Anthropic Messages Format

Create Message (Anthropic Format)

post

Creates a message using Anthropic's native Messages API format. Supports all Claude models through FastRouter with full feature parity including streaming, tool use, extended thinking, and vision.

Available at both:

• POST https://api.fastrouter.ai/api/v1/messages

• POST https://api.fastrouter.ai/v1/messages

Authentication: Use your FastRouter API key via either:

• x-api-key: YOUR_API_KEY (Anthropic style)

• Authorization: Bearer YOUR_API_KEY (OpenAI style)

Streaming: Set stream: true to receive Server-Sent Events (SSE) with Anthropic's native streaming format (message_start, content_block_start, content_block_delta, message_delta, etc.).

Cost tracking: The response usage object includes a cost field showing credits consumed.

Drop-in replacement: This endpoint is a drop-in replacement for Anthropic's /v1/messages API. Simply change the base URL to https://api.fastrouter.ai and use your FastRouter API key.

Authorizations

AuthorizationstringRequired

FastRouter API Key. Get yours at https://fastrouter.ai

Format: Authorization: Bearer YOUR_API_KEY

Body

modelstringRequired

Model ID. Use anthropic/ prefix or bare Anthropic model name. Examples: anthropic/claude-sonnet-4-20250514, claude-sonnet-4-20250514, anthropic/claude-opus-4-20250514

Example: anthropic/claude-sonnet-4-20250514

max_tokensintegerRequired

The maximum number of tokens to generate. Required for all requests.

Example: 1024

systemstringOptional

System prompt. Provides instructions or context to the model.

temperaturenumber · max: 1Optional

Randomness. Ranges from 0.0 to 1.0.

Example: 1

top_pnumberOptional

Nucleus sampling parameter.

Example: 0.999

top_kintegerOptional

Only sample from the top K options for each subsequent token.

stop_sequencesstring[]Optional

Custom text sequences that will cause the model to stop generating.

streambooleanOptional

Whether to stream the response using SSE.

Default: falseExample: false

Responses

200

Message created successfully

application/json

idstringOptional

Unique message identifier

Example: msg_01XFDUDYJgAACzvnptvVoYEL

typestring · enumOptionalExample: messagePossible values:

rolestring · enumOptionalExample: assistantPossible values:

modelstringOptionalExample: claude-sonnet-4-20250514

stop_reasonstring · enumOptionalExample: end_turnPossible values:

400

Bad Request - The request is malformed. This could be due to missing parameters, invalid formats, or routing errors.

Note: the 400 body shape is not uniform. Validation errors return the structured object below, but routing/model errors may instead return a plain string: { "error": "<message>" }. A few endpoints (e.g. some file operations) may return a plain-text body rather than JSON.

application/json

401

Invalid Credentials - Your API key is invalid, missing, or disabled. Check your credentials.

Note: the 401 error body uses code, message, param, and type (there is no status field), and type is invalid_request_error.

application/json

429

Rate Limited - You have exceeded your request limits (TPM/RPM). Slow down or increase your limits.

application/json

500

Internal Error - Something went wrong on our side. Retry the request, and contact support if the issue persists.

application/json

post/api/v1/messages

POST /api/v1/messages HTTP/1.1
Host: api.fastrouter.ai
Authorization: Bearer YOUR_SECRET_TOKEN
Content-Type: application/json
Accept: */*
Content-Length: 125

{
  "model": "anthropic/claude-sonnet-4-20250514",
  "max_tokens": 1024,
  "messages": [
    {
      "role": "user",
      "content": "Hello, how are you?"
    }
  ]
}

{
  "id": "msg_01XFDUDYJgAACzvnptvVoYEL",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Hello! I'm doing well, thank you for asking."
    }
  ],
  "model": "claude-sonnet-4-20250514",
  "stop_reason": "end_turn",
  "usage": {
    "input_tokens": 14,
    "output_tokens": 18,
    "cost": 0.000234
  }
}