciplinary reasoning.

The new Claude Mythos 5, by contrast, is a more restricted, upgraded version of the prior, similarly restricted Mythos Preview model. As such, it has certain safeguards lifted for approved users, including Anthropic's cybersecurity partners in its Project Glasswing effort, and select biology researchers. Anthropic says requests involving certain high-risk areas — including cybersecurity, biology and chemistry, and model distillation — are automatically routed to Claude Opus 4.8, Anthropic's previously flagship general model, instead, with users notified when that happens.

The company says more than 95% of Fable sessions run entirely on Fable’s own responses, with no fallback, and that internal and external red-teaming efforts found no “universal jailbreaks” after more than 1,000 hours of testing. Anthropic says Fable 5 is available to the general public today through its website, apps, and API, but that Mythos 5 will initially only be made available to users who already have access to the older Claude Mythos Preview. Pricing for both Fable 5 and Mythos 5 is $10 per million input tokens and $50 per million output tokens, which Anthropic says is less than half the price of Claude Mythos Preview.

For developers, Fable 5 is available through the Claude API as claude-fable-5. Anthropic says Fable 5 is fully available today on the Claude API and on consumption-based Enterprise plans. The launch also signals how Anthropic plans to bring frontier models with dangerous dual-use capabilities into the market: not by releasing all capabilities to everyone, and not by simply refusing risky questions, but by routing some requests to a less capable model while keeping the stronger model available for the majority of everyday work.

A major improvement in autonomous coding is one of the key benefits of Fable 5, with Anthropic saying it can work unattended for longer and with more independence than previous Claude models. On SWE-bench Pro, which measures a model's ability to complete difficult software engineering tasks, Anthropic says Fable 5 and Mythos 5 reach 80.3%, vastly outperforming OpenAI's latest and greatest general model GPT-5.5, which scored 58.6%. Stripe tested Fable 5 in a 50-million-line Ruby codebase and found that the model completed a codebase-wide migration in one day that otherwise would have taken a team more than two months by hand.