I believe I've tracked this down. Some of our internal build configuration within the project was using the Xcode variable TOOLCHAIN_DIR to reference the clang compiler. Apparently TOOLCHAIN_DIR is now pointed at the Metal toolchain directory, and we need to be using DT_TOOLCHAIN_DIR (which points at the default Xcode toolchain directory) instead.

We're still seeing this issue with the Xcode 26 RC build as of today (September 10). It is currently blocking us from switching to the new tools.

The two log sections that start with ExecuteExternalTool were under actions labelled Discovering version info for clang. There are other, additional actions with the same name that completed successfully, and have the correct toolchain directory in the Xcode app in their arguments. The other log section was under the Create build description action.

I haven't yet created an isolated sample project that demonstrates the issue. This is on an active commercial product that builds cleanly under Xcode 16.3.

xcrun -f clang gives a reasonable result: /Applications/Xcode-26-beta-3.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang I've attached the requested sysdiagnose to feedback request ID FB18864396. I just noticed there are a couple other sections of the build log containing failures, which could be relevant: ExecuteExternalTool /var/run/com.apple.security.cryptexd/mnt/com.apple.MobileAsset.MetalToolchain-v17.1.5276.7.Pb9SLL/Metal.xctoolchain/usr/bin/clang -v -E -dM -isysroot /Applications/Xcode-26-beta-3.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS26.0.sdk -x c -c /dev/null Command ExecuteExternalTool failed with a nonzero exit code ExecuteExternalTool /var/run/com.apple.security.cryptexd/mnt/com.apple.MobileAsset.MetalToolchain-v17.1.5276.7.Pb9SLL/Metal.xctoolchain/usr/bin/clang -v -E -dM -arch arm64 -isysroot /Applications/Xcode-26-beta-3.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS26.0.sdk -x c++ -c /dev/null Command ExecuteExternalTool failed with a nonzero exit code Build description signature: 30b31cde5cf1e21881d40077c4e3bf06 Build description path: /Users/<redacted>/Library/Developer/Xcode/DerivedData/Mobile-grpqdtycanuwrdavwfqbqzxsgmxo/Build/Intermediates.noindex/XCBuildData/30b31cde5cf1e21881d40077c4e3bf06.xcbuilddata Command CreateBuildDescription emitted errors but did not return a nonzero exit code to indicate failure I've also tried deleting ~/Library/Developer/Xcode/DerivedData and attempting the build again, with no change. Thanks!