Claude Fable 5.1 is Anthropic's most capable generally available model, built for demanding reasoning and long-horizon agentic work. On Magnus it is the brain for the jobs where Opus 5.5 still falls short, and the model Genius mode now pins every Claude agent to.
Released September 1, 2026 by Anthropic. Fable 5.1 has a 1M-token context window and adaptive thinking that is always on. It sits at the top of Anthropic's lineup, above Opus 5.5, at $10 per million input tokens and $50 per million output tokens.
On the All-inclusive plan a Fable 5.1 reply costs 5 credits, the same as Opus 5.5 and GPT-6 Astra.
Give Fable 5.1 the work that has to be right: multi-step research, long documents, spreadsheets and plans with many moving parts. It thinks longer than Opus 5.5, so replies take a little more time to start.