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Trace your LangChain applications by configuring a LangChain callback handler. 
npm install braintrust @braintrust/langchain-js @langchain/core @langchain/openai
trace-langchain.ts
import { BraintrustCallbackHandler } from "@braintrust/langchain-js";
import { ChatOpenAI } from "@langchain/openai";
import { initLogger } from "braintrust";

initLogger({
  projectName: "My Project",
  apiKey: process.env.BRAINTRUST_API_KEY,
});

const handler = new BraintrustCallbackHandler();

async function main() {
  const model = new ChatOpenAI({ modelName: "gpt-4o-mini" });

  await model.invoke("What is the capital of France?", {
    callbacks: [handler],
  });
}

main();
Learn more about LangChain callbacks in their documentation.