diff --git a/src/aphront/configuration/AphrontDefaultApplicationConfiguration.php b/src/aphront/configuration/AphrontDefaultApplicationConfiguration.php
index 97bd5740eb..b67954185a 100644
--- a/src/aphront/configuration/AphrontDefaultApplicationConfiguration.php
+++ b/src/aphront/configuration/AphrontDefaultApplicationConfiguration.php
@@ -1,6 +1,9 @@
 <?php
 
 /**
+ * NOTE: Do not extend this!
+ *
+ * @concrete-extensible
  * @group aphront
  */
 class AphrontDefaultApplicationConfiguration
diff --git a/src/applications/differential/mail/DifferentialReplyHandler.php b/src/applications/differential/mail/DifferentialReplyHandler.php
index e495269f4f..2e3b47327c 100644
--- a/src/applications/differential/mail/DifferentialReplyHandler.php
+++ b/src/applications/differential/mail/DifferentialReplyHandler.php
@@ -1,5 +1,10 @@
 <?php
 
+/**
+ * NOTE: Do not extend this!
+ *
+ * @concrete-extensible
+ */
 class DifferentialReplyHandler extends PhabricatorMailReplyHandler {
 
   private $receivedMail;