diff --git a/src/api/blackboard_routes.py b/src/api/blackboard_routes.py
index 0eb6ab6..f996c30 100644
--- a/src/api/blackboard_routes.py
+++ b/src/api/blackboard_routes.py
@@ -141,7 +141,10 @@ async def create_task(project_id: str, body: Dict[str, Any]):
         stages_json=body.get("stages_json", "[]"),
     )
     bb.create_task(task)
-    return {"ok": True, "task_id": task.id, "title": task.title}
+    result = {"ok": True, "task_id": task.id, "title": task.title}
+    if body.get("assignee") and not description_mentions:
+        result["warning"] = "assignee parameter is deprecated, use @mention in description"
+    return result
 
 
 async def _generate_title(description: str) -> str | None: