diff --git a/include/nc_connector_ws.inc.php b/include/nc_connector_ws.inc.php
index 9149ac4..733c255 100644
--- a/include/nc_connector_ws.inc.php
+++ b/include/nc_connector_ws.inc.php
@@ -24,8 +24,13 @@ function bratonien_tools_register_ws_methods($arr)
         'type' => WS_TYPE_ID,
         'info' => 'Piwigo storage site to synchronize. Default: 1.',
       ),
+      'simulate' => array(
+        'default' => true,
+        'type' => WS_TYPE_BOOL,
+        'info' => 'Simulation only. Productive API synchronization is disabled until parity has been verified.',
+      ),
     ),
-    'Synchronizes a local Piwigo filesystem site for the Bratonien NC Connector.',
+    'Simulates the local Piwigo filesystem synchronization used by the Bratonien NC Connector.',
     null,
     array(
       'admin_only' => true,
@@ -34,30 +39,61 @@ function bratonien_tools_register_ws_methods($arr)
   );
 }
 
+function bratonien_tools_ws_nc_sync_error(&$errors, $path, $type)
+{
+  $errors[] = array(
+    'path' => (string)$path,
+    'type' => (string)$type,
+  );
+}
+
 /**
- * Runs the same local filesystem synchronization currently used by Piwigo's
- * admin site_update page, but inside an authenticated Web API request.
+ * Read-only parity test for Piwigo 16.4.0 admin/site_update.php with the
+ * connector's fixed settings:
+ *   sync=files, privacy_level=0, sync_meta=1, subcats-included=1,
+ *   no cat, no caddie, no meta_all, no meta_empty_overrides.
+ *
+ * The simulation deliberately performs no INSERT/UPDATE/DELETE and triggers
+ * no Piwigo activity/events. It only calculates what the matching core admin
+ * synchronization would attempt to change.
  */
 function bratonien_tools_ws_nc_sync($params, &$service)
 {
-  global $conf, $template, $page, $user;
+  global $conf, $user, $logger;
 
   if (empty($conf['enable_synchronization']))
   {
     return new PwgError(403, 'Piwigo filesystem synchronization is disabled.');
   }
 
+  $piwigo_version = defined('PHPWG_VERSION') ? (string)PHPWG_VERSION : '';
+  if ($piwigo_version !== '16.4.0')
+  {
+    return new PwgError(
+      409,
+      'Bratonien API synchronization is not approved for Piwigo '.$piwigo_version.'. Use the administrator fallback until this Piwigo version has been verified.'
+    );
+  }
+
+  $simulate = !isset($params['simulate']) || (bool)$params['simulate'];
+  if (!$simulate)
+  {
+    return new PwgError(
+      409,
+      'Productive Bratonien API synchronization is not enabled yet. Run the simulation and verify parity with Piwigo admin synchronization first.'
+    );
+  }
+
   $site_id = isset($params['site_id']) ? (int)$params['site_id'] : 1;
   if ($site_id < 1)
   {
     return new PwgError(400, 'Invalid site_id.');
   }
 
-  $query = '
-SELECT galleries_url
-  FROM '.SITES_TABLE.'
-  WHERE id = '.$site_id.'
-  LIMIT 1';
+  include_once(PHPWG_ROOT_PATH.'admin/include/functions.php');
+  include_once(PHPWG_ROOT_PATH.'admin/site_reader_local.php');
+
+  $query = '\nSELECT galleries_url\n  FROM '.SITES_TABLE.'\n  WHERE id = '.$site_id.'\n  LIMIT 1';
   $result = pwg_query($query);
   if (!pwg_db_num_rows($result))
   {
@@ -70,56 +106,171 @@ SELECT galleries_url
     return new PwgError(400, 'Remote Piwigo sites are not supported by this synchronization method.');
   }
 
-  $saved_get = $_GET;
-  $saved_post = $_POST;
-
-  $_GET['site'] = (string)$site_id;
-  $_POST = array(
-    'sync' => 'files',
-    'display_info' => '1',
-    'privacy_level' => '0',
-    'sync_meta' => '1',
-    'simulate' => '0',
-    'subcats-included' => '1',
-    'submit' => '1',
-  );
-
-  $counts = array();
   $errors = array();
-  $infos = array();
-  $general_failure = true;
-
-  try
-  {
-    ob_start();
-    include(PHPWG_ROOT_PATH.'admin/site_update.php');
-    ob_end_clean();
-  }
-  catch (Throwable $e)
-  {
-    if (ob_get_level() > 0)
-    {
-      ob_end_clean();
-    }
-    $_GET = $saved_get;
-    $_POST = $saved_post;
-    return new PwgError(500, 'Piwigo synchronization failed: '.$e->getMessage());
-  }
-
-  $_GET = $saved_get;
-  $_POST = $saved_post;
-
-  if ($general_failure)
+  $site_reader = new LocalSiteReader($site_url);
+  if (!$site_reader->open())
   {
     return new PwgError(500, 'Piwigo could not open the configured local site.');
   }
 
+  $counts = array(
+    'new_categories' => 0,
+    'del_categories' => 0,
+    'new_elements' => 0,
+    'del_elements' => 0,
+    'upd_elements' => 0,
+    'new_formats' => 0,
+    'del_formats' => 0,
+    'metadata_candidates' => 0,
+  );
+
+  // -------------------------------------------------------------------
+  // Directories / physical categories
+  // -------------------------------------------------------------------
+  $query = '\nSELECT id, uppercats, global_rank, status, visible\n  FROM '.CATEGORIES_TABLE.'\n  WHERE dir IS NOT NULL\n    AND site_id = '.$site_id;
+  $db_categories = hash_from_query($query, 'id');
+  $db_fulldirs = get_fulldirs(array_keys($db_categories));
+  $basedir = preg_replace('#/*$#', '', $site_url);
+  $db_fulldirs = array_flip($db_fulldirs);
+
+  $fs_fulldirs = $site_reader->get_full_directories($basedir);
+  $next_id = pwg_db_nextval('id', CATEGORIES_TABLE);
+
+  foreach (array_diff($fs_fulldirs, array_keys($db_fulldirs)) as $fulldir)
+  {
+    $dir = basename($fulldir);
+    if (!preg_match($conf['sync_chars_regex'], $dir))
+    {
+      bratonien_tools_ws_nc_sync_error($errors, $fulldir, 'PWG-UPDATE-1');
+      continue;
+    }
+
+    // site_update.php also adds newly discovered categories to its in-memory
+    // maps during simulation so files below them can be detected in the same
+    // run. Synthetic IDs reproduce that behavior without database writes.
+    $virtual_id = $next_id++;
+    $db_fulldirs[$fulldir] = $virtual_id;
+    $db_categories[$virtual_id] = array('id'=>$virtual_id);
+    $counts['new_categories']++;
+  }
+
+  $to_delete = array();
+  foreach (array_diff(array_keys($db_fulldirs), $fs_fulldirs) as $fulldir)
+  {
+    // A just-added virtual directory is always in $fs_fulldirs and therefore
+    // cannot land here.
+    $to_delete[] = $db_fulldirs[$fulldir];
+  }
+  $counts['del_categories'] = count($to_delete);
+
+  // -------------------------------------------------------------------
+  // Files / images / formats
+  // -------------------------------------------------------------------
+  $fs = $site_reader->get_elements($basedir);
+  $cat_ids = array_diff(array_keys($db_categories), $to_delete);
+  $db_elements = array();
+
+  if (count($cat_ids) > 0)
+  {
+    $query = '\nSELECT id, path\n  FROM '.IMAGES_TABLE.'\n  WHERE storage_category_id IN ('.wordwrap(implode(', ', $cat_ids), 160, "\n").')';
+    $db_elements = simple_hash_from_query($query, 'id', 'path');
+  }
+
+  foreach (array_diff(array_keys($fs), $db_elements) as $path)
+  {
+    $dirname = dirname($path);
+    if (!isset($db_fulldirs[$dirname]))
+    {
+      continue;
+    }
+
+    $filename = basename($path);
+    if (!preg_match($conf['sync_chars_regex'], $filename))
+    {
+      bratonien_tools_ws_nc_sync_error($errors, $path, 'PWG-UPDATE-1');
+      continue;
+    }
+
+    $counts['new_elements']++;
+    if (!empty($conf['enable_formats']) && isset($fs[$path]['formats']) && is_array($fs[$path]['formats']))
+    {
+      $counts['new_formats'] += count($fs[$path]['formats']);
+    }
+  }
+
+  if (!empty($conf['enable_formats']) && count($db_elements) > 0)
+  {
+    $db_elements_flip = array_flip($db_elements);
+    $existing_ids = array();
+    foreach (array_intersect_key($fs, $db_elements_flip) as $path => $unused)
+    {
+      $existing_ids[] = $db_elements_flip[$path];
+    }
+
+    if (count($existing_ids) > 0)
+    {
+      $db_formats = array();
+      $query = '\nSELECT *\n  FROM '.IMAGE_FORMAT_TABLE.'\n  WHERE image_id IN ('.implode(',', $existing_ids).')';
+      $result = pwg_query($query);
+      while ($row = pwg_db_fetch_assoc($result))
+      {
+        if (!isset($db_formats[$row['image_id']]))
+        {
+          $db_formats[$row['image_id']] = array();
+        }
+        $db_formats[$row['image_id']][$row['ext']] = $row['format_id'];
+      }
+
+      foreach ($db_formats as $image_id => $formats)
+      {
+        $path = $db_elements[$image_id];
+        $filesystem_formats = isset($fs[$path]['formats']) && is_array($fs[$path]['formats']) ? $fs[$path]['formats'] : array();
+        $counts['del_formats'] += count(array_diff_key($formats, $filesystem_formats));
+      }
+
+      foreach ($existing_ids as $image_id)
+      {
+        $path = $db_elements[$image_id];
+        $known_formats = isset($db_formats[$image_id]) ? $db_formats[$image_id] : array();
+        $filesystem_formats = isset($fs[$path]['formats']) && is_array($fs[$path]['formats']) ? $fs[$path]['formats'] : array();
+        $counts['new_formats'] += count(array_diff_key($filesystem_formats, $known_formats));
+      }
+    }
+  }
+
+  $counts['del_elements'] = count(array_diff($db_elements, array_keys($fs)));
+
+  // site_update.php updates representative_ext for all currently registered
+  // files. In simulation newly found files/categories are not yet in the DB,
+  // so get_filelist() intentionally sees only the current database state.
+  $files = get_filelist('', $site_id, true, false);
+  $update_count = 0;
+  foreach ($files as $id => $file)
+  {
+    $data = $site_reader->get_element_update_attributes($file['path']);
+    if (is_array($data))
+    {
+      $update_count++;
+    }
+  }
+  $counts['upd_elements'] = $update_count;
+
+  // The connector does not set meta_all. Piwigo therefore processes only
+  // files whose date_metadata_update is still NULL.
+  $metadata_files = get_filelist('', $site_id, true, true);
+  $counts['metadata_candidates'] = count($metadata_files);
+
   return array(
+    'mode' => 'simulation',
+    'approved_piwigo_version' => '16.4.0',
+    'piwigo_version' => $piwigo_version,
     'site_id' => $site_id,
     'site_url' => $site_url,
     'counts' => $counts,
     'errors' => $errors,
-    'info_count' => count($infos),
+    'error_count' => count($errors),
+    'database_writes' => false,
+    'fallback' => 'administrator',
     'connected_with' => isset($_SESSION['connected_with']) ? (string)$_SESSION['connected_with'] : '',
     'username' => isset($user['username']) ? (string)$user['username'] : '',
     'status' => isset($user['status']) ? (string)$user['status'] : '',