If you’re building apps using Xcode 26 beta on macOS 26 beta, you should beware of the combination of their third betas. If you’re unlucky like me, you’ll discover those shiny new app icons generated by Icon Composer no longer work on any older version of macOS. This is mentioned in the Xcode 26 b3 release notes, and the workaround given is “none”.

Problem

I’ve built over 20 apps now using Icon Composer’s new icon files, with Xcode 26 b2 on macOS 26 b2. All look good in Tahoe and in previous versions of macOS, at least in Sonoma and Sequoia.

Today I built my first apps using icon files already generated by Icon Composer, but this time with Xcode 26 b3 on macOS 26 b3. In every case, I got a build warning that Xcode “failed to generate flattened icon stack for icon named…”. Although the app icon displays fine in macOS 26 b3, when moved across to Sequoia there’s just a generic icon. Inspecting the app bundle reveals that there’s no icns file inside. The Xcode warning implied this might be related to CoreSVG.

Workaround

I next tried building the same projects using Xcode 26 b2 on macOS 26 b3, but that made no difference at all.

What does appear to be effective is to build in Xcode 26 b2 on macOS 15.5. So far each of the apps I’m trying to build has then worked fine in both Tahoe and Sequoia, and that magic icns file is back.

I wish you success.