fix(chatgpt): normalize string responses input

litellm/llms/chatgpt/responses/transformation.py

@@ -73,6 +73,7 @@ class ChatGPTResponsesAPIConfig(OpenAIResponsesAPIConfig):
             litellm_params,
             headers,
         )
+        request["input"] = self._normalize_chatgpt_input(request.get("input"))
         base_instructions = get_chatgpt_default_instructions()
         existing_instructions = request.get("instructions")
         if existing_instructions:
@@ -105,6 +106,18 @@ class ChatGPTResponsesAPIConfig(OpenAIResponsesAPIConfig):
 
         return {k: v for k, v in request.items() if k in allowed_keys}
 
+    @staticmethod
+    def _normalize_chatgpt_input(input_value: Any) -> Any:
+        if isinstance(input_value, str):
+            return [
+                {
+                    "role": "user",
+                    "content": [{"type": "input_text", "text": input_value}],
+                }
+            ]
+
+        return input_value
+
     def transform_response_api_response(
         self,
         model: str,

tests/test_litellm/llms/chatgpt/responses/test_chatgpt_responses_transformation.py

@@ -107,6 +107,24 @@ class TestChatGPTResponsesAPITransformation:
             "You are Codex, based on GPT-5."
         )
 
+    def test_chatgpt_normalizes_string_input_for_responses_api(self):
+        config = ChatGPTResponsesAPIConfig()
+
+        request = config.transform_responses_api_request(
+            model="chatgpt/gpt-5.4",
+            input="hi",
+            response_api_optional_request_params={},
+            litellm_params=GenericLiteLLMParams(),
+            headers={},
+        )
+
+        assert request["input"] == [
+            {
+                "role": "user",
+                "content": [{"type": "input_text", "text": "hi"}],
+            }
+        ]
+
     @pytest.mark.parametrize(
         "model_name",
         [