diff --git a/EvoScientist/llm/models.py b/EvoScientist/llm/models.py
index 3efb6f5..b48dcf2 100644
--- a/EvoScientist/llm/models.py
+++ b/EvoScientist/llm/models.py
@@ -215,7 +215,8 @@ def _apply_auto_config(
             # ccproxy uses Chat Completions which doesn't support reasoning.
             pass
         else:
-            kwargs["reasoning"] = {"effort": "high", "summary": "auto"}
+            _eff = "xhigh" if ("5.4" in model_id or "codex" in model_id) else "high"
+            kwargs["reasoning"] = {"effort": _eff, "summary": "auto"}
 
     # Google GenAI: surface thinking traces
     if provider == "google-genai":
diff --git a/tests/test_llm.py b/tests/test_llm.py
index 3d535e2..37c91b6 100644
--- a/tests/test_llm.py
+++ b/tests/test_llm.py
@@ -771,15 +771,40 @@ class TestAutoConfig:
         assert call_kwargs["thinking"] == custom_thinking
 
     @patch("EvoScientist.llm.models.init_chat_model")
-    def test_openai_reasoning(self, mock_init, monkeypatch):
-        """Native OpenAI models get auto-reasoning."""
+    def test_openai_reasoning_xhigh(self, mock_init, monkeypatch):
+        """gpt-5.4+ and codex models get xhigh reasoning."""
+        mock_init.return_value = "mock_model"
+        monkeypatch.delenv("OPENAI_BASE_URL", raising=False)
+
+        get_chat_model("gpt-5.4", provider="openai")
+        assert mock_init.call_args[1]["reasoning"] == {
+            "effort": "xhigh",
+            "summary": "auto",
+        }
+
+        get_chat_model("gpt-5.3-codex", provider="openai")
+        assert mock_init.call_args[1]["reasoning"] == {
+            "effort": "xhigh",
+            "summary": "auto",
+        }
+
+    @patch("EvoScientist.llm.models.init_chat_model")
+    def test_openai_reasoning_high_fallback(self, mock_init, monkeypatch):
+        """Other OpenAI models get high reasoning effort."""
         mock_init.return_value = "mock_model"
         monkeypatch.delenv("OPENAI_BASE_URL", raising=False)
 
         get_chat_model("gpt-5-nano")
+        assert mock_init.call_args[1]["reasoning"] == {
+            "effort": "high",
+            "summary": "auto",
+        }
 
-        call_kwargs = mock_init.call_args[1]
-        assert call_kwargs["reasoning"] == {"effort": "high", "summary": "auto"}
+        get_chat_model("gpt-5.2", provider="openai")
+        assert mock_init.call_args[1]["reasoning"] == {
+            "effort": "high",
+            "summary": "auto",
+        }
 
     @patch("EvoScientist.llm.models.init_chat_model")
     def test_openai_base_url_override(self, mock_init, monkeypatch):