C# MCP Server Creation

Create Model Context Protocol servers using the official C# SDK (ModelContextProtocol NuGet package) and the dotnet new mcpserver project template. Servers expose tools, prompts, and resources that LLMs can discover and invoke via the MCP protocol.

When to Use

• Starting a new MCP server project from scratch
• Adding tools, prompts, or resources to an existing MCP server
• Choosing between stdio (--transport local) and HTTP (--transport remote) transport
• Setting up ASP.NET Core hosting for an HTTP MCP server
• Wrapping an external API or service as MCP tools

Stop Signals

• Server already exists and needs debugging? → Use mcp-csharp-debug
• Need tests or evaluations? → Use mcp-csharp-test
• Ready to publish? → Use mcp-csharp-publish
• Building an MCP client, not a server → This skill is server-side only

Inputs

| Input | Required | Description | |-------|----------|-------------| | Transport type | Yes | stdio (local/CLI) or http (remote/web). Ask user if not specified — default to stdio | | Project name | Yes | PascalCase name for the project (e.g., WeatherMcpServer) | | .NET SDK version | Recommended | .NET 10.0+ required. Check with dotnet --version | | Service/API to wrap | Recommended | External API or service the tools will interact with |

Workflow

> Commit strategy: Commit after completing each step so scaffolding and implementation are separately reviewable.

Step 1: Verify prerequisites

1. Confirm .NET 10+ SDK: dotnet --version (install from https://dotnet.microsoft.com if < 10.0)

1. Check if the MCP server template is already installed:

bash dotnet new list mcpserver If "No templates found" → install: dotnet new install Microsoft.McpServer.ProjectTemplates

Step 2: Choose transport

| Choose stdio if… | Choose HTTP if… | |----------------------|---------------------| | Local CLI tool or IDE plugin | Cloud/web service deployment | | Single user at a time | Multiple simultaneous clients | | Running as subprocess (VS Code, GitHub Copilot) | Cross-network access needed | | Simpler setup, no network config | Containerized deployment (Docker/Azure) |

Default: stdio — simpler, works for most local development. Users can add HTTP later.

Step 3: Scaffold the project

stdio server:

bash
dotnet new mcpserver -n <ProjectName>

If the template times out or is unavailable, use dotnet new console -n <ProjectName> and add dotnet add package ModelContextProtocol.

HTTP server:

bash
dotnet new web -n <ProjectName>
cd <ProjectName>
dotnet add package ModelContextProtocol.AspNetCore

This is the recommended approach — faster and more reliable than the template. The template also supports HTTP via dotnet new mcpserver -n <ProjectName> --transport remote, but dotnet new web gives you more control over the project structure.

Template flags reference: --transport local (stdio, default), --transport remote (ASP.NET Core HTTP), --aot, --self-contained.

Step 4: Implement tools

Tools are the primary way MCP servers expose functionality. Add a class with [McpServerToolType] and methods with [McpServerTool]:

csharp
using ModelContextProtocol.Server;
using System.ComponentModel;

[McpServerToolType]
public static class MyTools
{
    [McpServerTool, Description("Brief description of what the tool does.")]
    public static async Task<string> DoSomething(
        [Description("What this parameter controls")] string input,
        CancellationToken cancellationToken = default)
    {
        // Implementation
        return $"Result: {input}";
    }
}

Critical rules:

• Every tool method must have a [Description] attribute — LLMs use this to decide when to call the tool
• Every parameter must have a [Description] attribute
• Accept CancellationToken in all async tools
• Use [McpServerTool(Name = "custom_name")] only if the default method name is unclear

DI injection patterns — the SDK supports two styles:

1. Method parameter injection (static class): DI services appear as method parameters. The SDK resolves them automatically — they do not appear in the tool schema.

1. Constructor injection (non-static class): Use when tools need shared state or multiple services:

csharp
[McpServerToolType]
public class ApiTools(HttpClient httpClient, ILogger<ApiTools> logger)
{
    [McpServerTool, Description("Fetch a resource by ID.")]
    public async Task<string> FetchResource(
        [Description("Resource identifier")] string id,
        CancellationToken cancellationToken = default)
    {
        logger.LogInformation("Fetching {Id}", id);
        return await httpClient.GetStringAsync($"/api/{id}", cancellationToken);
    }
}

Register services in Program.cs:

csharp
var builder = Host.CreateApplicationBuilder(args);
builder.Logging.AddConsole(options =>
    options.LogToStandardErrorThreshold = LogLevel.Trace);

builder.Services.AddHttpClient();        // registers IHttpClientFactory + HttpClient
// ILogger<T> is registered by default — no extra setup needed.

builder.Services.AddMcpServer()
    .WithStdioServerTransport()
    .WithToolsFromAssembly();            // discovers non-static [McpServerToolType] classes

await builder.Build().RunAsync();

For the full attribute reference, return types, DI injection, and builder API patterns, see references/api-patterns.md.

Step 5: Add prompts and resources (optional)

Prompts — reusable LLM interaction templates:

csharp
[McpServerPromptType]
public static class MyPrompts
{
    [McpServerPrompt, Description("Summarize content into one sentence.")]
    public static ChatMessage Summarize(
        [Description("Content to summarize")] string content) =>
        new(ChatRole.User, $"Summarize this into one sentence: {content}");
}

Resources — data the LLM can read:

csharp
[McpServerResourceType]
public static class MyResources
{
    [McpServerResource(UriTemplate = "config://app", Name = "App Config",
        MimeType = "application/json"), Description("Application configuration")]
    public static string GetConfig() => JsonSerializer.Serialize(AppConfig.Current);
}

Step 6: Configure Program.cs

stdio transport:

csharp
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using ModelContextProtocol.Server;

var builder = Host.CreateApplicationBuilder(args);
builder.Logging.AddConsole(options =>
    options.LogToStandardErrorThreshold = LogLevel.Trace); // CRITICAL: stderr only

builder.Services.AddMcpServer()
    .WithStdioServerTransport()
    .WithToolsFromAssembly();

await builder.Build().RunAsync();

HTTP transport:

csharp
using ModelContextProtocol.Server;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddMcpServer()
    .WithHttpTransport()
    .WithToolsFromAssembly();

// Register services your tools need via DI
// builder.Services.AddHttpClient();
// builder.Services.AddSingleton<IMyService, MyService>();

var app = builder.Build();
app.MapMcp();                     // exposes MCP endpoint at /mcp (Streamable HTTP)
app.MapGet("/health", () => "ok"); // health check for container orchestrators
app.Run();

Key HTTP details: MapMcp() defaults to /mcp path. For containers, set ASPNETCORE_URLS=http://+:8080 and EXPOSE 8080. The MCP HTTP protocol uses Streamable HTTP — no special client config needed beyond the URL.

For transport configuration details (stateless mode, auth, path prefix, HttpContextAccessor), see references/transport-config.md.

Step 7: Verify the server starts

bash
cd <ProjectName>
dotnet build
dotnet run

For stdio: the process starts and waits for JSON-RPC input on stdin. For HTTP: the server listens on the configured port.

Validation

• ] Project builds with no errors (dotnet build)
• ] All tool classes have [McpServerToolType] attribute
• ] All tool methods have [McpServerTool] and [Description] attributes
• ] All parameters have [Description] attributes
• ] stdio: logging directed to stderr, not stdout
• ] HTTP: app.MapMcp() is called in Program.cs
• ] Server starts successfully with dotnet run

Common Pitfalls

| Pitfall | Solution | |---------|----------| | stdio server outputs garbage or hangs | Logging to stdout corrupts JSON-RPC protocol. Set LogToStandardErrorThreshold = LogLevel.Trace | | Tool not discovered by LLM clients | Missing [McpServerToolType] on the class or [McpServerTool] on the method. Verify .WithToolsFromAssembly() in Program.cs | | LLM doesn't understand when to use a tool | Add clear [Description] attributes on both the method and all parameters | | WithToolsFromAssembly() fails in AOT | Reflection-based discovery is incompatible with Native AOT. Use .WithTools<MyTools>() instead | | Parameters not appearing in tool schema | CancellationToken, IMcpServer, and DI services are injected automatically — they do not appear in the schema. Only parameters with [Description] are exposed | | HTTP server returns 404 | app.MapMcp() must be called. Check the request path matches the configured route |

Related Skills

• mcp-csharp-debug — Run, debug, and test with MCP Inspector
• mcp-csharp-test — Unit tests, integration tests, evaluations
• mcp-csharp-publish — NuGet, Docker, Azure deployment

Reference Files

• references/api-patterns.md — Complete attribute reference, return types, DI injection, builder API, dynamic tools, experimental APIs. Load when: implementing tools, prompts, or resources beyond the basic patterns shown above.
• references/transport-config.md — Detailed transport configuration: stateless HTTP mode, OAuth/auth, custom path prefix, HttpContextAccessor, OpenTelemetry observability. Load when: configuring advanced transport options or authentication.

More Info

• C# MCP SDK — Official SDK repository
• Build an MCP server (.NET) — Microsoft quickstart
• MCP Specification — Protocol specification