> ## Documentation Index
> Fetch the complete documentation index at: https://docs.anyfast.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# 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

<CodeGroup>
  ```bash cURL theme={null}
  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." }
      ]
    }'
  ```

  ```python Python theme={null}
  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)
  ```

  ```python Streaming theme={null}
  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="")
  ```
</CodeGroup>

## 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         |

<Note>
  `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.
</Note>

<Note>
  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.
</Note>

<Card title="API Reference" icon="code" href="/api-reference/model-api/anthropic/claude-fable-5">
  View the interactive API playground for Claude Fable 5.
</Card>