Practical MCP with FastMCP & LangChain

cat > main.py << EOF
import os
from typing import Annotated
from dotenv import load_dotenv
from pydantic import Field
from fastmcp import FastMCP
from fastmcp.exceptions import ToolError

load_dotenv()

mcp = FastMCP(
    name=os.getenv("MCP_NAME"),
    instructions=os.getenv("MCP_INSTRUCTIONS"),
)

def get_breed_multiplier(breed: str) -> int | None:
    """Fetch the age multiplier for a given dog breed."""
    breed_multipliers = {
        "labrador": 7,
        "chihuahua": 5,
        "german shepherd": 8,
        "bulldog": 6,
        # In reality there are over 350 recognized breeds,
        # but for brevity we only list a few here.
    }
    # Returns None if the breed is not found
    return breed_multipliers.get(breed.lower())

@mcp.tool
def dog_to_human_age(
    age: Annotated[int, Field(ge=0, le=30, description="The dog's age in years")],
    breed: Annotated[str, Field(description="The dog's breed")],
) -> int:
    """Calculate the real age of a dog in human years based on its breed."""
    multiplier = get_breed_multiplier(breed)
    if multiplier is None:
        raise ToolError