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Claude Fable 5.1: AI Large Language Models

Generate text with Claude Fable 5.1. Anthropic's flagship model with adaptive thinking, 1M context. Try free — pay per generation, no subscription. Claude Fable 5.1 is Anthropic's latest flagship model with adaptive extended thinking always enabled, a 1M-token context window, and support for long-horizon agentic coding, multistep research, and document/spreadsheet/slide analysis. Supports text and image inputs. Token-based pricing: $10.00/M input tokens, $50.00/M output tokens. Two endpoints: standard async (/claude-fable-5-1) and live streaming (/claude-fable-5-1/stream) via SSE.

Interactive model controls

Claude Fable 5.1 is Anthropic's latest flagship model with adaptive extended thinking always enabled, a 1M-token context window, and support for long-horizon agentic coding, multistep research, and document/spreadsheet/slide analysis. Supports text and image inputs. Token-based pricing: $10.00/M input tokens, $50.00/M output tokens. Two endpoints: standard async (/claude-fable-5-1) and live streaming (/claude-fable-5-1/stream) via SSE.

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Overview

About this model

Claude Fable 5.1 is Anthropic's newest flagship model, built for long-horizon agentic coding, multistep research, and document, spreadsheet, and slide work. It runs with adaptive extended thinking always enabled (steerable via effort, from low up to max) and a 1 million token context window with up to 128K tokens of output. It accepts both text and image inputs — charts, tables, PDFs, and screenshots — for multimodal analysis, and is a step up in reasoning depth from the earlier Claude Fable 5. Pricing is token-based: $10.00 per million input tokens and $50.00 per million output tokens.

1Agentic Coding: Plan and execute long-horizon coding tasks that span many files, tools, and steps without losing track of the goal.
2Multistep Research: Chain together searches, document reads, and analysis across a 1M-token context without re-summarizing earlier steps.
3Document & Spreadsheet Work: Read and reason over long PDFs, spreadsheets, and slide decks, including scanned pages and screenshots.
4Scientific Analysis: Work through multi-step scientific or technical problems that benefit from always-on extended thinking.
5Multimodal Analysis: Combine image and text inputs to extract insights from charts, tables, diagrams, and screenshots.
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Pricing & Value

Cost analysis

muapiapp$10.00/M input tokens, $50.00/M output tokens

Token-based billing at Anthropic's official published rate. Minimum $0.0008 per call.

Fal.aiNot available

Claude Fable 5.1 is not available on Fal.ai.

ReplicateNot available

Claude Fable 5.1 is not available on Replicate.

** Competitor pricing is estimated based on similar model architectures and usage tiers.

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Technical Details

Configuration schema

Promptstring

The user message or instruction.

Default ValueExplain quantum entanglement in simple terms.
Image URLstring

Optional image URL for multimodal requests.

Default Valueundefined
System Promptstring

Optional system-level instruction to guide model behavior.

Default ValueYou are a concise and precise assistant.
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Implementation Guide

Developer documentation

Standard (Async)

POST /api/v1/claude-fable-5-1 — returns request_id, poll for the result via /api/v1/predictions/{id}/result.

curl -X POST https://api.muapi.ai/api/v1/claude-fable-5-1 \
  -H "x-api-key: YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "prompt": "Summarize the key risks in this quarterly report and list follow-up questions.",
    "image_url": "https://example.com/report-page-1.png",
    "system_prompt": "You are a meticulous financial analyst."
  }'

Streaming (SSE)

POST /api/v1/claude-fable-5-1/stream — returns a live Server-Sent Events stream (Content-Type: text/event-stream). Each chunk has the format data: {"choices":[{"delta":{"content":"text"}}]}, ending with data: [DONE].

See Streaming Documentation for full code examples.

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Common Questions

Frequently asked

What is Claude Fable 5.1?

Claude Fable 5.1 is Anthropic's newest flagship language model, built for long-horizon agentic coding, multistep research, and document, spreadsheet, and slide analysis, with a 1M-token context window and always-on adaptive extended thinking.

How is pricing calculated?

Pricing is token-based: $10.00 per million input tokens and $50.00 per million output tokens. The minimum charge per call is $0.0008. Actual cost is deducted after each call based on token counts returned by the model.

What is the difference between /claude-fable-5-1 and /claude-fable-5-1/stream?

/claude-fable-5-1 is async — you receive a request_id and poll for the result. /claude-fable-5-1/stream returns a live SSE stream. Use streaming for chat UIs; use the async endpoint for workflows and automation.

Does Claude Fable 5.1 support images?

Yes. Pass an image_url in the request body to include an image — such as a chart, table, PDF page, or screenshot — as part of the user message for multimodal analysis.

How is Claude Fable 5.1 different from Claude Fable 5?

Claude Fable 5.1 adds a larger 1M-token context window, always-on adaptive extended thinking (steerable via effort), and stronger long-horizon agentic and research performance compared to [Claude Fable 5](/playground/claude-fable-5).

Can I control how much the model 'thinks' before responding?

Extended thinking is always enabled and cannot be turned off, but its depth is adaptive to the task — harder, multistep requests automatically get more reasoning effort.