... Fable 5.1 is here and it builds smallworlds Anthropic s newest model just went live in the Happycapy model picker. The benchmark story is that it more than doubled its predecessor on frontier science. The better story is what it does with a free afternoon. Fable 5.1, Anthropic s newest model, went live on September 1, 2026, and it s in Happycapy today. It s the latest model in the Claude 5 family and part of the Mythos-class ... ... scientific computing) 52.6% Fable 5: 24.7% Opus 5: 29.0% Context window 1,000,000 tokens hold a whole codebase at once Max output 128,000 tokens a full build in one pass Prompt-cache reads up to 75% cheaper