Anthropic Messages API

Chuizi.AI proxies the Anthropic Messages API as a native passthrough. Your request body goes directly to Anthropic with zero format conversion, and automatic failover ensures high availability. The response comes back in Anthropic's own format, including streaming SSE event types.

This matters if you use Claude Code, Cursor, Cline, or OpenCode. These tools expect the Anthropic Messages API, not OpenAI's format. With Chuizi.AI, you set two environment variables and everything works.

Endpoints

Method	Path	Description
`POST`	`/anthropic/v1/messages`	Create a message (streaming and non-streaming)
`GET`	`/anthropic/v1/models`	List available Anthropic models

Authentication

Chuizi.AI accepts your ck- API key through two headers. Use whichever your tool expects:

Header	Format	Example
`x-api-key`	`ck-your-key-here`	Anthropic SDK default
`Authorization`	`Bearer ck-your-key-here`	OpenAI convention

Both resolve to the same user account, balance, and rate limits.

Required Headers

Header	Value	Notes
`anthropic-version`	`2023-06-01`	Required. Passed through to upstream.
`content-type`	`application/json`	Required for POST requests.

How It Differs from OpenAI `/v1/chat/completions`

If you are used to the OpenAI Chat Completions format, these are the key structural differences in Anthropic's Messages API:

Concept	OpenAI Chat Completions	Anthropic Messages
System prompt	`messages` array entry with `role: "system"`	Top-level `system` field (string or array of blocks)
Content format	String or array of content parts	Always an array of content blocks (`[{type: "text", text: "..."}]`)
Stop indicator	`finish_reason: "stop"`	`stop_reason: "end_turn"`
Max tokens	Optional (`max_tokens` or `max_completion_tokens`)	Required (`max_tokens`)
Token usage	`usage.prompt_tokens`, `usage.completion_tokens`	`usage.input_tokens`, `usage.output_tokens`
Streaming events	`data: {"choices": [...]}` chunks	Typed events: `message_start`, `content_block_delta`, `message_delta`
Model prefix	`anthropic/claude-sonnet-4-6`	`claude-sonnet-4-6` (bare name) or `anthropic/claude-sonnet-4-6`

Request Format

config.json

json

{
  "model": "claude-sonnet-4-6",
  "max_tokens": 1024,
  "system": "You are a helpful assistant.",
  "messages": [
    {
      "role": "user",
      "content": "Explain how TCP handshakes work."
    }
  ]
}

Supported Parameters

Parameter	Type	Required	Description
`model`	string	Yes	Model name. Bare names (`claude-sonnet-4-6`) and prefixed names (`anthropic/claude-sonnet-4-6`) both work.
`max_tokens`	integer	Yes	Maximum tokens to generate.
`messages`	array	Yes	Conversation messages.
`system`	string or array	No	System prompt. Can be a string or an array of content blocks (useful for caching).
`stream`	boolean	No	Enable SSE streaming. Default: `false`.
`temperature`	number	No	Sampling temperature, 0-1.
`top_p`	number	No	Nucleus sampling threshold.
`top_k`	integer	No	Top-K sampling.
`stop_sequences`	array	No	Custom stop sequences.
`tools`	array	No	Tool definitions for function calling.
`tool_choice`	object	No	Tool selection strategy.
`metadata`	object	No	Request metadata (e.g., `user_id` for abuse tracking).

Response Format

Non-streaming