diff --git a/litellm/main.py b/litellm/main.py
index 43d021ebe8b..8a9e0e37ace 100644
--- a/litellm/main.py
+++ b/litellm/main.py
@@ -1022,14 +1022,20 @@ def responses_api_bridge_check(
     # ``reasoningSummary`` in ``extra_body``) must be bridged; Chat Completions rejects
     # those keys.
     #
-    # - gpt-5.4+: function tools with reasoning active must be bridged. OpenAI enables
+    # - gpt-5.4+: FUNCTION tools with reasoning active must be bridged. OpenAI enables
     #   reasoning by default for these models (unset reasoning_effort means medium
-    #   server-side), and Chat Completions rejects tools whenever reasoning is on
-    #   ("Function tools with reasoning_effort are not supported ... use /v1/responses
-    #   or set reasoning_effort to 'none'"), so only an explicit ``"none"`` keeps the
-    #   request chat-servable.
+    #   server-side), and Chat Completions rejects function tools whenever reasoning is
+    #   on ("Function tools with reasoning_effort are not supported ... use
+    #   /v1/responses or set reasoning_effort to 'none'"), so only an explicit
+    #   ``"none"`` keeps the request chat-servable. Custom (grammar) tools are served
+    #   natively by Chat Completions with reasoning on, so custom-only requests stay on
+    #   chat and keep their native custom tool_call response shape.
     # - Older GPT-5 names (e.g. ``gpt-5``, ``gpt-5.1``): bridge only when a reasoning
     #   summary alias is present with ``reasoning_effort`` (tools alone stay on chat).
+    has_function_tool = any(
+        (tool.get("type") == "function" if isinstance(tool, dict) else getattr(tool, "type", None) == "function")
+        for tool in (tools or [])
+    )
     if (
         custom_llm_provider in ("openai", "azure")
         and model_info.get("mode") != "responses"
@@ -1037,7 +1043,9 @@ def responses_api_bridge_check(
         and not OpenAIGPT5Config.is_model_gpt_5_search_model(model)
         and (
             (reasoning_effort is not None and reasoning_summary is not None)
-            or (OpenAIGPT5Config.is_model_gpt_5_4_plus_model(model) and tools and reasoning_effort != "none")
+            or (
+                OpenAIGPT5Config.is_model_gpt_5_4_plus_model(model) and has_function_tool and reasoning_effort != "none"
+            )
         )
     ):
         model_info["mode"] = "responses"
diff --git a/tests/test_litellm/test_main.py b/tests/test_litellm/test_main.py
index b4f94177f7d..4a0ec04bed1 100644
--- a/tests/test_litellm/test_main.py
+++ b/tests/test_litellm/test_main.py
@@ -889,6 +889,65 @@ def test_responses_api_bridge_check_reasoning_none_with_summary_still_routes_to_
     assert model_info.get("mode") == "responses"
 
 
+def test_responses_api_bridge_check_gpt_5_4_custom_tools_only_stays_chat():
+    """
+    Chat Completions serves custom (grammar) tools natively with reasoning on; only
+    FUNCTION tools trigger the OpenAI rejection. Custom-only requests must stay on chat
+    so responses keep the native custom tool_call shape instead of the bridge's
+    function-shaped mapping.
+    """
+    from litellm.main import responses_api_bridge_check
+
+    with patch("litellm.main._get_model_info_helper") as mock_get_model_info:
+        mock_get_model_info.return_value = {"max_tokens": 128000}
+        model_info, model = responses_api_bridge_check(
+            model="gpt-5.6",
+            custom_llm_provider="openai",
+            tools=[{"type": "custom", "custom": {"name": "ApplyPatch", "description": "V4A patch"}}],
+            reasoning_effort=None,
+        )
+
+    assert model == "gpt-5.6"
+    assert model_info.get("mode") != "responses"
+
+
+def test_responses_api_bridge_check_gpt_5_4_mixed_function_and_custom_tools_routes_to_responses():
+    """One function tool in the mix is enough to make chat unservable with reasoning on."""
+    from litellm.main import responses_api_bridge_check
+
+    with patch("litellm.main._get_model_info_helper") as mock_get_model_info:
+        mock_get_model_info.return_value = {"max_tokens": 128000}
+        model_info, model = responses_api_bridge_check(
+            model="gpt-5.6",
+            custom_llm_provider="openai",
+            tools=[
+                {"type": "custom", "custom": {"name": "ApplyPatch"}},
+                {"type": "function", "function": {"name": "shell"}},
+            ],
+            reasoning_effort=None,
+        )
+
+    assert model == "gpt-5.6"
+    assert model_info.get("mode") == "responses"
+
+
+def test_responses_api_bridge_check_gpt_5_4_flat_function_tool_routes_to_responses():
+    """Responses-style flat function tool defs still count as function tools."""
+    from litellm.main import responses_api_bridge_check
+
+    with patch("litellm.main._get_model_info_helper") as mock_get_model_info:
+        mock_get_model_info.return_value = {"max_tokens": 128000}
+        model_info, model = responses_api_bridge_check(
+            model="gpt-5.6",
+            custom_llm_provider="openai",
+            tools=[{"type": "function", "name": "shell", "parameters": {"type": "object"}}],
+            reasoning_effort=None,
+        )
+
+    assert model == "gpt-5.6"
+    assert model_info.get("mode") == "responses"
+
+
 def test_responses_api_bridge_check_older_gpt_5_tools_without_reasoning_stays_chat():
     """Pre-5.4 GPT-5 names keep the old boundary: tools alone never bridge."""
     from litellm.main import responses_api_bridge_check