Xcode 9 Bug: Cannot find cdtool

An Apple engineer reached out about this...

Those of you with cdtool errors in Xcode 8, I suspect you installed the iOS 10.3 Simulator runtime from Xcode 9. It was discovered this week that this causes a problem with cdtool in Xcode 8.3.

You can work around that by moving iOS 10.3.simruntime aside and restarting CoreSimulatorService (source):

sudo mkdir /Library/Developer/CoreSimulator/Profiles/Runtimes/Backup 

sudo mv /Library/Developer/CoreSimulator/Profiles/Runtimes/{,Backup/}iOS\ 10.3.simruntime 

sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService 

Then restart Xcode, Simulator, etc. Those of you that deleted CoreSimulator.framework and thus cannot run Xcode.app any more can reinstall CoreSimulator.framework with:

installer -pkg /Applications/Xcode-beta.app/Contents/Resources/Packages/XcodeSystemResources.pkg -target /

Didn't work for me because I also have a Watch app and got the error on the Watch SDK.

I ended up deleting both Xcode 8 and 9 Beta, deleting /Library/Developer and ~/Library/Developer. Then reinstalled Xcode 8 and it worked.