diff --git a/webdav-derivative-guard.php b/webdav-derivative-guard.php
new file mode 100644
index 0000000..41f73ce
--- /dev/null
+++ b/webdav-derivative-guard.php
@@ -0,0 +1,70 @@
+<?php
+define('PHPWG_ROOT_PATH', '../../');
+include_once(PHPWG_ROOT_PATH.'include/common.inc.php');
+require_once(PHPWG_ROOT_PATH.'include/derivative.inc.php');
+
+if (!defined('BRATONIEN_TOOLS_PATH'))
+{
+  define('BRATONIEN_TOOLS_ID', basename(__DIR__));
+  define('BRATONIEN_TOOLS_PATH', PHPWG_ROOT_PATH.'plugins/'.BRATONIEN_TOOLS_ID.'/');
+}
+require_once(BRATONIEN_TOOLS_PATH.'include/webdav_materialize_runtime.inc.php');
+
+$image_id = (int)($_GET['id'] ?? 0);
+if ($image_id < 1)
+{
+  http_response_code(400);
+  header('Content-Type: text/plain; charset=utf-8');
+  echo 'Ungueltige Derivatanforderung.';
+  exit;
+}
+
+$source = bratonien_tools_webdav_materialize_source_info($image_id);
+if (!$source)
+{
+  http_response_code(404);
+  header('Content-Type: text/plain; charset=utf-8');
+  echo 'WebDAV-Bildquelle nicht gefunden.';
+  exit;
+}
+
+$state_dir = rtrim((string)($source['state_dir'] ?? ''), '/');
+if ($state_dir === '')
+{
+  http_response_code(503);
+  header('Content-Type: text/plain; charset=utf-8');
+  echo 'Connector-State-Verzeichnis fehlt.';
+  exit;
+}
+
+$lock = @fopen($state_dir.'/webdav-sync.lock', 'c');
+if (!$lock)
+{
+  http_response_code(503);
+  header('Content-Type: text/plain; charset=utf-8');
+  echo 'Connector-Sync-Lock konnte nicht geoeffnet werden.';
+  exit;
+}
+
+// On-Demand bleibt parallel zu anderen Bildabrufen moeglich, verhindert aber
+// fuer die Dauer dieses einzelnen Derivataufrufs einen exklusiven Connector-
+// Tree-Swap. Der bestehende Bild-Lock im eigentlichen Endpoint koordiniert
+// weiterhin On-Demand und Warmup fuer dieselbe Bild-ID.
+if (!@flock($lock, LOCK_SH))
+{
+  fclose($lock);
+  http_response_code(503);
+  header('Content-Type: text/plain; charset=utf-8');
+  echo 'Connector-Sync-Schutz konnte nicht gesetzt werden.';
+  exit;
+}
+
+try
+{
+  include BRATONIEN_TOOLS_PATH.'webdav-derivative.php';
+}
+finally
+{
+  @flock($lock, LOCK_UN);
+  fclose($lock);
+}