diff --git a/litellm/llms/chatgpt/responses/transformation.py b/litellm/llms/chatgpt/responses/transformation.py
index 2352de35b5b..040b7b8c1d8 100644
--- a/litellm/llms/chatgpt/responses/transformation.py
+++ b/litellm/llms/chatgpt/responses/transformation.py
@@ -2,6 +2,7 @@ from collections.abc import Mapping
 from typing import TYPE_CHECKING, Any, Final, cast
 
 import httpx
+from openai.types.responses.response_input_param import ResponseInputItemParam
 
 from litellm.exceptions import AuthenticationError
 from litellm.litellm_core_utils.core_helpers import process_response_headers
@@ -42,6 +43,16 @@ if TYPE_CHECKING:
 _CHATGPT_SERVICE_TIERS: Final = {"default": "default", "priority": "priority", "fast": "priority"}
 
 
+def _system_item_as_developer(item: ResponseInputItemParam) -> ResponseInputItemParam:
+    # The Codex backend 400s on {"type": "message", "role": "system"} items ("System messages are not
+    # allowed") but accepts role=developer in the same position, which is what the Codex CLI sends.
+    if item.get("role") != "system":
+        return item
+    return cast(  # cast-ok: only the role changes, and developer is valid wherever system is
+        ResponseInputItemParam, {**item, "role": "developer"}
+    )
+
+
 class ChatGPTResponsesAPIConfig(OpenAIResponsesAPIConfig):
     def __init__(self, authenticator: Authenticator | None = None) -> None:
         super().__init__()
@@ -101,7 +112,9 @@ class ChatGPTResponsesAPIConfig(OpenAIResponsesAPIConfig):
     ) -> dict:
         request: Final = super().transform_responses_api_request(
             model,
-            [{"role": "user", "content": input}] if isinstance(input, str) else input,
+            [{"role": "user", "content": input}]
+            if isinstance(input, str)
+            else [_system_item_as_developer(item) for item in input],
             response_api_optional_request_params,
             litellm_params,
             headers,
diff --git a/tests/unit/llms/chatgpt/responses/test_chatgpt_responses_transformation.py b/tests/unit/llms/chatgpt/responses/test_chatgpt_responses_transformation.py
index d88bdfc0b41..8985d0b7fd8 100644
--- a/tests/unit/llms/chatgpt/responses/test_chatgpt_responses_transformation.py
+++ b/tests/unit/llms/chatgpt/responses/test_chatgpt_responses_transformation.py
@@ -74,6 +74,30 @@ class TestChatGPTResponsesAPITransformation:
 
         assert "service_tier" not in request
 
+    def test_chatgpt_sends_system_input_items_as_developer(self) -> None:
+        system_string_item: Final = {"role": "system", "content": "Be brief."}
+        system_blocks_item: Final = {
+            "type": "message",
+            "role": "system",
+            "content": [{"type": "input_text", "text": "Answer in French."}],
+        }
+        user_item: Final = {"role": "user", "content": "Hello"}
+        developer_item: Final = {"role": "developer", "content": "Use metric units."}
+        request: Final = ChatGPTResponsesAPIConfig().transform_responses_api_request(
+            model="chatgpt/gpt-6.1-sol",
+            input=[system_string_item, user_item, system_blocks_item, developer_item],
+            response_api_optional_request_params={},
+            litellm_params=GenericLiteLLMParams(),
+            headers={},
+        )
+
+        assert request["input"] == [
+            {**system_string_item, "role": "developer"},
+            user_item,
+            {**system_blocks_item, "role": "developer"},
+            developer_item,
+        ]
+
     @pytest.mark.parametrize(
         "model_name",
         [