Usage of multiple async lets crashes the app in a nondeterministic fashion. We are experiencing this crash in production, but it is rare. 0 libswift_Concurrency.dylib 0x20a8b89b4 swift_task_create_commonImpl(unsigned long, swift::TaskOptionRecord*, swift::TargetMetadata<swift::InProcess> const*, void (swift::AsyncContext* swift_async_context) swiftasynccall*, void*, unsigned long) + 384 1 libswift_Concurrency.dylib 0x20a8b6970 swift_asyncLet_begin + 36 We managed to isolate the issue, and we submitted a technical incident (Case-ID: 8007727). However, we were completely ignored, and referred to the developer forums. To reproduce the bug you need to run the code on a physical device and under instruments (we used swift concurrency). This bug is present on iOS 17 and 18, Xcode 15.1, 15.4 and 16 beta, swift 5 and 6, including strict concurrency. Here's the code for Swift 6 / Xcode 16 / strict concurrency: (I wanted to attach the project but for some reason I am unable to) typealias VoidHandler = () -> Void enum Fetching { case inProgress, idle } protocol PersonProviding: Sendable { func getPerson() async throws -> Person } actor PersonProvider: PersonProviding { func getPerson() async throws -> Person { async let first = getFirstName() async let last = getLastName() async let age = getAge() async let role = getRole() return try await Person(firstName: first, lastName: last, age: age, familyMemberRole: role) } private func getFirstName() async throws -> String { try await Task.sleep(nanoseconds: 1_000_000_000) return ["John", "Kate", "Alex"].randomElement()! } private func getLastName() async throws -> String { try await Task.sleep(nanoseconds: 1_400_000_000) return ["Kowalski", "McMurphy", "Grimm"].randomElement()! } private func getAge() async throws -> Int { try await Task.sleep(nanoseconds: 2_100_000_000) return [56, 24, 11].randomElement()! } private func getRole() async throws -> Person.Role { try await Task.sleep(nanoseconds: 500_000_000) return Person.Role.allCases.randomElement()! } } @MainActor final class ViewModel { private let provider: PersonProviding = PersonProvider() private var fetchingTask: Task<Void, Never>? let onFetchingChanged: (Fetching) -> Void let onPersonFetched: (Person) -> Void init(onFetchingChanged: @escaping (Fetching) -> Void, onPersonFetched: @escaping (Person) -> Void) { self.onFetchingChanged = onFetchingChanged self.onPersonFetched = onPersonFetched } func fetchData() { fetchingTask?.cancel() fetchingTask = Task { do { onFetchingChanged(.inProgress) let person = try await provider.getPerson() guard !Task.isCancelled else { return } onPersonFetched(person) onFetchingChanged(.idle) } catch { print(error) } } } } struct Person { enum Role: String, CaseIterable { case mum, dad, brother, sister } let firstName: String let lastName: String let age: Int let familyMemberRole: Role init(firstName: String, lastName: String, age: Int, familyMemberRole: Person.Role) { self.firstName = firstName self.lastName = lastName self.age = age self.familyMemberRole = familyMemberRole } } import UIKit class ViewController: UIViewController { @IBOutlet private var first: UILabel! @IBOutlet private var last: UILabel! @IBOutlet private var age: UILabel! @IBOutlet private var role: UILabel! @IBOutlet private var spinner: UIActivityIndicatorView! private lazy var viewModel = ViewModel(onFetchingChanged: { [weak self] state in switch state { case .idle: self?.spinner.stopAnimating() case .inProgress: self?.spinner.startAnimating() } }, onPersonFetched: { [weak self] person in guard let self else { return } first.text = person.firstName last.text = person.lastName age.text = "\(person.age)" role.text = person.familyMemberRole.rawValue }) @IBAction private func onTap() { viewModel.fetchData() } }