diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index 91b4fd5d20..34966ae0fa 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -6157,7 +6157,14 @@ def _merge_aux_extra_body(
         if reasoning_config.get("enabled") is False:
             merged_extra["reasoning"] = {"enabled": False}
         else:
-            merged_extra["reasoning"] = {"enabled": True, "effort": reasoning_config.get("effort") or "medium"}
+            # This fallback uses the OpenAI-compatible chat-completions wire. Hermes'
+            # internal ``ultra`` tier is not accepted there, including for MoA slots.
+            from agent.reasoning_effort import OPENAI_COMPAT_WIRE_EFFORTS, clamp_effort
+            effort = reasoning_config.get("effort") or "medium"
+            merged_extra["reasoning"] = {
+                "enabled": True,
+                "effort": clamp_effort(effort, OPENAI_COMPAT_WIRE_EFFORTS),
+            }
     # Portal tags + sticky session_id fallback when the profile didn't supply them; session_id
     # keeps aux calls on the main turn's upstream instance (cache warmth) — tags alone are not
     # enough on /v1/messages.
diff --git a/tests/agent/test_auxiliary_client.py b/tests/agent/test_auxiliary_client.py
index 0d11dfe2d5..3e51216365 100644
--- a/tests/agent/test_auxiliary_client.py
+++ b/tests/agent/test_auxiliary_client.py
@@ -2499,6 +2499,26 @@ class TestStaleBaseUrlWarning:
 
 
 class TestAuxiliaryTaskExtraBody:
+    @pytest.mark.parametrize("task", ["session_search", "moa_reference", "moa_aggregator"])
+    def test_generic_reasoning_fallback_clamps_ultra_for_auxiliary_and_moa_calls(self, task, monkeypatch):
+        """The OpenAI-compatible fallback must never put Hermes-only ``ultra`` on the wire."""
+        from agent.auxiliary_client import _ProfileProjection, _build_call_kwargs
+
+        monkeypatch.setattr(
+            "agent.auxiliary_client._project_provider_profile",
+            lambda *_args: _ProfileProjection({}, {}, {}, False),
+        )
+
+        kwargs = _build_call_kwargs(
+            provider="custom",
+            model="test-model",
+            messages=[{"role": "user", "content": "hello"}],
+            reasoning_config={"enabled": True, "effort": "ultra"},
+            task=task,
+        )
+
+        assert kwargs["extra_body"]["reasoning"] == {"enabled": True, "effort": "max"}
+
     def test_sync_call_merges_task_extra_body_from_config(self):
         client = MagicMock()
         client.base_url = "https://api.example.com/v1"