Program.cs

#!/usr/bin/env -S dotnet run --
#:package Microsoft.Extensions.Hosting
#:project ../../src/ModelContextProtocol/ModelContextProtocol.csproj

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using ModelContextProtocol;
using ModelContextProtocol.Server;
using System.ComponentModel;
using System.Globalization;
using System.Net.Http.Headers;
using System.Text.Json;

var builder = Host.CreateApplicationBuilder(args);

builder.Services.AddMcpServer()
    .WithStdioServerTransport()
    .WithTools<WeatherTools>();

builder.Logging.AddConsole(options =>
{
    options.LogToStandardErrorThreshold = LogLevel.Trace;
});

using var httpClient = new HttpClient { BaseAddress = new Uri("https://api.weather.gov") };
httpClient.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("weather-tool", "1.0"));
builder.Services.AddSingleton(httpClient);

await builder.Build().RunAsync();

// Weather Tools
[McpServerToolType]
file sealed class WeatherTools
{
    [McpServerTool, Description("Get weather alerts for a US state.")]
    public static async Task<string> GetAlerts(
        HttpClient client,
        [Description("The US state to get alerts for. Use the 2 letter abbreviation for the state (e.g. NY).")] string state)
    {
        using var jsonDocument = await client.ReadJsonDocumentAsync($"/alerts/active/area/{state}");
        var jsonElement = jsonDocument.RootElement;
        var alerts = jsonElement.GetProperty("features").EnumerateArray();

        if (!alerts.Any())
        {
            return "No active alerts for this state.";
        }

        return string.Join("\n--\n", alerts.Select(alert =>
        {
            JsonElement properties = alert.GetProperty("properties");
            return $"""
                    Event: {properties.GetProperty("event").GetString()}
                    Area: {properties.GetProperty("areaDesc").GetString()}
                    Severity: {properties.GetProperty("severity").GetString()}
                    Description: {properties.GetProperty("description").GetString()}
                    Instruction: {properties.GetProperty("instruction").GetString()}
                    """;
        }));
    }

    [McpServerTool, Description("Get weather forecast for a location.")]
    public static async Task<string> GetForecast(
        HttpClient client,
        [Description("Latitude of the location.")] double latitude,
        [Description("Longitude of the location.")] double longitude)
    {
        var pointUrl = string.Create(CultureInfo.InvariantCulture, $"/points/{latitude},{longitude}");
        using var locationDocument = await client.ReadJsonDocumentAsync(pointUrl);
        var forecastUrl = locationDocument.RootElement.GetProperty("properties").GetProperty("forecast").GetString()
            ?? throw new McpException($"No forecast URL provided by {client.BaseAddress}points/{latitude},{longitude}");

        using var forecastDocument = await client.ReadJsonDocumentAsync(forecastUrl);
        var periods = forecastDocument.RootElement.GetProperty("properties").GetProperty("periods").EnumerateArray();

        return string.Join("\n---\n", periods.Select(period => $"""
                {period.GetProperty("name").GetString()}
                Temperature: {period.GetProperty("temperature").GetInt32()}°F
                Wind: {period.GetProperty("windSpeed").GetString()} {period.GetProperty("windDirection").GetString()}
                Forecast: {period.GetProperty("detailedForecast").GetString()}
                """));
    }
}

// HttpClient Extension Methods
file static class HttpClientExt
{
    public static async Task<JsonDocument> ReadJsonDocumentAsync(this HttpClient client, string requestUri)
    {
        using var response = await client.GetAsync(requestUri);
        response.EnsureSuccessStatusCode();
        return await JsonDocument.ParseAsync(await response.Content.ReadAsStreamAsync());
    }
}