fix(responses-guardrails): keep a lone system message an input item when the rebuild empties input

When a guardrail's rewrite cannot be patched in place, the handler rebuilds
the request through the chat-to-Responses converter. A rebuild left holding
only the system message folded it into `instructions` over an empty `input`,
which the Responses API rejects. That already happened for string content;
folding list content made it happen for list content too, where main kept
the item. The rebuild now sends that message as a system item, as the chat
bridge already does for a system-only request.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

litellm/llms/openai/responses/guardrail_translation/handler.py

@@ -505,6 +505,15 @@ def _patch_or_convert_request_fields(
     input_items, converted_instructions = (
         LiteLLMResponsesTransformationHandler().convert_chat_completion_messages_to_responses_api(structured_messages)
     )
+    if not input_items and converted_instructions is not None:
+        # The Responses API rejects an empty input, so a lone system message stays a system item, as the chat
+        # bridge sends a system-only request.
+        system_item: Final = {
+            "type": "message",
+            "role": "system",
+            "content": [{"type": "input_text", "text": converted_instructions}],
+        }
+        return _RequestFields(input=(system_item,), instructions=None)
     return _RequestFields(input=tuple(input_items), instructions=converted_instructions)
 
 

tests/unit/llms/openai/responses/test_openai_responses_guardrail_handler.py

@@ -3128,6 +3128,27 @@ class TestPatchEdgeBranches:
             *trailing,
         ]
 
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize(
+        "system_content",
+        ["Answer from the memo only.", [{"type": "input_text", "text": "Answer from the memo only."}]],
+    )
+    async def test_fallback_leaving_only_the_system_message_keeps_it_an_input_item(self, system_content):
+        """The Responses API rejects an empty input, so a rebuild left holding only the system
+        message sends it as a system item rather than as instructions over an empty input."""
+        handler = OpenAIResponsesHandler()
+        data = {
+            "model": "gpt-5.6",
+            "input": [{"role": "system", "content": system_content}, {"role": "user", "content": "memo " * 400}],
+        }
+
+        result = await handler.process_input_messages(data, DroppingRewriteGuardrail())
+
+        assert "instructions" not in result
+        assert result["input"] == [
+            {"type": "message", "role": "system", "content": [{"type": "input_text", "text": "Answer from the memo only."}]}
+        ]
+
     def test_item_rewrite_field_ignores_non_string_type(self):
         from litellm.llms.openai.responses.guardrail_translation.handler import _item_rewrite_field