Anthropic Ships Claude Fable 5.1, More Than Doubling Its Predecessor on Key Benchmark

In brief

• Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, scoring 52.6% on Terminal-Bench-Science 0.1 against Fable 5's 24.7%, and 55.8% on Terminal-Bench 4.0 against 42.0%.
• Cache reads now cost 75% less, cutting typical workload costs by roughly 25% and highly agentic workloads by up to 45%, with base pricing unchanged at $10/$50 per million tokens.
• Fable 5.1 is not included in Pro plans or standard Team seats, which run it on usage credits; Max and premium Team or Enterprise seats get it included for up to 50% of weekly limits.

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on Tuesday, its first update to the Mythos-class model line since Fable 5 launched on June 9.

The company claims new AI models are the world's “most advanced for coding and knowledge work,” and it lands three months into a launch cycle that has already included an 18-day export-control shutdown, a mid-summer pricing fight over subscription access, and a July release of Claude Opus 5 that undercut Fable 5 on price.

Fable 5.1 and Mythos 5.1 are, per Anthropic, the same underlying model with different safety filters bolted on. Fable 5.1 is generally available to anyone with a Claude account. Mythos 5.1 stays restricted to vetted cybersecurity and life-sciences professionals through Anthropic's Cyber Verification Program and Life Sciences Verification Program, the successor to the Project Glasswing access track that gated Mythos 5.