diff --git a/code/src/Plugin.php b/code/src/Plugin.php
index 8b573ea..b328951 100644
--- a/code/src/Plugin.php
+++ b/code/src/Plugin.php
@@ -2014,12 +2014,20 @@ HTML
 
     private function serveCalDavPath(string $path, string $method): void
     {
+        $userAgent = (string) ($_SERVER['HTTP_USER_AGENT'] ?? '');
         $caldavUser = $this->resolveCalDavUserForRequest(null);
+        $this->caldavTrace('request', [
+            'method' => $method,
+            'path' => $path,
+            'user_agent' => $userAgent,
+            'authorized' => $caldavUser !== null,
+        ]);
         if ($caldavUser === null) {
             http_response_code(401);
             header('WWW-Authenticate: Basic realm="Calendar CalDAV"');
             header('Content-Type: application/xml; charset=utf-8');
             echo '<error><message>auth required</message></error>';
+            $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 401, 'reason' => 'auth_required']);
             return;
         }
 
@@ -2046,6 +2054,7 @@ HTML
                 header('Allow: OPTIONS, PROPFIND, REPORT, GET, PUT, DELETE, HEAD');
                 header('DAV: 1, calendar-access');
                 http_response_code(200);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'kind' => 'collection_head']);
                 return;
             }
             if (str_starts_with($path, $resourcePrefix) && str_ends_with($path, '.ics')) {
@@ -2053,11 +2062,13 @@ HTML
                 $obj = $this->calDavService->getObject($resource);
                 if ($obj === null) {
                     http_response_code(404);
+                    $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 404, 'resource' => $resource]);
                     return;
                 }
                 header('Content-Type: text/calendar; charset=utf-8');
                 header('ETag: ' . (string) ($obj['etag'] ?? ''));
                 http_response_code(200);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'resource' => $resource]);
                 return;
             }
         }
@@ -2078,6 +2089,7 @@ HTML
                 header('Allow: OPTIONS, PROPFIND, REPORT, GET, PUT, DELETE, HEAD');
                 header('DAV: 1, calendar-access');
                 http_response_code(200);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'kind' => 'collection_get']);
                 return;
             }
         }
@@ -2086,6 +2098,7 @@ HTML
             header('Allow: OPTIONS, PROPFIND, REPORT, GET, PUT, DELETE, HEAD');
             header('DAV: 1, calendar-access');
             http_response_code(200);
+            $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'kind' => 'options']);
             return;
         }
 
@@ -2094,18 +2107,22 @@ HTML
             http_response_code(207);
             if ($path === $root || $path === $root . '/') {
                 echo $this->caldavPropfindRootXml($root, $principal, $calendarsRoot, $collection);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_root']);
                 return;
             }
             if ($path === $principalCollection || $path === rtrim($principalCollection, '/')) {
                 echo $this->caldavPropfindPrincipalCollectionXml($principalCollection, $principal);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_principal_collection']);
                 return;
             }
             if ($path === $principal || $path === rtrim($principal, '/')) {
                 echo $this->caldavPropfindPrincipalXml($principal, $calendarsRoot);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_principal']);
                 return;
             }
             if ($path === $calendarsRoot || $path === rtrim($calendarsRoot, '/')) {
                 echo $this->caldavPropfindCalendarsRootXml($calendarsRoot, $collection);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_calendars_root']);
                 return;
             }
             if ($path === rtrim($collection, '/')) {
@@ -2113,6 +2130,7 @@ HTML
             }
             if ($path === $collection) {
                 echo $this->caldavPropfindCollectionXml($collection, $this->caldavSyncToken(), true);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_collection']);
                 return;
             }
             if (str_starts_with($path, $resourcePrefix) && str_ends_with($path, '.ics')) {
@@ -2121,21 +2139,33 @@ HTML
                 if ($obj === null) {
                     http_response_code(404);
                     echo '<error><message>not found</message></error>';
+                    $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 404, 'resource' => $resource, 'kind' => 'propfind_object']);
                     return;
                 }
                 echo $this->caldavPropfindObjectXml($collection . $resource, (string) ($obj['etag'] ?? ''));
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'resource' => $resource, 'kind' => 'propfind_object']);
                 return;
             }
             http_response_code(404);
             echo '<error><message>not found</message></error>';
+            $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 404, 'kind' => 'propfind_not_found']);
             return;
         }
 
         if ($method === 'REPORT' && ($path === $collection || $path === rtrim($collection, '/'))) {
             $body = (string) file_get_contents('php://input');
+            $reportType = $this->caldavReportType($body);
             header('Content-Type: application/xml; charset=utf-8');
             http_response_code(207);
             echo $this->caldavReportXml($collection, $body, $this->caldavSyncToken());
+            $this->caldavTrace('response', [
+                'method' => $method,
+                'path' => $path,
+                'status' => 207,
+                'kind' => 'report',
+                'report_type' => $reportType,
+                'body_bytes' => strlen($body),
+            ]);
             return;
         }
 
@@ -2145,12 +2175,14 @@ HTML
                 $obj = $this->calDavService->getObject($resource);
                 if ($obj === null) {
                     http_response_code(404);
+                    $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 404, 'resource' => $resource, 'kind' => 'object_get']);
                     return;
                 }
                 header('Content-Type: text/calendar; charset=utf-8');
                 header('ETag: ' . (string) ($obj['etag'] ?? ''));
                 http_response_code(200);
                 echo (string) ($obj['ics'] ?? '');
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'resource' => $resource, 'kind' => 'object_get']);
                 return;
             }
             if ($method === 'PUT') {
@@ -2169,6 +2201,14 @@ HTML
                     http_response_code((int) ($error['status'] ?? 500));
                     header('Content-Type: application/xml; charset=utf-8');
                     echo '<error><message>' . esc_html((string) ($error['message'] ?? 'error')) . '</message></error>';
+                    $this->caldavTrace('response', [
+                        'method' => $method,
+                        'path' => $path,
+                        'status' => (int) ($error['status'] ?? 500),
+                        'resource' => $resource,
+                        'kind' => 'object_put',
+                        'body_bytes' => strlen($raw),
+                    ]);
                     return;
                 }
                 $status = (int) ($result['status'] ?? 204);
@@ -2177,6 +2217,14 @@ HTML
                     header('ETag: ' . (string) $event['etag']);
                 }
                 http_response_code($status);
+                $this->caldavTrace('response', [
+                    'method' => $method,
+                    'path' => $path,
+                    'status' => $status,
+                    'resource' => $resource,
+                    'kind' => 'object_put',
+                    'body_bytes' => strlen($raw),
+                ]);
                 return;
             }
             if ($method === 'DELETE') {
@@ -2184,15 +2232,24 @@ HTML
                 if (isset($result['error'])) {
                     $error = (array) $result['error'];
                     http_response_code((int) ($error['status'] ?? 500));
+                    $this->caldavTrace('response', [
+                        'method' => $method,
+                        'path' => $path,
+                        'status' => (int) ($error['status'] ?? 500),
+                        'resource' => $resource,
+                        'kind' => 'object_delete',
+                    ]);
                     return;
                 }
                 http_response_code(204);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 204, 'resource' => $resource, 'kind' => 'object_delete']);
                 return;
             }
         }
 
         http_response_code(405);
         header('Allow: OPTIONS, PROPFIND, REPORT, GET, PUT, DELETE, HEAD');
+        $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 405, 'reason' => 'method_not_allowed']);
     }
 
     private function caldavPropfindRootXml(string $root, string $principal, string $calendarsRoot, string $collection): string
@@ -2319,6 +2376,10 @@ HTML
         $bodyLower = strtolower($xmlBody);
         $resources = array_map(static fn(array $r): string => (string) ($r['resource'] ?? ''), $this->calDavService->listResources());
         $items = [];
+        $reportType = $this->caldavReportType($xmlBody);
+        $includeDeleted = false;
+        $deletedCount = 0;
+        $clientSyncToken = '';
 
         if (str_contains($bodyLower, 'sync-collection')) {
             $clientSyncToken = $this->extractSyncCollectionToken($xmlBody);
@@ -2326,9 +2387,18 @@ HTML
             if ($clientSyncToken !== '' && $clientSyncToken === $syncToken) {
                 $items = [];
             } else {
-                // Fallback implementation: emit current objects for initial/out-of-date tokens.
-                // Avoid emitting historical tombstones because many clients treat large 404 sets as transient failures.
+                // Emit current objects for initial/out-of-date tokens.
                 $items = $this->calDavService->multiget($resources);
+                // For incremental syncs, include a bounded set of deleted hrefs so clients can remove local copies.
+                if ($clientSyncToken !== '') {
+                    $includeDeleted = true;
+                    // Keep incremental deletes bounded to avoid overwhelming strict clients.
+                    $deletedResources = $this->calDavService->listDeletedResources(20);
+                    $deletedCount = count($deletedResources);
+                    foreach ($deletedResources as $deletedResource) {
+                        $items[] = ['resource' => $deletedResource, 'status' => 404];
+                    }
+                }
             }
         } elseif (str_contains($bodyLower, 'calendar-query')) {
             $items = $this->calDavService->multiget($resources);
@@ -2359,17 +2429,38 @@ HTML
         }
 
         $responses = '';
+        $okCount = 0;
+        $notFoundCount = 0;
         foreach ($items as $item) {
             $status = (int) ($item['status'] ?? 404);
             $resource = (string) ($item['resource'] ?? '');
-            $responses .= '<D:response><D:href>' . htmlspecialchars($collection . $resource, ENT_XML1 | ENT_QUOTES, 'UTF-8') . '</D:href><D:propstat><D:prop>';
+            $responses .= '<D:response><D:href>' . htmlspecialchars($collection . $resource, ENT_XML1 | ENT_QUOTES, 'UTF-8') . '</D:href>';
             if ($status === 200) {
+                $okCount++;
+                $responses .= '<D:propstat><D:prop>';
                 $responses .= '<D:getetag>' . htmlspecialchars((string) ($item['etag'] ?? ''), ENT_XML1 | ENT_QUOTES, 'UTF-8') . '</D:getetag>'
                     . '<C:calendar-data xmlns:C="urn:ietf:params:xml:ns:caldav">' . htmlspecialchars((string) ($item['ics'] ?? ''), ENT_XML1 | ENT_QUOTES, 'UTF-8') . '</C:calendar-data>';
+                $responses .= '</D:prop><D:status>HTTP/1.1 200 OK</D:status></D:propstat>';
+            } else {
+                $notFoundCount++;
+                // For sync-collection deletions, clients expect bare status (no propstat block).
+                if ($reportType === 'sync-collection') {
+                    $responses .= '<D:status>HTTP/1.1 404 Not Found</D:status>';
+                } else {
+                    $responses .= '<D:propstat><D:prop></D:prop><D:status>HTTP/1.1 404 Not Found</D:status></D:propstat>';
+                }
             }
-            $responses .= '</D:prop><D:status>HTTP/1.1 ' . $status . ($status === 200 ? ' OK' : ' Not Found')
-                . '</D:status></D:propstat></D:response>';
+            $responses .= '</D:response>';
         }
+        $this->caldavTrace('report', [
+            'report_type' => $reportType,
+            'client_sync_token_present' => $clientSyncToken !== '',
+            'client_sync_token_matches' => $clientSyncToken !== '' && $clientSyncToken === $syncToken,
+            'include_deleted' => $includeDeleted,
+            'deleted_included_count' => $deletedCount,
+            'response_ok_count' => $okCount,
+            'response_not_found_count' => $notFoundCount,
+        ]);
         return '<?xml version="1.0" encoding="utf-8"?><D:multistatus xmlns:D="DAV:"><D:sync-token>'
             . htmlspecialchars($syncToken, ENT_XML1 | ENT_QUOTES, 'UTF-8')
             . '</D:sync-token>'
@@ -2425,6 +2516,34 @@ HTML
         return $token;
     }
 
+    private function caldavReportType(string $xmlBody): string
+    {
+        $bodyLower = strtolower($xmlBody);
+        if (str_contains($bodyLower, 'sync-collection')) {
+            return 'sync-collection';
+        }
+        if (str_contains($bodyLower, 'calendar-query')) {
+            return 'calendar-query';
+        }
+        if (str_contains($bodyLower, 'calendar-multiget')) {
+            return 'calendar-multiget';
+        }
+        return 'other';
+    }
+
+    private function caldavTrace(string $event, array $context = []): void
+    {
+        if (!$this->isDiagnosticsEnabled()) {
+            return;
+        }
+        $payload = [
+            'event' => $event,
+            'at' => gmdate('c'),
+            'context' => $context,
+        ];
+        error_log('[calendar-plugin][caldav] ' . wp_json_encode($payload));
+    }
+
     private function unwrapServiceResult(array $result): array|\WP_Error
     {
         if (isset($result['error']) && is_array($result['error'])) {
diff --git a/package/calendar-plugin-0.1.15.manifest.sha256 b/package/calendar-plugin-0.1.15.manifest.sha256
index 99df439..6c1e8a9 100644
--- a/package/calendar-plugin-0.1.15.manifest.sha256
+++ b/package/calendar-plugin-0.1.15.manifest.sha256
@@ -15,6 +15,6 @@ e65577c707c5a66e2097faa7720170180c79a9d9df219f0e1061fdcec55be744  ./src/Infrastr
 68c0ca15ad2c8b6363a2578b85f8daf0d3a094e612a120a2cdd2a2bfd8fe5e3c  ./src/Infrastructure/WordPress/WordPressDatabaseAdapter.php
 8da85db3c1e69c2c5f01aaa2f558aa8f0323446d8af0aec5b34d4607cd4afe1b  ./src/Infrastructure/WordPress/WordPressHttpAdapter.php
 cf9fddcecb07af2c03ad2c0e448be12a6b45dd936efc8bc1fd46a52b4af864ea  ./src/Infrastructure/WordPress/WordPressOptionsAdapter.php
-0cc96b252ac86e91a724e73beecfe6f57a1afdb6f5d9a26e44aed00a3f0e20f0  ./src/Plugin.php
+b0e95460e705b9059bf2f197c17f17c51c9bd5492ca69eaa073ebbb808542fe0  ./src/Plugin.php
 4e6930c79d9ce1045be6863ddd7546e19f7ef7e839d41a41014a3efb3183eaf0  ./src/bootstrap.php
 893c6df62beed87a981d372c473e5012d1b5d1c254d23b39cda44ae8a08cd16c  ./uninstall.php
diff --git a/package/calendar-plugin-0.1.15.zip b/package/calendar-plugin-0.1.15.zip
index d501ca0..1c36308 100644
Binary files a/package/calendar-plugin-0.1.15.zip and b/package/calendar-plugin-0.1.15.zip differ
diff --git a/package/staging/calendar-plugin/src/Plugin.php b/package/staging/calendar-plugin/src/Plugin.php
index 8b573ea..b328951 100644
--- a/package/staging/calendar-plugin/src/Plugin.php
+++ b/package/staging/calendar-plugin/src/Plugin.php
@@ -2014,12 +2014,20 @@ HTML
 
     private function serveCalDavPath(string $path, string $method): void
     {
+        $userAgent = (string) ($_SERVER['HTTP_USER_AGENT'] ?? '');
         $caldavUser = $this->resolveCalDavUserForRequest(null);
+        $this->caldavTrace('request', [
+            'method' => $method,
+            'path' => $path,
+            'user_agent' => $userAgent,
+            'authorized' => $caldavUser !== null,
+        ]);
         if ($caldavUser === null) {
             http_response_code(401);
             header('WWW-Authenticate: Basic realm="Calendar CalDAV"');
             header('Content-Type: application/xml; charset=utf-8');
             echo '<error><message>auth required</message></error>';
+            $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 401, 'reason' => 'auth_required']);
             return;
         }
 
@@ -2046,6 +2054,7 @@ HTML
                 header('Allow: OPTIONS, PROPFIND, REPORT, GET, PUT, DELETE, HEAD');
                 header('DAV: 1, calendar-access');
                 http_response_code(200);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'kind' => 'collection_head']);
                 return;
             }
             if (str_starts_with($path, $resourcePrefix) && str_ends_with($path, '.ics')) {
@@ -2053,11 +2062,13 @@ HTML
                 $obj = $this->calDavService->getObject($resource);
                 if ($obj === null) {
                     http_response_code(404);
+                    $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 404, 'resource' => $resource]);
                     return;
                 }
                 header('Content-Type: text/calendar; charset=utf-8');
                 header('ETag: ' . (string) ($obj['etag'] ?? ''));
                 http_response_code(200);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'resource' => $resource]);
                 return;
             }
         }
@@ -2078,6 +2089,7 @@ HTML
                 header('Allow: OPTIONS, PROPFIND, REPORT, GET, PUT, DELETE, HEAD');
                 header('DAV: 1, calendar-access');
                 http_response_code(200);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'kind' => 'collection_get']);
                 return;
             }
         }
@@ -2086,6 +2098,7 @@ HTML
             header('Allow: OPTIONS, PROPFIND, REPORT, GET, PUT, DELETE, HEAD');
             header('DAV: 1, calendar-access');
             http_response_code(200);
+            $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'kind' => 'options']);
             return;
         }
 
@@ -2094,18 +2107,22 @@ HTML
             http_response_code(207);
             if ($path === $root || $path === $root . '/') {
                 echo $this->caldavPropfindRootXml($root, $principal, $calendarsRoot, $collection);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_root']);
                 return;
             }
             if ($path === $principalCollection || $path === rtrim($principalCollection, '/')) {
                 echo $this->caldavPropfindPrincipalCollectionXml($principalCollection, $principal);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_principal_collection']);
                 return;
             }
             if ($path === $principal || $path === rtrim($principal, '/')) {
                 echo $this->caldavPropfindPrincipalXml($principal, $calendarsRoot);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_principal']);
                 return;
             }
             if ($path === $calendarsRoot || $path === rtrim($calendarsRoot, '/')) {
                 echo $this->caldavPropfindCalendarsRootXml($calendarsRoot, $collection);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_calendars_root']);
                 return;
             }
             if ($path === rtrim($collection, '/')) {
@@ -2113,6 +2130,7 @@ HTML
             }
             if ($path === $collection) {
                 echo $this->caldavPropfindCollectionXml($collection, $this->caldavSyncToken(), true);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'kind' => 'propfind_collection']);
                 return;
             }
             if (str_starts_with($path, $resourcePrefix) && str_ends_with($path, '.ics')) {
@@ -2121,21 +2139,33 @@ HTML
                 if ($obj === null) {
                     http_response_code(404);
                     echo '<error><message>not found</message></error>';
+                    $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 404, 'resource' => $resource, 'kind' => 'propfind_object']);
                     return;
                 }
                 echo $this->caldavPropfindObjectXml($collection . $resource, (string) ($obj['etag'] ?? ''));
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 207, 'resource' => $resource, 'kind' => 'propfind_object']);
                 return;
             }
             http_response_code(404);
             echo '<error><message>not found</message></error>';
+            $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 404, 'kind' => 'propfind_not_found']);
             return;
         }
 
         if ($method === 'REPORT' && ($path === $collection || $path === rtrim($collection, '/'))) {
             $body = (string) file_get_contents('php://input');
+            $reportType = $this->caldavReportType($body);
             header('Content-Type: application/xml; charset=utf-8');
             http_response_code(207);
             echo $this->caldavReportXml($collection, $body, $this->caldavSyncToken());
+            $this->caldavTrace('response', [
+                'method' => $method,
+                'path' => $path,
+                'status' => 207,
+                'kind' => 'report',
+                'report_type' => $reportType,
+                'body_bytes' => strlen($body),
+            ]);
             return;
         }
 
@@ -2145,12 +2175,14 @@ HTML
                 $obj = $this->calDavService->getObject($resource);
                 if ($obj === null) {
                     http_response_code(404);
+                    $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 404, 'resource' => $resource, 'kind' => 'object_get']);
                     return;
                 }
                 header('Content-Type: text/calendar; charset=utf-8');
                 header('ETag: ' . (string) ($obj['etag'] ?? ''));
                 http_response_code(200);
                 echo (string) ($obj['ics'] ?? '');
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 200, 'resource' => $resource, 'kind' => 'object_get']);
                 return;
             }
             if ($method === 'PUT') {
@@ -2169,6 +2201,14 @@ HTML
                     http_response_code((int) ($error['status'] ?? 500));
                     header('Content-Type: application/xml; charset=utf-8');
                     echo '<error><message>' . esc_html((string) ($error['message'] ?? 'error')) . '</message></error>';
+                    $this->caldavTrace('response', [
+                        'method' => $method,
+                        'path' => $path,
+                        'status' => (int) ($error['status'] ?? 500),
+                        'resource' => $resource,
+                        'kind' => 'object_put',
+                        'body_bytes' => strlen($raw),
+                    ]);
                     return;
                 }
                 $status = (int) ($result['status'] ?? 204);
@@ -2177,6 +2217,14 @@ HTML
                     header('ETag: ' . (string) $event['etag']);
                 }
                 http_response_code($status);
+                $this->caldavTrace('response', [
+                    'method' => $method,
+                    'path' => $path,
+                    'status' => $status,
+                    'resource' => $resource,
+                    'kind' => 'object_put',
+                    'body_bytes' => strlen($raw),
+                ]);
                 return;
             }
             if ($method === 'DELETE') {
@@ -2184,15 +2232,24 @@ HTML
                 if (isset($result['error'])) {
                     $error = (array) $result['error'];
                     http_response_code((int) ($error['status'] ?? 500));
+                    $this->caldavTrace('response', [
+                        'method' => $method,
+                        'path' => $path,
+                        'status' => (int) ($error['status'] ?? 500),
+                        'resource' => $resource,
+                        'kind' => 'object_delete',
+                    ]);
                     return;
                 }
                 http_response_code(204);
+                $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 204, 'resource' => $resource, 'kind' => 'object_delete']);
                 return;
             }
         }
 
         http_response_code(405);
         header('Allow: OPTIONS, PROPFIND, REPORT, GET, PUT, DELETE, HEAD');
+        $this->caldavTrace('response', ['method' => $method, 'path' => $path, 'status' => 405, 'reason' => 'method_not_allowed']);
     }
 
     private function caldavPropfindRootXml(string $root, string $principal, string $calendarsRoot, string $collection): string
@@ -2319,6 +2376,10 @@ HTML
         $bodyLower = strtolower($xmlBody);
         $resources = array_map(static fn(array $r): string => (string) ($r['resource'] ?? ''), $this->calDavService->listResources());
         $items = [];
+        $reportType = $this->caldavReportType($xmlBody);
+        $includeDeleted = false;
+        $deletedCount = 0;
+        $clientSyncToken = '';
 
         if (str_contains($bodyLower, 'sync-collection')) {
             $clientSyncToken = $this->extractSyncCollectionToken($xmlBody);
@@ -2326,9 +2387,18 @@ HTML
             if ($clientSyncToken !== '' && $clientSyncToken === $syncToken) {
                 $items = [];
             } else {
-                // Fallback implementation: emit current objects for initial/out-of-date tokens.
-                // Avoid emitting historical tombstones because many clients treat large 404 sets as transient failures.
+                // Emit current objects for initial/out-of-date tokens.
                 $items = $this->calDavService->multiget($resources);
+                // For incremental syncs, include a bounded set of deleted hrefs so clients can remove local copies.
+                if ($clientSyncToken !== '') {
+                    $includeDeleted = true;
+                    // Keep incremental deletes bounded to avoid overwhelming strict clients.
+                    $deletedResources = $this->calDavService->listDeletedResources(20);
+                    $deletedCount = count($deletedResources);
+                    foreach ($deletedResources as $deletedResource) {
+                        $items[] = ['resource' => $deletedResource, 'status' => 404];
+                    }
+                }
             }
         } elseif (str_contains($bodyLower, 'calendar-query')) {
             $items = $this->calDavService->multiget($resources);
@@ -2359,17 +2429,38 @@ HTML
         }
 
         $responses = '';
+        $okCount = 0;
+        $notFoundCount = 0;
         foreach ($items as $item) {
             $status = (int) ($item['status'] ?? 404);
             $resource = (string) ($item['resource'] ?? '');
-            $responses .= '<D:response><D:href>' . htmlspecialchars($collection . $resource, ENT_XML1 | ENT_QUOTES, 'UTF-8') . '</D:href><D:propstat><D:prop>';
+            $responses .= '<D:response><D:href>' . htmlspecialchars($collection . $resource, ENT_XML1 | ENT_QUOTES, 'UTF-8') . '</D:href>';
             if ($status === 200) {
+                $okCount++;
+                $responses .= '<D:propstat><D:prop>';
                 $responses .= '<D:getetag>' . htmlspecialchars((string) ($item['etag'] ?? ''), ENT_XML1 | ENT_QUOTES, 'UTF-8') . '</D:getetag>'
                     . '<C:calendar-data xmlns:C="urn:ietf:params:xml:ns:caldav">' . htmlspecialchars((string) ($item['ics'] ?? ''), ENT_XML1 | ENT_QUOTES, 'UTF-8') . '</C:calendar-data>';
+                $responses .= '</D:prop><D:status>HTTP/1.1 200 OK</D:status></D:propstat>';
+            } else {
+                $notFoundCount++;
+                // For sync-collection deletions, clients expect bare status (no propstat block).
+                if ($reportType === 'sync-collection') {
+                    $responses .= '<D:status>HTTP/1.1 404 Not Found</D:status>';
+                } else {
+                    $responses .= '<D:propstat><D:prop></D:prop><D:status>HTTP/1.1 404 Not Found</D:status></D:propstat>';
+                }
             }
-            $responses .= '</D:prop><D:status>HTTP/1.1 ' . $status . ($status === 200 ? ' OK' : ' Not Found')
-                . '</D:status></D:propstat></D:response>';
+            $responses .= '</D:response>';
         }
+        $this->caldavTrace('report', [
+            'report_type' => $reportType,
+            'client_sync_token_present' => $clientSyncToken !== '',
+            'client_sync_token_matches' => $clientSyncToken !== '' && $clientSyncToken === $syncToken,
+            'include_deleted' => $includeDeleted,
+            'deleted_included_count' => $deletedCount,
+            'response_ok_count' => $okCount,
+            'response_not_found_count' => $notFoundCount,
+        ]);
         return '<?xml version="1.0" encoding="utf-8"?><D:multistatus xmlns:D="DAV:"><D:sync-token>'
             . htmlspecialchars($syncToken, ENT_XML1 | ENT_QUOTES, 'UTF-8')
             . '</D:sync-token>'
@@ -2425,6 +2516,34 @@ HTML
         return $token;
     }
 
+    private function caldavReportType(string $xmlBody): string
+    {
+        $bodyLower = strtolower($xmlBody);
+        if (str_contains($bodyLower, 'sync-collection')) {
+            return 'sync-collection';
+        }
+        if (str_contains($bodyLower, 'calendar-query')) {
+            return 'calendar-query';
+        }
+        if (str_contains($bodyLower, 'calendar-multiget')) {
+            return 'calendar-multiget';
+        }
+        return 'other';
+    }
+
+    private function caldavTrace(string $event, array $context = []): void
+    {
+        if (!$this->isDiagnosticsEnabled()) {
+            return;
+        }
+        $payload = [
+            'event' => $event,
+            'at' => gmdate('c'),
+            'context' => $context,
+        ];
+        error_log('[calendar-plugin][caldav] ' . wp_json_encode($payload));
+    }
+
     private function unwrapServiceResult(array $result): array|\WP_Error
     {
         if (isset($result['error']) && is_array($result['error'])) {