diff --git a/src/workflow/ArcanistBackoutWorkflow.php b/src/workflow/ArcanistBackoutWorkflow.php
--- a/src/workflow/ArcanistBackoutWorkflow.php
+++ b/src/workflow/ArcanistBackoutWorkflow.php
@@ -88,15 +88,16 @@
    */
   private function getDiffusionCommit($commit_id) {
     $result = $this->getConduit()->callMethodSynchronous(
-      'diffusion.getcommits',
+      'diffusion.querycommits',
       array(
-        'commits' => array($commit_id),
+        'names' => array($commit_id),
       ));
-    $commit = $result[$commit_id];
+    $phid = idx($result['identifierMap'], $commit_id);
     // This commit was not found in Diffusion
-    if (array_key_exists('error', $commit)) {
+    if (!$phid) {
       return null;
     }
+    $commit = $result['data'][$phid];
     return $commit;
   }
 
@@ -157,7 +158,7 @@
       $revision_id = $matches[1];
       $commit_id = $this->getCommitIDFromRevisionID($revision_id);
       $commit = $this->getDiffusionCommit($commit_id);
-      $commit_hash = $commit['commitIdentifier'];
+      $commit_hash = $commit['identifier'];
       // Convert commit hash from SVN to Git/HG (for FB case)
       if ($is_git_svn || $is_hg_svn) {
         $commit_hash = $repository_api->
diff --git a/src/workflow/ArcanistWorkflow.php b/src/workflow/ArcanistWorkflow.php
--- a/src/workflow/ArcanistWorkflow.php
+++ b/src/workflow/ArcanistWorkflow.php
@@ -1008,10 +1008,12 @@
     $repository = $this->loadProjectRepository();
     if ($repository) {
       $callsign = $repository['callsign'];
-      $known_commits = $this->getConduit()->callMethodSynchronous(
-        'diffusion.getcommits',
-        array('commits' => array('r'.$callsign.$commit['commit'])));
-      if (ifilter($known_commits, 'error', $negate = true)) {
+      $commit_name = 'r'.$callsign.$commit['commit'];
+      $result = $this->getConduit()->callMethodSynchronous(
+        'diffusion.querycommits',
+        array('names' => array($commit_name)));
+      $known_commit = idx($result['identifierMap'], $commit_name);
+      if (!$known_commit) {
         return false;
       }
     }