Create a message | OpenRouter

Creates a message using the Anthropic Messages API format. Supports text, images, PDFs, tools, and extended thinking.

Request

This endpoint expects an object.

modelstringRequired

max_tokensdoubleRequired

messageslist of objectsRequired

systemstring or list of objectsOptional

metadataobjectOptional

stop_sequenceslist of stringsOptional

streambooleanOptional

temperaturedoubleOptional

top_pdoubleOptional

top_kdoubleOptional

toolslist of objectsOptional

tool_choiceobjectOptional

thinkingobjectOptional

service_tierenumOptional

Allowed values:

providerobject or nullOptional

When multiple model providers are available, optionally indicate your routing preference.

pluginslist of objectsOptional

Plugins you want to enable for this request, including their settings.

userstringOptional<=128 characters

A unique identifier representing your end-user, which helps distinguish between different users of your app. This allows your app to identify specific users in case of abuse reports, preventing your entire app from being affected by the actions of individual users. Maximum of 128 characters.

session_idstringOptional<=128 characters

A unique identifier for grouping related requests (e.g., a conversation or agent workflow) for observability. If provided in both the request body and the x-session-id header, the body value takes precedence. Maximum of 128 characters.

traceobjectOptional

Metadata for observability and tracing. Known keys (trace_id, trace_name, span_name, generation_name, parent_span_id) have special handling. Additional keys are passed through as custom metadata to configured broadcast destinations.

modelslist of stringsOptional

output_configobjectOptional

Configuration for controlling output behavior. Currently supports the effort parameter for Claude Opus 4.5.

Response

Successful response

contentlist of objects

idstring

modelstring

roleenum

Allowed values:

stop_reasonenum or null

stop_sequencestring or null

typeenum

Allowed values:

usageobject

1	import requests
2
3	url = "https://openrouter.ai/api/v1/messages"
4
5	payload = {
6	"model": "anthropic/claude-4.5-sonnet-20250929",
7	"max_tokens": 1024,
8	"messages": [
9	{
10	"role": "user",
11	"content": "Hello, how are you?"
12	}
13	],
14	"temperature": 0.7
15	}
16	headers = {"Content-Type": "application/json"}
17
18	response = requests.post(url, json=payload, headers=headers)
19
20	print(response.json())

1	{
2	"content": [
3	{
4	"type": "text",
5	"text": "Hello! I'm doing well, thank you for asking.",
6	"citations": null
7	}
8	],
9	"id": "msg_01XFDUDYJgAACzvnptvVoYEL",
10	"model": "claude-sonnet-4-5-20250929",
11	"role": "assistant",
12	"stop_reason": "end_turn",
13	"stop_sequence": null,
14	"type": "message",
15	"usage": {
16	"input_tokens": 12,
17	"output_tokens": 15,
18	"cache_creation_input_tokens": null,
19	"cache_read_input_tokens": null,
20	"cache_creation": null,
21	"inference_geo": null,
22	"server_tool_use": null,
23	"service_tier": "standard"
24	}
25	}

Request

Response

Errors