The Mythos-Class Tier

Anthropic has introduced a new capability tier called "Mythos-class," which sits above the existing Opus class. The first models in this tier, Claude Fable 5 and Claude Mythos 5, share the same underlying architecture but differ significantly in their safety configurations and availability. Both models support a 1M token context window and up to 128k output tokens per request, priced at $10 per million input tokens and $50 per million output tokens.

Capability and Performance

Fable 5 is positioned as Anthropic's most capable generally available model, showing state-of-the-art performance in complex reasoning, software engineering, and scientific research.

  • Software Engineering: In a test by Stripe, Fable 5 completed a codebase-wide migration in a 50-million-line Ruby codebase in one day, a task estimated to take a human team over two months.
  • Scientific Research: Mythos 5 has demonstrated the ability to generate novel scientific hypotheses, with experts preferring its molecular biology hypotheses 80% of the time in blinded comparisons. It also successfully conducted autonomous genomics research, outperforming a published model despite being 100 times smaller.
  • Vision and Memory: Fable 5 exhibits strong vision capabilities, such as rebuilding web app source code from screenshots, and utilizes persistent file-based memory to maintain focus over long-context tasks.

Safeguards and Deployment

To manage the risks associated with such high-capability models, Anthropic employs a system of separate AI classifiers for Fable 5.

  • Fallback Mechanism: When the classifiers detect potential misuse in cybersecurity, biology, or chemistry, the request is automatically routed to Claude Opus 4.8. This fallback occurs in less than 5% of sessions, meaning 95% of users experience the full capability of the model without interruption.
  • Mythos 5 Access: Mythos 5 is the unrestricted version of the model, with cyber-related safeguards removed. It is currently available only through "Project Glasswing," a limited-access program conducted in collaboration with the US government. Anthropic also plans a trusted access program to provide approved researchers with access to Fable 5 without biological and chemical safeguards.