Anthropic releases Claude Fable 5.1 and Mythos 5.1

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on 1 September 2026. Both names refer to the same underlying model; the difference is safeguard level.

Headline pricing is unchanged from Fable 5 at $10 per million input tokens and $50 per million output. Cache reads dropped 75% to $0.25 per million, down from $1.

Anthropic estimates Fable 5.1 costs about 25% less than Fable 5 for typical workloads and up to approximately 45% less for highly agentic work. The figures come from the company's own four-week usage measurement in August 2026.

OpenAI launched GPT-6 Astra two days later at the same $10/$50 headline rates without a comparable cache-read discount.

Anthropic describes the models as the world's most advanced for coding and knowledge work. At Low or Medium effort, Fable 5.1 achieves results similar to or better than Fable 5 at much lower cost; higher effort tiers deliver materially more performance. Defaults are High in Claude Code and Medium in Claude Cowork and on Claude.ai.

On three targets in Adaptyv Bio's protein design competitions, Anthropic reports binding affinities ten times higher than the best submitted designs.

The model is generally available as claude-fable-5-1 across the Claude API, Amazon Web Services, Google Cloud and Microsoft Azure, and in Claude, Claude Code and Cursor. Mythos 5.1 is available through two trusted access programmes aimed at vetted cybersecurity and life-sciences organisations.

Anthropic notes that production safeguards can affect scores, and that its August 2026 OSWorld task release is not directly comparable with some previously published results.

Under Fable's production safeguards, requests flagged by classifiers relating to cybersecurity, biology and chemistry, or model distillation are handled by the less capable Claude Opus instead of Fable.

Anthropic publicly released Claude Fable 5 in June 2026 alongside the restricted Claude Mythos 5. The 5.1 release follows the same two-track structure.

Some coverage dates the launch to 2 September, reflecting announcement versus publication timing; Anthropic's own materials point to 1 September.