Claude Fable 5.1

Name: Claude Fable 5.1
Author: Anthropic

by Anthropic

APIAvailable through a hosted API — pay per token, no self-hosting required

Released September 20261000K context≈ 750,000 words

Claude Fable 5.1 is the latest iteration of Anthropic's most capable widely available model. It retains the always-on extended thinking and million-token context window of Fable 5 while delivering measurably stronger reasoning, coding, and instruction-following. Cache-read pricing drops 75 percent to $0.25 per million tokens, making long-context agentic workflows significantly cheaper. Fable 5.1 also reduces false refusals, following nuanced instructions more reliably than its predecessor.

Claude Fable 5.1

by Anthropic

APIAvailable through a hosted API — pay per token, no self-hosting required

Released September 20261000K context≈ 750,000 words

Claude Fable 5.1 is the latest iteration of Anthropic's most capable widely available model. It retains the always-on extended thinking and million-token context window of Fable 5 while delivering measurably stronger reasoning, coding, and instruction-following. Cache-read pricing drops 75 percent to $0.25 per million tokens, making long-context agentic workflows significantly cheaper. Fable 5.1 also reduces false refusals, following nuanced instructions more reliably than its predecessor.

Glossary

AgenticA model designed to act autonomously by making decisions, selecting actions, and using tools to accomplish multi-step tasks.Agentic WorkflowsProcesses where a model autonomously plans and executes multiple steps or tool calls to accomplish a goal, rather than responding to a single prompt.Context WindowThe maximum number of tokens a model can process in a single conversation or prompt.Extended ThinkingA reasoning technique where a model works through a problem step-by-step internally before providing an answer, improving accuracy on complex tasks.Instruction-FollowingThe ability of a model to understand and execute specific tasks or commands given in natural language prompts.Long-ContextThe ability of a model to process and understand very long sequences of text while maintaining coherence across distant parts of the input.ReasoningThe model's ability to work through multi-step logical problems and provide justified answers rather than just pattern-matching.TokenA small unit of text (a word, subword, or punctuation mark) that a language model breaks input into for processing.TokensThe basic units of text that a language model processes, typically representing words or word fragments.