diff --git a/dev/custom_components/sws12500/data.py b/dev/custom_components/sws12500/data.py
new file mode 100644
index 0000000..1dec410
--- /dev/null
+++ b/dev/custom_components/sws12500/data.py
@@ -0,0 +1,17 @@
+"""Shared keys and helpers for storing integration runtime state in hass.data.
+
+This integration uses `hass.data[DOMAIN][entry_id]` as a per-entry dictionary.
+Keeping keys in one place prevents subtle bugs where different modules
+store different value types under the same key.
+"""
+
+from __future__ import annotations
+
+from typing import Final
+
+
+# Per-entry dict keys stored under hass.data[DOMAIN][entry_id]
+ENTRY_COORDINATOR: Final[str] = "coordinator"
+ENTRY_ADD_ENTITIES: Final[str] = "async_add_entities"
+ENTRY_DESCRIPTIONS: Final[str] = "sensor_descriptions"
+ENTRY_LAST_OPTIONS: Final[str] = "last_options"