diff --git a/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/_shared_macros.j2 b/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/_shared_macros.j2
index 755e4530e7ba..5f1b236f1780 100644
--- a/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/_shared_macros.j2
+++ b/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/_shared_macros.j2
@@ -28,11 +28,7 @@
 {% if method_settings is not none %}
 {% for auto_populated_field in method_settings.auto_populated_fields %}
     {% set is_proto3_optional = method.input.fields[auto_populated_field].proto3_optional %}
-    {% if is_async %}
-        self._client._setup_request_id(request, '{{ auto_populated_field }}', {{ is_proto3_optional }})
-    {% else %}
-        self._setup_request_id(request, '{{ auto_populated_field }}', {{ is_proto3_optional }})
-    {% endif %}
+        setup_request_id(request, '{{ auto_populated_field }}', {{ is_proto3_optional }})
 {% endfor %}
 {% endif %}{# if method_settings is not none #}
 {% endwith %}{# method_settings #}
diff --git a/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/async_client.py.j2 b/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/async_client.py.j2
index 5fe416902b86..19a39aa53f3c 100644
--- a/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/async_client.py.j2
+++ b/packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/async_client.py.j2
@@ -4,13 +4,18 @@
 {% import "%namespace/%name_%version/%sub/services/%service/_client_macros.j2" as macros %}
 {% import "%namespace/%name_%version/%sub/services/%service/_shared_macros.j2" as shared_macros %}
 
+{% set has_auto_populated_fields = namespace(value=false) %}
+{% for method in service.methods.values() %}
+  {% set method_settings = api.all_method_settings.get(method.meta.address.proto) %}
+  {% if method_settings and method_settings.auto_populated_fields %}
+    {% set has_auto_populated_fields.value = true %}
+  {% endif %}
+{% endfor %}
+
 import logging as std_logging
 from collections import OrderedDict
 import re
 from typing import Dict, Callable, Mapping, MutableMapping, MutableSequence, Optional, {% if service.any_server_streaming %}AsyncIterable, Awaitable, {% endif %}{% if service.any_client_streaming %}AsyncIterator, {% endif %}Sequence, Tuple, Type, Union
-{% if api.all_method_settings.values()|map(attribute="auto_populated_fields", default=[])|select|list %}
-import uuid
-{% endif %}
 {% if service.any_deprecated %}
 import warnings
 {% endif %}
@@ -21,6 +26,9 @@ from {{package_path}} import gapic_version as package_version
 from google.api_core.client_options import ClientOptions
 from google.api_core import exceptions as core_exceptions
 from google.api_core import gapic_v1
+{% if has_auto_populated_fields.value %}
+from google.api_core.gapic_v1.request import setup_request_id
+{% endif %}
 from google.api_core import retry_async as retries
 from google.auth import credentials as ga_credentials   # type: ignore
 from google.oauth2 import service_account              # type: ignore
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..a29896142286 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
@@ -6,6 +6,14 @@
 {% import "%namespace/%name_%version/%sub/services/%service/_client_macros.j2" as macros %}
 {% import "%namespace/%name_%version/%sub/services/%service/_shared_macros.j2" as shared_macros %}
 
+{% set has_auto_populated_fields = namespace(value=false) %}
+{% for method in service.methods.values() %}
+  {% set method_settings = api.all_method_settings.get(method.meta.address.proto) %}
+  {% if method_settings and method_settings.auto_populated_fields %}
+    {% set has_auto_populated_fields.value = true %}
+  {% endif %}
+{% endfor %}
+
 from collections import OrderedDict
 {% if service.any_extended_operations_methods %}
 import functools
@@ -16,9 +24,6 @@ import logging as std_logging
 import os
 import re
 from typing import Dict, Callable, Mapping, MutableMapping, MutableSequence, Optional, {% if service.any_server_streaming %}Iterable, {% endif %}{% if service.any_client_streaming %}Iterator, {% endif %}Sequence, Tuple, Type, Union, cast
-{% if api.all_method_settings.values()|map(attribute="auto_populated_fields", default=[])|select|list %}
-import uuid
-{% endif %}
 import warnings
 
 {% set package_path = api.naming.module_namespace|join('.') + "." + api.naming.versioned_module_name %}
@@ -30,6 +35,9 @@ from google.api_core import exceptions as core_exceptions
 from google.api_core import extended_operation
 {% endif %}
 from google.api_core import gapic_v1
+{% if has_auto_populated_fields.value %}
+from google.api_core.gapic_v1.request import setup_request_id
+{% endif %}
 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
@@ -453,37 +461,7 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         # NOTE (b/349488459): universe validation is disabled until further notice.
         return True
 
-    {% if api.all_method_settings.values()|map(attribute="auto_populated_fields", default=[])|select|list %}
-    @staticmethod
-    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.
-        """
-        if isinstance(request, dict):
-            if is_proto3_optional:
-                if field_name not in request:
-                    request[field_name] = str(uuid.uuid4())
-            elif not request.get(field_name):
-                request[field_name] = str(uuid.uuid4())
-            return
 
-        if is_proto3_optional:
-            try:
-                # Pure protobuf messages
-                if not request.HasField(field_name):
-                    setattr(request, field_name, str(uuid.uuid4()))
-            except (AttributeError, ValueError):
-                # Proto-plus messages or other objects
-                if field_name not in request:
-                    setattr(request, field_name, str(uuid.uuid4()))
-        else:
-            if not getattr(request, field_name):
-                setattr(request, field_name, str(uuid.uuid4()))
-    {% endif %}
 
     def _add_cred_info_for_auth_errors(
         self,
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..aa7c48173f5c 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
@@ -6,9 +6,17 @@
 {% import "tests/unit/gapic/%name_%version/%sub/test_macros.j2" as test_macros %}
 {% import "%namespace/%name_%version/%sub/services/%service/_shared_macros.j2" as shared_macros %}
 
+{% set has_auto_populated_fields = namespace(value=false) %}
+{% for method in service.methods.values() %}
+  {% set method_settings = api.all_method_settings.get(method.meta.address.proto) %}
+  {% if method_settings and method_settings.auto_populated_fields %}
+    {% set has_auto_populated_fields.value = true %}
+  {% endif %}
+{% endfor %}
+
 import os
 import asyncio
-{% if api.all_method_settings.values()|map(attribute="auto_populated_fields", default=[])|select|list %}
+{% if has_auto_populated_fields.value %}
 import re
 {% endif %}
 from unittest import mock
@@ -107,7 +115,7 @@ CRED_INFO_JSON = {
     "principal": "service-account@example.com",
 }
 CRED_INFO_STRING = json.dumps(CRED_INFO_JSON)
-{% if api.all_method_settings.values()|map(attribute="auto_populated_fields", default=[])|select|list %}
+{% if has_auto_populated_fields.value %}
 _UUID4_RE = re.compile(r"{{ uuid4_re }}")
 {% endif %}
 
@@ -434,84 +442,6 @@ def test__add_cred_info_for_auth_errors_no_get_cred_info(error_code):
     client._add_cred_info_for_auth_errors(error)
     assert error.details == []
 
-{% if api.all_method_settings.values()|map(attribute="auto_populated_fields", default=[])|select|list %}
-def test__setup_request_id():
-    class MockRequest:
-        def __init__(self, **kwargs):
-            for k, v in kwargs.items():
-                setattr(self, k, v)
-        def __contains__(self, key):
-            return hasattr(self, key)
-
-    class MockProtoRequest:
-        def __init__(self, **kwargs):
-            for k, v in kwargs.items():
-                setattr(self, k, v)
-        def HasField(self, key):
-            return hasattr(self, key)
-
-    # Test with proto3 optional field not in request
-    request = MockRequest()
-    {{ service.client_name }}._setup_request_id(request, "request_id", True)
-    assert re.match(r"{{ test_macros.get_uuid4_re() }}", request.request_id)
-
-    # Test with proto3 optional field already in request
-    request = MockRequest(request_id="already_set")
-    {{ service.client_name }}._setup_request_id(request, "request_id", True)
-    assert request.request_id == "already_set"
-
-    # Test with non-proto3 optional field empty
-    request = MockRequest(request_id="")
-    {{ service.client_name }}._setup_request_id(request, "request_id", False)
-    assert re.match(r"{{ test_macros.get_uuid4_re() }}", request.request_id)
-
-    # Test with non-proto3 optional field already set
-    request = MockRequest(request_id="already_set")
-    {{ service.client_name }}._setup_request_id(request, "request_id", False)
-    assert request.request_id == "already_set"
-
-    # Test with proto3 optional field not in request (MockProtoRequest)
-    request = MockProtoRequest()
-    {{ service.client_name }}._setup_request_id(request, "request_id", True)
-    assert re.match(r"{{ test_macros.get_uuid4_re() }}", request.request_id)
-
-    # Test with proto3 optional field already in request (MockProtoRequest)
-    request = MockProtoRequest(request_id="already_set")
-    {{ service.client_name }}._setup_request_id(request, "request_id", True)
-    assert request.request_id == "already_set"
-
-    # Test with ValueError
-    class MockValueErrorRequest:
-        def HasField(self, key):
-            raise ValueError("Mismatched field")
-        def __contains__(self, key):
-            return hasattr(self, key)
-
-    request = MockValueErrorRequest()
-    {{ service.client_name }}._setup_request_id(request, "request_id", True)
-    assert re.match(r"{{ test_macros.get_uuid4_re() }}", request.request_id)
-
-    # Test with dict and proto3 optional field not in request
-    request = {}
-    {{ service.client_name }}._setup_request_id(request, "request_id", True)
-    assert re.match(r"{{ test_macros.get_uuid4_re() }}", request["request_id"])
-
-    # Test with dict and proto3 optional field already in request
-    request = {"request_id": "already_set"}
-    {{ service.client_name }}._setup_request_id(request, "request_id", True)
-    assert request["request_id"] == "already_set"
-
-    # Test with dict and non-proto3 optional field empty
-    request = {"request_id": ""}
-    {{ service.client_name }}._setup_request_id(request, "request_id", False)
-    assert re.match(r"{{ test_macros.get_uuid4_re() }}", request["request_id"])
-
-    # Test with dict and non-proto3 optional field already set
-    request = {"request_id": "already_set"}
-    {{ service.client_name }}._setup_request_id(request, "request_id", False)
-    assert request["request_id"] == "already_set"
-
-{% endif %}
 @pytest.mark.parametrize("client_class,transport_name", [
     {% if 'grpc' in opts.transport %}
     ({{ service.client_name }}, "grpc"),
diff --git a/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/google/cloud/storagebatchoperations_v1/services/storage_batch_operations/client.py b/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/google/cloud/storagebatchoperations_v1/services/storage_batch_operations/client.py
index 5f79cf8e016a..95ad1da16ea1 100755
--- a/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/google/cloud/storagebatchoperations_v1/services/storage_batch_operations/client.py
+++ b/packages/gapic-generator/tests/integration/goldens/storagebatchoperations/google/cloud/storagebatchoperations_v1/services/storage_batch_operations/client.py
@@ -28,6 +28,7 @@
 from google.api_core import client_options as client_options_lib
 from google.api_core import exceptions as core_exceptions
 from google.api_core import gapic_v1
+from google.api_core.gapic_v1 import request
 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
@@ -480,26 +481,7 @@ def _setup_request_id(request, field_name: str, is_proto3_optional: bool):
             field_name (str): The name of the field to populate.
             is_proto3_optional (bool): Whether the field is proto3 optional.
         """
-        if isinstance(request, dict):
-            if is_proto3_optional:
-                if field_name not in request:
-                    request[field_name] = str(uuid.uuid4())
-            elif not request.get(field_name):
-                request[field_name] = str(uuid.uuid4())
-            return
-
-        if is_proto3_optional:
-            try:
-                # Pure protobuf messages
-                if not request.HasField(field_name):
-                    setattr(request, field_name, str(uuid.uuid4()))
-            except (AttributeError, ValueError):
-                # Proto-plus messages or other objects
-                if field_name not in request:
-                    setattr(request, field_name, str(uuid.uuid4()))
-        else:
-            if not getattr(request, field_name):
-                setattr(request, field_name, str(uuid.uuid4()))
+        request.setup_request_id(request, field_name, is_proto3_optional)
 
     def _add_cred_info_for_auth_errors(
         self,
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..3697bd9a4839 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
@@ -351,82 +351,6 @@ def test__add_cred_info_for_auth_errors_no_get_cred_info(error_code):
     client._add_cred_info_for_auth_errors(error)
     assert error.details == []
 
-def test__setup_request_id():
-    class MockRequest:
-        def __init__(self, **kwargs):
-            for k, v in kwargs.items():
-                setattr(self, k, v)
-        def __contains__(self, key):
-            return hasattr(self, key)
-
-    class MockProtoRequest:
-        def __init__(self, **kwargs):
-            for k, v in kwargs.items():
-                setattr(self, k, v)
-        def HasField(self, key):
-            return hasattr(self, key)
-
-    # Test with proto3 optional field not in request
-    request = MockRequest()
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", True)
-    assert re.match(r"[a-f0-9]{8}-?[a-f0-9]{4}-?4[a-f0-9]{3}-?[89ab][a-f0-9]{3}-?[a-f0-9]{12}", request.request_id)
-
-    # Test with proto3 optional field already in request
-    request = MockRequest(request_id="already_set")
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", True)
-    assert request.request_id == "already_set"
-
-    # Test with non-proto3 optional field empty
-    request = MockRequest(request_id="")
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", False)
-    assert re.match(r"[a-f0-9]{8}-?[a-f0-9]{4}-?4[a-f0-9]{3}-?[89ab][a-f0-9]{3}-?[a-f0-9]{12}", request.request_id)
-
-    # Test with non-proto3 optional field already set
-    request = MockRequest(request_id="already_set")
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", False)
-    assert request.request_id == "already_set"
-
-    # Test with proto3 optional field not in request (MockProtoRequest)
-    request = MockProtoRequest()
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", True)
-    assert re.match(r"[a-f0-9]{8}-?[a-f0-9]{4}-?4[a-f0-9]{3}-?[89ab][a-f0-9]{3}-?[a-f0-9]{12}", request.request_id)
-
-    # Test with proto3 optional field already in request (MockProtoRequest)
-    request = MockProtoRequest(request_id="already_set")
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", True)
-    assert request.request_id == "already_set"
-
-    # Test with ValueError
-    class MockValueErrorRequest:
-        def HasField(self, key):
-            raise ValueError("Mismatched field")
-        def __contains__(self, key):
-            return hasattr(self, key)
-
-    request = MockValueErrorRequest()
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", True)
-    assert re.match(r"[a-f0-9]{8}-?[a-f0-9]{4}-?4[a-f0-9]{3}-?[89ab][a-f0-9]{3}-?[a-f0-9]{12}", request.request_id)
-
-    # Test with dict and proto3 optional field not in request
-    request = {}
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", True)
-    assert re.match(r"[a-f0-9]{8}-?[a-f0-9]{4}-?4[a-f0-9]{3}-?[89ab][a-f0-9]{3}-?[a-f0-9]{12}", request["request_id"])
-
-    # Test with dict and proto3 optional field already in request
-    request = {"request_id": "already_set"}
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", True)
-    assert request["request_id"] == "already_set"
-
-    # Test with dict and non-proto3 optional field empty
-    request = {"request_id": ""}
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", False)
-    assert re.match(r"[a-f0-9]{8}-?[a-f0-9]{4}-?4[a-f0-9]{3}-?[89ab][a-f0-9]{3}-?[a-f0-9]{12}", request["request_id"])
-
-    # Test with dict and non-proto3 optional field already set
-    request = {"request_id": "already_set"}
-    StorageBatchOperationsClient._setup_request_id(request, "request_id", False)
-    assert request["request_id"] == "already_set"
-
 @pytest.mark.parametrize("client_class,transport_name", [
     (StorageBatchOperationsClient, "grpc"),
     (StorageBatchOperationsAsyncClient, "grpc_asyncio"),
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 48a27ec21d24..010ff6ed3ef7 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
@@ -25,9 +25,10 @@
 # Older Python versions safely ignore this variable.
 __lazy_modules__: Set[str] = {
     "google.api_core.gapic_v1.client_info",
+    "google.api_core.gapic_v1.request",
     "google.api_core.gapic_v1.routing_header",
 }
-__all__ = ["client_info", "routing_header"]
+__all__ = ["client_info", "request", "routing_header"]
 
 if _has_grpc:
     __lazy_modules__.update(
@@ -41,6 +42,7 @@
 
 from google.api_core.gapic_v1 import (  # noqa: E402
     client_info,
+    request,
     routing_header,
 )
 
diff --git a/packages/google-api-core/google/api_core/gapic_v1/request.py b/packages/google-api-core/google/api_core/gapic_v1/request.py
new file mode 100644
index 000000000000..90c135d00f8c
--- /dev/null
+++ b/packages/google-api-core/google/api_core/gapic_v1/request.py
@@ -0,0 +1,72 @@
+# -*- 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.
+#
+
+"""Helpers for preparing and structuring API requests.
+
+This module provides utilities to preprocess request parameters and objects
+before invoking API methods, such as automatically generating request IDs
+if they are not already set.
+"""
+
+import uuid
+from typing import Union
+
+import google.protobuf.message
+
+
+def setup_request_id(
+    request: Union[google.protobuf.message.Message, dict, None],
+    field_name: str,
+    is_proto3_optional: bool,
+) -> None:
+    """Populate a UUID4 field in the request if it is not already set.
+
+    This helper is used to ensure request idempotency by automatically
+    generating a unique identifier (such as `request_id`) for requests
+    that support it. If a request is retried, the same identifier can be
+    sent on subsequent retries, allowing the server to recognize the retried
+    request and prevent duplicate processing (e.g., creating duplicate
+    resources).
+
+    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.
+    """
+    if request is None:
+        return
+
+    if isinstance(request, dict):
+        if is_proto3_optional:
+            if field_name not in request:
+                request[field_name] = str(uuid.uuid4())
+        elif not request.get(field_name):
+            request[field_name] = str(uuid.uuid4())
+        return
+
+    if is_proto3_optional:
+        try:
+            # Pure protobuf messages
+            if not request.HasField(field_name):
+                setattr(request, field_name, str(uuid.uuid4()))
+        except (AttributeError, ValueError):
+            # Proto-plus messages or other objects
+            if not getattr(request, field_name, None):
+                setattr(request, field_name, str(uuid.uuid4()))
+    else:
+        if not getattr(request, field_name, None):
+            setattr(request, field_name, str(uuid.uuid4()))
diff --git a/packages/google-api-core/tests/unit/gapic/test_request.py b/packages/google-api-core/tests/unit/gapic/test_request.py
new file mode 100644
index 000000000000..14ec9bb5dd24
--- /dev/null
+++ b/packages/google-api-core/tests/unit/gapic/test_request.py
@@ -0,0 +1,114 @@
+# -*- 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 re
+
+import pytest
+
+from google.api_core.gapic_v1.request import setup_request_id
+
+
+# --- Mock Request Helper Classes ---
+
+
+class MockRequest:
+    def __init__(self, **kwargs):
+        for k, v in kwargs.items():
+            setattr(self, k, v)
+
+    def __contains__(self, key):
+        return hasattr(self, key)
+
+
+class MockProtoRequest:
+    def __init__(self, **kwargs):
+        for k, v in kwargs.items():
+            setattr(self, k, v)
+
+    def HasField(self, key):
+        return hasattr(self, key)
+
+
+class MockValueErrorRequest:
+    def HasField(self, key):
+        raise ValueError("Mismatched field")
+
+    def __contains__(self, key):
+        return hasattr(self, key)
+
+
+# --- Parameterized Test ---
+
+UUID_REGEX = (
+    r"[a-f0-9]{8}-?[a-f0-9]{4}-?4[a-f0-9]{3}-?[89ab][a-f0-9]{3}-?[a-f0-9]{12}"
+)
+
+
+@pytest.mark.parametrize(
+    "request_obj, is_proto3_optional, expected",
+    [
+        # MockRequest cases
+        (MockRequest(), True, "uuid"),
+        (MockRequest(request_id="already_set"), True, "already_set"),
+        (MockRequest(request_id=""), False, "uuid"),
+        (MockRequest(request_id="already_set"), False, "already_set"),
+        # MockProtoRequest cases
+        (MockProtoRequest(), True, "uuid"),
+        (MockProtoRequest(request_id="already_set"), True, "already_set"),
+        # ValueError case
+        (MockValueErrorRequest(), True, "uuid"),
+        # Dict cases
+        ({}, True, "uuid"),
+        ({"request_id": "already_set"}, True, "already_set"),
+        ({"request_id": ""}, False, "uuid"),
+        ({"request_id": "already_set"}, False, "already_set"),
+        # None case
+        (None, True, "none"),
+    ],
+    ids=[
+        "proto3_optional_not_in_request",
+        "proto3_optional_already_in_request",
+        "non_proto3_optional_empty",
+        "non_proto3_optional_already_set",
+        "proto3_optional_not_in_request_proto",
+        "proto3_optional_already_in_request_proto",
+        "value_error_fallback",
+        "dict_proto3_optional_not_in_request",
+        "dict_proto3_optional_already_in_request",
+        "dict_non_proto3_optional_empty",
+        "dict_non_proto3_optional_already_set",
+        "none_request",
+    ],
+)
+def test_setup_request_id(request_obj, is_proto3_optional, expected):
+    # Act
+    setup_request_id(request_obj, "request_id", is_proto3_optional)
+
+    # Assert
+    if expected == "none":
+        assert request_obj is None
+        return
+
+    # Extract the resulting value depending on container type
+    value = (
+        request_obj["request_id"]
+        if isinstance(request_obj, dict)
+        else request_obj.request_id
+    )
+
+    if expected == "uuid":
+        assert re.match(UUID_REGEX, value)
+    else:
+        assert value == expected