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Claude Fable 5.1

AnthropicLarge Language ModelProprietary

Anthropic's most capable widely available model, released September 2026 as the successor to Claude Fable 5. Built for long-running agentic coding, multistep research, and 1M-context work, with cache reads 75% cheaper than Fable 5 and built-in text watermarking with C2PA content credentials.

Abilities

Text GenerationCode GenerationReasoningMathVisionFunction CallingStructured OutputLong ContextMultilingual

Use Cases

Coding AssistantResearchEnterpriseData AnalysisContent Writing

Available in Tools

Claude Codehttps://docs.anthropic.com/en/docs/claude-code
Claude Desktophttps://claude.ai/download
Claude.aihttps://claude.ai

Availability

APIhttps://console.anthropic.com
Claude Webhttps://claude.ai
Claude Mobile
Amazon Bedrock
Google Vertex AI
Microsoft Foundry

How to Use

Use the Anthropic Messages API with model ID `claude-fable-5-1`. Adaptive thinking is always on. Note the breaking changes vs Fable 5: forced tool use (`tool_choice: any/tool`) returns an error, and thinking blocks are readable only by 5.1 itself. Requires 30-day data retention (no zero-data-retention option).

Pros

  • Anthropic's most capable widely available model — strongest at long-running agentic coding and multistep research
  • Cache reads at $0.25 per MTok — 75% cheaper than Fable 5, a major saving for agentic loops
  • 1M token context window with 128K max output — processes entire codebases and book-length documents
  • Excellent at documents, spreadsheets, and slides generation plus vision tasks
  • Built-in text watermarking and C2PA content credentials for provenance
  • New API features: per-message effort, turn-scoped system messages, and streaming progress updates (beta)

Cons

  • Most expensive widely available Claude model at $10/$50 per MTok
  • Breaking API changes vs Fable 5 — forced tool use returns an error and thinking blocks are non-portable
  • Requires 30-day data retention — not available under zero-data-retention agreements
  • Slower response times than Sonnet/Haiku due to deeper reasoning
  • Closed source — no model weights or training details available

What to Use It For

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Perfect For

Long-running autonomous coding and agentic tasks

Built for long-horizon work with cheap cache reads that make extended agentic loops far more economical

Multistep research across large document sets

1M context plus top-tier reasoning for synthesizing across many long sources

Generating documents, spreadsheets, and slides

Officially highlighted strength of the 5.1 release for office-document workflows

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Good For

Vision and document intelligence

Excellent at extracting structured data from images, PDFs, and complex visual documents

Computer use automation

Strong computer-use capabilities carried over and improved from Fable 5

warning

Not Recommended

High-volume batch processing

At $10/1M input it is too expensive for bulk classification or extraction pipelines

Try instead: Claude Haiku 4.5, Claude Sonnet 5

Workloads requiring forced tool choice

`tool_choice: any/tool` returns an error on 5.1 — pipelines relying on forced tool use need Fable 5 or Opus 5

Try instead: Claude Fable 5, Claude Opus 5

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Do Not Use For

Image generation

Text and vision LLM only — no image generation capabilities

Try instead: GPT Image 2.5, FLUX.2 Pro

Self-hosting or offline use

Closed source cloud-only model — completely dependent on the Anthropic API

Try instead: Llama 4 Maverick, Qwen 3

Technical Details

Pricing$10 / 1M input tokens, $50 / 1M output tokens (cache reads $0.25 / 1M)
ParametersUndisclosed
detail.contextWindow1M tokens (128K max output)

Links

language Website description API Documentation