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LangChain is a framework for developing applications powered by language models. Braintrust integrates with LangChain using callback handlers to automatically trace chains, agents, and LLM calls.

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Setup

Install the Braintrust LangChain integration alongside your LangChain packages: 
# pnpm
pnpm add braintrust @braintrust/langchain-js @langchain/core @langchain/openai
# npm
npm install braintrust @braintrust/langchain-js @langchain/core @langchain/openai

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Trace with LangChain

Braintrust provides callback handlers that automatically capture LangChain operations including chains, agents, retrievers, and individual LLM calls. To trace your LangChain application, configure the BraintrustCallbackHandler:
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();

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