diff --git a/automatic_updates_extensions/src/BatchProcessor.php b/automatic_updates_extensions/src/BatchProcessor.php
index fe67608019bcdbbd0b59d0e5fc94c3f861d03cd0..49c5e3ae724d36391cbaefebfe1505646e6c4d46 100644
--- a/automatic_updates_extensions/src/BatchProcessor.php
+++ b/automatic_updates_extensions/src/BatchProcessor.php
@@ -8,8 +8,12 @@ use Symfony\Component\HttpFoundation\RedirectResponse;
 
 /**
  * A batch processor for updates.
+ *
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
  */
-class BatchProcessor {
+final class BatchProcessor {
 
   /**
    * The session key under which the stage ID is stored.
diff --git a/automatic_updates_extensions/src/ExtensionUpdater.php b/automatic_updates_extensions/src/ExtensionUpdater.php
index 8e1046bac81724c9c3a7ba6cb2d45934f43c92c8..6e532e75d7a230beefd9649c29fc8f07efd75f27 100644
--- a/automatic_updates_extensions/src/ExtensionUpdater.php
+++ b/automatic_updates_extensions/src/ExtensionUpdater.php
@@ -10,6 +10,10 @@ use Drupal\package_manager\Stage;
 
 /**
  * Defines a service to perform updates for modules and themes.
+ *
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
  */
 class ExtensionUpdater extends Stage {
 
diff --git a/automatic_updates_extensions/src/Form/UpdateReady.php b/automatic_updates_extensions/src/Form/UpdateReady.php
index 1265da77166c2b3f1e649476d9fccdaf0c737912..6fe7c737acb992070b8963de5181409df93dadf9 100644
--- a/automatic_updates_extensions/src/Form/UpdateReady.php
+++ b/automatic_updates_extensions/src/Form/UpdateReady.php
@@ -21,7 +21,7 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
  * @internal
  *   Form classes are internal.
  */
-class UpdateReady extends FormBase {
+final class UpdateReady extends FormBase {
 
   /**
    * The updater service.
diff --git a/automatic_updates_extensions/src/Form/UpdaterForm.php b/automatic_updates_extensions/src/Form/UpdaterForm.php
index 90d218c33b74505341590318e1140503738630fd..b059c9df51b33bea40f4a50539568811ab96beec 100644
--- a/automatic_updates_extensions/src/Form/UpdaterForm.php
+++ b/automatic_updates_extensions/src/Form/UpdaterForm.php
@@ -16,8 +16,11 @@ use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 
 /**
  * A form for selecting extension updates.
+ *
+ * @internal
+ *   Form classes are internal.
  */
-class UpdaterForm extends FormBase {
+final class UpdaterForm extends FormBase {
 
   use ReadinessTrait;
 
diff --git a/automatic_updates_extensions/src/Validator/UpdateReleaseValidator.php b/automatic_updates_extensions/src/Validator/UpdateReleaseValidator.php
index 9870fbd8c5b61b0261486df3d66ef1636e3ed488..600afe7dbde773a6052b129b10e4f322198ba95c 100644
--- a/automatic_updates_extensions/src/Validator/UpdateReleaseValidator.php
+++ b/automatic_updates_extensions/src/Validator/UpdateReleaseValidator.php
@@ -11,8 +11,12 @@ use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
 /**
  * Validates that updated projects are secure and supported.
+ *
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
  */
-class UpdateReleaseValidator implements EventSubscriberInterface {
+final class UpdateReleaseValidator implements EventSubscriberInterface {
 
   use StringTranslationTrait;