Apple is now accepting App Store submissions of apps optimized for iPhone Duo, its first foldable iPhone. As the company announced on Monday, October 5, on its developer site, developers just need to recompile with Xcode 27.1 and submit through App Store Connect, the dashboard where App Store apps are managed. It's the last step before launch: the phone ships with iOS 27.1 and hits stores on October 23.
Why the iOS 27.1 SDK matters
The SDK (the set of tools and libraries an app is compiled against) determines how your app behaves on the foldable. Apple lays it out in its guide to preparing apps for iPhone Duo with three scenarios:
- iOS 26 SDK or earlier: the app runs but doesn't adapt. With the phone open, it sits in the middle of the inner display with empty space around it.
- iOS 27 SDK: the app fills most of the inner display but still avoids the status bar, which sits on the right edge when the Duo is open.
- iOS 27.1 SDK or later: the app is optimized and uses the full display; toolbars and tab bars move to a vertical layout below the status bar.
That last change is where the work is. Apple warns that before shipping a build compiled with the iOS 27.1 SDK, you should make sure your content adapts to the bars going vertical. Xcode 27.1 is at the Release Candidate stage, meaning the build expected to become final, and it includes an iPhone Duo simulator that Apple calls the best way to test the full experience. Device Hub, which includes a resizable iOS simulator, lets you check how your app behaves in every pose and orientation.
What Apple wants you to check in your code
The guide lists the usual culprits. The big one is sizing your app with UIScreen.main: on the Duo, your app's size changes every time the phone folds or unfolds, and it doesn't always match the screen, so you should size from the view or window and re-read it whenever it changes. Apple also says not to base layout on device orientation or on whether the device is an iPhone or an iPad. On the Duo, an iPhone app can end up with any combination of size classes (the "compact" and "regular" categories iOS uses to describe available space), and on the inner display it should feel like a natural extension of its iPad layout. Hardcoded screen-size comparisons should go too, since they won't match the Duo.
To find these issues, Xcode ships a "skill" for its coding assistant: ask it to "get my app ready for iPhone Duo" and it scans the project, explains each problem, and suggests a fix. Apple notes it catches most issues, not all of them.
App Store screenshots
Screenshot specifications now include iPhone Duo. According to the , the outer display uses 1398 x 2034 pixels in portrait (2034 x 1398 in landscape) and the inner display uses 2007 x 2853 (2853 x 2007 in landscape). They're optional for now, but starting in April 2027 every app or game submitted will need iPhone Duo screenshots. App Store Connect also adds a tool to preview how your product page looks on the foldable, per Apple's . reports that optimized apps will get a badge on their store page; Apple doesn't mention it in the official pages we reviewed.
