diff --git a/src/Halo/HaloRenderer.php b/src/Halo/HaloRenderer.php
index 1dc7bdd..7cf6e93 100644
--- a/src/Halo/HaloRenderer.php
+++ b/src/Halo/HaloRenderer.php
@@ -15,6 +15,7 @@
 
 use function array_walk_recursive;
 use function assert;
+use function error_log;
 use function explode;
 use function get_class;
 use function highlight_string;
@@ -63,6 +64,8 @@ public function __construct(
     public function render(ResourceObject $ro)
     {
         if (! $this->isEableHalo()) {
+            error_log('Debug: ?_halo=1 query enables Halo. See https://bearsunday.github.io/manuals/1.0/en/html.html');
+
             return $this->renderer->render($ro);
         }