diff --git a/src/a2a/utils/proto_utils.py b/src/a2a/utils/proto_utils.py
index 1e0003582..f3042e38e 100644
--- a/src/a2a/utils/proto_utils.py
+++ b/src/a2a/utils/proto_utils.py
@@ -23,7 +23,7 @@
 
 from google.api.field_behavior_pb2 import FieldBehavior, field_behavior
 from google.protobuf.descriptor import FieldDescriptor
-from google.protobuf.json_format import Parse, ParseDict
+from google.protobuf.json_format import Parse, ParseDict, ParseError
 from google.protobuf.message import Message as ProtobufMessage
 from google.rpc import error_details_pb2
 
@@ -225,6 +225,10 @@ def parse_params(params: QueryParams, message: ProtobufMessage) -> None:
             if raw_val is not None:
                 parsed_val: Any = raw_val
                 if field.type == field.TYPE_BOOL and isinstance(raw_val, str):
+                    if raw_val.lower() not in ('true', 'false'):
+                        raise ParseError(
+                            f'Invalid boolean query parameter {k}: expected "true" or "false".'
+                        )
                     parsed_val = raw_val.lower() == 'true'
                 processed[k] = parsed_val
 
diff --git a/tests/server/routes/test_rest_dispatcher.py b/tests/server/routes/test_rest_dispatcher.py
index e0d72c4f2..4865b0cb5 100644
--- a/tests/server/routes/test_rest_dispatcher.py
+++ b/tests/server/routes/test_rest_dispatcher.py
@@ -3,6 +3,7 @@
 from collections.abc import AsyncIterator
 from unittest.mock import AsyncMock, MagicMock
 
+import httpx
 import pytest
 
 from a2a.server.context import ServerCallContext
@@ -11,9 +12,11 @@
 from a2a.server.routes.rest_dispatcher import (
     RestDispatcher,
 )
+from a2a.server.routes.rest_routes import create_rest_routes
 from a2a.types.a2a_pb2 import (
     AgentCapabilities,
     AgentCard,
+    Artifact,
     ListTaskPushNotificationConfigsResponse,
     ListTasksResponse,
     Message,
@@ -23,6 +26,7 @@
 from a2a.utils.errors import (
     UnsupportedOperationError,
 )
+from starlette.applications import Starlette
 from starlette.requests import Request
 from starlette.responses import JSONResponse
 
@@ -225,6 +229,21 @@ async def test_list_tasks(self, rest_dispatcher_instance, mock_handler):
                 True,
                 id='included',
             ),
+            pytest.param(
+                QueryParams({'includeArtifacts': 'false'}),
+                False,
+                id='excluded',
+            ),
+            pytest.param(
+                QueryParams({'includeArtifacts': 'TRUE'}),
+                True,
+                id='uppercase-true',
+            ),
+            pytest.param(
+                QueryParams({'includeArtifacts': 'False'}),
+                False,
+                id='mixed-case-false',
+            ),
         ],
     )
     async def test_list_tasks_artifact_presence(
@@ -235,7 +254,12 @@ async def test_list_tasks_artifact_presence(
         artifacts_expected: bool,
     ) -> None:
         mock_handler.on_list_tasks.return_value = ListTasksResponse(
-            tasks=[Task(id='test_task')]
+            tasks=[
+                Task(
+                    id='test_task',
+                    artifacts=[Artifact(artifact_id='test-artifact')],
+                )
+            ]
         )
         req = make_mock_request(method='GET', query_params=query_params)
 
@@ -243,6 +267,33 @@ async def test_list_tasks_artifact_presence(
 
         task = json.loads(response.body)['tasks'][0]
         assert ('artifacts' in task) is artifacts_expected
+        if artifacts_expected:
+            assert task['artifacts'][0]['artifactId'] == 'test-artifact'
+
+    @pytest.mark.parametrize(
+        'value', ['garbage', '', '0', '1', ' ', ' true ', 'private-query-value']
+    )
+    async def test_list_tasks_rejects_invalid_boolean_query(
+        self, mock_handler: AsyncMock, value: str
+    ) -> None:
+        app = Starlette(routes=create_rest_routes(mock_handler))
+        async with httpx.AsyncClient(
+            transport=httpx.ASGITransport(app=app),
+            base_url='http://test',
+            trust_env=False,
+        ) as client:
+            response = await client.get(
+                '/tasks',
+                params={'includeArtifacts': value},
+                headers={'a2a-version': '1.0'},
+            )
+
+        assert response.status_code == 400
+        assert response.json()['error']['status'] == 'INVALID_ARGUMENT'
+        assert 'includeArtifacts' in response.json()['error']['message']
+        if value.strip():
+            assert value not in response.json()['error']['message']
+        mock_handler.on_list_tasks.assert_not_awaited()
 
     async def test_get_push_notification(
         self, rest_dispatcher_instance, mock_handler
diff --git a/tests/utils/test_proto_utils.py b/tests/utils/test_proto_utils.py
index 031aa2e89..6673096e2 100644
--- a/tests/utils/test_proto_utils.py
+++ b/tests/utils/test_proto_utils.py
@@ -222,6 +222,50 @@ class FakeOldFieldOptional:
 class TestRestParams:
     """Unit tests for REST parameter conversion."""
 
+    @pytest.mark.parametrize(
+        ('values', 'expected'),
+        [
+            ([], None),
+            (['true'], True),
+            (['false'], False),
+            (['TRUE'], True),
+            (['False'], False),
+            (['true', 'false'], False),
+            (['false', 'true'], True),
+            (['garbage', 'true'], True),
+        ],
+    )
+    def test_boolean_query_values(
+        self, values: list[str], expected: bool | None
+    ) -> None:
+        message = ListTasksRequest()
+
+        proto_utils.parse_params(
+            QueryParams([('includeArtifacts', value) for value in values]),
+            message,
+        )
+
+        assert message.HasField('include_artifacts') is (expected is not None)
+        assert message.include_artifacts is bool(expected)
+
+    @pytest.mark.parametrize(
+        'value', ['garbage', '', '0', '1', ' ', ' true ', 'private-query-value']
+    )
+    @pytest.mark.parametrize('prefix', [[], ['true']])
+    def test_invalid_boolean_query_values(
+        self, value: str, prefix: list[str]
+    ) -> None:
+        with pytest.raises(ParseError, match='includeArtifacts') as error:
+            proto_utils.parse_params(
+                QueryParams(
+                    [('includeArtifacts', item) for item in [*prefix, value]]
+                ),
+                ListTasksRequest(),
+            )
+
+        if value.strip():
+            assert value not in str(error.value)
+
     def test_rest_params_roundtrip(self):
         """Test the comprehensive roundtrip conversion for REST parameters."""