Create Message

Send a message to an Anthropic-compatible model and receive a response

Send a message to an Anthropic-compatible model and receive a response. This endpoint follows the Anthropic Messages API format and supports all Anthropic models as well as compatible models from other providers through Requesty’s routing.

Base URL

https://router.requesty.ai/v1/messages

Authentication

Include your Requesty API key in the request headers using Anthropic’s standard format:

x-api-key: YOUR_REQUESTY_API_KEY

Headers

Header	Required	Description
x-api-key	✅	Your Requesty API key (Anthropic format)
Content-Type	✅	Must be application/json
anthropic-version	❌	API version (defaults to 2023-06-01)

Example Request

curl https://router.requesty.ai/v1/messages \
  -H "Content-Type: application/json" \
  -H "x-api-key: YOUR_REQUESTY_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "anthropic/claude-sonnet-4-20250514",
    "max_tokens": 1024,
    "messages": [
      {
        "role": "user",
        "content": "Hello, Claude!"
      }
    ]
  }'

Model Selection

You can use any model available in the Model Library. Examples:

• Anthropic Models: anthropic/claude-sonnet-4-20250514, anthropic/claude-3-7-sonnet
• OpenAI Models: openai/gpt-4o, openai/gpt-4o-mini
• Google Models: google/gemini-2.0-flash-exp
• Other Providers: mistral/mistral-large-2411, meta/llama-3.3-70b-instruct

While this endpoint uses the Anthropic Messages format, Requesty automatically handles format conversion for non-Anthropic models, so you can use any supported model with this endpoint.

Streaming

Enable streaming responses by setting stream: true:

{
	"model": "anthropic/claude-sonnet-4-20250514",
	"max_tokens": 1024,
	"stream": true,
	"messages": [
		{
			"role": "user",
			"content": "Write a short story"
		}
	]
}

Vision Support

Send images using the content blocks format:

{
	"model": "anthropic/claude-sonnet-4-20250514",
	"max_tokens": 1024,
	"messages": [
		{
			"role": "user",
			"content": [
				{
					"type": "text",
					"text": "What do you see in this image?"
				},
				{
					"type": "image",
					"source": {
						"type": "base64",
						"media_type": "image/jpeg",
						"data": "/9j/4AAQSkZJRgABAQAAAQABAAD..."
					}
				}
			]
		}
	]
}

PDF Support

You can send PDFs, encoded in base 64 format:

{
	"model": "anthropic/claude-sonnet-4-20250514",
	"max_tokens": 1024,
	"messages": [
		{
			"role": "user",
			"content": [
				{
					"type": "text",
					"text": "What is in this PDF?"
				},
                {
                    "type": "document",
                    "source": {
                        "type": "base64",
                        "media_type": "application/pdf",
                        "data": "JVBERi0=..."
                    }
				}
			]
		}
	]
}

Tool Use

Define tools that the model can call:

{
	"model": "anthropic/claude-sonnet-4-20250514",
	"max_tokens": 1024,
	"tools": [
		{
			"name": "get_weather",
			"description": "Get the current weather in a given location",
			"input_schema": {
				"type": "object",
				"properties": {
					"location": {
						"type": "string",
						"description": "The city and state, e.g. San Francisco, CA"
					}
				},
				"required": ["location"]
			}
		}
	],
	"messages": [
		{
			"role": "user",
			"content": "What's the weather like in New York?"
		}
	]
}

System Prompts

Include system instructions using the system parameter:

{
	"model": "anthropic/claude-sonnet-4-20250514",
	"max_tokens": 1024,
	"system": "You are a helpful assistant that always responds in a friendly, professional manner.",
	"messages": [
		{
			"role": "user",
			"content": "Hello!"
		}
	]
}

Error Handling

The API returns standard HTTP status codes:

• 200 - Success
• 400 - Bad Request (invalid parameters)
• 401 - Unauthorized (invalid API key)
• 403 - Forbidden (insufficient permissions)
• 429 - Rate Limited
• 500 - Internal Server Error

Example error response:

{
	"error": {
		"type": "invalid_request_error",
		"message": "max_tokens is required"
	}
}

Response Format

Successful responses follow the Anthropic Messages format:

{
	"id": "msg_01ABC123",
	"type": "message",
	"role": "assistant",
	"content": [
		{
			"type": "text",
			"text": "Hello! I'm Claude, an AI assistant. How can I help you today?"
		}
	],
	"model": "anthropic/claude-sonnet-4-20250514",
	"stop_reason": "end_turn",
	"usage": {
		"input_tokens": 12,
		"output_tokens": 18
	}
}

Key Differences from OpenAI Chat Completions

• Authentication: Uses x-api-key header instead of Authorization: Bearer
• Required max_tokens: Unlike OpenAI’s API, the max_tokens parameter is required
• Content Blocks: Messages use content blocks for rich content (text, images, tool calls)
• System Parameter: System prompts are specified as a separate system parameter, not as a message
• Role Restrictions: Only user and assistant roles are supported in messages (no system role)

For the most seamless experience with Anthropic models, use this endpoint. For broader compatibility across all providers, consider using the Chat Completions endpoint instead.

Headers

x-api-key · string · required

Your Requesty API key

anthropic-version · string · default: 2023-06-01

The version of the Anthropic API to use

Example:

"2023-06-01"

Body

application/json

model · string · default: anthropic/claude-sonnet-4-20250514 · required

The model to use for the completion

Example:

"anthropic/claude-sonnet-4-20250514"

max_tokens · integer · required

The maximum number of tokens to generate before stopping

Required range: x >= 1

Example:

1024

messages · object[] · required

Input messages

Show child attributes

system · string

System prompt to be used for the completion

temperature · number

Amount of randomness injected into the response

Required range: 0 <= x <= 2

top_p · number

Use nucleus sampling

Required range: 0 <= x <= 1

top_k · integer

Only sample from the top K options for each subsequent token

Required range: x >= 0

stream · boolean

Whether to incrementally stream the response using server-sent events

stop_sequences · string[]

Custom text sequences that will cause the model to stop generating

tools · object[]

Definitions of tools that the model may use

Show child attributes

tool_choice

How the model should use the provided tools

Available options:

auto,

any

Response

200

application/json

Message response

id · string · required

Unique object identifier

type · enum<string> · required

Object type

Available options:

message

role · enum<string> · required

Conversational role of the generated message

Available options:

assistant

content · object[] · required

Content generated by the model

Option 1

Show child attributes

model · string · required

The model that handled the request

stop_reason · enum<string> · required

The reason that we stopped

Available options:

end_turn,

max_tokens,

stop_sequence,

tool_use

usage · object · required

Show child attributes

stop_sequence · string

Which custom stop sequence was generated