diff --git a/src/evohomeasync2/const.py b/src/evohomeasync2/const.py
index 18e7ee6..9625563 100644
--- a/src/evohomeasync2/const.py
+++ b/src/evohomeasync2/const.py
@@ -11,6 +11,8 @@
 SZ_ALLOWED_MODES: Final = "allowed_modes"
 SZ_ALLOWED_SETPOINT_MODES: Final = "allowed_setpoint_modes"
 SZ_ALLOWED_SYSTEM_MODES: Final = "allowed_system_modes"
+SZ_CAN_BE_PERMANENT: Final = "can_be_permanent"
+SZ_CAN_BE_TEMPORARY: Final = "can_be_temporary"
 SZ_COUNTRY: Final = "country"
 SZ_CURRENT_OFFSET_MINUTES: Final = "current_offset_minutes"
 SZ_DAILY_SCHEDULES: Final = "daily_schedules"
@@ -53,6 +55,7 @@
 SZ_TIME_UNTIL: Final = "time_until"
 SZ_TIME_ZONE: Final = "time_zone"
 SZ_TIME_ZONE_ID: Final = "time_zone_id"
+SZ_TIMING_MODE: Final = "timing_mode"
 SZ_UNTIL: Final = "until"
 SZ_USE_DAYLIGHT_SAVE_SWITCHING: Final = "use_daylight_save_switching"
 SZ_USER_ID: Final = "user_id"
diff --git a/src/evohomeasync2/schemas/typedefs.py b/src/evohomeasync2/schemas/typedefs.py
index d03349e..7bd0bc9 100644
--- a/src/evohomeasync2/schemas/typedefs.py
+++ b/src/evohomeasync2/schemas/typedefs.py
@@ -110,9 +110,9 @@ class EvoAllowedSystemModesResponseT(TypedDict):
     system_mode: SystemMode
     can_be_permanent: Literal[True]
     can_be_temporary: bool
-    max_duration: NotRequired[str]
-    timing_resolution: NotRequired[str]
-    timing_mode: NotRequired[str]
+    max_duration: NotRequired[str]  # when can_be_temporary is True
+    timing_resolution: NotRequired[str]  # when can_be_temporary is True
+    timing_mode: NotRequired[str]  # when can_be_temporary is True
 
 
 class EvoTcsConfigResponseT(EvoTcsConfigEntryT):