Messages

Method and path

POST/v1/messages

Anthropic SDKs take the base URL https://api.relayvia.dev. Send the key as x-api-key: sk-... or Authorization: Bearer sk-..., together with the anthropic-version header your SDK already sends.

Common parameters

Requests are forwarded in the original format, so the original API's parameters work unless the next section says otherwise. This table lists the common ones; see the official reference for the rest.

Differences from the original API

• tools[] only accepts tools your code runs: no type, or type: custom. Server tools (web fetch, code execution and similar) and mcp_servers return 400 unsupported_parameter.
• Web search is not available. web_search_* tools are removed and the request runs without search. If web search is the only tool, as in Claude Code's WebSearch requests, or tool_choice names it, the request returns 400.
• Files from the Files API (source.type: file) and documents by URL (source.type: url) return 400 unsupported_parameter; send them as base64.
• anthropic-beta values Relayvia does not support are dropped without an error; the rest are forwarded.
• Message Batches (/v1/messages/batches) are not provided and return 400.
• metadata.user_id is scoped to your account before forwarding, so it is never shared with other accounts.
• Events are forwarded as they arrive, and message_delta carries the final output usage. A : keepalive comment is sent after every 15 seconds without data. message.model shows the name you sent.
• A non-streaming request with no response within 95 seconds returns 500 upstream_timeout; use streaming for long outputs. Or send the header X-Robi-Keepalive: 1: if there is no response within 30 seconds you get a 200 right away and a space every 15 seconds until the JSON arrives, and a later failure arrives as an error object in the body.

Request examples

curl https://api.relayvia.dev/v1/messages \
  -H "x-api-key: sk-..." \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{"model":"claude-opus-5","max_tokens":1024,"messages":[{"role":"user","content":"Hello!"}]}'

import anthropic

client = anthropic.Anthropic(base_url="https://api.relayvia.dev", api_key="sk-...")
resp = client.messages.create(model="claude-opus-5", max_tokens=1024, messages=[{"role": "user", "content": "Hello!"}])
print(resp.content[0].text)

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

const client = new Anthropic({ baseURL: "https://api.relayvia.dev", apiKey: "sk-..." });
const resp = await client.messages.create({
  model: "claude-opus-5",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Hello!" }],
});
console.log(resp.content[0].text);

Response and streaming examples

{
  "id": "msg_abc123",
  "type": "message",
  "role": "assistant",
  "model": "claude-opus-5",
  "content": [
    {
      "type": "text",
      "text": "Hello! How can I help?"
    }
  ],
  "stop_reason": "end_turn",
  "usage": {
    "input_tokens": 9,
    "output_tokens": 8
  }
}

event: message_start
data: {"type":"message_start","message":{"id":"msg_abc123","model":"claude-opus-5","usage":{"input_tokens":9,"output_tokens":1}}}

event: content_block_delta
data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hello!"}}

event: message_delta
data: {"type":"message_delta","delta":{"stop_reason":"end_turn"},"usage":{"output_tokens":8}}

event: message_stop
data: {"type":"message_stop"}

Errors this endpoint returns

Every code, its fixed message and what to do are on the error codes page.