Practical MCP with FastMCP & LangChain

Prototyping an MCP Server

In this section, we will prototype a simple MCP server that provides a single tool to the model: a set of calculator functions that can perform basic sum operation.

This is the flow we will follow:

• The model will receive a user query that requires performing a calculation, for example: What is the sum of 5 and 10?.

• The model will recognize that it needs to perform a calculation and our application will call the appropriate tool provided by our MCP server, for example: sum(5, 10).

• The MCP server will execute the tool, perform the calculation, and return the result to the application, for example: 15.

Let's start by the sum tool. The following command creates a new file called calculator_server.py and adds the code for the sum tool:

cat <<EOF > $HOME/workspace/mcp-server/calculator_server.py
# Import the FastMCP class from the MCP SDK
from mcp.server.fastmcp import FastMCP

# Create a new MCP server instance
mcp_server = FastMCP("Calculator Server")

# Add a tool
@mcp_server.tool()