How FoldCheck decides if an app works on iPhone Duo

Signal 1: when the current version was built

Apps can only use the full iPhone Duo inner display when they are built with the iOS 27.1 SDK or later. Apple began accepting those builds on October 5, 2026.

So if the version currently on the App Store was released before October 5, it can't be built for Duo, and it will open letterboxed on the inner display. We mark those apps "Letterboxed" with high confidence.

A release after October 5 is necessary but not sufficient: developers can still ship an update built with an older SDK. Those apps are marked "Updated, unconfirmed" until we have more evidence.

Signal 2: what the release notes say

Every three hours we look up every tracked app in the US and China App Stores and store each new version with its release notes.

If the notes mention iPhone Duo, a foldable layout or the inner display, we mark the app "Built for Duo" and highlight the update on the update tracker. It is still a claim by the developer, so owner reports can override it.

Signal 3: reports from Duo owners

Anyone with an iPhone Duo can report how an app looks folded, unfolded and half-open. One report per network connection per app version.

Once three or more reports for the current version agree, that result replaces the automatic verdict. When the app ships a new version, the count starts again, because a new build can change everything.

What we track

The top 100 free apps and the top grossing apps in the US and China App Stores, refreshed every three hours. Any other app is looked up live when someone searches for it.

Limits

We can't see inside an app binary, so we don't know the exact SDK an app was built with. The build date rules apps out; it can't rule them in.

Reports come from the public. We allow one report per network connection per app version and require three matching reports before they count.