Model Context Protocol (MCP)

Model Context Protocol (MCP) 是一个开源协议，它标准化了应用程序如何向 LLMs 提供工具和上下文。LangChain agent 可以使用定义在 MCP 服务器上的工具，通过 langchain-mcp-adapters 库实现。

快速开始

安装 langchain-mcp-adapters 库：

pip

pip install langchain-mcp-adapters

uv add langchain-mcp-adapters

langchain-mcp-adapters 使 agent 能够使用一个或多个 MCP 服务器上定义的工具。

MultiServerMCPClient 默认是无状态的。每次工具调用都会创建一个全新的 MCP ClientSession，执行工具，然后进行清理。详情请参阅 状态会话 部分。

访问多个 MCP 服务器

import asyncio
from langchain_mcp_adapters.client import MultiServerMCPClient  
from langchain.agents import create_agent

async def main():
    client = MultiServerMCPClient(
        {
            "math": {
                "transport": "stdio",  # 本地子进程通信
                "command": "python",
                # math_server.py 文件的绝对路径
                "args": ["/path/to/math_server.py"],
            },
            "weather": {
                "transport": "http",  # HTTP 远程服务器
                # 确保在端口 8000 启动 weather 服务器
                "url": "http://localhost:8000/mcp",
            }
        }
    )

    tools = await client.get_tools()
    agent = create_agent(
        "claude-sonnet-4-6",
        tools  
    )
    math_response = await agent.ainvoke(
        {"messages": [{"role": "user", "content": "what's (3 + 5) x 12?"}]}
    )
    weather_response = await agent.ainvoke(
        {"messages": [{"role": "user", "content": "what is the weather in nyc?"}]}
    )
    print(math_response)
    print(weather_response)

if __name__ == "__main__":
    asyncio.run(main())

自定义服务器

要创建一个自定义 MCP 服务器，请使用 FastMCP 库：

pip

pip install fastmcp

uv add fastmcp

要用 MCP 工具服务器测试你的 agent，请使用以下示例：

数学服务器 (stdio 传输)

from fastmcp import FastMCP

mcp = FastMCP("Math")

@mcp.tool()
def add(a: int, b: int) -> int:
    """相加两个数字"""
    return a + b

@mcp.tool()
def multiply(a: int, b: int) -> int:
    """相乘两个数字"""
    return a * b

if __name__ == "__main__":
    mcp.run(transport="stdio")

from fastmcp import FastMCP

mcp = FastMCP("Weather")

@mcp.tool()
async def get_weather(location: str) -> str:
    """获取地点天气."""
    return "纽约总是晴天"

if __name__ == "__main__":
    mcp.run(transport="streamable-http")

:::js

数学服务器 (stdio 传输)

import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import {
    CallToolRequestSchema,
    ListToolsRequestSchema,
} from "@modelcontextprotocol/sdk/types.js";

const server = new Server(
    {
        name: "math-server",
        version: "0.1.0",
    },
    {
        capabilities: {
            tools: {},
        },
    }
);

server.setRequestHandler(ListToolsRequestSchema, async () => {
    return {
        tools: [
        {
            name: "add",
            description: "相加两个数字",
            inputSchema: {
                type: "object",
                properties: {
                    a: {
                        type: "number",
                        description: "第一个数字",
                    },
                    b: {
                        type: "number",
                        description: "第二个数字",
                    },
                },
                required: ["a", "b"],
            },
        },
        {
            name: "multiply",
            description: "相乘两个数字",
            inputSchema: {
                type: "object",
                properties: {
                    a: {
                        type: "number",
                        description: "第一个数字",
                    },
                    b: {
                        type: "number",
                        description: "第二个数字",
                    },
                },
                required: ["a", "b"],
            },
        },
        ],
    };
});

server.setRequestHandler(CallToolRequestSchema, async (request) => {
    switch (request.params.name) {
        case "add": {
            const { a, b } = request.params.arguments as { a: number; b: number };
            return {
                content: [
                {
                    type: "text",
                    text: String(a + b),
                },
                ],
            };
        }
        case "multiply": {
            const { a, b } = request.params.arguments as { a: number; b: number };
            return {
                content: [
                {
                    type: "text",
                    text: String(a * b),
                },
                ],
            };
        }
        default:
            throw new Error(`Unknown tool: ${request.params.name}`);
    }
});

async function main() {
    const transport = new StdioServerTransport();
    await server.connect(transport);
    console.error("Math MCP server running on stdio");
}

main();

天气服务器 (SSE 传输)

import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";
import {
    CallToolRequestSchema,
    ListToolsRequestSchema,
} from "@modelcontextprotocol/sdk/types.js";
import express from "express";

const app = express();
app.use(express.json());

const server = new Server(
    {
        name: "weather-server",
        version: "0.1.0",
    },
    {
        capabilities: {
            tools: {},
        },
    }
);

server.setRequestHandler(ListToolsRequestSchema, async () => {
    return {
        tools: [
        {
            name: "get_weather",
            description: "获取地点天气",
            inputSchema: {
            type: "object",
            properties: {
                location: {
                type: "string",
                description: "要获取天气的地点",
                },
            },
            required: ["location"],
            },
        },
        ],
    };
});

server.setRequestHandler(CallToolRequestSchema, async (request) => {
    switch (request.params.name) {
        case "get_weather": {
            const { location } = request.params.arguments as { location: string };
            return {
                content: [
                    {
                        type: "text",
                        text: `It's always sunny in ${location}`,
                    },
                ],
            };
        }
        default:
            throw new Error(`Unknown tool: ${request.params.name}`);
    }
});

app.post("/mcp", async (req, res) => {
    const transport = new SSEServerTransport("/mcp", res

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