diff --git a/Package.swift b/Package.swift
index aa5694a..f3fa273 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version: 5.10
+// swift-tools-version: 6.0
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription
@@ -24,7 +24,8 @@ let package = Package(
         .target(
             name: "SnapAuth",
             swiftSettings: [
-                .define("HARDWARE_KEY_SUPPORT", .when(platforms: [.iOS, .macOS]))
+                .define("HARDWARE_KEY_SUPPORT", .when(platforms: [.iOS, .macOS])),
+                .swiftLanguageVersion(.v6),
             ]),
         .testTarget(
             name: "SnapAuthTests",
diff --git a/Sources/SnapAuth/Errors.swift b/Sources/SnapAuth/Errors.swift
index 3742060..72d4380 100644
--- a/Sources/SnapAuth/Errors.swift
+++ b/Sources/SnapAuth/Errors.swift
@@ -84,16 +84,13 @@ extension ASAuthorizationError.Code {
         case .invalidResponse: return .invalidResponse
         case .notHandled: return .notHandled
         case .notInteractive: return .notInteractive
-        @unknown default:
-            /* This is (AFAICT) correct, but doesn't seem to work on the Github
-               Actions runner version.
+        default:
             // This case only exists on new OS platforms
             if #available(iOS 18, visionOS 2, macOS 15, tvOS 18, *) {
                 if case .matchedExcludedCredential = self {
                     return .matchedExcludedCredential
                 }
             }
-             */
             return .unknown
         }
     }
diff --git a/Sources/SnapAuth/PresentationAnchor.swift b/Sources/SnapAuth/PresentationAnchor.swift
index 6ddc55d..52fb7bd 100644
--- a/Sources/SnapAuth/PresentationAnchor.swift
+++ b/Sources/SnapAuth/PresentationAnchor.swift
@@ -1,14 +1,20 @@
 import AuthenticationServices
 
+extension ASPresentationAnchor {
+    /// A platform-specific anchor, intended to be used by ASAuthorizationController
+    static var `default`: ASPresentationAnchor {
 #if os(macOS)
-// FIXME: Figure out better fallback mechanisms here.
-// This will cause a new window to open _and remain open_
-fileprivate let defaultPresentationAnchor: ASPresentationAnchor = NSApplication.shared.mainWindow ?? ASPresentationAnchor()
+        // FIXME: Figure out better fallback mechanisms here.
+        // This will cause a new window to open _and remain open_
+        return NSApplication.shared.mainWindow ?? ASPresentationAnchor()
 #else
-fileprivate let defaultPresentationAnchor: ASPresentationAnchor = (UIApplication.shared.connectedScenes.first as? UIWindowScene)?.windows.first?.rootViewController?.view.window ?? ASPresentationAnchor()
+        return (UIApplication.shared.connectedScenes.first as? UIWindowScene)?
+            .windows
+            .first?
+            .rootViewController?
+            .view
+            .window
+        ?? ASPresentationAnchor()
 #endif
-
-extension ASPresentationAnchor {
-    /// A platform-specific anchor, intended to be used by ASAuthorizationController
-    static let `default` = defaultPresentationAnchor
+    }
 }
diff --git a/Sources/SnapAuth/SnapAuth+ASACD.swift b/Sources/SnapAuth/SnapAuth+ASACD.swift
index 3646ce4..47c83e6 100644
--- a/Sources/SnapAuth/SnapAuth+ASACD.swift
+++ b/Sources/SnapAuth/SnapAuth+ASACD.swift
@@ -6,39 +6,42 @@ extension SnapAuth: ASAuthorizationControllerDelegate {
 
     /// Delegate method for ASAuthorizationController.
     /// This should not be called directly.
-    public func authorizationController(
+    nonisolated public func authorizationController(
         controller: ASAuthorizationController,
         didCompleteWithError error: Error
     ) {
         logger.debug("ASACD error")
-        guard let asError = error as? ASAuthorizationError else {
-            logger.error("authorizationController didCompleteWithError error was not an ASAuthorizationError")
-            sendError(.unknown)
-            return
-        }
+        Task { @MainActor in
+            guard let asError = error as? ASAuthorizationError else {
+                logger.error("authorizationController didCompleteWithError error was not an ASAuthorizationError")
+                sendError(.unknown)
+                return
+            }
 
-        sendError(asError.code.snapAuthError)
-        // The start call can SILENTLY produce this error which never makes it into this handler
-        // ASAuthorizationController credential request failed with error: Error Domain=com.apple.AuthenticationServices.AuthorizationError Code=1004 "(null)"
+            sendError(asError.code.snapAuthError)
+            // The start call can SILENTLY produce this error which never makes it into this handler
+            // ASAuthorizationController credential request failed with error: Error Domain=com.apple.AuthenticationServices.AuthorizationError Code=1004 "(null)"
+        }
     }
 
     /// Delegate method for ASAuthorizationController.
     /// This should not be called directly.
-    public func authorizationController(
+    nonisolated public func authorizationController(
         controller: ASAuthorizationController,
         didCompleteWithAuthorization authorization: ASAuthorization
     ) {
         logger.debug("ASACD did complete")
 
-
-        switch authorization.credential {
-        case is ASAuthorizationPublicKeyCredentialAssertion:
-            handleAssertion(authorization.credential as! ASAuthorizationPublicKeyCredentialAssertion)
-        case is ASAuthorizationPublicKeyCredentialRegistration:
-            handleRegistration(authorization.credential as! ASAuthorizationPublicKeyCredentialRegistration)
-        default:
-            logger.error("Unexpected credential type \(String(describing: type(of: authorization.credential)))")
-            sendError(.unexpectedAuthorizationType)
+        Task { @MainActor in
+            switch authorization.credential {
+            case is ASAuthorizationPublicKeyCredentialAssertion:
+                handleAssertion(authorization.credential as! ASAuthorizationPublicKeyCredentialAssertion)
+            case is ASAuthorizationPublicKeyCredentialRegistration:
+                handleRegistration(authorization.credential as! ASAuthorizationPublicKeyCredentialRegistration)
+            default:
+                logger.error("Unexpected credential type \(String(describing: type(of: authorization.credential)))")
+                sendError(.unexpectedAuthorizationType)
+            }
         }
     }
 
@@ -157,4 +160,3 @@ extension SnapAuth: ASAuthorizationControllerDelegate {
 //        }
 //    }
 }
-
diff --git a/Sources/SnapAuth/SnapAuth.swift b/Sources/SnapAuth/SnapAuth.swift
index 20faf1c..6bcf31c 100644
--- a/Sources/SnapAuth/SnapAuth.swift
+++ b/Sources/SnapAuth/SnapAuth.swift
@@ -7,6 +7,7 @@ import os
 /// This is used to start the passkey registration and authentication processes,
 /// typically in the `action` of a `Button`
 @available(macOS 12.0, iOS 15.0, tvOS 16.0, *)
+@MainActor
 public class SnapAuth: NSObject { // NSObject for ASAuthorizationControllerDelegate
 
     internal let api: SnapAuthClient
@@ -40,7 +41,7 @@ public class SnapAuth: NSObject { // NSObject for ASAuthorizationControllerDeleg
     }
 
     /// Permitted authenticator types
-    public enum Authenticator: CaseIterable {
+    public enum Authenticator: CaseIterable, Sendable {
         /// Allow all available authenticator types to be used
         public static let all = Set(Authenticator.allCases)
 
@@ -231,7 +232,7 @@ public class SnapAuth: NSObject { // NSObject for ASAuthorizationControllerDeleg
 }
 
 /// A representation of the user that is trying to authenticate.
-public enum AuthenticatingUser {
+public enum AuthenticatingUser: Sendable {
     /// Your application's internal identifier for the user (usually a primary key)
     case id(String)
     /// The user's handle, such as a username or email address