diff --git a/litellm/llms/custom_httpx/llm_http_handler.py b/litellm/llms/custom_httpx/llm_http_handler.py
index a21d2d81284..c2df5613e8e 100644
--- a/litellm/llms/custom_httpx/llm_http_handler.py
+++ b/litellm/llms/custom_httpx/llm_http_handler.py
@@ -11199,7 +11199,7 @@ class BaseLLMHTTPHandler:
         )
 
         if extra_body:
-            data = _merge_extra_body_preserving_model(data, extra_body)
+            data.update(extra_body)
 
         ## LOGGING
         logging_obj.pre_call(
@@ -11314,7 +11314,7 @@ class BaseLLMHTTPHandler:
         )
 
         if extra_body:
-            data = _merge_extra_body_preserving_model(data, extra_body)
+            data.update(extra_body)
 
         ## LOGGING
         logging_obj.pre_call(
diff --git a/tests/test_litellm/llms/custom_httpx/test_llm_http_handler.py b/tests/test_litellm/llms/custom_httpx/test_llm_http_handler.py
index 64cc5860958..d88e0b173c1 100644
--- a/tests/test_litellm/llms/custom_httpx/test_llm_http_handler.py
+++ b/tests/test_litellm/llms/custom_httpx/test_llm_http_handler.py
@@ -272,6 +272,81 @@ async def test_async_response_api_handler_streams_when_provider_transform_adds_s
     assert client.post.call_args.kwargs["json"]["stream"] is True
 
 
+def test_response_api_handler_extra_body_cannot_override_model():
+    handler = BaseLLMHTTPHandler()
+    config = Mock()
+    config.validate_environment.return_value = {}
+    config.get_complete_url.return_value = "https://chatgpt.example.com/responses"
+    config.transform_responses_api_request.return_value = {
+        "model": "gpt-5.3-codex",
+        "input": "hi",
+        "stream": True,
+    }
+    config.sign_request.return_value = ({}, None)
+    client = HTTPHandler(client=httpx.Client())
+    client.post = Mock(
+        return_value=httpx.Response(
+            200,
+            request=httpx.Request("POST", "https://chatgpt.example.com/responses"),
+        )
+    )
+    logging_obj = Mock()
+
+    handler.response_api_handler(
+        model="gpt-5.3-codex",
+        input="hi",
+        responses_api_provider_config=config,
+        response_api_optional_request_params={},
+        custom_llm_provider="chatgpt",
+        litellm_params=GenericLiteLLMParams(),
+        logging_obj=logging_obj,
+        extra_body={"model": "attacker-model", "passthrough": "kept"},
+        client=client,
+    )
+
+    sent_body = client.post.call_args.kwargs["json"]
+    assert sent_body["model"] == "gpt-5.3-codex"
+    assert sent_body["passthrough"] == "kept"
+
+
+@pytest.mark.asyncio
+async def test_async_response_api_handler_extra_body_cannot_override_model():
+    handler = BaseLLMHTTPHandler()
+    config = Mock()
+    config.validate_environment.return_value = {}
+    config.get_complete_url.return_value = "https://chatgpt.example.com/responses"
+    config.transform_responses_api_request.return_value = {
+        "model": "gpt-5.3-codex",
+        "input": "hi",
+        "stream": True,
+    }
+    config.sign_request.return_value = ({}, None)
+    client = AsyncHTTPHandler()
+    client.post = AsyncMock(
+        return_value=httpx.Response(
+            200,
+            request=httpx.Request("POST", "https://chatgpt.example.com/responses"),
+        )
+    )
+    logging_obj = Mock()
+
+    await handler.async_response_api_handler(
+        model="gpt-5.3-codex",
+        input="hi",
+        responses_api_provider_config=config,
+        response_api_optional_request_params={},
+        custom_llm_provider="chatgpt",
+        litellm_params=GenericLiteLLMParams(),
+        logging_obj=logging_obj,
+        extra_body={"model": "attacker-model", "passthrough": "kept"},
+        client=client,
+    )
+
+    sent_body = client.post.call_args.kwargs["json"]
+    assert sent_body["model"] == "gpt-5.3-codex"
+    assert sent_body["passthrough"] == "kept"
+
+
 def test_merge_extra_body_preserving_model_blocks_model_override():
     merged = _merge_extra_body_preserving_model(
         {"model": "authorized", "messages": []},