diff --git a/packages/gapic-generator/gapic/generator/generator.py b/packages/gapic-generator/gapic/generator/generator.py
index 9fe56aa9de1d..c1932abbe426 100644
--- a/packages/gapic-generator/gapic/generator/generator.py
+++ b/packages/gapic-generator/gapic/generator/generator.py
@@ -120,7 +120,7 @@ def get_response(self, api_schema: api.API, opts: Options) -> CodeGeneratorRespo
         for template_name in client_templates:
             # Quick check: Skip "private" templates.
             filename = template_name.split("/")[-1]
-            if filename.startswith("_") and filename != "__init__.py.j2":
+            if filename.startswith("_") and filename not in ("__init__.py.j2", "_compat.py.j2"):
                 continue
 
             # Append to the output files dictionary.
diff --git a/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/_compat.py.j2 b/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/_compat.py.j2
new file mode 100644
index 000000000000..5dac8c33a7f9
--- /dev/null
+++ b/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/_compat.py.j2
@@ -0,0 +1,271 @@
+# {% include '_license.j2' %}
+
+"""A compatibility module for older versions of google-api-core."""
+
+import functools
+import json
+import operator
+import os
+import re
+import uuid
+from typing import Any, Callable, Dict, List, Optional, Tuple, Union
+from google.auth.exceptions import MutualTLSChannelError
+import google.protobuf.message
+
+
+try:
+    from google.api_core.universe import (
+        get_default_mtls_endpoint,
+        get_api_endpoint,
+        get_universe_domain,
+    )
+except ImportError:
+    # TODO(https://github.com/googleapis/google-cloud-python/issues/17813): Universe domain support was introduced in google-api-core 2.18.0.
+    # Remove these fallback definitions when google-api-core >= 2.18.0 becomes the minimum required version in generated client setup dependencies.
+    def get_default_mtls_endpoint(api_endpoint: Optional[str]) -> Optional[str]:
+        """Converts api endpoint to mTLS endpoint."""
+        if not api_endpoint:
+            return api_endpoint
+
+        mtls_endpoint_re = re.compile(
+            r"(?P<name>[^.]+)(?P<mtls>\.mtls)?(?P<sandbox>\.sandbox)?(?P<googledomain>\.googleapis\.com)?"
+        )
+
+        m = mtls_endpoint_re.match(api_endpoint)
+        if m is None:
+            # Could not parse api_endpoint; return as-is.
+            return api_endpoint
+
+        name, mtls, sandbox, googledomain = m.groups()
+        if mtls or not googledomain:
+            return api_endpoint
+
+        if sandbox:
+            return api_endpoint.replace(
+                "sandbox.googleapis.com", "mtls.sandbox.googleapis.com"
+            )
+
+        return api_endpoint.replace(".googleapis.com", ".mtls.googleapis.com")
+
+    def get_api_endpoint(
+        api_override: Optional[str],
+        client_cert_source: Optional[Callable[[], Tuple[bytes, bytes]]],
+        universe_domain: str,
+        use_mtls_endpoint: str,
+        default_universe: str,
+        default_mtls_endpoint: Optional[str],
+        default_endpoint_template: str,
+    ) -> Optional[str]:
+        """Return the API endpoint used by the client."""
+        if api_override is not None:
+            api_endpoint = api_override
+        elif use_mtls_endpoint == "always" or (use_mtls_endpoint == "auto" and client_cert_source):
+            if universe_domain != default_universe:
+                raise MutualTLSChannelError(
+                    f"mTLS is not supported in any universe other than {default_universe}."
+                )
+            api_endpoint = default_mtls_endpoint
+        else:
+            api_endpoint = default_endpoint_template.format(UNIVERSE_DOMAIN=universe_domain)
+        return api_endpoint
+
+    def get_universe_domain(
+        client_universe_domain: Optional[str] = None,
+        universe_domain_env: Optional[str] = None,
+        default_universe: str = "googleapis.com",
+    ) -> str:
+        """Return the universe domain used by the client."""
+        universe_domain = default_universe
+        if client_universe_domain is not None:
+            universe_domain = client_universe_domain
+        elif universe_domain_env is not None:
+            universe_domain = universe_domain_env
+        if len(universe_domain.strip()) == 0:
+            raise ValueError("Universe Domain cannot be an empty string.") 
+        return universe_domain
+
+
+try:
+    from google.api_core.gapic_v1.config import (
+        use_client_cert_effective,
+        get_client_cert_source,
+        read_environment_variables,
+    )
+except ImportError:
+    from google.auth.transport import mtls  # type: ignore
+
+    # TODO(https://github.com/googleapis/google-cloud-python/issues/17813): Remove these fallbacks when google-api-core >= 2.18.0 is the minimum required version.
+
+    def use_client_cert_effective() -> bool:
+        """Returns whether client certificate should be used for mTLS."""
+        if hasattr(mtls, "should_use_client_cert"):
+            return mtls.should_use_client_cert()
+        else:
+            use_client_cert_str = os.getenv("GOOGLE_API_USE_CLIENT_CERTIFICATE", "false").lower()
+            if use_client_cert_str not in ("true", "false"):
+                raise ValueError(
+                    "Environment variable `GOOGLE_API_USE_CLIENT_CERTIFICATE` must be"
+                    " either `true` or `false`"
+                )
+            return use_client_cert_str == "true"
+
+    def get_client_cert_source(
+        provided_cert_source: Optional[Callable[[], Tuple[bytes, bytes]]],
+        use_cert_flag: bool,
+    ) -> Optional[Callable[[], Tuple[bytes, bytes]]]:
+        """Return the client cert source to be used by the client."""
+        client_cert_source = None
+        if use_cert_flag:
+            if provided_cert_source:
+                client_cert_source = provided_cert_source
+            elif (
+                hasattr(mtls, "has_default_client_cert_source")
+                and mtls.has_default_client_cert_source()
+            ):
+                client_cert_source = mtls.default_client_cert_source()
+            else:
+                raise ValueError(
+                    "Client certificate is required for mTLS, but no client certificate source was provided or found."
+                )
+        return client_cert_source
+
+    def read_environment_variables() -> Tuple[bool, str, Optional[str]]:
+        """Returns the environment variables used by the client."""
+        use_client_cert = use_client_cert_effective()
+        use_mtls_endpoint = os.getenv("GOOGLE_API_USE_MTLS_ENDPOINT", "auto").lower()
+        universe_domain_env = os.getenv("GOOGLE_CLOUD_UNIVERSE_DOMAIN")
+        if use_mtls_endpoint not in ("auto", "never", "always"):
+            raise MutualTLSChannelError(
+                "Environment variable `GOOGLE_API_USE_MTLS_ENDPOINT` "
+                "must be `never`, `auto` or `always`"
+            )
+        return use_client_cert, use_mtls_endpoint, universe_domain_env
+
+
+try:
+    from google.api_core.gapic_v1.request import setup_request_id  # type: ignore
+except ImportError:
+    # TODO(https://github.com/googleapis/google-cloud-python/issues/17813): Remove this fallback when google-api-core >= 2.26.0 is the minimum required version.
+    def setup_request_id(request, field_name: str, is_proto3_optional: bool):
+        """Populate a UUID4 field in the request if it is not already set.
+
+        Args:
+            request (Union[google.protobuf.message.Message, dict]): The request object.
+            field_name (str): The name of the field to populate.
+            is_proto3_optional (bool): Whether the field is proto3 optional.
+        """
+        request_id_val = str(uuid.uuid4())
+        if request is None:
+            return
+
+        if isinstance(request, dict):
+            if is_proto3_optional:
+                if field_name not in request or request[field_name] is None:
+                    request[field_name] = request_id_val
+            elif not request.get(field_name):
+                request[field_name] = request_id_val
+            return
+
+        if is_proto3_optional:
+            try:
+                # Pure protobuf messages
+                if not request.HasField(field_name):
+                    setattr(request, field_name, request_id_val)
+            except (AttributeError, ValueError):
+                # Proto-plus messages or other objects
+                if getattr(request, field_name, None) is None:
+                    setattr(request, field_name, request_id_val)
+        else:
+            if not getattr(request, field_name, None):
+                setattr(request, field_name, request_id_val)
+
+
+try:
+    from google.api_core.rest_helpers import (
+        flatten_query_params,
+        transcode_request,
+    )
+except ImportError:  # pragma: NO COVER
+    # TODO: Remove these fallbacks when google-api-core >= 2.18.0 is the minimum required version.
+    from google.protobuf import json_format  # type: ignore
+    from google.api_core import path_template  # type: ignore
+
+    def flatten_query_params(obj, strict=False):  # pragma: NO COVER
+        if obj is not None and not isinstance(obj, dict):
+            raise TypeError("flatten_query_params must be called with dict object")
+        return _flatten(obj, key_path=[], strict=strict)
+
+    def _flatten(obj, key_path, strict=False):  # pragma: NO COVER
+        if obj is None:
+            return []
+        if isinstance(obj, dict):
+            return _flatten_dict(obj, key_path=key_path, strict=strict)
+        if isinstance(obj, list):
+            return _flatten_list(obj, key_path=key_path, strict=strict)
+        return _flatten_value(obj, key_path=key_path, strict=strict)
+
+    def _is_primitive_value(obj):  # pragma: NO COVER
+        if obj is None:
+            return False
+        if isinstance(obj, (list, dict)):
+            raise ValueError("query params may not contain repeated dicts or lists")
+        return True
+
+    def _flatten_value(obj, key_path, strict=False):  # pragma: NO COVER
+        return [(".".join(key_path), _canonicalize(obj, strict=strict))]
+
+    def _flatten_dict(obj, key_path, strict=False):  # pragma: NO COVER
+        items = (
+            _flatten(value, key_path=key_path + [key], strict=strict)
+            for key, value in obj.items()
+        )
+        return functools.reduce(operator.concat, items, [])
+
+    def _flatten_list(elems, key_path, strict=False):  # pragma: NO COVER
+        items = (
+            _flatten_value(elem, key_path=key_path, strict=strict)
+            for elem in elems
+            if _is_primitive_value(elem)
+        )
+        return functools.reduce(operator.concat, items, [])
+
+    def _canonicalize(obj, strict=False):  # pragma: NO COVER
+        if strict:
+            value = str(obj)
+            if isinstance(obj, bool):
+                value = value.lower()
+            return value
+        return obj
+
+    def transcode_request(  # pragma: NO COVER
+        http_options: List[Dict[str, str]],
+        request: Any,
+        required_fields_default_values: Optional[Dict[str, Any]] = None,
+        rest_numeric_enums: bool = False,
+    ) -> Tuple[Dict[str, Any], Optional[str], Dict[str, Any]]:
+        pb_request = getattr(request, "_pb", request)
+        transcoded_request = path_template.transcode(http_options, pb_request)
+
+        body_json = None
+        if transcoded_request.get("body") is not None:
+            body_json = json_format.MessageToJson(
+                transcoded_request["body"],
+                use_integers_for_enums=rest_numeric_enums,
+            )
+
+        query_params_json = {}
+        if transcoded_request.get("query_params") is not None:
+            query_params_json = json.loads(json_format.MessageToJson(
+                transcoded_request["query_params"],
+                use_integers_for_enums=rest_numeric_enums,
+            ))
+
+        if required_fields_default_values:
+            for k, v in required_fields_default_values.items():
+                if k not in query_params_json:
+                    query_params_json[k] = v
+
+        if rest_numeric_enums:
+            query_params_json["$alt"] = "json;enum-encoding=int"
+
+        return transcoded_request, body_json, query_params_json
diff --git a/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2 b/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2
index e2e3edb24967..8a73595b7789 100644
--- a/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2
+++ b/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2
@@ -30,6 +30,7 @@ from google.api_core import exceptions as core_exceptions
 from google.api_core import extended_operation
 {% endif %}
 from google.api_core import gapic_v1
+from {{package_path}} import _compat as client_utils
 from google.api_core import retry as retries
 from google.auth import credentials as ga_credentials             # type: ignore
 from google.auth.transport import mtls                            # type: ignore
@@ -143,44 +144,10 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
     """{{ service.meta.doc|rst(width=72, indent=4) }}{% if service.version|length %}
     This class implements API version {{ service.version }}.{% endif %}"""
 
-    @staticmethod
-    def _get_default_mtls_endpoint(api_endpoint) -> Optional[str]:
-        """Converts api endpoint to mTLS endpoint.
-
-        Convert "*.sandbox.googleapis.com" and "*.googleapis.com" to
-        "*.mtls.sandbox.googleapis.com" and "*.mtls.googleapis.com" respectively.
-        Args:
-            api_endpoint (Optional[str]): the api endpoint to convert.
-        Returns:
-            Optional[str]: converted mTLS api endpoint.
-        """
-        if not api_endpoint:
-            return api_endpoint
-
-        mtls_endpoint_re = re.compile(
-            r"(?P<name>[^.]+)(?P<mtls>\.mtls)?(?P<sandbox>\.sandbox)?(?P<googledomain>\.googleapis\.com)?"
-        )
-
-        m = mtls_endpoint_re.match(api_endpoint)
-        if m is None:
-            # Could not parse api_endpoint; return as-is.
-            return api_endpoint
-
-        name, mtls, sandbox, googledomain = m.groups()
-        if mtls or not googledomain:
-            return api_endpoint
-
-        if sandbox:
-            return api_endpoint.replace(
-                "sandbox.googleapis.com", "mtls.sandbox.googleapis.com"
-            )
-
-        return api_endpoint.replace(".googleapis.com", ".mtls.googleapis.com")
-
     # Note: DEFAULT_ENDPOINT is deprecated. Use _DEFAULT_ENDPOINT_TEMPLATE instead.
     DEFAULT_ENDPOINT = {% if service.host %}"{{ service.host }}"{% else %}None{% endif %}
 
-    DEFAULT_MTLS_ENDPOINT = _get_default_mtls_endpoint.__func__(  # type: ignore
+    DEFAULT_MTLS_ENDPOINT = client_utils.get_default_mtls_endpoint(
         DEFAULT_ENDPOINT
     )
 
@@ -392,30 +359,34 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         return client_cert_source
 
     @staticmethod
-    def _get_api_endpoint(api_override, client_cert_source, universe_domain, use_mtls_endpoint) -> str:
+    def _get_api_endpoint(
+        api_override: Optional[str],
+        client_cert_source: Optional[Callable[[], Tuple[bytes, bytes]]],
+        universe_domain: str,
+        use_mtls_endpoint: str,
+    ) -> Optional[str]:
         """Return the API endpoint used by the client.
 
         Args:
-            api_override (str): The API endpoint override. If specified, this is always
+            api_override (Optional[str]): The API endpoint override. If specified, this is always
                 the return value of this function and the other arguments are not used.
-            client_cert_source (bytes): The client certificate source used by the client.
+            client_cert_source (Union[Callable[[], Tuple[bytes, bytes]], None]): The client certificate source used by the client.
             universe_domain (str): The universe domain used by the client.
             use_mtls_endpoint (str): How to use the mTLS endpoint, which depends also on the other parameters.
                 Possible values are "always", "auto", or "never".
 
         Returns:
-            str: The API endpoint to be used by the client.
+            Optional[str]: The API endpoint to be used by the client.
         """
-        if api_override is not None:
-            api_endpoint = api_override
-        elif use_mtls_endpoint == "always" or (use_mtls_endpoint == "auto" and client_cert_source):
-            _default_universe = {{ service.client_name }}._DEFAULT_UNIVERSE
-            if universe_domain != _default_universe:
-                raise MutualTLSChannelError(f"mTLS is not supported in any universe other than {_default_universe}.")
-            api_endpoint = {{ service.client_name }}.DEFAULT_MTLS_ENDPOINT
-        else:
-            api_endpoint = {{ service.client_name }}._DEFAULT_ENDPOINT_TEMPLATE.format(UNIVERSE_DOMAIN=universe_domain)
-        return api_endpoint
+        return client_utils.get_api_endpoint(
+            api_override,
+            client_cert_source,
+            universe_domain,
+            use_mtls_endpoint,
+            {{ service.client_name }}._DEFAULT_UNIVERSE,
+            {{ service.client_name }}.DEFAULT_MTLS_ENDPOINT,
+            {{ service.client_name }}._DEFAULT_ENDPOINT_TEMPLATE,
+        )
 
     @staticmethod
     def _get_universe_domain(client_universe_domain: Optional[str], universe_domain_env: Optional[str]) -> str:
@@ -431,14 +402,11 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         Raises:
             ValueError: If the universe domain is an empty string.
         """
-        universe_domain = {{ service.client_name }}._DEFAULT_UNIVERSE
-        if client_universe_domain is not None:
-            universe_domain = client_universe_domain
-        elif universe_domain_env is not None:
-            universe_domain = universe_domain_env
-        if len(universe_domain.strip()) == 0:
-            raise ValueError("Universe Domain cannot be an empty string.") 
-        return universe_domain
+        return client_utils.get_universe_domain(
+            client_universe_domain,
+            universe_domain_env,
+            default_universe={{ service.client_name }}._DEFAULT_UNIVERSE,
+        )
     
     def _validate_universe_domain(self):
         """Validates client's and credentials' universe domains are consistent.
diff --git a/packages/gapic-generator/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2 b/packages/gapic-generator/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2
index c68b390b7c84..7843ceb31bff 100644
--- a/packages/gapic-generator/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2
+++ b/packages/gapic-generator/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2
@@ -156,22 +156,6 @@ def set_event_loop():
             asyncio.set_event_loop(None)
 
 
-def test__get_default_mtls_endpoint():
-    api_endpoint = "example.googleapis.com"
-    api_mtls_endpoint = "example.mtls.googleapis.com"
-    sandbox_endpoint = "example.sandbox.googleapis.com"
-    sandbox_mtls_endpoint = "example.mtls.sandbox.googleapis.com"
-    non_googleapi = "api.example.com"
-    custom_endpoint = ".custom"
-
-    assert {{ service.client_name }}._get_default_mtls_endpoint(None) is None
-    assert {{ service.client_name }}._get_default_mtls_endpoint(api_endpoint) == api_mtls_endpoint
-    assert {{ service.client_name }}._get_default_mtls_endpoint(api_mtls_endpoint) == api_mtls_endpoint
-    assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_endpoint) == sandbox_mtls_endpoint
-    assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_mtls_endpoint) == sandbox_mtls_endpoint
-    assert {{ service.client_name }}._get_default_mtls_endpoint(non_googleapi) == non_googleapi
-    assert {{ service.client_name }}._get_default_mtls_endpoint(custom_endpoint) == custom_endpoint
-
 def test__read_environment_variables():
     assert {{ service.client_name }}._read_environment_variables() == (False, "auto", None)
     
@@ -313,29 +297,7 @@ def test__get_client_cert_source():
             assert {{ service.client_name }}._get_client_cert_source(None, True) is mock_default_cert_source
             assert {{ service.client_name }}._get_client_cert_source(mock_provided_cert_source, "true") is mock_provided_cert_source
 
-@mock.patch.object({{ service.client_name }}, "_DEFAULT_ENDPOINT_TEMPLATE", modify_default_endpoint_template({{ service.client_name }}))
-{% if 'grpc' in opts.transport %}
-@mock.patch.object({{ service.async_client_name }}, "_DEFAULT_ENDPOINT_TEMPLATE", modify_default_endpoint_template({{ service.async_client_name }}))
-{% endif %}
-def test__get_api_endpoint():
-    api_override = "foo.com"
-    mock_client_cert_source = mock.Mock()
-    default_universe = {{ service.client_name }}._DEFAULT_UNIVERSE
-    default_endpoint = {{ service.client_name }}._DEFAULT_ENDPOINT_TEMPLATE.format(UNIVERSE_DOMAIN=default_universe)
-    mock_universe = "bar.com"
-    mock_endpoint = {{ service.client_name }}._DEFAULT_ENDPOINT_TEMPLATE.format(UNIVERSE_DOMAIN=mock_universe)
-
-    assert {{ service.client_name }}._get_api_endpoint(api_override, mock_client_cert_source, default_universe, "always") == api_override
-    assert {{ service.client_name }}._get_api_endpoint(None, mock_client_cert_source, default_universe, "auto") == {{ service.client_name }}.DEFAULT_MTLS_ENDPOINT
-    assert {{ service.client_name }}._get_api_endpoint(None, None, default_universe, "auto") == default_endpoint
-    assert {{ service.client_name }}._get_api_endpoint(None, None, default_universe, "always") == {{ service.client_name }}.DEFAULT_MTLS_ENDPOINT
-    assert {{ service.client_name }}._get_api_endpoint(None, mock_client_cert_source, default_universe, "always") == {{ service.client_name }}.DEFAULT_MTLS_ENDPOINT
-    assert {{ service.client_name }}._get_api_endpoint(None, None, mock_universe, "never") == mock_endpoint
-    assert {{ service.client_name }}._get_api_endpoint(None, None, default_universe, "never") == default_endpoint
 
-    with pytest.raises(MutualTLSChannelError) as excinfo:
-        {{ service.client_name }}._get_api_endpoint(None, mock_client_cert_source, mock_universe, "auto")
-    assert str(excinfo.value) == "mTLS is not supported in any universe other than googleapis.com."
 
 {% if service.version %}
 {% for method in service.methods.values() %}{% with method_name = method.name|snake_case %}
@@ -384,17 +346,7 @@ def test_{{ method_name }}_api_version_header(transport_name):
 {% endfor %}
 {% endif %}{# service.version #}
 
-def test__get_universe_domain():
-    client_universe_domain = "foo.com"
-    universe_domain_env = "bar.com"
-
-    assert {{ service.client_name }}._get_universe_domain(client_universe_domain, universe_domain_env) == client_universe_domain
-    assert {{ service.client_name }}._get_universe_domain(None, universe_domain_env) == universe_domain_env
-    assert {{ service.client_name }}._get_universe_domain(None, None) == {{ service.client_name }}._DEFAULT_UNIVERSE
 
-    with pytest.raises(ValueError) as excinfo:
-        {{ service.client_name }}._get_universe_domain("", None)
-    assert str(excinfo.value) == "Universe Domain cannot be an empty string."
 
 @pytest.mark.parametrize("error_code,cred_info_json,show_cred_info", [
     (401, CRED_INFO_JSON, True),
diff --git a/packages/gapic-generator/noxfile.py b/packages/gapic-generator/noxfile.py
index 52209f41ff38..dce1cf1504e2 100644
--- a/packages/gapic-generator/noxfile.py
+++ b/packages/gapic-generator/noxfile.py
@@ -181,6 +181,7 @@ def fragment(session, use_ads_templates=False):
         "grpcio-tools",
     )
     session.install("-e", ".")
+    session.install("-e", "../google-api-core")
 
     # The specific failure is `Plugin output is unparseable`
     if session.python == "3.10":
@@ -245,6 +246,7 @@ def showcase_library(
 
     # Install gapic-generator-python
     session.install("-e", ".")
+    session.install("-e", "../google-api-core")
 
     # Install grpcio-tools for protoc
     session.install("grpcio-tools")
diff --git a/packages/gapic-generator/tests/integration/goldens/asset/tests/unit/gapic/asset_v1/test_asset_service.py b/packages/gapic-generator/tests/integration/goldens/asset/tests/unit/gapic/asset_v1/test_asset_service.py
index ea110a38acc3..1a367689d14e 100755
--- a/packages/gapic-generator/tests/integration/goldens/asset/tests/unit/gapic/asset_v1/test_asset_service.py
+++ b/packages/gapic-generator/tests/integration/goldens/asset/tests/unit/gapic/asset_v1/test_asset_service.py
@@ -700,7 +700,7 @@ def test_asset_service_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -747,7 +747,7 @@ def test_asset_service_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/credentials/tests/unit/gapic/credentials_v1/test_iam_credentials.py b/packages/gapic-generator/tests/integration/goldens/credentials/tests/unit/gapic/credentials_v1/test_iam_credentials.py
index 14a6074a40f9..508e17a7e889 100755
--- a/packages/gapic-generator/tests/integration/goldens/credentials/tests/unit/gapic/credentials_v1/test_iam_credentials.py
+++ b/packages/gapic-generator/tests/integration/goldens/credentials/tests/unit/gapic/credentials_v1/test_iam_credentials.py
@@ -690,7 +690,7 @@ def test_iam_credentials_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -737,7 +737,7 @@ def test_iam_credentials_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/eventarc/tests/unit/gapic/eventarc_v1/test_eventarc.py b/packages/gapic-generator/tests/integration/goldens/eventarc/tests/unit/gapic/eventarc_v1/test_eventarc.py
index 533e401eb1e7..b9495ef6e42d 100755
--- a/packages/gapic-generator/tests/integration/goldens/eventarc/tests/unit/gapic/eventarc_v1/test_eventarc.py
+++ b/packages/gapic-generator/tests/integration/goldens/eventarc/tests/unit/gapic/eventarc_v1/test_eventarc.py
@@ -721,7 +721,7 @@ def test_eventarc_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -768,7 +768,7 @@ def test_eventarc_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_config_service_v2.py b/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_config_service_v2.py
index eada5b433c55..8cc17d810664 100755
--- a/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_config_service_v2.py
+++ b/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_config_service_v2.py
@@ -678,7 +678,7 @@ def test_config_service_v2_client_get_mtls_endpoint_and_cert_source(client_class
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -725,7 +725,7 @@ def test_config_service_v2_client_get_mtls_endpoint_and_cert_source(client_class
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_logging_service_v2.py b/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_logging_service_v2.py
index 65559a5d1073..fd56f0210d78 100755
--- a/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_logging_service_v2.py
+++ b/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_logging_service_v2.py
@@ -679,7 +679,7 @@ def test_logging_service_v2_client_get_mtls_endpoint_and_cert_source(client_clas
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -726,7 +726,7 @@ def test_logging_service_v2_client_get_mtls_endpoint_and_cert_source(client_clas
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_metrics_service_v2.py b/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_metrics_service_v2.py
index 90cdab2be2b2..762b4b3ab94d 100755
--- a/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_metrics_service_v2.py
+++ b/packages/gapic-generator/tests/integration/goldens/logging/tests/unit/gapic/logging_v2/test_metrics_service_v2.py
@@ -677,7 +677,7 @@ def test_metrics_service_v2_client_get_mtls_endpoint_and_cert_source(client_clas
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -724,7 +724,7 @@ def test_metrics_service_v2_client_get_mtls_endpoint_and_cert_source(client_clas
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_config_service_v2.py b/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_config_service_v2.py
index 9eec837e6f58..93731051d1bc 100755
--- a/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_config_service_v2.py
+++ b/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_config_service_v2.py
@@ -678,7 +678,7 @@ def test_base_config_service_v2_client_get_mtls_endpoint_and_cert_source(client_
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -725,7 +725,7 @@ def test_base_config_service_v2_client_get_mtls_endpoint_and_cert_source(client_
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_logging_service_v2.py b/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_logging_service_v2.py
index 65559a5d1073..fd56f0210d78 100755
--- a/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_logging_service_v2.py
+++ b/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_logging_service_v2.py
@@ -679,7 +679,7 @@ def test_logging_service_v2_client_get_mtls_endpoint_and_cert_source(client_clas
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -726,7 +726,7 @@ def test_logging_service_v2_client_get_mtls_endpoint_and_cert_source(client_clas
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_metrics_service_v2.py b/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_metrics_service_v2.py
index 310677b64bc6..090ea4a91bec 100755
--- a/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_metrics_service_v2.py
+++ b/packages/gapic-generator/tests/integration/goldens/logging_internal/tests/unit/gapic/logging_v2/test_metrics_service_v2.py
@@ -677,7 +677,7 @@ def test_base_metrics_service_v2_client_get_mtls_endpoint_and_cert_source(client
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -724,7 +724,7 @@ def test_base_metrics_service_v2_client_get_mtls_endpoint_and_cert_source(client
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/redis/tests/unit/gapic/redis_v1/test_cloud_redis.py b/packages/gapic-generator/tests/integration/goldens/redis/tests/unit/gapic/redis_v1/test_cloud_redis.py
index 8ca1fb5194a6..d58d99d1fdb8 100755
--- a/packages/gapic-generator/tests/integration/goldens/redis/tests/unit/gapic/redis_v1/test_cloud_redis.py
+++ b/packages/gapic-generator/tests/integration/goldens/redis/tests/unit/gapic/redis_v1/test_cloud_redis.py
@@ -708,7 +708,7 @@ def test_cloud_redis_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -755,7 +755,7 @@ def test_cloud_redis_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/redis_selective/tests/unit/gapic/redis_v1/test_cloud_redis.py b/packages/gapic-generator/tests/integration/goldens/redis_selective/tests/unit/gapic/redis_v1/test_cloud_redis.py
index 3f6b7aa521f3..780964608350 100755
--- a/packages/gapic-generator/tests/integration/goldens/redis_selective/tests/unit/gapic/redis_v1/test_cloud_redis.py
+++ b/packages/gapic-generator/tests/integration/goldens/redis_selective/tests/unit/gapic/redis_v1/test_cloud_redis.py
@@ -708,7 +708,7 @@ def test_cloud_redis_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -755,7 +755,7 @@ def test_cloud_redis_client_get_mtls_endpoint_and_cert_source(client_class):
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/tests/unit/gapic/storagebatchoperations_v1/test__compat.py b/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/tests/unit/gapic/storagebatchoperations_v1/test__compat.py
new file mode 100644
index 000000000000..e10021c90673
--- /dev/null
+++ b/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/tests/unit/gapic/storagebatchoperations_v1/test__compat.py
@@ -0,0 +1,57 @@
+# -*- coding: utf-8 -*-
+# 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 builtins
+import sys
+
+import mock
+import pytest
+
+from google.cloud.storagebatchoperations_v1 import _compat
+
+
+def test_setup_request_id_fallback():
+    with mock.patch("sys.modules", dict(sys.modules)):
+        if "google.api_core.gapic_v1.request" in sys.modules:
+            del sys.modules["google.api_core.gapic_v1.request"]
+            
+        real_import = builtins.__import__
+
+        def mock_import(name, globals=None, locals=None, fromlist=(), level=0):
+            if name == "google.api_core.gapic_v1.request":
+                raise ImportError("Mocked ImportError for gapic_v1.request")
+            return real_import(name, globals, locals, fromlist, level)
+
+        with mock.patch("builtins.__import__", side_effect=mock_import):
+            # Reload _compat to trigger the fallback path
+            import importlib
+            importlib.reload(_compat)
+            
+            # The fallback method should be defined
+            assert hasattr(_compat, "setup_request_id")
+            
+            # Test it works on a dict
+            request = {}
+            _compat.setup_request_id(request, "request_id", True)
+            assert "request_id" in request
+            
+            # Test it works on a dict when proto3 optional is False
+            request = {}
+            _compat.setup_request_id(request, "request_id", False)
+            assert "request_id" in request
+            
+            # Reset the module state
+            importlib.reload(_compat)
diff --git a/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/tests/unit/gapic/storagebatchoperations_v1/test_storage_batch_operations.py b/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/tests/unit/gapic/storagebatchoperations_v1/test_storage_batch_operations.py
index 5c53e97f8d12..892375775385 100755
--- a/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/tests/unit/gapic/storagebatchoperations_v1/test_storage_batch_operations.py
+++ b/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/tests/unit/gapic/storagebatchoperations_v1/test_storage_batch_operations.py
@@ -776,7 +776,7 @@ def test_storage_batch_operations_client_get_mtls_endpoint_and_cert_source(clien
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
@@ -823,7 +823,7 @@ def test_storage_batch_operations_client_get_mtls_endpoint_and_cert_source(clien
                     config_filename = "mock_certificate_config.json"
                     config_file_content = json.dumps(config_data)
                     m = mock.mock_open(read_data=config_file_content)
-                    with mock.patch("builtins.open", m):
+                    with mock.patch("builtins.open", m), mock.patch("os.path.exists", side_effect=lambda path: os.path.basename(path) == config_filename):
                         with mock.patch.dict(
                             os.environ, {"GOOGLE_API_CERTIFICATE_CONFIG": config_filename}
                         ):
diff --git a/packages/gapic-generator/tests/unit/generator/test_generator.py b/packages/gapic-generator/tests/unit/generator/test_generator.py
index 9d8545c4192f..f1566ee6e5a7 100644
--- a/packages/gapic-generator/tests/unit/generator/test_generator.py
+++ b/packages/gapic-generator/tests/unit/generator/test_generator.py
@@ -117,6 +117,8 @@ def test_get_response_ignores_private_files():
         list_templates.return_value = [
             "foo/bar/baz.py.j2",
             "foo/bar/_base.py.j2",
+            "foo/bar/__init__.py.j2",
+            "foo/bar/_compat.py.j2",
             "molluscs/squid/sample.py.j2",
         ]
         with mock.patch.object(jinja2.Environment, "get_template") as get_template:
@@ -128,12 +130,13 @@ def test_get_response_ignores_private_files():
             get_template.assert_has_calls(
                 [
                     mock.call("molluscs/squid/sample.py.j2"),
+                    mock.call("foo/bar/__init__.py.j2"),
+                    mock.call("foo/bar/_compat.py.j2"),
                     mock.call("foo/bar/baz.py.j2"),
-                ]
+                ],
+                any_order=True,
             )
-            assert len(cgr.file) == 1
-            assert cgr.file[0].name == "foo/bar/baz.py"
-            assert cgr.file[0].content == "I am a template result.\n"
+            assert len(cgr.file) == 3
 
 
 def test_get_response_fails_invalid_file_paths():
diff --git a/packages/google-api-core/google/api_core/gapic_v1/requests.py b/packages/google-api-core/google/api_core/gapic_v1/requests.py
index 76f5e916716d..82a2fbac39ec 100644
--- a/packages/google-api-core/google/api_core/gapic_v1/requests.py
+++ b/packages/google-api-core/google/api_core/gapic_v1/requests.py
@@ -65,6 +65,13 @@ def setup_request_id(
                 setattr(request, field_name, str(uuid.uuid4()))
         except (AttributeError, ValueError):
             # Proto-plus messages or other objects
+            if hasattr(request, "_pb"):
+                try:
+                    if not request._pb.HasField(field_name):
+                        setattr(request, field_name, str(uuid.uuid4()))
+                    return
+                except (AttributeError, ValueError):
+                    pass
             if getattr(request, field_name, None) is None:
                 setattr(request, field_name, str(uuid.uuid4()))
     else:
diff --git a/packages/google-api-core/tests/unit/gapic/test_requests.py b/packages/google-api-core/tests/unit/gapic/test_requests.py
index a69bff5e16d7..f03843b1d6d7 100644
--- a/packages/google-api-core/tests/unit/gapic/test_requests.py
+++ b/packages/google-api-core/tests/unit/gapic/test_requests.py
@@ -113,15 +113,18 @@ def test_setup_request_id(request_obj, is_proto3_optional, expected):
         assert value == expected
 
 
-def test_setup_request_id_assertion_strictness(mocker):
-    # Mock uuid.uuid4 to return a UUID with trailing characters
-    mock_uuid = mocker.patch("uuid.uuid4")
-    mock_uuid.return_value.__str__.return_value = (
-        "12345678-1234-4123-8123-123456789012-extra"
-    )
+from unittest import mock
 
-    # We expect test_setup_request_id to fail (raise AssertionError) because the UUID is invalid.
-    # If test_setup_request_id uses re.fullmatch, it will fail (raise AssertionError), so pytest.raises passes.
-    # If test_setup_request_id uses re.match, it will NOT fail, so pytest.raises fails!
-    with pytest.raises(AssertionError):
-        test_setup_request_id(MockRequest(), is_proto3_optional=True, expected="uuid")
+
+def test_setup_request_id_assertion_strictness():
+    # Mock uuid.uuid4 to return a UUID with trailing characters
+    with mock.patch("uuid.uuid4") as mock_uuid:
+        mock_uuid.return_value.__str__.return_value = (
+            "12345678-1234-4123-8123-123456789012-extra"
+        )
+
+        # We expect test_setup_request_id to fail (raise AssertionError) because the UUID is invalid.
+        # If test_setup_request_id uses re.fullmatch, it will fail (raise AssertionError), so pytest.raises passes.
+        # If test_setup_request_id uses re.match, it will NOT fail, so pytest.raises fails!
+        with pytest.raises(AssertionError):
+            test_setup_request_id(MockRequest(), is_proto3_optional=True, expected="uuid")