feat(responses): add use_responses_api_bridge flag for openai/ models with custom api_base

Allows openai/-prefixed models with a custom api_base pointing to a
third-party OpenAI-compatible provider to opt-in to the
/responses → /chat/completions bridge, rather than forwarding requests
natively to /v1/responses (which may not be supported by the provider).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

litellm/responses/main.py

@@ -754,6 +754,7 @@ def responses(
         litellm_logging_obj: LiteLLMLoggingObj = kwargs.get("litellm_logging_obj")  # type: ignore
         litellm_call_id: Optional[str] = kwargs.get("litellm_call_id", None)
         _is_async = kwargs.pop("aresponses", False) is True
+        use_responses_api_bridge = kwargs.pop("use_responses_api_bridge", None)
 
         # Convert text_format to text parameter if provided
         text = ResponsesAPIRequestUtils.convert_text_format_to_text_param(
@@ -871,6 +872,7 @@ def responses(
 
         if _has_file_search_tool(tools) and (
             responses_api_provider_config is None
+            or use_responses_api_bridge is True
             or not responses_api_provider_config.supports_native_file_search()
         ):
             from litellm.responses.file_search.emulated_handler import (
@@ -919,7 +921,7 @@ def responses(
                 **emulated_kwargs,
             )
 
-        if responses_api_provider_config is None:
+        if responses_api_provider_config is None or use_responses_api_bridge is True:
             return litellm_completion_transformation_handler.response_api_handler(
                 model=model,
                 input=input,

litellm/types/router.py

@@ -199,6 +199,7 @@ class GenericLiteLLMParams(CredentialLiteLLMParams, CustomPricingLiteLLMParams):
     budget_duration: Optional[str] = None
     use_in_pass_through: Optional[bool] = False
     use_litellm_proxy: Optional[bool] = False
+    use_responses_api_bridge: Optional[bool] = None
     model_config = ConfigDict(extra="allow", arbitrary_types_allowed=True)
     merge_reasoning_content_in_choices: Optional[bool] = False
     model_info: Optional[Dict] = None
@@ -318,6 +319,8 @@ class LiteLLMParamsTypedDict(TypedDict, total=False):
     configurable_clientside_auth_params: CONFIGURABLE_CLIENTSIDE_AUTH_PARAMS  # for allowing api base switching on finetuned models
     ## DROP PARAMS ##
     drop_params: Optional[bool]
+    ## RESPONSES API BRIDGE ##
+    use_responses_api_bridge: Optional[bool]
     ## UNIFIED PROJECT/REGION ##
     region_name: Optional[str]
     ## VERTEX AI ##

tests/test_litellm/responses/test_responses_api_bridge_flag.py

@@ -0,0 +1,107 @@
+"""
+Tests for the `use_responses_api_bridge` flag that allows openai/ models
+with custom api_base to opt-in to the /responses → /chat/completions bridge.
+"""
+
+import os
+import sys
+from unittest.mock import MagicMock, patch
+
+sys.path.insert(
+    0, os.path.abspath("../../..")
+)  # Adds the parent directory to the system path
+
+import litellm
+
+
+class TestUseResponsesApiBridgeFlag:
+    """Test that use_responses_api_bridge forces the chat completions bridge."""
+
+    @patch(
+        "litellm.responses.main.litellm_completion_transformation_handler.response_api_handler"
+    )
+    @patch(
+        "litellm.responses.main.ProviderConfigManager.get_provider_responses_api_config"
+    )
+    def test_bridge_used_when_flag_is_true(self, mock_get_config, mock_bridge_handler):
+        """When use_responses_api_bridge=True, the bridge handler should be called
+        even though the provider (openai) has native responses API support."""
+        # Setup: provider config returns a non-None config (native support exists)
+        mock_get_config.return_value = litellm.OpenAIResponsesAPIConfig()
+
+        mock_bridge_handler.return_value = MagicMock()
+
+        litellm.responses(
+            model="openai/my-custom-model",
+            input="Hello",
+            use_responses_api_bridge=True,
+            litellm_logging_obj=MagicMock(),
+        )
+
+        mock_bridge_handler.assert_called_once()
+
+    @patch("litellm.responses.main.base_llm_http_handler.response_api_handler")
+    @patch(
+        "litellm.responses.main.ProviderConfigManager.get_provider_responses_api_config"
+    )
+    def test_native_forwarding_when_flag_absent(
+        self, mock_get_config, mock_native_handler
+    ):
+        """When use_responses_api_bridge is not set, openai/ models should use
+        native responses API forwarding (existing behavior)."""
+        mock_get_config.return_value = litellm.OpenAIResponsesAPIConfig()
+        mock_native_handler.return_value = MagicMock()
+
+        litellm.responses(
+            model="openai/gpt-4o",
+            input="Hello",
+            litellm_logging_obj=MagicMock(),
+        )
+
+        mock_native_handler.assert_called_once()
+
+    @patch(
+        "litellm.responses.main.litellm_completion_transformation_handler.response_api_handler"
+    )
+    @patch(
+        "litellm.responses.main.ProviderConfigManager.get_provider_responses_api_config"
+    )
+    def test_flag_does_not_leak_into_kwargs(self, mock_get_config, mock_bridge_handler):
+        """The use_responses_api_bridge flag should be popped from kwargs and not
+        passed through to the bridge handler."""
+        mock_get_config.return_value = litellm.OpenAIResponsesAPIConfig()
+        mock_bridge_handler.return_value = MagicMock()
+
+        litellm.responses(
+            model="openai/my-custom-model",
+            input="Hello",
+            use_responses_api_bridge=True,
+            litellm_logging_obj=MagicMock(),
+        )
+
+        call_kwargs = mock_bridge_handler.call_args
+        # The flag should not appear in the kwargs passed to the bridge handler
+        all_kwargs = call_kwargs.kwargs if call_kwargs.kwargs else {}
+        assert "use_responses_api_bridge" not in all_kwargs
+
+    @patch(
+        "litellm.responses.main.litellm_completion_transformation_handler.response_api_handler"
+    )
+    @patch(
+        "litellm.responses.main.ProviderConfigManager.get_provider_responses_api_config"
+    )
+    def test_bridge_used_when_provider_config_none(
+        self, mock_get_config, mock_bridge_handler
+    ):
+        """When the provider has no native responses API config (returns None),
+        the bridge should be used regardless of the flag (existing behavior)."""
+        mock_get_config.return_value = None
+        mock_bridge_handler.return_value = MagicMock()
+
+        litellm.responses(
+            model="anthropic/claude-3-haiku",
+            input="Hello",
+            litellm_logging_obj=MagicMock(),
+        )
+
+        mock_bridge_handler.assert_called_once()