@@ -368,7 +368,7 @@ async def list_sessions(
       self, *, app_name: str, user_id: Optional[str] = None
   ) -> ListSessionsResponse:
     """Lists sessions from Firestore."""
-    if user_id:
+    if user_id is not None:
       query = self._get_sessions_ref(app_name, user_id).where(
           filter=FieldFilter("appName", "==", app_name)
       )
@@ -392,7 +392,7 @@ def _iter_sessions_data() -> Iterator[dict[str, Any]]:
     app_state = app_doc.to_dict() if app_doc.exists else {}
 
     user_states_map = {}
-    if user_id:
+    if user_id is not None:
       user_ref = (
           self.client.collection(self.user_state_collection)
           .document(app_name)
 
@@ -961,6 +961,32 @@ async def mock_get_all(refs):
   assert _where_filter(where) == ("appName", "==", app_name)
 
 
+@pytest.mark.asyncio
+async def test_list_sessions_with_empty_user_id_lists_only_that_user(
+    mock_firestore_client,
+):
+  service = FirestoreSessionService(client=mock_firestore_client)
+  root_coll = mock_firestore_client.collection.return_value
+  users_coll = root_coll.document.return_value.collection.return_value
+  user_doc_ref = users_coll.document.return_value
+  user_doc_ref.collection.return_value.where.return_value.get = mock.AsyncMock(
+      return_value=[]
+  )
+  user_doc_ref.get = mock.AsyncMock(return_value=_stored_snapshot(None))
+  root_coll.document.return_value.get = mock.AsyncMock(
+      return_value=_stored_snapshot(None)
+  )
+  mock_firestore_client.collection_group.return_value.where.return_value.get = (
+      mock.AsyncMock(return_value=[])
+  )
+
+  response = await service.list_sessions(app_name="test_app", user_id="")
+
+  assert response.sessions == []
+  mock_firestore_client.collection_group.assert_not_called()
+  users_coll.document.assert_any_call("")
+
+
 @pytest.mark.asyncio
 async def test_get_user_state_returns_stored_user_state(mock_firestore_client):
   """get_user_state returns the user_states document for the app and user."""