Add a Typed Tool

Add a tool between the FastMCP instance and the run block:

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("mcp-server-demo")


@mcp.tool()
def add(a: int, b: int) -> int:
    """Add two integers and return the result."""
    return a + b


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

What the decorator contributes

The decorator registers add as a tool. The SDK uses the function name, type annotations, and docstring to construct the tool description and input schema exposed to clients.

This first tool is intentionally deterministic and side-effect free. It lets you verify discovery and invocation without introducing credentials, network calls, or destructive behavior.

• Both parameters have explicit integer types
• The return value has an explicit type
• The docstring describes the result in plain language
• The tool performs no hidden network or file-system work

Recheck the module

uv run python -m py_compile server.py