Langfuse.Core 1.1.0

Langfuse .NET SDK (Unofficial)

Unofficial .NET SDK for Langfuse - the open-source LLM engineering platform.

Packages

Langfuse.OpenTelemetry

Export .NET OpenTelemetry traces to Langfuse. Works with any OTEL-instrumented library including Semantic Kernel.

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  # EU region (default)
# LANGFUSE_BASE_URL=https://us.cloud.langfuse.com  # US region

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 with Langfuse exporter
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 Options

// Option 1: Environment variables (recommended)
.AddLangfuseExporter()

// Option 2: Manual configuration
.AddLangfuseExporter(options =>
{
    options.PublicKey = "pk-lf-...";
    options.SecretKey = "sk-lf-...";
    options.BaseUrl = "https://cloud.langfuse.com";
})

// Option 3: From IConfiguration (appsettings.json)
.AddLangfuseExporter(configuration)

Langfuse.Client

Access Langfuse features like Prompt Management directly from .NET with built-in caching.

Quick Start

1. Install

dotnet add package Langfuse.Client

2. Set environment variables

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

3. Use prompts

using Langfuse.Client;

var client = new LangfuseClient();

// Fetch a text prompt (cached for 60s by default)
var prompt = await client.GetPromptAsync("movie-critic");

// Compile with variables
var compiled = prompt.Compile(new Dictionary<string, string>
{
    ["criticlevel"] = "expert",
    ["movie"] = "Dune 2"
});
// -> "As an expert movie critic, do you like Dune 2?"

// Fetch a chat prompt
var chatPrompt = await client.GetChatPromptAsync("movie-critic-chat");
var messages = chatPrompt.Compile(("criticlevel", "expert"), ("movie", "Dune 2"));
// -> [{ role: "system", content: "..." }, { role: "user", content: "..." }]

Features

• Text & Chat prompts - Full support for both prompt types
• Variable compilation - {{variable}} syntax support
• Version/Label selection - Fetch specific versions or labels (production, staging)
• Client-side caching - 60s TTL by default, configurable
• Fallback prompts - Graceful degradation when API fails
• Config access - Access prompt config (model, temperature, etc.)

// Get specific version
var v1 = await client.GetPromptAsync("my-prompt", version: 1);

// Get by label
var staging = await client.GetPromptAsync("my-prompt", label: "staging");

// With fallback
var fallback = TextPrompt.CreateFallback("default", "Fallback prompt text");
var prompt = await client.GetPromptAsync("my-prompt", fallback: fallback);

// Access config
var model = prompt.GetConfigValue<string>("model");
var temperature = prompt.GetConfigValue<double>("temperature", 0.7);

Documentation

• Testing Guide - How to run tests
• Features - Implemented features with Langfuse docs links
• Contributing - How to contribute

Running the Sample

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

# Run sample
cd samples/SemanticKernel.Sample
dotnet run

Check your Langfuse dashboard to see the traces.

License

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

Links

• Langfuse - Open-source LLM engineering platform
• Langfuse Docs - Official documentation
• OpenTelemetry Integration - OTEL docs
• Prompt Management - Prompts docs