Shawn G
November 3, 2026
5
min. read
and updated on:
November 3, 2026
Apple's first foldable iPhone went on sale on October 23. Closed, iPhone Duo is a 5.4-inch phone. Open, it's a 7.6-inch screen. Your app already runs on it. How well it runs depends mostly on which version of Apple's tools it was last built with.
This post is for founders and product owners. It covers what your app looks like on iPhone Duo today, the code assumptions that break on it, and what to ask whoever maintains your app. The technical names are included so you can pass them straight to your developer.
iPhone Duo in one minute
- Apple announced it on September 9, 2026. It went on sale October 23 in more than 70 countries, and 28 more followed on October 30.
- It starts at $1,999 in the US.
- It has two screens: a 5.4-inch outer display and a 7.6-inch inner display. Apple says both share the same aspect ratio, “so content scales proportionally.”
- People use it closed, open flat, or propped up partly folded on a desk.
- It ships with iOS 27.1. For the first time on iPhone, Split View lets people run two apps side by side.
What your app looks like on it today
Apple's developer guide sorts apps into three groups, by the SDK the app was built with.
| Last built with | What happens when someone opens the phone |
|---|---|
| iOS 26 SDK or earlier | The app sits in the center of the inner display, with empty space around it |
| iOS 27 SDK | The app resizes to fill most of the inner display |
| iOS 27.1 SDK or later (Xcode 27.1) | Apple treats the app as optimized for iPhone Duo, and it can carry the iPhone Duo badge on the App Store |
If your last App Store update was built with the iOS 26 SDK or earlier, your users are looking at a phone-sized app floating in the middle of a much bigger screen. A rebuild in Xcode 27.1 is the first step. Apple's own instruction is short: “Recompile existing apps with Xcode 27.1.”
What tends to break
Recompiling gets the app to resize. Whether it resizes well depends on the assumptions inside the code. In Apple's words: “Hardcoded screen sizes, fixed orientations, and device assumptions can break layouts.” These are the ones its guide names:
- Reading the screen size. Code that asks the device for its screen size (
UIScreen.main) gets the wrong answer once the app can change size. Layouts should take their size from the window or view they're drawn in. - Hardcoded heights. A check like “if the screen is 844 points tall” was written for one phone. On iPhone Duo it matches nothing.
- Phone or tablet checks. The inner display reports a regular width and a regular height, the same size class iPad apps see. Apple's note for developers: “Your app may react to different size class boundaries, but it is still an iPhone app.” Code that switches layouts on “is this an iPad?” can pick the wrong one.
- Portrait lock. Apple says the inner display “doesn't honor your supported interface orientations.” An app locked to portrait still gets a wide window when the phone opens.
- Hero images and video. Media set to fill its frame (
.scaleAspectFill) can crop faces and text on the wider screen. - Margins. Apple says safe areas are “often asymmetric,” especially on iPhone Duo. Layouts that assume the same margin on every side need a check.
Apps built on Apple's standard navigation (split views and tab bars) adapt to every pose on their own. Custom navigation needs testing on both screens and in Split View.
Apple also added an “App Resizability” skill to Xcode 27.1 that a developer can prompt with “get my app ready for iPhone Duo.” It gives a first pass. Someone still has to open every screen on the iPhone Duo simulator, closed and open, before release.
What you get for doing the work
- The badge. Once you submit an optimized build, your App Store page shows a badge saying the app is optimized for iPhone Duo.
- A better featuring pitch. When you nominate the app for App Store featuring, you can say it supports all device poses.
- Split View. People can now run two iPhone apps side by side. An app that resizes cleanly works there too.
Screenshots become required in April 2027
Apple's developer news, October 5: “Starting April 2027, any apps or games submitted will need to include screenshots for iPhone Duo.”
That applies to every submission, bug fixes included. From April, you need iPhone Duo screenshots to ship any update at all, so the layout has to look right before then.
Five questions to ask your developer
- Which SDK was our last App Store build made with?
- Have you run the app in the iPhone Duo simulator in Xcode 27.1? What does it look like closed, open and in Split View?
- Does the code read the screen size, or check for a specific device or screen height anywhere?
- Is the app locked to portrait, and what does it do on the inner display?
- Who is making the iPhone Duo screenshots, and when, before April 2027?
Not sure where your app stands?
Send us your app and a senior engineer will check it for iPhone Duo: the SDK it's built on, the screens that break when the phone opens, and what it takes to fix them. It's free, with no commitment.
FAQ
Does my app work on iPhone Duo without an update? Yes, it runs. An app built with the iOS 26 SDK or earlier appears in the center of the inner display with empty space around it. Apps built with the iOS 27 SDK or later resize to fill most of the screen.
What do I need to build with to be optimized for iPhone Duo? Xcode 27.1 and the iOS 27.1 SDK. Apps built that way are treated as optimized and can show the iPhone Duo badge on the App Store.
When are iPhone Duo screenshots required? From April 2027, for every app or game submitted to the App Store.
How big are the iPhone Duo screens? 5.4 inches closed and 7.6 inches open. Both have the same aspect ratio.
