Program.cs

using System.Text;
using System.Threading.Channels;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddCors();

builder.Services.AddSingleton(
    Channel.CreateUnbounded<string>()
);

var app = builder.Build();

app.UseCors(e => e.AllowAnyHeader().AllowAnyOrigin().AllowAnyMethod());


app.MapPost("/send", async (Channel<string> channel, HttpContext context) =>
{
    using var reader = new StreamReader(context.Request.Body);
    var message = await reader.ReadToEndAsync();

    if (string.IsNullOrWhiteSpace(message))
        return Results.BadRequest();

    await channel.Writer.WriteAsync(message);
    return Results.Accepted();
});


app.MapGet("/sse", async (
    HttpContext context,
    Channel<string> channel) =>
{
    try
    {
        var id = context.Request.Query["id"].ToString();
        var token = context.Request.Query["token"].ToString();

        context.Response.Headers.Append("Content-Type", "text/event-stream");
        context.Response.Headers.Append("Cache-Control", "no-cache");
        context.Response.Headers.Append("Connection", "keep-alive");

        await foreach (var msg in channel.Reader.ReadAllAsync(
            context.RequestAborted))
        {
            await context.Response.WriteAsync($"data: {msg}\n\n");
            await context.Response.Body.FlushAsync();
        }
    }
    catch (TaskCanceledException ex)
    {

    }
    catch (OperationCanceledException ex)
    {

    }
    catch (Exception ex)
    {

        throw;
    }
});




await app.RunAsync();