> ## Documentation Index
> Fetch the complete documentation index at: https://langwatch.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# LangChain Instrumentation

> Instrument LangChain applications with the LangWatch TypeScript SDK to trace chains, RAG flows, and agent evaluation metrics.

<Tip>
  **Quick setup?** Instead of following these steps manually, [copy a prompt](/skills/code-prompts#instrument-my-code) into your coding agent and it will set this up for you automatically.
</Tip>

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

## Installation

<CodeGroup>
  ```bash npm theme={null}
  npm i langwatch @langchain/openai @langchain/core
  ```

  ```bash pnpm theme={null}
  pnpm add langwatch @langchain/openai @langchain/core
  ```

  ```bash yarn theme={null}
  yarn add langwatch @langchain/openai @langchain/core
  ```

  ```bash bun theme={null}
  bun add langwatch @langchain/openai @langchain/core
  ```
</CodeGroup>

## Usage

<Info>
  The LangWatch API key is configured by default via the `LANGWATCH_API_KEY` environment variable.
</Info>

Use `LangWatchCallbackHandler` to capture Langchain events as spans within your trace.

```typescript  theme={null}
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()`.

## Related

* [Capturing RAG](/integration/typescript/tutorials/capturing-rag) - Learn how to capture RAG data from LangChain retrievers and tools
* [Capturing Metadata and Attributes](/integration/typescript/tutorials/capturing-metadata) - Add custom metadata and attributes to your traces and spans
* [Capturing Evaluations & Guardrails](/integration/python/tutorials/capturing-evaluations-guardrails) - Log evaluations and implement guardrails in your LangChain applications