Claude Fable 5.1

Claude Fable 5.1 is Anthropic’s most capable generally available model for demanding reasoning, long-running agentic coding, multistep research, and complex knowledge work. Call claude-fable-5-1 through MixRoute’s Anthropic-compatible Messages API; the OpenAI-compatible Chat Completions API is also supported.

Key capabilities

Long context - Supports a 1M-token context window and up to 128K output tokens
Adaptive thinking - Always-on adaptive thinking with high default effort
Long-horizon work - Stronger agentic coding, multistep research, and document-heavy workflows
Multimodal input - Accepts text and images and returns text
Dual API support - Works through Anthropic-compatible Messages and OpenAI-compatible Chat Completions

Quick example

Messages API

cURL
Python
Streaming

curl "https://api.mixroute.ai/v1/messages" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{
  "model": "claude-fable-5-1",
  "max_tokens": 256,
  "messages": [
    {
      "role": "user",
      "content": "Explain quantum entanglement in simple terms."
    }
  ]
}'

from anthropic import Anthropic

client = Anthropic(api_key="YOUR_API_KEY", base_url="https://api.mixroute.ai")

message = client.messages.create(
    model="claude-fable-5-1",
    max_tokens=256,
    messages=[{"role": "user", "content": "Explain quantum entanglement in simple terms."}],
)

print(message.content[0].text)

from anthropic import Anthropic

client = Anthropic(api_key="YOUR_API_KEY", base_url="https://api.mixroute.ai")

with client.messages.stream(
    model="claude-fable-5-1",
    max_tokens=256,
    messages=[{"role": "user", "content": "Write a short poem about the sea."}],
) as stream:
    for text in stream.text_stream:
        print(text, end="")

OpenAI-compatible API

cURL
Python
Streaming

curl "https://api.mixroute.ai/v1/chat/completions" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
  "model": "claude-fable-5-1",
  "messages": [
    {
      "role": "user",
      "content": "Explain quantum entanglement in simple terms."
    }
  ],
  "max_tokens": 256
}'

from openai import OpenAI

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

response = client.chat.completions.create(
    model="claude-fable-5-1",
    messages=[{"role": "user", "content": "Explain quantum entanglement in simple terms."}],
    max_tokens=256,
)

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

from openai import OpenAI

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

stream = client.chat.completions.create(
    model="claude-fable-5-1",
    messages=[{"role": "user", "content": "Write a short poem about the sea."}],
    max_tokens=256,
    stream=True,
)

for chunk in stream:
    print(chunk.choices[0].delta.content or "", end="")

Parameters

Messages API

Parameter	Type	Required	Description
`model`	string	Yes	Must be `claude-fable-5-1`.
`messages`	array	Yes	Input messages with role and content.
`max_tokens`	integer	Yes	Maximum tokens to generate.
`system`	string \| array	No	Top-level system instruction.
`stream`	boolean	No	Enable SSE streaming.
`tools`	array	No	Anthropic tool definitions.
`thinking`	object	No	Extended-thinking configuration when supported.

OpenAI-compatible API

Parameter	Type	Required	Description
`model`	string	Yes	Must be `claude-fable-5-1`.
`messages`	array	Yes	Conversation messages with role and content.
`max_tokens`	integer	No	Maximum generated tokens.
`stream`	boolean	No	Enable SSE streaming. Default: false.
`temperature`	number	No	Sampling temperature when supported by the model.
`top_p`	number	No	Nucleus sampling threshold when supported.
`stop`	string \| array	No	Sequences that stop generation.
`tools`	array	No	OpenAI-compatible tool definitions.

Vendor reference

Claude Fable 5.1 documentation

Home

Model Introduction

Key capabilities

Quick example

Messages API

OpenAI-compatible API

Parameters

Messages API

OpenAI-compatible API

Vendor reference

​Key capabilities

​Quick example

​Messages API

​OpenAI-compatible API

​Parameters

​Messages API

​OpenAI-compatible API

​Vendor reference

Key capabilities

Quick example

Messages API

OpenAI-compatible API

Parameters

Messages API

OpenAI-compatible API

Vendor reference