I also upgraded to Xcode 13 and have the same error warning: Could not read serialized diagnostics file: error("Invalid diagnostics signature") (in target .... Command CompileSwift failed with a nonzero exit code I deleted the DerivedData folder and also cleaned the build folder, it makes no difference. Every time I compile the app I get the same error.

I am looking in the DerivedData/.....build/Objects-normal/arm64/ folder For classes that compiled successfully, 7 files were created with the same name and different extensions, as follows .d .dia .o .swiftdeps ~partial.swiftdoc ~partial.swiftmodule ~partial.swiftsourceinfo However for the classes that did not compile only the following two files were created .d .dia Also the .dia file was a file of zero bytes So it appears to have fallen over when creating the .dia file