diff --git a/tests/agent/test_auxiliary_client.py b/tests/agent/test_auxiliary_client.py
index a42b6fc877..cf75d0b1f8 100644
--- a/tests/agent/test_auxiliary_client.py
+++ b/tests/agent/test_auxiliary_client.py
@@ -1675,6 +1675,18 @@ class TestCodexAdapterReasoningTranslation:
         assert captured.get("reasoning") == {"effort": "medium", "summary": "auto"}
         assert captured.get("include") == ["reasoning.encrypted_content"]
 
+    def test_reasoning_effort_zero_falls_back_to_medium(self):
+        """Numeric ``0`` is also falsy — the docstring lists it explicitly,
+        so cover the contract.  Codex would reject ``{"effort": 0}`` the
+        same way it rejects ``null``."""
+        adapter, captured = self._build_adapter()
+        adapter.create(
+            messages=[{"role": "user", "content": "hi"}],
+            extra_body={"reasoning": {"effort": 0}},
+        )
+        assert captured.get("reasoning") == {"effort": "medium", "summary": "auto"}
+        assert captured.get("include") == ["reasoning.encrypted_content"]
+
 
 class TestVisionAutoSkipsKimiCoding:
     """_resolve_auto vision branch skips providers that have no vision on