Anthropic releases Claude Fable 5.1 and Mythos 5.1

Anthropic/Claude models

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1: the same model with different safeguards. Fable 5.1 is generally available; Mythos 5.1, with looser limits on cyber and biology work, goes only to vetted US organisations in defensive security and life sciences. Base prices stay at $10/$50 per million tokens, but cheaper cache reads cut typical bills by about 25%.

• Terminal-Bench 4.0: 55.8% for Fable 5.1 vs 42.0% for Fable 5; Mythos 5.1 reaches 60.9%
• Terminal-Bench-Science 0.1 more than doubles, 52.6% vs 24.7%; OSWorld 2.0 41.7% vs 36.1%
• Cache reads fall 75% to $0.25 per million tokens; savings up to ~45% on heavily agentic work
• New safeguards block 60% fewer false positives in cybersecurity and fire 85% less often on benign medical questions
• Mythos access runs through a Cyber Verification Program and a Life Sciences Verification Program built with the US government