fix(bigframes): update GeminiTextGenerator default model to gemini-2.5-flash (#18060)

Fixes #<544873054> 🦕

Author: shuoweil

packages/bigframes/bigframes/ml/llm.py

@@ -440,15 +440,15 @@ class GeminiTextGenerator(base.RetriableRemotePredictor):
         gemini-1.5-X are going to be deprecated. Use gemini-2.5-X (https://cloud.google.com/python/docs/reference/bigframes/latest/bigframes.ml.llm.GeminiTextGenerator) instead.
 
     Args:
-        model_name (str, Default to "gemini-2.0-flash-001"):
+        model_name (str, Default to "gemini-2.5-flash"):
             The model for natural language tasks. Accepted values are
             "gemini-1.5-pro-preview-0514", "gemini-1.5-flash-preview-0514",
             "gemini-1.5-pro-001", "gemini-1.5-pro-002", "gemini-1.5-flash-001",
             "gemini-1.5-flash-002", "gemini-2.0-flash-exp",
             "gemini-2.0-flash-lite-001", "gemini-2.0-flash-001",
             "gemini-2.5-pro", "gemini-2.5-flash", "gemini-2.5-flash-lite",
             "gemini-3.1-flash-lite" and "gemini-3.5-flash".
-            If no setting is provided, "gemini-2.0-flash-001" will be used by
+            If no setting is provided, "gemini-2.5-flash" will be used by
             default and a warning will be issued.
 
         .. note::
@@ -505,7 +505,7 @@ def __init__(
             warnings.warn(msg, category=exceptions.PreviewWarning)
 
         if model_name is None:
-            model_name = "gemini-2.0-flash-001"
+            model_name = "gemini-2.5-flash"
             msg = exceptions.format_message(_REMOVE_DEFAULT_MODEL_WARNING)
             warnings.warn(msg, category=FutureWarning, stacklevel=2)
 

packages/bigframes/tests/unit/ml/test_llm.py

@@ -0,0 +1,50 @@
+# Copyright 2026 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.
+
+from unittest import mock
+
+import pytest
+from google.cloud import bigquery
+
+import bigframes.session
+from bigframes.ml import llm
+
+
+def test_gemini_text_generator_default_model():
+    mock_session = mock.create_autospec(spec=bigframes.session.Session)
+    mock_session._create_bq_connection.return_value = (
+        "projects/test-project/locations/us-central1/connections/test-conn"
+    )
+    mock_session._anonymous_dataset = bigquery.DatasetReference(
+        "test-project", "test_dataset"
+    )
+    mock_job = mock.MagicMock()
+    mock_job.destination.project = "test-project"
+    mock_job.destination.dataset_id = "test_dataset"
+    mock_job.destination.table_id = "test_model"
+    mock_session._start_query_ml_ddl.return_value = (None, mock_job)
+    mock_session.bqclient.get_model.return_value = mock.MagicMock(spec=bigquery.Model)
+
+    with pytest.warns(
+        FutureWarning, match="default model will be removed in BigFrames 3.0"
+    ):
+        model = llm.GeminiTextGenerator(
+            session=mock_session,
+            connection_name="test-conn",
+        )
+
+    assert model.model_name == "gemini-2.5-flash"
+    mock_session._start_query_ml_ddl.assert_called_once()
+    generated_sql = mock_session._start_query_ml_ddl.call_args[0][0]
+    assert "gemini-2.5-flash" in generated_sql