Claude Sonnet 5

Anthropic’s Claude Sonnet 5 via AnyFast. The latest Sonnet model with a 1M context window, adaptive thinking, and strong coding and agent capabilities.

Claude Sonnet 5 is Anthropic’s latest Sonnet model, available through AnyFast via the native Anthropic Messages API. It combines Sonnet-class speed with stronger coding, agentic execution, long-context reasoning, and analysis capabilities, making it well suited for production assistants, developer tools, and complex multi-step workflows.

Key capabilities

• 1M context window — 1M tokens by default, with 128K max output tokens
• Adaptive thinking — On by default; control depth with output_config.effort, or use thinking: {"type": "disabled"} to turn it off
• Same API shape — Requests, responses, and streaming keep the same shape as Claude Sonnet 4.6
• New tokenizer — The same text produces about 30% more tokens than Claude Sonnet 4.6
• Model ID — Use claude-sonnet-5

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-5",
    "max_tokens": 1024,
    "messages": [
      { "role": "user", "content": "Write a Python function to merge two sorted arrays." }
    ]
  }'

import anthropic

client = anthropic.Anthropic(
    api_key="YOUR_API_KEY",
    base_url="https://www.anyfast.ai"
)

message = client.messages.create(
    model="claude-sonnet-5",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Write a Python function to merge two sorted arrays."}
    ]
)

print(message.content[0].text)

import anthropic

client = anthropic.Anthropic(
    api_key="YOUR_API_KEY",
    base_url="https://www.anyfast.ai"
)

with client.messages.stream(
    model="claude-sonnet-5",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Explain the difference between async and sync programming."}
    ],
) as stream:
    for text in stream.text_stream:
        print(text, end="")

Parameters

Parameter	Type	Required	Description
model	string	Yes	Must be claude-sonnet-5
messages	array	Yes	List of { role, content } objects
max_tokens	integer	Yes	Maximum tokens to generate. Claude Sonnet 5 supports up to 128K output tokens.
output_config	object	No	Use {"effort":"low" | "medium" | "high" | "xhigh" | "max"} to control adaptive thinking depth. Default: high
thinking	object	No	Omit it to use default adaptive thinking, or use {"type":"disabled"} to turn thinking off.
stream	boolean	No	Enable SSE streaming. Default: false
stop_sequences	array	No	Sequences that stop generation

Claude Sonnet 5 uses adaptive thinking by default. Use output_config.effort to tune reasoning depth (low, medium, high, xhigh, or max). Manual extended thinking (thinking: {type: "enabled", budget_tokens: N}) returns a 400 error, and non-default temperature, top_p, or top_k values also return a 400 error. Use thinking: {type: "disabled"} to turn thinking off.

Claude Sonnet 5 uses a new tokenizer. The same text produces approximately 30% more tokens than on Claude Sonnet 4.6, so prompt counts and max_tokens budgets should be recalculated before migrating.

Requests involving prohibited or high-risk cybersecurity topics may be refused. Refusals return stop_reason: "refusal".

API Reference

View the interactive API playground for Claude Sonnet 5.