dotnet

MCP stdio Server - .NET Solution

⚠️ Important: This solution has been updated to use the stdio transport as recommended by MCP Specification 2025-06-18. The original SSE transport has been deprecated.

Overview

This .NET solution demonstrates how to build an MCP server using the current stdio transport. The stdio transport is simpler, more secure, and provides better performance than the deprecated SSE approach.

Prerequisites

• .NET 9.0 SDK or later
• Basic understanding of .NET dependency injection

Setup Instructions

Step 1: Restore dependencies

dotnet restore

Step 2: Build the project

dotnet build

Running the Server

The stdio server runs differently than the old HTTP-based server. Instead of starting a web server, it communicates through stdin/stdout:

dotnet run

Important: The server will appear to hang - this is normal! It's waiting for JSON-RPC messages from stdin.

Testing the Server

Method 1: Using the MCP Inspector (Recommended)

npx @modelcontextprotocol/inspector dotnet run

This will:

1. Launch your server as a subprocess
2. Open a web interface for testing
3. Allow you to test all server tools interactively

Method 2: Direct command line testing

You can also test by launching the Inspector directly:

npx @modelcontextprotocol/inspector dotnet run --project .

Available Tools

The server provides these tools:

• AddNumbers(a, b): Add two numbers together
• MultiplyNumbers(a, b): Multiply two numbers together
• GetGreeting(name): Generate a personalized greeting
• GetServerInfo(): Get information about the server

Testing with Claude Desktop

To use this server with Claude Desktop, add this configuration to your claude_desktop_config.json:

{
  "mcpServers": {
    "example-stdio-server": {
      "command": "dotnet",
      "args": ["run", "--project", "path/to/server.csproj"]
    }
  }
}

Project Structure

dotnet/
├── Program.cs           # Main server setup and configuration
├── Tools.cs            # Tool implementations
├── server.csproj       # Project file with dependencies
├── server.sln         # Solution file
├── Properties/         # Project properties
└── README.md          # This file

Key Differences from HTTP/SSE

stdio transport (Current):

• ✅ Simpler setup - no web server needed
• ✅ Better security - no HTTP endpoints
• ✅ Uses Host.CreateApplicationBuilder() instead of WebApplication.CreateBuilder()
• ✅ WithStdioTransport() instead of WithHttpTransport()
• ✅ Console application instead of web application
• ✅ Better performance

HTTP/SSE transport (Deprecated):

• ❌ Required ASP.NET Core web server
• ❌ Needed app.MapMcp() routing setup
• ❌ More complex configuration and dependencies
• ❌ Additional security considerations
• ❌ Now deprecated in MCP 2025-06-18

Development Features

• Dependency Injection: Full DI support for services and logging
• Structured Logging: Proper logging to stderr using ILogger<T>
• Tool Attributes: Clean tool definition using [McpServerTool] attributes
• Async Support: All tools support async operations
• Error Handling: Graceful error handling and logging

Development Tips

• Use ILogger for logging (never write to stdout directly)
• Build with dotnet build before testing
• Test with the Inspector for visual debugging
• All logging goes to stderr automatically
• The server handles graceful shutdown signals

This solution follows the current MCP specification and demonstrates best practices for stdio transport implementation using .NET.