Claude Sonnet 4.5 Thinking

Anthropic’s Claude Sonnet 4.5 with extended thinking enabled. Enhanced reasoning with the best coding model’s speed and capability.

Claude Sonnet 4.5 Thinking is the extended thinking variant of Claude Sonnet 4.5, available through AnyFast via an OpenAI-compatible interface. It combines Sonnet 4.5’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
• Best coding model — Top performance on code generation, refactoring, and debugging
• Enhanced reasoning — Improved accuracy on complex, multi-step tasks
• 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-4-5-20250929-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-4-5-20250929-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-4-5-20250929-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-4-5-20250929-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 4.5 Thinking.