Module @langfuse/langchain

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@langfuse/langchain

Langfuse is the open-source LLM engineering platform: tracing & evaluations for LLM and agent applications, prompt management, datasets & experiments, and evaluation (scores). This package provides the CallbackHandler for tracing LangChain and LangGraph invocations with Langfuse. Spans are exported by the LangfuseSpanProcessor from @langfuse/otel; prompt management, datasets/experiments, and evals/scores live in @langfuse/client.

Important

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

Install

npm install @langfuse/langchain @langfuse/otel @opentelemetry/sdk-trace-node

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

// Register the span processor once at startup
import { LangfuseSpanProcessor } from "@langfuse/otel";
import { NodeTracerProvider } from "@opentelemetry/sdk-trace-node";

new NodeTracerProvider({
  spanProcessors: [new LangfuseSpanProcessor()],
}).register();

// Pass the CallbackHandler to your LangChain invocations
import { CallbackHandler } from "@langfuse/langchain";
import { ChatOpenAI } from "@langchain/openai";

const handler = new CallbackHandler({
  sessionId: "session-123",
  userId: "user-456",
});

const model = new ChatOpenAI({ model: "gpt-5.1" });
const response = await model.invoke("What is Langfuse?", {
  callbacks: [handler],
});

Packages

Package	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