diff --git a/test_additional.py b/test_additional.py
new file mode 100644
index 0000000..648fd9a
--- /dev/null
+++ b/test_additional.py
@@ -0,0 +1,134 @@
+"""Throwaway script: test SYNO.FileStation.List::list additional field formats.
+
+Run with:
+    uv run python test_additional.py
+
+Reads config from the default location and credentials from the OS keyring,
+exactly as the MCP server does. Tests three additional variants against a
+real share path and prints the raw DSM response for each.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import json
+import sys
+
+import httpx  # noqa: F401 — used at runtime for the HTTP client reference
+
+# ── config ────────────────────────────────────────────────────────────────
+
+# Change this to any share that exists on your NAS (as returned by list_shares)
+TEST_PATH = "/dev"
+
+# ──────────────────────────────────────────────────────────────────────────
+
+
+async def run() -> None:
+    from mcp_synology_filestation.auth import AuthManager
+    from mcp_synology_filestation.client import FileStationClient
+    from mcp_synology_filestation.config import load_config
+
+    config = load_config()
+    auth = AuthManager(config)
+
+    async with FileStationClient(
+        config.base_url,
+        config.connection.verify_ssl,
+        config.connection.timeout,
+    ) as client:
+        await client.query_api_info()
+        sid = await auth.login(client)
+        client.sid = sid
+
+        info = client._api_cache.get("SYNO.FileStation.List", {})
+        print(
+            f"SYNO.FileStation.List — path={info.get('path')!r} "
+            f"v{info.get('minVersion')}-v{info.get('maxVersion')}\n"
+        )
+
+        variants: list[tuple[str, dict]] = [
+            (
+                "1 — no additional",
+                {
+                    "folder_path": TEST_PATH,
+                    "offset": 0,
+                    "limit": 5,
+                    "sort_by": "name",
+                    "sort_direction": "asc",
+                },
+            ),
+            (
+                '2 — additional="size,time" (comma-separated string)',
+                {
+                    "folder_path": TEST_PATH,
+                    "offset": 0,
+                    "limit": 5,
+                    "sort_by": "name",
+                    "sort_direction": "asc",
+                    "additional": "size,time",
+                },
+            ),
+            (
+                '3 — additional=json.dumps(["size","time"]) (JSON array)',
+                {
+                    "folder_path": TEST_PATH,
+                    "offset": 0,
+                    "limit": 5,
+                    "sort_by": "name",
+                    "sort_direction": "asc",
+                    "additional": json.dumps(["size", "time"]),
+                },
+            ),
+        ]
+
+        for label, params in variants:
+            print(f"{'─' * 60}")
+            print(f"Variant {label}")
+            print(f"  params: {params}")
+            try:
+                # Call the low-level HTTP layer directly so we see the raw body,
+                # bypassing any error-mapping in client.request()
+                api = "SYNO.FileStation.List"
+                method = "list"
+                api_info = client._api_cache[api]
+                version = api_info["maxVersion"]
+                url = f"{client._base_url}/webapi/{api_info['path']}"
+
+                req_params: dict = {
+                    "api": api,
+                    "version": str(version),
+                    "method": method,
+                    "_sid": client.sid,
+                }
+                req_params.update(params)
+
+                http = client._http
+                resp = await http.get(url, params=req_params)
+                body = resp.json()
+                print(f"  HTTP {resp.status_code}")
+                print(f"  success: {body.get('success')}")
+                if body.get("success"):
+                    data = body.get("data", {})
+                    files = data.get("files", [])
+                    print(f"  total: {data.get('total')}  returned: {len(files)}")
+                    if files:
+                        first = files[0]
+                        print(f"  first file keys: {list(first.keys())}")
+                        add = first.get("additional")
+                        print(f"  first file additional: {add!r}")
+                else:
+                    print(f"  RAW error body: {body!r}")
+            except Exception as exc:
+                print(f"  EXCEPTION: {exc!r}")
+            print()
+
+        await auth.logout(client)
+
+
+if __name__ == "__main__":
+    try:
+        asyncio.run(run())
+    except Exception as e:
+        print(f"Fatal: {e}", file=sys.stderr)
+        sys.exit(1)