What is Claude Fable 5.1 and Mythos 5.1?

Large language models are computer programs trained on vast amounts of text so they can work with human language: writing, summarizing, translating and analyzing it. One goal in this field is getting such a model to carry out long, multi-step tasks that need careful reasoning rather than a single quick answer. Claude Fable 5.1 is a model in Anthropic's Claude family, which Anthropic describes as state-of-the-art large language models. Anthropic's documentation says to use Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, meaning tasks where the model takes many steps toward a goal. It accepts text and image input, produces text output and can use tools, meaning it can call outside software to help with a task.

Written from primary sources and reference pages, and checked against them by an independent AI fact-checker. Drafted with AI assistance.