[Bug]: A2ARESTFastAPIApplication omits state field from task status when task is rejected

[benedict-khoo-sap]

What happened?

Steps to reproduce

1. Install a2a-sdk[http-server] (tested with v0.3.22) and uvicorn (tested with v0.40.0) using Python 3.12:

uv init -p 3.12
uv add "a2a-sdk[http-server]" uvicorn

2. Paste the following code in main.py

from a2a.server.agent_execution import AgentExecutor, RequestContext
from a2a.server.apps import A2ARESTFastAPIApplication, A2AStarletteApplication
from a2a.server.events import EventQueue
from a2a.server.request_handlers import DefaultRequestHandler
from a2a.server.tasks import InMemoryTaskStore, TaskUpdater
from a2a.types import (
    AgentCapabilities,
    AgentCard,
    AgentInterface,
    AgentSkill,
    TransportProtocol,
    UnsupportedOperationError,
)
from a2a.utils import new_agent_text_message, new_task
from a2a.utils.errors import ServerError
import uvicorn

SUPPORTED_CONTENT_TYPES = ["text", "text/plain"]


class TheAgentExecutor(AgentExecutor):
    async def execute(
        self,
        context: RequestContext,
        event_queue: EventQueue,
    ) -> None:
        task = context.current_task
        if not task:
            task = new_task(context.message)  # type: ignore
            await event_queue.enqueue_event(task)

        context_id = context.context_id or task.context_id
        updater = TaskUpdater(event_queue, task.id, context_id)

        await updater.reject(message=new_agent_text_message("I don't want to work"))

    async def cancel(self, context: RequestContext, event_queue: EventQueue) -> None:
        raise ServerError(error=UnsupportedOperationError())


def create_server(transport_protocol: TransportProtocol):
    capabilities = AgentCapabilities(streaming=True, push_notifications=False)
    skill = AgentSkill(
        id="convert_currency",
        name="Currency Exchange Rates Tool",
        description="Helps with exchange values between various currencies",
        tags=["currency conversion", "currency exchange"],
        examples=["What is exchange rate between USD and GBP?"],
    )
    url = "http://localhost:10000"
    agent_card = AgentCard(
        name="Currency Agent",
        description="Helps with exchange rates for currencies",
        url=url,
        version="1.0.0",
        default_input_modes=SUPPORTED_CONTENT_TYPES,
        default_output_modes=SUPPORTED_CONTENT_TYPES,
        capabilities=capabilities,
        skills=[skill],
        preferred_transport=transport_protocol,
        additional_interfaces=[AgentInterface(transport=transport_protocol, url=url)],
    )

    request_handler = DefaultRequestHandler(
        agent_executor=TheAgentExecutor(),
        task_store=InMemoryTaskStore(),
    )
    match transport_protocol:
        case TransportProtocol.http_json:
            return A2ARESTFastAPIApplication(
                agent_card=agent_card, http_handler=request_handler
            )
        case TransportProtocol.jsonrpc:
            return A2AStarletteApplication(
                agent_card=agent_card, http_handler=request_handler
            )
        case _:
            raise NotImplementedError(
                f"Unsupported server transport protocol: {transport_protocol}"
            )

server = create_server(transport_protocol=TransportProtocol.http_json)

if __name__ == "__main__":
    uvicorn.run(server.build(), host="0.0.0.0", port=10000)

3. Run it: uv run main.py
4. Open a terminal and send a message to the server:

curl --request POST \
  --url http://127.0.0.1:10000/v1/message:send \
  --header 'content-type: application/json' \
  --data '{
  "message": {
    "messageId": "c52d542b-06cb-4e9a-b36e-15ee6cb7eee4",
    "role": "ROLE_USER",
    "content": [
      {
        "text": "Exchange USD"
      }
    ]
  }
}'

5. Wait a few seconds then get the task, replacing <TASK_ID> with the task ID you got inresponse to the send message request:

curl --request GET \
  --url http://127.0.0.1:10000/v1/tasks/<TASK_ID>

Actual Output

The Task response is not valid. It contains a status field with no state sub-field.

Example (IDs might be different for you, but the shape should be the same):

{
  "id": "dd8b8e5b-1831-422c-a8a0-46560ef59e59",
  "contextId": "d076a8f4-e1a2-4a25-8516-3e853433e8bb",
  "status": {
    "message": {
      "messageId": "c7e0d4aa-8a8d-4e31-bafe-e57e19585803",
      "role": "ROLE_AGENT",
      "content": [
        {
          "text": "I don't want to work"
        }
      ]
    }
  },
  "history": [
    {
      "messageId": "c52d542b-06cb-4e9a-b36e-15ee6cb7eee4",
      "contextId": "d076a8f4-e1a2-4a25-8516-3e853433e8bb",
      "taskId": "dd8b8e5b-1831-422c-a8a0-46560ef59e59",
      "role": "ROLE_USER",
      "content": [
        {
          "text": "Exchange USD"
        }
      ],
      "metadata": {}
    }
  ]
}

Expected Output

The response should be a task object that contains a status field. Within status, there should be a state field with a valid TaskState value. According to the A2A specs, state is a required field: https://a2a-protocol.org/dev/specification/#412-taskstatus

Other notes

• If you change the server code to use TransportProtocol.jsonrpc, the state field appears as expected.
• If you change TheAgentExecutor to complete the task instead of rejecting it, the state field appears as expected: i.e. replace

await updater.reject(message=new_agent_text_message("I don't want to work"))

with

await updater.complete(message=new_agent_text_message("I don't want to work"))

Relevant log output

Code of Conduct

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

[ryanaiagent]

Hi @benedict-khoo-sap , Thanks for the report! We’ve received the issue and have added it to our backlog for review. We’ll take a look as soon as we can.

[ryanaiagent]

Hi @benedict-khoo-sap,
The issue resides within the RESTHandler implementation. Specifically, the on_get_task method utilizes google.protobuf.json_format.MessageToDict to serialize the Protobuf Task object into a Python dictionary.
Although the TaskUpdater correctly assigns TaskState.rejected to the state field, the standard behavior of MessageToDict is to omit fields containing default values unless explicitly preserved. Since TaskState.rejected functions as the default enum value, the serializer excludes the field from the final dictionary, causing the status to be absent from the REST response.
Would you be interested in working towards a fix for this?

[benedict-khoo-sap]

Hi @ryanaiagent, sure I can work on a fix. Will let you know when a PR is ready.