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Anthropic: Claude Fable 5.1
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Anthropic: Claude Fable 5.1

Anthropic's frontier model for multi-hour autonomous agents, whole-codebase coding, and long-horizon knowledge work.

Context window: 
1000000
M tokens
Output: 
128000
 tokens
Modality:
Text
Image
PDF
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Claude Fable 5.1: Anthropic's Frontier Model for Long-Horizon Coding Agents Claude Fable 5.1 is Anthropic's most capable generally available model, sitting at the top of the Claude lineup above Opus, Sonnet, and Haiku. It's a Mythos-class model built for ambitious, long-running work: features that span an entire codebase, code review, and multi-hour autonomous agent sessions across many tools. Fable 5.1 improves most on agentic coding, long-running workflows, and knowledge work, and adds vision to interpret charts, diagrams, and dense PDFs. It suits teams running unattended agents rather than everyday chat or drafting, where lighter Claude models are more economical. Start building with Claude Fable 5.1 via the AnyAPI.ai API.

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Anthropic: Claude Fable 5.1

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Answers to common questions about integrating and using this AI model via AnyAPI.ai

Claude Fable 5.1 has a 1,000,000-token context window, which is both the default and maximum, billed at standard per-token rates across the entire window. Maximum output is 128,000 tokens per response. Unlike Opus 5 and Sonnet 5, it is not listed for the Batch API 300K extended-output beta.

Yes, but reasoning cannot be turned off. Fable 5.1 uses adaptive thinking that is always on, unlike earlier Claude models where extended thinking was opt-in. You control depth via an effort parameter with five levels — low, medium, high, xhigh, and max — with a default of high, trading latency and cost for reasoning depth.

Anthropic positions Fable 5.1 as leading Opus 5 on every benchmark it published, and independent testing found Fable 5.1 solved shared coding tasks with 58% fewer output tokens and 36% faster than Opus 5. However, Fable 5.1 sits higher on list price. Opus 5 is the more economical default for everyday reasoning and chat.

Fable 5.1 includes blocking classifiers for dual-use cybersecurity and life-sciences content, and Anthropic notes refusal rates are materially higher than earlier Claude models. Blocked requests return HTTP 200 with stop_reason: "refusal". Many bio- and cyber-adjacent queries are rerouted to Opus models. Developers should handle refusal as a primary response path.

Claude Fable 5.1 accepts text, image, and file inputs, including PDFs, and returns text output. Its vision is designed for dense filings, charts, diagrams, and tables nested in documents, and it can use vision to check its own coding outputs against a design or goal. It does not generate images or audio.