Claude Fable 5.1 API

Claude Fable 5.1 is Anthropic's most capable model for demanding reasoning and long-horizon agentic work. Features a 1M token context window, always-on adaptive thinking, and stronger long-running agentic coding, multistep research and document work than Claude Fable 5.

Modalities: TextText
In / Out price: $10/$50per 1M
Context window: 1M tokens
Released: Sep 1, 2026

Claude Fable 5.1 overview

Claude Fable 5.1 by Anthropic

Claude Fable 5.1 is Anthropic's most capable model for demanding reasoning and long-horizon agentic work.

Claude Fable 5.1 specs

Model ID: anthropic/claude-fable-5.1
Developer: Anthropic
Context window: 1,000,000 tokens
Max output: 128,000 tokens
Input: Text
Output: Text
API: OpenAI-compatible, base URL https://api.aimlapi.com/v1

What Is Claude Fable 5.1 API?

Claude Fable 5.1 API is Anthropic's most capable model for demanding reasoning and long-horizon agentic work, available through AI/ML API on the standard chat completions endpoint. It carries a 1M token context window, always-on adaptive thinking, and stronger long-running agentic coding, multistep research and document work than Claude Fable 5. The model was released on September 1, 2026.

Always-On Adaptive Thinking

Adaptive thinking is always on rather than being a mode you opt into per request. The model allocates more internal deliberation to harder steps on its own, which suits work where the difficulty of each step is not known in advance.

Long-Horizon Agentic Work

Anthropic positions this release specifically around tasks that run for a long time without supervision: agentic coding sessions, multistep research, and document work that spans many turns. These are the areas where it is described as stronger than Claude Fable 5.

Large Context Window

The model accepts up to 1,000,000 tokens of input and returns up to 128,000 tokens of output. That input budget is large enough to hold entire repositories, long document sets or extended agent transcripts in a single request, without an external retrieval step.

Agentic Coding

Long-running coding sessions where the model plans, edits and verifies across many steps rather than answering a single prompt.

Multistep Research

Research tasks that require gathering, comparing and reconciling information across many sources before producing a conclusion.

Document Work

Processing large document sets end to end, where the 1M token window removes the need to chunk material and stitch partial answers back together.

Use cases

Claude Fable 5.1 is documented as suited to long-running agentic coding, knowledge work, and research.

It is also described as a model for ambitious coding, long-horizon agents, and enterprise knowledge work, including software projects that span an entire codebase and multi-hour jobs across many tools.

The model is further documented for multistep research and document, spreadsheet, and slide work.

It is also listed as useful for scientific research, financial analysis, and vision tasks.

Capabilities

Streaming

Claude Fable 5.1 benchmarks by Artificial Analysis

See how Claude Fable 5.1 compares in intelligence, generation speed and cost per task. Data retrieved Nov 3, 2026.

Intelligence#9 / 5049Index v5.0 Speed#148 / 21969.6Output tokens / second Cost per task#44 / 44$12.99USD / benchmark task Humanity's Last Exam#2 / 45859.1%Score

Intelligence

Index v5.0

Speed

Output tokens / second

Cost per task

USD / benchmark task

Benchmark results

Compare 8 models available on AI/ML API across 5 independent benchmarks.

Humanity's Last Exam

Expert-written questions across academic subjects.

Claude Fable 5.1 API pricing

Claude Fable 5.1 pricing in US dollars
Usage	Price	Unit
Input	$10	1M tokens
Output	$50	1M tokens
Cached input	$0.25	1M tokens

Prices from the AI/ML API price list, checked on Nov 4, 2026.

Claude Fable 5.1 vs other models: token prices

AI/ML API input and output rates for 5 models, per million tokens.

AI/ML API model prices in USD per million tokens
Model	Input	Output
Claude Fable 5.1	$10	$50
Claude Haiku 5.5	$0.1	$0.5
Claude Sonnet 5.5	$2	$10
Claude Opus 5.5	$4	$20
Claude Opus 5	$5	$25

How to use the Claude Fable 5.1 API

View docs

Build with your agent

Give your agent the model details, code and documentation.

1Get your API key

Create API key

export AIMLAPI_KEY="8f2b4c7d9a1e6b3f…"

2Make your first request

curl 'https://api.aimlapi.com/v1/chat/completions' \
  -H "Authorization: Bearer $AIMLAPI_KEY" \
  -H 'Content-Type: application/json' \
  -d '{
  "model": "anthropic/claude-fable-5.1",
  "messages": [
    {
      "role": "user",
      "content": "Write a haiku about the ocean."
    }
  ]
}'

import os
import requests

headers = {
    "Authorization": f"Bearer {os.environ['AIMLAPI_KEY']}",
    "Content-Type": "application/json",
}
payload = {
    "model": "anthropic/claude-fable-5.1",
    "messages": [
        {
            "role": "user",
            "content": "Write a haiku about the ocean."
        }
    ]
}

response = requests.post("https://api.aimlapi.com/v1/chat/completions", headers=headers, json=payload, timeout=120)
response.raise_for_status()
print(response.json())

const response = await fetch("https://api.aimlapi.com/v1/chat/completions", {
  method: "POST",
  headers: {
    Authorization: `Bearer ${process.env.AIMLAPI_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "model": "anthropic/claude-fable-5.1",
    "messages": [
      {
        "role": "user",
        "content": "Write a haiku about the ocean."
      }
    ]
  }),
});

if (!response.ok) throw new Error(`HTTP ${response.status}`);
console.log(await response.json());

All posts

Claude Fable 5.1 FAQ

What is the context window of Claude Fable 5.1?

Claude Fable 5.1 has a 1,000,000 tokens context window and can return up to 128,000 tokens.

What inputs and outputs does Claude Fable 5.1 support?

Claude Fable 5.1 takes text as input and returns text.

What is the model ID for Claude Fable 5.1?

Use anthropic/claude-fable-5.1 as the model id. Requests go to https://api.aimlapi.com/v1/chat/completions.

When was Claude Fable 5.1 released?

Claude Fable 5.1 became available on September 1, 2026.

How much does Claude Fable 5.1 cost?

Claude Fable 5.1 costs $10 input and $50 output per 1M tokens on AI/ML API, billed as you go with no subscription.

Does Claude Fable 5.1 support streaming?

Yes, Claude Fable 5.1 can stream responses as they are generated.

Who created Claude Fable 5.1?

Claude Fable 5.1 was built by Anthropic.

What tasks is Claude Fable 5.1 designed for?

It is built for demanding reasoning and long-horizon agentic work, including coding, multistep research, and document tasks.

Start building with Claude Fable 5.1

Get API key

Claude Fable 5.1 API

anthropic/claude-fable-5.1

Build with your agent

Give your agent the model details, code and documentation.

1Get your API key

Create API key

export AIMLAPI_KEY="8f2b4c7d9a1e6b3f…"

2Make your first request

curl 'https://api.aimlapi.com/v1/chat/completions' \
  -H "Authorization: Bearer $AIMLAPI_KEY" \
  -H 'Content-Type: application/json' \
  -d '{
  "model": "anthropic/claude-fable-5.1",
  "messages": [
    {
      "role": "user",
      "content": "Write a haiku about the ocean."
    }
  ]
}'

import os
import requests

headers = {
    "Authorization": f"Bearer {os.environ['AIMLAPI_KEY']}",
    "Content-Type": "application/json",
}
payload = {
    "model": "anthropic/claude-fable-5.1",
    "messages": [
        {
            "role": "user",
            "content": "Write a haiku about the ocean."
        }
    ]
}

response = requests.post("https://api.aimlapi.com/v1/chat/completions", headers=headers, json=payload, timeout=120)
response.raise_for_status()
print(response.json())

const response = await fetch("https://api.aimlapi.com/v1/chat/completions", {
  method: "POST",
  headers: {
    Authorization: `Bearer ${process.env.AIMLAPI_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "model": "anthropic/claude-fable-5.1",
    "messages": [
      {
        "role": "user",
        "content": "Write a haiku about the ocean."
      }
    ]
  }),
});

if (!response.ok) throw new Error(`HTTP ${response.status}`);
console.log(await response.json());

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Claude Fable 5.1 API

Claude Fable 5.1 overview

Claude Fable 5.1 by Anthropic

Claude Fable 5.1 specs

What Is Claude Fable 5.1 API?

Always-On Adaptive Thinking

Long-Horizon Agentic Work

Large Context Window

Agentic Coding

Multistep Research

Document Work

Use cases

Capabilities

Claude Fable 5.1 benchmarks by Artificial Analysis

Intelligence

Speed

Cost per task

Benchmark results

Humanity's Last Exam

GPQA Diamond

Terminal-Bench 4.0

AA-LCR

SciCode

Claude Fable 5.1 API pricing

Claude Fable 5.1 vs other models: token prices

How to use the Claude Fable 5.1 API

Build with your agent

1Get your API key

2Make your first request

Related articles

Claude Fable 5.1 FAQ

Start building with Claude Fable 5.1

Claude Fable 5.1 API

Build with your agent

1Get your API key

2Make your first request

Claude Fable 5.1 API

Claude Fable 5.1 overview

Claude Fable 5.1 by Anthropic

Claude Fable 5.1 specs

What Is Claude Fable 5.1 API?

Always-On Adaptive Thinking

Long-Horizon Agentic Work

Large Context Window

Agentic Coding

Multistep Research

Document Work

Use cases

Capabilities

Intelligence

Speed

Cost per task

Benchmark results

Humanity's Last Exam

GPQA Diamond

Terminal-Bench 4.0

AA-LCR

SciCode

Claude Fable 5.1 API pricing

Claude Fable 5.1 vs other models: token prices

How to use the Claude Fable 5.1 API

Build with your agent

1Get your API key

2Make your first request

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