AI model releases · Anthropic

Claude Fable 5.1: Anthropic's top coding model, now cheaper to run

Anthropic has released Claude Fable 5.1, which it calls its most advanced model for coding and knowledge work. It launched alongside Claude Mythos 5.1. Anthropic says the two are the same underlying model with different levels of safeguards.

Fable 5.1 is generally available. Mythos 5.1 is limited to trusted access programs, and its safeguards are designed for cybersecurity and life sciences work. Anthropic says Fable 5.1 can now be used to find software vulnerabilities, though not to develop exploits for them.

On price, Anthropic says Fable 5.1 will cost an estimated 25% less than Fable 5 for typical workloads billed by token, because cache read pricing is lower. For highly agentic work, it says savings can reach approximately 45%. Anthropic also says its newest cybersecurity safeguards block 60% fewer false positives than before.

In Anthropic's own benchmark table, Fable 5.1 scores 52.6% on Terminal-Bench-Science 0.1 against 24.7% for Fable 5, and 73.4% on CursorBench 3.2.0 against 70.5%. Anthropic says Fable 5.1 at Low or Medium effort matches or beats Fable 5 results at a much lower cost.

Key facts

  • Released alongside Claude Mythos 5.1, which is the same model with different safeguards.
  • Fable 5.1 is generally available; Mythos 5.1 is limited to trusted access programs.
  • Estimated 25% lower cost than Fable 5 for typical workloads, up to about 45% for agentic work.
  • Cybersecurity safeguards block 60% fewer false positives than before, per Anthropic.
  • Defaults to High effort in Claude Code and Medium in Claude Cowork and Claude.ai.
  • Enterprise Frontier Safeguards roll out in phases from later this fall; eligible customers can use zero data retention until then.

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