diff --git a/agent/transports/codex_app_server.py b/agent/transports/codex_app_server.py
index ee39918276..055d0b6aba 100644
--- a/agent/transports/codex_app_server.py
+++ b/agent/transports/codex_app_server.py
@@ -281,6 +281,9 @@ class CodexAppServerClient:
 
     def stderr_tail(self, n: int = 20) -> list[str]:
         """Return last n lines of codex's stderr (for error reports)."""
+        if self._proc.poll() is not None:
+            # The reader can still hold the crash text when the exit is first observed (#121467).
+            self._stderr_reader.join(timeout=1.0)
         with self._stderr_lock:
             return list(self._stderr_lines[-n:])