[Bug]: REST client creates A2A-incompatible payloads

[martina-zxy]

What happened?

The REST transport's _prepare_send_message method (line 81-101 in rest.py) uses MessageToDict() from google.protobuf.json_format to convert protobuf messages to JSON payloads. This creates payloads that are not compatible with the A2A REST API specification. _apply_interceptors might have been written to deal with this, but the method is still empty.

Location

a2a/client/transports/rest.py:95

async def _prepare_send_message(
    self, request: MessageSendParams, context: ClientCallContext | None
) -> tuple[dict[str, Any], dict[str, Any]]:
    pb = a2a_pb2.SendMessageRequest(
        request=proto_utils.ToProto.message(request.message),
        configuration=proto_utils.ToProto.message_send_configuration(
            request.configuration
        ),
        metadata=(
            proto_utils.ToProto.metadata(request.metadata)
            if request.metadata
            else None
        ),
    )
    payload = MessageToDict(pb) 
    payload, modified_kwargs = await self._apply_interceptors(
        payload,
        self._get_http_args(context),
        context,
    )
    return payload, modified_kwargs

a2a/client/transports/rest.py:65-74

async def _apply_interceptors(
    self,
    request_payload: dict[str, Any],
    http_kwargs: dict[str, Any] | None,
    context: ClientCallContext | None,
) -> tuple[dict[str, Any], dict[str, Any]]:
    final_http_kwargs = http_kwargs or {}
    final_request_payload = request_payload
    # TODO: Implement interceptors for other transports
    return final_request_payload, final_http_kwargs

Impact

This causes the REST transport to send message payload that is invalid according to the specification:

• send parts as content
• use the wrong enum of role, e.g., ROLE_USER instead of user

Example payload

{
  "message": {
    "messageId": "msg-1763370750463-w0jfn5g2n",
    "role": "ROLE_USER",
    "content": [
      {
        "text": "hello"
      }
    ],
    "metadata": {}
  },
  "configuration": {
    "blocking": true
  }
}

This affects both send_message() (line 103-118) and send_message_streaming() (line 120-156) methods.

Related Code

Similar issues may exist in other methods using MessageToDict:

• cancel_task() - line 233
• set_task_callback() - line 258
• get_task_callback() - line 281

Relevant log output

Code of Conduct

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

[lkawka]

Hi Martina,

I think that the issue stems from the fact there was a drift between a2a.json and a2a.proto definitions in the specification, parts vs content being one example of that. The JSON-RPC implementation is based on the a2a.json definition and the REST implementation is based on the a2a.proto definition, hence the discrepancy.

This was recently addressed on the specification level by making the a2a.proto the ultimate source of truth and generating a2a.json from it. We plan on addressing it in the SDK soon.

Please let me know if you have any further questions.