Skip to main content
LangWatch integrates with Langchain to provide detailed observability into your chains, agents, LLM calls, and tool usage.

​
Installation

npm i langwatch @langchain/openai @langchain/core

​
Usage

The LangWatch API key is configured by default via the LANGWATCH_API_KEY environment variable.
Use LangWatchCallbackHandler to capture Langchain events as spans within your trace. 
import { setupObservability } from "langwatch/observability/node";
import { LangWatchCallbackHandler } from "langwatch/observability/instrumentation/langchain";
import { ChatOpenAI } from "@langchain/openai";
import { HumanMessage } from "@langchain/core/messages";

setupObservability({ serviceName: "<project_name>" });

async function main(message: string): Promise<string> {
  const chatModel = new ChatOpenAI({ model: "gpt-5" }).withConfig({
    callbacks: [new LangWatchCallbackHandler()],
  });

  const result = await chatModel.invoke([new HumanMessage(message)]);
  return result.content as string;
}

console.log(await main("Hey, tell me a joke"));
The LangWatchCallbackHandler captures Langchain events and converts them into detailed LangWatch spans. Pass the callback handler to your Langchain components via the callbacks option in withConfig().