Create Anthropic Messages

OpenAI-compatible gateway endpoint for Anthropic Messages API payloads.

POST/v1/anthropic/messages

Request Parameters

Name	Type	Required	Description
model	string	Required	Anthropic model ID, for example anthropic/claude-sonnet-4.5.
messages	array	Required	Anthropic-formatted message list.
max_tokens	integer	Required	Maximum tokens to generate.
system	string \| array		System prompt string or array of system content blocks.
temperature	number		Sampling temperature between 0 and 1.
top_p	number		Nucleus sampling threshold.
top_k	integer		Top-K sampling: only consider the K most likely tokens.
stop_sequences	array		Sequences that will cause the model to stop generating.
stream	boolean		If true, returns a streaming SSE response. Defaults to false.
metadata	object		Arbitrary metadata object passed through with the request.
tools	array		Tool definitions available to the model (Anthropic tool format).
tool_choice	object		How the model selects tools: {"type": "auto"}, {"type": "any"}, or {"type": "tool", "name": "..."}.

Authentication

This endpoint accepts both Authorization: Bearer and x-api-key headers. Use x-api-key for drop-in compatibility with the native Anthropic SDK.

Request Example

bash

curl -X POST https://api.therouter.ai/v1/anthropic/messages
  -H "x-api-key: $THEROUTER_API_KEY"
  -H "Content-Type: application/json"
  -d '{
    "model": "anthropic/claude-sonnet-4.5",
    "max_tokens": 256,
    "messages": [{"role": "user", "content": "Explain retrieval augmented generation."}]
  }'

Response

json

{
  "id": "msg_abc123",
  "type": "message",
  "role": "assistant",
  "content": [{"type": "text", "text": "RAG combines..."}],
  "model": "anthropic/claude-sonnet-4.5",
  "stop_reason": "end_turn",
  "usage": {"input_tokens": 22, "output_tokens": 81}
}

Streaming

Set stream: true to receive the response as server-sent events (SSE). The gateway supports two routing paths, both producing Anthropic-format SSE events:

Name	Type	Required	Description
Native passthrough			When the resolved provider is Anthropic, events are streamed directly from the upstream API.
Translation path			When routing to a non-Anthropic provider, the gateway translates OpenAI SSE chunks into Anthropic-format events in real time.

Events follow the standard Anthropic streaming format:

text

event: message_start
event: content_block_start
event: content_block_delta     # repeated for each text chunk
event: content_block_stop
event: message_delta           # final usage and stop_reason
event: message_stop

Notes

Use this route when migrating existing Anthropic SDK payloads. Set your SDK base URL to https://api.therouter.ai/v1/anthropic and use your AI Router API key.