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Anthropic Messages

Send a request through RunAPI's Anthropic-compatible Messages operation.

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01

Overview

Use RunAPI's Anthropic-compatible Messages operation with a compatible model and the protocol's request and response shapes.

Quick start

  1. Create an API key and set it as RUNAPI_API_KEY.
  2. Choose a compatible model, then send the documented path parameters and JSON body.
  3. Read the JSON response or server-sent events shown for this operation, and handle protocol errors.

Endpoint

POST /v1/messages
Base URL
https://runapi.ai
Contract version
v1
Preferred authentication
x-api-key: YOUR_API_TOKEN
02

Authentication carriers

RunAPI accepts each carrier listed below. Generated samples use the protocol's preferred header.

header
Authorization: Bearer YOUR_API_TOKEN
header - Preferred
x-api-key: YOUR_API_TOKEN
header
x-goog-api-key: YOUR_API_TOKEN
query
?key=YOUR_API_TOKEN
03

Compatible models

Open a model page for current pricing, rate limits, and commercial-usage details.

Show 53 compatible models
04

Request contract

JSON body

Only fields with both public protocol and RunAPI support evidence are listed. Additional JSON body fields are passed through.

JSON body10 fields
max_tokensinteger
Required

Maximum number of tokens to generate before stopping.

messagesarray
Required

Conversation turns in Anthropic Messages format.

messages[].content["string", "array"]
Required
messages[].rolestring
Required
modelstring
Required

RunAPI model identifier returned by the compatible model catalog.

streamboolean
Optional

Return typed Messages events as server-sent events.

Default: false
system["string", "array"]
Optional

System instructions supplied separately from conversational messages.

temperaturenumber
Optional

Sampling temperature; model support can vary.

toolsarray
Optional

Tools the model may call.

top_pnumber
Optional

Nucleus sampling probability; model support can vary.

Request example

JSON 
{
  "max_tokens": 64,
  "messages": [
    {
      "content": "Say hello in one sentence.",
      "role": "user"
    }
  ],
  "model": "claude-sonnet-4-6"
}
05

Synchronous response

HTTP 200

Schema

JSON 
{
  "additionalProperties": true,
  "properties": {
    "content": {
      "type": "array"
    },
    "id": {
      "type": "string"
    },
    "model": {
      "type": "string"
    },
    "role": {
      "const": "assistant",
      "type": "string"
    },
    "stop_reason": {
      "type": [
        "string",
        "null"
      ]
    },
    "type": {
      "const": "message",
      "type": "string"
    },
    "usage": {
      "additionalProperties": true,
      "properties": {
        "input_tokens": {
          "type": "integer"
        },
        "output_tokens": {
          "type": "integer"
        }
      },
      "required": [
        "input_tokens",
        "output_tokens"
      ],
      "type": "object"
    }
  },
  "required": [
    "id",
    "type",
    "role",
    "content",
    "model",
    "stop_reason",
    "usage"
  ],
  "type": "object"
}

Example

JSON 
{
  "content": [
    {
      "text": "Hello! How can I help today?",
      "type": "text"
    }
  ],
  "id": "msg_example",
  "model": "claude-sonnet-4-6",
  "role": "assistant",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "type": "message",
  "usage": {
    "input_tokens": 12,
    "output_tokens": 8
  }
}
06

SSE event: message_start

text/event-stream

Schema

JSON 
{
  "additionalProperties": true,
  "properties": {
    "message": {
      "type": "object"
    },
    "type": {
      "const": "message_start",
      "type": "string"
    }
  },
  "required": [
    "type",
    "message"
  ],
  "type": "object"
}

Example

JSON 
{
  "message": {
    "content": [],
    "id": "msg_example",
    "model": "claude-sonnet-4-6",
    "role": "assistant",
    "stop_reason": null,
    "stop_sequence": null,
    "type": "message",
    "usage": {
      "input_tokens": 12,
      "output_tokens": 1
    }
  },
  "type": "message_start"
}
07

SSE event: content_block_delta

text/event-stream

Schema

JSON 
{
  "additionalProperties": true,
  "properties": {
    "delta": {
      "type": "object"
    },
    "index": {
      "type": "integer"
    },
    "type": {
      "const": "content_block_delta",
      "type": "string"
    }
  },
  "required": [
    "type",
    "index",
    "delta"
  ],
  "type": "object"
}

Example

JSON 
{
  "delta": {
    "text": "Hello",
    "type": "text_delta"
  },
  "index": 0,
  "type": "content_block_delta"
}
08

SSE event: message_delta

text/event-stream

Schema

JSON 
{
  "additionalProperties": true,
  "properties": {
    "delta": {
      "type": "object"
    },
    "type": {
      "const": "message_delta",
      "type": "string"
    },
    "usage": {
      "additionalProperties": true,
      "properties": {
        "output_tokens": {
          "type": "integer"
        }
      },
      "required": [
        "output_tokens"
      ],
      "type": "object"
    }
  },
  "required": [
    "type",
    "delta",
    "usage"
  ],
  "type": "object"
}

Example

JSON 
{
  "delta": {
    "stop_reason": "end_turn",
    "stop_sequence": null
  },
  "type": "message_delta",
  "usage": {
    "output_tokens": 8
  }
}
09

SSE event: message_stop

text/event-stream

Schema

JSON 
{
  "additionalProperties": true,
  "properties": {
    "type": {
      "const": "message_stop",
      "type": "string"
    }
  },
  "required": [
    "type"
  ],
  "type": "object"
}

Example

JSON 
{
  "type": "message_stop"
}
10

Protocol error: invalid_request

HTTP 400

Schema

JSON 
{
  "additionalProperties": true,
  "properties": {
    "error": {
      "additionalProperties": true,
      "properties": {
        "message": {
          "type": "string"
        },
        "type": {
          "type": "string"
        }
      },
      "required": [
        "type",
        "message"
      ],
      "type": "object"
    },
    "request_id": {
      "type": "string"
    },
    "type": {
      "const": "error",
      "type": "string"
    }
  },
  "required": [
    "type",
    "error"
  ],
  "type": "object"
}

Example

JSON 
{
  "error": {
    "message": "max_tokens is required",
    "type": "invalid_request_error"
  },
  "request_id": "req_example",
  "type": "error"
}
11

Usage

Schema

JSON 
{
  "additionalProperties": true,
  "properties": {
    "input_tokens": {
      "type": "integer"
    },
    "output_tokens": {
      "type": "integer"
    }
  },
  "required": [
    "input_tokens",
    "output_tokens"
  ],
  "type": "object"
}

Example

JSON 
{
  "input_tokens": 12,
  "output_tokens": 8
}
12

Generated Code Samples

Each cURL, JavaScript, and Python sample sends the validated request from this contract without requiring a protocol-specific SDK.

CURL 
curl -X POST https://runapi.ai/v1/messages \
  -H x-api-key:\ YOUR_API_TOKEN \
  -H Content-Type:\ application/json \
  -d \{\"model\":\"claude-sonnet-4-6\",\"max_tokens\":64,\"messages\":\[\{\"role\":\"user\",\"content\":\"Say\ hello\ in\ one\ sentence.\"\}\]\}
JAVASCRIPT 
const response = await fetch("https://runapi.ai/v1/messages", {
  method: "POST",
  headers: {
  "x-api-key": "YOUR_API_TOKEN",
  "Content-Type": "application/json"
},
  body: JSON.stringify({
  "model": "claude-sonnet-4-6",
  "max_tokens": 64,
  "messages": [
    {
      "role": "user",
      "content": "Say hello in one sentence."
    }
  ]
})
});
if (!response.ok) throw new Error(`HTTP ${response.status}`);
const data = await response.json();
console.log(data);

Install

BASH 
pip install requests
PYTHON 
import requests

response = requests.post(
    "https://runapi.ai/v1/messages",
    headers={"x-api-key": "YOUR_API_TOKEN", "Content-Type": "application/json"},
    json={"model": "claude-sonnet-4-6", "max_tokens": 64, "messages": [{"role": "user", "content": "Say hello in one sentence."}]}
)
response.raise_for_status()
print(response.json())