@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.

Install

npm install @langfuse/client
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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
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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");
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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