I have a large mixed language (obj-c, swift & c++) project. It builds and runs fine on all platforms "Build for previews" also seems to complete fine. Opening preview of a particular swift file fails with 'Failed to launch swiftc' The diagnostics gives: Failed to launch swiftc. ================================== | HumanReadableNSError: NSInvalidArgumentException | | com.apple.dt.PreviewsFoundation.ExceptionError (0): | ==NSLocalizedFailureReason: too many arguments (4670) -- limit is 4096