diff --git a/packages/google-api-core/google/api_core/gapic_v1/__init__.py b/packages/google-api-core/google/api_core/gapic_v1/__init__.py
index 78937c032670..0d7e53b54c5e 100644
--- a/packages/google-api-core/google/api_core/gapic_v1/__init__.py
+++ b/packages/google-api-core/google/api_core/gapic_v1/__init__.py
@@ -30,6 +30,7 @@
 }
 __all__ = ["client_info", "requests", "routing_header"]
 
+
 if _has_grpc:
     __lazy_modules__.update(
         {
diff --git a/packages/google-api-core/tests/conftest.py b/packages/google-api-core/tests/conftest.py
new file mode 100644
index 000000000000..ffa69d61f1a2
--- /dev/null
+++ b/packages/google-api-core/tests/conftest.py
@@ -0,0 +1,30 @@
+# 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.
+
+import os
+from unittest import mock
+
+import pytest
+
+
+@pytest.fixture(scope="session", autouse=True)
+def mock_mtls_env():
+    """Autouse session-scoped fixture to isolate unit tests from workstation mTLS environments."""
+    with mock.patch.dict(
+        os.environ,
+        {
+            "GOOGLE_API_USE_CLIENT_CERTIFICATE": "false",
+        },
+    ):
+        yield