[Bug]: ListTasks breaks the `includeArtifacts=false` spec "MUST omit" requirement

[lkk7]

Specification (3.1.4)

When includeArtifacts is false (the default), the artifacts field MUST be omitted entirely from each Task object in the response. The field should not be present as an empty array or null value.

Actual behavior:

from google.protobuf.json_format import MessageToDict

from a2a.types import ListTasksResponse, Task, TaskState, TaskStatus

task = Task(
    id="task-1",
    context_id="context-1",
    status=TaskStatus(state=TaskState.TASK_STATE_WORKING),
)
task.ClearField("artifacts")

response = ListTasksResponse(tasks=[task])
data = MessageToDict(
    response,
    preserving_proto_field_name=False,
    always_print_fields_with_no_presence=True,
)

print(data["tasks"][0]["artifacts"]) 

Returns [].

Code of Conduct

• I agree to follow this project's Code of Conduct

[dawNotPoi]

I can take this and add a regression test for the serialized ListTasks response.

[rohityan]

Hi @lkk7 , Thank you for reporting this and calling out the strict compliance requirement from Section 3.1.4 of the A2A specification.
Thanks @dawNotPoi for the contribution towards a clean fix and for aligning the ListTasks REST and JSON-RPC serialization behavior with the A2A spec.
While the PR is reviewed, upstream clients or middleware can strip or ignore empty artifacts keys before strict schema validation:

• Client-side Sanitization: Post-process the JSON response before schema parsing by stripping "artifacts" keys when the list is empty or when include_artifacts was set to False.T