diff --git a/src/daemon/inbox.py b/src/daemon/inbox.py
index 249da12..f76d9ca 100644
--- a/src/daemon/inbox.py
+++ b/src/daemon/inbox.py
@@ -168,8 +168,8 @@ class InboxWatcher:
     def _truncate(self) -> None:
         """清空文件内容（不删除文件）"""
         try:
-            with open(self.inbox_path, "w", encoding="utf-8") as f:
-                f.truncate(0)
+            with open(self.inbox_path, "w", encoding="utf-8"):
+                pass  # "w" mode already truncates
         except OSError:
             logger.exception("Failed to truncate inbox: %s", self.inbox_path)