# LangChain

LangChain is a framework for developing applications that are powered by Large
Language Models (LLMs). It implements a standard interface for large language
models and related technologies, such as embedding models and vector stores, and
integrates with hundreds of providers.

## Prerequisites

In order to use the AI Gateway with any LangChain powered application you will
need to complete these steps first:

<Stepper>

1. Create a [new provider](/ai-gateway/managing-providers) in the AI Gateway for
   the provider you want to use with LangChain

2. [Set up a new team](/ai-gateway/managing-teams)

3. Create a [new app](/ai-gateway/managing-apps) to use specifically with
   LangChain and assign it to the team you created

4. Copy the API Key for the app you created, as well as the Gateway URL

</Stepper>

## Configure LangChain

In this example we will configure LangChain to work with OpenAI (or OpenAI
compatible models).

To work with OpenAI in LangChain it's recommended to use their
[ChatOpenAI integration](https://python.langchain.com/docs/integrations/chat/openai/).

```python
import os
from langchain_openai import ChatOpenAI

def init_chat_model():
    """Initialize the ChatOpenAI model"""
    api_key = os.getenv("ZUPLO_AI_GATEWAY_API_KEY")
    if not api_key:
        print("❌ Error: Please set your ZUPLO_AI_GATEWAY_API_KEY in a .env file")
        exit(1)

    # Check for custom BASE_URL - this is the AI Gateway URL from Zuplo
    base_url = os.getenv("BASE_URL")

    if base_url:
        return ChatOpenAI(api_key=api_key, model="gpt-4o", base_url=base_url)
    else:
        return ChatOpenAI(api_key=api_key, model="gpt-4o")
```

In the code above, checks are performed for two environment variables:

- `ZUPLO_AI_GATEWAY_API_KEY` - This is the API key of the app you have
  configured to use with LangChain in Zuplo
- `BASE_URL` - This is the Gateway URL of your AI Gateway project in Zuplo

Both of these values are passed to `ChatOpenAI` when it's instantiated,
switching the configuration from using default OpenAI APIs to using Zuplo's AI
Gateway for all `gpt-4o` requests that the LangChain SDK will make.