diff --git a/py/plugins/vertex-ai/src/genkit/plugins/vertex_ai/model_garden/client.py b/py/plugins/vertex-ai/src/genkit/plugins/vertex_ai/model_garden/client.py
index a28d03aaf..93519d00a 100644
--- a/py/plugins/vertex-ai/src/genkit/plugins/vertex_ai/model_garden/client.py
+++ b/py/plugins/vertex-ai/src/genkit/plugins/vertex_ai/model_garden/client.py
@@ -14,7 +14,7 @@
 #
 # SPDX-License-Identifier: Apache-2.0
 
-from google.auth import default, transport
+from google import auth
 from openai import OpenAI as _OpenAI
 
 
@@ -26,10 +26,10 @@ def __new__(cls, **openai_params) -> _OpenAI:
         location = openai_params.get('location')
         project_id = openai_params.get('project_id')
         if project_id:
-            credentials, _ = default()
+            credentials, _ = auth.default()
         else:
-            credentials, project_id = default()
+            credentials, project_id = auth.default()
 
-        credentials.refresh(transport.requests.Request())
+        credentials.refresh(auth.transport.requests.Request())
         base_url = f'https://{location}-aiplatform.googleapis.com/v1beta1/projects/{project_id}/locations/{location}/endpoints/openapi'
         return _OpenAI(api_key=credentials.token, base_url=base_url)
diff --git a/py/plugins/vertex-ai/tests/model_garden/test_client.py b/py/plugins/vertex-ai/tests/model_garden/test_client.py
new file mode 100644
index 000000000..4c663e28f
--- /dev/null
+++ b/py/plugins/vertex-ai/tests/model_garden/test_client.py
@@ -0,0 +1,75 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+"""Unittests for VertexAI Model Garden OpenAI Client."""
+
+from unittest.mock import MagicMock, patch
+
+from genkit.plugins.vertex_ai.model_garden.client import OpenAIClient
+
+
+@patch('google.auth.default')
+@patch('google.auth.transport.requests.Request')
+@patch('openai.OpenAI')
+def test_client_initialization_with_explicit_project_id(
+    mock_openai_cls, mock_request_cls, mock_default_auth
+):
+    """Unittests for init client."""
+    mock_location = "location"
+    mock_project_id = "project_id"
+    mock_token = "token"
+
+    mock_credentials = MagicMock()
+    mock_credentials.token = mock_token
+
+    mock_default_auth.return_value = (mock_credentials, "project_id")
+
+    client_instance = OpenAIClient(
+        location=mock_location,
+        project_id=mock_project_id
+    )
+
+    mock_default_auth.assert_called_once()
+    mock_credentials.refresh.assert_called_once()
+    mock_request_cls.assert_called_once()
+
+    assert client_instance is not None
+
+
+@patch('google.auth.default')
+@patch('google.auth.transport.requests.Request')
+@patch('openai.OpenAI')
+def test_client_initialization_without_explicit_project_id(
+    mock_openai_cls, mock_request_cls, mock_default_auth
+):
+    """Unittests for init client."""
+    mock_location = "location"
+    mock_token = "token"
+
+    mock_credentials = MagicMock()
+    mock_credentials.token = mock_token
+
+    mock_default_auth.return_value = (mock_credentials, "project_id")
+
+    client_instance = OpenAIClient(
+        location=mock_location,
+    )
+
+    mock_default_auth.assert_called_once()
+    mock_credentials.refresh.assert_called_once()
+    mock_request_cls.assert_called_once()
+
+    assert client_instance is not None
diff --git a/py/plugins/vertex-ai/tests/model_garden/test_model_garden.py b/py/plugins/vertex-ai/tests/model_garden/test_model_garden.py
new file mode 100644
index 000000000..d98df68e0
--- /dev/null
+++ b/py/plugins/vertex-ai/tests/model_garden/test_model_garden.py
@@ -0,0 +1,18 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+"""Unittests for VertexAI Model Garden Models."""
+