In an AppKit document-based project created by Xcode, setting canConcurrentlyReadDocuments to true allows new documents to open normally in Swift 5, but switching to Swift 6 causes an error. Judging from the error message, it seems to be a threading issue, but I’m not sure how to adjust the code to support the Swift 6 environment. The project is the most basic code from an Xcode-created document-based project without any modifications, except for changing the Swift version to 6 and setting canConcurrentlyReadDocuments to true. Source code： https://drive.google.com/file/d/1ryb2TaU6IX884q0h5joJqqZwSX95Q335/view?usp=sharing AppDelegate.swift import Cocoa @main class AppDelegate: NSObject, NSApplicationDelegate { func applicationDidFinishLaunching(_ aNotification: Notification) { // Insert code here to initialize your application } func applicationWillTerminate(_ aNotification: Notification) { // Insert code here to tear down your application } func applicationSupportsSecureRestorableState(_ app: NSApplication) -> Bool { return true } } Document.swift import Cocoa class Document: NSDocument { override init() { super.init() // Add your subclass-specific initialization here. } override class var autosavesInPlace: Bool { return true } override class func canConcurrentlyReadDocuments(ofType typeName: String) -> Bool { true } override func canAsynchronouslyWrite(to url: URL, ofType typeName: String, for saveOperation: NSDocument.SaveOperationType) -> Bool { true } override func makeWindowControllers() { // Returns the Storyboard that contains your Document window. let storyboard = NSStoryboard(name: NSStoryboard.Name("Main"), bundle: nil) let windowController = storyboard.instantiateController(withIdentifier: NSStoryboard.SceneIdentifier("Document Window Controller")) as! NSWindowController self.addWindowController(windowController) } override func data(ofType typeName: String) throws -> Data { // Insert code here to write your document to data of the specified type, throwing an error in case of failure. // Alternatively, you could remove this method and override fileWrapper(ofType:), write(to:ofType:), or write(to:ofType:for:originalContentsURL:) instead. // throw NSError(domain: NSOSStatusErrorDomain, code: unimpErr, userInfo: nil) return Data() } override func read(from data: Data, ofType typeName: String) throws { // Insert code here to read your document from the given data of the specified type, throwing an error in case of failure. // Alternatively, you could remove this method and override read(from:ofType:) instead. // If you do, you should also override isEntireFileLoaded to return false if the contents are lazily loaded. // throw NSError(domain: NSOSStatusErrorDomain, code: unimpErr, userInfo: nil) } } ViewController.swift import Cocoa class ViewController: NSViewController { override func viewDidLoad() { super.viewDidLoad() // Do any additional setup after loading the view. } override var representedObject: Any? { didSet { // Update the view, if already loaded. } } }