diff --git a/src/fabledassistant/services/tools/calendar.py b/src/fabledassistant/services/tools/calendar.py
index 0218ddc..2f15de9 100644
--- a/src/fabledassistant/services/tools/calendar.py
+++ b/src/fabledassistant/services/tools/calendar.py
@@ -149,7 +149,17 @@ def _validate_weekday(start_dt_utc: datetime, user_tz, expected: str | None) ->
         "When the user names a weekday ('this Friday', 'next Monday'), "
         "state the resolved calendar date in your reply BEFORE calling "
         "this tool, and pass `expected_weekday` so the server can verify "
-        "the date falls on the day you intended."
+        "the date falls on the day you intended.\n\n"
+        "DON'T call this tool with placeholder values. If the user "
+        "mentions an event without giving you concrete details (a real "
+        "title, a specific time, and location when it applies), record "
+        "a moment instead and ask for the missing pieces — do NOT create "
+        "an event with a stand-in title like 'Appointment', 'Meeting', "
+        "or 'Event' and a description that says 'details TBD'. Wait for "
+        "the user's reply, then call create_event ONCE with the actual "
+        "title, time, and location. Premature placeholder events pollute "
+        "the calendar and require an immediate update_event to fix — "
+        "both visible to the user, neither what they asked for."
     ),
     parameters={
         "title": {"type": "string", "description": "A descriptive event title"},