Building Your First MCP Server

python server.py

from datetime import datetime, timedelta
import random

@server.tool(
    name="get_forecast",
    description="Get weather forecast for the next N days",
)
async def get_forecast(
    city: Annotated[str, Field(description="City name")],
    days: Annotated[int, Field(ge=1, le=7, description="Number of days")] = 3,
) -> dict:
    city_key = city.lower()
    current = weather_data.get(city_key)
    
    if not current:
        return {"error": f"Weather data not available for {city}"}
    
    forecast = []
    for i in range(days):
        date = datetime.now() + timedelta(days=i)
        forecast.append({
            "day": i + 1,
            "date": date.strftime("%Y-%m-%d"),
            "temp": current["temp"] + random.randint(-5, 5),
            "condition": current["condition"],
        })
    
    return {
        "city": city,
        "forecast": forecast,
    }

import json

@server.resource(
    name="supported-cities",
    uri="weather://cities",
    title="Supported Cities",
    description="List of cities with weather data",
)
async def get_supported_cities() -> dict:
    cities = [c.title() for c in weather_data.keys()]
    return {
        "cities": cities,
        "count": len(cities),
    }

@server.resource(
    name="city-info",
    uri="weather://cities/{city}",
    title="City Weather Info",
)
async def get_city_info(city: str) -> str:
    city_key = city.lower()
    weather = weather_data.get(city_key)
    
    if not weather:
        return f"No data for {city}"
    
    return f"""
# Weather in {city}

- **Temperature**: {weather['temp']}°F
- **Condition**: {weather['condition']}
- **Humidity**: {weather['humidity']}%

*Last updated: {datetime.now().strftime('%Y-%m-%d %H:%M:%S')}*
    """

@server.prompt(
    name="weather-report",
    description="Generate a comprehensive weather report",
)
async def weather_report_prompt(
    city: str,
    include_advice: bool = True,
) -> str:
    advice = "4. Provide practical advice (what to wear, activities, etc.)" if include_advice else ""
    
    return f"""
Generate a weather report for {city}:

1. Get current weather conditions
2. Get 3-day forecast
3. Summarize the information in a friendly tone
{advice}

Format the report in a clear, conversational style.
    """

@server.prompt(
    name="compare-cities",
    description="Compare weather between two cities",
)
async def compare_cities_prompt(
    city1: str,
    city2: str,
) -> str:
    return f"""
Compare the weather between {city1} and {city2}:

1. Get current weather for both cities
2. Compare temperatures, conditions, and humidity
3. Recommend which city has better weather today
4. Explain your reasoning
    """

@server.tool(
    name="get_current_weather",
    description="Get current weather for a city",
)
async def get_current_weather(
    city: Annotated[str, Field(min_length=1, description="City name")],
    units: Annotated[str, Field(description="Temperature units")] = "fahrenheit",
) -> dict:
    try:
        city_key = city.lower().strip()
        
        if not city_key:
            return {
                "error": "City name cannot be empty",
                "code": "INVALID_INPUT",
            }
        
        weather = weather_data.get(city_key)
        
        if not weather:
            return {
                "error": f"Weather data not available for '{city}'",
                "code": "CITY_NOT_FOUND",
                "available_cities": list(weather_data.keys()),
            }
        
        temp = weather["temp"]
        if units == "celsius":
            temp = round((temp - 32) * 5/9)
        
        return {
            "success": True,
            "city": city,
            "temperature": temp,
            "units": units,
            "condition": weather["condition"],
            "humidity": weather["humidity"],
        }
    except Exception as e:
        return {
            "error": "An unexpected error occurred",
            "code": "INTERNAL_ERROR",
            "message": str(e),
        }