diff --git a/tests/test_tui_gateway_server.py b/tests/test_tui_gateway_server.py
index a33e27f8071..d41ef105a14 100644
--- a/tests/test_tui_gateway_server.py
+++ b/tests/test_tui_gateway_server.py
@@ -153,7 +153,7 @@ def test_write_json_returns_false_on_broken_pipe(monkeypatch):
 
 
 def test_write_json_drops_detached_ws_frames(monkeypatch):
-    out = _ThreadUnsafeStdout()
+    out = _ChunkyStdout()
     monkeypatch.setattr(server, "_real_stdout", out)
     server._sessions["detached-sid"] = {"transport": server._detached_ws_transport}
     try: