diff --git a/app/api/routers/tasks.py b/app/api/routers/tasks.py
index a250e6a..d07e17d 100644
--- a/app/api/routers/tasks.py
+++ b/app/api/routers/tasks.py
@@ -209,6 +209,21 @@ def update_task(task_id: int, task_update: schemas.TaskUpdate, db: Session = Dep
     body_fields = {k for k in update_data.keys() if k not in _always_allowed}
 
     if body_fields:
+        # P3.6 supplement: feature story tasks locked after milestone freeze
+        task_type = task.task_type.value if hasattr(task.task_type, 'value') else (task.task_type or "")
+        task_subtype = task.task_subtype or ""
+        if task_type == "story" and task_subtype == "feature" and task.milestone_id:
+            from app.models.milestone import Milestone
+            ms = db.query(Milestone).filter(Milestone.id == task.milestone_id).first()
+            if ms:
+                ms_status = ms.status.value if hasattr(ms.status, 'value') else ms.status
+                if ms_status in ("freeze", "undergoing", "completed", "closed"):
+                    raise HTTPException(
+                        status_code=400,
+                        detail=f"Feature story task cannot be edited: milestone is '{ms_status}'. "
+                               f"Blocked fields: {sorted(body_fields)}",
+                    )
+
         # undergoing/completed/closed: body edits forbidden
         if current_status in ("undergoing", "completed", "closed"):
             raise HTTPException(