diff --git a/webdav-derivative-test.php b/webdav-derivative-test.php
new file mode 100644
index 0000000..ff35f90
--- /dev/null
+++ b/webdav-derivative-test.php
@@ -0,0 +1,350 @@
+<?php
+define('PHPWG_ROOT_PATH', '../../');
+include_once(PHPWG_ROOT_PATH.'include/common.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_image_runtime.inc.php');
+require_once(PHPWG_ROOT_PATH.'include/derivative.inc.php');
+
+function bratonien_tools_webdav_derivative_test_abort($status, $message)
+{
+  http_response_code((int)$status);
+  header('Content-Type: text/plain; charset=utf-8');
+  header('Cache-Control: no-store');
+  echo $message;
+  exit;
+}
+
+function bratonien_tools_webdav_derivative_test_decrypt_secret($blob, $hex_key)
+{
+  $hex_key = trim((string)$hex_key);
+  if (!preg_match('/^[a-f0-9]{64}$/', $hex_key)) return null;
+
+  $outer = base64_decode(trim((string)$blob), true);
+  $payload = is_string($outer) ? json_decode($outer, true) : null;
+  if (!is_array($payload) || (int)($payload['v'] ?? 0) !== 1) return null;
+
+  $iv = base64_decode((string)($payload['iv'] ?? ''), true);
+  $tag = base64_decode((string)($payload['tag'] ?? ''), true);
+  $cipher = base64_decode((string)($payload['data'] ?? ''), true);
+  if (!is_string($iv) || !is_string($tag) || !is_string($cipher)) return null;
+
+  $plain = openssl_decrypt($cipher, 'aes-256-gcm', hex2bin($hex_key), OPENSSL_RAW_DATA, $iv, $tag);
+  if ($plain === false) return null;
+
+  $decoded = json_decode((string)$plain, true);
+  return is_array($decoded) ? $decoded : null;
+}
+
+function bratonien_tools_webdav_derivative_test_quote_path($path)
+{
+  $parts = array_values(array_filter(explode('/', trim((string)$path, '/')), 'strlen'));
+  return implode('/', array_map('rawurlencode', $parts));
+}
+
+function bratonien_tools_webdav_derivative_test_download(array $source, $destination, &$detail=null)
+{
+  $detail = '';
+  $table = $GLOBALS['prefixeTable'].'bratonien_tools_nc_connections';
+  $result = pwg_query('SELECT config_json, secret_blob FROM `'.$table.'` WHERE id='.(int)$source['connection_id'].' LIMIT 1');
+  if (!pwg_db_num_rows($result))
+  {
+    $detail = 'WebDAV-Verbindung nicht gefunden.';
+    return false;
+  }
+
+  $row = pwg_db_fetch_assoc($result);
+  $config = json_decode((string)$row['config_json'], true);
+  if (!is_array($config))
+  {
+    $detail = 'WebDAV-Konfiguration ist ungueltig.';
+    return false;
+  }
+
+  $key_result = pwg_query("SELECT value FROM ".$GLOBALS['prefixeTable']."config WHERE param='bratonien_nc_connector_secret' LIMIT 1");
+  if (!pwg_db_num_rows($key_result))
+  {
+    $detail = 'Connector-Schluessel fehlt.';
+    return false;
+  }
+  $key_row = pwg_db_fetch_assoc($key_result);
+  $credentials = bratonien_tools_webdav_derivative_test_decrypt_secret((string)$row['secret_blob'], (string)$key_row['value']);
+  if (!is_array($credentials))
+  {
+    $detail = 'WebDAV-Zugangsdaten konnten nicht gelesen werden.';
+    return false;
+  }
+
+  $base_url = rtrim((string)($config['nextcloud_url'] ?? ''), '/');
+  $user = trim((string)($credentials['nextcloud_user'] ?? ''));
+  $password = (string)($credentials['nextcloud_password'] ?? '');
+  $webdav_path = trim((string)($source['webdav_path'] ?? ''), '/');
+  if ($base_url === '' || $user === '' || $password === '' || $webdav_path === '')
+  {
+    $detail = 'WebDAV-Bildquelle ist unvollstaendig.';
+    return false;
+  }
+
+  $fp = @fopen($destination, 'xb');
+  if (!$fp)
+  {
+    $detail = 'Temporaere Quelldatei konnte nicht angelegt werden.';
+    return false;
+  }
+
+  $url = $base_url.'/remote.php/dav/files/'.rawurlencode($user).'/'.bratonien_tools_webdav_derivative_test_quote_path($webdav_path);
+  $ch = curl_init($url);
+  curl_setopt_array($ch, array(
+    CURLOPT_FOLLOWLOCATION => false,
+    CURLOPT_CONNECTTIMEOUT => 10,
+    CURLOPT_TIMEOUT => 180,
+    CURLOPT_HTTPAUTH => CURLAUTH_BASIC,
+    CURLOPT_USERPWD => $user.':'.$password,
+    CURLOPT_RETURNTRANSFER => false,
+    CURLOPT_FAILONERROR => false,
+    CURLOPT_FILE => $fp,
+    CURLOPT_USERAGENT => 'Bratonien-Tools-WebDAV-Derivative-Test/0.9.6.3',
+  ));
+
+  $ok = curl_exec($ch);
+  $errno = curl_errno($ch);
+  $error = curl_error($ch);
+  $http = (int)curl_getinfo($ch, CURLINFO_HTTP_CODE);
+  curl_close($ch);
+  fclose($fp);
+
+  if ($ok === false || $errno !== 0 || $http < 200 || $http >= 300)
+  {
+    @unlink($destination);
+    $detail = 'Nextcloud-Download fehlgeschlagen (HTTP '.$http.', cURL '.$errno.($error !== '' ? ': '.$error : '').').';
+    return false;
+  }
+
+  if (!is_file($destination) || filesize($destination) < 1 || @getimagesize($destination) === false)
+  {
+    @unlink($destination);
+    $detail = 'Nextcloud-Datei ist kein lesbares Bild.';
+    return false;
+  }
+
+  return true;
+}
+
+function bratonien_tools_webdav_derivative_test_inner_url($derivative_path)
+{
+  $derivative_root = PHPWG_ROOT_PATH.PWG_DERIVATIVE_DIR;
+  if (strpos($derivative_path, $derivative_root) !== 0) return null;
+
+  $location = ltrim(substr($derivative_path, strlen($derivative_root)), '/');
+  if ($location === '') return null;
+
+  $segments = array_map('rawurlencode', explode('/', $location));
+  return rtrim(get_absolute_root_url(), '/').'/i.php?/'.implode('/', $segments);
+}
+
+function bratonien_tools_webdav_derivative_test_call_i($url, &$response_body, &$response_type, &$response_status, &$detail=null)
+{
+  $detail = '';
+  $response_body = '';
+  $response_type = 'application/octet-stream';
+  $response_status = 500;
+
+  $ch = curl_init($url);
+  curl_setopt_array($ch, array(
+    CURLOPT_FOLLOWLOCATION => false,
+    CURLOPT_CONNECTTIMEOUT => 10,
+    CURLOPT_TIMEOUT => 180,
+    CURLOPT_RETURNTRANSFER => true,
+    CURLOPT_FAILONERROR => false,
+    CURLOPT_USERAGENT => 'Bratonien-Tools-Derivative-Gate/0.9.6.3',
+  ));
+
+  $body = curl_exec($ch);
+  $errno = curl_errno($ch);
+  $error = curl_error($ch);
+  $status = (int)curl_getinfo($ch, CURLINFO_HTTP_CODE);
+  $content_type = (string)curl_getinfo($ch, CURLINFO_CONTENT_TYPE);
+  curl_close($ch);
+
+  $response_status = $status;
+  if ($content_type !== '') $response_type = $content_type;
+  if (is_string($body)) $response_body = $body;
+
+  if ($body === false || $errno !== 0)
+  {
+    $detail = 'Interner i.php-Aufruf fehlgeschlagen (cURL '.$errno.($error !== '' ? ': '.$error : '').').';
+    return false;
+  }
+  if ($status < 200 || $status >= 400)
+  {
+    $detail = 'i.php antwortete mit HTTP '.$status.'.';
+    return false;
+  }
+
+  return true;
+}
+
+global $user;
+if (!isset($user['status']) || !in_array($user['status'], array('admin', 'webmaster'), true))
+{
+  bratonien_tools_webdav_derivative_test_abort(403, 'Dieser Test-Endpunkt ist nur fuer Administratoren verfuegbar.');
+}
+
+$image_id = (int)($_GET['id'] ?? 0);
+if ($image_id < 1)
+{
+  bratonien_tools_webdav_derivative_test_abort(400, 'Bild-ID fehlt.');
+}
+
+$type = trim((string)($_GET['type'] ?? IMG_THUMB));
+$defined = ImageStdParams::get_defined_type_map();
+if (!isset($defined[$type]))
+{
+  bratonien_tools_webdav_derivative_test_abort(400, 'Unbekannter oder deaktivierter Derivat-Typ: '.$type);
+}
+
+$result = pwg_query('SELECT * FROM '.IMAGES_TABLE.' WHERE id='.$image_id.' LIMIT 1');
+if (!pwg_db_num_rows($result))
+{
+  bratonien_tools_webdav_derivative_test_abort(404, 'Bild nicht gefunden.');
+}
+$image_row = pwg_db_fetch_assoc($result);
+$src_image = new SrcImage($image_row);
+$source = bratonien_tools_webdav_image_source_info($image_id);
+if (!$source)
+{
+  bratonien_tools_webdav_derivative_test_abort(404, 'Keine WebDAV-Quelle fuer dieses Bild gefunden.');
+}
+
+$derivative = new DerivativeImage($defined[$type], $src_image);
+if ($derivative->same_as_source())
+{
+  bratonien_tools_webdav_derivative_test_abort(409, 'Dieser Typ ist fuer das Bild identisch mit der Quelle und erzeugt kein Derivat.');
+}
+$derivative_path = $derivative->get_path();
+if (is_file($derivative_path) && is_readable($derivative_path))
+{
+  header('X-Bratonien-WebDAV-Test: derivative-already-exists');
+  header('Content-Type: '.(function_exists('mime_content_type') ? (mime_content_type($derivative_path) ?: 'application/octet-stream') : 'application/octet-stream'));
+  header('Content-Length: '.filesize($derivative_path));
+  header('Cache-Control: no-store');
+  readfile($derivative_path);
+  exit;
+}
+
+$image_path = (string)($image_row['path'] ?? '');
+$absolute_image_path = $image_path;
+if (strpos($absolute_image_path, '/') !== 0)
+{
+  $absolute_image_path = PHPWG_ROOT_PATH.ltrim(preg_replace('#^\./#', '', $absolute_image_path), '/');
+}
+$materialize_path = realpath($absolute_image_path);
+if ($materialize_path === false || !is_file($materialize_path))
+{
+  bratonien_tools_webdav_derivative_test_abort(500, 'Placeholder-Quelldatei konnte nicht aufgeloest werden.');
+}
+
+$normalized_materialize = str_replace('\\', '/', $materialize_path);
+if (!preg_match('#/nc-webdav-source/connection-'.(int)$source['connection_id'].'/#', $normalized_materialize))
+{
+  bratonien_tools_webdav_derivative_test_abort(500, 'Aufgeloester Placeholder-Pfad passt nicht zur WebDAV-Verbindung.');
+}
+
+$lock_path = $materialize_path.'.bratonien-materialize.lock';
+$lock = @fopen($lock_path, 'c');
+if (!$lock || !flock($lock, LOCK_EX))
+{
+  if ($lock) fclose($lock);
+  bratonien_tools_webdav_derivative_test_abort(503, 'Materialisierungs-Lock konnte nicht gesetzt werden.');
+}
+
+$temp_path = $materialize_path.'.bratonien-real.'.getmypid().'.'.bin2hex(random_bytes(4)).'.part';
+$backup_path = $materialize_path.'.bratonien-placeholder.'.getmypid().'.'.bin2hex(random_bytes(4));
+$materialized = false;
+$backup_created = false;
+$cleaned = false;
+
+$cleanup = function() use (&$cleaned, &$materialized, &$backup_created, $materialize_path, $backup_path, $temp_path, $lock)
+{
+  if ($cleaned) return;
+  $cleaned = true;
+
+  if ($materialized && $backup_created && is_file($backup_path))
+  {
+    @rename($backup_path, $materialize_path);
+  }
+  elseif ($backup_created && is_file($backup_path) && !is_file($materialize_path))
+  {
+    @rename($backup_path, $materialize_path);
+  }
+
+  if (is_file($temp_path)) @unlink($temp_path);
+  if (is_file($backup_path)) @unlink($backup_path);
+
+  @flock($lock, LOCK_UN);
+  @fclose($lock);
+};
+register_shutdown_function($cleanup);
+
+try
+{
+  $detail = '';
+  if (!bratonien_tools_webdav_derivative_test_download($source, $temp_path, $detail))
+  {
+    bratonien_tools_webdav_derivative_test_abort(502, $detail);
+  }
+
+  // Preserve the exact placeholder inode via a second hard link. Replacing the
+  // real source entry with rename() is then atomic and does not overwrite the
+  // shared placeholder inode used by the WebDAV source tree.
+  if (!@link($materialize_path, $backup_path))
+  {
+    bratonien_tools_webdav_derivative_test_abort(500, 'Placeholder konnte nicht sicher per Hardlink gesichert werden.');
+  }
+  $backup_created = true;
+
+  if (!@rename($temp_path, $materialize_path))
+  {
+    bratonien_tools_webdav_derivative_test_abort(500, 'Nextcloud-Bild konnte nicht atomar materialisiert werden.');
+  }
+  $materialized = true;
+  clearstatcache(true, $materialize_path);
+
+  $inner_url = bratonien_tools_webdav_derivative_test_inner_url($derivative_path);
+  if ($inner_url === null)
+  {
+    bratonien_tools_webdav_derivative_test_abort(500, 'Piwigo-i.php-URL konnte nicht bestimmt werden.');
+  }
+
+  $body = '';
+  $content_type = 'application/octet-stream';
+  $status = 500;
+  if (!bratonien_tools_webdav_derivative_test_call_i($inner_url, $body, $content_type, $status, $detail))
+  {
+    bratonien_tools_webdav_derivative_test_abort(502, $detail."\n".$body);
+  }
+
+  // Restore before anything is sent to the client. The derivative itself stays
+  // in Piwigo; only the transient Nextcloud original disappears again.
+  $cleanup();
+  clearstatcache(true, $derivative_path);
+  if (!is_file($derivative_path) || !is_readable($derivative_path))
+  {
+    bratonien_tools_webdav_derivative_test_abort(500, 'i.php lieferte eine Antwort, aber das Piwigo-Derivat wurde nicht gefunden.');
+  }
+
+  header('X-Bratonien-WebDAV-Test: generated-by-piwigo-i.php');
+  header('Content-Type: '.$content_type);
+  header('Content-Length: '.strlen($body));
+  header('Cache-Control: no-store');
+  http_response_code($status);
+  echo $body;
+}
+finally
+{
+  $cleanup();
+}