In this tutorial, we’ll build a simple MCP weather server and connect it to a host, Claude for Desktop. We’ll start with a basic setup, and then progress to more complex use cases.

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What we’ll be building

Many LLMs (including Claude) do not currently have the ability to fetch the forecast and severe weather alerts. Let’s use MCP to solve that!

We’ll build a server that exposes two tools: get-alerts and get-forecast. Then we’ll connect the server to an MCP host (in this case, Claude for Desktop):

Servers can connect to any client. We’ve chosen Claude for Desktop here for simplicity, but we also have guides on building your own client as well as a list of other clients here.

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Core MCP Concepts

MCP servers can provide three main types of capabilities:

  1. Resources: File-like data that can be read by clients (like API responses or file contents)
  2. Tools: Functions that can be called by the LLM (with user approval)
  3. Prompts: Pre-written templates that help users accomplish specific tasks

This tutorial will primarily focus on tools.

Let’s get started with building our weather server! You can find the complete code for what we’ll be building here.

Prerequisite knowledge

This quickstart assumes you have familiarity with:

  • Python
  • LLMs like Claude

System requirements

For Python, make sure you have Python 3.9 or higher installed.

Set up your environment

First, let’s install uv and set up our Python project and environment:

Make sure to restart your terminal afterwards to ensure that the uv command gets picked up.

Now, let’s create and set up our project:

Add this code to pyproject.toml:

...rest of config

[build-system]
requires = [ "hatchling",]
build-backend = "hatchling.build"

[project.scripts]
weather = "weather:main"

Add this code to __init__.py:

src/weather/__init__.py
from . import server
import asyncio

def main():
    """Main entry point for the package."""
    asyncio.run(server.main())

# Optionally expose other important items at package level
__all__ = ['main', 'server']

Now let’s dive into building your server.

Building your server

Importing packages

Add these to the top of your server.py:

from typing import Any
import asyncio
import httpx
from mcp.server.models import InitializationOptions
import mcp.types as types
from mcp.server import NotificationOptions, Server
import mcp.server.stdio

Setting up the instance

Then initialize the server instance and the base URL for the NWS API:

NWS_API_BASE = "https://api.weather.gov"
USER_AGENT = "weather-app/1.0"

server = Server("weather")

Implementing tool listing

We need to tell clients what tools are available. The list_tools() decorator registers this handler:

@server.list_tools()
async def handle_list_tools() -> list[types.Tool]:
    """
    List available tools.
    Each tool specifies its arguments using JSON Schema validation.
    """
    return [
        types.Tool(
            name="get-alerts",
            description="Get weather alerts for a state",
            inputSchema={
                "type": "object",
                "properties": {
                    "state": {
                        "type": "string",
                        "description": "Two-letter state code (e.g. CA, NY)",
                    },
                },
                "required": ["state"],
            },
        ),
        types.Tool(
            name="get-forecast",
            description="Get weather forecast for a location",
            inputSchema={
                "type": "object",
                "properties": {
                    "latitude": {
                        "type": "number",
                        "description": "Latitude of the location",
                    },
                    "longitude": {
                        "type": "number",
                        "description": "Longitude of the location",
                    },
                },
                "required": ["latitude", "longitude"],
            },
        ),
    ]

This defines our two tools: get-alerts and get-forecast.

Helper functions

Next, let’s add our helper functions for querying and formatting the data from the National Weather Service API:

async def make_nws_request(client: httpx.AsyncClient, url: str) -> dict[str, Any] | None:
    """Make a request to the NWS API with proper error handling."""
    headers = {
        "User-Agent": USER_AGENT,
        "Accept": "application/geo+json"
    }

    try:
        response = await client.get(url, headers=headers, timeout=30.0)
        response.raise_for_status()
        return response.json()
    except Exception:
        return None

def format_alert(feature: dict) -> str:
    """Format an alert feature into a concise string."""
    props = feature["properties"]
    return (
        f"Event: {props.get('event', 'Unknown')}\n"
        f"Area: {props.get('areaDesc', 'Unknown')}\n"
        f"Severity: {props.get('severity', 'Unknown')}\n"
        f"Status: {props.get('status', 'Unknown')}\n"
        f"Headline: {props.get('headline', 'No headline')}\n"
        "---"
    )

Implementing tool execution

The tool execution handler is responsible for actually executing the logic of each tool. Let’s add it:

@server.call_tool()
async def handle_call_tool(
    name: str, arguments: dict | None
) -> list[types.TextContent | types.ImageContent | types.EmbeddedResource]:
    """
    Handle tool execution requests.
    Tools can fetch weather data and notify clients of changes.
    """
    if not arguments:
        raise ValueError("Missing arguments")
  
    if name == "get-alerts":
        state = arguments.get("state")
        if not state:
            raise ValueError("Missing state parameter")

        # Convert state to uppercase to ensure consistent format
        state = state.upper()
        if len(state) != 2:
            raise ValueError("State must be a two-letter code (e.g. CA, NY)")

        async with httpx.AsyncClient() as client:
            alerts_url = f"{NWS_API_BASE}/alerts?area={state}"
            alerts_data = await make_nws_request(client, alerts_url)

            if not alerts_data:
                return [types.TextContent(type="text", text="Failed to retrieve alerts data")]

            features = alerts_data.get("features", [])
            if not features:
                return [types.TextContent(type="text", text=f"No active alerts for {state}")]

            # Format each alert into a concise string
            formatted_alerts = [format_alert(feature) for feature in features[:20]] # only take the first 20 alerts
            alerts_text = f"Active alerts for {state}:\n\n" + "\n".join(formatted_alerts)

            return [
                types.TextContent(
                    type="text",
                    text=alerts_text
                )
            ]
    elif name == "get-forecast":
        try:
            latitude = float(arguments.get("latitude"))
            longitude = float(arguments.get("longitude"))
        except (TypeError, ValueError):
            return [types.TextContent(
                type="text",
                text="Invalid coordinates. Please provide valid numbers for latitude and longitude."
            )]
            
        # Basic coordinate validation
        if not (-90 <= latitude <= 90) or not (-180 <= longitude <= 180):
            return [types.TextContent(
                type="text",
                text="Invalid coordinates. Latitude must be between -90 and 90, longitude between -180 and 180."
            )]

        async with httpx.AsyncClient() as client:
            # First get the grid point
            lat_str = f"{latitude}"
            lon_str = f"{longitude}"
            points_url = f"{NWS_API_BASE}/points/{lat_str},{lon_str}"
            points_data = await make_nws_request(client, points_url)

            if not points_data:
                return [types.TextContent(type="text", text=f"Failed to retrieve grid point data for coordinates: {latitude}, {longitude}. This location may not be supported by the NWS API (only US locations are supported).")]

            # Extract forecast URL from the response
            properties = points_data.get("properties", {})
            forecast_url = properties.get("forecast")
            
            if not forecast_url:
                return [types.TextContent(type="text", text="Failed to get forecast URL from grid point data")]

            # Get the forecast
            forecast_data = await make_nws_request(client, forecast_url)
            
            if not forecast_data:
                return [types.TextContent(type="text", text="Failed to retrieve forecast data")]

            # Format the forecast periods
            periods = forecast_data.get("properties", {}).get("periods", [])
            if not periods:
                return [types.TextContent(type="text", text="No forecast periods available")]

            # Format each period into a concise string
            formatted_forecast = []
            for period in periods:
                forecast_text = (
                    f"{period.get('name', 'Unknown')}:\n"
                    f"Temperature: {period.get('temperature', 'Unknown')}°{period.get('temperatureUnit', 'F')}\n"
                    f"Wind: {period.get('windSpeed', 'Unknown')} {period.get('windDirection', '')}\n"
                    f"{period.get('shortForecast', 'No forecast available')}\n"
                    "---"
                )
                formatted_forecast.append(forecast_text)

            forecast_text = f"Forecast for {latitude}, {longitude}:\n\n" + "\n".join(formatted_forecast)

            return [types.TextContent(
                type="text",
                text=forecast_text
            )]
    else:
        raise ValueError(f"Unknown tool: {name}")

Running the server

Finally, implement the main function to run the server:

async def main():
    # Run the server using stdin/stdout streams
    async with mcp.server.stdio.stdio_server() as (read_stream, write_stream):
        await server.run(
            read_stream,
            write_stream,
            InitializationOptions(
                server_name="weather",
                server_version="0.1.0",
                capabilities=server.get_capabilities(
                    notification_options=NotificationOptions(),
                    experimental_capabilities={},
                ),
            ),
        )

# This is needed if you'd like to connect to a custom client
if __name__ == "__main__":
    asyncio.run(main())

Your server is complete! Run uv run src/weather/server.py to confirm that everything’s working.

Let’s now test your server from an existing MCP host, Claude for Desktop.

Testing your server with Claude for Desktop

Claude for Desktop is not yet available on Linux. Linux users can proceed to the Building a client tutorial to build an MCP client that connects to the server we just built.

First, make sure you have Claude for Desktop installed. You can install the latest version here. If you already have Claude for Desktop, make sure it’s updated to the latest version.

We’ll need to configure Claude for Desktop for whichever MCP servers you want to use. To do this, open your Claude for Desktop App configuration at ~/Library/Application Support/Claude/claude_desktop_config.json in a text editor. Make sure to create the file if it doesn’t exist.

For example, if you have VS Code installed:

code ~/Library/Application\ Support/Claude/claude_desktop_config.json

You’ll then add your servers in the mcpServers key. The MCP UI elements will only show up in Claude for Desktop if at least one server is properly configured.

In this case, we’ll add our single weather server like so:

Python
{
    "mcpServers": {
        "weather": {
            "command": "uv",
            "args": [
                "--directory",
                "/ABSOLUTE/PATH/TO/PARENT/FOLDER/weather",
                "run",
                "weather"
            ]
        }
    }
}

Make sure you pass in the absolute path to your server.

This tells Claude for Desktop:

  1. There’s an MCP server named “weather”
  2. To launch it by running uv --directory /ABSOLUTE/PATH/TO/PARENT/FOLDER/weather run weather

Save the file, and restart Claude for Desktop.

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Test with commands

Let’s make sure Claude for Desktop is picking up the two tools we’ve exposed in our weather server. You can do this by looking for the hammer icon:

After clicking on the hammer icon, you should see two tools listed:

If your server isn’t being picked up by Claude for Desktop, proceed to the Troubleshooting section for debugging tips.

If the hammer icon has shown up, you can now test your server by running the following commands in Claude for Desktop:

  • What’s the weather in Sacramento?
  • What are the active weather alerts in Texas?

Since this is the US National Weather service, the queries will only work for US locations.

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What’s happening under the hood

When you ask a question:

  1. The client sends your question to Claude
  2. Claude analyzes the available tools and decides which one(s) to use
  3. The client executes the chosen tool(s) through the MCP server
  4. The results are sent back to Claude
  5. Claude formulates a natural language response
  6. The response is displayed to you!

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Troubleshooting

For more advanced troubleshooting, check out our guide on Debugging MCP

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Next steps

Building a client

Learn how to build your an MCP client that can connect to your server

Example servers

Check out our gallery of official MCP servers and implementations

Debugging Guide

Learn how to effectively debug MCP servers and integrations

Building MCP with LLMs

Learn how to use LLMs like Claude to speed up your MCP development

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