What's new in Claude Fable 5.1
Claude Fable 5.1 and Mythos 5.1 have been released with improvements in agentic coding, research capabilities, and cost-efficient cache reads. The update introduces breaking changes regarding tool use and thinking blocks, requiring developers to adjust their integration strategies.
Why it matters
These updates represent significant shifts in how developers interact with high-end AI models, impacting the cost and architectural design of agentic software applications.
Overview of new features, breaking changes, and capability improvements in Claude Fable 5.1 and Claude Mythos 5.1.
Claude Fable 5.1 extends Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost, and brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work. For most workloads, start with Claude Opus 5 (see Choosing a model ). Use Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Claude Opus 5 at higher effort still fall short. Claude Mythos 5.1 offers the same capabilities to Project Glasswing participants only.
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