MCP的通信協議 / MCP Communication Protocol

Protocol Overview

MCP utilizes JSON-RPC 2.0 as its communication format. The protocol enables structured communication between clients and servers, with clear patterns for requests, responses, notifications, and error handling.

1. JSON-RPC 2.0 Basic Structure

// Example Request:
{
  "jsonrpc": "2.0",
  "method": "subtract",
  "params": [42, 23],
  "id": 1
}
// Example Response:
{
  "jsonrpc": "2.0",
  "result": 19,
  "id": 1
}

Communication Patterns

Client
Request (method, params, id)
Server
Client
Response (result, id)
Server
Client
Notification (method, params)
No response
Server
Client
Error (code, message, id)
Server

JSON-RPC 2.0 Operation Examples

Here's a flow illustrating the actual operation of the protocol:

Client → Server: Request
  {"jsonrpc": "2.0", "method": "subtract", "params": [42, 23], "id": 1}

Server → Client: Response
  {"jsonrpc": "2.0", "result": 19, "id": 1}

Client → Server: Notification
  {"jsonrpc": "2.0", "method": "update", "params": [1, 2, 3, 4]}
  // No response required

Client → Server: Request (error case)
  {"jsonrpc": "2.0", "method": "unknownMethod", "id": 2}

Server → Client: Error Response
  {"jsonrpc": "2.0", "error": {"code": -32601, "message": "Method not found"}, "id": 2}

MCP Transport Methods

1. STD IO (Standard Input/Output)

  • Use Case: Local MCP servers (subprocesses or CLI tools)
  • Communication: Via stdin/stdout
  • Advantages: High efficiency, no network delay
  • Implementation: Client starts server as child process using OS pipes
  • Message Format: UTF-8 encoded JSON-RPC with newline delimiters
  • Communication Mode: Synchronous, messages processed in order

2. HTTP SSE (Server-Sent Events)

  • Use Case: Remote servers or cloud services
  • Key Feature: Supports server pushing events to clients
  • Ideal For: Long connections, large result transmission, real-time updates
  • Implementation:
    • Client and Server communicate over HTTP protocol
    • Server uses SSE to establish long-lived connections for pushing data
    • Client sends requests using HTTP POST to specific endpoints
  • Communication Mode: Asynchronous, event-driven model
  • Server Requirements: Two endpoints
    • SSE endpoint (e.g., /sse): For establishing event streams
    • HTTP POST endpoint (e.g., /messages): For receiving client requests

Advantages of the MCP Design

The MCP communication protocol offers several key benefits:

  • Lightweight & Language-agnostic: JSON is universally supported across programming languages
  • Flexibility: Supports both local and remote operation through different transport methods
  • Standardized: Based on the established JSON-RPC 2.0 specification
  • Debugging-friendly: Human-readable format makes troubleshooting easier
  • Cross-platform compatibility: Works consistently across different operating systems
  • Scalability: Can handle simple local interactions as well as distributed systems

協議概述

MCP 採用 JSON-RPC 2.0 作為通訊格式。該協議實現了客戶端和服務器之間的結構化通信，具有清晰的請求、響應、通知和錯誤處理模式。

1. JSON-RPC 2.0 基本結構

// 請求示例:
{
  "jsonrpc": "2.0",
  "method": "subtract",
  "params": [42, 23],
  "id": 1
}
// 回應示例:
{
  "jsonrpc": "2.0",
  "result": 19,
  "id": 1
}

通信模式

客戶端
請求 (method, params, id)
服務器
客戶端
回應 (result, id)
服務器
客戶端
通知 (method, params)
無需回應
服務器
客戶端
錯誤 (code, message, id)
服務器

JSON-RPC 2.0 實際運作過程

這是一個說明協議實際運作的流程：

客戶端 → 服務器: 請求
  {"jsonrpc": "2.0", "method": "subtract", "params": [42, 23], "id": 1}

服務器 → 客戶端: 回應
  {"jsonrpc": "2.0", "result": 19, "id": 1}

客戶端 → 服務器: 通知
  {"jsonrpc": "2.0", "method": "update", "params": [1, 2, 3, 4]}
  // 無需回應

客戶端 → 服務器: 請求 (錯誤範例)
  {"jsonrpc": "2.0", "method": "unknownMethod", "id": 2}

服務器 → 客戶端: 錯誤回應
  {"jsonrpc": "2.0", "error": {"code": -32601, "message": "Method not found"}, "id": 2}

MCP 傳輸方式

1. STDIO傳輸 (標準輸入/輸出)

  • 應用場景: 適用於本地運行的 MCP 伺服器，例如子程序或 CLI 工具
  • 通信方式: 透過標準輸入/輸出 (stdin/stdout) 進行通信
  • 優點: 無網路延遲，效率高
  • 實現機制: Client 啟動 Server 為子進程，利用作業系統管道傳輸資料
  • 消息格式: 使用 UTF-8 編碼的 JSON-RPC 格式，以換行符 \n 分隔
  • 通訊模式: 同步阻塞，訊息需依序處理

2. HTTP SSE (Server-Sent Events)

  • 應用場景: 適用於遠端伺服器或雲端服務
  • 功能特點: 支援伺服器主動推送事件給 Client
  • 適合場景: 適合長連線、大型結果的分段傳輸或即時狀態更新
  • 實現機制:
    • Client 與 Server 透過 HTTP 協議進行跨網路通信
    • Server 透過 SSE (Server-Sent Events) 建立長連接主動推送資料
    • Client 使用 HTTP POST 向指定端點發送請求
  • 通訊模式: 採用非同步事件驅動模型，支援即時或近即時互動
  • 伺服器實作要求: 提供兩個端點
    • SSE 端點 (如 /sse): 用於建立長連線接收事件流
    • HTTP POST 端點 (如 /messages): 用於接收 Client 的請求

MCP 設計的優勢

MCP 通信協議提供了多項重要優勢：

  • 輕量且語言無關: JSON 在各種程式語言中普遍支援
  • 靈活性: 通過不同的傳輸方法支援本地和遠端操作
  • 標準化: 基於已建立的 JSON-RPC 2.0 規範
  • 便於調試: 人類可讀的格式使故障排除更容易
  • 跨平台兼容性: 在不同操作系統上一致運作
  • 可擴展性: 可以處理簡單的本地互動以及分散式系統