Claude Fable 5.1

Claude Fable 5.1 is the highest-priced model in Anthropic's current lineup, intended for demanding reasoning and long-horizon agentic work. Anthropic describes Fable 5.1 and Claude Mythos 5.1 as the same model with different levels of safeguards, and Mythos 5.1 is limited to organizations in Anthropic's verification programs. It reads text, images and PDFs across a 1M-token context window.

By AnthropicReleased September 1, 2026Data checked October 10, 2026

CurrentClosedFrontier

Specifications

Context window: 1Mabout 750,000 words, or 1,500 pages
Max output: 125Ktokens per response
Input price: $10per 1M tokens
Output price: $50per 1M tokens
Cached input: $0.25per 1M tokens
Weights: ClosedProprietary
Parameters: Undisclosed
Input types: Text, Images, PDF
Output types: Text
Reasoning mode: Yes
Tool calling: Yes
Released: Sep 1, 2026Current

Standard rates; the full 1M-token window is billed at the same rate. Batch API 50% off. Cache writes cost $12.50 (5-minute) or $20 (1-hour) per 1M tokens. US-only inference adds 1.1x.

What it costs in practice

Workload	Monthly cost at list price
Support chatbot30,000 conversations a month, 2,000 input and 400 output tokens each	$1,200
Document summaries2,000 documents a month, 12,000 input and 800 output tokens each	$320
Coding agent1,000 tasks a month, 60,000 input and 6,000 output tokens each	$900

Standard rates, no caching or batch discounts. Run your own numbers.

Strengths

1M-token context window and 128K-token output limit
Adaptive thinking is always on, with high effort by default
Cache reads cost $0.25 per million tokens, a quarter of Claude Fable 5's rate

Trade-offs to weigh

$10 input and $50 output per million tokens, and Anthropic rates its latency as slower than the other current Claude models
Safeguards redirect some cybersecurity and life-sciences requests, such as exploit development, to Opus models

Where it fits

Anthropic positions Claude Fable 5.1 for:

reasoningagentscodingresearchlong documents

Where you can use it

API
Claude apps
Amazon Bedrock
Google Vertex AI
Microsoft Azure AI Foundry
Claude Platform on AWS

Alternatives to consider

Model	Tier	Context	Input / output per 1M
GPT-6 AstraOpenAI	Frontier	1.05M	$10 / $50	Compare
Gemini 3.1 ProGoogle	Frontier	1M	$2 / $12	Compare
Kimi K3Moonshot AI	Frontier	1M	$3 / $15	Compare
Claude Opus 5.5Anthropic	Frontier	1M	$4 / $20	Compare

Frequently asked questions

How much does Claude Fable 5.1 cost?

Claude Fable 5.1 lists at $10 per million input tokens and $50 per million output tokens, with cached input at $0.25 (checked October 10, 2026). Standard rates; the full 1M-token window is billed at the same rate. Batch API 50% off. Cache writes cost $12.50 (5-minute) or $20 (1-hour) per 1M tokens. US-only inference adds 1.1x. Confirm current rates on the provider's pricing page before you commit.

What is the context window of Claude Fable 5.1?

Claude Fable 5.1 accepts up to 1,000,000 tokens of context (about 750,000 words, or 1,500 pages), and it can write up to 125K tokens in a single response. Long prompts cost more and can be slower, so most apps send only what the task needs.

Is Claude Fable 5.1 open source?

No. Claude Fable 5.1 is a proprietary model from Anthropic; its weights aren't published. You can use it through the Anthropic API and through Claude apps, Amazon Bedrock, Google Vertex AI, Microsoft Azure AI Foundry, Claude Platform on AWS.

What is Claude Fable 5.1 best for?

Anthropic positions Claude Fable 5.1 for reasoning, agents, coding, research. It accepts text, images, pdf as input, has a reasoning (thinking) mode and supports tool calling. Test it on a sample of your own tasks before you choose it for production.

Sources

Facts checked against these pages on October 10, 2026. Spot an error? Tell us.

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