diff --git a/litellm/responses/litellm_completion_transformation/transformation.py b/litellm/responses/litellm_completion_transformation/transformation.py
index 878f493b58e..6b9931b7df2 100644
--- a/litellm/responses/litellm_completion_transformation/transformation.py
+++ b/litellm/responses/litellm_completion_transformation/transformation.py
@@ -27,9 +27,10 @@ from openai.types.chat.chat_completion_named_tool_choice_param import (
 )
 from openai.types.responses import ResponseFunctionToolCall
 from openai.types.responses.response_create_params import ResponseInputParam
+from openai.types.responses.tool_choice_custom_param import ToolChoiceCustomParam
 from openai.types.responses.tool_choice_function_param import ToolChoiceFunctionParam
 from openai.types.responses.tool_param import FunctionToolParam
-from pydantic import TypeAdapter
+from pydantic import TypeAdapter, ValidationError
 from typing_extensions import ReadOnly, TypedDict
 
 from litellm._logging import verbose_logger
@@ -272,13 +273,21 @@ class LiteLLMCompletionResponsesConfig:
 
     @staticmethod
     def _transform_tool_choice_for_responses_api_response(tool_choice: object) -> ToolChoice:
-        normalized: Final = LiteLLMCompletionResponsesConfig._transform_tool_choice(tool_choice)
-        match normalized:
-            case None:
-                return "auto"
-            case {"type": "function", "function": {"name": str(function_name)}}:
+        if tool_choice is None:
+            return "auto"
+        try:
+            return _RESPONSES_API_TOOL_CHOICE_ADAPTER.validate_python(tool_choice)
+        except ValidationError:
+            return LiteLLMCompletionResponsesConfig._chat_tool_choice_as_responses_api_tool_choice(tool_choice)
+
+    @staticmethod
+    def _chat_tool_choice_as_responses_api_tool_choice(tool_choice: object) -> ToolChoice:
+        match tool_choice, LiteLLMCompletionResponsesConfig._transform_tool_choice(tool_choice):
+            case {"type": "custom"}, {"function": {"name": str(custom_name)}}:
+                return ToolChoiceCustomParam(type="custom", name=custom_name)
+            case _, {"type": "function", "function": {"name": str(function_name)}}:
                 return ToolChoiceFunctionParam(type="function", name=function_name)
-            case _:
+            case _, normalized:
                 return _RESPONSES_API_TOOL_CHOICE_ADAPTER.validate_python(normalized)
 
     @staticmethod
diff --git a/tests/test_litellm/responses/litellm_completion_transformation/test_litellm_completion_responses.py b/tests/test_litellm/responses/litellm_completion_transformation/test_litellm_completion_responses.py
index 2aab660b5b7..8f2937cb252 100644
--- a/tests/test_litellm/responses/litellm_completion_transformation/test_litellm_completion_responses.py
+++ b/tests/test_litellm/responses/litellm_completion_transformation/test_litellm_completion_responses.py
@@ -1,4 +1,5 @@
 import json
+from typing import Final
 
 import pytest
 
@@ -1426,7 +1427,9 @@ class TestToolChoiceTransformation:
         [
             ({"type": "function", "name": "run_command"}, {"type": "function", "name": "run_command"}),
             ({"type": "function", "function": {"name": "run_command"}}, {"type": "function", "name": "run_command"}),
-            ({"type": "custom", "name": "ApplyPatch"}, {"type": "function", "name": "ApplyPatch"}),
+            ({"type": "custom", "name": "ApplyPatch"}, {"type": "custom", "name": "ApplyPatch"}),
+            ({"type": "custom", "custom": {"name": "ApplyPatch"}}, {"type": "custom", "name": "ApplyPatch"}),
+            ({"type": "function"}, "required"),
             ({"type": "tool"}, "required"),
             ({"type": "auto"}, "auto"),
             ("required", "required"),
@@ -1434,14 +1437,16 @@ class TestToolChoiceTransformation:
             (None, "auto"),
         ],
     )
-    def test_transform_tool_choice_for_responses_api_response(self, request_tool_choice, expected):
-        result = LiteLLMCompletionResponsesConfig._transform_tool_choice_for_responses_api_response(
+    def test_transform_tool_choice_for_responses_api_response(
+        self, request_tool_choice: object, expected: str | dict[str, str]
+    ) -> None:
+        result: Final = LiteLLMCompletionResponsesConfig._transform_tool_choice_for_responses_api_response(
             request_tool_choice
         )
         assert result == expected
 
-    def test_non_streamed_response_echoes_named_tool_choice_in_responses_api_shape(self):
-        chat_completion_response = ModelResponse(
+    def test_non_streamed_response_echoes_named_tool_choice_in_responses_api_shape(self) -> None:
+        chat_completion_response: Final = ModelResponse(
             id="chatcmpl-named-tool-choice",
             created=1748575031,
             model="claude-haiku-4-5",
@@ -1465,7 +1470,7 @@ class TestToolChoiceTransformation:
             ],
         )
 
-        responses_api_response = LiteLLMCompletionResponsesConfig.transform_chat_completion_response_to_responses_api_response(
+        responses_api_response: Final = LiteLLMCompletionResponsesConfig.transform_chat_completion_response_to_responses_api_response(
             request_input="Run the command pwd.",
             responses_api_request={"tool_choice": {"type": "function", "name": "run_command"}},
             chat_completion_response=chat_completion_response,
diff --git a/tests/test_litellm/responses/litellm_completion_transformation/test_streaming_iterator_transformation.py b/tests/test_litellm/responses/litellm_completion_transformation/test_streaming_iterator_transformation.py
index d4b565f82a1..29061db97dc 100644
--- a/tests/test_litellm/responses/litellm_completion_transformation/test_streaming_iterator_transformation.py
+++ b/tests/test_litellm/responses/litellm_completion_transformation/test_streaming_iterator_transformation.py
@@ -11,6 +11,7 @@ spend tracking stores, so a follow-up previous_response_id still finds the conve
 """
 
 import json
+from typing import Final
 from unittest.mock import AsyncMock, MagicMock
 
 import pytest
@@ -657,8 +658,8 @@ def _tool_call_chunk(finish_reason: str | None = None) -> ModelResponseStream:
     )
 
 
-def test_streamed_named_tool_choice_is_echoed_in_responses_api_shape():
-    iterator = LiteLLMCompletionStreamingIterator(
+def test_streamed_named_tool_choice_is_echoed_in_responses_api_shape() -> None:
+    iterator: Final = LiteLLMCompletionStreamingIterator(
         model="claude-haiku-4-5",
         litellm_custom_stream_wrapper=_FakeStreamWrapper([_tool_call_chunk(finish_reason="tool_calls")]),
         request_input="Run the command pwd.",
@@ -670,9 +671,9 @@ def test_streamed_named_tool_choice_is_echoed_in_responses_api_shape():
         litellm_metadata={},
     )
 
-    events = list(iterator)
+    events: Final = list(iterator)
 
-    response_events = [event for event in events if getattr(event, "type", None) in RESPONSE_ID_EVENT_TYPES]
+    response_events: Final = [event for event in events if getattr(event, "type", None) in RESPONSE_ID_EVENT_TYPES]
     assert [event.type for event in response_events] == [
         "response.created",
         "response.in_progress",