Index: conf/default.conf.php
===================================================================
--- conf/default.conf.php
+++ conf/default.conf.php
@@ -827,14 +827,6 @@
 
   'differential.revision-custom-detail-renderer'  => null,
 
-  // Array for custom remarkup rules. The array should have a list of
-  // class names of classes that extend PhutilRemarkupRule
-  'differential.custom-remarkup-rules' => null,
-
-  // Array for custom remarkup block rules. The array should have a list of
-  // class names of classes that extend PhutilRemarkupEngineBlockRule
-  'differential.custom-remarkup-block-rules' => null,
-
   // List of file regexps where whitespace is meaningful and should not
   // use 'ignore-all' by default
   'differential.whitespace-matters' => array(
Index: src/__phutil_library_map__.php
===================================================================
--- src/__phutil_library_map__.php
+++ src/__phutil_library_map__.php
@@ -1592,6 +1592,8 @@
     'PhabricatorRemarkupBlockInterpreterFiglet' => 'infrastructure/markup/interpreter/PhabricatorRemarkupBlockInterpreterFiglet.php',
     'PhabricatorRemarkupBlockInterpreterGraphviz' => 'infrastructure/markup/interpreter/PhabricatorRemarkupBlockInterpreterGraphviz.php',
     'PhabricatorRemarkupControl' => 'view/form/control/PhabricatorRemarkupControl.php',
+    'PhabricatorRemarkupCustomBlockRule' => 'infrastructure/markup/rule/PhabricatorRemarkupCustomBlockRule.php',
+    'PhabricatorRemarkupCustomInlineRule' => 'infrastructure/markup/rule/PhabricatorRemarkupCustomInlineRule.php',
     'PhabricatorRemarkupRuleEmbedFile' => 'applications/files/remarkup/PhabricatorRemarkupRuleEmbedFile.php',
     'PhabricatorRemarkupRuleImageMacro' => 'applications/macro/remarkup/PhabricatorRemarkupRuleImageMacro.php',
     'PhabricatorRemarkupRuleMeme' => 'applications/macro/remarkup/PhabricatorRemarkupRuleMeme.php',
@@ -3886,6 +3888,8 @@
     'PhabricatorRemarkupBlockInterpreterFiglet' => 'PhutilRemarkupBlockInterpreter',
     'PhabricatorRemarkupBlockInterpreterGraphviz' => 'PhutilRemarkupBlockInterpreter',
     'PhabricatorRemarkupControl' => 'AphrontFormTextAreaControl',
+    'PhabricatorRemarkupCustomBlockRule' => 'PhutilRemarkupEngineBlockRule',
+    'PhabricatorRemarkupCustomInlineRule' => 'PhutilRemarkupRule',
     'PhabricatorRemarkupRuleEmbedFile' => 'PhabricatorRemarkupRuleObject',
     'PhabricatorRemarkupRuleImageMacro' => 'PhutilRemarkupRule',
     'PhabricatorRemarkupRuleMeme' => 'PhutilRemarkupRule',
Index: src/applications/config/check/PhabricatorSetupCheckExtraConfig.php
===================================================================
--- src/applications/config/check/PhabricatorSetupCheckExtraConfig.php
+++ src/applications/config/check/PhabricatorSetupCheckExtraConfig.php
@@ -141,6 +141,11 @@
 
     $ancient_config = array_fill_keys($auth_config, $reason_auth);
 
+    $markup_reason = pht(
+      'Custom remarkup rules are now added by subclassing '.
+      'PhabricatorRemarkupCustomInlineRule or '.
+      'PhabricatorRemarkupCustomBlockRule.');
+
     $ancient_config += array(
       'phid.external-loaders' =>
         pht(
@@ -155,6 +160,8 @@
           'Maniphest fields are now defined in '.
           '`maniphest.custom-field-definitions`. Existing definitions have '.
           'been migrated.'),
+      'differential.custom-remarkup-rules' => $markup_reason,
+      'differential.custom-remarkup-block-rules' => $markup_reason,
     );
 
     return $ancient_config;
Index: src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
===================================================================
--- src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
+++ src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
@@ -20,25 +20,6 @@
         ->setBaseClass('DifferentialRevisionDetailRenderer')
         ->setDescription(pht("Custom revision detail renderer.")),
       $this->newOption(
-        'differential.custom-remarkup-rules',
-        'list<string>',
-        array())
-        ->setSummary(pht('Custom remarkup rules.'))
-        ->setDescription(
-          pht(
-            "Array for custom remarkup rules. The array should have a list ".
-            "of class names of classes that extend PhutilRemarkupRule")),
-      $this->newOption(
-        'differential.custom-remarkup-block-rules',
-        'list<string>',
-        array())
-        ->setSummary(pht('Custom remarkup block rules.'))
-        ->setDescription(
-          pht(
-            "Array for custom remarkup block rules. The array should have a ".
-            "list of class names of classes that extend ".
-            "PhutilRemarkupEngineBlockRule")),
-      $this->newOption(
         'differential.whitespace-matters',
         'list<regex>',
         array(
Index: src/infrastructure/markup/PhabricatorMarkupEngine.php
===================================================================
--- src/infrastructure/markup/PhabricatorMarkupEngine.php
+++ src/infrastructure/markup/PhabricatorMarkupEngine.php
@@ -201,7 +201,16 @@
   private function getMarkupFieldKey(
     PhabricatorMarkupInterface $object,
     $field) {
-    return $object->getMarkupFieldKey($field).'@'.$this->version;
+
+    $custom = array_merge(
+      self::loadCustomInlineRules(),
+      self::loadCustomBlockRules());
+
+    $custom = mpull($custom, 'getRuleVersion', null);
+    asort($custom);
+    $custom = PhabricatorHash::digestForIndex(serialize($custom));
+
+    return $object->getMarkupFieldKey($field).'@'.$this->version.'@'.$custom;
   }
 
 
@@ -328,10 +337,6 @@
    */
   public static function newDifferentialMarkupEngine(array $options = array()) {
     return self::newMarkupEngine(array(
-      'custom-inline' => PhabricatorEnv::getEnvConfig(
-        'differential.custom-remarkup-rules'),
-      'custom-block'  => PhabricatorEnv::getEnvConfig(
-        'differential.custom-remarkup-block-rules'),
       'differential.diff' => idx($options, 'differential.diff'),
     ));
   }
@@ -381,8 +386,6 @@
       'pygments'      => PhabricatorEnv::getEnvConfig('pygments.enabled'),
       'youtube'       => PhabricatorEnv::getEnvConfig(
         'remarkup.enable-embedded-youtube'),
-      'custom-inline' => array(),
-      'custom-block'  => array(),
       'differential.diff' => null,
       'header.generate-toc' => false,
       'macros'        => true,
@@ -419,12 +422,6 @@
     $rules[] = new PhutilRemarkupRuleEscapeRemarkup();
     $rules[] = new PhutilRemarkupRuleMonospace();
 
-    $custom_rule_classes = $options['custom-inline'];
-    if ($custom_rule_classes) {
-      foreach ($custom_rule_classes as $custom_rule_class) {
-        $rules[] = newv($custom_rule_class, array());
-      }
-    }
 
     $rules[] = new PhutilRemarkupRuleDocumentLink();
 
@@ -450,6 +447,10 @@
     $rules[] = new PhutilRemarkupRuleItalic();
     $rules[] = new PhutilRemarkupRuleDel();
 
+    foreach (self::loadCustomInlineRules() as $rule) {
+      $rules[] = $rule;
+    }
+
     $blocks = array();
     $blocks[] = new PhutilRemarkupEngineRemarkupQuotesBlockRule();
     $blocks[] = new PhutilRemarkupEngineRemarkupLiteralBlockRule();
@@ -461,16 +462,12 @@
     $blocks[] = new PhutilRemarkupEngineRemarkupTableBlockRule();
     $blocks[] = new PhutilRemarkupEngineRemarkupSimpleTableBlockRule();
     $blocks[] = new PhutilRemarkupEngineRemarkupInterpreterRule();
+    $blocks[] = new PhutilRemarkupEngineRemarkupDefaultBlockRule();
 
-    $custom_block_rule_classes = $options['custom-block'];
-    if ($custom_block_rule_classes) {
-      foreach ($custom_block_rule_classes as $custom_block_rule_class) {
-        $blocks[] = newv($custom_block_rule_class, array());
-      }
+    foreach (self::loadCustomBlockRules() as $rule) {
+      $blocks[] = $rule;
     }
 
-    $blocks[] = new PhutilRemarkupEngineRemarkupDefaultBlockRule();
-
     foreach ($blocks as $block) {
       $block->setMarkupRules($rules);
     }
@@ -564,4 +561,16 @@
     return $best;
   }
 
+  private static function loadCustomInlineRules() {
+    return id(new PhutilSymbolLoader())
+      ->setAncestorClass('PhabricatorRemarkupCustomInlineRule')
+      ->loadObjects();
+  }
+
+  private static function loadCustomBlockRules() {
+    return id(new PhutilSymbolLoader())
+      ->setAncestorClass('PhabricatorRemarkupCustomBlockRule')
+      ->loadObjects();
+  }
+
 }
Index: src/infrastructure/markup/rule/PhabricatorRemarkupCustomBlockRule.php
===================================================================
--- /dev/null
+++ src/infrastructure/markup/rule/PhabricatorRemarkupCustomBlockRule.php
@@ -0,0 +1,10 @@
+<?php
+
+abstract class PhabricatorRemarkupCustomBlockRule
+  extends PhutilRemarkupEngineBlockRule {
+
+  public function getRuleVersion() {
+    return 1;
+  }
+
+}
Index: src/infrastructure/markup/rule/PhabricatorRemarkupCustomInlineRule.php
===================================================================
--- /dev/null
+++ src/infrastructure/markup/rule/PhabricatorRemarkupCustomInlineRule.php
@@ -0,0 +1,10 @@
+<?php
+
+abstract class PhabricatorRemarkupCustomInlineRule
+  extends PhutilRemarkupRule {
+
+  public function getRuleVersion() {
+    return 1;
+  }
+
+}