diff --git a/litellm/llms/chatgpt/responses/transformation.py b/litellm/llms/chatgpt/responses/transformation.py
index 8e4bbf1d3c9..e9094b94eda 100644
--- a/litellm/llms/chatgpt/responses/transformation.py
+++ b/litellm/llms/chatgpt/responses/transformation.py
@@ -38,6 +38,18 @@ class ChatGPTResponsesAPIConfig(OpenAIResponsesAPIConfig):
     def custom_llm_provider(self) -> LlmProviders:
         return LlmProviders.CHATGPT
 
+    @staticmethod
+    def _normalize_string_input(request: dict) -> None:
+        """Convert Responses API string shorthand to ChatGPT's message-list form."""
+        request_input = request.get("input")
+        if isinstance(request_input, str):
+            request["input"] = [
+                {
+                    "role": "user",
+                    "content": [{"type": "input_text", "text": request_input}],
+                }
+            ]
+
     def validate_environment(
         self,
         headers: dict,
@@ -73,6 +85,7 @@ class ChatGPTResponsesAPIConfig(OpenAIResponsesAPIConfig):
             litellm_params,
             headers,
         )
+        self._normalize_string_input(request)
         base_instructions: Final = get_chatgpt_default_instructions()
         existing_instructions: Final = request.get("instructions")
         if existing_instructions:
diff --git a/tests/test_litellm/llms/chatgpt/responses/test_chatgpt_responses_transformation.py b/tests/test_litellm/llms/chatgpt/responses/test_chatgpt_responses_transformation.py
index 8e0415d50de..f0ca0ad6d74 100644
--- a/tests/test_litellm/llms/chatgpt/responses/test_chatgpt_responses_transformation.py
+++ b/tests/test_litellm/llms/chatgpt/responses/test_chatgpt_responses_transformation.py
@@ -101,9 +101,38 @@ class TestChatGPTResponsesAPITransformation:
         )
 
         assert request["stream"] is True
+        assert request["input"] == [
+            {
+                "role": "user",
+                "content": [{"type": "input_text", "text": "hi"}],
+            }
+        ]
         assert "reasoning.encrypted_content" in request["include"]
         assert request["instructions"].startswith("You are Codex, based on GPT-5.")
 
+    def test_chatgpt_normalizes_string_input_with_tools(self):
+        config = ChatGPTResponsesAPIConfig()
+        tools = [{"type": "function", "name": "echo", "parameters": {}}]
+        request = config.transform_responses_api_request(
+            model="chatgpt/gpt-5.3-codex",
+            input="Call echo with ping.",
+            response_api_optional_request_params={
+                "tools": tools,
+                "tool_choice": "required",
+            },
+            litellm_params=GenericLiteLLMParams(),
+            headers={},
+        )
+
+        assert request["input"] == [
+            {
+                "role": "user",
+                "content": [{"type": "input_text", "text": "Call echo with ping."}],
+            }
+        ]
+        assert request["tools"] == tools
+        assert request["tool_choice"] == "required"
+
     @pytest.mark.parametrize(
         "model_name",
         [