How to tell if an app is optimized for iPhone Duo

Why some apps don't use the big screen

Every iPhone app runs on iPhone Duo. How much of the inner display it uses depends on the SDK its developer built it with, according to Apple:

• Older than iOS 27: the app appears in the middle of the display, with empty space around the window.
• iOS 27 SDK: the app fills most of the inner display, but keeps clear of the status bar on the right edge.
• iOS 27.1 SDK or later: the app uses the whole display, with toolbars and tab bars running vertically. Apple started accepting these builds on October 5, 2026.

Unlike some Android foldables, iPhone Duo doesn't switch to an app's iPad version instead. Each app needs its own update, so on launch day the picture will be mixed. Apple named Netflix, Zoom and Slack as already taking advantage of the new design; other apps get there as their developers ship updates.

The four things you might see

• Built for Duo: Uses the full inner display with a layout designed for it, and adapts when you fold, unfold or prop the phone half-open.
• Fills the screen: Uses all or most of the inner display, but with a stretched iPhone layout rather than one designed for Duo.
• Letterboxed: Runs in a smaller window in the middle of the inner display, with empty space around it. The app hasn't been updated for Duo yet.
• Broken: Crashes, freezes or is unusable on iPhone Duo.

A 30-second test

1. Unfold and open the app. Look at the edges of the inner display. A smaller window with empty space around it means letterboxed. A strip left empty by the status bar usually means an iOS 27 build that hasn't been updated for 27.1 yet.
2. Look at the layout. If everything is just bigger, with wide margins and stretched lists, the app fills the screen without a Duo layout. If it shows more at once, such as a sidebar, two columns or a list beside the open item, it was designed for Duo.
3. Turn the phone. A Duo-ready app rearranges itself in landscape instead of rotating a phone-shaped window.
4. Half-fold it. Prop the phone open on a table. Apps designed for this posture usually move controls to the lower half and keep the content on top.
5. Fold it closed while the app is open. You should land in the same place on the outer screen, without the app restarting or losing what you typed.

Did that? Report what you saw. It takes a few seconds, needs no account, and tells everyone else what to expect.

Is there an App Store badge?

Some news reports said the App Store would mark optimized apps with a badge. Apple's own announcement to developers doesn't mention one, and we haven't seen one in the App Store's data yet. Our tracker checks for it every few hours, and we'll update this guide if one appears.

Do release notes tell you?

Sometimes. A developer may write “now optimized for iPhone Duo” in the update notes, but many just say “bug fixes and improvements”. Our update tracker reads the release notes of every app we list and highlights the ones that mention iPhone Duo.

What to do if an app is still letterboxed

1. Update it. Open the App Store and install any pending updates. Duo support only arrives in an update.
2. Check its page here. Find the app in the directory to see what other owners report and when it last updated.
3. Ask for it. Tap “I want this on Duo” on the app's page. The most-wanted list shows developers which apps people are waiting on.
4. Tell the developer. Most App Store listings have an “App Support” link. A short, polite message from a real user often helps.