What happens on day one
Nothing breaks overnight. Apps built with the iOS 26 SDK or earlier run on iPhone Duo but do not adapt: on the inner display they sit in the centre with empty space around them. Apps built with the iOS 27 SDK fill most of the inner display. Builds with the iOS 27.1 SDK or later use the whole display, with toolbars and tab bars moved to the side.
So the first question is simple: which SDK was your current App Store build made with?
Worth doing now
- Open your app in the iPhone Duo simulator. It comes with Xcode 27.1, and Apple calls it the best way to test the full native experience. Ten minutes on each main screen tells you whether there is work to do.
- Run Xcode’s resizability check. Ask the coding assistant in Xcode to “get my app ready for iPhone Duo” and it scans the project for the usual problems. Apple notes that it finds most issues, not all of them.
- Search for the patterns that break.
UIScreen.main, layout decisions based on device orientation or onuserInterfaceIdiom, and fixed screen heights. Our iPhone Duo guide explains each one. - Check full-bleed images. Photos set to fill can lose their subject on the wider inner display. In a photo app, that is the first thing people see.
What can wait
- Screenshots. Apple says submissions will need iPhone Duo screenshots starting April 2027. Until then they are optional, though submitting them adds a badge to your product page saying the app is optimised for iPhone Duo. Our screenshot size checker has the sizes.
- A redesign. Apple does not ask for a layout for each pose. A compact layout for the outer display and a regular one for the inner display cover them all, as long as they are built on size classes.
One thing to plan for
From April 2027, every upload to App Store Connect must be built with the iOS 27 SDK or later. So within months, every app that ships an update will be resized on iPhone Duo, whether its team planned for it or not. Testing now costs less than fixing layouts in a hurry before that deadline. More in our post on the April 2027 upload rules.
Sources: Apple’s Prepare and submit your apps for iPhone Duo (5 October 2026), Three steps to make your app shine on iPhone Duo and App Store submissions now open for the latest OS releases (9 September 2026).
Related: iPhone Duo: how to get your app ready · App Store screenshot sizes · iOS UX audit