fix: return mandatory fields from list_tasks

According to [the spec](https://a2a-protocol.org/latest/specification/#314-list-tasks) all response fields are mandatory. Update implementation and add tests to cover.

Author: ishymko

src/a2a/server/request_handlers/default_request_handler.py

@@ -137,25 +137,15 @@ async def on_list_tasks(
     ) -> ListTasksResponse:
         """Default handler for 'tasks/list'."""
         page = await self.task_store.list(params, context)
-        processed_tasks = []
         for task in page.tasks:
-            processed_task = task
-
             if not params.include_artifacts:
-                new_task = Task()
-                new_task.CopyFrom(processed_task)
-                new_task.ClearField('artifacts')
-                processed_task = new_task
-
-            if params.history_length > 0:
-                processed_task = apply_history_length(
-                    processed_task, params.history_length
-                )
-            processed_tasks.append(processed_task)
-        return ListTasksResponse(
-            next_page_token=page.next_page_token or '',
-            tasks=processed_tasks,
-        )
+                task.ClearField('artifacts')
+
+            updated_task = apply_history_length(task, params.history_length)
+            if updated_task is not task:
+                task.CopyFrom(updated_task)
+
+        return page
 
     async def on_cancel_task(
         self,

src/a2a/server/tasks/database_task_store.py

@@ -261,7 +261,8 @@ async def list(
             return a2a_pb2.ListTasksResponse(
                 tasks=tasks[:page_size],
                 total_size=total_count,
-                next_page_token=next_page_token or '',
+                next_page_token=next_page_token,
+                page_size=page_size,
             )
 
     async def delete(

src/a2a/server/tasks/inmemory_task_store.py

@@ -106,7 +106,8 @@ async def list(
                     break
             if not valid_token:
                 raise ValueError(f'Invalid page token: {params.page_token}')
-        end_idx = start_idx + (params.page_size or DEFAULT_LIST_TASKS_PAGE_SIZE)
+        page_size = params.page_size or DEFAULT_LIST_TASKS_PAGE_SIZE
+        end_idx = start_idx + page_size
         next_page_token = (
             encode_page_token(tasks[end_idx].id)
             if end_idx < total_size
@@ -115,9 +116,10 @@ async def list(
         tasks = tasks[start_idx:end_idx]
 
         return a2a_pb2.ListTasksResponse(
-            next_page_token=next_page_token or '',
+            next_page_token=next_page_token,
             tasks=tasks,
             total_size=total_size,
+            page_size=page_size,
         )
 
     async def delete(

tests/integration/test_client_server_integration.py

@@ -96,6 +96,8 @@
 LIST_TASKS_RESPONSE = ListTasksResponse(
     tasks=[TASK_FROM_BLOCKING, GET_TASK_RESPONSE],
     next_page_token='page-2',
+    total_size=12,
+    page_size=10,
 )
 
 
@@ -480,6 +482,8 @@ async def test_http_transport_list_tasks(
 
     assert len(result.tasks) == 2
     assert result.next_page_token == 'page-2'
+    assert result.total_size == 12
+    assert result.page_size == 10
     handler.on_list_tasks.assert_awaited_once()
 
     if hasattr(transport, 'close'):

tests/integration/test_end_to_end.py

@@ -304,6 +304,8 @@ async def test_end_to_end_list_tasks(transport_setups):
 
         list_response = await transport.list_tasks(request=list_request)
         assert 0 < len(list_response.tasks) <= page_size
+        assert list_response.total_size == total_items
+        assert list_response.page_size == page_size
 
         for task in list_response.tasks:
             unique_task_ids.add(task.id)

tests/server/tasks/test_database_task_store.py

@@ -30,6 +30,7 @@
     TaskState,
     TaskStatus,
 )
+from a2a.utils.constants import DEFAULT_LIST_TASKS_PAGE_SIZE
 
 
 # DSNs for different databases
@@ -305,6 +306,7 @@ async def test_list_tasks(
     assert retrieved_ids == expected_ids
     assert page.total_size == total_count
     assert page.next_page_token == (next_page_token or '')
+    assert page.page_size == (params.page_size or DEFAULT_LIST_TASKS_PAGE_SIZE)
 
     # Cleanup
     for task in tasks_to_create:

tests/server/tasks/test_inmemory_task_store.py

@@ -3,6 +3,7 @@
 
 from a2a.server.tasks import InMemoryTaskStore
 from a2a.types.a2a_pb2 import Task, TaskState, TaskStatus, ListTasksRequest
+from a2a.utils.constants import DEFAULT_LIST_TASKS_PAGE_SIZE
 
 
 def create_minimal_task(
@@ -167,6 +168,7 @@ async def test_list_tasks(
     assert retrieved_ids == expected_ids
     assert page.total_size == total_count
     assert page.next_page_token == (next_page_token or '')
+    assert page.page_size == (params.page_size or DEFAULT_LIST_TASKS_PAGE_SIZE)
 
     # Cleanup
     for task in tasks_to_create: