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[Bug]: Client transports reject responses with unrecognized fields (spec 5.7)

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Dal 15/10/2026.

  • #1343 di @jw-ond — aperta

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Descrizione

What happened?

The 1.0 client transports reject any response that carries a field they don't know. A SendMessage result with one extra key fails with ParseError and the caller gets no answer, even though the message in it is valid.

Spec 5.7 says: "Implementations SHOULD ignore unrecognized fields in messages, allowing for forward compatibility as the protocol evolves."

#1273 applied this to the server (jsonrpc_dispatcher.py, rest_dispatcher.py). The client side is still strict:

  • client/transports/jsonrpc.py: every json_format.ParseDict(json_rpc_response.result, ...) (send_message, get_task, list_tasks, cancel_task, the push-notification config methods, get_extended_agent_card) and the streaming ParseDict(..., StreamResponse()).
  • client/transports/rest.py: the same set of ParseDict(response_data, ...) calls.

The rest of the client is already lenient: client/card_resolver.py (parse_agent_card) and the v0.3 compat transport (compat/v0_3/rest_transport.py) both pass ignore_unknown_fields=True. So a client talking to a 1.0 agent is stricter than the same client talking to a 0.3 agent.

Expected: unknown fields in a response are ignored and the valid payload is returned, as on the server since #1273.

Why it matters: a server one minor version ahead (or one that adds a vendor field) breaks every client on an older SDK. Gateways built on this client inherit it: LiteLLM's A2A proxy calls client.send_message, so an agent whose SendMessage result has an extra key cannot be called through LiteLLM at all. We hit exactly this with a third-party agent that returns {"message": {...}, "status": "COMPLETED"}.

Reproduction (a2a-sdk 1.2.2, also present on main; no network needed):

import asyncio
import json
import uuid

import httpx
from a2a.client import ClientConfig, ClientFactory
from a2a.client.card_resolver import parse_agent_card
from a2a.types import a2a_pb2

CARD = {
    "name": "demo",
    "description": "demo",
    "version": "1.0.0",
    "capabilities": {},
    "defaultInputModes": ["text/plain"],
    "defaultOutputModes": ["text/plain"],
    "skills": [{"id": "s", "name": "s", "description": "s", "tags": []}],
    "supportedInterfaces": [
        {"url": "https://agent.example", "protocolBinding": "JSONRPC", "protocolVersion": "1.0"}
    ],
}


def agent(request: httpx.Request) -> httpx.Response:
    return httpx.Response(200, json={
        "jsonrpc": "2.0",
        "id": json.loads(request.content)["id"],
        "result": {
            "message": {"messageId": "r1", "role": "ROLE_AGENT", "parts": [{"text": "hello"}]},
            # A field this SDK version does not know (e.g. added by a newer spec minor version).
            "someNewField": "x",
        },
    })


async def main() -> None:
    async with httpx.AsyncClient(transport=httpx.MockTransport(agent)) as http:
        client = ClientFactory(ClientConfig(httpx_client=http, streaming=False)).create(parse_agent_card(CARD))
        request = a2a_pb2.SendMessageRequest(message=a2a_pb2.Message(
            role=a2a_pb2.Role.ROLE_USER, message_id=uuid.uuid4().hex, parts=[a2a_pb2.Part(text="hi")]))
        async for event in client.send_message(request):
            print(event)


asyncio.run(main())

Removing someNewField makes it print the message.

Suggested fix: pass ignore_unknown_fields=True at the response-parsing call sites in client/transports/jsonrpc.py and client/transports/rest.py, mirroring #1273. The enum caveat discussed there (an unknown enum value becomes 0) applies here too.

Relevant log output
google.protobuf.json_format.ParseError: Message type "lf.a2a.v1.SendMessageResponse" has no field named "someNewField" at "SendMessageResponse".
 Available Fields(except extensions): "['task', 'message']"
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