API reference

Create a message (Anthropic Messages compatibility)

POST /v1/messages request, response, and error contract.

View Markdown

POST /v1/messages

Anthropic Messages API compatibility endpoint. Accepts Anthropic-format requests and returns Anthropic-format responses. Supports tools, system prompts, and streaming. Authenticate with either Authorization: Bearer or x-api-key header.

Authentication¶

Send Authorization: Bearer $KOSCOMPUTE_API_KEY.

Request¶

Content type: application/json

Field	Type	Required	Description
`model`	string	Yes	See the endpoint guide for behavior and model support.
`messages`	array	Yes	See the endpoint guide for behavior and model support.
`max_tokens`	integer	Yes	See the endpoint guide for behavior and model support.
`system`	string · array	No	See the endpoint guide for behavior and model support.
`temperature`	number	No	See the endpoint guide for behavior and model support.
`top_p`	number	No	See the endpoint guide for behavior and model support.
`top_k`	integer	No	See the endpoint guide for behavior and model support.
`stop_sequences`	array	No	See the endpoint guide for behavior and model support.
`tools`	array	No	See the endpoint guide for behavior and model support.
`tool_choice`	object	No	See the endpoint guide for behavior and model support.
`metadata`	object	No	See the endpoint guide for behavior and model support.

Examples¶

cURL¶

curl https://api.koscompute.com/v1/messages \
  -H "x-api-key: $KOSCOMPUTE_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model":"qwen/qwen3.8-27b","max_tokens":120,"messages":[{"role":"user","content":"Reply with one sentence."}]}'

Python¶

import os
import requests

response = requests.post(
    "https://api.koscompute.com/v1/messages",
    headers={"Authorization": f"Bearer {os.environ['KOSCOMPUTE_API_KEY']}"},
    json={
  "model": "qwen/qwen3.8-27b",
  "max_tokens": 120,
  "messages": [
    {
      "role": "user",
      "content": "Reply with one sentence."
    }
  ]
},
)
print(response.json())

JavaScript¶

const response = await fetch("https://api.koscompute.com/v1/messages", {
  method: "POST",
  headers: {
    Authorization: `Bearer ${process.env.KOSCOMPUTE_API_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({"model":"qwen/qwen3.8-27b","max_tokens":120,"messages":[{"role":"user","content":"Reply with one sentence."}]}),
});
console.log(await response.json());

Responses¶

Status	Meaning
`200`	Successful response
`400`	Bad request — invalid parameters or payload
`401`	Invalid or missing API key
`403`	Model not allowed for this API key
`404`	Public model not found
`413`	Request body exceeds the limit
`429`	Rate limit or admission capacity is full. The message states when to retry; vision saturation also reports active/total vision concurrency.
`500`	Internal server error
`529`	Anthropic-compatible provider unavailable or upstream failure

Next steps¶

Use the endpoint guide for complete workflows, model capability notes, streaming behavior, and retry advice.