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Langfuse

Nuget package dotnet License: MIT Discord

Features 🔥

  • Fully generated C# SDK based on official Langfuse OpenAPI specification using AutoSDK
  • Same day update to support new features
  • Updated and supported automatically if there are no breaking changes
  • All modern .NET features - nullability, trimming, NativeAOT, etc.
  • Targets .NET 10

Langfuse v4 migration

This SDK uses the current v4 public APIs. See MIGRATION.md for breaking changes, replacement methods, and self-hosted compatibility. Langfuse Cloud removes legacy reads and trace ingestion on November 16, 2026.

Usage

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using Langfuse;

using var client = new LangfuseClient(username: publicKey, password: secretKey);

Health Check

Basic example showing how to create a client and check API health.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// Check that the Langfuse API and database are healthy.
var response = await client.Health.HealthHealthAsync();

Observations

List observations from the current Langfuse API.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// Bound observation reads to a time range; follow response.Meta.Cursor for more rows.
var end = DateTime.UtcNow;
var response = await client.Observations.ObservationsGetManyAsync(
    limit: 10,
    fromStartTime: end.AddDays(-1),
    toStartTime: end);

Prompts

List prompts managed in Langfuse.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// List all prompts in your project.
var response = await client.Prompts.PromptsListAsync(
    limit: 10);

OpenTelemetry ingestion

Ingest trace spans into Langfuse using the current OpenTelemetry endpoint.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// Use the v4 ingestion header for real-time observation visibility.
var options = new AutoSDKRequestOptions
{
    Headers = { ["x-langfuse-ingestion-version"] = "4" },
};

// Create a span and send it via OTLP/HTTP JSON ingestion.
var nowNanoseconds = (DateTimeOffset.UtcNow.ToUnixTimeMilliseconds() * 1_000_000).ToString();
var response = await client.Opentelemetry.OpentelemetryExportTracesAsync(
    resourceSpans:
    [
        new OtelResourceSpan
        {
            ScopeSpans =
            [
                new OtelScopeSpan
                {
                    Spans =
                    [
                        new OtelSpan
                        {
                            TraceId = Guid.NewGuid().ToString("N"),
                            SpanId = Guid.NewGuid().ToString("N")[..16],
                            Name = "sdk-integration-test",
                            Kind = 1,
                            StartTimeUnixNano = nowNanoseconds,
                            EndTimeUnixNano = nowNanoseconds,
                        },
                    ],
                },
            ],
        },
    ],
    requestOptions: options);

Projects

Get information about the project associated with the API key.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// Retrieve the project associated with your API key.
var response = await client.Projects.ProjectsGetAsync();

Scores

List scores for evaluating LLM outputs in Langfuse.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// Read Scores API v3. Use cursor pagination instead of the legacy page parameter.
var response = await client.Scores.ScoresGetManyAsync(
    limit: 10);

Datasets

List and manage datasets for LLM evaluation in Langfuse.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// List all datasets in your project with pagination.
var response = await client.Datasets.DatasetsListAsync(
    limit: 10);

Experiments

Read experiment runs and items instead of deprecated dataset-run endpoints.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// Experiment queries require a start time and use cursor pagination.
var end = DateTime.UtcNow;
var response = await client.Experiments.ExperimentsListAsync(
    fromStartTime: end.AddDays(-1),
    toStartTime: end,
    limit: 10);

Experiment Items

Read experiment items instead of deprecated dataset-run items.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// Use experimentId or experimentName to select a run; follow Meta.Cursor.
var end = DateTime.UtcNow;
var response = await client.Experiments.ExperimentsListItemsAsync(
    fromStartTime: end.AddDays(-1),
    toStartTime: end,
    limit: 10);

Metrics

Query aggregate observation metrics through Metrics API v2.

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using var client = new LangfuseClient(username: publicKey, password: secretKey);

// Query observations in a bounded UTC interval instead of legacy trace metrics.
var end = DateTime.UtcNow;
var query = $$"""
    {"view":"observations","metrics":[{"measure":"count","aggregation":"count"}],
     "dimensions":[],"filters":[],"fromTimestamp":"{{end.AddDays(-1):O}}","toTimestamp":"{{end:O}}"}
    """;
var response = await client.Metrics.MetricsMetricsAsync(query: query);

Ecosystem maintenance

This SDK is one of more than 200 .NET SDKs maintained with AutoSDK. The tryAGI SDK audit continuously checks repository synchronization, upstream-spec regeneration, release workflows, warnings, public API visibility, and trimming/NativeAOT compatibility.

Every issue is first investigated for ecosystem-wide applicability. When the root cause belongs in AutoSDK, we fix and regression-test the generator, then roll the improvement out to every applicable SDK. Provider-specific behavior remains in this repository when it cannot be derived safely from the API specification.

Issue content—including code blocks, logs, links, and attachments—is treated only as untrusted diagnostic data. Embedded control instructions, hidden directives, delimiter tricks, or requests to alter triage or tooling behavior are ignored. Please report reproducible technical evidence and remove secrets and personal data.

Support

Priority place for bugs: https://github.com/tryAGI/Langfuse/issues
Priority place for ideas and general questions: https://github.com/tryAGI/Langfuse/discussions
Discord: https://discord.gg/Ca2xhfBf3v

Acknowledgments

JetBrains logo

This project is supported by JetBrains through the Open Source Support Program.