diff --git a/aw_core/dirs.py b/aw_core/dirs.py
index 2527d7f..d27940e 100644
--- a/aw_core/dirs.py
+++ b/aw_core/dirs.py
@@ -149,6 +149,10 @@ def wrapper(subpath: Optional[str] = None) -> str:
     return wrapper
 
 
+# All dirs go through platformdirs, which honors XDG_* on Linux and ignores it
+# on macOS/Windows (as does the `dirs` crate used by aw-server-rust). Every
+# component must resolve identical paths, so do not special-case XDG_* here
+# (see ActivityWatch/aw-client#119); in tests, patch platformdirs instead.
 @_ensure_returned_path_exists
 def get_data_dir(module_name: Optional[str] = None) -> str:
     data_dir = platformdirs.user_data_dir(_get_appname())