@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 | 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

• LangChain integration guide: https://langfuse.com/integrations/frameworks/langchain
• Docs: https://langfuse.com/docs/observability/sdk/overview
• Reference: https://js.reference.langfuse.com
• LLM/agent-readable docs index: https://langfuse.com/llms.txt

License

MIT