diff --git a/src/mcp_synology_filestation/tools/filestation.py b/src/mcp_synology_filestation/tools/filestation.py
index 3ef2748..dedbfb5 100644
--- a/src/mcp_synology_filestation/tools/filestation.py
+++ b/src/mcp_synology_filestation/tools/filestation.py
@@ -151,7 +151,7 @@ def register_filestation(
                     "limit": limit,
                     "sort_by": sort_by,
                     "sort_direction": sort_direction,
-                    "additional": json.dumps(["size", "time"]),  # DEBUG: size + time
+                    "additional": "size,time",  # DEBUG: comma-separated (no json.dumps)
                 },
             )
         except SynologyError as e: