Anthropic Messages API

/ API docs

POST https://api.opencheese.dev/v1/messages is the Anthropic Messages API, the one Claude Code, Claude Desktop and the Anthropic SDKs speak. Its base URL is https://api.opencheese.dev, without /v1: the SDKs append /v1/messages themselves.

A first request

bash

curl https://api.opencheese.dev/v1/messages \
  -H "x-api-key: $OPENCHEESE_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{"model": "MODEL_ID", "max_tokens": 1024, "system": "Answer in one sentence.", "messages": [{"role": "user", "content": "What is a gateway?"}]}'

Authorization: Bearer works as well as x-api-key. The answer has the Anthropic shape:

json

{
  "id": "msg_01AbCdEf",
  "type": "message",
  "role": "assistant",
  "model": "MODEL_ID",
  "content": [{ "type": "text", "text": "A gateway passes your requests on to the models behind it." }],
  "stop_reason": "end_turn",
  "usage": { "input_tokens": 24, "output_tokens": 13, "cache_read_input_tokens": 0, "cache_creation_input_tokens": 0 }
}

Parameters

Field	Notes
`model`	Required. An `id` from `/v1/models`; dated Claude IDs such as `...-20251001` resolve to the undated model.
`messages`	Required. `user` and `assistant` turns with text, image, `tool_use` and `tool_result` blocks.
`system`	A string or text blocks.
`stream`	`true` for server-sent events.
`tools`, `tool_choice`	Tool use, where the model supports tools.
`output_config.effort`	A level from the model's `supported_reasoning_levels`.
`max_tokens`, `temperature`, `top_p`, `thinking`	Optional. If you don't set them, the model's defaults are used.

Reasoning effort

Choose how hard the model thinks with Anthropic's output_config.effort (or reasoning_effort, which is read too). If you don't set one, the model's default is used.

bash

curl https://api.opencheese.dev/v1/messages \
  -H "x-api-key: $OPENCHEESE_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{"model": "MODEL_ID", "max_tokens": 4096, "output_config": {"effort": "high"}, "messages": [{"role": "user", "content": "Plan a three-day trip to Lyon."}]}'

A level the model doesn't offer is refused with unsupported_reasoning_effort, and the error's supported_reasoning_levels lists the levels it does offer. Thinking, where the model returns it, arrives as thinking content blocks.

Streaming

With "stream": true the answer arrives as the Anthropic events: message_start, content_block_start, content_block_delta (text in delta.text), content_block_stop, message_delta with the stop_reason and usage, and message_stop. A failure after the stream has started is an error event.

text

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"One, two"}}

event: message_delta
data: {"type":"message_delta","delta":{"stop_reason":"end_turn"},"usage":{"output_tokens":6}}

Images and tools

Images go in as image blocks with a base64 source or a url source, for models whose modalities.input includes image. Tools follow the Anthropic format: tools with an input_schema, a tool_use block in the answer, and your tool_result block in the next user turn.

json

{
  "model": "MODEL_ID",
  "max_tokens": 1024,
  "tools": [
    {
      "name": "get_weather",
      "description": "Current weather for a city",
      "input_schema": { "type": "object", "properties": { "city": { "type": "string" } }, "required": ["city"] }
    }
  ],
  "messages": [{ "role": "user", "content": "What is the weather in Paris?" }]
}

Count tokens

POST /v1/messages/count_tokens takes the same body and returns an estimate of its input tokens. It is free: nothing is charged and no transaction is recorded.

bash

curl https://api.opencheese.dev/v1/messages/count_tokens \
  -H "x-api-key: $OPENCHEESE_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{"model": "MODEL_ID", "messages": [{"role": "user", "content": "How many tokens is this?"}]}'

json

{ "input_tokens": 9 }

SDKs

Python:

python

import anthropic

client = anthropic.Anthropic(base_url="https://api.opencheese.dev", api_key="YOUR_OPENCHEESE_API_KEY")
message = client.messages.create(
    model="MODEL_ID",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Write a haiku about cheese."}],
)
print(message.content[0].text)

JavaScript:

javascript

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic({ baseURL: "https://api.opencheese.dev", apiKey: process.env.OPENCHEESE_API_KEY });
const message = await client.messages.create({
  model: "MODEL_ID",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Write a haiku about cheese." }]
});
console.log(message.content[0].text);

Errors use the Anthropic envelope, {"type": "error", "error": {...}}; see Errors and rate limits. Add the opencheese-session-uuid header to keep a conversation together.

The cheese has holes. Your infrastructure should not.