diff --git a/src/applications/config/option/PhabricatorCoreConfigOptions.php b/src/applications/config/option/PhabricatorCoreConfigOptions.php
--- a/src/applications/config/option/PhabricatorCoreConfigOptions.php
+++ b/src/applications/config/option/PhabricatorCoreConfigOptions.php
@@ -92,6 +92,7 @@
             'create a collision preventing you from logging in.'))
         ->addExample('dev', pht('Prefix cookie with "dev"')),
       $this->newOption('phabricator.show-prototypes', 'bool', false)
+        ->setLocked(true)
         ->setBoolOptions(
           array(
             pht('Enable Prototypes'),