diff --git a/litellm/llms/custom_httpx/llm_http_handler.py b/litellm/llms/custom_httpx/llm_http_handler.py
index 138f2410c89..a21d2d81284 100644
--- a/litellm/llms/custom_httpx/llm_http_handler.py
+++ b/litellm/llms/custom_httpx/llm_http_handler.py
@@ -10,6 +10,7 @@ from typing import (
     Dict,
     List,
     Literal,
+    Mapping,
     Optional,
     Tuple,
     Union,
@@ -194,6 +195,15 @@ def _responses_api_optional_request_param_names() -> frozenset[str]:
     return frozenset(get_type_hints(ResponsesAPIOptionalRequestParams).keys())
 
 
+def _merge_extra_body_preserving_model(
+    transformed_body: Mapping[str, object], extra_body: Mapping[str, object]
+) -> dict[str, object]:
+    merged = {**transformed_body, **extra_body}
+    if "model" in transformed_body:
+        return {**merged, "model": transformed_body["model"]}
+    return merged
+
+
 def _custom_logger_callbacks(logging_obj: Any) -> list[Any]:
     from litellm.integrations.custom_logger import CustomLogger
     from litellm.litellm_core_utils.litellm_logging import (
@@ -496,7 +506,7 @@ class BaseLLMHTTPHandler:
         )
 
         if extra_body is not None:
-            data = {**data, **extra_body}
+            data = _merge_extra_body_preserving_model(data, extra_body)
 
         headers, signed_json_body = provider_config.sign_request(
             headers=headers,
@@ -2451,7 +2461,7 @@ class BaseLLMHTTPHandler:
         data = BaseResponsesAPIConfig.normalize_responses_api_request_dict(data)
 
         if extra_body:
-            data.update(extra_body)
+            data = _merge_extra_body_preserving_model(data, extra_body)
         stream = bool(stream or data.get("stream"))
 
         # Preserve the OpenAI-style request context (not sent to the provider) for streaming
@@ -2638,7 +2648,7 @@ class BaseLLMHTTPHandler:
         data = BaseResponsesAPIConfig.normalize_responses_api_request_dict(data)
 
         if extra_body:
-            data.update(extra_body)
+            data = _merge_extra_body_preserving_model(data, extra_body)
         stream = bool(stream or data.get("stream"))
 
         # Preserve the OpenAI-style request context (not sent to the provider) for streaming
@@ -11189,7 +11199,7 @@ class BaseLLMHTTPHandler:
         )
 
         if extra_body:
-            data.update(extra_body)
+            data = _merge_extra_body_preserving_model(data, extra_body)
 
         ## LOGGING
         logging_obj.pre_call(
@@ -11304,7 +11314,7 @@ class BaseLLMHTTPHandler:
         )
 
         if extra_body:
-            data.update(extra_body)
+            data = _merge_extra_body_preserving_model(data, 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 64ae30daa70..64cc5860958 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
@@ -18,6 +18,7 @@ from litellm.llms.custom_httpx.http_handler import AsyncHTTPHandler, HTTPHandler
 from litellm.llms.custom_httpx.llm_http_handler import (
     BaseLLMHTTPHandler,
     _google_genai_streaming_hidden_params,
+    _merge_extra_body_preserving_model,
 )
 from litellm.types.llms.openai import ResponsesAPIResponse
 from litellm.types.router import GenericLiteLLMParams
@@ -271,6 +272,25 @@ 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_merge_extra_body_preserving_model_blocks_model_override():
+    merged = _merge_extra_body_preserving_model(
+        {"model": "authorized", "messages": []},
+        {"model": "attacker", "passthrough": "kept"},
+    )
+    assert merged["model"] == "authorized"
+    assert merged["passthrough"] == "kept"
+
+
+def test_merge_extra_body_preserving_model_passes_through_when_no_model():
+    merged = _merge_extra_body_preserving_model(
+        {"input": "hi"},
+        {"model": "from-extra-body", "extra": "kept"},
+    )
+    assert merged["model"] == "from-extra-body"
+    assert merged["extra"] == "kept"
+    assert merged["input"] == "hi"
+
+
 def test_get_agentic_loop_settings_defaults_and_overrides():
     handler = BaseLLMHTTPHandler()
 
diff --git a/tests/test_litellm/test_main.py b/tests/test_litellm/test_main.py
index 113e1bc0df8..1e6f536caf5 100644
--- a/tests/test_litellm/test_main.py
+++ b/tests/test_litellm/test_main.py
@@ -433,6 +433,28 @@ def test_custom_provider_with_extra_body():
         }
 
 
+def test_extra_body_cannot_override_authorized_model():
+    from litellm.llms.custom_httpx.http_handler import HTTPHandler
+
+    client = HTTPHandler()
+    with patch.object(client, "post") as mock_post:
+        try:
+            litellm.completion(
+                model="deepseek/deepseek-chat",
+                messages=[{"role": "user", "content": "Hello, how are you?"}],
+                extra_body={"model": "deepseek/some-other-model", "passthrough": "kept"},
+                api_key="fake-key-for-testing",
+                client=client,
+            )
+        except Exception:
+            pass
+
+        mock_post.assert_called_once()
+        sent_body = json.loads(mock_post.call_args.kwargs["data"])
+        assert sent_body["model"] == "deepseek-chat"
+        assert sent_body["passthrough"] == "kept"
+
+
 @pytest.fixture(autouse=True)
 def set_openrouter_api_key():
     original_api_key = os.environ.get("OPENROUTER_API_KEY")