diff --git a/src/applications/maniphest/editor/ManiphestEditEngine.php b/src/applications/maniphest/editor/ManiphestEditEngine.php
index ada1eef8d1..9f78e0c4b8 100644
--- a/src/applications/maniphest/editor/ManiphestEditEngine.php
+++ b/src/applications/maniphest/editor/ManiphestEditEngine.php
@@ -1,122 +1,114 @@
 <?php
 
 final class ManiphestEditEngine
   extends PhabricatorEditEngine {
 
   const ENGINECONST = 'maniphest.task';
 
   public function getEngineName() {
     return pht('Maniphest Tasks');
   }
 
   public function getEngineApplicationClass() {
     return 'PhabricatorManiphestApplication';
   }
 
   protected function newEditableObject() {
     return ManiphestTask::initializeNewTask($this->getViewer());
   }
 
   protected function newObjectQuery() {
     return id(new ManiphestTaskQuery());
   }
 
   protected function getObjectCreateTitleText($object) {
     return pht('Create New Task');
   }
 
   protected function getObjectEditTitleText($object) {
     return pht('Edit %s %s', $object->getMonogram(), $object->getTitle());
   }
 
   protected function getObjectEditShortText($object) {
     return $object->getMonogram();
   }
 
   protected function getObjectCreateShortText() {
     return pht('Create Task');
   }
 
   protected function getCommentViewHeaderText($object) {
     $is_serious = PhabricatorEnv::getEnvConfig('phabricator.serious-business');
     if (!$is_serious) {
       return pht('Weigh In');
     }
 
     return parent::getCommentViewHeaderText($object);
   }
 
   protected function getObjectViewURI($object) {
     return '/'.$object->getMonogram();
   }
 
   protected function buildCustomEditFields($object) {
     // See T4819.
     $status_map = ManiphestTaskStatus::getTaskStatusMap();
     $dup_status = ManiphestTaskStatus::getDuplicateStatus();
     if ($object->getStatus() != $dup_status) {
       unset($status_map[$dup_status]);
     }
 
-    $owner_phid = $object->getOwnerPHID();
-    if ($owner_phid) {
-      $owner_value = array($owner_phid);
-    } else {
-      $owner_value = array();
-    }
-
     $priority_map = ManiphestTaskPriority::getTaskPriorityMap();
 
     // TODO: Restore these or toss them:
-    // - Require a single owner.
     // - Default owner to viewer.
     // - Don't show "change status" for closed tasks.
     // - Don't show "change owner" for closed tasks.
     // - Don't let users change a task status to "Duplicate".
     // - When closing an unassigned task, assign the closing user.
     // - Make sure implicit CCs on actions are working reasonably.
 
     return array(
       id(new PhabricatorTextEditField())
         ->setKey('title')
         ->setLabel(pht('Title'))
         ->setDescription(pht('Name of the task.'))
         ->setTransactionType(ManiphestTransaction::TYPE_TITLE)
         ->setIsRequired(true)
         ->setValue($object->getTitle()),
       id(new PhabricatorSelectEditField())
         ->setKey('status')
         ->setLabel(pht('Status'))
         ->setDescription(pht('Status of the task.'))
         ->setTransactionType(ManiphestTransaction::TYPE_STATUS)
         ->setValue($object->getStatus())
         ->setOptions($status_map),
       id(new PhabricatorUsersEditField())
-        ->setKey('assigned')
-        ->setAliases(array('assign', 'assignee'))
+        ->setKey('owner')
+        ->setAliases(array('ownerPHID', 'assign', 'assigned'))
         ->setLabel(pht('Assigned To'))
         ->setDescription(pht('User who is responsible for the task.'))
         ->setTransactionType(ManiphestTransaction::TYPE_OWNER)
-        ->setValue($owner_value),
+        ->setSingleValue($object->getOwnerPHID()),
       id(new PhabricatorSelectEditField())
         ->setKey('priority')
         ->setLabel(pht('Priority'))
         ->setDescription(pht('Priority of the task.'))
         ->setTransactionType(ManiphestTransaction::TYPE_PRIORITY)
         ->setValue($object->getPriority())
         ->setOptions($priority_map),
       id(new PhabricatorRemarkupEditField())
         ->setKey('description')
         ->setLabel(pht('Description'))
         ->setDescription(pht('Task description.'))
         ->setTransactionType(ManiphestTransaction::TYPE_DESCRIPTION)
         ->setValue($object->getDescription()),
     );
   }
 
   protected function getEditorURI() {
     // TODO: Remove when cutting over.
     return $this->getApplication()->getApplicationURI('editpro/');
   }
 
 }
diff --git a/src/applications/transactions/editfield/PhabricatorPHIDListEditField.php b/src/applications/transactions/editfield/PhabricatorPHIDListEditField.php
index b1ce19a1e0..85e0dc8fb1 100644
--- a/src/applications/transactions/editfield/PhabricatorPHIDListEditField.php
+++ b/src/applications/transactions/editfield/PhabricatorPHIDListEditField.php
@@ -1,114 +1,144 @@
 <?php
 
 abstract class PhabricatorPHIDListEditField
   extends PhabricatorEditField {
 
   private $useEdgeTransactions;
   private $transactionDescriptions = array();
+  private $isSingleValue;
 
   public function setUseEdgeTransactions($use_edge_transactions) {
     $this->useEdgeTransactions = $use_edge_transactions;
     return $this;
   }
 
   public function getUseEdgeTransactions() {
     return $this->useEdgeTransactions;
   }
 
   public function setEdgeTransactionDescriptions($add, $rem, $set) {
     $this->transactionDescriptions = array(
       '+' => $add,
       '-' => $rem,
       '=' => $set,
     );
     return $this;
   }
 
+  public function setSingleValue($value) {
+    if ($value === null) {
+      $value = array();
+    } else {
+      $value = array($value);
+    }
+
+    $this->isSingleValue = true;
+    return $this->setValue($value);
+  }
+
+  public function getIsSingleValue() {
+    return $this->isSingleValue;
+  }
+
   protected function newHTTPParameterType() {
     return new AphrontPHIDListHTTPParameterType();
   }
 
   public function getValueForTransaction() {
     $new = parent::getValueForTransaction();
 
+    if ($this->getIsSingleValue()) {
+      if ($new) {
+        return head($new);
+      } else {
+        return null;
+      }
+    }
+
     if (!$this->getUseEdgeTransactions()) {
       return $new;
     }
 
     $old = $this->getInitialValue();
     if ($old === null) {
       return array(
         '=' => array_fuse($new),
       );
     }
 
     // If we're building an edge transaction and the request has data about the
     // original value the user saw when they loaded the form, interpret the
     // edit as a mixture of "+" and "-" operations instead of a single "="
     // operation. This limits our exposure to race conditions by making most
     // concurrent edits merge correctly.
 
     $add = array_diff($new, $old);
     $rem = array_diff($old, $new);
 
     $value = array();
 
     if ($add) {
       $value['+'] = array_fuse($add);
     }
     if ($rem) {
       $value['-'] = array_fuse($rem);
     }
 
     return $value;
   }
 
   protected function newEditType() {
     if ($this->getUseEdgeTransactions()) {
       return new PhabricatorEdgeEditType();
     }
 
-    return parent::newEditType();
+    $type = parent::newEditType();
+
+    if ($this->getIsSingleValue()) {
+      $type->setValueType('phid');
+    }
+
+    return $type;
   }
 
   public function getConduitEditTypes() {
     if (!$this->getUseEdgeTransactions()) {
       return parent::getConduitEditTypes();
     }
 
     $transaction_type = $this->getTransactionType();
     if ($transaction_type === null) {
       return array();
     }
 
     $type_key = $this->getEditTypeKey();
     $strings = $this->transactionDescriptions;
 
     $base = $this->getEditType();
 
     $add = id(clone $base)
       ->setEditType($type_key.'.add')
       ->setEdgeOperation('+')
       ->setDescription(idx($strings, '+'))
       ->setValueDescription(pht('List of PHIDs to add.'));
 
     $rem = id(clone $base)
       ->setEditType($type_key.'.remove')
       ->setEdgeOperation('-')
       ->setDescription(idx($strings, '-'))
       ->setValueDescription(pht('List of PHIDs to remove.'));
 
     $set = id(clone $base)
       ->setEditType($type_key.'.set')
       ->setEdgeOperation('=')
       ->setDescription(idx($strings, '='))
       ->setValueDescription(pht('List of PHIDs to set.'));
 
     return array(
       $add,
       $rem,
       $set,
     );
   }
 
 }
diff --git a/src/applications/transactions/editfield/PhabricatorTokenizerEditField.php b/src/applications/transactions/editfield/PhabricatorTokenizerEditField.php
index 3fe45fc5b6..0ac5ba5083 100644
--- a/src/applications/transactions/editfield/PhabricatorTokenizerEditField.php
+++ b/src/applications/transactions/editfield/PhabricatorTokenizerEditField.php
@@ -1,73 +1,77 @@
 <?php
 
 abstract class PhabricatorTokenizerEditField
   extends PhabricatorPHIDListEditField {
 
   private $commentActionLabel;
 
   abstract protected function newDatasource();
 
   public function setCommentActionLabel($label) {
     $this->commentActionLabel = $label;
     return $this;
   }
 
   public function getCommentActionLabel() {
     return $this->commentActionLabel;
   }
 
   protected function newControl() {
     $control = id(new AphrontFormTokenizerControl())
       ->setDatasource($this->newDatasource());
 
     $initial_value = $this->getInitialValue();
     if ($initial_value !== null) {
       $control->setOriginalValue($initial_value);
     }
 
+    if ($this->getIsSingleValue()) {
+      $control->setLimit(1);
+    }
+
     return $control;
   }
 
   protected function getInitialValueFromSubmit(AphrontRequest $request, $key) {
     return $request->getArr($key.'.original');
   }
 
   protected function newEditType() {
     $type = parent::newEditType();
 
     if ($this->getUseEdgeTransactions()) {
       $datasource = $this->newDatasource()
         ->setViewer($this->getViewer());
       $type->setDatasource($datasource);
     }
 
     return $type;
   }
 
   public function getCommentEditTypes() {
     if (!$this->getUseEdgeTransactions()) {
       return parent::getCommentEditTypes();
     }
 
     $transaction_type = $this->getTransactionType();
     if ($transaction_type === null) {
       return array();
     }
 
     $label = $this->getCommentActionLabel();
     if ($label === null) {
       return array();
     }
 
     $type_key = $this->getEditTypeKey();
     $base = $this->getEditType();
 
     $add = id(clone $base)
       ->setEditType($type_key.'.add')
       ->setEdgeOperation('+')
       ->setLabel($label);
 
     return array($add);
   }
 
 }