Advanced MCP: C# Servers, Auth and Ecosystem

After the basics: how to build an MCP server in .NET, how OAuth authorization works for remote MCP servers, and what is in the MCP ecosystem.

18.1 MCP server in C# (official C# SDK)

dotnet new console
dotnet add package ModelContextProtocol
dotnet add package Microsoft.Extensions.Hosting

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using ModelContextProtocol.Server;
using System.ComponentModel;

var builder = Host.CreateApplicationBuilder(args);
builder.Logging.AddConsole(o =>
{
    o.LogToStandardErrorThreshold = LogLevel.Trace;   // with stdio, logs go to stderr — stdout is for the protocol
});
builder.Services
    .AddMcpServer()
    .WithStdioServerTransport()
    .WithToolsFromAssembly();          // registers [McpServerToolType] classes automatically
await builder.Build().RunAsync();

[McpServerToolType]
public static class EmployeeTools
{
    [McpServerTool, Description("Returns the department of an employee by employee ID.")]
    public static string GetDepartment(int empId) => empId == 1 ? "IT" : "HR";
}

# Add it to Claude Code
claude mcp add employee-tools -- dotnet run --project ./EmployeeMcp

18.2 MCP Authorization (remote / HTTP servers)

Authorization is optional. For servers with HTTP transport the spec is based on OAuth 2.1. stdio servers do not follow OAuth — they take credentials from the environment.

• PKCE is required; the token never goes in the URL query string, only in the Authorization header.
• The server must validate the token's audience — it must not accept a token meant for another service or pass it on (token passthrough).
• On insufficient scope the server returns 403 + error="insufficient_scope"; the client does step-up authorization.
• Dynamic Client Registration is now deprecated; Client ID Metadata Documents are preferred.

18.3 MCP ecosystem