diff --git a/src/applications/differential/xaction/DifferentialRevisionPlanChangesTransaction.php b/src/applications/differential/xaction/DifferentialRevisionPlanChangesTransaction.php
index 231adc5bf8..e53426c8ba 100644
--- a/src/applications/differential/xaction/DifferentialRevisionPlanChangesTransaction.php
+++ b/src/applications/differential/xaction/DifferentialRevisionPlanChangesTransaction.php
@@ -1,100 +1,111 @@
 <?php
 
 final class DifferentialRevisionPlanChangesTransaction
   extends DifferentialRevisionActionTransaction {
 
   const TRANSACTIONTYPE = 'differential.revision.plan';
   const ACTIONKEY = 'plan-changes';
 
   protected function getRevisionActionLabel() {
     return pht('Plan Changes');
   }
 
   protected function getRevisionActionDescription(
     DifferentialRevision $revision) {
     return pht(
       'This revision will be removed from review queues until it is revised.');
   }
 
   public function getIcon() {
     return 'fa-headphones';
   }
 
   public function getColor() {
     return 'red';
   }
 
   protected function getRevisionActionOrder() {
     return 200;
   }
 
   public function getActionName() {
     return pht('Planned Changes');
   }
 
   public function getCommandKeyword() {
     return 'planchanges';
   }
 
   public function getCommandAliases() {
     return array(
       'rethink',
     );
   }
 
   public function getCommandSummary() {
     return pht('Plan changes to a revision.');
   }
 
   public function generateOldValue($object) {
     return $object->isChangePlanned();
   }
 
   public function applyInternalEffects($object, $value) {
     $status_planned = DifferentialRevisionStatus::CHANGES_PLANNED;
     $object->setModernRevisionStatus($status_planned);
   }
 
   protected function validateAction($object, PhabricatorUser $viewer) {
     if ($object->isDraft()) {
-      throw new Exception(
-        pht('You can not plan changes to a draft revision.'));
+
+      // See PHI346. Until the "Draft" state fully unprototypes, allow drafts
+      // to be moved to "changes planned" via the API. This preserves the
+      // behavior of "arc diff --plan-changes". We still prevent this
+      // transition from the web UI.
+      // TODO: Remove this once drafts leave prototype.
+
+      $editor = $this->getEditor();
+      $type_web = PhabricatorWebContentSource::SOURCECONST;
+      if ($editor->getContentSource()->getSource() == $type_web) {
+        throw new Exception(
+          pht('You can not plan changes to a draft revision.'));
+      }
     }
 
     if ($object->isChangePlanned()) {
       throw new Exception(
         pht(
           'You can not request review of this revision because this '.
           'revision is already under review and the action would have '.
           'no effect.'));
     }
 
     if ($object->isClosed()) {
       throw new Exception(
         pht(
           'You can not plan changes to this this revision because it has '.
           'already been closed.'));
     }
 
     if (!$this->isViewerRevisionAuthor($object, $viewer)) {
       throw new Exception(
         pht(
           'You can not plan changes to this revision because you do not '.
           'own it. Only the author of a revision can plan changes to it.'));
     }
   }
 
   public function getTitle() {
     return pht(
       '%s planned changes to this revision.',
       $this->renderAuthor());
   }
 
   public function getTitleForFeed() {
     return pht(
       '%s planned changes to %s.',
       $this->renderAuthor(),
       $this->renderObject());
   }
 
 }