Claude Fable 5

Anthropic’s most capable model via OpenAI-compatible API. Adaptive thinking always on for complex reasoning and long-horizon agentic work.

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

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.