diff --git a/Firebase.podspec b/Firebase.podspec
index e20903da021..c11b0b7021a 100644
--- a/Firebase.podspec
+++ b/Firebase.podspec
@@ -28,7 +28,7 @@ Simplify your app development, grow your user base, and monetize more effectivel
 
   s.cocoapods_version = '>= 1.12.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.default_subspec = 'Core'
 
diff --git a/FirebaseABTesting.podspec b/FirebaseABTesting.podspec
index 767da46ca85..612d9be3afc 100644
--- a/FirebaseABTesting.podspec
+++ b/FirebaseABTesting.podspec
@@ -35,7 +35,7 @@ Firebase Cloud Messaging and Firebase Remote Config in your app.
   s.cocoapods_version = '>= 1.12.0'
   s.prefix_header_file = false
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   base_dir = "FirebaseABTesting/Sources/"
   s.source_files = [
diff --git a/FirebaseAnalytics.podspec b/FirebaseAnalytics.podspec
index dcd6a9ee85b..907690f2442 100644
--- a/FirebaseAnalytics.podspec
+++ b/FirebaseAnalytics.podspec
@@ -17,7 +17,7 @@ Pod::Spec.new do |s|
     }
 
     s.cocoapods_version = '>= 1.12.0'
-    s.swift_version     = '5.9'
+    s.swift_version     = '6.0'
 
     s.ios.deployment_target  = '12.0'
     s.osx.deployment_target  = '10.15'
diff --git a/FirebaseAppCheck.podspec b/FirebaseAppCheck.podspec
index 3b22cf6f5c3..3e7569875bc 100644
--- a/FirebaseAppCheck.podspec
+++ b/FirebaseAppCheck.podspec
@@ -22,7 +22,7 @@ Pod::Spec.new do |s|
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseAppDistribution.podspec b/FirebaseAppDistribution.podspec
index e9ca3a73ed0..f40e93920db 100644
--- a/FirebaseAppDistribution.podspec
+++ b/FirebaseAppDistribution.podspec
@@ -17,7 +17,7 @@ iOS SDK for App Distribution for Firebase.
 
   s.ios.deployment_target = '13.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.cocoapods_version = '>= 1.12.0'
   s.prefix_header_file = false
diff --git a/FirebaseAuth.podspec b/FirebaseAuth.podspec
index db99c757eba..122186f516f 100644
--- a/FirebaseAuth.podspec
+++ b/FirebaseAuth.podspec
@@ -24,7 +24,7 @@ supports email and password accounts, as well as several 3rd party authenticatio
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseAuth/CHANGELOG.md b/FirebaseAuth/CHANGELOG.md
index 04ff2a7322f..5bddc4f16b4 100644
--- a/FirebaseAuth/CHANGELOG.md
+++ b/FirebaseAuth/CHANGELOG.md
@@ -1,3 +1,6 @@
+# Unreleased
+- [fixed] Replaced unsafe uses of `os_unfair_lock` (#14548).
+
 # 11.12.0
 - [fixed] Fix a `fatalError` unenrolling from MFA. An invalid user token now throws an
   `invalidUserToken` error instead of crashing. (#14663)
diff --git a/FirebaseAuth/Interop/Public/FirebaseAuthInterop/FIRAuthInterop.h b/FirebaseAuth/Interop/Public/FirebaseAuthInterop/FIRAuthInterop.h
index a50669da433..53b0f745926 100644
--- a/FirebaseAuth/Interop/Public/FirebaseAuthInterop/FIRAuthInterop.h
+++ b/FirebaseAuth/Interop/Public/FirebaseAuthInterop/FIRAuthInterop.h
@@ -28,13 +28,12 @@ typedef void (^FIRTokenCallback)(NSString *_Nullable_result token, NSError *_Nul
     NS_SWIFT_UNAVAILABLE("Use Swift's closure syntax instead.");
 
 /// Common methods for Auth interoperability.
-NS_SWIFT_NAME(AuthInterop)
-@protocol FIRAuthInterop
+NS_SWIFT_NAME(AuthInterop) NS_SWIFT_SENDABLE @protocol FIRAuthInterop
 
 /// Retrieves the Firebase authentication token, possibly refreshing it if it has expired.
 - (void)getTokenForcingRefresh:(BOOL)forceRefresh
-                  withCallback:
-                      (void (^)(NSString *_Nullable_result token, NSError *_Nullable error))callback
+                  withCallback:(void (^NS_SWIFT_UI_ACTOR)(NSString *_Nullable_result token,
+                                                          NSError *_Nullable error))callback
     NS_SWIFT_NAME(getToken(forcingRefresh:completion:));
 
 /// Get the current Auth user's UID. Returns nil if there is no user signed in.
diff --git a/FirebaseAuth/Sources/Swift/Auth/Auth.swift b/FirebaseAuth/Sources/Swift/Auth/Auth.swift
index 5d8050cc891..f88a1864990 100644
--- a/FirebaseAuth/Sources/Swift/Auth/Auth.swift
+++ b/FirebaseAuth/Sources/Swift/Auth/Auth.swift
@@ -18,6 +18,7 @@ import FirebaseAppCheckInterop
 import FirebaseAuthInterop
 import FirebaseCore
 import FirebaseCoreExtension
+import FirebaseCoreInternal
 #if COCOAPODS
   internal import GoogleUtilities
 #else
@@ -81,7 +82,8 @@ extension Auth: AuthInterop {
   /// This method is not for public use. It is for Firebase clients of AuthInterop.
   @objc(getTokenForcingRefresh:withCallback:)
   public func getToken(forcingRefresh forceRefresh: Bool,
-                       completion callback: @escaping (String?, Error?) -> Void) {
+                       completion callback: @escaping @MainActor @Sendable (String?, Error?)
+                         -> Void) {
     kAuthGlobalWorkQueue.async { [weak self] in
       if let strongSelf = self {
         // Enable token auto-refresh if not already enabled.
@@ -144,8 +146,7 @@ extension Auth: AuthInterop {
 ///
 /// This class is thread-safe.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@preconcurrency
-@objc(FIRAuth) open class Auth: NSObject {
+@objc(FIRAuth) open class Auth: NSObject, @unchecked Sendable /* TODO: sendable */ {
   /// Gets the auth object for the default Firebase app.
   ///
   /// The default Firebase app must have already been configured or an exception will be raised.
@@ -226,14 +227,15 @@ extension Auth: AuthInterop {
   ///   - user: The user object to be set as the current user of the calling Auth instance.
   ///   - completion: Optionally; a block invoked after the user of the calling Auth instance has
   ///             been updated or an error was encountered.
-  @objc open func updateCurrentUser(_ user: User?, completion: ((Error?) -> Void)? = nil) {
+  @objc open func updateCurrentUser(_ user: User?,
+                                    completion: (@MainActor (Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       guard let user else {
         let error = AuthErrorUtils.nullUserError(message: nil)
         Auth.wrapMainAsync(completion, error)
         return
       }
-      let updateUserBlock: (User) -> Void = { user in
+      let updateUserBlock: @Sendable (User) -> Void = { user in
         do {
           try self.updateCurrentUser(user, byForce: true, savingToDisk: true)
           Auth.wrapMainAsync(completion, nil)
@@ -293,7 +295,7 @@ extension Auth: AuthInterop {
     )
   #endif // !FIREBASE_CI
   @objc open func fetchSignInMethods(forEmail email: String,
-                                     completion: (([String]?, Error?) -> Void)? = nil) {
+                                     completion: (@MainActor ([String]?, Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       let request = CreateAuthURIRequest(identifier: email,
                                          continueURI: "http://www.google.com/",
@@ -357,7 +359,7 @@ extension Auth: AuthInterop {
   /// or is canceled. Invoked asynchronously on the main thread in the future.
   @objc open func signIn(withEmail email: String,
                          password: String,
-                         completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                         completion: (@Sendable (AuthDataResult?, Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       let decoratedCallback = self.signInFlowAuthDataResultCallback(byDecorating: completion)
       Task {
@@ -456,7 +458,7 @@ extension Auth: AuthInterop {
   /// or is canceled. Invoked asynchronously on the main thread in the future.
   @objc open func signIn(withEmail email: String,
                          link: String,
-                         completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                         completion: (@MainActor (AuthDataResult?, Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       let decoratedCallback = self.signInFlowAuthDataResultCallback(byDecorating: completion)
       let credential = EmailAuthCredential(withEmail: email, link: link)
@@ -535,7 +537,7 @@ extension Auth: AuthInterop {
     @objc(signInWithProvider:UIDelegate:completion:)
     open func signIn(with provider: FederatedAuthProvider,
                      uiDelegate: AuthUIDelegate?,
-                     completion: ((AuthDataResult?, Error?) -> Void)?) {
+                     completion: (@Sendable (AuthDataResult?, Error?) -> Void)?) {
       kAuthGlobalWorkQueue.async {
         Task {
           let decoratedCallback = self.signInFlowAuthDataResultCallback(byDecorating: completion)
@@ -636,7 +638,7 @@ extension Auth: AuthInterop {
   /// or is canceled. Invoked asynchronously on the main thread in the future.
   @objc(signInWithCredential:completion:)
   open func signIn(with credential: AuthCredential,
-                   completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                   completion: (@MainActor (AuthDataResult?, Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       let decoratedCallback = self.signInFlowAuthDataResultCallback(byDecorating: completion)
       Task {
@@ -706,7 +708,8 @@ extension Auth: AuthInterop {
   /// not enabled. Enable them in the Auth section of the Firebase console.
   /// - Parameter completion: Optionally; a block which is invoked when the sign in finishes, or is
   /// canceled. Invoked asynchronously on the main thread in the future.
-  @objc open func signInAnonymously(completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+  @objc open func signInAnonymously(completion: (@MainActor (AuthDataResult?, Error?) -> Void)? =
+    nil) {
     kAuthGlobalWorkQueue.async {
       let decoratedCallback = self.signInFlowAuthDataResultCallback(byDecorating: completion)
       if let currentUser = self._currentUser, currentUser.isAnonymous {
@@ -773,7 +776,7 @@ extension Auth: AuthInterop {
   /// - Parameter completion: Optionally; a block which is invoked when the sign in finishes, or is
   ///    canceled. Invoked asynchronously on the main thread in the future.
   @objc open func signIn(withCustomToken token: String,
-                         completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                         completion: (@MainActor (AuthDataResult?, Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       let decoratedCallback = self.signInFlowAuthDataResultCallback(byDecorating: completion)
       let request = VerifyCustomTokenRequest(token: token,
@@ -843,7 +846,7 @@ extension Auth: AuthInterop {
   /// or is canceled. Invoked asynchronously on the main thread in the future.
   @objc open func createUser(withEmail email: String,
                              password: String,
-                             completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                             completion: (@Sendable (AuthDataResult?, Error?) -> Void)? = nil) {
     guard password.count > 0 else {
       if let completion {
         completion(nil, AuthErrorUtils.weakPasswordError(serverResponseReason: "Missing password"))
@@ -884,7 +887,10 @@ extension Auth: AuthInterop {
 
   func internalCreateUserWithEmail(request: SignUpNewUserRequest,
                                    inResponse: SignUpNewUserResponse? = nil,
-                                   decoratedCallback: @escaping (Result<AuthDataResult, Error>)
+                                   decoratedCallback: @escaping @Sendable (Result<
+                                     AuthDataResult,
+                                     Error
+                                   >)
                                      -> Void) {
     Task {
       do {
@@ -957,7 +963,7 @@ extension Auth: AuthInterop {
   /// - Parameter completion: Optionally; a block which is invoked when the request finishes.
   /// Invoked asynchronously on the main thread in the future.
   @objc open func confirmPasswordReset(withCode code: String, newPassword: String,
-                                       completion: @escaping (Error?) -> Void) {
+                                       completion: @MainActor @escaping (Error?) -> Void) {
     kAuthGlobalWorkQueue.async {
       let request = ResetPasswordRequest(oobCode: code,
                                          newPassword: newPassword,
@@ -997,7 +1003,8 @@ extension Auth: AuthInterop {
   /// Invoked
   /// asynchronously on the main thread in the future.
   @objc open func checkActionCode(_ code: String,
-                                  completion: @escaping (ActionCodeInfo?, Error?) -> Void) {
+                                  completion: @MainActor @escaping (ActionCodeInfo?, Error?)
+                                    -> Void) {
     kAuthGlobalWorkQueue.async {
       let request = ResetPasswordRequest(oobCode: code,
                                          newPassword: nil,
@@ -1042,7 +1049,8 @@ extension Auth: AuthInterop {
   /// - Parameter completion: Optionally; a block which is invoked when the request finishes.
   /// Invoked asynchronously on the main thread in the future.
   @objc open func verifyPasswordResetCode(_ code: String,
-                                          completion: @escaping (String?, Error?) -> Void) {
+                                          completion: @escaping @MainActor (String?, Error?)
+                                            -> Void) {
     checkActionCode(code) { info, error in
       if let error {
         completion(nil, error)
@@ -1075,7 +1083,8 @@ extension Auth: AuthInterop {
   /// - Parameter code: The out of band code to be applied.
   /// - Parameter completion: Optionally; a block which is invoked when the request finishes.
   /// Invoked asynchronously on the main thread in the future.
-  @objc open func applyActionCode(_ code: String, completion: @escaping (Error?) -> Void) {
+  @objc open func applyActionCode(_ code: String,
+                                  completion: @escaping @MainActor (Error?) -> Void) {
     kAuthGlobalWorkQueue.async {
       let request = SetAccountInfoRequest(requestConfiguration: self.requestConfiguration)
       request.oobCode = code
@@ -1120,7 +1129,7 @@ extension Auth: AuthInterop {
   /// Invoked
   /// asynchronously on the main thread in the future.
   @objc open func sendPasswordReset(withEmail email: String,
-                                    completion: ((Error?) -> Void)? = nil) {
+                                    completion: (@MainActor (Error?) -> Void)? = nil) {
     sendPasswordReset(withEmail: email, actionCodeSettings: nil, completion: completion)
   }
 
@@ -1153,7 +1162,7 @@ extension Auth: AuthInterop {
   /// Invoked asynchronously on the main thread in the future.
   @objc open func sendPasswordReset(withEmail email: String,
                                     actionCodeSettings: ActionCodeSettings?,
-                                    completion: ((Error?) -> Void)? = nil) {
+                                    completion: (@MainActor (Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       let request = GetOOBConfirmationCodeRequest.passwordResetRequest(
         email: email,
@@ -1222,7 +1231,7 @@ extension Auth: AuthInterop {
   /// Invoked asynchronously on the main thread in the future.
   @objc open func sendSignInLink(toEmail email: String,
                                  actionCodeSettings: ActionCodeSettings,
-                                 completion: ((Error?) -> Void)? = nil) {
+                                 completion: (@MainActor (Error?) -> Void)? = nil) {
     if !actionCodeSettings.handleCodeInApp {
       fatalError("The handleCodeInApp flag in ActionCodeSettings must be true for Email-link " +
         "Authentication.")
@@ -1304,7 +1313,7 @@ extension Auth: AuthInterop {
     /// If you change the tenant ID of the `Auth` instance, the configuration will be
     /// reloaded.
     @objc(initializeRecaptchaConfigWithCompletion:)
-    open func initializeRecaptchaConfig(completion: ((Error?) -> Void)?) {
+    open func initializeRecaptchaConfig(completion: (@Sendable (Error?) -> Void)?) {
       Task {
         do {
           try await initializeRecaptchaConfig()
@@ -1347,7 +1356,7 @@ extension Auth: AuthInterop {
   /// the main thread, even for it's initial invocation after having been added as a listener.
   /// - Returns: A handle useful for manually unregistering the block as a listener.
   @objc(addAuthStateDidChangeListener:)
-  open func addStateDidChangeListener(_ listener: @escaping (Auth, User?) -> Void)
+  open func addStateDidChangeListener(_ listener: @escaping @MainActor (Auth, User?) -> Void)
     -> NSObjectProtocol {
     var firstInvocation = true
     var previousUserID: String?
@@ -1387,7 +1396,8 @@ extension Auth: AuthInterop {
   /// - Parameter listener: The block to be invoked. The block is always invoked asynchronously on
   /// the main thread, even for it's initial invocation after having been added as a listener.
   /// - Returns: A handle useful for manually unregistering the block as a listener.
-  @objc open func addIDTokenDidChangeListener(_ listener: @escaping (Auth, User?) -> Void)
+  @objc open func addIDTokenDidChangeListener(_ listener: @MainActor @escaping (Auth, User?)
+    -> Void)
     -> NSObjectProtocol {
     let handle = NotificationCenter.default.addObserver(
       forName: Auth.authStateDidChangeNotification,
@@ -1395,7 +1405,10 @@ extension Auth: AuthInterop {
       queue: OperationQueue.main
     ) { notification in
       if let auth = notification.object as? Auth {
-        listener(auth, auth._currentUser)
+        // MainActor is guaranteed by the queue parameter above.
+        MainActor.assumeIsolated {
+          listener(auth, auth._currentUser)
+        }
       }
     }
     objc_sync_enter(Auth.self)
@@ -1443,7 +1456,7 @@ extension Auth: AuthInterop {
   /// - Parameter completion: (Optional) the block invoked when the request to revoke the token is
   /// complete, or fails. Invoked asynchronously on the main thread in the future.
   @objc open func revokeToken(withAuthorizationCode authorizationCode: String,
-                              completion: ((Error?) -> Void)? = nil) {
+                              completion: (@MainActor (Error?) -> Void)? = nil) {
     _currentUser?.internalGetToken(backend: backend) { idToken, error in
       if let error {
         Auth.wrapMainAsync(completion, error)
@@ -1605,13 +1618,11 @@ extension Auth: AuthInterop {
     /// so the caller should ignore the URL from further processing, and `false` means the
     /// the URL is for the app (or another library) so the caller should continue handling
     /// this URL as usual.
-    @objc(canHandleURL:) open func canHandle(_ url: URL) -> Bool {
-      kAuthGlobalWorkQueue.sync {
-        guard let authURLPresenter = self.authURLPresenter as? AuthURLPresenter else {
-          return false
-        }
-        return authURLPresenter.canHandle(url: url)
+    @MainActor @objc(canHandleURL:) open func canHandle(_ url: URL) -> Bool {
+      guard let authURLPresenter = authURLPresenter as? AuthURLPresenter else {
+        return false
       }
+      return authURLPresenter.canHandle(url: url)
     }
   #endif
 
@@ -1819,22 +1830,23 @@ extension Auth: AuthInterop {
   /// This map is needed for looking up the keychain service name after the FirebaseApp instance
   /// is deleted, to remove the associated keychain item. Accessing should occur within a
   /// @synchronized([FIRAuth class]) context.
-  fileprivate static var gKeychainServiceNameForAppName: [String: String] = [:]
+  fileprivate static let gKeychainServiceNameForAppName =
+    FIRAllocatedUnfairLock<[String: String]>(initialState: [:])
 
   /// Gets the keychain service name global data for the particular app by
   /// name, creating an entry for one if it does not exist.
   /// - Parameter app: The Firebase app to get the keychain service name for.
   /// - Returns: The keychain service name for the given app.
   static func keychainServiceName(for app: FirebaseApp) -> String {
-    objc_sync_enter(Auth.self)
-    defer { objc_sync_exit(Auth.self) }
-    let appName = app.name
-    if let serviceName = gKeychainServiceNameForAppName[appName] {
-      return serviceName
-    } else {
-      let serviceName = "firebase_auth_\(app.options.googleAppID)"
-      gKeychainServiceNameForAppName[appName] = serviceName
-      return serviceName
+    return gKeychainServiceNameForAppName.withLock { map in
+      let appName = app.name
+      if let serviceName = map[appName] {
+        return serviceName
+      } else {
+        let serviceName = "firebase_auth_\(app.options.googleAppID)"
+        map[appName] = serviceName
+        return serviceName
+      }
     }
   }
 
@@ -1842,13 +1854,13 @@ extension Auth: AuthInterop {
   /// - Parameter appName: The name of the Firebase app to delete keychain service name for.
   /// - Returns: The deleted keychain service name, if any.
   static func deleteKeychainServiceNameForAppName(_ appName: String) -> String? {
-    objc_sync_enter(Auth.self)
-    defer { objc_sync_exit(Auth.self) }
-    guard let serviceName = gKeychainServiceNameForAppName[appName] else {
-      return nil
+    return gKeychainServiceNameForAppName.withLock { map in
+      guard let serviceName = map[appName] else {
+        return nil
+      }
+      map.removeValue(forKey: appName)
+      return serviceName
     }
-    gKeychainServiceNameForAppName.removeValue(forKey: appName)
-    return serviceName
   }
 
   func signOutByForce(withUserID userID: String) throws {
@@ -1872,17 +1884,18 @@ extension Auth: AuthInterop {
       // Schedule new refresh task after successful attempt.
       scheduleAutoTokenRefresh()
     }
-    var internalNotificationParameters: [String: Any] = [:]
-    if let app = app {
-      internalNotificationParameters[FIRAuthStateDidChangeInternalNotificationAppKey] = app
-    }
-    if let token, token.count > 0 {
-      internalNotificationParameters[FIRAuthStateDidChangeInternalNotificationTokenKey] = token
-    }
-    internalNotificationParameters[FIRAuthStateDidChangeInternalNotificationUIDKey] = _currentUser?
-      .uid
-    let notifications = NotificationCenter.default
     DispatchQueue.main.async {
+      var internalNotificationParameters: [String: Any] = [:]
+      if let app = self.app {
+        internalNotificationParameters[FIRAuthStateDidChangeInternalNotificationAppKey] = app
+      }
+      if let token, token.count > 0 {
+        internalNotificationParameters[FIRAuthStateDidChangeInternalNotificationTokenKey] = token
+      }
+      internalNotificationParameters[FIRAuthStateDidChangeInternalNotificationUIDKey] = self
+        ._currentUser?
+        .uid
+      let notifications = NotificationCenter.default
       notifications.post(name: NSNotification.Name.FIRAuthStateDidChangeInternal,
                          object: self,
                          userInfo: internalNotificationParameters)
@@ -2242,7 +2255,8 @@ extension Auth: AuthInterop {
   /// Invoked asynchronously on the main thread in the future.
   /// - Returns: Returns a block that updates the current user.
   func signInFlowAuthDataResultCallback(byDecorating callback:
-    ((AuthDataResult?, Error?) -> Void)?) -> (Result<AuthDataResult, Error>) -> Void {
+    (@MainActor (AuthDataResult?, Error?) -> Void)?)
+    -> @Sendable (Result<AuthDataResult, Error>) -> Void {
     return { result in
       switch result {
       case let .success(authResult):
@@ -2258,7 +2272,8 @@ extension Auth: AuthInterop {
     }
   }
 
-  private func wrapAsyncRPCTask(_ request: any AuthRPCRequest, _ callback: ((Error?) -> Void)?) {
+  private func wrapAsyncRPCTask(_ request: any AuthRPCRequest,
+                                _ callback: (@MainActor (Error?) -> Void)?) {
     Task {
       do {
         let _ = try await self.backend.call(with: request)
@@ -2269,7 +2284,7 @@ extension Auth: AuthInterop {
     }
   }
 
-  class func wrapMainAsync(_ callback: ((Error?) -> Void)?, _ error: Error?) {
+  class func wrapMainAsync(_ callback: (@MainActor (Error?) -> Void)?, _ error: Error?) {
     if let callback {
       DispatchQueue.main.async {
         callback(error)
@@ -2277,8 +2292,8 @@ extension Auth: AuthInterop {
     }
   }
 
-  class func wrapMainAsync<T: Any>(callback: ((T?, Error?) -> Void)?,
-                                   with result: Result<T, Error>) -> Void {
+  class func wrapMainAsync<T: Sendable>(callback: (@MainActor (T?, Error?) -> Void)?,
+                                        with result: Result<T, Error>) -> Void {
     guard let callback else { return }
     DispatchQueue.main.async {
       switch result {
@@ -2291,7 +2306,7 @@ extension Auth: AuthInterop {
   #if os(iOS)
     private func wrapInjectRecaptcha<T: AuthRPCRequest>(request: T,
                                                         action: AuthRecaptchaAction,
-                                                        _ callback: @escaping (
+                                                        _ callback: @escaping @Sendable (
                                                           (T.Response?, Error?) -> Void
                                                         )) {
       Task {
diff --git a/FirebaseAuth/Sources/Swift/Auth/AuthComponent.swift b/FirebaseAuth/Sources/Swift/Auth/AuthComponent.swift
index 649d274c294..36bb008d06c 100644
--- a/FirebaseAuth/Sources/Swift/Auth/AuthComponent.swift
+++ b/FirebaseAuth/Sources/Swift/Auth/AuthComponent.swift
@@ -18,6 +18,7 @@ import FirebaseAppCheckInterop
 import FirebaseAuthInterop
 import FirebaseCore
 import FirebaseCoreExtension
+import FirebaseCoreInternal
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
 @objc(FIRAuthComponent)
@@ -33,7 +34,7 @@ class AuthComponent: NSObject, Library, ComponentLifecycleMaintainer {
   private var instances: [String: Auth] = [:]
 
   /// Lock to manage access to the instances array to avoid race conditions.
-  private var instancesLock: os_unfair_lock = .init()
+  private let instancesLock: FirebaseCoreInternal.FIRAllocatedUnfairLock<Void> = .init()
 
   // MARK: - Initializers
 
@@ -58,10 +59,10 @@ class AuthComponent: NSObject, Library, ComponentLifecycleMaintainer {
   // MARK: - AuthProvider conformance
 
   @discardableResult func auth() -> Auth {
-    os_unfair_lock_lock(&instancesLock)
+    instancesLock.lock()
 
     // Unlock before the function returns.
-    defer { os_unfair_lock_unlock(&instancesLock) }
+    defer { instancesLock.unlock() }
 
     if let instance = instances[app.name] {
       return instance
diff --git a/FirebaseAuth/Sources/Swift/Auth/AuthDataResult.swift b/FirebaseAuth/Sources/Swift/Auth/AuthDataResult.swift
index 9d6dacd2687..bd4bdc73af9 100644
--- a/FirebaseAuth/Sources/Swift/Auth/AuthDataResult.swift
+++ b/FirebaseAuth/Sources/Swift/Auth/AuthDataResult.swift
@@ -22,7 +22,8 @@ extension AuthDataResult: NSSecureCoding {}
 ///
 /// It contains references to a `User` instance and an `AdditionalUserInfo` instance.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIRAuthDataResult) open class AuthDataResult: NSObject {
+@objc(FIRAuthDataResult) open class AuthDataResult: NSObject,
+  @unchecked Sendable /* TODO: sendable */ {
   /// The signed in user.
   @objc public let user: User
 
diff --git a/FirebaseAuth/Sources/Swift/Auth/AuthTokenResult.swift b/FirebaseAuth/Sources/Swift/Auth/AuthTokenResult.swift
index 90be2649ef6..496cc6b2c98 100644
--- a/FirebaseAuth/Sources/Swift/Auth/AuthTokenResult.swift
+++ b/FirebaseAuth/Sources/Swift/Auth/AuthTokenResult.swift
@@ -20,7 +20,8 @@ extension AuthTokenResult: NSSecureCoding {}
 /// A data class containing the ID token JWT string and other properties associated with the
 /// token including the decoded payload claims.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIRAuthTokenResult) open class AuthTokenResult: NSObject {
+@objc(FIRAuthTokenResult) open class AuthTokenResult: NSObject,
+  @unchecked Sendable /* TODO: sendable */ {
   /// Stores the JWT string of the ID token.
   @objc open var token: String
 
diff --git a/FirebaseAuth/Sources/Swift/AuthProvider/FederatedAuthProvider.swift b/FirebaseAuth/Sources/Swift/AuthProvider/FederatedAuthProvider.swift
index 0399b0584ec..76894d52c27 100644
--- a/FirebaseAuth/Sources/Swift/AuthProvider/FederatedAuthProvider.swift
+++ b/FirebaseAuth/Sources/Swift/AuthProvider/FederatedAuthProvider.swift
@@ -16,7 +16,7 @@ import Foundation
 
 /// Utility type for constructing federated auth provider credentials.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIRFederatedAuthProvider) public protocol FederatedAuthProvider: NSObjectProtocol {
+@objc(FIRFederatedAuthProvider) public protocol FederatedAuthProvider: NSObjectProtocol, Sendable {
   #if os(iOS)
 
     /// Used to obtain an auth credential via a mobile web flow.
diff --git a/FirebaseAuth/Sources/Swift/AuthProvider/OAuthProvider.swift b/FirebaseAuth/Sources/Swift/AuthProvider/OAuthProvider.swift
index b8cca1f5fca..d397fdca48c 100644
--- a/FirebaseAuth/Sources/Swift/AuthProvider/OAuthProvider.swift
+++ b/FirebaseAuth/Sources/Swift/AuthProvider/OAuthProvider.swift
@@ -17,7 +17,8 @@ import Foundation
 
 /// Utility class for constructing OAuth Sign In credentials.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIROAuthProvider) open class OAuthProvider: NSObject, FederatedAuthProvider {
+@objc(FIROAuthProvider) open class OAuthProvider: NSObject, FederatedAuthProvider,
+  @unchecked Sendable /* TODO: sendable */ {
   @objc public static let id = "OAuth"
 
   /// Array used to configure the OAuth scopes.
@@ -268,7 +269,7 @@ import Foundation
     /// - Parameter completion: Optionally; a block which is invoked asynchronously on the main
     /// thread when the mobile web flow is completed.
     open func getCredentialWith(_ uiDelegate: AuthUIDelegate?,
-                                completion: ((AuthCredential?, Error?) -> Void)? = nil) {
+                                completion: (@MainActor (AuthCredential?, Error?) -> Void)? = nil) {
       guard let urlTypes = auth.mainBundleUrlTypes,
             AuthWebUtils.isCallbackSchemeRegistered(forCustomURLScheme: callbackScheme,
                                                     urlTypes: urlTypes) else {
@@ -281,13 +282,12 @@ import Foundation
         let eventID = AuthWebUtils.randomString(withLength: 10)
         let sessionID = AuthWebUtils.randomString(withLength: 10)
 
-        let callbackOnMainThread: ((AuthCredential?, Error?) -> Void) = { credential, error in
-          if let completion {
-            DispatchQueue.main.async {
+        let callbackOnMainThread: (@MainActor (AuthCredential?, Error?) -> Void) =
+          { credential, error in
+            if let completion {
               completion(credential, error)
             }
           }
-        }
         Task {
           do {
             guard let headfulLiteURL = try await self.getHeadfulLiteUrl(eventID: eventID,
@@ -296,15 +296,15 @@ import Foundation
                 "FirebaseAuth Internal Error: Both error and headfulLiteURL return are nil"
               )
             }
-            let callbackMatcher: (URL?) -> Bool = { callbackURL in
+            let callbackMatcher: @Sendable (URL?) -> Bool = { callbackURL in
               AuthWebUtils.isExpectedCallbackURL(callbackURL,
                                                  eventID: eventID,
                                                  authType: "signInWithRedirect",
                                                  callbackScheme: self.callbackScheme)
             }
-            self.auth.authURLPresenter.present(headfulLiteURL,
-                                               uiDelegate: uiDelegate,
-                                               callbackMatcher: callbackMatcher) { callbackURL, error in
+            await self.auth.authURLPresenter.present(headfulLiteURL,
+                                                     uiDelegate: uiDelegate,
+                                                     callbackMatcher: callbackMatcher) { callbackURL, error in
               if let error {
                 callbackOnMainThread(nil, error)
                 return
@@ -328,7 +328,7 @@ import Foundation
               callbackOnMainThread(credential, nil)
             }
           } catch {
-            callbackOnMainThread(nil, error)
+            await callbackOnMainThread(nil, error)
           }
         }
       }
diff --git a/FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift b/FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift
index 61e5693f374..3d9a4ac9e34 100644
--- a/FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift
+++ b/FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift
@@ -19,7 +19,8 @@ import Foundation
 ///
 /// This class is available on iOS only.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIRPhoneAuthProvider) open class PhoneAuthProvider: NSObject {
+@objc(FIRPhoneAuthProvider) open class PhoneAuthProvider: NSObject,
+  @unchecked Sendable /* TODO: sendable */ {
   /// A string constant identifying the phone identity provider.
   @objc public static let id = "phone"
   private static let recaptchaVersion = "RECAPTCHA_ENTERPRISE"
@@ -56,7 +57,7 @@ import Foundation
     @objc(verifyPhoneNumber:UIDelegate:completion:)
     open func verifyPhoneNumber(_ phoneNumber: String,
                                 uiDelegate: AuthUIDelegate? = nil,
-                                completion: ((_: String?, _: Error?) -> Void)?) {
+                                completion: (@Sendable (_: String?, _: Error?) -> Void)?) {
       verifyPhoneNumber(phoneNumber,
                         uiDelegate: uiDelegate,
                         multiFactorSession: nil,
@@ -75,7 +76,7 @@ import Foundation
     open func verifyPhoneNumber(_ phoneNumber: String,
                                 uiDelegate: AuthUIDelegate? = nil,
                                 multiFactorSession: MultiFactorSession? = nil,
-                                completion: ((_: String?, _: Error?) -> Void)?) {
+                                completion: (@Sendable (String?, Error?) -> Void)?) {
       Task {
         do {
           let verificationID = try await verifyPhoneNumber(
@@ -135,7 +136,7 @@ import Foundation
     open func verifyPhoneNumber(with multiFactorInfo: PhoneMultiFactorInfo,
                                 uiDelegate: AuthUIDelegate? = nil,
                                 multiFactorSession: MultiFactorSession?,
-                                completion: ((_: String?, _: Error?) -> Void)?) {
+                                completion: (@Sendable (String?, Error?) -> Void)?) {
       Task {
         do {
           let verificationID = try await verifyPhoneNumber(
@@ -531,7 +532,7 @@ import Foundation
           "Internal error: reCAPTCHAURL returned neither a value nor an error. Report issue"
         )
       }
-      let callbackMatcher: (URL?) -> Bool = { callbackURL in
+      let callbackMatcher: @Sendable (URL?) -> Bool = { callbackURL in
         AuthWebUtils.isExpectedCallbackURL(
           callbackURL,
           eventID: eventID,
@@ -540,17 +541,19 @@ import Foundation
         )
       }
 
-      return try await withUnsafeThrowingContinuation { continuation in
-        self.auth.authURLPresenter.present(url,
-                                           uiDelegate: uiDelegate,
-                                           callbackMatcher: callbackMatcher) { callbackURL, error in
-          if let error {
-            continuation.resume(throwing: error)
-          } else {
-            do {
-              try continuation.resume(returning: self.reCAPTCHAToken(forURL: callbackURL))
-            } catch {
+      return try await withCheckedThrowingContinuation { continuation in
+        DispatchQueue.main.async {
+          self.auth.authURLPresenter.present(url,
+                                             uiDelegate: uiDelegate,
+                                             callbackMatcher: callbackMatcher) { callbackURL, error in
+            if let error {
               continuation.resume(throwing: error)
+            } else {
+              do {
+                try continuation.resume(returning: self.reCAPTCHAToken(forURL: callbackURL))
+              } catch {
+                continuation.resume(throwing: error)
+              }
             }
           }
         }
diff --git a/FirebaseAuth/Sources/Swift/Backend/AuthBackend.swift b/FirebaseAuth/Sources/Swift/Backend/AuthBackend.swift
index 7a0c39340ae..5232e5afa4c 100644
--- a/FirebaseAuth/Sources/Swift/Backend/AuthBackend.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/AuthBackend.swift
@@ -23,7 +23,7 @@ import Foundation
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
 protocol AuthBackendProtocol: Sendable {
-  func call<T: AuthRPCRequest>(with request: T) async throws -> T.Response
+  func call<T: AuthRPCRequest>(with request: sending T) async throws -> T.Response
 }
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
@@ -48,7 +48,7 @@ final class AuthBackend: AuthBackendProtocol {
   /// * See FIRAuthInternalErrorCodeRPCResponseDecodingError
   /// - Parameter request: The request.
   /// - Returns: The response.
-  func call<T: AuthRPCRequest>(with request: T) async throws -> T.Response {
+  func call<T: AuthRPCRequest>(with request: sending T) async throws -> T.Response {
     let response = try await callInternal(with: request)
     if let auth = request.requestConfiguration().auth,
        let mfaError = Self.generateMFAError(response: response, auth: auth) {
diff --git a/FirebaseAuth/Sources/Swift/Backend/AuthRPCRequest.swift b/FirebaseAuth/Sources/Swift/Backend/AuthRPCRequest.swift
index eeacfbea897..de253b8d56d 100644
--- a/FirebaseAuth/Sources/Swift/Backend/AuthRPCRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/AuthRPCRequest.swift
@@ -16,7 +16,7 @@ import Foundation
 
 /// The generic interface for an RPC request needed by AuthBackend.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-protocol AuthRPCRequest {
+protocol AuthRPCRequest: Sendable {
   associatedtype Response: AuthRPCResponse
 
   /// Gets the request's full URL.
diff --git a/FirebaseAuth/Sources/Swift/Backend/AuthRequestConfiguration.swift b/FirebaseAuth/Sources/Swift/Backend/AuthRequestConfiguration.swift
index 91f99c266f8..1a98aa66333 100644
--- a/FirebaseAuth/Sources/Swift/Backend/AuthRequestConfiguration.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/AuthRequestConfiguration.swift
@@ -19,7 +19,7 @@ import FirebaseCoreExtension
 
 /// Defines configurations to be added to a request to Firebase Auth's backend.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-final class AuthRequestConfiguration {
+final class AuthRequestConfiguration: @unchecked Sendable /* TODO: sendable */ {
   /// The Firebase Auth API key used in the request.
   let apiKey: String
 
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/CreateAuthURIRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/CreateAuthURIRequest.swift
index 987b7d28fe6..d9a1656b1a6 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/CreateAuthURIRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/CreateAuthURIRequest.swift
@@ -44,7 +44,8 @@ private let kTenantIDKey = "tenantId"
 /// Represents the parameters for the createAuthUri endpoint.
 /// See https://developers.google.com/identity/toolkit/web/reference/relyingparty/createAuthUri
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class CreateAuthURIRequest: IdentityToolkitRequest, AuthRPCRequest {
+class CreateAuthURIRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = CreateAuthURIResponse
 
   /// The email or federated ID of the user.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/DeleteAccountRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/DeleteAccountRequest.swift
index 543d5af3b8f..62343c6726b 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/DeleteAccountRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/DeleteAccountRequest.swift
@@ -26,7 +26,8 @@ private let kIDTokenKey = "idToken"
 private let kLocalIDKey = "localId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class DeleteAccountRequest: IdentityToolkitRequest, AuthRPCRequest {
+class DeleteAccountRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = DeleteAccountResponse
 
   /// The STS Access Token of the authenticated user.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/EmailLinkSignInRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/EmailLinkSignInRequest.swift
index 72a23517985..7ac70918036 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/EmailLinkSignInRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/EmailLinkSignInRequest.swift
@@ -33,7 +33,7 @@ private let kPostBodyKey = "postBody"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class EmailLinkSignInRequest: IdentityToolkitRequest, AuthRPCRequest {
+class EmailLinkSignInRequest: IdentityToolkitRequest, AuthRPCRequest, @unchecked Sendable {
   typealias Response = EmailLinkSignInResponse
 
   let email: String
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/GetAccountInfoRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/GetAccountInfoRequest.swift
index 5dba854b172..d824a617769 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/GetAccountInfoRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/GetAccountInfoRequest.swift
@@ -24,7 +24,8 @@ private let kIDTokenKey = "idToken"
 /// Represents the parameters for the getAccountInfo endpoint.
 /// See https://developers.google.com/identity/toolkit/web/reference/relyingparty/getAccountInfo
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class GetAccountInfoRequest: IdentityToolkitRequest, AuthRPCRequest {
+class GetAccountInfoRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = GetAccountInfoResponse
 
   /// The STS Access Token for the authenticated user.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/GetOOBConfirmationCodeRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/GetOOBConfirmationCodeRequest.swift
index 3dd4a07b59b..2241b9cb768 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/GetOOBConfirmationCodeRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/GetOOBConfirmationCodeRequest.swift
@@ -112,7 +112,8 @@ protocol SuppressWarning {
 extension ActionCodeSettings: SuppressWarning {}
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class GetOOBConfirmationCodeRequest: IdentityToolkitRequest, AuthRPCRequest {
+class GetOOBConfirmationCodeRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = GetOOBConfirmationCodeResponse
 
   /// The types of OOB Confirmation Code to request.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/GetProjectConfigRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/GetProjectConfigRequest.swift
index 1a1b011ff01..f4169a98d56 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/GetProjectConfigRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/GetProjectConfigRequest.swift
@@ -18,7 +18,8 @@ import Foundation
 private let kGetProjectConfigEndPoint = "getProjectConfig"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class GetProjectConfigRequest: IdentityToolkitRequest, AuthRPCRequest {
+class GetProjectConfigRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = GetProjectConfigResponse
 
   init(requestConfiguration: AuthRequestConfiguration) {
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/GetRecaptchaConfigRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/GetRecaptchaConfigRequest.swift
index 075d8273a15..3c9fc7e5af1 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/GetRecaptchaConfigRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/GetRecaptchaConfigRequest.swift
@@ -44,7 +44,8 @@ private let kVersionKey = "version"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class GetRecaptchaConfigRequest: IdentityToolkitRequest, AuthRPCRequest {
+class GetRecaptchaConfigRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = GetRecaptchaConfigResponse
 
   required init(requestConfiguration: AuthRequestConfiguration) {
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Enroll/FinalizeMFAEnrollmentRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Enroll/FinalizeMFAEnrollmentRequest.swift
index 0c1154d5a68..9430391ba58 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Enroll/FinalizeMFAEnrollmentRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Enroll/FinalizeMFAEnrollmentRequest.swift
@@ -20,7 +20,8 @@ private let kFinalizeMFAEnrollmentEndPoint = "accounts/mfaEnrollment:finalize"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class FinalizeMFAEnrollmentRequest: IdentityToolkitRequest, AuthRPCRequest {
+class FinalizeMFAEnrollmentRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = FinalizeMFAEnrollmentResponse
 
   let idToken: String?
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Enroll/StartMFAEnrollmentRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Enroll/StartMFAEnrollmentRequest.swift
index a36c7bb3bc6..e6a1844a96d 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Enroll/StartMFAEnrollmentRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Enroll/StartMFAEnrollmentRequest.swift
@@ -32,7 +32,11 @@ private let kRecaptchaVersion = "recaptchaVersion"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class StartMFAEnrollmentRequest: IdentityToolkitRequest, AuthRPCRequest {
+final class StartMFAEnrollmentRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
+  // This and the other request classes could be made into structs, which would make
+  // Sendable conformance easier.
+
   typealias Response = StartMFAEnrollmentResponse
 
   let idToken: String?
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/FinalizeMFASignInRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/FinalizeMFASignInRequest.swift
index 7e8b67eca96..a2808af9d30 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/FinalizeMFASignInRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/FinalizeMFASignInRequest.swift
@@ -20,7 +20,8 @@ private let kFinalizeMFASignInEndPoint = "accounts/mfaSignIn:finalize"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class FinalizeMFASignInRequest: IdentityToolkitRequest, AuthRPCRequest {
+class FinalizeMFASignInRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = FinalizeMFAEnrollmentResponse
 
   let mfaPendingCredential: String?
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/StartMFASignInRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/StartMFASignInRequest.swift
index ae28451fb9e..b460a69594b 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/StartMFASignInRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/StartMFASignInRequest.swift
@@ -21,7 +21,8 @@ private let kStartMFASignInEndPoint = "accounts/mfaSignIn:start"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class StartMFASignInRequest: IdentityToolkitRequest, AuthRPCRequest {
+class StartMFASignInRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = StartMFASignInResponse
 
   let MFAPendingCredential: String?
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Unenroll/WithdrawMFARequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Unenroll/WithdrawMFARequest.swift
index f2324db3ca2..c82f4972b3b 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Unenroll/WithdrawMFARequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Unenroll/WithdrawMFARequest.swift
@@ -20,7 +20,8 @@ private let kWithdrawMFAEndPoint = "accounts/mfaEnrollment:withdraw"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class WithdrawMFARequest: IdentityToolkitRequest, AuthRPCRequest {
+class WithdrawMFARequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = WithdrawMFAResponse
 
   let idToken: String?
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/ResetPasswordRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/ResetPasswordRequest.swift
index c0b2629683d..3f85e471a11 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/ResetPasswordRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/ResetPasswordRequest.swift
@@ -27,7 +27,8 @@ private let kCurrentPasswordKey = "newPassword"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class ResetPasswordRequest: IdentityToolkitRequest, AuthRPCRequest {
+class ResetPasswordRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = ResetPasswordResponse
 
   /// The oobCode sent in the request.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/RevokeTokenRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/RevokeTokenRequest.swift
index 94e6deeeabb..8bbcc3dd18c 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/RevokeTokenRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/RevokeTokenRequest.swift
@@ -33,7 +33,8 @@ private let kIDTokenKey = "idToken"
 ///
 /// See https: // developers.google.com/identity/toolkit/web/reference/relyingparty/verifyPassword
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class RevokeTokenRequest: IdentityToolkitRequest, AuthRPCRequest {
+class RevokeTokenRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = RevokeTokenResponse
 
   /// The provider that issued the token to revoke.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/SecureTokenRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/SecureTokenRequest.swift
index cd26caa83f5..a6449895f9f 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/SecureTokenRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/SecureTokenRequest.swift
@@ -60,7 +60,7 @@ private nonisolated(unsafe) var gAPIHost = "securetoken.googleapis.com"
 
 /// Represents the parameters for the token endpoint.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class SecureTokenRequest: AuthRPCRequest {
+class SecureTokenRequest: AuthRPCRequest, @unchecked Sendable /* TODO: sendable */ {
   typealias Response = SecureTokenResponse
 
   /// The type of grant requested.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/SendVerificationTokenRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/SendVerificationTokenRequest.swift
index 729cf1d200d..953769618e0 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/SendVerificationTokenRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/SendVerificationTokenRequest.swift
@@ -49,7 +49,8 @@ enum CodeIdentity: Equatable {
 }
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class SendVerificationCodeRequest: IdentityToolkitRequest, AuthRPCRequest {
+class SendVerificationCodeRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = SendVerificationCodeResponse
 
   /// The phone number to which the verification code should be sent.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/SetAccountInfoRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/SetAccountInfoRequest.swift
index 5e310d4a656..4ceedd5ca50 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/SetAccountInfoRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/SetAccountInfoRequest.swift
@@ -79,7 +79,8 @@ private let kTenantIDKey = "tenantId"
 /// Represents the parameters for the setAccountInfo endpoint.
 /// See https://developers.google.com/identity/toolkit/web/reference/relyingparty/setAccountInfo
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class SetAccountInfoRequest: IdentityToolkitRequest, AuthRPCRequest {
+class SetAccountInfoRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = SetAccountInfoResponse
 
   /// The STS Access Token of the authenticated user.
@@ -89,19 +90,19 @@ class SetAccountInfoRequest: IdentityToolkitRequest, AuthRPCRequest {
   var displayName: String?
 
   /// The local ID of the user.
-  var localID: String? = nil
+  var localID: String?
 
   /// The email of the user.
-  var email: String? = nil
+  var email: String?
 
   /// The photoURL of the user.
   var photoURL: URL?
 
   /// The new password of the user.
-  var password: String? = nil
+  var password: String?
 
   /// The associated identity providers of the user.
-  var providers: [String]? = nil
+  var providers: [String]?
 
   /// The out-of-band code of the change email request.
   var oobCode: String?
@@ -113,16 +114,16 @@ class SetAccountInfoRequest: IdentityToolkitRequest, AuthRPCRequest {
   var upgradeToFederatedLogin: Bool = false
 
   /// The captcha challenge.
-  var captchaChallenge: String? = nil
+  var captchaChallenge: String?
 
   /// Response to the captcha.
-  var captchaResponse: String? = nil
+  var captchaResponse: String?
 
   /// The list of user attributes to delete.
   ///
   /// Every element of the list must be one of the predefined constant starts with
   /// `SetAccountInfoUserAttribute`.
-  var deleteAttributes: [String]? = nil
+  var deleteAttributes: [String]?
 
   /// The list of identity providers to delete.
   var deleteProviders: [String]?
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/SignInWithGameCenterRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/SignInWithGameCenterRequest.swift
index 34f94f87489..ba7684fbaad 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/SignInWithGameCenterRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/SignInWithGameCenterRequest.swift
@@ -18,7 +18,7 @@ private let kSignInWithGameCenterEndPoint = "signInWithGameCenter"
 
 /// The request to sign in with Game Center account
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class SignInWithGameCenterRequest: IdentityToolkitRequest, AuthRPCRequest {
+class SignInWithGameCenterRequest: IdentityToolkitRequest, AuthRPCRequest, @unchecked Sendable {
   typealias Response = SignInWithGameCenterResponse
 
   /// The playerID to verify.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/SignUpNewUserRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/SignUpNewUserRequest.swift
index 68cfe6b5e38..8f2e7301803 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/SignUpNewUserRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/SignUpNewUserRequest.swift
@@ -45,7 +45,8 @@ private let kReturnSecureTokenKey = "returnSecureToken"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class SignUpNewUserRequest: IdentityToolkitRequest, AuthRPCRequest {
+class SignUpNewUserRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = SignUpNewUserResponse
 
   /// The email of the user.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyAssertionRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyAssertionRequest.swift
index 30c924ff403..3c86fa38463 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyAssertionRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyAssertionRequest.swift
@@ -80,7 +80,8 @@ private let kLastNameKey = "lastName"
 /// Represents the parameters for the verifyAssertion endpoint.
 /// See https://developers.google.com/identity/toolkit/web/reference/relyingparty/verifyAssertion
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class VerifyAssertionRequest: IdentityToolkitRequest, AuthRPCRequest {
+class VerifyAssertionRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = VerifyAssertionResponse
 
   /// The URI to which the IDP redirects the user back. It may contain federated login result
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyCustomTokenRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyCustomTokenRequest.swift
index b8a97278908..61d4a746185 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyCustomTokenRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyCustomTokenRequest.swift
@@ -27,7 +27,8 @@ private let kReturnSecureTokenKey = "returnSecureToken"
 private let kTenantIDKey = "tenantId"
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class VerifyCustomTokenRequest: IdentityToolkitRequest, AuthRPCRequest {
+class VerifyCustomTokenRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = VerifyCustomTokenResponse
 
   let token: String
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyPasswordRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyPasswordRequest.swift
index 4c1096d5fbe..45eeca9f888 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyPasswordRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyPasswordRequest.swift
@@ -47,7 +47,8 @@ private let kTenantIDKey = "tenantId"
 /// Represents the parameters for the verifyPassword endpoint.
 /// See https: // developers.google.com/identity/toolkit/web/reference/relyingparty/verifyPassword
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class VerifyPasswordRequest: IdentityToolkitRequest, AuthRPCRequest {
+class VerifyPasswordRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = VerifyPasswordResponse
 
   /// The email of the user.
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyPhoneNumberRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyPhoneNumberRequest.swift
index 07ddf167527..acbf03adc39 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyPhoneNumberRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/VerifyPhoneNumberRequest.swift
@@ -57,7 +57,8 @@ extension AuthOperationType {
 }
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class VerifyPhoneNumberRequest: IdentityToolkitRequest, AuthRPCRequest {
+class VerifyPhoneNumberRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = VerifyPhoneNumberResponse
 
   /// The verification ID obtained from the response of `sendVerificationCode`.
diff --git a/FirebaseAuth/Sources/Swift/Backend/VerifyClientRequest.swift b/FirebaseAuth/Sources/Swift/Backend/VerifyClientRequest.swift
index 5821fc55c7e..e909c65f26b 100644
--- a/FirebaseAuth/Sources/Swift/Backend/VerifyClientRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/VerifyClientRequest.swift
@@ -15,7 +15,8 @@
 import Foundation
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class VerifyClientRequest: IdentityToolkitRequest, AuthRPCRequest {
+class VerifyClientRequest: IdentityToolkitRequest, AuthRPCRequest,
+  @unchecked Sendable /* TODO: sendable */ {
   typealias Response = VerifyClientResponse
 
   /// The endpoint for the verifyClient request.
diff --git a/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactor.swift b/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactor.swift
index 17ec6b18731..250212d13af 100644
--- a/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactor.swift
+++ b/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactor.swift
@@ -33,7 +33,8 @@ import Foundation
     /// - Parameter completion: A block with the session identifier for a second factor enrollment
     /// operation.
     @objc(getSessionWithCompletion:)
-    open func getSessionWithCompletion(_ completion: ((MultiFactorSession?, Error?) -> Void)?) {
+    open func getSessionWithCompletion(_ completion: ((sending MultiFactorSession?, Error?)
+        -> Void)?) {
       let session = MultiFactorSession.session(for: user)
       if let completion {
         completion(session, nil)
@@ -65,7 +66,7 @@ import Foundation
     @objc(enrollWithAssertion:displayName:completion:)
     open func enroll(with assertion: MultiFactorAssertion,
                      displayName: String?,
-                     completion: ((Error?) -> Void)?) {
+                     completion: (@Sendable (Error?) -> Void)?) {
       // TODO: Refactor classes so this duplicated code isn't necessary for phone and totp.
 
       guard
@@ -185,7 +186,7 @@ import Foundation
     /// complete, or fails.
     @objc(unenrollWithInfo:completion:)
     open func unenroll(with factorInfo: MultiFactorInfo,
-                       completion: ((Error?) -> Void)?) {
+                       completion: (@Sendable (Error?) -> Void)?) {
       unenroll(withFactorUID: factorInfo.uid, completion: completion)
     }
 
@@ -202,7 +203,7 @@ import Foundation
     /// complete, or fails.
     @objc(unenrollWithFactorUID:completion:)
     open func unenroll(withFactorUID factorUID: String,
-                       completion: ((Error?) -> Void)?) {
+                       completion: (@Sendable (Error?) -> Void)?) {
       guard let user = user, let auth = user.auth else {
         fatalError("Internal Auth error: failed to get user unenrolling in MultiFactor")
       }
diff --git a/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactorResolver.swift b/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactorResolver.swift
index 223c1f9f5f5..2a3fe152943 100644
--- a/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactorResolver.swift
+++ b/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactorResolver.swift
@@ -22,7 +22,7 @@ import Foundation
   /// This class is available on iOS only.
   @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
   @objc(FIRMultiFactorResolver)
-  open class MultiFactorResolver: NSObject {
+  open class MultiFactorResolver: NSObject, @unchecked Sendable /* TODO: sendable */ {
     /// The opaque session identifier for the current sign-in flow.
     @objc public let session: MultiFactorSession
 
@@ -39,7 +39,7 @@ import Foundation
     /// - Parameter completion: The block invoked when the request is complete, or fails.
     @objc(resolveSignInWithAssertion:completion:)
     open func resolveSignIn(with assertion: MultiFactorAssertion,
-                            completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                            completion: (@Sendable (AuthDataResult?, Error?) -> Void)? = nil) {
       var finalizedMFARequestInfo: AuthProto?
       if let totpAssertion = assertion as? TOTPMultiFactorAssertion {
         switch totpAssertion.secretOrID {
diff --git a/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactorSession.swift b/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactorSession.swift
index c4bda3eba36..889207eb22c 100644
--- a/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactorSession.swift
+++ b/FirebaseAuth/Sources/Swift/MultiFactor/MultiFactorSession.swift
@@ -25,7 +25,8 @@ import Foundation
   ///
   /// This class is available on iOS only.
   @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-  @objc(FIRMultiFactorSession) open class MultiFactorSession: NSObject {
+  @objc(FIRMultiFactorSession) open class MultiFactorSession: NSObject,
+    @unchecked Sendable /* TODO: Sendable */ {
     /// The ID token for an enroll flow. This has to be retrieved after recent authentication.
     var idToken: String?
 
diff --git a/FirebaseAuth/Sources/Swift/MultiFactor/Phone/PhoneMultiFactorInfo.swift b/FirebaseAuth/Sources/Swift/MultiFactor/Phone/PhoneMultiFactorInfo.swift
index 58e5fc5fb8b..be209725797 100644
--- a/FirebaseAuth/Sources/Swift/MultiFactor/Phone/PhoneMultiFactorInfo.swift
+++ b/FirebaseAuth/Sources/Swift/MultiFactor/Phone/PhoneMultiFactorInfo.swift
@@ -21,7 +21,12 @@ import Foundation
   /// The identifier of this second factor is "phone".
   ///
   /// This class is available on iOS only.
-  @objc(FIRPhoneMultiFactorInfo) open class PhoneMultiFactorInfo: MultiFactorInfo {
+  @objc(FIRPhoneMultiFactorInfo) public final class PhoneMultiFactorInfo: MultiFactorInfo,
+    @unchecked Sendable {
+    // In order for this class to remain safely Sendable, all of its parameters must be immutable
+    // Sendable types. If you add a parameter here that is either mutable or not Sendable, please
+    // update the unchecked Sendable above.
+
     /// The string identifier for using phone as a second factor.
     @objc(FIRPhoneMultiFactorID) public static let PhoneMultiFactorID = "phone"
 
@@ -29,7 +34,7 @@ import Foundation
     @objc(FIRTOTPMultiFactorID) public static let TOTPMultiFactorID = "totp"
 
     /// This is the phone number associated with the current second factor.
-    @objc open var phoneNumber: String
+    @objc final let phoneNumber: String
 
     init(proto: AuthProtoMFAEnrollment) {
       guard let phoneInfo = proto.phoneInfo else {
diff --git a/FirebaseAuth/Sources/Swift/MultiFactor/TOTP/TOTPMultiFactorGenerator.swift b/FirebaseAuth/Sources/Swift/MultiFactor/TOTP/TOTPMultiFactorGenerator.swift
index bf3b07634ca..69e6d4ffb9b 100644
--- a/FirebaseAuth/Sources/Swift/MultiFactor/TOTP/TOTPMultiFactorGenerator.swift
+++ b/FirebaseAuth/Sources/Swift/MultiFactor/TOTP/TOTPMultiFactorGenerator.swift
@@ -30,7 +30,8 @@ import Foundation
     /// - Parameter completion: Completion block
     @objc(generateSecretWithMultiFactorSession:completion:)
     open class func generateSecret(with session: MultiFactorSession,
-                                   completion: @escaping (TOTPSecret?, Error?) -> Void) {
+                                   completion: @escaping @Sendable (sending TOTPSecret?, Error?)
+                                     -> Void) {
       guard let currentUser = session.currentUser, let auth = currentUser.auth else {
         let error = AuthErrorUtils.error(code: AuthErrorCode.internalError,
                                          userInfo: [NSLocalizedDescriptionKey:
diff --git a/FirebaseAuth/Sources/Swift/User/User.swift b/FirebaseAuth/Sources/Swift/User/User.swift
index 857dc5060a2..bfdd8e4f2af 100644
--- a/FirebaseAuth/Sources/Swift/User/User.swift
+++ b/FirebaseAuth/Sources/Swift/User/User.swift
@@ -27,7 +27,7 @@ extension User: NSSecureCoding {}
 ///
 /// This class is thread-safe.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIRUser) open class User: NSObject, UserInfo {
+@objc(FIRUser) open class User: NSObject, UserInfo, @unchecked Sendable /* TODO: unchecked */ {
   /// Indicates the user represents an anonymous user.
   @objc public internal(set) var isAnonymous: Bool
 
@@ -101,7 +101,7 @@ extension User: NSSecureCoding {}
     )
   #endif // !FIREBASE_CI
   @objc(updateEmail:completion:)
-  open func updateEmail(to email: String, completion: ((Error?) -> Void)? = nil) {
+  open func updateEmail(to email: String, completion: (@Sendable (Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       self.updateEmail(email: email, password: nil) { error in
         User.callInMainThreadWithError(callback: completion, error: error)
@@ -173,7 +173,7 @@ extension User: NSSecureCoding {}
   /// - Parameter completion: Optionally; the block invoked when the user profile change has
   /// finished.
   @objc(updatePassword:completion:)
-  open func updatePassword(to password: String, completion: ((Error?) -> Void)? = nil) {
+  open func updatePassword(to password: String, completion: (@Sendable (Error?) -> Void)? = nil) {
     guard password.count > 0 else {
       if let completion {
         completion(AuthErrorUtils.weakPasswordError(serverResponseReason: "Missing Password"))
@@ -234,7 +234,7 @@ extension User: NSSecureCoding {}
     /// finished.
     @objc(updatePhoneNumberCredential:completion:)
     open func updatePhoneNumber(_ credential: PhoneAuthCredential,
-                                completion: ((Error?) -> Void)? = nil) {
+                                completion: (@Sendable (Error?) -> Void)? = nil) {
       kAuthGlobalWorkQueue.async {
         self.internalUpdateOrLinkPhoneNumber(credential: credential,
                                              isLinkOperation: false) { error in
@@ -303,7 +303,7 @@ extension User: NSSecureCoding {}
   /// `updateEmail(to:)`.
   /// - Parameter completion: Optionally; the block invoked when the reload has finished. Invoked
   ///   asynchronously on the main thread in the future.
-  @objc open func reload(completion: ((Error?) -> Void)? = nil) {
+  @objc open func reload(completion: (@Sendable (Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       self.getAccountInfoRefreshingCache { user, error in
         User.callInMainThreadWithError(callback: completion, error: error)
@@ -361,7 +361,7 @@ extension User: NSSecureCoding {}
   /// finished. Invoked asynchronously on the main thread in the future.
   @objc(reauthenticateWithCredential:completion:)
   open func reauthenticate(with credential: AuthCredential,
-                           completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                           completion: (@Sendable (AuthDataResult?, Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       Task {
         do {
@@ -463,7 +463,7 @@ extension User: NSSecureCoding {}
     @objc(reauthenticateWithProvider:UIDelegate:completion:)
     open func reauthenticate(with provider: FederatedAuthProvider,
                              uiDelegate: AuthUIDelegate?,
-                             completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                             completion: (@Sendable (AuthDataResult?, Error?) -> Void)? = nil) {
       kAuthGlobalWorkQueue.async {
         Task {
           do {
@@ -507,7 +507,7 @@ extension User: NSSecureCoding {}
   /// - Parameter completion: Optionally; the block invoked when the token is available. Invoked
   ///    asynchronously on the main thread in the future.
   @objc(getIDTokenWithCompletion:)
-  open func getIDToken(completion: ((String?, Error?) -> Void)?) {
+  open func getIDToken(completion: (@Sendable (String?, Error?) -> Void)?) {
     // |getIDTokenForcingRefresh:completion:| is also a public API so there is no need to dispatch to
     // global work queue here.
     getIDTokenForcingRefresh(false, completion: completion)
@@ -523,7 +523,7 @@ extension User: NSSecureCoding {}
   ///    asynchronously on the main thread in the future.
   @objc(getIDTokenForcingRefresh:completion:)
   open func getIDTokenForcingRefresh(_ forceRefresh: Bool,
-                                     completion: ((String?, Error?) -> Void)?) {
+                                     completion: (@Sendable (String?, Error?) -> Void)?) {
     getIDTokenResult(forcingRefresh: forceRefresh) { tokenResult, error in
       if let completion {
         DispatchQueue.main.async {
@@ -563,7 +563,7 @@ extension User: NSSecureCoding {}
   /// - Parameter completion: Optionally; the block invoked when the token is available. Invoked
   ///    asynchronously on the main thread in the future.
   @objc(getIDTokenResultWithCompletion:)
-  open func getIDTokenResult(completion: ((AuthTokenResult?, Error?) -> Void)?) {
+  open func getIDTokenResult(completion: (@Sendable (AuthTokenResult?, Error?) -> Void)?) {
     getIDTokenResult(forcingRefresh: false) { tokenResult, error in
       if let completion {
         DispatchQueue.main.async {
@@ -584,7 +584,7 @@ extension User: NSSecureCoding {}
   /// asynchronously on the main thread in the future.
   @objc(getIDTokenResultForcingRefresh:completion:)
   open func getIDTokenResult(forcingRefresh: Bool,
-                             completion: ((AuthTokenResult?, Error?) -> Void)?) {
+                             completion: (@Sendable (AuthTokenResult?, Error?) -> Void)?) {
     kAuthGlobalWorkQueue.async {
       self.internalGetToken(forceRefresh: forcingRefresh, backend: self.backend) { token, error in
         var tokenResult: AuthTokenResult?
@@ -660,7 +660,7 @@ extension User: NSSecureCoding {}
   /// fails.
   @objc(linkWithCredential:completion:)
   open func link(with credential: AuthCredential,
-                 completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                 completion: (@Sendable (AuthDataResult?, Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       if self.providerDataRaw[credential.provider] != nil {
         User.callInMainThreadWithAuthDataResultAndError(
@@ -747,7 +747,7 @@ extension User: NSSecureCoding {}
     @objc(linkWithProvider:UIDelegate:completion:)
     open func link(with provider: FederatedAuthProvider,
                    uiDelegate: AuthUIDelegate?,
-                   completion: ((AuthDataResult?, Error?) -> Void)? = nil) {
+                   completion: (@Sendable (AuthDataResult?, Error?) -> Void)? = nil) {
       kAuthGlobalWorkQueue.async {
         Task {
           do {
@@ -801,7 +801,7 @@ extension User: NSSecureCoding {}
   /// - Parameter completion: Optionally; the block invoked when the unlinking is complete, or
   /// fails.
   @objc open func unlink(fromProvider provider: String,
-                         completion: ((User?, Error?) -> Void)? = nil) {
+                         completion: (@Sendable (User?, Error?) -> Void)? = nil) {
     Task {
       do {
         let user = try await unlink(fromProvider: provider)
@@ -847,7 +847,7 @@ extension User: NSSecureCoding {}
   /// - Parameter completion: Optionally; the block invoked when the request to send an email
   /// verification is complete, or fails. Invoked asynchronously on the main thread in the future.
   @objc(sendEmailVerificationWithCompletion:)
-  open func __sendEmailVerification(withCompletion completion: ((Error?) -> Void)?) {
+  open func __sendEmailVerification(withCompletion completion: (@Sendable (Error?) -> Void)?) {
     sendEmailVerification(completion: completion)
   }
 
@@ -867,7 +867,7 @@ extension User: NSSecureCoding {}
   /// verification is complete, or fails. Invoked asynchronously on the main thread in the future.
   @objc(sendEmailVerificationWithActionCodeSettings:completion:)
   open func sendEmailVerification(with actionCodeSettings: ActionCodeSettings? = nil,
-                                  completion: ((Error?) -> Void)? = nil) {
+                                  completion: (@Sendable (Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       self.internalGetToken(backend: self.backend) { accessToken, error in
         if let error {
@@ -932,7 +932,7 @@ extension User: NSSecureCoding {}
   /// `reauthenticate(with:)`.
   /// - Parameter completion: Optionally; the block invoked when the request to delete the account
   /// is complete, or fails. Invoked asynchronously on the main thread in the future.
-  @objc open func delete(completion: ((Error?) -> Void)? = nil) {
+  @objc open func delete(completion: (@Sendable (Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       self.internalGetToken(backend: self.backend) { accessToken, error in
         if let error {
@@ -985,7 +985,8 @@ extension User: NSSecureCoding {}
   /// - Parameter completion: Optionally; the block invoked when the request to send the
   /// verification email is complete, or fails.
   @objc(sendEmailVerificationBeforeUpdatingEmail:completion:)
-  open func __sendEmailVerificationBeforeUpdating(email: String, completion: ((Error?) -> Void)?) {
+  open func __sendEmailVerificationBeforeUpdating(email: String,
+                                                  completion: (@Sendable (Error?) -> Void)?) {
     sendEmailVerification(beforeUpdatingEmail: email, completion: completion)
   }
 
@@ -997,7 +998,7 @@ extension User: NSSecureCoding {}
   /// verification email is complete, or fails.
   @objc open func sendEmailVerification(beforeUpdatingEmail email: String,
                                         actionCodeSettings: ActionCodeSettings? = nil,
-                                        completion: ((Error?) -> Void)? = nil) {
+                                        completion: (@Sendable (Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       self.internalGetToken(backend: self.backend) { accessToken, error in
         if let error {
@@ -1157,7 +1158,7 @@ extension User: NSSecureCoding {}
 
   private func updateEmail(email: String?,
                            password: String?,
-                           callback: @escaping (Error?) -> Void) {
+                           callback: @escaping @Sendable (Error?) -> Void) {
     let hadEmailPasswordCredential = hasEmailPasswordCredential
     executeUserUpdateWithChanges(changeBlock: { user, request in
       if let email {
@@ -1233,9 +1234,10 @@ extension User: NSSecureCoding {}
   /// - Parameter changeBlock: A block responsible for mutating a template `SetAccountInfoRequest`
   /// - Parameter callback: A block to invoke when the change is complete. Invoked asynchronously on
   /// the auth global work queue in the future.
-  func executeUserUpdateWithChanges(changeBlock: @escaping (GetAccountInfoResponse.User,
-                                                            SetAccountInfoRequest) -> Void,
-                                    callback: @escaping (Error?) -> Void) {
+  func executeUserUpdateWithChanges(changeBlock: @escaping @Sendable (GetAccountInfoResponse.User,
+                                                                      SetAccountInfoRequest)
+      -> Void,
+    callback: @escaping @Sendable (Error?) -> Void) {
     Task {
       do {
         try await userProfileUpdate.executeUserUpdateWithChanges(user: self,
@@ -1254,8 +1256,8 @@ extension User: NSSecureCoding {}
   /// Gets the users' account data from the server, updating our local values.
   /// - Parameter callback: Invoked when the request to getAccountInfo has completed, or when an
   /// error has been detected. Invoked asynchronously on the auth global work queue in the future.
-  func getAccountInfoRefreshingCache(callback: @escaping (GetAccountInfoResponse.User?,
-                                                          Error?) -> Void) {
+  func getAccountInfoRefreshingCache(callback: @escaping @Sendable (GetAccountInfoResponse.User?,
+                                                                    Error?) -> Void) {
     Task {
       do {
         let responseUser = try await userProfileUpdate.getAccountInfoRefreshingCache(self)
@@ -1318,7 +1320,7 @@ extension User: NSSecureCoding {}
     /// finished.
     private func internalUpdateOrLinkPhoneNumber(credential: PhoneAuthCredential,
                                                  isLinkOperation: Bool,
-                                                 completion: @escaping (Error?) -> Void) {
+                                                 completion: @escaping @Sendable (Error?) -> Void) {
       internalGetToken(backend: backend) { accessToken, error in
         if let error {
           completion(error)
@@ -1380,7 +1382,7 @@ extension User: NSSecureCoding {}
   private func link(withEmail email: String,
                     password: String,
                     authResult: AuthDataResult,
-                    _ completion: ((AuthDataResult?, Error?) -> Void)?) {
+                    _ completion: (@Sendable (AuthDataResult?, Error?) -> Void)?) {
     internalGetToken(backend: backend) { accessToken, error in
       guard let requestConfiguration = self.auth?.requestConfiguration else {
         fatalError("Internal auth error: missing auth on User")
@@ -1428,7 +1430,7 @@ extension User: NSSecureCoding {}
   }
 
   private func link(withEmailCredential emailCredential: EmailAuthCredential,
-                    completion: ((AuthDataResult?, Error?) -> Void)?) {
+                    completion: (@Sendable (AuthDataResult?, Error?) -> Void)?) {
     if hasEmailPasswordCredential {
       User.callInMainThreadWithAuthDataResultAndError(
         callback: completion,
@@ -1489,7 +1491,7 @@ extension User: NSSecureCoding {}
 
   #if !os(watchOS)
     private func link(withGameCenterCredential gameCenterCredential: GameCenterAuthCredential,
-                      completion: ((AuthDataResult?, Error?) -> Void)?) {
+                      completion: (@Sendable (AuthDataResult?, Error?) -> Void)?) {
       internalGetToken(backend: backend) { accessToken, error in
         guard let requestConfiguration = self.auth?.requestConfiguration,
               let publicKeyURL = gameCenterCredential.publicKeyURL,
@@ -1537,7 +1539,7 @@ extension User: NSSecureCoding {}
 
   #if os(iOS)
     private func link(withPhoneCredential phoneCredential: PhoneAuthCredential,
-                      completion: ((AuthDataResult?, Error?) -> Void)?) {
+                      completion: (@Sendable (AuthDataResult?, Error?) -> Void)?) {
       internalUpdateOrLinkPhoneNumber(credential: phoneCredential,
                                       isLinkOperation: true) { error in
         if let error {
@@ -1591,7 +1593,7 @@ extension User: NSSecureCoding {}
   /// on the  global work thread in the future.
   func internalGetToken(forceRefresh: Bool = false,
                         backend: AuthBackend,
-                        callback: @escaping (String?, Error?) -> Void) {
+                        callback: @escaping @Sendable (String?, Error?) -> Void) {
     Task {
       do {
         let token = try await internalGetTokenAsync(forceRefresh: forceRefresh, backend: backend)
@@ -1636,7 +1638,7 @@ extension User: NSSecureCoding {}
   /// - Parameter callback: The callback to be called in main thread.
   /// - Parameter error: The error to pass to callback.
 
-  class func callInMainThreadWithError(callback: ((Error?) -> Void)?, error: Error?) {
+  class func callInMainThreadWithError(callback: (@MainActor (Error?) -> Void)?, error: Error?) {
     if let callback {
       DispatchQueue.main.async {
         callback(error)
@@ -1648,9 +1650,10 @@ extension User: NSSecureCoding {}
   /// - Parameter callback: The callback to be called in main thread.
   /// - Parameter user: The user to pass to callback if there is no error.
   /// - Parameter error: The error to pass to callback.
-  private class func callInMainThreadWithUserAndError(callback: ((User?, Error?) -> Void)?,
-                                                      user: User,
-                                                      error: Error?) {
+  private class func callInMainThreadWithUserAndError(callback: (@MainActor (User?, Error?)
+                                                        -> Void)?,
+  user: User,
+  error: Error?) {
     if let callback {
       DispatchQueue.main.async {
         callback((error != nil) ? nil : user, error)
@@ -1661,7 +1664,7 @@ extension User: NSSecureCoding {}
   /// Calls a callback in main thread with user and error.
   /// - Parameter callback: The callback to be called in main thread.
   private class func callInMainThreadWithAuthDataResultAndError(callback: (
-    (AuthDataResult?, Error?) -> Void
+    @MainActor (AuthDataResult?, Error?) -> Void
   )?,
   result: AuthDataResult? = nil,
   error: Error? = nil) {
diff --git a/FirebaseAuth/Sources/Swift/User/UserProfileChangeRequest.swift b/FirebaseAuth/Sources/Swift/User/UserProfileChangeRequest.swift
index 493f3d80f92..3270d124cb1 100644
--- a/FirebaseAuth/Sources/Swift/User/UserProfileChangeRequest.swift
+++ b/FirebaseAuth/Sources/Swift/User/UserProfileChangeRequest.swift
@@ -19,7 +19,8 @@ import Foundation
 /// Properties are marked as being part of a profile update when they are set. Setting a
 /// property value to nil is not the same as leaving the property unassigned.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIRUserProfileChangeRequest) open class UserProfileChangeRequest: NSObject {
+@objc(FIRUserProfileChangeRequest) open class UserProfileChangeRequest: NSObject,
+  @unchecked Sendable /* TODO: sendable */ {
   /// The name of the user.
   @objc open var displayName: String? {
     get { return _displayName }
@@ -59,7 +60,7 @@ import Foundation
   /// This method should only be called once.Once called, property values should not be changed.
   /// - Parameter completion: Optionally; the block invoked when the user profile change has been
   /// applied.
-  @objc open func commitChanges(completion: ((Error?) -> Void)? = nil) {
+  @objc open func commitChanges(completion: (@Sendable (Error?) -> Void)? = nil) {
     kAuthGlobalWorkQueue.async {
       if self.consumed {
         fatalError("Internal Auth Error: commitChanges should only be called once.")
diff --git a/FirebaseAuth/Sources/Swift/Utilities/AuthDefaultUIDelegate.swift b/FirebaseAuth/Sources/Swift/Utilities/AuthDefaultUIDelegate.swift
index 4106977a5e1..767e5d22cc1 100644
--- a/FirebaseAuth/Sources/Swift/Utilities/AuthDefaultUIDelegate.swift
+++ b/FirebaseAuth/Sources/Swift/Utilities/AuthDefaultUIDelegate.swift
@@ -26,7 +26,7 @@
   ///
   /// This class should be used in the case that a UIDelegate was expected and necessary to
   /// continue a given flow, but none was provided.
-  final class AuthDefaultUIDelegate: NSObject, AuthUIDelegate {
+  final class AuthDefaultUIDelegate: NSObject, AuthUIDelegate, Sendable {
     /// Returns a default AuthUIDelegate object.
     /// - Returns: The default AuthUIDelegate object.
     @MainActor static func defaultUIDelegate() -> AuthUIDelegate? {
@@ -76,12 +76,12 @@
       self.viewController = viewController
     }
 
-    func present(_ viewControllerToPresent: UIViewController, animated flag: Bool,
-                 completion: (() -> Void)? = nil) {
+    @MainActor func present(_ viewControllerToPresent: UIViewController, animated flag: Bool,
+                            completion: (() -> Void)? = nil) {
       viewController?.present(viewControllerToPresent, animated: flag, completion: completion)
     }
 
-    func dismiss(animated flag: Bool, completion: (() -> Void)? = nil) {
+    @MainActor func dismiss(animated flag: Bool, completion: (() -> Void)? = nil) {
       viewController?.dismiss(animated: flag, completion: completion)
     }
 
diff --git a/FirebaseAuth/Sources/Swift/Utilities/AuthRecaptchaVerifier.swift b/FirebaseAuth/Sources/Swift/Utilities/AuthRecaptchaVerifier.swift
index b0c61e4dfb4..7a89cbd5e2a 100644
--- a/FirebaseAuth/Sources/Swift/Utilities/AuthRecaptchaVerifier.swift
+++ b/FirebaseAuth/Sources/Swift/Utilities/AuthRecaptchaVerifier.swift
@@ -72,7 +72,7 @@
     private(set) var agentConfig: AuthRecaptchaConfig?
     private(set) var tenantConfigs: [String: AuthRecaptchaConfig] = [:]
     private(set) var recaptchaClient: RCARecaptchaClientProtocol?
-    private static var _shared = AuthRecaptchaVerifier()
+    private nonisolated(unsafe) static var _shared = AuthRecaptchaVerifier()
     private let kRecaptchaVersion = "RECAPTCHA_ENTERPRISE"
     init() {}
 
@@ -154,7 +154,7 @@
       #endif // !(COCOAPODS || SWIFT_PACKAGE)
     }
 
-    private static var recaptchaClient: (any RCARecaptchaClientProtocol)?
+    private nonisolated(unsafe) static var recaptchaClient: (any RCARecaptchaClientProtocol)?
 
     #if COCOAPODS || SWIFT_PACKAGE // No recaptcha on internal build system.
       private func recaptchaToken(siteKey: String,
diff --git a/FirebaseAuth/Sources/Swift/Utilities/AuthUIDelegate.swift b/FirebaseAuth/Sources/Swift/Utilities/AuthUIDelegate.swift
index 94025574ab4..444978f3de7 100644
--- a/FirebaseAuth/Sources/Swift/Utilities/AuthUIDelegate.swift
+++ b/FirebaseAuth/Sources/Swift/Utilities/AuthUIDelegate.swift
@@ -20,14 +20,14 @@
   /// A protocol to handle user interface interactions for Firebase Auth.
   ///
   /// This protocol is available on iOS, macOS Catalyst, and tvOS only.
-  @objc(FIRAuthUIDelegate) public protocol AuthUIDelegate: NSObjectProtocol {
+  @objc(FIRAuthUIDelegate) public protocol AuthUIDelegate: NSObjectProtocol, Sendable {
     /// If implemented, this method will be invoked when Firebase Auth needs to display a view
     /// controller.
     /// - Parameter viewControllerToPresent: The view controller to be presented.
     /// - Parameter flag: Decides whether the view controller presentation should be animated.
     /// - Parameter completion: The block to execute after the presentation finishes.
     /// This block has no return value and takes no parameters.
-    @objc(presentViewController:animated:completion:)
+    @MainActor @objc(presentViewController:animated:completion:)
     func present(_ viewControllerToPresent: UIViewController,
                  animated flag: Bool,
                  completion: (() -> Void)?)
@@ -37,19 +37,19 @@
     /// - Parameter flag: Decides whether removing the view controller should be animated or not.
     /// - Parameter completion: The block to execute after the presentation finishes.
     /// This block has no return value and takes no parameters.
-    @objc(dismissViewControllerAnimated:completion:)
+    @MainActor @objc(dismissViewControllerAnimated:completion:)
     func dismiss(animated flag: Bool, completion: (() -> Void)?)
   }
 
   // Extension to support default argument variations.
   extension AuthUIDelegate {
-    func present(_ viewControllerToPresent: UIViewController,
-                 animated flag: Bool,
-                 completion: (() -> Void)? = nil) {
+    @MainActor func present(_ viewControllerToPresent: UIViewController,
+                            animated flag: Bool,
+                            completion: (() -> Void)? = nil) {
       return present(viewControllerToPresent, animated: flag, completion: nil)
     }
 
-    func dismiss(animated flag: Bool, completion: (() -> Void)? = nil) {
+    @MainActor func dismiss(animated flag: Bool, completion: (() -> Void)? = nil) {
       return dismiss(animated: flag, completion: nil)
     }
   }
diff --git a/FirebaseAuth/Sources/Swift/Utilities/AuthURLPresenter.swift b/FirebaseAuth/Sources/Swift/Utilities/AuthURLPresenter.swift
index c59ced1cfe1..5745350dabc 100644
--- a/FirebaseAuth/Sources/Swift/Utilities/AuthURLPresenter.swift
+++ b/FirebaseAuth/Sources/Swift/Utilities/AuthURLPresenter.swift
@@ -22,17 +22,18 @@
   /// A Class responsible for presenting URL via SFSafariViewController or WKWebView.
   @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
   class AuthURLPresenter: NSObject,
-    SFSafariViewControllerDelegate, AuthWebViewControllerDelegate {
+    @preconcurrency SFSafariViewControllerDelegate,
+    AuthWebViewControllerDelegate {
     /// Presents an URL to interact with user.
     /// - Parameter url: The URL to present.
     /// - Parameter uiDelegate: The UI delegate to present view controller.
     /// - Parameter completion: A block to be called either synchronously if the presentation fails
     /// to start, or asynchronously in future on an unspecified thread once the presentation
     /// finishes.
-    func present(_ url: URL,
-                 uiDelegate: AuthUIDelegate?,
-                 callbackMatcher: @escaping (URL?) -> Bool,
-                 completion: @escaping (URL?, Error?) -> Void) {
+    @MainActor func present(_ url: URL,
+                            uiDelegate: AuthUIDelegate?,
+                            callbackMatcher: @Sendable @escaping (URL?) -> Bool,
+                            completion: @MainActor @escaping (URL?, Error?) -> Void) {
       if isPresenting {
         // Unable to start a new presentation on top of another.
         // Invoke the new completion closure and leave the old one as-is
@@ -74,7 +75,7 @@
     /// Determines if a URL was produced by the currently presented URL.
     /// - Parameter url: The URL to handle.
     /// - Returns: Whether the URL could be handled or not.
-    func canHandle(url: URL) -> Bool {
+    @MainActor func canHandle(url: URL) -> Bool {
       if isPresenting,
          let callbackMatcher = callbackMatcher,
          callbackMatcher(url) {
@@ -86,45 +87,38 @@
 
     // MARK: SFSafariViewControllerDelegate
 
-    func safariViewControllerDidFinish(_ controller: SFSafariViewController) {
-      kAuthGlobalWorkQueue.async {
-        if controller == self.safariViewController {
-          self.safariViewController = nil
-          // TODO: Ensure that the SFSafariViewController is actually removed from the screen
-          // before invoking finishPresentation
-          self.finishPresentation(withURL: nil,
-                                  error: AuthErrorUtils.webContextCancelledError(message: nil))
-        }
+    @MainActor func safariViewControllerDidFinish(_ controller: SFSafariViewController) {
+      if controller == safariViewController {
+        safariViewController = nil
+        // TODO: Ensure that the SFSafariViewController is actually removed from the screen
+        // before invoking finishPresentation
+        finishPresentation(withURL: nil,
+                           error: AuthErrorUtils.webContextCancelledError(message: nil))
       }
     }
 
     // MARK: AuthWebViewControllerDelegate
 
-    func webViewControllerDidCancel(_ controller: AuthWebViewController) {
-      kAuthGlobalWorkQueue.async {
-        if self.webViewController == controller {
-          self.finishPresentation(withURL: nil,
-                                  error: AuthErrorUtils.webContextCancelledError(message: nil))
-        }
+    @MainActor func webViewControllerDidCancel(_ controller: AuthWebViewController) {
+      if webViewController == controller {
+        finishPresentation(withURL: nil,
+                           error: AuthErrorUtils.webContextCancelledError(message: nil))
       }
     }
 
-    func webViewController(_ controller: AuthWebViewController, canHandle url: URL) -> Bool {
+    @MainActor func webViewController(_ controller: AuthWebViewController,
+                                      canHandle url: URL) -> Bool {
       var result = false
-      kAuthGlobalWorkQueue.sync {
-        if self.webViewController == controller {
-          result = self.canHandle(url: url)
-        }
+      if webViewController == controller {
+        result = canHandle(url: url)
       }
       return result
     }
 
-    func webViewController(_ controller: AuthWebViewController,
-                           didFailWithError error: Error) {
-      kAuthGlobalWorkQueue.async {
-        if self.webViewController == controller {
-          self.finishPresentation(withURL: nil, error: error)
-        }
+    @MainActor func webViewController(_ controller: AuthWebViewController,
+                                      didFailWithError error: Error) {
+      if webViewController == controller {
+        finishPresentation(withURL: nil, error: error)
       }
     }
 
@@ -159,7 +153,7 @@
 
     // MARK: Private methods
 
-    private func finishPresentation(withURL url: URL?, error: Error?) {
+    @MainActor private func finishPresentation(withURL url: URL?, error: Error?) {
       callbackMatcher = nil
       let uiDelegate = self.uiDelegate
       self.uiDelegate = nil
@@ -170,12 +164,10 @@
       let webViewController = self.webViewController
       self.webViewController = nil
       if safariViewController != nil || webViewController != nil {
-        DispatchQueue.main.async {
-          uiDelegate?.dismiss(animated: true) {
-            self.isPresenting = false
-            if let completion {
-              completion(url, error)
-            }
+        uiDelegate?.dismiss(animated: true) {
+          self.isPresenting = false
+          if let completion {
+            completion(url, error)
           }
         }
       } else {
diff --git a/FirebaseAuth/Sources/Swift/Utilities/AuthWebViewController.swift b/FirebaseAuth/Sources/Swift/Utilities/AuthWebViewController.swift
index ee954b0029f..7fc50c82226 100644
--- a/FirebaseAuth/Sources/Swift/Utilities/AuthWebViewController.swift
+++ b/FirebaseAuth/Sources/Swift/Utilities/AuthWebViewController.swift
@@ -23,17 +23,19 @@
   protocol AuthWebViewControllerDelegate: AnyObject {
     /// Notifies the delegate that the web view controller is being cancelled by the user.
     /// - Parameter webViewController: The web view controller in question.
-    func webViewControllerDidCancel(_ controller: AuthWebViewController)
+    @MainActor func webViewControllerDidCancel(_ controller: AuthWebViewController)
 
     /// Determines if a URL should be handled by the delegate.
     /// - Parameter url: The URL to handle.
     /// - Returns: Whether the URL could be handled or not.
-    func webViewController(_ controller: AuthWebViewController, canHandle url: URL) -> Bool
+    @MainActor func webViewController(_ controller: AuthWebViewController, canHandle url: URL)
+      -> Bool
 
     /// Notifies the delegate that the web view controller failed to load a page.
     /// - Parameter webViewController: The web view controller in question.
     /// - Parameter error: The error that has occurred.
-    func webViewController(_ controller: AuthWebViewController, didFailWithError error: Error)
+    @MainActor func webViewController(_ controller: AuthWebViewController,
+                                      didFailWithError error: Error)
 
     /// Presents an URL to interact with user.
     /// - Parameter url: The URL to present.
@@ -41,10 +43,10 @@
     /// - Parameter completion: A block to be called either synchronously if the presentation fails
     /// to start, or asynchronously in future on an unspecified thread once the presentation
     /// finishes.
-    func present(_ url: URL,
-                 uiDelegate: AuthUIDelegate?,
-                 callbackMatcher: @escaping (URL?) -> Bool,
-                 completion: @escaping (URL?, Error?) -> Void)
+    @MainActor func present(_ url: URL,
+                            uiDelegate: AuthUIDelegate?,
+                            callbackMatcher: @Sendable @escaping (URL?) -> Bool,
+                            completion: @MainActor @escaping (URL?, Error?) -> Void)
   }
 
   @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
diff --git a/FirebaseAuthTestingSupport.podspec b/FirebaseAuthTestingSupport.podspec
index 3cf9b5128b5..34e00912a14 100644
--- a/FirebaseAuthTestingSupport.podspec
+++ b/FirebaseAuthTestingSupport.podspec
@@ -22,7 +22,7 @@ Pod::Spec.new do |s|
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseCombineSwift.podspec b/FirebaseCombineSwift.podspec
index dc198ef47e0..e3897512ae6 100644
--- a/FirebaseCombineSwift.podspec
+++ b/FirebaseCombineSwift.podspec
@@ -19,7 +19,7 @@ for internal testing only. It should not be published.
 
   s.social_media_url = 'https://twitter.com/Firebase'
 
-  s.swift_version       = '5.9'
+  s.swift_version       = '6.0'
 
   ios_deployment_target = '13.0'
   osx_deployment_target = '10.15'
diff --git a/FirebaseCore.podspec b/FirebaseCore.podspec
index e0a86f26df0..cf730563819 100644
--- a/FirebaseCore.podspec
+++ b/FirebaseCore.podspec
@@ -40,7 +40,7 @@ Firebase Core includes FIRApp and FIROptions which provide central configuration
     "#{s.module_name}_Privacy" => 'FirebaseCore/Sources/Resources/PrivacyInfo.xcprivacy'
   }
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.public_header_files = 'FirebaseCore/Sources/Public/FirebaseCore/*.h'
 
diff --git a/FirebaseCore/Internal/Sources/FIRAllocatedUnfairLock.swift b/FirebaseCore/Internal/Sources/FIRAllocatedUnfairLock.swift
new file mode 100644
index 00000000000..ae52faefce6
--- /dev/null
+++ b/FirebaseCore/Internal/Sources/FIRAllocatedUnfairLock.swift
@@ -0,0 +1,66 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import Foundation
+import os.lock
+
+/// A reference wrapper around `os_unfair_lock`. Replace this class with
+/// `OSAllocatedUnfairLock` once we support only iOS 16+. For an explanation
+/// on why this is necessary, see the docs:
+/// https://developer.apple.com/documentation/os/osallocatedunfairlock
+public final class FIRAllocatedUnfairLock<State>: @unchecked Sendable {
+  private var lockPointer: UnsafeMutablePointer<os_unfair_lock>
+  private var state: State
+
+  public init(initialState: sending State) {
+    lockPointer = UnsafeMutablePointer<os_unfair_lock>
+      .allocate(capacity: 1)
+    lockPointer.initialize(to: os_unfair_lock())
+    state = initialState
+  }
+
+  public convenience init() where State == Void {
+    self.init(initialState: ())
+  }
+
+  public func lock() {
+    os_unfair_lock_lock(lockPointer)
+  }
+
+  public func unlock() {
+    os_unfair_lock_unlock(lockPointer)
+  }
+
+  @discardableResult
+  public func withLock<R>(_ body: (inout State) throws -> R) rethrows -> R {
+    let value: R
+    lock()
+    defer { unlock() }
+    value = try body(&state)
+    return value
+  }
+
+  @discardableResult
+  public func withLock<R>(_ body: () throws -> R) rethrows -> R {
+    let value: R
+    lock()
+    defer { unlock() }
+    value = try body()
+    return value
+  }
+
+  deinit {
+    lockPointer.deallocate()
+  }
+}
diff --git a/FirebaseCoreExtension.podspec b/FirebaseCoreExtension.podspec
index 480568da10c..641e73fec87 100644
--- a/FirebaseCoreExtension.podspec
+++ b/FirebaseCoreExtension.podspec
@@ -20,7 +20,7 @@ Pod::Spec.new do |s|
     }
     s.social_media_url = 'https://twitter.com/Firebase'
 
-    s.swift_version = '5.9'
+    s.swift_version = '6.0'
 
     s.ios.deployment_target = '12.0'
     s.osx.deployment_target = '10.15'
diff --git a/FirebaseCoreInternal.podspec b/FirebaseCoreInternal.podspec
index 18c528b5e39..6382dfbf32c 100644
--- a/FirebaseCoreInternal.podspec
+++ b/FirebaseCoreInternal.podspec
@@ -36,7 +36,7 @@ Pod::Spec.new do |s|
     "#{s.module_name}_Privacy" => 'FirebaseCore/Internal/Sources/Resources/PrivacyInfo.xcprivacy'
   }
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.dependency 'GoogleUtilities/NSData+zlib', '~> 8.1'
 
diff --git a/FirebaseCrashlytics.podspec b/FirebaseCrashlytics.podspec
index 9e1c2cbcc70..8ebb67356df 100644
--- a/FirebaseCrashlytics.podspec
+++ b/FirebaseCrashlytics.podspec
@@ -16,7 +16,7 @@ Pod::Spec.new do |s|
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseDatabase.podspec b/FirebaseDatabase.podspec
index 3a2a9b899f2..df134fad8d9 100644
--- a/FirebaseDatabase.podspec
+++ b/FirebaseDatabase.podspec
@@ -22,7 +22,7 @@ Simplify your iOS development, grow your user base, and monetize more effectivel
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseDynamicLinks.podspec b/FirebaseDynamicLinks.podspec
index bd7db3429db..cf9a691ad5a 100644
--- a/FirebaseDynamicLinks.podspec
+++ b/FirebaseDynamicLinks.podspec
@@ -18,7 +18,7 @@ Firebase Dynamic Links are deep links that enhance user experience and increase
   s.social_media_url = 'https://twitter.com/Firebase'
   s.ios.deployment_target = '13.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   # See https://firebase.google.com/support/dynamic-links-faq
   s.deprecated = true
diff --git a/FirebaseFirestore.podspec b/FirebaseFirestore.podspec
index a582e1c1aaa..83edebfc7ce 100644
--- a/FirebaseFirestore.podspec
+++ b/FirebaseFirestore.podspec
@@ -17,7 +17,7 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
   s.osx.deployment_target = '10.15'
   s.tvos.deployment_target = '13.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.weak_framework = 'FirebaseFirestoreInternal'
 
diff --git a/FirebaseFirestoreInternal.podspec b/FirebaseFirestoreInternal.podspec
index 8567f74fe7d..ec3b59e6796 100644
--- a/FirebaseFirestoreInternal.podspec
+++ b/FirebaseFirestoreInternal.podspec
@@ -20,7 +20,7 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
   s.osx.deployment_target = '10.15'
   s.tvos.deployment_target = '13.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.cocoapods_version = '>= 1.12.0'
   s.prefix_header_file = false
diff --git a/FirebaseFirestoreTestingSupport.podspec b/FirebaseFirestoreTestingSupport.podspec
index a28ab450c3f..23764501713 100644
--- a/FirebaseFirestoreTestingSupport.podspec
+++ b/FirebaseFirestoreTestingSupport.podspec
@@ -22,7 +22,7 @@ Pod::Spec.new do |s|
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseFunctions.podspec b/FirebaseFunctions.podspec
index 5bd8f2c5990..3f547ac2991 100644
--- a/FirebaseFunctions.podspec
+++ b/FirebaseFunctions.podspec
@@ -16,7 +16,7 @@ Cloud Functions for Firebase.
     :tag => 'CocoaPods-' + s.version.to_s
   }
 
-  s.swift_version    = '5.9'
+  s.swift_version    = '6.0'
 
   ios_deployment_target = '13.0'
   osx_deployment_target = '10.15'
diff --git a/FirebaseInAppMessaging.podspec b/FirebaseInAppMessaging.podspec
index 52b9b0b79cd..0a491c2794e 100644
--- a/FirebaseInAppMessaging.podspec
+++ b/FirebaseInAppMessaging.podspec
@@ -20,7 +20,7 @@ See more product details at https://firebase.google.com/products/in-app-messagin
   s.ios.deployment_target = '13.0'
   s.tvos.deployment_target = '13.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.cocoapods_version = '>= 1.12.0'
   s.prefix_header_file = false
diff --git a/FirebaseInstallations.podspec b/FirebaseInstallations.podspec
index 328da56178e..07e8b67711a 100644
--- a/FirebaseInstallations.podspec
+++ b/FirebaseInstallations.podspec
@@ -22,7 +22,7 @@ Pod::Spec.new do |s|
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseMLModelDownloader.podspec b/FirebaseMLModelDownloader.podspec
index 38b3563d84b..490b3f741ab 100644
--- a/FirebaseMLModelDownloader.podspec
+++ b/FirebaseMLModelDownloader.podspec
@@ -16,7 +16,7 @@ Pod::Spec.new do |s|
     :tag => 'CocoaPods-' + s.version.to_s
   }
   s.social_media_url = 'https://twitter.com/Firebase'
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   ios_deployment_target = '13.0'
   osx_deployment_target = '10.15'
diff --git a/FirebaseMessaging.podspec b/FirebaseMessaging.podspec
index 79eabe7f453..22d900576e7 100644
--- a/FirebaseMessaging.podspec
+++ b/FirebaseMessaging.podspec
@@ -25,7 +25,7 @@ device, and it is completely free.
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseMessaging/Sources/FIRMessagingExtensionHelper.m b/FirebaseMessaging/Sources/FIRMessagingExtensionHelper.m
index 7c987279e28..bacc2a3684a 100644
--- a/FirebaseMessaging/Sources/FIRMessagingExtensionHelper.m
+++ b/FirebaseMessaging/Sources/FIRMessagingExtensionHelper.m
@@ -97,24 +97,18 @@ - (NSData *)transportBytes {
 
 @end
 
-@interface FIRMessagingExtensionHelper ()
-@property(nonatomic, strong) void (^contentHandler)(UNNotificationContent *contentToDeliver);
-@property(nonatomic, strong) UNMutableNotificationContent *bestAttemptContent;
-
-@end
-
 @implementation FIRMessagingExtensionHelper
 
 - (void)populateNotificationContent:(UNMutableNotificationContent *)content
                  withContentHandler:(void (^)(UNNotificationContent *_Nonnull))contentHandler {
-  self.contentHandler = [contentHandler copy];
-  self.bestAttemptContent = content;
+  __block void (^handler)(UNNotificationContent *_Nonnull) = [contentHandler copy];
+  __block UNMutableNotificationContent *bestAttemptContent = [content mutableCopy];
 
   // The `userInfo` property isn't available on newer versions of tvOS.
 #if !TARGET_OS_TV
   NSObject *currentImageURL = content.userInfo[kPayloadOptionsName][kPayloadOptionsImageURLName];
   if (!currentImageURL || currentImageURL == [NSNull null]) {
-    [self deliverNotification];
+    [self deliverNotificationWithContent:bestAttemptContent handler:handler];
     return;
   }
   NSURL *attachmentURL = [NSURL URLWithString:(NSString *)currentImageURL];
@@ -122,14 +116,14 @@ - (void)populateNotificationContent:(UNMutableNotificationContent *)content
     [self loadAttachmentForURL:attachmentURL
              completionHandler:^(UNNotificationAttachment *attachment) {
                if (attachment != nil) {
-                 self.bestAttemptContent.attachments = @[ attachment ];
+                 bestAttemptContent.attachments = @[ attachment ];
                }
-               [self deliverNotification];
+               [self deliverNotificationWithContent:bestAttemptContent handler:handler];
              }];
   } else {
     FIRMessagingLoggerError(kFIRMessagingServiceExtensionImageInvalidURL,
                             @"The Image URL provided is invalid %@.", currentImageURL);
-    [self deliverNotification];
+    [self deliverNotificationWithContent:bestAttemptContent handler:handler];
   }
 #else   // !TARGET_OS_TV
   [self deliverNotification];
@@ -150,14 +144,15 @@ - (NSString *)fileExtensionForResponse:(NSURLResponse *)response {
 }
 
 - (void)loadAttachmentForURL:(NSURL *)attachmentURL
-           completionHandler:(void (^)(UNNotificationAttachment *))completionHandler {
-  __block UNNotificationAttachment *attachment = nil;
-
+           completionHandler:
+               (void (^NS_SWIFT_SENDABLE)(UNNotificationAttachment *))completionHandler
+    NS_SWIFT_SENDABLE {
   NSURLSession *session = [NSURLSession
       sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
   [[session
       downloadTaskWithURL:attachmentURL
         completionHandler:^(NSURL *temporaryFileLocation, NSURLResponse *response, NSError *error) {
+          UNNotificationAttachment *attachment = nil;
           if (error != nil) {
             FIRMessagingLoggerError(kFIRMessagingServiceExtensionImageNotDownloaded,
                                     @"Failed to download image given URL %@, error: %@\n",
@@ -196,9 +191,10 @@ - (void)loadAttachmentForURL:(NSURL *)attachmentURL
 }
 #endif  // !TARGET_OS_TV
 
-- (void)deliverNotification {
-  if (self.contentHandler) {
-    self.contentHandler(self.bestAttemptContent);
+- (void)deliverNotificationWithContent:(UNNotificationContent *)content
+                               handler:(void (^_Nullable)(UNNotificationContent *_Nonnull))handler {
+  if (handler) {
+    handler(content);
   }
 }
 
diff --git a/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessaging+ExtensionHelper.h b/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessaging+ExtensionHelper.h
index 7f0fe7c7688..b4a2d7254f8 100644
--- a/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessaging+ExtensionHelper.h
+++ b/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessaging+ExtensionHelper.h
@@ -32,7 +32,8 @@ NS_ASSUME_NONNULL_BEGIN
  *
  * @return An instance of MessagingExtensionHelper that handles the extensions API.
  */
-+ (FIRMessagingExtensionHelper *)extensionHelper NS_SWIFT_NAME(serviceExtension());
++ (FIRMessagingExtensionHelper *)extensionHelper NS_SWIFT_NAME(serviceExtension())
+    NS_SWIFT_SENDABLE;
 
 @end
 
diff --git a/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessaging.h b/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessaging.h
index fded673f5fb..1c2e5e746d5 100644
--- a/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessaging.h
+++ b/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessaging.h
@@ -154,7 +154,7 @@ NS_SWIFT_NAME(MessagingDelegate)
 /// * Subscribing to any topics.
 - (void)messaging:(FIRMessaging *)messaging
     didReceiveRegistrationToken:(nullable NSString *)fcmToken
-    NS_SWIFT_NAME(messaging(_:didReceiveRegistrationToken:));
+    NS_SWIFT_NAME(messaging(_:didReceiveRegistrationToken:)) NS_SWIFT_UI_ACTOR;
 @end
 
 /**
@@ -174,7 +174,7 @@ NS_SWIFT_NAME(Messaging)
 /**
  * Delegate to handle FCM token refreshes, and remote data messages received via FCM direct channel.
  */
-@property(nonatomic, weak, nullable) id<FIRMessagingDelegate> delegate;
+@property(nonatomic, weak, nullable) id<FIRMessagingDelegate> delegate NS_SWIFT_UI_ACTOR;
 
 /**
  *  FIRMessaging
@@ -203,7 +203,7 @@ NS_SWIFT_NAME(Messaging)
  *  If you would like to set the type of the APNs token, rather than relying on
  *  automatic detection, see `setAPNSToken(_:type:)`.
  */
-@property(nonatomic, copy, nullable) NSData *APNSToken NS_SWIFT_NAME(apnsToken);
+@property(nonatomic, copy, nullable) NSData *APNSToken NS_SWIFT_NAME(apnsToken) NS_SWIFT_UI_ACTOR;
 
 /**
  *  Set the APNs token for the application. This token will be used to register
@@ -216,7 +216,7 @@ NS_SWIFT_NAME(Messaging)
  *  `MessagingAPNSTokenTypeUnknown` to have the type automatically
  *  detected based on your provisioning profile.
  */
-- (void)setAPNSToken:(NSData *)apnsToken type:(FIRMessagingAPNSTokenType)type;
+- (void)setAPNSToken:(NSData *)apnsToken type:(FIRMessagingAPNSTokenType)type NS_SWIFT_UI_ACTOR;
 
 #pragma mark - FCM Tokens
 
@@ -236,7 +236,7 @@ NS_SWIFT_NAME(Messaging)
  * default (for example, because you want to prompt the user before getting a token),
  * set `FirebaseMessagingAutoInitEnabled` to NO in your application's Info.plist.
  */
-@property(nonatomic, assign, getter=isAutoInitEnabled) BOOL autoInitEnabled;
+@property(nonatomic, assign, getter=isAutoInitEnabled) BOOL autoInitEnabled NS_SWIFT_UI_ACTOR;
 
 /**
  * The FCM registration token is used to identify this device so that FCM can send notifications to
@@ -251,7 +251,8 @@ NS_SWIFT_NAME(Messaging)
  * Once you have an FCM registration token, you should send it to your application server, where
  * it can be used to send notifications to your device.
  */
-@property(nonatomic, readonly, nullable) NSString *FCMToken NS_SWIFT_NAME(fcmToken);
+@property(nonatomic, readonly, nullable) NSString *FCMToken NS_SWIFT_NAME(fcmToken)
+    NS_SWIFT_UI_ACTOR;
 
 /**
  * Asynchronously gets the default FCM registration token.
@@ -263,9 +264,9 @@ NS_SWIFT_NAME(Messaging)
  *
  * @param completion The completion handler to handle the token request.
  */
-
-- (void)tokenWithCompletion:(void (^)(NSString *_Nullable token,
-                                      NSError *_Nullable error))completion;
+- (void)tokenWithCompletion:
+    (void (^NS_SWIFT_UI_ACTOR)(NSString *_Nullable token, NSError *_Nullable error))completion
+    NS_SWIFT_UI_ACTOR;
 
 /**
  * Asynchronously deletes the default FCM registration token.
@@ -275,8 +276,8 @@ NS_SWIFT_NAME(Messaging)
  *
  * @param completion The completion handler to handle the token deletion.
  */
-
-- (void)deleteTokenWithCompletion:(void (^)(NSError *_Nullable error))completion;
+- (void)deleteTokenWithCompletion:(void (^NS_SWIFT_UI_ACTOR)(NSError *_Nullable error))completion
+    NS_SWIFT_UI_ACTOR;
 
 /**
  *  Retrieves an FCM registration token for a particular Sender ID. This can be used to allow
@@ -298,9 +299,9 @@ NS_SWIFT_NAME(Messaging)
  *  @param completion The completion handler to handle the token request.
  */
 - (void)retrieveFCMTokenForSenderID:(NSString *)senderID
-                         completion:(void (^)(NSString *_Nullable FCMToken,
-                                              NSError *_Nullable error))completion
-    NS_SWIFT_NAME(retrieveFCMToken(forSenderID:completion:));
+                         completion:(void (^NS_SWIFT_UI_ACTOR)(NSString *_Nullable FCMToken,
+                                                               NSError *_Nullable error))completion
+    NS_SWIFT_NAME(retrieveFCMToken(forSenderID:completion:)) NS_SWIFT_UI_ACTOR;
 
 /**
  * Invalidates an FCM token for a particular Sender ID. That Sender ID cannot no longer send
@@ -311,8 +312,8 @@ NS_SWIFT_NAME(Messaging)
  * @param completion The completion handler to handle the token deletion.
  */
 - (void)deleteFCMTokenForSenderID:(NSString *)senderID
-                       completion:(void (^)(NSError *_Nullable error))completion
-    NS_SWIFT_NAME(deleteFCMToken(forSenderID:completion:));
+                       completion:(void (^NS_SWIFT_UI_ACTOR)(NSError *_Nullable error))completion
+    NS_SWIFT_NAME(deleteFCMToken(forSenderID:completion:)) NS_SWIFT_UI_ACTOR;
 
 #pragma mark - Topics
 
@@ -323,7 +324,7 @@ NS_SWIFT_NAME(Messaging)
  *
  * @param topic The name of the topic, for example, @"sports".
  */
-- (void)subscribeToTopic:(NSString *)topic NS_SWIFT_NAME(subscribe(toTopic:));
+- (void)subscribeToTopic:(NSString *)topic NS_SWIFT_NAME(subscribe(toTopic:)) NS_SWIFT_UI_ACTOR;
 
 /**
  * Asynchronously subscribe to the provided topic, retrying on failure. This uses the default FCM
@@ -337,7 +338,8 @@ NS_SWIFT_NAME(Messaging)
  *                    appropriate error object is returned.
  */
 - (void)subscribeToTopic:(nonnull NSString *)topic
-              completion:(void (^_Nullable)(NSError *_Nullable error))completion;
+              completion:(void (^_Nullable NS_SWIFT_UI_ACTOR)(NSError *_Nullable error))completion
+    NS_SWIFT_UI_ACTOR;
 
 /**
  * Asynchronously unsubscribe from a topic.  This uses a FCM Token
@@ -346,7 +348,8 @@ NS_SWIFT_NAME(Messaging)
  *
  * @param topic The name of the topic, for example @"sports".
  */
-- (void)unsubscribeFromTopic:(NSString *)topic NS_SWIFT_NAME(unsubscribe(fromTopic:));
+- (void)unsubscribeFromTopic:(NSString *)topic
+    NS_SWIFT_NAME(unsubscribe(fromTopic:)) NS_SWIFT_UI_ACTOR;
 
 /**
  * Asynchronously unsubscribe from the provided topic, retrying on failure. This uses a FCM Token
@@ -359,7 +362,9 @@ NS_SWIFT_NAME(Messaging)
  *                     appropriate error object is returned.
  */
 - (void)unsubscribeFromTopic:(nonnull NSString *)topic
-                  completion:(void (^_Nullable)(NSError *_Nullable error))completion;
+                  completion:
+                      (void (^_Nullable NS_SWIFT_UI_ACTOR)(NSError *_Nullable error))completion
+    NS_SWIFT_UI_ACTOR;
 
 #pragma mark - Analytics
 
@@ -374,7 +379,7 @@ NS_SWIFT_NAME(Messaging)
  *
  *  @return Information about the downstream message.
  */
-- (FIRMessagingMessageInfo *)appDidReceiveMessage:(NSDictionary *)message;
+- (FIRMessagingMessageInfo *)appDidReceiveMessage:(NSDictionary *)message NS_SWIFT_UI_ACTOR;
 
 #pragma mark - GDPR
 /**
@@ -387,7 +392,8 @@ NS_SWIFT_NAME(Messaging)
  * @param completion A completion handler which is invoked when the operation completes. `error ==
  * nil` indicates success.
  */
-- (void)deleteDataWithCompletion:(void (^)(NSError *__nullable error))completion;
+- (void)deleteDataWithCompletion:(void (^NS_SWIFT_UI_ACTOR)(NSError *__nullable error))completion
+    NS_SWIFT_UI_ACTOR;
 
 @end
 
diff --git a/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessagingExtensionHelper.h b/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessagingExtensionHelper.h
index eb25919ced3..ac24cc19e6c 100644
--- a/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessagingExtensionHelper.h
+++ b/FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessagingExtensionHelper.h
@@ -30,14 +30,17 @@ NS_ASSUME_NONNULL_BEGIN
 /// specified in the notification body via the `image` parameter. Images and other
 /// rich content can be populated manually without the use of this class. See the
 /// `UNNotificationServiceExtension` type for more details.
-__OSX_AVAILABLE(10.14) @interface FIRMessagingExtensionHelper : NSObject
+__OSX_AVAILABLE(10.14) NS_SWIFT_SENDABLE @interface FIRMessagingExtensionHelper : NSObject
 
 /// Call this API to complete your notification content modification. If you like to
 /// overwrite some properties of the content instead of using the default payload,
-/// make sure to make your customized motification to the content before passing it to
-/// this call.
-- (void)populateNotificationContent:(UNMutableNotificationContent *)content
-                 withContentHandler:(void (^)(UNNotificationContent *_Nonnull))contentHandler;
+/// make sure to make your customized modification to the content before passing it to
+/// this call. The content returned in the content handler after populating will be a
+/// different instance than the value passed in to `content`.
+- (void)populateNotificationContent:(UNNotificationContent *)content
+                 withContentHandler:
+                     (void (^NS_SWIFT_SENDABLE)(UNNotificationContent *_Nonnull))contentHandler
+    NS_SWIFT_SENDABLE;
 
 /// Exports delivery metrics to BigQuery. Call this API to enable logging delivery of alert
 /// notification or background notification and export to BigQuery.
diff --git a/FirebasePerformance.podspec b/FirebasePerformance.podspec
index b6ce91be3b4..3e340732a40 100644
--- a/FirebasePerformance.podspec
+++ b/FirebasePerformance.podspec
@@ -20,7 +20,7 @@ Firebase Performance library to measure performance of Mobile and Web Apps.
   ios_deployment_target = '13.0'
   tvos_deployment_target = '13.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.tvos.deployment_target = tvos_deployment_target
diff --git a/FirebaseRemoteConfig.podspec b/FirebaseRemoteConfig.podspec
index 7866379e8ae..283b25d36a5 100644
--- a/FirebaseRemoteConfig.podspec
+++ b/FirebaseRemoteConfig.podspec
@@ -23,7 +23,7 @@ app update.
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseRemoteConfigInterop.podspec b/FirebaseRemoteConfigInterop.podspec
index e1eb447a4db..03fa5527141 100644
--- a/FirebaseRemoteConfigInterop.podspec
+++ b/FirebaseRemoteConfigInterop.podspec
@@ -20,7 +20,7 @@ Pod::Spec.new do |s|
     :tag => 'CocoaPods-' + s.version.to_s
   }
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
   s.cocoapods_version = '>= 1.12.0'
   s.prefix_header_file = false
 
diff --git a/FirebaseSessions.podspec b/FirebaseSessions.podspec
index 9f7729162dc..ce92eced680 100644
--- a/FirebaseSessions.podspec
+++ b/FirebaseSessions.podspec
@@ -23,7 +23,7 @@ Pod::Spec.new do |s|
   tvos_deployment_target = '13.0'
   watchos_deployment_target = '7.0'
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target
diff --git a/FirebaseSharedSwift.podspec b/FirebaseSharedSwift.podspec
index fd5ded5eafd..4f483f7165a 100644
--- a/FirebaseSharedSwift.podspec
+++ b/FirebaseSharedSwift.podspec
@@ -18,7 +18,7 @@ Firebase products. FirebaseSharedSwift is not supported for non-Firebase usage.
     :tag => 'CocoaPods-' + s.version.to_s
   }
 
-  s.swift_version           = '5.9'
+  s.swift_version           = '6.0'
 
   ios_deployment_target = '13.0'
   osx_deployment_target = '10.15'
diff --git a/FirebaseStorage.podspec b/FirebaseStorage.podspec
index d8563d48502..2b9e520256e 100644
--- a/FirebaseStorage.podspec
+++ b/FirebaseStorage.podspec
@@ -27,7 +27,7 @@ Firebase Storage provides robust, secure file uploads and downloads from Firebas
   s.tvos.deployment_target = tvos_deployment_target
   s.watchos.deployment_target = watchos_deployment_target
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.cocoapods_version = '>= 1.12.0'
   s.prefix_header_file = false
diff --git a/FirebaseStorage/CHANGELOG.md b/FirebaseStorage/CHANGELOG.md
index 9e940e34310..43ad424d1e2 100644
--- a/FirebaseStorage/CHANGELOG.md
+++ b/FirebaseStorage/CHANGELOG.md
@@ -2,6 +2,7 @@
 - [fixed] `putFile` now works in App Clips. Similarly to app extensions, background session
   configurations are not used in App Clips (#14794).
 
+
 # 11.1.0
 - [fixed] Fix a potential data race in Storage initialization. (#13369)
 
diff --git a/FirebaseStorage/Sources/Internal/StorageFetcherService.swift b/FirebaseStorage/Sources/Internal/StorageFetcherService.swift
index 809d258f87d..267f4e15933 100644
--- a/FirebaseStorage/Sources/Internal/StorageFetcherService.swift
+++ b/FirebaseStorage/Sources/Internal/StorageFetcherService.swift
@@ -15,9 +15,9 @@
 import Foundation
 
 #if COCOAPODS
-  import GTMSessionFetcher
+  @preconcurrency import GTMSessionFetcher
 #else
-  import GTMSessionFetcherCore
+  @preconcurrency import GTMSessionFetcherCore
 #endif
 
 /// Manage Storage's fetcherService
diff --git a/FirebaseStorage/Sources/Internal/StorageGetDownloadURLTask.swift b/FirebaseStorage/Sources/Internal/StorageGetDownloadURLTask.swift
index be4e99f4066..abe66e7376e 100644
--- a/FirebaseStorage/Sources/Internal/StorageGetDownloadURLTask.swift
+++ b/FirebaseStorage/Sources/Internal/StorageGetDownloadURLTask.swift
@@ -19,7 +19,7 @@ import Foundation
 enum StorageGetDownloadURLTask {
   static func getDownloadURLTask(reference: StorageReference,
                                  queue: DispatchQueue,
-                                 completion: ((_: URL?, _: Error?) -> Void)?) {
+                                 completion: (@Sendable (_: URL?, _: Error?) -> Void)?) {
     StorageInternalTask(reference: reference,
                         queue: queue,
                         httpMethod: "GET",
diff --git a/FirebaseStorage/Sources/Internal/StorageGetMetadataTask.swift b/FirebaseStorage/Sources/Internal/StorageGetMetadataTask.swift
index dffbe26c46c..85348b0aebc 100644
--- a/FirebaseStorage/Sources/Internal/StorageGetMetadataTask.swift
+++ b/FirebaseStorage/Sources/Internal/StorageGetMetadataTask.swift
@@ -19,7 +19,7 @@ import Foundation
 enum StorageGetMetadataTask {
   static func getMetadataTask(reference: StorageReference,
                               queue: DispatchQueue,
-                              completion: ((_: StorageMetadata?, _: Error?) -> Void)?) {
+                              completion: (@Sendable (_: StorageMetadata?, _: Error?) -> Void)?) {
     StorageInternalTask(reference: reference,
                         queue: queue,
                         httpMethod: "GET",
diff --git a/FirebaseStorage/Sources/Internal/StorageInternalTask.swift b/FirebaseStorage/Sources/Internal/StorageInternalTask.swift
index aac100bb5e1..6cdc47f7cbb 100644
--- a/FirebaseStorage/Sources/Internal/StorageInternalTask.swift
+++ b/FirebaseStorage/Sources/Internal/StorageInternalTask.swift
@@ -15,15 +15,15 @@
 import Foundation
 
 #if COCOAPODS
-  import GTMSessionFetcher
+  @preconcurrency import GTMSessionFetcher
 #else
-  import GTMSessionFetcherCore
+  @preconcurrency import GTMSessionFetcherCore
 #endif
 
 /// Implement StorageTasks that are not directly exposed via the public API.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
 @preconcurrency
-class StorageInternalTask: StorageTask {
+class StorageInternalTask: StorageTask, @unchecked Sendable /* TODO: sendable */ {
   private var fetcher: GTMSessionFetcher?
 
   @discardableResult
@@ -32,7 +32,7 @@ class StorageInternalTask: StorageTask {
        request: URLRequest? = nil,
        httpMethod: String,
        fetcherComment: String,
-       completion: ((_: Data?, _: Error?) -> Void)?) {
+       completion: (@Sendable (_: Data?, _: Error?) -> Void)?) {
     super.init(reference: reference, queue: queue)
 
     // Prepare a task and begins execution.
diff --git a/FirebaseStorage/Sources/Internal/StorageTokenAuthorizer.swift b/FirebaseStorage/Sources/Internal/StorageTokenAuthorizer.swift
index 3dec3a3872a..e35fb678eee 100644
--- a/FirebaseStorage/Sources/Internal/StorageTokenAuthorizer.swift
+++ b/FirebaseStorage/Sources/Internal/StorageTokenAuthorizer.swift
@@ -14,63 +14,85 @@
 
 import Foundation
 
-import FirebaseAppCheckInterop
+@preconcurrency import FirebaseAppCheckInterop /* TODO: sendable */
 import FirebaseAuthInterop
 import FirebaseCore
 internal import FirebaseCoreExtension
 
 #if COCOAPODS
-  import GTMSessionFetcher
+  @preconcurrency import GTMSessionFetcher
 #else
-  import GTMSessionFetcherCore
+  @preconcurrency import GTMSessionFetcherCore
 #endif
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-class StorageTokenAuthorizer: NSObject, GTMSessionFetcherAuthorizer {
+final class StorageTokenAuthorizer: NSObject, GTMSessionFetcherAuthorizer, Sendable {
   func authorizeRequest(_ request: NSMutableURLRequest?,
-                        completionHandler handler: @escaping (Error?) -> Void) {
+                        completionHandler handler: @escaping @Sendable (Error?) -> Void) {
+    if let request = request {
+      Task {
+        do {
+          try await self._authorizeRequest(request)
+          handler(nil)
+        } catch {
+          handler(error)
+        }
+      }
+    }
+  }
+
+  private func _authorizeRequest(_ request: NSMutableURLRequest) async throws {
     // Set version header on each request
     let versionString = "ios/\(FirebaseVersion())"
-    request?.setValue(versionString, forHTTPHeaderField: "x-firebase-storage-version")
+    request.setValue(versionString, forHTTPHeaderField: "x-firebase-storage-version")
 
     // Set GMP ID on each request
-    request?.setValue(googleAppID, forHTTPHeaderField: "x-firebase-gmpid")
+    request.setValue(googleAppID, forHTTPHeaderField: "x-firebase-gmpid")
 
-    var tokenError: NSError?
-    let fetchTokenGroup = DispatchGroup()
     if let auth {
-      fetchTokenGroup.enter()
-      auth.getToken(forcingRefresh: false) { token, error in
-        if let error = error as? NSError {
-          var errorDictionary = error.userInfo
-          errorDictionary["ResponseErrorDomain"] = error.domain
-          errorDictionary["ResponseErrorCode"] = error.code
-          tokenError = StorageError.unauthenticated(serverError: errorDictionary) as NSError
-        } else if let token {
-          let firebaseToken = "Firebase \(token)"
-          request?.setValue(firebaseToken, forHTTPHeaderField: "Authorization")
+      let token: String = try await withCheckedThrowingContinuation { continuation in
+        auth.getToken(forcingRefresh: false) { token, error in
+          if let error = error as? NSError {
+            var errorDictionary = error.userInfo
+            errorDictionary["ResponseErrorDomain"] = error.domain
+            errorDictionary["ResponseErrorCode"] = error.code
+            let wrappedError = StorageError.unauthenticated(serverError: errorDictionary) as Error
+            continuation.resume(throwing: wrappedError)
+          } else if let token {
+            let firebaseToken = "Firebase \(token)"
+            continuation.resume(returning: firebaseToken)
+          } else {
+            let underlyingError: [String: Any]
+            if let error = error {
+              underlyingError = [NSUnderlyingErrorKey: error]
+            } else {
+              underlyingError = [:]
+            }
+            let unknownError = StorageError.unknown(
+              message: "Auth token fetch returned no token or error: \(token ?? "nil")",
+              serverError: underlyingError
+            ) as Error
+            continuation.resume(throwing: unknownError)
+          }
         }
-        fetchTokenGroup.leave()
       }
+      request.setValue(token, forHTTPHeaderField: "Authorization")
     }
     if let appCheck {
-      fetchTokenGroup.enter()
-      appCheck.getToken(forcingRefresh: false) { tokenResult in
-        request?.setValue(tokenResult.token, forHTTPHeaderField: "X-Firebase-AppCheck")
-
-        if let error = tokenResult.error {
-          FirebaseLogger.log(
-            level: .debug,
-            service: "[FirebaseStorage]",
-            code: "I-STR000001",
-            message: "Failed to fetch AppCheck token. Error: \(error)"
-          )
+      let token = await withCheckedContinuation { continuation in
+        appCheck.getToken(forcingRefresh: false) { tokenResult in
+          if let error = tokenResult.error {
+            FirebaseLogger.log(
+              level: .debug,
+              service: "[FirebaseStorage]",
+              code: "I-STR000001",
+              message: "Failed to fetch AppCheck token. Error: \(error)"
+            )
+          }
+          continuation.resume(returning: tokenResult.token)
         }
-        fetchTokenGroup.leave()
       }
-    }
-    fetchTokenGroup.notify(queue: callbackQueue) {
-      handler(tokenError)
+      request.setValue(token, forHTTPHeaderField: "X-Firebase-AppCheck")
     }
   }
 
@@ -98,7 +120,7 @@ class StorageTokenAuthorizer: NSObject, GTMSessionFetcherAuthorizer {
     return authHeader.hasPrefix("Firebase")
   }
 
-  var userEmail: String?
+  let userEmail: String?
 
   let callbackQueue: DispatchQueue
   private let googleAppID: String
diff --git a/FirebaseStorage/Sources/Internal/StorageUpdateMetadataTask.swift b/FirebaseStorage/Sources/Internal/StorageUpdateMetadataTask.swift
index 12f4924576f..d4fed476f23 100644
--- a/FirebaseStorage/Sources/Internal/StorageUpdateMetadataTask.swift
+++ b/FirebaseStorage/Sources/Internal/StorageUpdateMetadataTask.swift
@@ -20,7 +20,8 @@ enum StorageUpdateMetadataTask {
   static func updateMetadataTask(reference: StorageReference,
                                  queue: DispatchQueue,
                                  metadata: StorageMetadata,
-                                 completion: ((_: StorageMetadata?, _: Error?) -> Void)?) {
+                                 completion: (@Sendable (_: StorageMetadata?, _: Error?)
+                                   -> Void)?) {
     var request = StorageUtils.defaultRequestForReference(reference: reference)
     let updateData = try? JSONSerialization.data(withJSONObject: metadata.updatedMetadata())
     request.httpBody = updateData
diff --git a/FirebaseStorage/Sources/Result.swift b/FirebaseStorage/Sources/Result.swift
index af2f11961a8..12250f76961 100644
--- a/FirebaseStorage/Sources/Result.swift
+++ b/FirebaseStorage/Sources/Result.swift
@@ -23,8 +23,9 @@ import Foundation
 /// - Returns: A closure parameterized with an optional generic and optional `Error` to match
 ///            Objective-C APIs.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-private func getResultCallback<T>(completion: @escaping (Result<T, Error>) -> Void) -> (_: T?,
-                                                                                        _: Error?)
+private func getResultCallback<T>(completion: @escaping @Sendable (Result<T, Error>) -> Void)
+  -> @Sendable (_: T?,
+                _: Error?)
   -> Void {
   return { (value: T?, error: Error?) in
     if let value {
@@ -48,7 +49,7 @@ public extension StorageReference {
   ///
   /// - Parameters:
   ///   - completion: A completion block returning a `Result` enum with either a URL or an `Error`.
-  func downloadURL(completion: @escaping (Result<URL, Error>) -> Void) {
+  func downloadURL(completion: @escaping @Sendable (Result<URL, Error>) -> Void) {
     downloadURL(completion: getResultCallback(completion: completion))
   }
 
@@ -64,7 +65,7 @@ public extension StorageReference {
   ///
   /// - Returns: A StorageDownloadTask that can be used to monitor or manage the download.
   @discardableResult
-  func getData(maxSize: Int64, completion: @escaping (Result<Data, Error>) -> Void)
+  func getData(maxSize: Int64, completion: @escaping @Sendable (Result<Data, Error>) -> Void)
     -> StorageDownloadTask {
     return getData(maxSize: maxSize, completion: getResultCallback(completion: completion))
   }
@@ -74,7 +75,7 @@ public extension StorageReference {
   /// - Parameters:
   ///   - completion: A completion block which returns a `Result` enum with either the
   ///                 object metadata or an `Error`.
-  func getMetadata(completion: @escaping (Result<StorageMetadata, Error>) -> Void) {
+  func getMetadata(completion: @escaping @Sendable (Result<StorageMetadata, Error>) -> Void) {
     getMetadata(completion: getResultCallback(completion: completion))
   }
 
@@ -97,7 +98,7 @@ public extension StorageReference {
   ///                with either the list or an `Error`.
   func list(maxResults: Int64,
             pageToken: String,
-            completion: @escaping (Result<StorageListResult, Error>) -> Void) {
+            completion: @escaping @Sendable (Result<StorageListResult, Error>) -> Void) {
     list(maxResults: maxResults,
          pageToken: pageToken,
          completion: getResultCallback(completion: completion))
@@ -118,7 +119,7 @@ public extension StorageReference {
   ///                prefixes under the current `StorageReference`. It returns a `Result` enum
   ///                with either the list or an `Error`.
   func list(maxResults: Int64,
-            completion: @escaping (Result<StorageListResult, Error>) -> Void) {
+            completion: @escaping @Sendable (Result<StorageListResult, Error>) -> Void) {
     list(maxResults: maxResults,
          completion: getResultCallback(completion: completion))
   }
@@ -135,7 +136,7 @@ public extension StorageReference {
   ///   - completion: A completion handler that will be invoked with all items and prefixes
   ///                under the current StorageReference. It returns a `Result` enum with either the
   ///                list or an `Error`.
-  func listAll(completion: @escaping (Result<StorageListResult, Error>) -> Void) {
+  func listAll(completion: @escaping @Sendable (Result<StorageListResult, Error>) -> Void) {
     listAll(completion: getResultCallback(completion: completion))
   }
 
@@ -154,7 +155,7 @@ public extension StorageReference {
   @discardableResult
   func putData(_ uploadData: Data,
                metadata: StorageMetadata? = nil,
-               completion: @escaping (Result<StorageMetadata, Error>) -> Void)
+               completion: @escaping @Sendable (Result<StorageMetadata, Error>) -> Void)
     -> StorageUploadTask {
     return putData(uploadData,
                    metadata: metadata,
@@ -175,7 +176,7 @@ public extension StorageReference {
   @discardableResult
   func putFile(from: URL,
                metadata: StorageMetadata? = nil,
-               completion: @escaping (Result<StorageMetadata, Error>) -> Void)
+               completion: @escaping @Sendable (Result<StorageMetadata, Error>) -> Void)
     -> StorageUploadTask {
     return putFile(from: from,
                    metadata: metadata,
@@ -189,7 +190,7 @@ public extension StorageReference {
   ///   - completion: A completion block which returns a `Result` enum with either the
   ///                object metadata or an `Error`.
   func updateMetadata(_ metadata: StorageMetadata,
-                      completion: @escaping (Result<StorageMetadata, Error>) -> Void) {
+                      completion: @escaping @Sendable (Result<StorageMetadata, Error>) -> Void) {
     updateMetadata(metadata, completion: getResultCallback(completion: completion))
   }
 
@@ -203,7 +204,7 @@ public extension StorageReference {
   ///
   /// - Returns: A `StorageDownloadTask` that can be used to monitor or manage the download.
   @discardableResult
-  func write(toFile: URL, completion: @escaping (Result<URL, Error>)
+  func write(toFile: URL, completion: @escaping @Sendable (Result<URL, Error>)
     -> Void) -> StorageDownloadTask {
     return write(toFile: toFile, completion: getResultCallback(completion: completion))
   }
diff --git a/FirebaseStorage/Sources/Storage.swift b/FirebaseStorage/Sources/Storage.swift
index b4eefff1ff1..a46c0fec9b4 100644
--- a/FirebaseStorage/Sources/Storage.swift
+++ b/FirebaseStorage/Sources/Storage.swift
@@ -17,6 +17,7 @@ import Foundation
 import FirebaseAppCheckInterop
 import FirebaseAuthInterop
 import FirebaseCore
+import FirebaseCoreInternal
 
 // Avoids exposing internal FirebaseCore APIs to Swift users.
 internal import FirebaseCoreExtension
@@ -249,13 +250,13 @@ internal import FirebaseCoreExtension
     private var instances: [String: Storage] = [:]
 
     /// Lock to manage access to the instances array to avoid race conditions.
-    private var instancesLock: os_unfair_lock = .init()
+    private let instancesLock = FIRAllocatedUnfairLock<Void>()
 
     private init() {}
 
     func storage(app: FirebaseApp, bucket: String) -> Storage {
-      os_unfair_lock_lock(&instancesLock)
-      defer { os_unfair_lock_unlock(&instancesLock) }
+      instancesLock.lock()
+      defer { instancesLock.unlock() }
 
       if let instance = instances[bucket] {
         return instance
diff --git a/FirebaseStorage/Sources/StorageDownloadTask.swift b/FirebaseStorage/Sources/StorageDownloadTask.swift
index 70a2e64629e..8bfb663ab0e 100644
--- a/FirebaseStorage/Sources/StorageDownloadTask.swift
+++ b/FirebaseStorage/Sources/StorageDownloadTask.swift
@@ -15,9 +15,9 @@
 import Foundation
 
 #if COCOAPODS
-  import GTMSessionFetcher
+  @preconcurrency import GTMSessionFetcher
 #else
-  import GTMSessionFetcherCore
+  @preconcurrency import GTMSessionFetcherCore
 #endif
 
 /**
@@ -34,7 +34,8 @@ import Foundation
  */
 @objc(FIRStorageDownloadTask)
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-open class StorageDownloadTask: StorageObservableTask, StorageTaskManagement {
+open class StorageDownloadTask: StorageObservableTask, StorageTaskManagement,
+  @unchecked Sendable /* TODO: sendable */ {
   /**
    * Prepares a task and begins execution.
    */
diff --git a/FirebaseStorage/Sources/StorageError.swift b/FirebaseStorage/Sources/StorageError.swift
index fb863afa14d..22c52d66d12 100644
--- a/FirebaseStorage/Sources/StorageError.swift
+++ b/FirebaseStorage/Sources/StorageError.swift
@@ -104,7 +104,11 @@ public let StorageErrorDomain: String = "FIRStorageErrorDomain"
 
 /// Firebase Storage errors
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-public enum StorageError: Error, CustomNSError {
+public enum StorageError: Error, CustomNSError, @unchecked Sendable {
+  // It would be nice to make this type a checked sendable,
+  // but the Any params we get from NSErrors are not easily
+  // castable to Sendable even though they're all Foundation types
+  // and should be Sendable.
   case unknown(message: String, serverError: [String: Any])
   case objectNotFound(object: String, serverError: [String: Any])
   case bucketNotFound(bucket: String)
diff --git a/FirebaseStorage/Sources/StorageListResult.swift b/FirebaseStorage/Sources/StorageListResult.swift
index 418b3c25f20..e69efead4a2 100644
--- a/FirebaseStorage/Sources/StorageListResult.swift
+++ b/FirebaseStorage/Sources/StorageListResult.swift
@@ -16,7 +16,7 @@ import Foundation
 
 /** Contains the prefixes and items returned by a `StorageReference.list()` call. */
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIRStorageListResult) open class StorageListResult: NSObject {
+@objc(FIRStorageListResult) public final class StorageListResult: NSObject, Sendable {
   /**
    * The prefixes (folders) returned by a `list()` operation.
    */
@@ -35,7 +35,7 @@ import Foundation
 
   // MARK: - NSObject overrides
 
-  @objc override open func copy() -> Any {
+  @objc override public func copy() -> Any {
     return StorageListResult(withPrefixes: prefixes,
                              items: items,
                              pageToken: pageToken)
diff --git a/FirebaseStorage/Sources/StorageMetadata.swift b/FirebaseStorage/Sources/StorageMetadata.swift
index d0e9d43df4b..3e8b8a779b1 100644
--- a/FirebaseStorage/Sources/StorageMetadata.swift
+++ b/FirebaseStorage/Sources/StorageMetadata.swift
@@ -23,7 +23,8 @@ import Foundation
  * [GCS documentation](https://cloud.google.com/storage/docs/json_api/v1/objects#resource)
  */
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIRStorageMetadata) open class StorageMetadata: NSObject {
+@objc(FIRStorageMetadata) open class StorageMetadata: NSObject,
+  @unchecked Sendable /* TODO: sendable */ {
   // MARK: - Public APIs
 
   /**
diff --git a/FirebaseStorage/Sources/StorageReference.swift b/FirebaseStorage/Sources/StorageReference.swift
index 26d4b27ee5a..4d3a9409360 100644
--- a/FirebaseStorage/Sources/StorageReference.swift
+++ b/FirebaseStorage/Sources/StorageReference.swift
@@ -18,7 +18,8 @@ import Foundation
 /// upload and download objects, as well as get/set object metadata, and delete an object at the
 /// path. See the [Cloud docs](https://cloud.google.com/storage/)  for more details.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
-@objc(FIRStorageReference) open class StorageReference: NSObject {
+@objc(FIRStorageReference) open class StorageReference: NSObject,
+  @unchecked Sendable /* TODO: sendable */ {
   // MARK: - Public APIs
 
   /// The `Storage` service object which created this reference.
@@ -235,7 +236,7 @@ import Foundation
   /// - Parameter completion: A completion block that either returns the URL on success,
   ///     or an error on failure.
   @objc(downloadURLWithCompletion:)
-  open func downloadURL(completion: @escaping ((_: URL?, _: Error?) -> Void)) {
+  open func downloadURL(completion: @escaping (@Sendable (_: URL?, _: Error?) -> Void)) {
     StorageGetDownloadURLTask.getDownloadURLTask(reference: self,
                                                  queue: storage.dispatchQueue,
                                                  completion: completion)
@@ -435,7 +436,7 @@ import Foundation
   /// - Parameter completion: A completion block which returns the object metadata on success,
   ///   or an error on failure.
   @objc(metadataWithCompletion:)
-  open func getMetadata(completion: @escaping ((_: StorageMetadata?, _: Error?) -> Void)) {
+  open func getMetadata(completion: @escaping @Sendable (_: StorageMetadata?, _: Error?) -> Void) {
     StorageGetMetadataTask.getMetadataTask(reference: self,
                                            queue: storage.dispatchQueue,
                                            completion: completion)
@@ -459,7 +460,7 @@ import Foundation
   ///     or an error on failure.
   @objc(updateMetadata:completion:)
   open func updateMetadata(_ metadata: StorageMetadata,
-                           completion: ((_: StorageMetadata?, _: Error?) -> Void)?) {
+                           completion: (@Sendable (_: StorageMetadata?, _: Error?) -> Void)?) {
     StorageUpdateMetadataTask.updateMetadataTask(reference: self,
                                                  queue: storage.dispatchQueue,
                                                  metadata: metadata,
diff --git a/FirebaseStorage/Sources/StorageUploadTask.swift b/FirebaseStorage/Sources/StorageUploadTask.swift
index 80a25209b4e..d420028f5a4 100644
--- a/FirebaseStorage/Sources/StorageUploadTask.swift
+++ b/FirebaseStorage/Sources/StorageUploadTask.swift
@@ -21,9 +21,9 @@ import Foundation
 #endif // COCOAPODS
 
 #if COCOAPODS
-  import GTMSessionFetcher
+  @preconcurrency import GTMSessionFetcher
 #else
-  import GTMSessionFetcherCore
+  @preconcurrency import GTMSessionFetcherCore
 #endif
 
 /**
@@ -40,7 +40,7 @@ import Foundation
  */
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
 @objc(FIRStorageUploadTask) open class StorageUploadTask: StorageObservableTask,
-  StorageTaskManagement {
+  StorageTaskManagement, @unchecked Sendable /* TODO: sendable */ {
   /**
    * Prepares a task and begins execution.
    */
diff --git a/FirebaseVertexAI.podspec b/FirebaseVertexAI.podspec
index 4305c116ad5..c3f26477dc0 100644
--- a/FirebaseVertexAI.podspec
+++ b/FirebaseVertexAI.podspec
@@ -36,7 +36,7 @@ Firebase SDK.
     'FirebaseVertexAI/Sources/**/*.swift',
   ]
 
-  s.swift_version = '5.9'
+  s.swift_version = '6.0'
 
   s.framework = 'Foundation'
   s.ios.framework = 'UIKit'
diff --git a/scripts/test_catalyst.sh b/scripts/test_catalyst.sh
index ae28da6875c..ea955c48b69 100755
--- a/scripts/test_catalyst.sh
+++ b/scripts/test_catalyst.sh
@@ -53,8 +53,8 @@ args=(
   "SUPPORTS_MACCATALYST=YES"
   # Run on macOS.
   "-sdk" "macosx" "-destination platform=\"OS X\"" "TARGETED_DEVICE_FAMILY=2"
-  # Disable signing.
-  "CODE_SIGN_IDENTITY=-" "CODE_SIGNING_REQUIRED=NO" "CODE_SIGNING_ALLOWED=NO"
+  # Use ad-hoc signing for macOS tests.
+  "CODE_SIGN_IDENTITY=-"
   # GHA is still running 10.15.
   "MACOSX_DEPLOYMENT_TARGET=10.15"
 )
diff --git a/scripts/xcresult_logs.py b/scripts/xcresult_logs.py
index 207f513e96b..34f8ef1efe7 100755
--- a/scripts/xcresult_logs.py
+++ b/scripts/xcresult_logs.py
@@ -245,7 +245,9 @@ def export_log(xcresult_path, log_id):
   Returns:
     The logged output, as a string.
   """
-  contents = xcresulttool_json('get', '--path', xcresult_path, '--id', log_id)
+  # Note: --legacy is required for Xcode 16.
+  contents = xcresulttool_json(
+      'get', '--path', xcresult_path, '--id', log_id, '--legacy')
 
   result = []
   collect_log_output(contents, result)