The problem was indeed the EnableSwiftBuildSystemIntegration. Setting it to 0 was not working, though. I got a response from an engineer through feedback assistant. Completely removing the key fixed this issue. This worked for me: defaults delete com.apple.dt.XCBuild EnableSwiftBuildSystemIntegration restart Xcode

I had the same issue like @darrylfromredwoodcity described. My problem was that I hat installed multiple versions of Xcode and the wrong Command Line Tools were selected. Changing the CLT fixed it for me: Xcode > Preferences > Location > Command Line Tools -> Select correct version