Also facing this issue when trying to initialize LanguageModelSession in our project when using Xcode 26 Beta 5. I had no issues using Beta 3 version. When running the FoundationModels sample projects and a basic SwiftUI project, I am also crashing at launch with the following error. I am on macOS Sequoia 15.5 running Xcode 26 Beta 5 on and iPhone 16 - iOS 26 device. Thank you!

Thank you @DTS Engineer! This was exactly what I had, I was building with Xcode Beta 5 and running on previous iOS 26 versions. Because of the symbol changes, building with Xcode 26 Beta 5 and running on macOS / iOS 26 Beta 4 / 3 may trigger the crash. I was able to run properly after upgrading my iOS device to Beta 5 and no more crashes! It seems you'll need to be on both Xcode 26 Beta 5 + iOS 26 Beta 5.