Model Context Protocol (MCP) in C#: Build Servers and Clients

using System.ComponentModel;
using System.Security.Claims;
using Microsoft.AspNetCore.Authorization;
using Microsoft.Extensions.Logging;
using ModelContextProtocol.Server;

public enum Warehouse { Sea, Ams, Sin }

public sealed record StockLevel(string Sku, Warehouse Warehouse, int OnHand, int Reserved);

[McpServerToolType]
public sealed class InventoryTools(IInventoryService inventory, ILogger<InventoryTools> log)
{
    [McpServerTool(Name = "get_stock_level", ReadOnly = true, Idempotent = true,
        UseStructuredContent = true)]
    [Description("Returns on-hand and reserved quantities for a SKU in one warehouse.")]
    public async Task<StockLevel> GetStockLevelAsync(
        [Description("Product SKU, for example BK-1042.")] string sku,
        [Description("Warehouse code.")] Warehouse warehouse,
        ClaimsPrincipal? user,
        CancellationToken cancellationToken)
    {
        log.LogInformation("Stock lookup {Sku} by {User}", sku, user?.Identity?.Name);
        return await inventory.GetAsync(sku, warehouse, cancellationToken);
    }

    [McpServerTool(Name = "reserve_stock", Destructive = false, Idempotent = false)]
    [Description("Reserves units of a SKU for an order. Requires the Fulfillment role.")]
    [Authorize(Roles = "Fulfillment")]
    public Task<string> ReserveStockAsync(string sku, Warehouse warehouse,
        [Description("Units to reserve, 1 to 500.")] int quantity, string orderId,
        CancellationToken cancellationToken) =>
        inventory.ReserveAsync(sku, warehouse, Math.Clamp(quantity, 1, 500), orderId,
            cancellationToken);
}

using Microsoft.Extensions.AI; // ChatMessage and ChatRole for prompts

[McpServerResourceType]
public sealed class RunbookResources(IRunbookStore store)
{
    [McpServerResource(UriTemplate = "runbook://{service}", Name = "Service runbook",
        MimeType = "text/markdown")]
    [Description("The operational runbook for a service.")]
    public async Task<string> GetRunbookAsync(string service, CancellationToken ct) =>
        await store.FindAsync(service, ct) ?? throw new McpException($"No runbook for {service}");
}

[McpServerPromptType]
public static class IncidentPrompts
{
    [McpServerPrompt, Description("Drafts a customer-facing incident update.")]
    public static ChatMessage IncidentUpdate(
        [Description("Affected service.")] string service,
        [Description("Current status, for example Investigating.")] string status) =>
        new(ChatRole.User,
            $"Write a short, factual status update for {service}. Status: {status}. " +
            "Avoid speculation about root cause.");
}