DuoDiscoverUnderstanding iPhone Duo app compatibility
Every App Store app opens on iPhone Duo. What changes from one app to the next is how much of the 7.6-inch inner display it uses, and whether it does anything with the fold at all.
What decides how an app looks
On iPhone Duo, an app’s layout depends mostly on which version of Apple’s software development kit (SDK) the developer last built it with. Apple sets out the three cases in its iPhone Duo preparation guide for developers. In short:
- Built with the iOS 26 SDK or earlier. The app still runs, but it does not adjust to every layout. With the phone open, it sits in the middle of the inner display with unused space around it. With the phone closed, it appears on the outer display beside the status bar and camera.
- Built with the iOS 27 SDK. The app grows to cover most of the inner display, while keeping clear of the status bar along the right edge. There is far less empty space than before.
- Built with the iOS 27.1 SDK or later. Apple treats these apps as optimized for iPhone Duo. They use the whole display, and toolbars and tab bars move into a vertical position under the status bar.
Apple also asks developers to stop assuming a fixed screen size or orientation, because folding or unfolding can resize an app while it is open. Apps that skip this work may crop images or place controls awkwardly even after a rebuild.
DuoDiscover’s three tiers
The SDK only tells part of the story. An app can fill the screen and still ignore the fold, so DuoDiscover sorts each listing into one of three tiers:
- Built for Duo Built for Duo. Also called Made for iPhone Duo. Does something specific with the hardware: a different layout folded and unfolded, Split View, the half-open position or the outer display.
- Adapted Adapted. Rebuilt with the iOS 27 SDK or later, so it resizes to the 7.6-inch inner display instead of showing black borders. With the iOS 27.1 SDK it can use the whole screen.
- Compatible Compatible. Runs unchanged. It looks normal on the outer display and keeps its phone layout, with black borders, when unfolded.
When other sites or developers talk about apps that are “optimized” or “Made for iPhone Duo”, they usually mean what DuoDiscover calls Built for Duo: an app that changes its behaviour when you fold or unfold the phone, rather than one that simply fills the larger display.
How to tell whether an app has been adapted
The quickest check is to open the app and unfold the phone. If the app sits inside black borders on the inner display, it has not been rebuilt for iPhone Duo and is running in its old phone layout. If it reaches the edges, it has been adapted. If it shows a different layout folded and unfolded, or puts something useful on each display, it is likely Built for Duo.
The App Store page can also help: Apple lets developers submit iPhone Duo screenshots, which tell you how the app is meant to look on the device. Screenshots are a statement of intent, though, so they are worth confirming on the phone itself.
How DuoDiscover verifies apps
Every listing shows its tier, the date it was checked and how: on a device, in the iPhone Duo simulator, or from a statement by the developer. The full process is described in how apps get verified.
An example of a Built for Duo app
NostalgicPod is a retro music player that uses each display differently: unfolded, the inner display shows a cover flow-style album carousel; folded, the outer display switches to an iPod-style menu with a click wheel. Its page includes screenshots of both layouts and a short video of the switch.
Finding more apps
Browse all listed iPhone Duo apps by tier and category, or follow the launch day tracker for apps that Apple and developers have announced but that DuoDiscover has not verified yet.