Claude Fable 5

Claude Fable 5 is Anthropic’s most capable widely released model, available through AnyFast via an OpenAI-compatible interface. It is designed for the most demanding reasoning, long-horizon agentic coding, and high-autonomy work.

Key capabilities

Most capable Claude — Anthropic’s top-tier model for reasoning and agentic coding
Adaptive thinking always on — Smart reasoning built into every request, no configuration needed
1M context window — 128K max output tokens
OpenAI-compatible — Works as a drop-in replacement with the OpenAI SDK
Safety classifiers — May decline certain requests with finish_reason: "refusal", not billed when refusals occur before output
Fallback support — Pass fallbacks to automatically retry refused requests on another model
Streaming — Supports real-time token streaming via SSE

Quick example

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

from openai import OpenAI

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

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

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

from openai import OpenAI

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

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

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

Parameters

Parameter	Type	Required	Description
`model`	string	Yes	Must be `claude-fable-5`
`messages`	array	Yes	List of `{ role, content }` objects
`max_tokens`	integer	No	Maximum tokens to generate
`stream`	boolean	No	Enable SSE streaming. Default: `false`
`stop`	string / array	No	Sequences that stop generation

temperature, top_p, and top_k are not supported on Claude Fable 5. Setting them to non-default values returns a 400 error. Use prompting to guide the model’s behavior.

Claude Fable 5 includes safety classifiers that can return finish_reason: "refusal". You are not billed for refused requests. Use the fallbacks parameter to automatically retry refused requests on other Claude models.

API Reference

View the interactive API playground for Claude Fable 5.

Getting started

Model Introduction

Resources

Key capabilities

Quick example

Parameters

API Reference

​Key capabilities

​Quick example

​Parameters

API Reference

Key capabilities

Quick example

Parameters