LongCat API Platform Documentation

Overview

The LongCat API Platform provides AI model proxy services exclusively for the LongCat-Flash-Chat model, while maintaining compatibility with OpenAI and Anthropic API formats. This documentation follows standard API format conventions.

Base URLs

Production Endpoint: https://api.longcat.chat

Authentication

All API requests require authentication using an API key in the Authorization header:

Authorization: Bearer YOUR_API_KEY

Endpoints

Chat Completions

POST /openai/v1/chat/completions

Create a chat completion using OpenAI-compatible format.

Headers

Authorization: Bearer YOUR_API_KEY (required)
Content-Type: application/json

Request Body

Field	Type	Required	Description
`model`	string	Yes	Model identifier (only LongCat-Flash-Chat is supported)
`messages`	array	Yes	Array of message objects, only text inputs are allowed
`stream`	boolean	No	Whether to stream the response (default: false)
`max_tokens`	integer	No	Maximum number of tokens to generate, default to 1024
`temperature`	number	No	Sampling temperature between 0 and 1
`top_p`	number	No	Nucleus sampling parameter

Message Object

Field	Type	Required	Description
`role`	string	Yes	The role of the message author. Must be one of: • `system` - Sets the behavior and context for the assistant • `user` - Messages from the human user • `assistant` - Messages from the AI assistant (for conversation history)
`content`	string	Yes	The message content. A string for simple text messages.

Example Request

{
  "model": "LongCat-Flash-Chat",
  "messages": [
    {
      "role": "system",
      "content": "You are a helpful assistant."
    },
    {
      "role": "user",
      "content": "Hello, how are you?"
    }
  ],
  "stream": false,
  "max_tokens": 150,
  "temperature": 0.7
}

Response (Non-streaming)

{
  "id": "chatcmpl-123",
  "object": "chat.completion",
  "created": 1677652288,
  "model": "LongCat-Flash-Chat",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "content": "Hello! I'm doing well, thank you for asking. How can I help you today?"
      },
      "finish_reason": "stop"
    }
  ],
  "usage": {
    "prompt_tokens": 20,
    "completion_tokens": 15,
    "total_tokens": 35
  }
}

Response (Streaming)

When stream: true, the response is returned as Server-Sent Events (SSE):

Content-Type: text/event-stream

data: {"id":"chatcmpl-123","object":"chat.completion.chunk","created":1677652288,"model":"LongCat-Flash-Chat","choices":[{"index":0,"delta":{"role":"assistant","content":""},"finish_reason":null}]}

data: {"id":"chatcmpl-123","object":"chat.completion.chunk","created":1677652288,"model":"LongCat-Flash-Chat","choices":[{"index":0,"delta":{"content":"Hello"},"finish_reason":null}]}

data: {"id":"chatcmpl-123","object":"chat.completion.chunk","created":1677652288,"model":"LongCat-Flash-Chat","choices":[{"index":0,"delta":{"content":"!"},"finish_reason":null}]}

data: [DONE]

Anthropic Messages

POST /anthropic/v1/messages

Create a message using Anthropic's Claude API format.

Headers

Authorization: Bearer YOUR_API_KEY (required)
Content-Type: application/json

Request Body

Field	Type	Required	Description
`model`	string	Yes	The Claude model to use
`messages`	array	Yes	Array of message objects
`max_tokens`	integer	No	Maximum number of tokens to generate
`stream`	boolean	No	Whether to stream the response (default: false)
`temperature`	number	No	Sampling temperature between 0 and 1
`top_p`	number	No	Nucleus sampling parameter
`system`	string	No	System message to set context

Message Object

Field	Type	Required	Description
`role`	string	Yes	The role of the message author. Must be one of: • `user` - Messages from the human user • `assistant` - Messages from Claude (for conversation history) Note: System messages are passed separately via the `system` parameter
`content`	string	Yes	The message content. A string for text-only messages

Example Request

{
  "model": "LongCat-Flash-Chat",
  "max_tokens": 1000,
  "messages": [
    {
      "role": "user",
      "content": "Hello, LongCat"
    }
  ],
  "stream": false,
  "temperature": 0.7
}

Response (Non-streaming)

{
  "id": "msg_123",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Hello! How can I help you today?"
    }
  ],
  "model": "LongCat-Flash-Chat",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 10,
    "output_tokens": 8
  }
}

Response (Streaming)

When stream: true, the response follows Anthropic's SSE format:

Content-Type: text/event-stream

event: message_start
data: {"type": "message_start", "message": {"id": "msg_123", "type": "message", "role": "assistant", "content": [], "model": "LongCat-Flash-Chat", "stop_reason": null, "stop_sequence": null, "usage": {"input_tokens": 10, "output_tokens": 0}}}

event: content_block_start
data: {"type": "content_block_start", "index": 0, "content_block": {"type": "text", "text": ""}}

event: content_block_delta
data: {"type": "content_block_delta", "index": 0, "delta": {"type": "text_delta", "text": "Hello"}}

event: content_block_delta
data: {"type": "content_block_delta", "index": 0, "delta": {"type": "text_delta", "text": "!"}}

event: content_block_stop
data: {"type": "content_block_stop", "index": 0}

event: message_delta
data: {"type": "message_delta", "delta": {"stop_reason": "end_turn", "stop_sequence": null}, "usage": {"output_tokens": 8}}

event: message_stop
data: {"type": "message_stop"}

Error Responses

The API uses conventional HTTP response codes to indicate success or failure:

HTTP Status Codes

Status Code	Status Name	Description
`200`	OK	Request successful
`400`	Bad Request	Invalid request parameters or malformed JSON
`401`	Unauthorized	Invalid or missing API key
`403`	Forbidden	API key doesn't have permission for the requested resource
`429`	Too Many Requests	Rate limit exceeded
`500`	Internal Server Error	Server encountered an unexpected condition
`502`	Bad Gateway	Invalid response from upstream server
`503`	Service Unavailable	Server temporarily unavailable

Error Response Format

All errors return a JSON object with the following structure:

{
  "error": {
    "message": "Human-readable error description",
    "type": "error_type_identifier", 
    "code": "specific_error_code",
    "param": "parameter_name_if_applicable"
  }
}

Error Types and Codes

Error Type	Error Code	HTTP Status	Description
`authentication_error`	`invalid_api_key`	401	Invalid API key provided
`authentication_error`	`missing_api_key`	401	No API key provided
`permission_error`	`insufficient_quota`	403	API key has insufficient quota
`invalid_request_error`	`invalid_parameter`	400	Invalid parameter value
`invalid_request_error`	`missing_parameter`	400	Required parameter missing
`invalid_request_error`	`invalid_json`	400	Invalid JSON format
`rate_limit_error`	`rate_limit_exceeded`	429	Too many requests in a short period
`server_error`	`internal_error`	500	Internal server error

Example Error Responses

Invalid API Key

{
  "error": {
    "message": "Invalid API key provided",
    "type": "authentication_error",
    "code": "invalid_api_key"
  }
}

Missing Required Parameter

{
  "error": {
    "message": "Missing required parameter: 'messages'",
    "type": "invalid_request_error", 
    "code": "missing_parameter",
    "param": "messages"
  }
}

Rate Limit Exceeded

{
  "error": {
    "message": "Rate limit exceeded. Please try again in 60 seconds",
    "type": "rate_limit_error",
    "code": "rate_limit_exceeded"
  }
}

Rate Limiting

Rate limits are enforced per API key. When exceeded, you'll receive a 429 status code.

SDK Compatibility

This API is designed to be compatible with:

OpenAI Python SDK (for /openai/ endpoints)
Anthropic Python SDK (for /anthropic/ endpoints)
Any HTTP client that supports the respective API formats

Examples

Using with OpenAI Python SDK

import openai

# Configure for LongCat-Flash-Chat API
openai.api_base = "https://api.longcat.chat/openai"
openai.api_key = "your-api-key"

response = openai.ChatCompletion.create(
    model="LongCat-Flash-Chat",
    messages=[
        {"role": "user", "content": "Hello!"}
    ]
)

Using with Anthropic Python SDK

import anthropic

# Configure for LongCat-Flash-Chat API  
client = anthropic.Anthropic(
    api_key="Bearer your-api-key",
    base_url="https://api.longcat.chat"
)
default_headers={
        "Content-Type": "application/json",
        "Authorization": "Bearer your-api-key",
    }


message = client.messages.create(
    model="LongCat-Flash-Chat",
    max_tokens=150,
    messages=[
        {"role": "user", "content": "Hello, LongCat!"}
    ]
)

Using with cURL

# OpenAI-style request
curl -X POST https://api.longcat.chat/openai/v1/chat/completions \
  -H "Authorization: Bearer your-api-key" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "LongCat-Flash-Chat",
    "messages": [{"role": "user", "content": "Hello!"}],
    "stream": false
  }'

# Anthropic-style request
curl -X POST https://api.longcat.chat/anthropic/v1/messages \
  -H "Authorization: Bearer your-api-key" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "LongCat-Flash-Chat",
    "max_tokens": 1000,
    "messages": [{"role": "user", "content": "Hello!"}]
  }'

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