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

cURL

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?" }
    ]
  }'

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)

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="")

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

API Reference

View the interactive API playground for Claude Sonnet 4 Thinking.