Practical MCP with FastMCP & LangChain

cat > $HOME/workspace/puppy_guide/server/main.py << EOF
import os
from difflib import get_close_matches
from typing import Annotated

from dotenv import load_dotenv
from pydantic import Field

from fastmcp import Context
from fastmcp import FastMCP
from fastmcp.exceptions import ToolError

load_dotenv()

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


BREED_MULTIPLIERS = {
    "labrador": 7,
    "chihuahua": 5,
    "german shepherd": 8,
    "bulldog": 6,
}


def get_breed_multiplier(breed: str) -> int | None:
    """Fetch the age multiplier for a given dog breed."""
    return BREED_MULTIPLIERS.get(breed.lower())


@mcp.tool
async 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")],
    ctx: Context,
    # New optional parameter for the dog's name
    name: Annotated[str | None, Field(description="The dog's name")] = None,
) -> str:
    """Calculate the real age of a dog in human years based on its breed."""

    total_steps = 4

    # If a name is provided, store it in session state for future calls
    if name:
        await ctx.set_state("dog_name", name)
    else:
        # No name provided — check if we stored one in a previous call
        name = await ctx.get_state("dog_name")

    # Progress: breed lookup
    await ctx.report_progress(
        progress=0,
        total=total_steps,
        message=f"Looking up breed multiplier for '{breed}'",
    )
    await ctx.info(f"Looking up breed multiplier for '{breed}'")

    multiplier = get_breed_multiplier(breed)

    if multiplier is None:
        # Look for 1 similar breed name
        suggestions = get_close_matches(
            breed.lower(), BREED_MULTIPLIERS.keys(), n=1, cutoff=0.6
        )

        if suggestions:
            await ctx.info(
                f"Breed '{breed}' not found, but found similar: {suggestions}"
            )
            result