diff --git a/ai/shared.md b/ai/shared.md
index b696249..a6fe77d 100644
--- a/ai/shared.md
+++ b/ai/shared.md
@@ -12,6 +12,11 @@ these rules.
 - Schedules target whole sets. Users choose an element kind as the assignment
   granularity. To schedule a subset, create a separate named set containing
   that subset.
+- Schedules are immutable historical snapshots. Copy the set name and each
+  assignment's name, kind, and full path when creating a schedule.
+- Persisted schedules must not reference their source set, and persisted
+  assignments must not reference their source elements. Later source edits or
+  deletions must not change an existing schedule.
 - Schedule recalculation must preserve completed work while redistributing
   unfinished assignments across the remaining dates.
 - Planned features in `README.md` are future ideas, not authorized scope.