diff --git a/gateway/session.py b/gateway/session.py
index 16de296e0e..be393e48e6 100644
--- a/gateway/session.py
+++ b/gateway/session.py
@@ -1276,8 +1276,9 @@ class SessionStore:
         
         # Also write legacy JSONL (keeps existing tooling working during transition)
         transcript_path = self.get_transcript_path(session_id)
-        with open(transcript_path, "a", encoding="utf-8") as f:
-            f.write(json.dumps(message, ensure_ascii=False) + "\n")
+        with self._lock:
+            with open(transcript_path, "a", encoding="utf-8") as f:
+                f.write(json.dumps(message, ensure_ascii=False) + "\n")
     
     def rewrite_transcript(self, session_id: str, messages: List[Dict[str, Any]]) -> None:
         """Replace the entire transcript for a session with new messages.