LangChain Starter

A simple LangChain starter with a FastAPI backend and modern chat UI interface.

Quickstart

# 1) Create & activate venv
python -m venv venv
source venv/bin/activate        # Windows: venv\Scripts\activate

# 2) Install dependencies
pip install -r requirements.txt

# 3) Configure OpenAI
cp .env.example .env
# Edit .env and set OPENAI_API_KEY

# 4) Run the backend server
python -m src.backend

# 5) Open your browser
# Visit http://localhost:8000 to use the chat interface

Project Structure

langchain-starter/
├── src/
│   ├── __init__.py
│   ├── agent.py          # LangChain agent
│   └── backend.py        # FastAPI server with chat endpoints
├── frontend/
│   ├── index.html        # Chat interface
│   ├── styles.css        # Modern UI styling
│   └── script.js         # Frontend chat logic

├── requirements.txt     # Python dependencies
└── README.md

API Endpoints

• GET / - Serves the chat interface
• POST /api/chat - Chat with the LangChain agent
• GET /api/health - Health check endpoint

Chat API Usage

# Send a message to the agent
curl -X POST "http://localhost:8000/api/chat" \
     -H "Content-Type: application/json" \
     -d '{"message": "Who are you?"}'

What's inside?

Backend (src/)

• agent.py — LangChain agent
• backend.py — FastAPI server with:
  • Chat endpoint for processing messages
  • CORS middleware for frontend communication
  • Static file serving for the chat interface

Frontend (frontend/)

• Modern chat interface with:
  • Real-time message exchange
  • Typing indicators
  • Responsive design
  • Keyboard shortcuts

Extending the Agent

To add new tools to your agent, edit src/agent.py:

Development

Running in Development Mode

# Backend with auto-reload
python -m src.backend

# Or using uvicorn directly
uvicorn src.backend:app --reload --host 0.0.0.0 --port 8000

Learn More

• LangChain Documentation - Learn about LangChain
• FastAPI Documentation - Learn about FastAPI
• OpenAI API Documentation - Learn about OpenAI's API
• BuildKit Documentation - Learn about BuildKit

Notes

• Uses langchain-openai provider package (recommended modern setup)
• FastAPI provides automatic API documentation at /docs
• Frontend uses vanilla JavaScript (no framework dependencies)
• CORS enabled for development (configure for production)