diff --git a/src/applications/conduit/controller/PhabricatorConduitAPIController.php b/src/applications/conduit/controller/PhabricatorConduitAPIController.php
--- a/src/applications/conduit/controller/PhabricatorConduitAPIController.php
+++ b/src/applications/conduit/controller/PhabricatorConduitAPIController.php
@@ -211,9 +211,14 @@
         ->withIsActive(true)
         ->executeOne();
       if (!$stored_key) {
+        $key_summary = id(new PhutilUTF8StringTruncator())
+          ->setMaximumBytes(64)
+          ->truncateString($raw_key);
         return array(
           'ERR-INVALID-AUTH',
-          pht('No user or device is associated with that public key.'),
+          pht(
+            'No user or device is associated with the public key "%s".',
+            $key_summary),
         );
       }