Program.cs

global using Microsoft.AspNetCore.Connections;
global using Microsoft.AspNetCore.StaticFiles;
global using System.Runtime.InteropServices;
global using System.Text;
using Microsoft.AspNetCore.Server.Kestrel.Core;

var builder = WebApplication.CreateSlimBuilder(args);

builder.Services.AddTransient<GeminiRequestHandler>();
builder.WebHost.ConfigureKestrel(o =>
{

    o.ListenAnyIP(1965, b =>
    {
        b.Protocols = HttpProtocols.None;
        b.UseHttps(Path.Combine(AppContext.BaseDirectory, "certificate.pfx"), "", o =>
        {
            o.SslProtocols = System.Security.Authentication.SslProtocols.Tls12 | System.Security.Authentication.SslProtocols.Tls13;
            o.AllowAnyClientCertificate();
        });

        b.UseConnectionHandler(() => b.ApplicationServices.GetRequiredService<GeminiRequestHandler>());
    });
});

var app = builder.Build();
app.Run();

public static class Ext
{
    public static IConnectionBuilder UseConnectionHandler<T>(this IConnectionBuilder connectionBuilder, Func<T> factory)
        where T : ConnectionHandler => connectionBuilder.Run(connection
            => factory().OnConnectedAsync(connection));
}