lc2mcp

Convert LangChain tools to FastMCP tools — in one line of code.

Stop rewriting your tools. Just adapt them.

lc2mcp is a lightweight adapter that converts existing LangChain tools into FastMCP tools, enabling you to quickly build MCP servers accessible to Claude, Cursor, and any MCP-compatible client.

✨ Features

| Feature | Description | |---------|-------------| | 🔄 Instant Conversion | One function call to convert any LangChain tool to FastMCP tool | | 📦 Ecosystem Access | Unlock 1000+ LangChain community tools (Search, Wikipedia, SQL, APIs...) | | 🎯 Zero Boilerplate | Automatic Pydantic → JSON Schema conversion | | 🔐 Context Injection | Pass auth, user info, and request context to tools | | 📊 Progress & Logging | Full support for MCP progress notifications and logging | | 🏷️ Namespace Support | Prefix tool names and handle conflicts automatically |

🚀 Quick Start

Installation

pip install lc2mcp

3 Lines to MCP

from langchain_core.tools import tool
from fastmcp import FastMCP
from lc2mcp import register_tools

@tool
def get_weather(city: str) -> str:
    """Get current weather for a city."""
    return f"Sunny, 25°C in {city}"

mcp = FastMCP("weather-server")
register_tools(mcp, [get_weather])  # ← That's it!

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

Your tool is now available to Claude, Cursor, and any MCP client.

Tool Parameter Descriptions

To include parameter descriptions in the MCP tool schema, you have two options:

Option 1: Use parse_docstring=True (Recommended for simplicity)

@tool(parse_docstring=True)
def get_weather(city: str, unit: str = "celsius") -> str:
    """Get current weather for a city.

    Args:
        city: The name of the city to query
        unit: Temperature unit (celsius or fahrenheit)
    """
    return f"Sunny, 25°C in {city}"

Option 2: Use args_schema (Recommended for complex types)

from pydantic import BaseModel, Field

class WeatherInput(BaseModel):
    city: str = Field(description="The name of the city to query")
    unit: str = Field(default="celsius", description="Temperature unit")

@tool(args_schema=WeatherInput)
def get_weather(city: str, unit: str = "celsius") -> str:
    """Get current weather for a city."""
    return f"Sunny, 25°C in {city}"

Note: Without parse_docstring=True or args_schema, parameter descriptions from docstrings will not be extracted.

🔌 How It Works

┌─────────────────┐      ┌─────────────┐      ┌─────────────────┐
│  LangChain Tool │ ───▶ │   lc2mcp    │ ───▶ │  FastMCP Tool   │
│  (@tool, etc.)  │      │  (adapter)  │      │                 │
└─────────────────┘      └─────────────┘      └────────┬────────┘
                                                       │
                                                       ▼
                                              ┌─────────────────┐
                                              │  FastMCP Server │
                                              └────────┬────────┘
                                                       │
                                                       ▼
                                          ┌───────────────────────┐
                                          │      MCP Clients      │
                                          │ (Claude, Cursor, ...) │
                                          └───────────────────────┘

🔄 lc2mcp vs langchain-mcp-adapters

LangChain and MCP ecosystems can be connected in both directions:

| Direction | Tool | Description | |-----------|------|--------

... View full README on GitHub