diff --git a/src/Validator/CronFrequencyValidator.php b/src/Validator/CronFrequencyValidator.php
index 4003a7273e7c036dcc0b7207843a82222f35d49e..e431158047e2d46e9be358f1225e518e60cad280 100644
--- a/src/Validator/CronFrequencyValidator.php
+++ b/src/Validator/CronFrequencyValidator.php
@@ -15,6 +15,10 @@ use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
 /**
  * Validates that cron runs frequently enough to perform automatic updates.
+ *
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
  */
 class CronFrequencyValidator implements EventSubscriberInterface {
 
diff --git a/src/Validator/InstalledVersionValidator.php b/src/Validator/InstalledVersionValidator.php
index abb30dc61260d6f255e73bd162cdbd87fca4f6e5..0a827de58baed618974b1c81baa48ed0f8efed2b 100644
--- a/src/Validator/InstalledVersionValidator.php
+++ b/src/Validator/InstalledVersionValidator.php
@@ -13,6 +13,10 @@ use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
 /**
  * Validates that the site can update from the installed version of Drupal.
+ *
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
  */
 class InstalledVersionValidator implements EventSubscriberInterface {
 
diff --git a/src/Validator/PackageManagerReadinessCheck.php b/src/Validator/PackageManagerReadinessCheck.php
index 1a6bb48a80f91b6500dbaf43a4baf92fe48d1a88..960e97b2ea896c16b32af760e1078e17eac98199 100644
--- a/src/Validator/PackageManagerReadinessCheck.php
+++ b/src/Validator/PackageManagerReadinessCheck.php
@@ -12,6 +12,10 @@ use Symfony\Component\EventDispatcher\EventSubscriberInterface;
  * This class exists to facilitate re-use of Package Manager's stage validators
  * during update readiness checks, in addition to whatever events they normally
  * subscribe to.
+ *
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
  */
 class PackageManagerReadinessCheck implements EventSubscriberInterface {
 
diff --git a/src/Validator/SettingsValidator.php b/src/Validator/SettingsValidator.php
index 4d09e58df725e2123f7eb302900e6f0fcab43a79..6da6f14f32276e55243ee44801907685e9d6e4f0 100644
--- a/src/Validator/SettingsValidator.php
+++ b/src/Validator/SettingsValidator.php
@@ -11,6 +11,11 @@ use Drupal\package_manager\Event\PreCreateEvent;
 use Drupal\package_manager\Event\PreOperationStageEvent;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
+/**
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
+ */
 class SettingsValidator implements EventSubscriberInterface {
 
   use StringTranslationTrait;
diff --git a/src/Validator/StagedDatabaseUpdateValidator.php b/src/Validator/StagedDatabaseUpdateValidator.php
index 45e55b07c2030cc02022318543f921ae47d2607e..3c1cedd9d82842f9477535524ac9c48d84cd9efa 100644
--- a/src/Validator/StagedDatabaseUpdateValidator.php
+++ b/src/Validator/StagedDatabaseUpdateValidator.php
@@ -14,6 +14,10 @@ use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
 /**
  * Validates that there are no database updates in a staged update.
+ *
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
  */
 class StagedDatabaseUpdateValidator implements EventSubscriberInterface {
 
diff --git a/src/Validator/StagedProjectsValidator.php b/src/Validator/StagedProjectsValidator.php
index eec9e8f9fb14228f0a1b26ce8d8814afde294884..44e130bf30891a782e4b3392989d5e7d3dd51818 100644
--- a/src/Validator/StagedProjectsValidator.php
+++ b/src/Validator/StagedProjectsValidator.php
@@ -11,6 +11,10 @@ use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
 /**
  * Validates the staged Drupal projects.
+ *
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
  */
 final class StagedProjectsValidator implements EventSubscriberInterface {
 
diff --git a/src/Validator/UpdateReleaseValidator.php b/src/Validator/UpdateReleaseValidator.php
index d501d1cda78fad7d7026eb4880f39e2b181fc559..97449b36d3f4a70554a3d2807667ef7e6edbde5e 100644
--- a/src/Validator/UpdateReleaseValidator.php
+++ b/src/Validator/UpdateReleaseValidator.php
@@ -10,6 +10,10 @@ use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
 /**
  * Validates that the target release of Drupal core is 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 {
 
diff --git a/src/Validator/XdebugValidator.php b/src/Validator/XdebugValidator.php
index 098dc3348177e67689d620787771efdb4d55a869..95551c5f551194cc38bae34249e3af79b9f6ab3d 100644
--- a/src/Validator/XdebugValidator.php
+++ b/src/Validator/XdebugValidator.php
@@ -8,6 +8,10 @@ use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
 /**
  * Flags a warning if Xdebug is enabled.
+ *
+ * @internal
+ *   This class is an internal part of the module's update handling and
+ *   should not be used by external code.
  */
 class XdebugValidator implements EventSubscriberInterface {