See it on your phone · Build Your Card App

Your app runs on your phone through Expo Go, a free app. Your computer sends the app over your home Wi-Fi, and the phone shows it. No cable, nothing else installed on the phone, and no developer account yet. The cable, the "Trust This Computer?" question and the paid accounts come in Part 12, for the real version. This works the same from a Mac and from a Windows PC.

Install Expo Go

On iPhone

On your iPhone, open the App Store and search for Expo Go. The maker is Expo. The direct link is https://apps.apple.com/us/app/expo-go/id982107779. It needs iOS 16.4 or newer and is about 120 MB.
Tap Get, confirm with Face ID or your password, then Open.
iPhones also need a free Expo account. Make one at expo.dev/signup with an email address, a username and a password, or Continue with Google, GitHub or Apple. Use your usual email and a strong password saved where your browser or the Passwords app keeps them.
In Expo Go, tap the person icon at the top right and sign in with that account.
Tell Claude: "I have an Expo account and I'm signed in on the phone. Sign this computer in to the same account." Claude hands you npx expo login for the Terminal pane. Click + in the pane's header for a second Terminal tab, so the running server stays. The command opens a sign-in page in your browser, or asks for your username and password in the Terminal (nothing shows while you type the password).

Why the account: on an iPhone, Expo Go only opens a project from your computer when the computer and the app are signed in to the same Expo account; Android does not need this yet.

On Android

On your Android phone, open the Play Store and search for Expo Go. The maker is Expo. The direct link is https://play.google.com/store/apps/details?id=host.exp.exponent.
Tap Install, then Open.
No account is needed. If Expo Go offers to sign in, skip it.

Same Wi-Fi

Your phone and your computer must be on the same Wi-Fi network, with the exact same name if your home has two (the phone basics step at the end of Part 1).

On a Windows PC

One more thing on a PC: Windows must treat that Wi-Fi as a private network. On a new connection Windows picks Public, which hides the PC from every other device, including your phone. To check: Settings, Network & internet, Wi-Fi, then your network's name (or Properties), then under Network profile type choose Private. Only do this for your home Wi-Fi, never for a café or hotel network.

Open your app

The development server from step 1 is still running in the Terminal pane with its QR code. If the prompt line has come back, it stopped: ask Claude for the start command again.

On iPhone

Open the Camera app on your iPhone. Not Expo Go: the Camera.
Point it at the QR code on the computer screen, about a hand's width away, with the whole square in view.
A small yellow label appears saying something like Open in Expo Go. Tap it.
Expo Go opens. The first time, the iPhone asks whether Expo Go may find devices on your local network. Tap Allow; without it the phone can't reach the computer.
A progress bar runs while the app is sent over. Then your first screen appears.

On Android

Open Expo Go on your phone.
Tap Scan QR code. Allow camera use if asked.
Point the phone at the QR code on the computer screen until it reads it.
A progress bar runs while the app is sent over. Then your first screen appears.

For Binder, you see Collection value at the top with a total, and two cards below it: name, set and number, condition, price, and a colored rectangle where the picture will go.

Scroll it. It is a real app, running on your phone, that did not exist an hour ago. Leave it open; the next step changes it while you watch.

Advanced

This is optional and harder, and only for iPhone builders on a Mac whose phone won't connect: a pretend iPhone on the Mac screen, the Simulator. It needs Xcode fully installed (Part 4, step 1) plus one extra download inside Xcode. Ask Claude: "Open my app in the iOS Simulator and tell me what to install first." Expo installs its app on the pretend iPhone for you the first time; nothing to sign in to. There is no pretend iPhone on a Windows PC; if your phone won't connect there, use the tunnel from the trouble box below.

If something goes wrong

Expo Go says "Project is incompatible with this version of Expo Go" or asks for a newer version. Your project and your Expo Go are on different versions. Send Claude the exact words: "Expo Go on my phone says: [the message]. Change the project to match the version my Expo Go supports." Then scan again.

Nothing happens after scanning, or it says it can't connect. Check both devices are on the same Wi-Fi name, and turn off any VPN. On a PC, also check the Wi-Fi is set to Private (above) and that you allowed Node.js through the firewall (step 1). Then ask Claude: "The phone can't connect. Please restart the server as a tunnel." It uses npx expo start --tunnel, which works across networks but updates more slowly, and gives you a new QR code.

On iPhone, Expo Go says I need to sign in, or shows an account error. Sign in on the phone and on the computer to the same account (above). Then tell Claude: "Both are signed in now. Please restart the server", and scan again.

The Camera shows no yellow label. Move the phone closer or further, with the whole square visible. On an older iPhone, open Settings, then Camera, and turn on Scan QR Codes.

Expo's own website says I need a paid Apple account for Expo Go on an iPhone. Expo's own setup page currently tells iPhone users to build a private copy of Expo Go with the paid Apple Developer Program and TestFlight; the free App Store copy still works when its version matches your project. That is true on a Mac and on a PC alike. If the free copy ever stops opening your project, tell Claude the exact message; it can match versions, use the tunnel, or on a Mac use the Simulator above.

A red screen with an error appears on the phone. Take a screenshot on the phone, send it to your computer (the phone basics step at the end of Part 1), and drop it into the chat: "This appeared on my phone." Claude fixes it; the phone updates by itself.