Claude Sonnet 4

Anthropic’s Claude Sonnet 4 via OpenAI-compatible API. The best balance of intelligence and speed for everyday coding and complex tasks.

Claude Sonnet 4 is Anthropic’s best coding model, available through AnyFast via an OpenAI-compatible interface. It delivers an ideal balance of intelligence, speed, and cost — excelling at software engineering, complex reasoning, and day-to-day development tasks.

Key capabilities

• OpenAI-compatible — Works as a drop-in replacement with the OpenAI SDK
• Best coding model — Top performance on code generation, debugging, and refactoring
• Fast and efficient — Lower latency and cost compared to Opus models
• Strong reasoning — Excellent at multi-step problem solving and analysis
• 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",
    "messages": [
      { "role": "user", "content": "Write a Python function to merge two sorted arrays." }
    ]
  }'

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",
    messages=[
        {"role": "user", "content": "Write a Python function to merge two sorted arrays."}
    ]
)

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",
    messages=[
        {"role": "user", "content": "Explain the difference between async and sync programming."}
    ],
    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
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.