diff --git a/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutBridge.swift b/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutBridge.swift
index 1cc839dc..3a15f69d 100644
--- a/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutBridge.swift
+++ b/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutBridge.swift
@@ -1,5 +1,6 @@
 import WebKit
 
+@MainActor
 protocol CheckoutBridgeProtocol {
     @discardableResult @MainActor static func sendResponse(_ webView: WKWebView, messageBody: String) async -> Bool
 }
diff --git a/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutViewController.swift b/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutViewController.swift
index 2109d7c3..43ef9b36 100644
--- a/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutViewController.swift
+++ b/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutViewController.swift
@@ -4,6 +4,7 @@
 import SwiftUI
 import UIKit
 
+@MainActor
 public class CheckoutViewController: UINavigationController {
     public init(checkout url: URL, delegate: (any CheckoutDelegate)? = nil, client: (any CheckoutCommunicationProtocol)? = nil) {
         let rootViewController = CheckoutWebViewController(checkoutURL: url, delegate: delegate, client: client, entryPoint: nil)
@@ -80,6 +81,7 @@ public struct ShopifyCheckout: UIViewControllerRepresentable, CheckoutConfigurab
     }
 }
 
+@MainActor
 public protocol CheckoutConfigurable {
     func backgroundColor(_ color: UIColor) -> Self
     func colorScheme(_ colorScheme: ShopifyCheckoutKit.Configuration.ColorScheme) -> Self
diff --git a/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutWebView.swift b/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutWebView.swift
index 08140113..956027b4 100644
--- a/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutWebView.swift
+++ b/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutWebView.swift
@@ -4,18 +4,21 @@
 import UIKit
 import WebKit
 
+@MainActor
 protocol CheckoutWebViewDelegate: AnyObject {
     func checkoutViewDidStartNavigation()
     func checkoutViewDidFinishNavigation()
     func checkoutViewDidFailWithError(error: CheckoutError)
 }
 
+@MainActor
 class CheckoutWebView: WKWebView {
     var timer: Date?
 
     var checkoutBridge: CheckoutBridgeProtocol.Type = CheckoutBridge.self
 
     var isBridgeAttached = false
+    private var bridgeRegistration: ScriptMessageHandlerRegistration?
 
     var client: (any CheckoutCommunicationProtocol)?
 
@@ -94,7 +97,6 @@ class CheckoutWebView: WKWebView {
 
     deinit {
         OSLogger.shared.debug("De-allocating webview")
-        detachBridge()
     }
 
     @available(*, unavailable)
@@ -104,16 +106,20 @@ class CheckoutWebView: WKWebView {
 
     private func connectBridge() {
         OSLogger.shared.debug("Bridging communication to checkout")
-        configuration.userContentController
-            .add(MessageHandler(delegate: self), name: CheckoutBridge.messageHandler)
-
+        bridgeRegistration = ScriptMessageHandlerRegistration(
+            userContentController: configuration.userContentController,
+            name: CheckoutBridge.messageHandler,
+            handler: MessageHandler(delegate: self)
+        )
         isBridgeAttached = true
     }
 
     public func detachBridge() {
+        guard isBridgeAttached else { return }
+
         OSLogger.shared.debug("Detaching bridge")
-        configuration.userContentController
-            .removeScriptMessageHandler(forName: CheckoutBridge.messageHandler)
+        bridgeRegistration?.detach()
+        bridgeRegistration = nil
         isBridgeAttached = false
     }
 
@@ -132,6 +138,43 @@ class CheckoutWebView: WKWebView {
     }
 }
 
+/// Holds a WebKit script-message registration outside CheckoutWebView deinit.
+///
+/// Swift 6.0 does not support isolated deinit, so this token captures the
+/// WebKit registration while on the main actor and schedules fallback teardown
+/// back onto the main actor from deinit. Replace this with an isolated deinit
+/// on CheckoutWebView once Swift 6.2+ is the minimum supported compiler.
+private final class ScriptMessageHandlerRegistration {
+    private let userContentController: WKUserContentController
+    private let name: String
+    private var isAttached = true
+
+    @MainActor
+    init(userContentController: WKUserContentController, name: String, handler: WKScriptMessageHandler) {
+        self.userContentController = userContentController
+        self.name = name
+        userContentController.add(handler, name: name)
+    }
+
+    @MainActor
+    func detach() {
+        guard isAttached else { return }
+
+        userContentController.removeScriptMessageHandler(forName: name)
+        isAttached = false
+    }
+
+    deinit {
+        guard isAttached else { return }
+
+        let userContentController = userContentController
+        let name = name
+        Task { @MainActor in
+            userContentController.removeScriptMessageHandler(forName: name)
+        }
+    }
+}
+
 extension CheckoutWebView: WKScriptMessageHandler {
     func userContentController(_: WKUserContentController, didReceive message: WKScriptMessage) {
         guard let body = message.body as? String else {
@@ -139,13 +182,13 @@ extension CheckoutWebView: WKScriptMessageHandler {
         }
 
         if let response = CheckoutProtocol.acknowledgeReady(body) {
-            Task {
+            Task { @MainActor in
                 await checkoutBridge.sendResponse(self, messageBody: response)
             }
             return
         }
 
-        Task {
+        Task { @MainActor in
             let composedClient = ComposedCheckoutCommunicationClient(
                 merchant: client,
                 defaults: defaultClientBindings
diff --git a/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutWebViewController.swift b/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutWebViewController.swift
index c478be82..d612f809 100644
--- a/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutWebViewController.swift
+++ b/platforms/swift/Sources/ShopifyCheckoutKit/CheckoutWebViewController.swift
@@ -1,6 +1,7 @@
 import UIKit
 import WebKit
 
+@MainActor
 class CheckoutWebViewController: UIViewController, UIAdaptivePresentationControllerDelegate {
     var onCancel: (() -> Void)?
     var onFail: ((CheckoutError) -> Void)?
diff --git a/platforms/swift/Sources/ShopifyCheckoutKit/ConfettiCannon.swift b/platforms/swift/Sources/ShopifyCheckoutKit/ConfettiCannon.swift
index 36d4e0b2..df33f0ad 100644
--- a/platforms/swift/Sources/ShopifyCheckoutKit/ConfettiCannon.swift
+++ b/platforms/swift/Sources/ShopifyCheckoutKit/ConfettiCannon.swift
@@ -1,5 +1,6 @@
 import UIKit
 
+@MainActor
 enum ConfettiCannon {
     static func fire(in view: UIView) {
         let layerName = "shopify-confetti"
diff --git a/platforms/swift/Sources/ShopifyCheckoutKit/ProgressBarView.swift b/platforms/swift/Sources/ShopifyCheckoutKit/ProgressBarView.swift
index be5121a6..c9513617 100644
--- a/platforms/swift/Sources/ShopifyCheckoutKit/ProgressBarView.swift
+++ b/platforms/swift/Sources/ShopifyCheckoutKit/ProgressBarView.swift
@@ -1,5 +1,6 @@
 import UIKit
 
+@MainActor
 class ProgressBarView: UIView {
     lazy var progressBar: UIProgressView = {
         let progressBar = UIProgressView(progressViewStyle: .bar)
diff --git a/platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift b/platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift
index ce871ea3..5ee84e63 100644
--- a/platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift
+++ b/platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift
@@ -18,6 +18,7 @@ public func configure(_ block: (inout Configuration) -> Void) {
     block(&configuration)
 }
 
+@MainActor
 @discardableResult
 public func present(checkout url: URL, from: UIViewController, delegate: (any CheckoutDelegate)? = nil, client: (any CheckoutCommunicationProtocol)? = nil) -> CheckoutViewController {
     let decorated = CheckoutProtocol.url(for: url)
@@ -26,6 +27,7 @@ public func present(checkout url: URL, from: UIViewController, delegate: (any Ch
     return viewController
 }
 
+@MainActor
 @discardableResult
 package func present(checkout url: URL, from: UIViewController, entryPoint: MetaData.EntryPoint, delegate: (any CheckoutDelegate)? = nil, client: (any CheckoutCommunicationProtocol)? = nil) -> CheckoutViewController {
     let decorated = CheckoutProtocol.url(for: url)
diff --git a/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutBridgeTests.swift b/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutBridgeTests.swift
index 5b3b2eed..287451d8 100644
--- a/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutBridgeTests.swift
+++ b/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutBridgeTests.swift
@@ -2,6 +2,7 @@
 import WebKit
 import XCTest
 
+@MainActor
 class CheckoutBridgeTests: XCTestCase {
     func testApplicationNameDelegatesToUserAgent() {
         XCTAssertEqual(
@@ -50,6 +51,7 @@ class CheckoutBridgeTests: XCTestCase {
     }
 }
 
+@MainActor
 private class MockWebView: WKWebView {
     var evaluatedScript: String?
 
diff --git a/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutViewControllerTests.swift b/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutViewControllerTests.swift
index 867fb287..9a631634 100644
--- a/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutViewControllerTests.swift
+++ b/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutViewControllerTests.swift
@@ -2,13 +2,15 @@
 import WebKit
 import XCTest
 
+@MainActor
 class CheckoutViewDelegateTests: XCTestCase {
     private var customTitle: String?
     private let checkoutURL = URL(string: "https://checkout-sdk.myshopify.com")!
     private var viewController: MockCheckoutWebViewController!
     private var navigationController: UINavigationController!
 
-    override func setUp() {
+    override func setUp() async throws {
+        try await super.setUp()
         ShopifyCheckoutKit.configure {
             $0.title = customTitle ?? "Checkout"
         }
@@ -19,9 +21,9 @@ class CheckoutViewDelegateTests: XCTestCase {
         navigationController = UINavigationController(rootViewController: viewController)
     }
 
-    override func tearDown() {
+    override func tearDown() async throws {
         customTitle = nil
-        super.tearDown()
+        try await super.tearDown()
     }
 
     func testTitleIsSetToCheckout() {
@@ -30,7 +32,10 @@ class CheckoutViewDelegateTests: XCTestCase {
 
     func testTitleCanBeCustomized() {
         customTitle = "Custom title"
-        setUp()
+        ShopifyCheckoutKit.configure { $0.title = customTitle ?? "Checkout" }
+        viewController = MockCheckoutWebViewController(
+            checkoutURL: checkoutURL
+        )
         XCTAssertEqual(viewController.title, "Custom title")
     }
 
@@ -103,6 +108,7 @@ class CheckoutViewDelegateTests: XCTestCase {
     }
 }
 
+@MainActor
 protocol Dismissible: AnyObject {
     func dismiss(animated flag: Bool, completion: (() -> Void)?)
 }
diff --git a/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutWebViewControllerTests.swift b/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutWebViewControllerTests.swift
index a57581a0..0d9bed63 100644
--- a/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutWebViewControllerTests.swift
+++ b/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutWebViewControllerTests.swift
@@ -2,6 +2,7 @@
 import WebKit
 import XCTest
 
+@MainActor
 class TestableCheckoutWebViewController: CheckoutWebViewController {
     var dismissCalled = false
     var dismissAnimated: Bool = false
@@ -13,6 +14,7 @@ class TestableCheckoutWebViewController: CheckoutWebViewController {
     }
 }
 
+@MainActor
 class CheckoutWebViewControllerTests: XCTestCase {
     private let url = URL(string: "http://shopify1.shopify.com/checkouts/cn/123")!
 
diff --git a/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutWebViewTests.swift b/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutWebViewTests.swift
index 86156868..42d0c5e8 100644
--- a/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutWebViewTests.swift
+++ b/platforms/swift/Tests/ShopifyCheckoutKitTests/CheckoutWebViewTests.swift
@@ -3,12 +3,14 @@ import ShopifyCheckoutProtocol
 import WebKit
 import XCTest
 
+@MainActor
 class CheckoutWebViewTests: XCTestCase {
     private var view: CheckoutWebView!
     private var mockDelegate: MockCheckoutWebViewDelegate!
     private var url = URL(string: "http://shopify1.shopify.com/checkouts/cn/123")!
 
-    override func setUp() {
+    override func setUp() async throws {
+        try await super.setUp()
         view = CheckoutWebView.for(checkout: url)
         mockDelegate = MockCheckoutWebViewDelegate()
         view.viewDelegate = mockDelegate
@@ -16,9 +18,9 @@ class CheckoutWebViewTests: XCTestCase {
         MockCheckoutBridge.reset()
     }
 
-    override func tearDown() {
+    override func tearDown() async throws {
         view.viewDelegate = nil
-        super.tearDown()
+        try await super.tearDown()
     }
 
     func testCorrectlyConfiguresWebview() {
@@ -26,6 +28,15 @@ class CheckoutWebViewTests: XCTestCase {
         XCTAssertTrue(view.configuration.allowsInlineMediaPlayback)
     }
 
+    func testDetachBridgeIsIdempotent() {
+        XCTAssertTrue(view.isBridgeAttached)
+
+        view.detachBridge()
+        view.detachBridge()
+
+        XCTAssertFalse(view.isBridgeAttached)
+    }
+
     func testHTTPSLinkIsAllowed() throws {
         let link = try XCTUnwrap(URL(string: "https://www.shopify.com/legal/privacy/app-users"))
         let received = expectation(description: "policy decided")
@@ -503,6 +514,7 @@ class CheckoutWebViewTests: XCTestCase {
     }
 }
 
+@MainActor
 class MockCheckoutBridge: CheckoutBridgeProtocol {
     static var sendResponseCalled = false
     static var sendResponseCount = 0
diff --git a/platforms/swift/Tests/ShopifyCheckoutKitTests/ShopifyCheckoutKitTests.swift b/platforms/swift/Tests/ShopifyCheckoutKitTests/ShopifyCheckoutKitTests.swift
index 6f0a62fd..f4558f3f 100644
--- a/platforms/swift/Tests/ShopifyCheckoutKitTests/ShopifyCheckoutKitTests.swift
+++ b/platforms/swift/Tests/ShopifyCheckoutKitTests/ShopifyCheckoutKitTests.swift
@@ -1,6 +1,7 @@
 @testable import ShopifyCheckoutKit
 import XCTest
 
+@MainActor
 class ShopifyCheckoutKitTests: XCTestCase {
     func test_version_whenAccessed_shouldExist() {
         XCTAssertFalse(ShopifyCheckoutKit.version.isEmpty)
diff --git a/platforms/swift/Tests/ShopifyCheckoutKitTests/SwiftUITests.swift b/platforms/swift/Tests/ShopifyCheckoutKitTests/SwiftUITests.swift
index 2e76ef8b..a091378c 100644
--- a/platforms/swift/Tests/ShopifyCheckoutKitTests/SwiftUITests.swift
+++ b/platforms/swift/Tests/ShopifyCheckoutKitTests/SwiftUITests.swift
@@ -1,6 +1,7 @@
 @testable import ShopifyCheckoutKit
 import XCTest
 
+@MainActor
 class CheckoutViewControllerTests: XCTestCase {
     var checkoutURL: URL!
     var checkoutViewController: CheckoutViewController!
@@ -16,6 +17,7 @@ class CheckoutViewControllerTests: XCTestCase {
     }
 }
 
+@MainActor
 class ShopifyCheckoutTests: XCTestCase {
     var checkoutURL: URL!
     var shopifyCheckout: ShopifyCheckout!
@@ -58,6 +60,7 @@ class ShopifyCheckoutTests: XCTestCase {
     }
 }
 
+@MainActor
 class CheckoutConfigurableTests: XCTestCase {
     var checkoutURL: URL!
     var shopifyCheckout: ShopifyCheckout!
diff --git a/platforms/swift/api/ShopifyCheckoutKit.json b/platforms/swift/api/ShopifyCheckoutKit.json
index 6c5c1931..49d87b3c 100644
--- a/platforms/swift/api/ShopifyCheckoutKit.json
+++ b/platforms/swift/api/ShopifyCheckoutKit.json
@@ -1212,7 +1212,6 @@
             "mangledName": "$s18ShopifyCheckoutKit0B14ViewControllerC8checkout8delegate6clientAC10Foundation3URLV_AA0B8Delegate_pSgAA0B21CommunicationProtocol_pSgtcfc",
             "moduleName": "ShopifyCheckoutKit",
             "declAttributes": [
-              "Preconcurrency",
               "Custom"
             ],
             "init_kind": "Designated"
@@ -1286,7 +1285,6 @@
             "moduleName": "ShopifyCheckoutKit",
             "isInternal": true,
             "declAttributes": [
-              "Preconcurrency",
               "Custom"
             ],
             "init_kind": "Designated"
@@ -1396,7 +1394,6 @@
             "declAttributes": [
               "Dynamic",
               "ObjC",
-              "Preconcurrency",
               "Available",
               "Override",
               "Custom"
@@ -1431,7 +1428,6 @@
             "declAttributes": [
               "Dynamic",
               "ObjC",
-              "Preconcurrency",
               "Override",
               "Custom"
             ],
@@ -1487,7 +1483,6 @@
             "declAttributes": [
               "Dynamic",
               "ObjC",
-              "Preconcurrency",
               "Override",
               "Custom"
             ],
@@ -1499,7 +1494,6 @@
         "mangledName": "$s18ShopifyCheckoutKit0B14ViewControllerC",
         "moduleName": "ShopifyCheckoutKit",
         "declAttributes": [
-          "Preconcurrency",
           "Custom",
           "ObjC"
         ],
@@ -2094,6 +2088,9 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "protocolReq": true,
+            "declAttributes": [
+              "Custom"
+            ],
             "reqNewWitnessTableEntry": true,
             "funcSelfKind": "NonMutating"
           },
@@ -2120,6 +2117,9 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "protocolReq": true,
+            "declAttributes": [
+              "Custom"
+            ],
             "reqNewWitnessTableEntry": true,
             "funcSelfKind": "NonMutating"
           },
@@ -2146,6 +2146,9 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "protocolReq": true,
+            "declAttributes": [
+              "Custom"
+            ],
             "reqNewWitnessTableEntry": true,
             "funcSelfKind": "NonMutating"
           },
@@ -2172,6 +2175,9 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "protocolReq": true,
+            "declAttributes": [
+              "Custom"
+            ],
             "reqNewWitnessTableEntry": true,
             "funcSelfKind": "NonMutating"
           },
@@ -2206,6 +2212,9 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "protocolReq": true,
+            "declAttributes": [
+              "Custom"
+            ],
             "reqNewWitnessTableEntry": true,
             "funcSelfKind": "NonMutating"
           },
@@ -2232,7 +2241,8 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "declAttributes": [
-              "DiscardableResult"
+              "DiscardableResult",
+              "Custom"
             ],
             "isFromExtension": true,
             "funcSelfKind": "NonMutating"
@@ -2260,7 +2270,8 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "declAttributes": [
-              "DiscardableResult"
+              "DiscardableResult",
+              "Custom"
             ],
             "isFromExtension": true,
             "funcSelfKind": "NonMutating"
@@ -2288,7 +2299,8 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "declAttributes": [
-              "DiscardableResult"
+              "DiscardableResult",
+              "Custom"
             ],
             "isFromExtension": true,
             "funcSelfKind": "NonMutating"
@@ -2316,7 +2328,8 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "declAttributes": [
-              "DiscardableResult"
+              "DiscardableResult",
+              "Custom"
             ],
             "isFromExtension": true,
             "funcSelfKind": "NonMutating"
@@ -2352,7 +2365,8 @@
             "moduleName": "ShopifyCheckoutKit",
             "genericSig": "<Self where Self : ShopifyCheckoutKit.CheckoutConfigurable>",
             "declAttributes": [
-              "DiscardableResult"
+              "DiscardableResult",
+              "Custom"
             ],
             "isFromExtension": true,
             "funcSelfKind": "NonMutating"
@@ -2362,6 +2376,9 @@
         "usr": "s:18ShopifyCheckoutKit0B12ConfigurableP",
         "mangledName": "$s18ShopifyCheckoutKit0B12ConfigurableP",
         "moduleName": "ShopifyCheckoutKit",
+        "declAttributes": [
+          "Custom"
+        ],
         "conformances": [
           {
             "kind": "Conformance",
@@ -5499,7 +5516,8 @@
         "mangledName": "$s18ShopifyCheckoutKit7present8checkout4from8delegate6clientAA0B14ViewControllerC10Foundation3URLV_So06UIViewJ0CAA0B8Delegate_pSgAA0B21CommunicationProtocol_pSgtF",
         "moduleName": "ShopifyCheckoutKit",
         "declAttributes": [
-          "DiscardableResult"
+          "DiscardableResult",
+          "Custom"
         ],
         "funcSelfKind": "NonMutating"
       },
@@ -5569,7 +5587,8 @@
         "moduleName": "ShopifyCheckoutKit",
         "isInternal": true,
         "declAttributes": [
-          "DiscardableResult"
+          "DiscardableResult",
+          "Custom"
         ],
         "funcSelfKind": "NonMutating"
       },