On 9 September, Apple unveiled what it calls “the first foldable iPhone”. The iPhone Duo has been on sale since 23 October, starting at $1,999 in the US. Closed, it is a phone with a 5.4-inch outer display. Open, it unfolds a 7.6-inch inner display, close to a small tablet. For every business with an app on the App Store, the question comes quickly: do we need to do anything?
What happens to your app if you do nothing
Your app works, but how it looks depends on the version of Apple’s tools it was last built with. Apple distinguishes three cases:
- Built with the iOS 26 SDK or earlier: on the open display, the app appears in the centre, surrounded by empty space. Closed, its content sits to the left of the status bar and camera.
- Built with the iOS 27 SDK: the app fills most of the inner display, avoiding the status bar on the right edge.
- Built with the iOS 27.1 SDK or later: the app is treated as optimised. It uses the full display, and its toolbars and tab bars become vertical, below the status bar.
The first case is the one early reviews noticed. Several reviewers describe unadapted apps surrounded by thick black bars, with a phone-sized app in the middle of a screen nearly twice as large. Nothing is broken, but the app looks stuck in 2025.
The trap: rebuilding is not enough
The temptation is to update the tools, rebuild and ship. That is exactly where Apple urges caution: with the iOS 27.1 SDK, bars become vertical, and Apple asks developers to check that their content adapts before shipping. An app that assumed a tab bar at the bottom can end up with hidden or misplaced elements.
Two other assumptions fall. First, “iPhone” no longer means “narrow screen”: on the inner display, an iPhone app is treated as being on a large screen, much like an iPad, and Apple warns that code which reserved a layout for iPad will now show it on iPhone. Second, the app’s size can change mid-use: opening or closing the phone is enough. An app that measures the screen once at launch will put things in the wrong place at the first fold.
A foldable is not a screen question but a continuity one: the person opening their phone does not want to start over what they were doing.
What Apple makes easy, and what is left to do
The good news is that Apple has done part of the work. According to Apple, apps built with the standard components of its frameworks adapt automatically to each display, and Xcode 27.1 can simulate folding and rotation. To go further, new capabilities let an app know where the fold is and arrange content on either side, but an Apple engineer points out on the developer forums that not every app needs them.
What is left is more a product matter than a technical one. On a screen nearly twice as large, what do you show in addition? A list and its detail side by side, a wider map, two pages of a document? Above all, what happens at the moment of folding: the person must find exactly what they were looking at. That is the question we had to settle for Tome, our comics reader: open, the fold becomes the binding of a real two-page spread; closed, you read panel by panel; and either way, you stay on the page you were reading.
The App Store timeline
For now, nothing is mandatory. iPhone Duo screenshots are optional, but apps that provide them get a badge on their product page showing the app is optimised for the Duo. From April 2027, these screenshots will be required for any app built with the iOS 27.1 SDK or later. In other words, your app’s next serious update will raise the question anyway.
Apple also prepared the launch carefully: Netflix, Slack, Zoom, Instagram, Things and Flighty were adapted from day one, as was WEBTOON, and others are announced for the following weeks. On a device sold at this price, the early buyers are also the ones who will notice when an app falls behind.
How to decide
Adapting is not equally worthwhile for every app. Three questions help decide:
- Does your content benefit from being big? Reading, video, maps, tables, creative work: yes. A payment or settings app: much less.
- Are your users likely to own one? A device at this price reaches professionals and enthusiasts first.
- Is your app well built? An app that follows Apple’s components and layout rules often adapts in a few days. An app that assumes a fixed screen size everywhere needs real work, which the foldable merely exposes.
Either way, rebuild first with the recent tools in a test environment, and look at your app open, closed, then folded mid-use. Within an hour you will know whether it is a formality or a project. If it is a project, it is work we take on in product engineering and interface design. Tell us about your app: we will tell you what the Duo is going to ask of it.