diff --git a/tests/test_openai_responses_output_shaper.py b/tests/test_openai_responses_output_shaper.py
index b5ff4b093..b28db9373 100644
--- a/tests/test_openai_responses_output_shaper.py
+++ b/tests/test_openai_responses_output_shaper.py
@@ -10,10 +10,19 @@ pytest.importorskip("fastapi")
 
 from fastapi.testclient import TestClient  # noqa: E402
 
+from headroom.proxy import runtime_env  # noqa: E402
 from headroom.proxy.loopback_guard import require_loopback  # noqa: E402
 from headroom.proxy.server import ProxyConfig, create_app  # noqa: E402
 
 
+@pytest.fixture(autouse=True)
+def _isolate_runtime_env_overrides():
+    """Keep loopback hot-reload state from leaking into later test modules."""
+    runtime_env.clear_overrides()
+    yield
+    runtime_env.clear_overrides()
+
+
 def _make_client() -> TestClient:
     app = create_app(
         ProxyConfig(