diff --git a/litellm/llms/github_copilot/responses/transformation.py b/litellm/llms/github_copilot/responses/transformation.py
index 5a4bb798851..00e5d100d9e 100644
--- a/litellm/llms/github_copilot/responses/transformation.py
+++ b/litellm/llms/github_copilot/responses/transformation.py
@@ -240,6 +240,22 @@ class GithubCopilotResponsesAPIConfig(OpenAIResponsesAPIConfig):
                 message=str(e),
             )
 
+    def _validate_input_param(
+        self, input: Union[str, ResponseInputParam]
+    ) -> Union[str, ResponseInputParam]:
+        from litellm.litellm_core_utils.prompt_templates.common_utils import (
+            filter_value_from_dict,
+        )
+
+        validated_input = super()._validate_input_param(input)
+        if isinstance(validated_input, list):
+            for item in validated_input:
+                if isinstance(item, dict):
+                    filter_value_from_dict(
+                        item, "internal_chat_message_metadata_passthrough"
+                    )
+        return validated_input
+
     def get_complete_url(
         self,
         api_base: str | None,
diff --git a/tests/test_litellm/llms/github_copilot/responses/test_github_copilot_responses_transformation.py b/tests/test_litellm/llms/github_copilot/responses/test_github_copilot_responses_transformation.py
index 0174465b0cc..d7628831e75 100644
--- a/tests/test_litellm/llms/github_copilot/responses/test_github_copilot_responses_transformation.py
+++ b/tests/test_litellm/llms/github_copilot/responses/test_github_copilot_responses_transformation.py
@@ -317,6 +317,31 @@ class TestGithubCopilotResponsesAPITransformation:
         for param in expected_params:
             assert param in supported, f"{param} should be in supported params"
 
+    def test_transform_request_strips_codex_internal_metadata(self):
+        config = GithubCopilotResponsesAPIConfig()
+        transformed = config.transform_responses_api_request(
+            model="gpt-5.3-codex",
+            input=[
+                {
+                    "role": "user",
+                    "content": "hello",
+                    "internal_chat_message_metadata_passthrough": {"local": True},
+                    "nested": {
+                        "internal_chat_message_metadata_passthrough": "remove-me",
+                        "keep": "value",
+                    },
+                }
+            ],
+            response_api_optional_request_params={"max_output_tokens": 16},
+            litellm_params={},
+            headers={},
+        )
+
+        input_item = transformed["input"][0]
+        assert "internal_chat_message_metadata_passthrough" not in input_item
+        assert "internal_chat_message_metadata_passthrough" not in input_item["nested"]
+        assert input_item["nested"]["keep"] == "value"
+
     def test_handle_reasoning_item_preserves_encrypted_content(self):
         """Test that _handle_reasoning_item preserves encrypted_content for GitHub Copilot.