diff --git a/agent/transports/codex.py b/agent/transports/codex.py
index 4ee2277dc7..556aa6c133 100644
--- a/agent/transports/codex.py
+++ b/agent/transports/codex.py
@@ -220,6 +220,10 @@ def _alias_wire_tools(response_tools: Any, params: dict[str, Any], is_xai_respon
     return response_tools, wire_aliases
 
 
+# Models already warned that an explicit disable has no wire form on their route (one warning per process).
+_UNPROJECTABLE_DISABLE_WARNED: set[str] = set()
+
+
 def _resolve_reasoning(model: str, params: dict[str, Any]) -> tuple[Any, bool]:
     """``(effort, enabled)`` for the request, effort clamped (never escalated) to the endpoint's vocabulary.
 
@@ -261,6 +265,14 @@ def _resolve_reasoning(model: str, params: dict[str, Any]) -> tuple[Any, bool]:
             return None, False
     if not reasoning_enabled:
         has_none = any(str(level).strip().lower() == "none" for level in supported)
+        if not has_none and model not in _UNPROJECTABLE_DISABLE_WARNED:
+            # #75227: report the unsupported configuration instead of silently falling back.
+            _UNPROJECTABLE_DISABLE_WARNED.add(model)
+            logger.warning(
+                "reasoning_effort: none cannot be sent for %s — its route accepts only %s, so the model's "
+                "default effort stays on (an omitted reasoning field does not disable it).",
+                model, ", ".join(str(level) for level in supported),
+            )
         return ("none" if has_none else None), False
     return clamp_effort(reasoning_effort, supported), reasoning_enabled
 
diff --git a/tests/agent/transports/test_codex_transport.py b/tests/agent/transports/test_codex_transport.py
index 58c4a2be94..bfc3bc50d0 100644
--- a/tests/agent/transports/test_codex_transport.py
+++ b/tests/agent/transports/test_codex_transport.py
@@ -1886,6 +1886,18 @@ class TestOpenAIReasoningWireProjection:
         # Astra's vocabulary has no ``none``: nothing to send, never an escalated level.
         assert self._reasoning(transport, "gpt-6-astra", {"enabled": False}) is None
 
+    def test_disable_the_route_cannot_express_is_reported_once(self, transport, caplog):
+        """#75227: a disable the vocabulary cannot carry (Astra has no ``none``) is reported as an unsupported
+        configuration — the model's default effort stays on — instead of silently omitted; once per model."""
+        import logging
+        from agent.transports import codex as codex_transport
+        codex_transport._UNPROJECTABLE_DISABLE_WARNED.discard("gpt-6-astra")
+        with caplog.at_level(logging.WARNING, logger="agent.transports.codex"):
+            for _ in range(2):
+                assert self._reasoning(transport, "gpt-6-astra", {"enabled": False}) is None
+        warned = [r.getMessage() for r in caplog.records if "reasoning_effort: none" in r.getMessage()]
+        assert len(warned) == 1 and "gpt-6-astra" in warned[0], caplog.text
+
     @pytest.mark.parametrize("model", ["gpt-4o-mini", "gpt-4.1-mini", "openai/gpt-4o", "ft:gpt-4o-mini:acme::abc1"])
     def test_chat_era_openai_models_get_no_reasoning_field_on_the_official_origin(self, transport, model):
         for rc in (None, {"enabled": True, "effort": "high"}, {"enabled": False}):