Overview

Claude Fable 5.1 extends Claude Fable 5 at the same prices, with cheaper cache reads and stronger long-running agentic coding, multistep research, and document, spreadsheet and slide work. Anthropic suggests using it when evaluations of Claude Opus 5.5 at higher effort still fall short. Claude Mythos 5.1 offers the same capabilities to organizations verified through Anthropic’s programs.

Highlights

  • 1M-token context window
  • Up to 128K output tokens
  • Adaptive thinking, always on, with high effort by default
  • Text and image input
  • Cache reads at $0.25 per 1M tokens

Typical uses

Hard reasoning problems, long autonomous agent runs and complex research.

Things to consider

  • Slower and more expensive than Claude Opus 5.5.
  • Forced tool use returns an error, and thinking blocks are tied to the model that produced them.

Specifications are taken from the provider’s official announcement, documentation or model card. Benchmark results depend on evaluation settings; see how AI benchmarks work.