fix(responses): forward client headers to the provider on /v1/responses

litellm/responses/main.py

@@ -927,6 +927,13 @@ def responses(
         _is_async = kwargs.pop("aresponses", False) is True
         use_chat_completions_api = _pop_use_chat_completions_api_kw(kwargs)
 
+        client_headers = kwargs.get("headers")
+        extra_headers = ResponsesAPIRequestUtils.merge_client_forwarded_headers(
+            extra_headers=extra_headers,
+            client_headers=client_headers if isinstance(client_headers, dict) else None,
+        )
+        local_vars["extra_headers"] = extra_headers
+
         # Convert text_format to text parameter if provided
         text = ResponsesAPIRequestUtils.convert_text_format_to_text_param(text_format=text_format, text=text)
         if text is not None:

litellm/responses/utils.py

@@ -89,6 +89,28 @@ def merge_prompt_management_input(
         )
         return [*merged_input]
 
+    @staticmethod
+    def merge_client_forwarded_headers(
+        extra_headers: dict[str, Any] | None,
+        client_headers: dict[str, str] | None,
+    ) -> dict[str, Any] | None:
+        """
+        Merge headers forwarded by the proxy (`headers` kwarg, set when
+        `forward_client_headers_to_llm_api` is enabled) into `extra_headers`.
+
+        `extra_headers` wins on conflicts, since it is set explicitly by the caller.
+        Header names are compared case-insensitively, as HTTP defines them.
+        """
+        if not client_headers:
+            return extra_headers
+        if not extra_headers:
+            return dict(client_headers)
+        explicit_names = frozenset(name.lower() for name in extra_headers)
+        return {
+            **{name: value for name, value in client_headers.items() if name.lower() not in explicit_names},
+            **extra_headers,
+        }
+
     @staticmethod
     def _check_valid_arg(
         supported_params: Optional[List[str]],

tests/test_litellm/responses/test_responses_api_request_body.py

@@ -259,3 +259,63 @@ async def test_aresponses_bedrock_mantle_service_tier_raises_without_drop_params
         mock_post.assert_not_called()
         assert "drop_params" in str(excinfo.value)
         assert "priority" in str(excinfo.value)
+
+
+async def _aresponses_and_get_request_headers(**request_kwargs) -> dict:
+    with patch(
+        "litellm.llms.custom_httpx.http_handler.AsyncHTTPHandler.post",
+        new_callable=AsyncMock,
+    ) as mock_post:
+        mock_post.return_value = MockResponse(_minimal_responses_api_payload("resp_headers_test", "gpt-4o"), 200)
+
+        await litellm.aresponses(
+            model="openai/gpt-4o",
+            api_key="fake-api-key",
+            input="hi",
+            **request_kwargs,
+        )
+
+        mock_post.assert_called_once()
+        return dict(mock_post.call_args.kwargs["headers"])
+
+
+@pytest.mark.asyncio
+async def test_aresponses_forwards_client_headers_kwarg_to_provider():
+    """
+    The proxy passes client headers it forwards (`forward_client_headers_to_llm_api`)
+    as a `headers` kwarg; those must reach the provider request.
+    """
+    request_headers = await _aresponses_and_get_request_headers(headers={"x-my-new-header": "hello-from-client"})
+
+    assert request_headers["x-my-new-header"] == "hello-from-client"
+
+
+@pytest.mark.asyncio
+async def test_aresponses_merges_client_headers_with_extra_headers():
+    """
+    A `headers` kwarg and an explicit `extra_headers` are merged, with
+    `extra_headers` winning on conflicts.
+    """
+    request_headers = await _aresponses_and_get_request_headers(
+        headers={"x-my-new-header": "hello-from-client", "x-shared": "from-client"},
+        extra_headers={"x-explicit": "from-caller", "x-shared": "from-caller"},
+    )
+
+    assert request_headers["x-my-new-header"] == "hello-from-client"
+    assert request_headers["x-explicit"] == "from-caller"
+    assert request_headers["x-shared"] == "from-caller"
+
+
+@pytest.mark.asyncio
+async def test_aresponses_client_header_conflict_is_case_insensitive():
+    """
+    HTTP header names are case-insensitive, so a differently cased client header
+    must not survive alongside the explicit `extra_headers` value.
+    """
+    request_headers = await _aresponses_and_get_request_headers(
+        headers={"X-Shared": "from-client"},
+        extra_headers={"x-shared": "from-caller"},
+    )
+
+    assert [name for name in request_headers if name.lower() == "x-shared"] == ["x-shared"]
+    assert request_headers["x-shared"] == "from-caller"