Model Context Protocol (MCP)

Model Context Protocol (MCP) is an open protocol that standardizes how applications provide tools and context to language models. LangChain agents call tools defined on MCP servers through MCPAdapter, which discovers a server’s tools and adapts them into LangChain tools you can pass straight to create_agent. MCPAdapter is built on FastMCP, which handles transport inference, protocol negotiation, connection management, and authentication. This section covers the LangChain-specific layer and links out to the FastMCP client documentation for the connection details underneath.

The langchain.mcp namespace requires langchain[mcp]>=1.4.0 and is in beta. Importing from it raises a LangChainBetaWarning once per process. The API may change.If you used MCP before v1.4.0, see Migrate from langchain-mcp-adapters.

Install

Install LangChain with the mcp extra, which pulls in FastMCP:

pip install "langchain[mcp]"

uv add "langchain[mcp]"

Quickstart

Open an MCPAdapter, discover the server’s tools with list_tools(), and build the agent inside the context. The tools hold the client, so the agent stays usable after the context exits:

from langchain.agents import create_agent
from langchain.mcp import MCPAdapter


async def main():
    async with MCPAdapter("https://example.com/mcp") as adapter:
        tools = await adapter.list_tools()
        agent = create_agent("claude-sonnet-5", tools)
        return await agent.ainvoke({"messages": [{"role": "user", "content": "..."}]})

Transports

MCPAdapter infers the transport from the target you hand it, so the only thing that changes between an in-process server, a local script over stdio, and a remote URL is the target itself:

from pathlib import Path

from langchain.mcp import MCPAdapter

# An in-process FastMCP server: no subprocess, no socket. Ideal for tests.
in_memory = MCPAdapter(server)  # a FastMCP instance

# A script path is launched over stdio, one subprocess per adapter.
stdio = MCPAdapter(Path("weather_server.py"))

# A string must be an http(s) URL, reached over streamable HTTP.
http = MCPAdapter("https://example.com/mcp")

A target can be any of the following:

An http/https URL (str): reached over streamable HTTP.
A script path (Path): launched as a subprocess over stdio.
A transport object (StreamableTransport): a pre-configured transport object. See Client Transports.
An in-process FastMCP server: connected in-memory, with no subprocess or socket.
An MCPConfig dict ({"mcpServers": {...}}): several servers behind one adapter. See Connections.
A prebuilt fastmcp.Client: for full control over transport, caching, and protocol negotiation.

A str target must be an http or https URL. FastMCP resolves a string by testing it as a filesystem path before testing it as a URL, so a string naming an existing .py or .js file would launch that file as a subprocess. Because strings are the form a target most often arrives in from configuration or from a model, MCPAdapter rejects strings that don’t match the shape of a URL.

Next steps

Connections

Connection lifecycle, multiple servers, protocol eras, and caching.

Authentication

Bearer tokens, OAuth 2.1, and per-user server auth.

Tools

Load MCP tools into agents, control their execution, and handle their outputs.

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Get started

Core components

Middleware

Frontend

Advanced usage

Agent development

Production

Install

Quickstart

Transports

Next steps

Connections

Authentication

Tools

​Install

​Quickstart

​Transports

​Next steps

Connections

Authentication

Tools

Install

Quickstart

Transports

Next steps