Writing the MCP Server

In this chapter, we’ll learn how to write a simple MCP Server. We’ll create a server that provides basic user information such as name, age, location, and interests.

Basic Concepts

An MCP (Model Control Protocol) Server is a server that extends Cursor IDE’s functionality. It allows us to define custom tools that can be called by Cursor to retrieve information or perform specific actions.

Writing the Code

Let’s open our main.py file and write the following code:

from mcp.server.fastmcp import FastMCP

# Initialize FastMCP server
mcp = FastMCP("about-me")

@mcp.tool()
async def get_user_name() -> str:
    """Get the user's name.
    """
    return "Lucas Lee"

@mcp.tool()
async def get_user_age() -> int:
    """Get the user's age.
    """
    return 30

@mcp.tool()
async def get_user_location() -> str:
    """Get the user's location.
    """
    return "San Francisco, CA"

@mcp.tool()
async def get_user_interests() -> list[str]:
    """Get the user's interests.
    """
    return ["travel", "food", "technology"]

if __name__ == "__main__":
    # Initialize and run the server
    mcp.run(transport='stdio')

Let’s break down this code:

Code Explanation

1. Importing Required Module

   from mcp.server.fastmcp import FastMCP

   We import the FastMCP class from the MCP library, which is the main class for creating an MCP server.

2. Initializing the Server

   mcp = FastMCP("about-me")

   Create a new MCP server instance named “about-me”.

3. Defining Tool Functions

   @mcp.tool()
   async def get_user_name() -> str:

   • Use the @mcp.tool() decorator to mark this function as an MCP tool
   • Functions are async because MCP server is asynchronous
   • Use type annotations -> str to specify return type

4. Implementing Information Retrieval

   • get_user_name(): Returns the user’s name
   • get_user_age(): Returns the user’s age
   • get_user_location(): Returns the user’s location
   • get_user_interests(): Returns a list of user’s interests

5. Starting the Server

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

   • Use mcp.run() to start the server
   • transport='stdio' specifies using standard input/output for communication

Customizing Your Information

You can customize the returned values according to your needs:

• Modify get_user_name() to return your actual name
• Add your real interests to get_user_interests()
• Update get_user_location() to your location

Running the Server

After writing the code, you can run the server in your terminal:

python main.py

Next Steps

In the next chapter, we’ll learn how to:

• Test the MCP server
• Add more custom functionality
• Handle errors and exceptions
• Add configuration options

Common Issues

1. Server Won’t Start?

   • Ensure virtual environment is activated
   • Check all dependencies are installed
   • Verify there are no syntax errors

2. Function Return Type Errors?

   • Ensure return values match type annotations
   • Strings use str
   • Numbers use int or float
   • Lists use list[type]