I figured it out. My app is SwiftUI, so I'm testing both iOS and macOS. Originally, I was just using the macOS build to check if everything works, and I didn't download any of the extra simulator packages (iOS, visionOS, tvOS, etc). So I experienced the Metal Toolchain build issue. However, it did work after I downloaded the iOS 26 Simulator (I acutally downloaded them all). I then was able to build my app with no errors on iOS 26, and then subsequently the build for macOS 26 worked as well.

Relaunching Xcode 26 b5 doesn't resolve the issue for me (either with Xcode-beta.app nor xcodebuild) on macOS 15.6 M2 Pro Macbook Pro.

Relaunching doesn't do anything for me. Both Xcode 26 b5 and xcodebuild still fail to download the metal toolchain. macOS 15.6 on M2 Pro Macbook Pro.