Claude Fable 5.1

by September 1, 20261M context128K output

Claude Fable 5.1 is the next release in Anthropic’s Fable line, aimed at long-horizon coding, agentic execution, and multimodal research. It reads text and images, calls tools, and supports extended thinking, with a 1M-token context window and a 128K-token maximum output for work that has to hold an entire project in view. It fits document-heavy analysis, sustained agent runs, and engineering tasks that span many files.

Claude Fable 5.1 is served through Runware's compatible endpoints, which take another provider's request format. Point the SDK you already use at Runware by changing the base URL and the API key, and set model to anthropic:claude@fable-5.1.

Requests and responses keep the provider's format, so its reference documents every parameter.

Endpoint	Format
`/v1/chat/completions`	OpenAI Chat Completions
`/v1/messages`	Anthropic Messages

Chat Completions

cURL

curl -X POST https://api.runware.ai/v1/chat/completions \
  -H "Authorization: Bearer $RUNWARE_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "anthropic:claude@fable-5.1",
    "messages": [{"role": "user", "content": "What is the capital of France?"}]
  }'

Python

from openai import OpenAI

client = OpenAI(
    api_key="your_runware_api_key",
    base_url="https://api.runware.ai/v1",
)

response = client.chat.completions.create(
    model="anthropic:claude@fable-5.1",
    messages=[{"role": "user", "content": "What is the capital of France?"}],
)

print(response.choices[0].message.content)

TypeScript

import OpenAI from 'openai';

const client = new OpenAI({
  apiKey: 'your_runware_api_key',
  baseURL: 'https://api.runware.ai/v1',
});

const response = await client.chat.completions.create({
  model: 'anthropic:claude@fable-5.1',
  messages: [{ role: 'user', content: 'What is the capital of France?' }],
});

console.log(response.choices[0].message.content);

Compatible APIs covers streaming, tool calling and how the endpoint differs from OpenAI, and the OpenAI API reference lists every parameter.

Messages

cURL

curl -X POST https://api.runware.ai/v1/messages \
  -H "Authorization: Bearer $RUNWARE_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "anthropic:claude@fable-5.1",
    "max_tokens": 1024,
    "messages": [{"role": "user", "content": "What is the capital of France?"}]
  }'

Python

from anthropic import Anthropic

client = Anthropic(
    auth_token="your_runware_api_key",
    base_url="https://api.runware.ai",
)

message = client.messages.create(
    model="anthropic:claude@fable-5.1",
    max_tokens=1024,
    messages=[{"role": "user", "content": "What is the capital of France?"}],
)

print(message.content[0].text)

TypeScript

import Anthropic from '@anthropic-ai/sdk';

const client = new Anthropic({
  authToken: 'your_runware_api_key',
  baseURL: 'https://api.runware.ai',
});

const message = await client.messages.create({
  model: 'anthropic:claude@fable-5.1',
  max_tokens: 1024,
  messages: [{ role: 'user', content: 'What is the capital of France?' }],
});

console.log(message.content[0].text);

Compatible APIs covers how the Anthropic SDKs authenticate against Runware, and the Claude API reference lists every parameter.