Claude Fable 5.1 is Anthropic’s most capable generally available model, released on 1 September 2026 as an update to Claude Fable 5. Seg\u00fan Anthropic<\/a>, it is aimed at demanding reasoning and long-running agentic work: coding, multistep research, and document, spreadsheet and slide tasks. Claude Mythos 5.1 is the same model with different safeguards, offered only to organisations in Anthropic’s verification programmes.<\/p>\n API pricing is the same as Fable 5: $10 per million input tokens and $50 per million output tokens. Cache reads fell 75% to $0.25 per million, which Anthropic estimates makes typical workloads about 25% cheaper. It has a 1M-token context window and up to 128K output tokens, and runs on the Claude API as claude-fable-5-1 and on AWS, Google Cloud and Microsoft Azure.<\/p>\n For most work Anthropic now recommends starting with the cheaper Claude Opus 5.5<\/a> ($4 \/ $20) and moving to Fable 5.1 only when that falls short. Launch details are in our Fable 5.1 release story<\/a>, and the Calculadora de costos de API de IA<\/a> compares monthly costs.<\/p>","protected":false},"excerpt":{"rendered":" What is Claude Fable 5.1? 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