diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -1844,6 +1844,7 @@
     'PhabricatorChatLogQuery' => 'applications/chatlog/query/PhabricatorChatLogQuery.php',
     'PhabricatorChunkedFileStorageEngine' => 'applications/files/engine/PhabricatorChunkedFileStorageEngine.php',
     'PhabricatorClusterConfigOptions' => 'applications/config/option/PhabricatorClusterConfigOptions.php',
+    'PhabricatorCommentEditEngineExtension' => 'applications/transactions/editengineextension/PhabricatorCommentEditEngineExtension.php',
     'PhabricatorCommentEditField' => 'applications/transactions/editfield/PhabricatorCommentEditField.php',
     'PhabricatorCommentEditType' => 'applications/transactions/edittype/PhabricatorCommentEditType.php',
     'PhabricatorCommitBranchesField' => 'applications/repository/customfield/PhabricatorCommitBranchesField.php',
@@ -2132,6 +2133,8 @@
     'PhabricatorEditEngineConfigurationTransactionQuery' => 'applications/transactions/query/PhabricatorEditEngineConfigurationTransactionQuery.php',
     'PhabricatorEditEngineConfigurationViewController' => 'applications/transactions/controller/PhabricatorEditEngineConfigurationViewController.php',
     'PhabricatorEditEngineController' => 'applications/transactions/controller/PhabricatorEditEngineController.php',
+    'PhabricatorEditEngineExtension' => 'applications/transactions/editengineextension/PhabricatorEditEngineExtension.php',
+    'PhabricatorEditEngineExtensionModule' => 'applications/transactions/editengineextension/PhabricatorEditEngineExtensionModule.php',
     'PhabricatorEditEngineListController' => 'applications/transactions/controller/PhabricatorEditEngineListController.php',
     'PhabricatorEditEngineQuery' => 'applications/transactions/query/PhabricatorEditEngineQuery.php',
     'PhabricatorEditEngineSearchEngine' => 'applications/transactions/query/PhabricatorEditEngineSearchEngine.php',
@@ -2706,6 +2709,7 @@
     'PhabricatorPolicyDAO' => 'applications/policy/storage/PhabricatorPolicyDAO.php',
     'PhabricatorPolicyDataTestCase' => 'applications/policy/__tests__/PhabricatorPolicyDataTestCase.php',
     'PhabricatorPolicyEditController' => 'applications/policy/controller/PhabricatorPolicyEditController.php',
+    'PhabricatorPolicyEditEngineExtension' => 'applications/policy/editor/PhabricatorPolicyEditEngineExtension.php',
     'PhabricatorPolicyEditField' => 'applications/transactions/editfield/PhabricatorPolicyEditField.php',
     'PhabricatorPolicyException' => 'applications/policy/exception/PhabricatorPolicyException.php',
     'PhabricatorPolicyExplainController' => 'applications/policy/controller/PhabricatorPolicyExplainController.php',
@@ -2797,6 +2801,7 @@
     'PhabricatorProjectUserFunctionDatasource' => 'applications/project/typeahead/PhabricatorProjectUserFunctionDatasource.php',
     'PhabricatorProjectViewController' => 'applications/project/controller/PhabricatorProjectViewController.php',
     'PhabricatorProjectWatchController' => 'applications/project/controller/PhabricatorProjectWatchController.php',
+    'PhabricatorProjectsEditEngineExtension' => 'applications/project/editor/PhabricatorProjectsEditEngineExtension.php',
     'PhabricatorProjectsEditField' => 'applications/transactions/editfield/PhabricatorProjectsEditField.php',
     'PhabricatorProjectsPolicyRule' => 'applications/project/policyrule/PhabricatorProjectsPolicyRule.php',
     'PhabricatorProtocolAdapter' => 'infrastructure/daemon/bot/adapter/PhabricatorProtocolAdapter.php',
@@ -3087,6 +3092,7 @@
     'PhabricatorSubscriptionsAddSubscribersHeraldAction' => 'applications/subscriptions/herald/PhabricatorSubscriptionsAddSubscribersHeraldAction.php',
     'PhabricatorSubscriptionsApplication' => 'applications/subscriptions/application/PhabricatorSubscriptionsApplication.php',
     'PhabricatorSubscriptionsEditController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsEditController.php',
+    'PhabricatorSubscriptionsEditEngineExtension' => 'applications/subscriptions/editor/PhabricatorSubscriptionsEditEngineExtension.php',
     'PhabricatorSubscriptionsEditor' => 'applications/subscriptions/editor/PhabricatorSubscriptionsEditor.php',
     'PhabricatorSubscriptionsHeraldAction' => 'applications/subscriptions/herald/PhabricatorSubscriptionsHeraldAction.php',
     'PhabricatorSubscriptionsListController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsListController.php',
@@ -5896,6 +5902,7 @@
     'PhabricatorChatLogQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
     'PhabricatorChunkedFileStorageEngine' => 'PhabricatorFileStorageEngine',
     'PhabricatorClusterConfigOptions' => 'PhabricatorApplicationConfigOptions',
+    'PhabricatorCommentEditEngineExtension' => 'PhabricatorEditEngineExtension',
     'PhabricatorCommentEditField' => 'PhabricatorEditField',
     'PhabricatorCommentEditType' => 'PhabricatorEditType',
     'PhabricatorCommitBranchesField' => 'PhabricatorCommitCustomField',
@@ -6232,6 +6239,8 @@
     'PhabricatorEditEngineConfigurationTransactionQuery' => 'PhabricatorApplicationTransactionQuery',
     'PhabricatorEditEngineConfigurationViewController' => 'PhabricatorEditEngineController',
     'PhabricatorEditEngineController' => 'PhabricatorApplicationTransactionController',
+    'PhabricatorEditEngineExtension' => 'Phobject',
+    'PhabricatorEditEngineExtensionModule' => 'PhabricatorConfigModule',
     'PhabricatorEditEngineListController' => 'PhabricatorEditEngineController',
     'PhabricatorEditEngineQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
     'PhabricatorEditEngineSearchEngine' => 'PhabricatorApplicationSearchEngine',
@@ -6898,6 +6907,7 @@
     'PhabricatorPolicyDAO' => 'PhabricatorLiskDAO',
     'PhabricatorPolicyDataTestCase' => 'PhabricatorTestCase',
     'PhabricatorPolicyEditController' => 'PhabricatorPolicyController',
+    'PhabricatorPolicyEditEngineExtension' => 'PhabricatorEditEngineExtension',
     'PhabricatorPolicyEditField' => 'PhabricatorEditField',
     'PhabricatorPolicyException' => 'Exception',
     'PhabricatorPolicyExplainController' => 'PhabricatorPolicyController',
@@ -7014,6 +7024,7 @@
     'PhabricatorProjectUserFunctionDatasource' => 'PhabricatorTypeaheadCompositeDatasource',
     'PhabricatorProjectViewController' => 'PhabricatorProjectController',
     'PhabricatorProjectWatchController' => 'PhabricatorProjectController',
+    'PhabricatorProjectsEditEngineExtension' => 'PhabricatorEditEngineExtension',
     'PhabricatorProjectsEditField' => 'PhabricatorTokenizerEditField',
     'PhabricatorProjectsPolicyRule' => 'PhabricatorPolicyRule',
     'PhabricatorProtocolAdapter' => 'Phobject',
@@ -7360,6 +7371,7 @@
     'PhabricatorSubscriptionsAddSubscribersHeraldAction' => 'PhabricatorSubscriptionsHeraldAction',
     'PhabricatorSubscriptionsApplication' => 'PhabricatorApplication',
     'PhabricatorSubscriptionsEditController' => 'PhabricatorController',
+    'PhabricatorSubscriptionsEditEngineExtension' => 'PhabricatorEditEngineExtension',
     'PhabricatorSubscriptionsEditor' => 'PhabricatorEditor',
     'PhabricatorSubscriptionsHeraldAction' => 'HeraldAction',
     'PhabricatorSubscriptionsListController' => 'PhabricatorController',
diff --git a/src/applications/policy/editor/PhabricatorPolicyEditEngineExtension.php b/src/applications/policy/editor/PhabricatorPolicyEditEngineExtension.php
new file mode 100644
--- /dev/null
+++ b/src/applications/policy/editor/PhabricatorPolicyEditEngineExtension.php
@@ -0,0 +1,118 @@
+<?php
+
+final class PhabricatorPolicyEditEngineExtension
+  extends PhabricatorEditEngineExtension {
+
+  const EXTENSIONKEY = 'policy.policy';
+
+  public function getExtensionPriority() {
+    return 250;
+  }
+
+  public function isExtensionEnabled() {
+    return true;
+  }
+
+  public function getExtensionName() {
+    return pht('Policies');
+  }
+
+  public function supportsObject(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object) {
+    return ($object instanceof PhabricatorPolicyInterface);
+  }
+
+  public function buildCustomEditFields(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object) {
+
+    $viewer = $engine->getViewer();
+
+    $editor = $object->getApplicationTransactionEditor();
+    $types = $editor->getTransactionTypesForObject($object);
+    $types = array_fuse($types);
+
+    $policies = id(new PhabricatorPolicyQuery())
+      ->setViewer($viewer)
+      ->setObject($object)
+      ->execute();
+
+    $map = array(
+      PhabricatorTransactions::TYPE_VIEW_POLICY => array(
+        'key' => 'policy.view',
+        'aliases' => array('view'),
+        'capability' => PhabricatorPolicyCapability::CAN_VIEW,
+        'label' => pht('View Policy'),
+        'description' => pht('Controls who can view the object.'),
+        'edit' => 'view',
+      ),
+      PhabricatorTransactions::TYPE_EDIT_POLICY => array(
+        'key' => 'policy.edit',
+        'aliases' => array('edit'),
+        'capability' => PhabricatorPolicyCapability::CAN_EDIT,
+        'label' => pht('Edit Policy'),
+        'description' => pht('Controls who can edit the object.'),
+        'edit' => 'edit',
+      ),
+      PhabricatorTransactions::TYPE_JOIN_POLICY => array(
+        'key' => 'policy.join',
+        'aliases' => array('join'),
+        'capability' => PhabricatorPolicyCapability::CAN_JOIN,
+        'label' => pht('Join Policy'),
+        'description' => pht('Controls who can join the object.'),
+        'edit' => 'join',
+      ),
+    );
+
+    $fields = array();
+    foreach ($map as $type => $spec) {
+      if (empty($types[$type])) {
+        continue;
+      }
+
+      $capability = $spec['capability'];
+      $key = $spec['key'];
+      $aliases = $spec['aliases'];
+      $label = $spec['label'];
+      $description = $spec['description'];
+      $edit = $spec['edit'];
+
+      $policy_field = id(new PhabricatorPolicyEditField())
+        ->setKey($key)
+        ->setLabel($label)
+        ->setDescription($description)
+        ->setAliases($aliases)
+        ->setCapability($capability)
+        ->setPolicies($policies)
+        ->setTransactionType($type)
+        ->setEditTypeKey($edit)
+        ->setValue($object->getPolicy($capability));
+      $fields[] = $policy_field;
+
+      if (!($object instanceof PhabricatorSpacesInterface)) {
+        if ($capability == PhabricatorPolicyCapability::CAN_VIEW) {
+          $type_space = PhabricatorTransactions::TYPE_SPACE;
+          if (isset($types[$type_space])) {
+            $space_field = id(new PhabricatorSpaceEditField())
+              ->setKey('spacePHID')
+              ->setLabel(pht('Space'))
+              ->setEditTypeKey('space')
+              ->setDescription(
+                pht('Shifts the object in the Spaces application.'))
+              ->setIsReorderable(false)
+              ->setAliases(array('space', 'policy.space'))
+              ->setTransactionType($type_space)
+              ->setValue($object->getSpacePHID());
+            $fields[] = $space_field;
+
+            $policy_field->setSpaceField($space_field);
+          }
+        }
+      }
+    }
+
+    return $fields;
+  }
+
+}
diff --git a/src/applications/project/editor/PhabricatorProjectsEditEngineExtension.php b/src/applications/project/editor/PhabricatorProjectsEditEngineExtension.php
new file mode 100644
--- /dev/null
+++ b/src/applications/project/editor/PhabricatorProjectsEditEngineExtension.php
@@ -0,0 +1,60 @@
+<?php
+
+final class PhabricatorProjectsEditEngineExtension
+  extends PhabricatorEditEngineExtension {
+
+  const EXTENSIONKEY = 'projects.projects';
+
+  public function getExtensionPriority() {
+    return 500;
+  }
+
+  public function isExtensionEnabled() {
+    return PhabricatorApplication::isClassInstalled(
+      'PhabricatorProjectApplication');
+  }
+
+  public function getExtensionName() {
+    return pht('Projects');
+  }
+
+  public function supportsObject(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object) {
+
+    return ($object instanceof PhabricatorProjectInterface);
+  }
+
+  public function buildCustomEditFields(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object) {
+
+    $edge_type = PhabricatorTransactions::TYPE_EDGE;
+    $project_edge_type = PhabricatorProjectObjectHasProjectEdgeType::EDGECONST;
+
+    $object_phid = $object->getPHID();
+    if ($object_phid) {
+      $project_phids = PhabricatorEdgeQuery::loadDestinationPHIDs(
+        $object_phid,
+        $project_edge_type);
+      $project_phids = array_reverse($project_phids);
+    } else {
+      $project_phids = array();
+    }
+
+    $projects_field = id(new PhabricatorProjectsEditField())
+      ->setKey('projectPHIDs')
+      ->setLabel(pht('Projects'))
+      ->setEditTypeKey('projects')
+      ->setDescription(pht('Add or remove associated projects.'))
+      ->setAliases(array('project', 'projects'))
+      ->setTransactionType($edge_type)
+      ->setMetadataValue('edge:type', $project_edge_type)
+      ->setValue($project_phids);
+
+    return array(
+      $projects_field,
+    );
+  }
+
+}
diff --git a/src/applications/subscriptions/editor/PhabricatorSubscriptionsEditEngineExtension.php b/src/applications/subscriptions/editor/PhabricatorSubscriptionsEditEngineExtension.php
new file mode 100644
--- /dev/null
+++ b/src/applications/subscriptions/editor/PhabricatorSubscriptionsEditEngineExtension.php
@@ -0,0 +1,56 @@
+<?php
+
+final class PhabricatorSubscriptionsEditEngineExtension
+  extends PhabricatorEditEngineExtension {
+
+  const EXTENSIONKEY = 'subscriptions.subscribers';
+
+  public function getExtensionPriority() {
+    return 750;
+  }
+
+  public function isExtensionEnabled() {
+    return true;
+  }
+
+  public function getExtensionName() {
+    return pht('Subscriptions');
+  }
+
+  public function supportsObject(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object) {
+    return ($object instanceof PhabricatorSubscribableInterface);
+  }
+
+  public function buildCustomEditFields(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object) {
+
+    $subscribers_type = PhabricatorTransactions::TYPE_SUBSCRIBERS;
+
+    $object_phid = $object->getPHID();
+    if ($object_phid) {
+      $sub_phids = PhabricatorSubscribersQuery::loadSubscribersForPHID(
+        $object_phid);
+    } else {
+      // TODO: Allow applications to provide default subscribers? Maniphest
+      // does this at a minimum.
+      $sub_phids = array();
+    }
+
+    $subscribers_field = id(new PhabricatorSubscribersEditField())
+      ->setKey('subscriberPHIDs')
+      ->setLabel(pht('Subscribers'))
+      ->setEditTypeKey('subscribers')
+      ->setDescription(pht('Manage subscribers.'))
+      ->setAliases(array('subscriber', 'subscribers'))
+      ->setTransactionType($subscribers_type)
+      ->setValue($sub_phids);
+
+    return array(
+      $subscribers_field,
+    );
+  }
+
+}
diff --git a/src/applications/transactions/editengine/PhabricatorEditEngine.php b/src/applications/transactions/editengine/PhabricatorEditEngine.php
--- a/src/applications/transactions/editengine/PhabricatorEditEngine.php
+++ b/src/applications/transactions/editengine/PhabricatorEditEngine.php
@@ -73,163 +73,27 @@
 
   final protected function buildEditFields($object) {
     $viewer = $this->getViewer();
-    $editor = $object->getApplicationTransactionEditor();
-
-    $types = $editor->getTransactionTypesForObject($object);
-    $types = array_fuse($types);
 
     $fields = $this->buildCustomEditFields($object);
 
-    if ($object instanceof PhabricatorPolicyInterface) {
-      $policies = id(new PhabricatorPolicyQuery())
-        ->setViewer($viewer)
-        ->setObject($object)
-        ->execute();
-
-      $map = array(
-        PhabricatorTransactions::TYPE_VIEW_POLICY => array(
-          'key' => 'policy.view',
-          'aliases' => array('view'),
-          'capability' => PhabricatorPolicyCapability::CAN_VIEW,
-          'label' => pht('View Policy'),
-          'description' => pht('Controls who can view the object.'),
-          'edit' => 'view',
-        ),
-        PhabricatorTransactions::TYPE_EDIT_POLICY => array(
-          'key' => 'policy.edit',
-          'aliases' => array('edit'),
-          'capability' => PhabricatorPolicyCapability::CAN_EDIT,
-          'label' => pht('Edit Policy'),
-          'description' => pht('Controls who can edit the object.'),
-          'edit' => 'edit',
-        ),
-        PhabricatorTransactions::TYPE_JOIN_POLICY => array(
-          'key' => 'policy.join',
-          'aliases' => array('join'),
-          'capability' => PhabricatorPolicyCapability::CAN_JOIN,
-          'label' => pht('Join Policy'),
-          'description' => pht('Controls who can join the object.'),
-          'edit' => 'join',
-        ),
-      );
+    $extensions = PhabricatorEditEngineExtension::getAllEnabledExtensions();
+    foreach ($extensions as $extension) {
+      $extension->setViewer($viewer);
 
-      foreach ($map as $type => $spec) {
-        if (empty($types[$type])) {
-          continue;
-        }
-
-        $capability = $spec['capability'];
-        $key = $spec['key'];
-        $aliases = $spec['aliases'];
-        $label = $spec['label'];
-        $description = $spec['description'];
-        $edit = $spec['edit'];
-
-        $policy_field = id(new PhabricatorPolicyEditField())
-          ->setKey($key)
-          ->setLabel($label)
-          ->setDescription($description)
-          ->setAliases($aliases)
-          ->setCapability($capability)
-          ->setPolicies($policies)
-          ->setTransactionType($type)
-          ->setEditTypeKey($edit)
-          ->setValue($object->getPolicy($capability));
-        $fields[] = $policy_field;
-
-        if ($object instanceof PhabricatorSpacesInterface) {
-          if ($capability == PhabricatorPolicyCapability::CAN_VIEW) {
-            $type_space = PhabricatorTransactions::TYPE_SPACE;
-            if (isset($types[$type_space])) {
-              $space_field = id(new PhabricatorSpaceEditField())
-                ->setKey('spacePHID')
-                ->setLabel(pht('Space'))
-                ->setEditTypeKey('space')
-                ->setDescription(
-                  pht('Shifts the object in the Spaces application.'))
-                ->setIsReorderable(false)
-                ->setAliases(array('space', 'policy.space'))
-                ->setTransactionType($type_space)
-                ->setValue($object->getSpacePHID());
-              $fields[] = $space_field;
-
-              $policy_field->setSpaceField($space_field);
-            }
-          }
-        }
+      if (!$extension->supportsObject($this, $object)) {
+        continue;
       }
-    }
-
-    $edge_type = PhabricatorTransactions::TYPE_EDGE;
-    $object_phid = $object->getPHID();
-
-    $project_edge_type = PhabricatorProjectObjectHasProjectEdgeType::EDGECONST;
 
-    if ($object instanceof PhabricatorProjectInterface) {
-      if (isset($types[$edge_type])) {
-        if ($object_phid) {
-          $project_phids = PhabricatorEdgeQuery::loadDestinationPHIDs(
-            $object_phid,
-            $project_edge_type);
-          $project_phids = array_reverse($project_phids);
-        } else {
-          $project_phids = array();
-        }
-
-        $edge_field = id(new PhabricatorProjectsEditField())
-          ->setKey('projectPHIDs')
-          ->setLabel(pht('Projects'))
-          ->setEditTypeKey('projects')
-          ->setDescription(pht('Add or remove associated projects.'))
-          ->setAliases(array('project', 'projects'))
-          ->setTransactionType($edge_type)
-          ->setMetadataValue('edge:type', $project_edge_type)
-          ->setValue($project_phids);
-        $fields[] = $edge_field;
-      }
-    }
+      $extension_fields = $extension->buildCustomEditFields($this, $object);
 
-    $subscribers_type = PhabricatorTransactions::TYPE_SUBSCRIBERS;
-
-    if ($object instanceof PhabricatorSubscribableInterface) {
-      if (isset($types[$subscribers_type])) {
-        if ($object_phid) {
-          $sub_phids = PhabricatorSubscribersQuery::loadSubscribersForPHID(
-            $object_phid);
-        } else {
-          // TODO: Allow applications to provide default subscribers; Maniphest
-          // does this at a minimum.
-          $sub_phids = array();
-        }
+      // TODO: Validate this in more detail with a more tailored error.
+      assert_instances_of($extension_fields, 'PhabricatorEditField');
 
-        $subscribers_field = id(new PhabricatorSubscribersEditField())
-          ->setKey('subscriberPHIDs')
-          ->setLabel(pht('Subscribers'))
-          ->setEditTypeKey('subscribers')
-          ->setDescription(pht('Manage subscribers.'))
-          ->setAliases(array('subscriber', 'subscribers'))
-          ->setTransactionType($subscribers_type)
-          ->setValue($sub_phids);
-        $fields[] = $subscribers_field;
+      foreach ($extension_fields as $field) {
+        $fields[] = $field;
       }
     }
 
-    $xaction = $object->getApplicationTransactionTemplate();
-    $comment = $xaction->getApplicationTransactionCommentObject();
-    if ($comment) {
-      $comment_type = PhabricatorTransactions::TYPE_COMMENT;
-
-      $comment_field = id(new PhabricatorCommentEditField())
-        ->setKey('comment')
-        ->setLabel(pht('Comments'))
-        ->setDescription(pht('Add comments.'))
-        ->setAliases(array('comments'))
-        ->setIsHidden(true)
-        ->setTransactionType($comment_type)
-        ->setValue(null);
-      $fields[] = $comment_field;
-    }
-
     $config = $this->getEditEngineConfiguration();
     $fields = $config->applyConfigurationToFields($this, $fields);
 
@@ -678,6 +542,8 @@
     $validation_exception = null;
     if ($request->isFormPost()) {
       foreach ($fields as $field) {
+        $field->setIsSubmittedForm(true);
+
         if ($field->getIsLocked() || $field->getIsHidden()) {
           continue;
         }
@@ -709,6 +575,20 @@
           ->setURI($this->getObjectViewURI($object));
       } catch (PhabricatorApplicationTransactionValidationException $ex) {
         $validation_exception = $ex;
+
+        foreach ($fields as $field) {
+          $xaction_type = $field->getTransactionType();
+          if ($xaction_type === null) {
+            continue;
+          }
+
+          $message = $ex->getShortMessage($xaction_type);
+          if ($message === null) {
+            continue;
+          }
+
+          $field->setControlError($message);
+        }
       }
     } else {
       if ($this->getIsCreate()) {
diff --git a/src/applications/transactions/editengineextension/PhabricatorCommentEditEngineExtension.php b/src/applications/transactions/editengineextension/PhabricatorCommentEditEngineExtension.php
new file mode 100644
--- /dev/null
+++ b/src/applications/transactions/editengineextension/PhabricatorCommentEditEngineExtension.php
@@ -0,0 +1,55 @@
+<?php
+
+final class PhabricatorCommentEditEngineExtension
+  extends PhabricatorEditEngineExtension {
+
+  const EXTENSIONKEY = 'transactions.comment';
+
+  public function getExtensionPriority() {
+    return 9000;
+  }
+
+  public function isExtensionEnabled() {
+    return true;
+  }
+
+  public function getExtensionName() {
+    return pht('Comments');
+  }
+
+  public function supportsObject(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object) {
+
+    $xaction = $object->getApplicationTransactionTemplate();
+
+    try {
+      $comment = $xaction->getApplicationTransactionCommentObject();
+    } catch (PhutilMethodNotImplementedException $ex) {
+      $comment = null;
+    }
+
+    return (bool)$comment;
+  }
+
+  public function buildCustomEditFields(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object) {
+
+    $comment_type = PhabricatorTransactions::TYPE_COMMENT;
+
+    $comment_field = id(new PhabricatorCommentEditField())
+      ->setKey('comment')
+      ->setLabel(pht('Comments'))
+      ->setDescription(pht('Add comments.'))
+      ->setAliases(array('comments'))
+      ->setIsHidden(true)
+      ->setTransactionType($comment_type)
+      ->setValue(null);
+
+    return array(
+      $comment_field,
+    );
+  }
+
+}
diff --git a/src/applications/transactions/editengineextension/PhabricatorEditEngineExtension.php b/src/applications/transactions/editengineextension/PhabricatorEditEngineExtension.php
new file mode 100644
--- /dev/null
+++ b/src/applications/transactions/editengineextension/PhabricatorEditEngineExtension.php
@@ -0,0 +1,55 @@
+<?php
+
+abstract class PhabricatorEditEngineExtension extends Phobject {
+
+  private $viewer;
+
+  final public function getExtensionKey() {
+    return $this->getPhobjectClassConstant('EXTENSIONKEY');
+  }
+
+  final public function setViewer($viewer) {
+    $this->viewer = $viewer;
+    return $this;
+  }
+
+  final public function getViewer() {
+    return $this->viewer;
+  }
+
+  public function getExtensionPriority() {
+    return 1000;
+  }
+
+  abstract public function isExtensionEnabled();
+  abstract public function getExtensionName();
+
+  abstract public function supportsObject(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object);
+
+  abstract public function buildCustomEditFields(
+    PhabricatorEditEngine $engine,
+    PhabricatorApplicationTransactionInterface $object);
+
+  final public static function getAllExtensions() {
+    return id(new PhutilClassMapQuery())
+      ->setAncestorClass(__CLASS__)
+      ->setUniqueMethod('getExtensionKey')
+      ->setSortMethod('getExtensionPriority')
+      ->execute();
+  }
+
+  final public static function getAllEnabledExtensions() {
+    $extensions = self::getAllExtensions();
+
+    foreach ($extensions as $key => $extension) {
+      if (!$extension->isExtensionEnabled()) {
+        unset($extensions[$key]);
+      }
+    }
+
+    return $extensions;
+  }
+
+}
diff --git a/src/applications/transactions/editengineextension/PhabricatorEditEngineExtensionModule.php b/src/applications/transactions/editengineextension/PhabricatorEditEngineExtensionModule.php
new file mode 100644
--- /dev/null
+++ b/src/applications/transactions/editengineextension/PhabricatorEditEngineExtensionModule.php
@@ -0,0 +1,52 @@
+<?php
+
+final class PhabricatorEditEngineExtensionModule
+  extends PhabricatorConfigModule {
+
+  public function getModuleKey() {
+    return 'editengine';
+  }
+
+  public function getModuleName() {
+    return pht('EditEngine Extensions');
+  }
+
+  public function renderModuleStatus(AphrontRequest $request) {
+    $viewer = $request->getViewer();
+
+    $extensions = PhabricatorEditEngineExtension::getAllExtensions();
+
+    $rows = array();
+    foreach ($extensions as $extension) {
+      $rows[] = array(
+        $extension->getExtensionPriority(),
+        get_class($extension),
+        $extension->getExtensionName(),
+        $extension->isExtensionEnabled()
+          ? pht('Yes')
+          : pht('No'),
+      );
+    }
+
+    $table = id(new AphrontTableView($rows))
+      ->setHeaders(
+        array(
+          pht('Priority'),
+          pht('Class'),
+          pht('Name'),
+          pht('Enabled'),
+        ))
+      ->setColumnClasses(
+        array(
+          null,
+          null,
+          'wide pri',
+          null,
+        ));
+
+    return id(new PHUIObjectBoxView())
+      ->setHeaderText(pht('EditEngine Extensions'))
+      ->setTable($table);
+  }
+
+}
diff --git a/src/applications/transactions/editfield/PhabricatorDatasourceEditField.php b/src/applications/transactions/editfield/PhabricatorDatasourceEditField.php
--- a/src/applications/transactions/editfield/PhabricatorDatasourceEditField.php
+++ b/src/applications/transactions/editfield/PhabricatorDatasourceEditField.php
@@ -11,6 +11,9 @@
   }
 
   public function getDatasource() {
+    if (!$this->datasource) {
+      throw new PhutilInvalidStateException('setDatasource');
+    }
     return $this->datasource;
   }
 
diff --git a/src/applications/transactions/editfield/PhabricatorEditField.php b/src/applications/transactions/editfield/PhabricatorEditField.php
--- a/src/applications/transactions/editfield/PhabricatorEditField.php
+++ b/src/applications/transactions/editfield/PhabricatorEditField.php
@@ -13,12 +13,15 @@
   private $metadata = array();
   private $description;
   private $editTypeKey;
+  private $isRequired;
 
   private $isLocked;
   private $isHidden;
 
   private $isPreview;
   private $isEditDefaults;
+  private $isSubmittedForm;
+  private $controlError;
 
   private $isReorderable = true;
   private $isDefaultable = true;
@@ -145,6 +148,33 @@
     throw new PhutilMethodNotImplementedException();
   }
 
+  public function setIsSubmittedForm($is_submitted) {
+    $this->isSubmittedForm = $is_submitted;
+    return $this;
+  }
+
+  public function getIsSubmittedForm() {
+    return $this->isSubmittedForm;
+  }
+
+  public function setIsRequired($is_required) {
+    $this->isRequired = $is_required;
+    return $this;
+  }
+
+  public function getIsRequired() {
+    return $this->isRequired;
+  }
+
+  public function setControlError($control_error) {
+    $this->controlError = $control_error;
+    return $this;
+  }
+
+  public function getControlError() {
+    return $this->controlError;
+  }
+
   protected function renderControl() {
     $control = $this->newControl();
     if ($control === null) {
@@ -176,6 +206,16 @@
 
     $control->setDisabled($disabled);
 
+
+    if ($this->getIsSubmittedForm()) {
+      $error = $this->getControlError();
+      if ($error !== null) {
+        $control->setError($error);
+      }
+    } else if ($this->getIsRequired()) {
+      $control->setError(true);
+    }
+
     return $control;
   }