手写 MCP（Model Context Protocol）客户端：JSON-RPC 2.0 与跨进程工具发现

┌──────────────────────┐          JSON-RPC 2.0           ┌─────────────────────────┐
│  JS Agent (MCP Host) │ ◄────── (Stdio / HTTP) ───────► │  Git / Postgres / File  │
│  ┌────────────────┐  │   1. initialize 握手协商能力    │       MCP Server        │
│  │  McpStdioClient│  │   2. tools/list 动态拉取 Schema │  (独立进程 / 任何语言)   │
│  └────────────────┘  │   3. tools/call 跨进程调用执行  └─────────────────────────┘
└──────────────────────┘

import { spawn } from 'node:child_process'

export class McpStdioClient {
  constructor({ command, args = [], env = {} }) {
    this.command = command
    this.args = args
    this.env = { ...process.env, ...env }
    this.proc = null
    this.nextId = 1
    this.pending = new Map() // id -> { resolve, reject, timer }
    this.stdoutBuffer = ''
  }

  async connect() {
    this.proc = spawn(this.command, this.args, {
      env: this.env,
      stdio: ['pipe', 'pipe', 'pipe']
    })

    this.proc.stdout.setEncoding('utf-8')
    this.proc.stdout.on('data', (chunk) => this.#onStdoutData(chunk))
    this.proc.on('exit', (code) => {
      for (const [, p] of this.pending) {
        clearTimeout(p.timer)
        p.reject(new Error(`MCP Server 进程已退出 (code=${code})`))
      }
      this.pending.clear()
    })

    // 1. 发送 MCP initialize 握手请求
    const initResult = await this.request('initialize', {
      protocolVersion: '2024-11-05',
      capabilities: {},
      clientInfo: { name: 'js-agent-mcp-client', version: '1.0.0' }
    })

    // 2. 发送 initialized 通知（JSON-RPC Notification 不含 id）
    this.notify('notifications/initialized', {})
    return initResult
  }

  #onStdoutData(chunk) {
    this.stdoutBuffer += chunk
    const lines = this.stdoutBuffer.split('\n')
    this.stdoutBuffer = lines.pop() || ''

    for (const raw of lines) {
      const line = raw.trim()
      if (!line) continue
      try {
        const msg = JSON.parse(line)
        if (msg.id !== undefined && this.pending.has(msg.id)) {
          const req = this.pending.get(msg.id)
          this.pending.delete(msg.id)
          clearTimeout(req.timer)
          if (msg.error) req.reject(new Error(`MCP Error [${msg.error.code}]: ${msg.error.message}`))
          else req.resolve(msg.result)
        }
      } catch {
        // 忽略非 JSON 日志行
      }
    }
  }

  request(method, params = {}, timeoutMs = 15000) {
    const id = this.nextId++
    const payload = JSON.stringify({ jsonrpc: '2.0', id, method, params }) + '\n'
    return new Promise((resolve, reject) => {
      const timer = setTimeout(() => {
        this.pending.delete(id)
        reject(new Error(`MCP 请求 ${method} (id=${id}) 超时`))
      }, timeoutMs)
      this.pending.set(id, { resolve, reject, timer })
      this.proc.stdin.write(payload)
    })
  }

  notify(method, params = {}) {
    const payload = JSON.stringify({ jsonrpc: '2.0', method, params }) + '\n'
    this.proc.stdin.write(payload)
  }

  // 3. 动态发现 MCP Server 暴露的所有工具
  async listTools() {
    const res = await this.request('tools/list', {})
    return res.tools || []
  }

  // 4. 调用指定的 MCP 工具并提取文本结果
  async callTool(name, args = {}) {
    const res = await this.request('tools/call', { name, arguments: args })
    const textBlocks = (res.content || [])
      .filter((c) => c.type === 'text')
      .map((c) => c.text)
      .join('\n')
    if (res.isError) throw new Error(textBlocks || 'MCP 工具执行返回错误')
    return textBlocks
  }

  // 5. 将 MCP Server 的全部工具自动桥接注册到 Agent 的 ToolRegistry
  async mountToRegistry(registry, namespace = 'mcp') {
    const mcpTools = await this.listTools()
    for (const t of mcpTools) {
      registry.register({
        name: `${namespace}_${t.name}`,
        description: t.description || '',
        schema: t.inputSchema || { type: 'object', properties: {} },
        execute: (args) => this.callTool(t.name, args)
      })
    }
  }

  close() {
    this.proc?.kill()
  }
}