> ## 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 Sonnet 4 Thinking

> Anthropic's Claude Sonnet 4 with extended thinking enabled. Enhanced reasoning at Sonnet-level speed and cost.

Claude Sonnet 4 Thinking is the extended thinking variant of Claude Sonnet 4, available through Anyfast via an OpenAI-compatible interface. It combines Sonnet's excellent balance of speed and capability with step-by-step internal reasoning enabled by default, delivering improved accuracy on complex tasks while maintaining fast response times.

## Key capabilities

* **OpenAI-compatible** — Works as a drop-in replacement with the OpenAI SDK
* **Extended thinking** — Step-by-step internal reasoning enabled by default
* **Balanced performance** — Strong reasoning at Sonnet-level speed and cost
* **Advanced coding** — Excels at code generation, debugging, and refactoring
* **Long context** — Handles large documents and multi-turn conversations
* **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-sonnet-4-20250514-thinking",
      "messages": [
        { "role": "user", "content": "Solve step by step: If a train travels 120 km in 2 hours, then slows down and travels 80 km in 3 hours, what is the average speed for the entire trip?" }
      ]
    }'
  ```

  ```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-sonnet-4-20250514-thinking",
      messages=[
          {"role": "user", "content": "Solve step by step: If a train travels 120 km in 2 hours, then slows down and travels 80 km in 3 hours, what is the average speed for the entire trip?"}
      ]
  )

  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-sonnet-4-20250514-thinking",
      messages=[
          {"role": "user", "content": "Write a proof that there are infinitely many prime numbers."}
      ],
      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-sonnet-4-20250514-thinking` |
| `messages`    | array          | Yes      | List of `{ role, content }` objects         |
| `max_tokens`  | integer        | No       | Maximum tokens to generate                  |
| `temperature` | float          | No       | `0`–`2`. Controls randomness. Default: `1`  |
| `stream`      | boolean        | No       | Enable SSE streaming. Default: `false`      |
| `top_p`       | float          | No       | Nucleus sampling threshold. Default: `1`    |
| `stop`        | string / array | No       | Sequences that stop generation              |

<Card title="API Reference" icon="code" href="/api-reference/model-api/anthropic/claude-sonnet-4-20250514-thinking">
  View the interactive API playground for Claude Sonnet 4 Thinking.
</Card>

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