Program.cs

using System.ClientModel;
using System.ClientModel.Primitives;
using System.Diagnostics;
using System.Text;
using System.Text.Json;
using Microsoft.Agents.AI;
using Microsoft.Extensions.AI;
using OpenAI;
using OpenAI.Chat;
using OpenTelemetry.Exporter;
using OpenTelemetry.Resources;
using OpenTelemetry.Trace;
using OpenTelemetryLangFuseRepro;

var builder = WebApplication.CreateBuilder(args);
var cfg = builder.Configuration;

const string aiTelemetrySource = "Microsoft.Extensions.AI";
const string agentTelemetrySource = "Microsoft.Agents.AI";

string apiKey = RequireConfig(cfg, "LlmProvider:ApiKey");
string endpoint = RequireConfig(cfg, "LlmProvider:Endpoint");
string modelId = RequireConfig(cfg, "LlmProvider:Model");
string lfHost = RequireConfig(cfg, "Langfuse:HostUrl");
string lfPub = RequireConfig(cfg, "Langfuse:PublicKey");
string lfSec = RequireConfig(cfg, "Langfuse:SecretKey");
string version = RequireConfig(cfg, "App:Version");
string release = RequireConfig(cfg, "App:Release");

builder.Services.AddHttpClient("MyLlmProvider");

builder.Services.AddOpenTelemetry().ConfigureResource(r => r
        .AddService(serviceName: "langfuse-sandbox", serviceVersion: version))
    .WithTracing(t => t
        .AddSource(aiTelemetrySource)
        .AddSource(agentTelemetrySource)
        .AddAspNetCoreInstrumentation()
        .AddProcessor(new LangfuseBaggageProcessor())
        .AddConsoleExporter()
        .AddOtlpExporter(o =>
        {
            o.Endpoint = new Uri($"{lfHost.TrimEnd('/')}/api/public/otel/v1/traces");
            o.Protocol = OtlpExportProtocol.HttpProtobuf;
            var auth = Convert.ToBase64String(Encoding.UTF8.GetBytes($"{lfPub}:{lfSec}"));
            //o.Headers = $"Authorization=Basic {auth}";
            o.HttpClientFactory = () =>
            {
                HttpClient client = new HttpClient();
                client.DefaultRequestHeaders.Add("Authorization", $"Basic {auth}");
                return client;
            };
        }));

// IChatClient: OpenAI-compatible client pointed, with gen_ai OpenTelemetry
builder.Services.AddSingleton<IChatClient>(sp =>
{
    var http = sp.GetRequiredService<IHttpClientFactory>().CreateClient("MyLlmProvider");
    var options = new OpenAIClientOptions
    {
        Endpoint = new Uri(endpoint),
        Transport = new HttpClientPipelineTransport(http),
    };
    ChatClient chat = new OpenAIClient(new ApiKeyCredential(apiKey), options).GetChatClient(modelId);
    IChatClient inner = chat.AsIChatClient();
    return new ChatClientBuilder(inner)
        .UseOpenTelemetry(sourceName: aiTelemetrySource, configure: c => c.EnableSensitiveData = true)
        .Build();
});

// Simple C# coding agent, wrapped with the Agent Framework's OpenTelemetry instrumentation 
builder.Services.AddSingleton<AIAgent>(sp =>
{
    var chat = sp.GetRequiredService<IChatClient>();
    var baseAgent = new ChatClientAgent(
        chatClient: chat,
        instructions: "You are a concise C# coding assistant. Give accurate, modern .NET 10 C# advice. Prefer minimal, idiomatic code. Use short code blocks when helpful.",
        name: "CSharpHelper");
    return new OpenTelemetryAgent(baseAgent, agentTelemetrySource) { EnableSensitiveData = true };
});

var app = builder.Build();

// POST /chat -> streams the agent reply as SSE
app.MapPost("/chat", async (HttpContext ctx, AIAgent agent) =>
{
    var req = await ctx.Request.ReadFromJsonAsync<ChatRequest>(ctx.RequestAborted) ?? new ChatRequest("", null, null, null);
    string userId = string.IsNullOrWhiteSpace(req.UserId) ? "anonymous" : req.UserId;
    string sessionId = string.IsNullOrWhiteSpace(req.SessionId) ? Guid.NewGuid().ToString() : req.SessionId;
    string[] tags = req.Tags is { Length: > 0 } t ? t : ["sandbox", "csharp-agent"];

    var attrs = new Dictionary<string, object?>
    {
        ["langfuse.user.id"] = userId,
        ["langfuse.session.id"] = sessionId,
        ["langfuse.version"] = version,
        ["langfuse.release"] = release,
        ["langfuse.trace.name"] = "chat",
        ["langfuse.trace.tags"] = tags,
        ["langfuse.trace.metadata.source"] = "aspnet-webapi",
    };
    if (Activity.Current is { } root)
        foreach (var kv in attrs)
            root.SetTag(kv.Key, kv.Value);
    LangfuseContext.Current = attrs;

    ctx.Response.ContentType = "text/event-stream";
    ctx.Response.Headers["Cache-Control"] = "no-cache";

    var ct = ctx.RequestAborted;
    await foreach (var update in agent.RunStreamingAsync(req.Message).WithCancellation(ct))
    {
        var text = update.Text ?? "";
        if (text.Length == 0) continue;
        await ctx.Response.WriteAsync("data: ", ct);
        await ctx.Response.WriteAsync(JsonSerializer.Serialize(text), ct);
        await ctx.Response.WriteAsync("\n\n", ct);
        await ctx.Response.Body.FlushAsync(ct);
    }

    await ctx.Response.WriteAsync("data: [DONE]\n\n", ct);
    await ctx.Response.Body.FlushAsync(ct);
});

app.Run();

static string RequireConfig(IConfiguration cfg, string key) =>
    cfg[key] is { Length: > 0 } s
        ? s
        : throw new InvalidOperationException($"Missing required configuration value: '{key}'");

public sealed record ChatRequest(string Message, string? UserId, string? SessionId, string[]? Tags);