diff --git a/litellm/llms/openai/chat/gpt_5_transformation.py b/litellm/llms/openai/chat/gpt_5_transformation.py
index af02235c34..beb76f3d80 100644
--- a/litellm/llms/openai/chat/gpt_5_transformation.py
+++ b/litellm/llms/openai/chat/gpt_5_transformation.py
@@ -64,6 +64,12 @@ class OpenAIGPT5Config(OpenAIGPTConfig):
         model_name = model.split("/")[-1]
         return model_name.startswith("gpt-5.2") or model_name.startswith("gpt-5.4")
 
+    @classmethod
+    def is_model_gpt_5_4_model(cls, model: str) -> bool:
+        """Check if the model is a gpt-5.4 variant (including pro)."""
+        model_name = model.split("/")[-1]
+        return model_name.startswith("gpt-5.4")
+
     @classmethod
     def _supports_reasoning_effort_level(cls, model: str, level: str) -> bool:
         """Check if the model supports a specific reasoning_effort level.
@@ -179,6 +185,17 @@ class OpenAIGPT5Config(OpenAIGPTConfig):
                 "max_tokens"
             )
 
+        # gpt-5.4: function calls not supported when reasoning_effort != "none"
+        # Drop reasoning_effort when tools are present (small minority of volume)
+        if self.is_model_gpt_5_4_model(model):
+            has_tools = bool(
+                non_default_params.get("tools") or optional_params.get("tools")
+            )
+            if has_tools and reasoning_effort not in (None, "none"):
+                non_default_params.pop("reasoning_effort", None)
+                optional_params.pop("reasoning_effort", None)
+                reasoning_effort = None
+
         # gpt-5.1/5.2 support logprobs, top_p, top_logprobs only when reasoning_effort="none"
         supports_none = self._supports_reasoning_effort_level(model, "none")
         if supports_none:
diff --git a/tests/test_litellm/llms/openai/test_gpt5_transformation.py b/tests/test_litellm/llms/openai/test_gpt5_transformation.py
index 415325d0da..b136f8774b 100644
--- a/tests/test_litellm/llms/openai/test_gpt5_transformation.py
+++ b/tests/test_litellm/llms/openai/test_gpt5_transformation.py
@@ -349,6 +349,56 @@ def test_gpt5_normalizes_reasoning_effort_dict_from_optional_params(config: Open
     assert params["reasoning_effort"] == "medium"
 
 
+def test_gpt5_4_drops_reasoning_effort_when_tools_present(config: OpenAIConfig):
+    """gpt-5.4: function calls not supported with reasoning_effort != 'none'. Drop reasoning_effort."""
+    tools = [{"type": "function", "function": {"name": "test", "description": "test"}}]
+    params = config.map_openai_params(
+        non_default_params={"reasoning_effort": "high", "tools": tools},
+        optional_params={},
+        model="gpt-5.4",
+        drop_params=False,
+    )
+    assert "reasoning_effort" not in params
+    assert params["tools"] == tools
+
+
+def test_gpt5_4_keeps_reasoning_effort_when_no_tools(config: OpenAIConfig):
+    """reasoning_effort is kept when tools are not present."""
+    params = config.map_openai_params(
+        non_default_params={"reasoning_effort": "high"},
+        optional_params={},
+        model="gpt-5.4",
+        drop_params=False,
+    )
+    assert params["reasoning_effort"] == "high"
+
+
+def test_gpt5_4_keeps_reasoning_effort_none_with_tools(config: OpenAIConfig):
+    """reasoning_effort='none' is kept when tools are present."""
+    tools = [{"type": "function", "function": {"name": "test", "description": "test"}}]
+    params = config.map_openai_params(
+        non_default_params={"reasoning_effort": "none", "tools": tools},
+        optional_params={},
+        model="gpt-5.4",
+        drop_params=False,
+    )
+    assert params["reasoning_effort"] == "none"
+    assert params["tools"] == tools
+
+
+def test_gpt5_2_keeps_reasoning_effort_with_tools(config: OpenAIConfig):
+    """gpt-5.2: reasoning_effort drop only applies to gpt-5.4, not gpt-5.2."""
+    tools = [{"type": "function", "function": {"name": "test", "description": "test"}}]
+    params = config.map_openai_params(
+        non_default_params={"reasoning_effort": "high", "tools": tools},
+        optional_params={},
+        model="gpt-5.2",
+        drop_params=False,
+    )
+    assert params["reasoning_effort"] == "high"
+    assert params["tools"] == tools
+
+
 def test_gpt5_4_pro_rejects_non_default_temperature(config: OpenAIConfig):
     with pytest.raises(litellm.utils.UnsupportedParamsError):
         config.map_openai_params(