Claude Mythos 5.1

Claude Mythos 5.1 shipped alongside Claude Fable 5.1 on September 1, 2026, and the first thing to understand about it is that most developers cannot buy it. Its published rate is $10 per million input tokens and $50 output, identical to Fable 5.1, with cache reads at $0.25, a five-minute cache write at $12.50, a one-hour write at $20, and Batch at $5 and $25. That is a list price, not an order form. Mythos-class access runs through Anthropic's Cyber Verification Program and Life Sciences Verification Program, both built in coordination with the US government, and is limited to registered partners doing defensive security or life sciences research. Anthropic says it plans to broaden eligibility over time and is extending Project Glasswing to roughly 150 new organizations across more than fifteen countries. Specs otherwise track the Fable line: a 1 million token context window, up to 128K output tokens, and the same long-horizon coding, multistep research, and dense document targets. If you are comparing frontier options for a normal production workload, compare Fable 5.1 and treat Mythos 5.1 as a reference row until your organization is enrolled.