diff --git a/src/google/adk/models/google_llm.py b/src/google/adk/models/google_llm.py
index 2268d4da8..fd1d8e582 100644
--- a/src/google/adk/models/google_llm.py
+++ b/src/google/adk/models/google_llm.py
@@ -23,6 +23,7 @@
 from typing import AsyncGenerator
 from typing import cast
 from typing import TYPE_CHECKING
+from typing import Union
 
 from google.genai import Client
 from google.genai import types
@@ -244,9 +245,18 @@ async def connect(self, llm_request: LlmRequest) -> BaseLlmConnection:
 
   def _preprocess_request(self, llm_request: LlmRequest) -> None:
 
-    if llm_request.config and self._api_backend == GoogleLLMVariant.GEMINI_API:
+    if self._api_backend == GoogleLLMVariant.GEMINI_API:
       # Using API key from Google AI Studio to call model doesn't support labels.
-      llm_request.config.labels = None
+      if llm_request.config:
+        llm_request.config.labels = None
+
+      if llm_request.contents:
+        for content in llm_request.contents:
+          if not content.parts:
+            continue
+          for part in content.parts:
+            _remove_display_name_if_present(part.inline_data)
+            _remove_display_name_if_present(part.file_data)
 
 
 def _build_function_declaration_log(
@@ -324,3 +334,15 @@ def _build_response_log(resp: types.GenerateContentResponse) -> str:
 {resp.model_dump_json(exclude_none=True)}
 -----------------------------------------------------------
 """
+
+
+def _remove_display_name_if_present(
+    data_obj: Union[types.Blob, types.FileData, None],
+):
+  """Sets display_name to None for the Gemini API (non-Vertex) backend.
+
+  This backend does not support the display_name parameter for file uploads,
+  so it must be removed to prevent request failures.
+  """
+  if data_obj and data_obj.display_name:
+    data_obj.display_name = None
diff --git a/tests/unittests/models/test_google_llm.py b/tests/unittests/models/test_google_llm.py
index 349dd13b8..9278bee54 100644
--- a/tests/unittests/models/test_google_llm.py
+++ b/tests/unittests/models/test_google_llm.py
@@ -14,6 +14,7 @@
 
 import os
 import sys
+from typing import Optional
 from unittest import mock
 
 from google.adk import version as adk_version
@@ -23,6 +24,7 @@
 from google.adk.models.google_llm import Gemini
 from google.adk.models.llm_request import LlmRequest
 from google.adk.models.llm_response import LlmResponse
+from google.adk.utils.variant_utils import GoogleLLMVariant
 from google.genai import types
 from google.genai import version as genai_version
 from google.genai.types import Content
@@ -337,3 +339,84 @@ async def __aexit__(self, *args):
   ):
     async with gemini_llm.connect(llm_request) as connection:
       assert connection is mock_connection
+
+
+@pytest.mark.parametrize(
+    (
+        "api_backend, "
+        "expected_file_display_name, "
+        "expected_inline_display_name, "
+        "expected_labels"
+    ),
+    [
+        (
+            GoogleLLMVariant.GEMINI_API,
+            None,
+            None,
+            None,
+        ),
+        (
+            GoogleLLMVariant.VERTEX_AI,
+            "My Test PDF",
+            "My Test Image",
+            {"key": "value"},
+        ),
+    ],
+)
+def test_preprocess_request_handles_backend_specific_fields(
+    gemini_llm: Gemini,
+    api_backend: GoogleLLMVariant,
+    expected_file_display_name: Optional[str],
+    expected_inline_display_name: Optional[str],
+    expected_labels: Optional[str],
+):
+  """
+  Tests that _preprocess_request correctly sanitizes fields based on the API backend.
+
+  - For GEMINI_API, it should remove 'display_name' from file/inline data
+    and remove 'labels' from the config.
+  - For VERTEX_AI, it should leave these fields untouched.
+  """
+  # Arrange: Create a request with fields that need to be preprocessed.
+  llm_request_with_files = LlmRequest(
+      model="gemini-1.5-flash",
+      contents=[
+          Content(
+              role="user",
+              parts=[
+                  Part(
+                      file_data=types.FileData(
+                          file_uri="gs://bucket/file.pdf",
+                          mime_type="application/pdf",
+                          display_name="My Test PDF",
+                      )
+                  ),
+                  Part(
+                      inline_data=types.Blob(
+                          data=b"some_bytes",
+                          mime_type="image/png",
+                          display_name="My Test Image",
+                      )
+                  ),
+              ],
+          )
+      ],
+      config=types.GenerateContentConfig(labels={"key": "value"}),
+  )
+
+  # Mock the _api_backend property to control the test scenario
+  with mock.patch.object(
+      Gemini, "_api_backend", new_callable=mock.PropertyMock
+  ) as mock_backend:
+    mock_backend.return_value = api_backend
+
+    # Act: Run the preprocessing method
+    gemini_llm._preprocess_request(llm_request_with_files)
+
+    # Assert: Check if the fields were correctly processed
+    file_part = llm_request_with_files.contents[0].parts[0]
+    inline_part = llm_request_with_files.contents[0].parts[1]
+
+    assert file_part.file_data.display_name == expected_file_display_name
+    assert inline_part.inline_data.display_name == expected_inline_display_name
+    assert llm_request_with_files.config.labels == expected_labels