@langfuse/client

Langfuse is the open-source LLM engineering platform: tracing & evaluation for LLM and agent applications, prompt management, datasets & experiments, and evaluation (scores). This package provides the LangfuseClient — prompt management (langfuse.prompt), datasets (langfuse.dataset), experiments (langfuse.experiment), scores (langfuse.score), media (langfuse.media), and the full generated REST API client (langfuse.api). Tracing is intentionally separate: use @langfuse/tracing + @langfuse/otel for that.

[!IMPORTANT] This is the current SDK generation (@langfuse/* scoped packages). The unscoped langfuse npm package is the legacy v3 SDK — for new integrations use the @langfuse/* packages. Migration guides: v3 → v4, v4 → v5.

Install

npm install @langfuse/client

Environment variables

LANGFUSE_PUBLIC_KEY="pk-lf-..."
LANGFUSE_SECRET_KEY="sk-lf-..."
LANGFUSE_BASE_URL="https://cloud.langfuse.com" # 🇪🇺 EU region. 🇺🇸 US: https://us.cloud.langfuse.com

Quickstart

import { LangfuseClient } from "@langfuse/client";

const langfuse = new LangfuseClient(); // reads env vars

// Prompt management
const prompt = await langfuse.prompt.get("my-prompt");
const compiled = prompt.compile({ topic: "chickens" });

// Scores (evaluation / user feedback)
langfuse.score.create({ traceId: "trace-id", name: "quality", value: 0.9 });
await langfuse.flush(); // flush queued scores (tracing spans flush separately)

// Datasets & experiments
const dataset = await langfuse.dataset.get("my-dataset");
const result = await dataset.runExperiment({
  name: "My experiment",
  task: async ({ input }) => myModel.generate(input),
  evaluators: [
    async ({ output, expectedOutput }) => ({
      name: "exact_match",
      value: output === expectedOutput ? 1 : 0,
    }),
  ],
});
console.log(await result.format());

// Full REST API
const trace = await langfuse.api.trace.get("trace-id");

Packages

| Package | NPM | Description | Environments | | --------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------- | ------------ | | @langfuse/tracing | | OpenTelemetry-based tracing instrumentation | Node.js 20+ | | @langfuse/otel | | LangfuseSpanProcessor to export OpenTelemetry spans to Langfuse | Node.js 20+ | | @langfuse/client | | Prompt management, datasets, experiments, scores, full REST API | Universal JS | | @langfuse/openai | | observeOpenAI wrapper for tracing the OpenAI SDK | Universal JS | | @langfuse/langchain | | CallbackHandler for LangChain / LangGraph tracing | Universal JS | | @langfuse/vercel-ai-sdk | | Telemetry integration for Vercel AI SDK v7 | Universal JS | | @langfuse/browser | | Browser score ingestion with public-key auth | Browser | | @langfuse/core | | Shared core: generated API client, logger, utilities | Universal JS |

Documentation

• Prompt management: https://langfuse.com/docs/prompt-management/get-started
• Experiments: https://langfuse.com/docs/evaluation/experiments/experiments-via-sdk
• Scores: https://langfuse.com/docs/evaluation/evaluation-methods/scores-via-sdk
• Reference: https://js.reference.langfuse.com
• LLM/agent-readable docs index: https://langfuse.com/llms.txt

License

MIT