Tool Primitive

using System.ComponentModel;
using ModelContextProtocol.Server;

[McpServerToolType]
public class WeatherTools
{
    // Static or instance — both work. Instance methods get DI for the containing class.
    [McpServerTool, Description("Returns the current weather for a city.")]
    public static string GetWeather(
        [Description("City name, e.g. 'Brussels'")] string city,
        [Description("Units: 'celsius' or 'fahrenheit'")] string units = "celsius")
    {
        return $"{city}: 18°{units[0]}";
    }
}

.WithToolsFromAssembly()        // discovers all [McpServerToolType] in the calling assembly
.WithTools<WeatherTools>()      // explicit, single class

[McpServerTool(
    Name = "get_weather",                 // override the tool name
    Title = "Get current weather",        // human-readable display name
    Destructive = false,                  // hint: tool modifies state irreversibly
    Idempotent = true,                    // hint: same args ⇒ same result
    OpenWorld = true,                     // hint: interacts with external systems
    ReadOnly = true                       // hint: doesn't mutate any state
)]
[Description("Returns the current weather for a city.")]
public static string GetWeather(...) { ... }

[McpServerTool, Description("Fetches the latest commits for a repo.")]
public async Task<IEnumerable<Commit>> GetCommits(
    string owner,
    string repo,
    IGitHubClient github,                         // injected from DI
    CancellationToken cancellationToken)          // injected by the SDK
{
    return await github.GetCommitsAsync(owner, repo, cancellationToken);
}

public record Forecast(string City, double TempC, string Conditions);

[McpServerTool, Description("Returns a 3-day forecast.")]
public static Forecast[] GetForecast(string city) =>
    new[]
    {
        new Forecast(city, 18.0, "sunny"),
        new Forecast(city, 16.5, "cloudy"),
        new Forecast(city, 14.2, "rain"),
    };

[McpServerTool, Description("Generates a chart and returns it as a PNG.")]
public static ImageContentBlock RenderChart(string title)
{
    byte[] png = Renderer.Render(title);
    return ImageContentBlock.FromBytes(png, "image/png");
}

[McpServerTool, Description("Synthesises speech.")]
public static AudioContentBlock Speak(string text)
{
    byte[] wav = Tts.Synthesize(text);
    return AudioContentBlock.FromBytes(wav, "audio/wav");
}

[McpServerTool, Description("Returns the chart and a caption.")]
public static IEnumerable<ContentBlock> RenderAnnotatedChart(string title)
{
    byte[] png = Renderer.Render(title);
    return new ContentBlock[]
    {
        new TextContentBlock { Text = $"Chart for: {title}" },
        ImageContentBlock.FromBytes(png, "image/png"),
        new TextContentBlock { Text = "Generated at " + DateTime.UtcNow.ToString("u") }
    };
}

[McpServerTool, Description("Looks up a contract.")]
public static EmbeddedResourceBlock GetContract(string id)
{
    return new EmbeddedResourceBlock
    {
        Resource = new TextResourceContents
        {
            Uri = $"contracts://{id}",
            MimeType = "text/markdown",
            Text = LoadContract(id)
        }
    };
}

[McpServerTool, Description("Divides a by b.")]
public static double Divide(double a, double b)
{
    if (b == 0)
        throw new ArgumentException("Cannot divide by zero.");
    return a / b;
}

[McpServerTool, Description("…")]
public static CallToolResult Foo(...)
{
    return new CallToolResult
    {
        IsError = true,
        Content = [new TextContentBlock { Text = "Detailed error explanation for the LLM." }]
    };
}

[McpServerTool, Description("…")]
public static string Process(string input)
{
    if (string.IsNullOrWhiteSpace(input))
        throw new McpProtocolException("Missing required input", McpErrorCode.InvalidParams);
    return $"Processed: {input}";
}

await server.SendNotificationAsync(
    NotificationMethods.ToolListChangedNotification,
    new ToolListChangedNotificationParams(),
    cancellationToken);