Claude Sample App (Express + Claude API)

An Express server that talks to the Claude AI API, built one route at a time so you can see exactly how an app sends a prompt and gets an answer back.

This is a backend-only project. There is no frontend to build — you'll test every route with Thunder Client in VS Code.

What You'll Learn

• How to set up an Express server that accepts JSON request bodies
• How to keep an API key out of your code using a .env file
• How to install and use the official Claude SDK (@anthropic-ai/sdk)
• How to send a prompt to Claude and read the response back
• How a system prompt shapes Claude's personality and answer format
• How to build POST routes that take input from a request body and return JSON

Prerequisites

• Node.js version 20 or newer (the --env-file flag needs it)
• npm (installed automatically with Node.js)
• VS Code with the Thunder Client extension
• A Claude API key from console.anthropic.com

Getting Started

1. Clone the repository

git clone https://github.com/clydebankmedia/express-claude-sample-app.git
cd express-claude-sample-app

2. Install dependencies

npm install

3. Create your .env file

Copy .env.example to a new file named .env, then paste in your own key:

CLAUDE_API_KEY=sk-ant-your-key-here

Your .env file is listed in .gitignore, so your key never gets pushed to GitHub. Keep it that way.

4. Start the server

node --env-file=.env server.js

Express API → http://localhost:3000

The --env-file flag is built into Node. It loads your .env file for you — no dotenv package needed.

Project Structure

├── server.js      Express server, Claude client, and the routes you'll build
├── package.json   Dependencies
├── .gitignore     Ignores node_modules and .env
├── .env.example   Template for your own .env file
└── README.md      You are here

What You'll Build

The setup is done — Express, the JSON middleware, the Claude client, the welcome route at /, and app.listen are all in place. Your job is to write the four routes underneath.

Each spot is marked with a TODO comment telling you exactly what to do, what the request body looks like, and what to send back.

TODOs

1. GET /api/hello — a warm-up route with no Claude call. Return a JSON object with a message like "Hello from the API!" so you can confirm your route works before adding AI to the mix.

2. POST /api/ask — your first Claude call. Pull prompt out of req.body, pass it to client.messages.create() with the model claude-sonnet-5 and max_tokens: 1024, then send Claude's text back as JSON.

3. POST /api/cats — same idea, plus a personality. Pull question out of req.body and add a system prompt: "You are a cat expert. Answer everything about cats in 2-3 sentences." Notice how the system prompt changes the answer without changing the question.

4. POST /api/summarize — a practical use case. Pull text out of req.body, send it to Claude with the system prompt "Summarize the following text in 2-3 bullet points.", and return the summary as JSON.

Testing Your Routes

Thunder Client is a REST client that lives inside VS Code, so you never have to leave your editor to test a route.

1. Install the Thunder Client extension from the VS Code Extensions panel
2. Click the thunderbolt icon in the sidebar
3. Click New Request
4. Set the method (GET or POST) and paste in the URL
5. For POST requests, open the Body tab, choose JSON, and type your JSON
6. Click Send and read the response on the right

Make sure your server is running in the terminal before you send a request.

Say hello

GET http://localhost:3000/api/hello

Ask Claude anything

POST http://localhost:3000/api/ask
Content-Type: application/json

{ "prompt": "Why do cats purr?" }

Ask the cat expert

POST http://localhost:3000/api/cats
Content-Type: application/json

{ "question": "Why do cats knead blankets?" }

Summarize some text

POST http://localhost:3000/api/summarize
Content-Type: application/json

{ "text": "Paste a few paragraphs here and see what comes back." }

Troubleshooting

• Cannot read properties of undefined — your .env file is missing or you started the server without --env-file=.env
• 401 authentication error — double-check the key in your .env file for typos or extra spaces
• Cannot POST /api/ask — the route isn't defined yet, or the server needs a restart to pick up your changes
• Empty request body — make sure app.use(express.json()) is above your routes and that Thunder Client is sending JSON
• not_found_error about the model — the model ID is wrong or has been retired. Check the current model list and update the model value in server.js

QuickStart Guides Academy

This project is part of the QuickStart Guides Academy curriculum. Learn more at quickstartguides.com