Your app already runs on iPhone Duo. Nobody has to rebuild anything for it to launch. What you control is how much of the two displays it uses, and whether it keeps working when someone folds the phone halfway through a task.
What your app looks like today
Apple describes three tiers, set only by the SDK your shipping binary was built with:
| Linked against | Outer display (closed) | Inner display (open) |
|---|---|---|
| iOS 26 SDK or earlier | Shown to the left of the status bar and camera | Centered at a familiar iPhone size, with empty space around it |
| iOS 27 SDK | Resizes | Fills most of the display, but stays clear of the status bar on the right edge |
| iOS 27.1 SDK or later | Full display | Full display. Toolbars and tab bars move into a vertical bar below the status bar |
The first tier is what visitors to this site call letterboxed. In Apple’s compatibility mode an old app keeps an iPhone mini aspect ratio, centered on the inner display.13
Apple’s warning, in its own words: before you ship a build with the iOS 27.1 SDK, make sure your content adapts to the bars becoming vertical. Linking against the new SDK changes your layout the moment it ships, so test before you submit.1
The three steps Apple recommends
1. Run the App Resizability skill
Xcode 27.1 ships a coding-assistant skill that scans a project for the patterns that break on iPhone Duo. In Xcode, ask the assistant to “get my app ready for iPhone Duo”.1 If you use another coding agent, export the skill from Terminal:
xcrun agent skills exportApple notes that the skill doesn’t catch every issue, so treat its output as a first pass and go through the checklist below anyway.3
2. Fix the patterns that break resizing
Almost every layout bug on iPhone Duo comes from one of three assumptions:
- Sizing from the main screen.
UIScreen.mainis ambiguous on a phone with two displays. See Size from the window, not the screen. - Using orientation to decide layout. The inner display doesn’t honor your supported orientations, so orientation says nothing about your app’s shape.
- Treating idiom or size class as the device. An iPhone app can now get any combination of size classes. A regular width no longer means iPad. See Size classes, safe areas and standard containers.
If you build custom toolbars or tab bars, also read Vertical toolbars and tab bars before linking against iOS 27.1.
3. Test resizing
Apple suggests three tools, from cheapest to most faithful:
- The resizable iOS simulator in Device Hub.
- iPhone Mirroring on macOS 27, resized to extreme sizes in both directions.
- The iPhone Duo simulator in Xcode 27.1, which can open, close, rotate and fold the device. This is the only one that shows the real vertical bars and reserved regions.1
Testing and App Store submission covers what to check in each pose.
Do you need iPhone Duo-only code?
Usually not. Apple’s guidance is to treat iPhone Duo as one adaptable canvas: build with size classes, safe areas and standard containers, and the same code serves the outer display, the inner display, Split View and iPhone Mirroring. You can’t ship an app that runs only on iPhone Duo, and you don’t need to support iPad, although an existing iPad layout gives you a head start on the inner display.3
Hinge-aware layouts, the outer display for camera apps and multiple windows are optional extras for apps that benefit from them. Gate those APIs with #available(iOS 27.1, *) so you keep your current deployment target.4
Key dates
- October 5, 2026: Apple started accepting iPhone Duo-optimized apps in App Store Connect.2
- October 23, 2026: iPhone Duo goes on sale.
- April 2027: iPhone Duo screenshots become required. Apple’s screenshot specifications say this applies to apps built with the iOS 27.1 SDK or later.2