Langfuse.OpenTelemetry 1.0.0

.NET Langfuse OTEL Exporter

Export .NET OpenTelemetry traces to Langfuse with one line of code. Agentic Frameworks like Semantic Kernel are instrumented to emit OpenTelemetry and this library aims to collect OTEL data and export it to LangFuse's OTEL endpoints.

Quick Start

1. Install

dotnet add package Langfuse.OpenTelemetry

2. Set environment variables

LANGFUSE_PUBLIC_KEY=pk-lf-...
LANGFUSE_SECRET_KEY=sk-lf-...
LANGFUSE_BASE_URL=https://cloud.langfuse.com

3. Add to your app

using Langfuse.OpenTelemetry;
using Microsoft.SemanticKernel;
using OpenTelemetry;
using OpenTelemetry.Trace;

// Enable GenAI diagnostics (prompts, tokens, completions)
AppContext.SetSwitch("Microsoft.SemanticKernel.Experimental.GenAI.EnableOTelDiagnosticsSensitive", true);

// Setup OpenTelemetry
using var tracerProvider = Sdk.CreateTracerProviderBuilder()
    .AddSource("Microsoft.SemanticKernel*")
    .AddLangfuseExporter()
    .Build();

// Use Semantic Kernel as normal
var kernel = Kernel.CreateBuilder()
    .AddOpenAIChatCompletion("gpt-4o-mini", apiKey)
    .Build();

var result = await kernel.InvokePromptAsync("Hello!");

Configuration

Option 1: Environment Variables (Recommended)

Set these environment variables and call .AddLangfuseExporter() with no parameters:

LANGFUSE_PUBLIC_KEY=pk-lf-...
LANGFUSE_SECRET_KEY=sk-lf-...
LANGFUSE_BASE_URL=https://cloud.langfuse.com

Option 2: Manual Configuration

Pass configuration directly to the exporter:

using var tracerProvider = Sdk.CreateTracerProviderBuilder()
    .AddSource("Microsoft.SemanticKernel*")
    .AddLangfuseExporter(options =>
    {
        options.PublicKey = "pk-lf-...";
        options.SecretKey = "sk-lf-...";
        options.BaseUrl = "https://cloud.langfuse.com";
    })
    .Build();

Option 3: IConfiguration

Load from appsettings.json or other configuration sources:

// In appsettings.json:
// {
//   "Langfuse": {
//     "PublicKey": "pk-lf-...",
//     "SecretKey": "sk-lf-...",
//     "BaseUrl": "https://cloud.langfuse.com"
//   }
// }

using var tracerProvider = Sdk.CreateTracerProviderBuilder()
    .AddSource("Microsoft.SemanticKernel*")
    .AddLangfuseExporter(configuration.GetSection("Langfuse"))
    .Build();

Test

# Set environment variables
$env:OPENAI_API_KEY="sk-..."
$env:LANGFUSE_PUBLIC_KEY="pk-lf-..."
$env:LANGFUSE_SECRET_KEY="sk-lf-..."
$env:LANGFUSE_BASE_URL="https://cloud.langfuse.com"

# Run sample
cd samples/SemanticKernel.Sample
dotnet run

Check your Langfuse dashboard to see the traces.

Contributing

Contributions are welcome! Please see CONTRIBUTING.md for details.

License

This project is licensed under the MIT License - see the LICENSE file for details.

Links

• Langfuse OpenTelemetry Docs
• OpenTelemetry GenAI Conventions
• Semantic Kernel