BerriAI/litellm · error · RuntimeError

A2A send_message failed: non-streaming message/send expects…

Error message

A2A send_message failed: non-streaming message/send expects the agent's final event to be a Message or Task result.

What it means

After collecting the final event of a non-streaming message/send, LiteLLM converts it via the a2a.compat.v0_3 layer and requires the result to be a Message or Task — the terminal results the A2A protocol defines for that method. If the agent's final event yields anything else (a status-update event, artifact event, or unexpected compat object), this RuntimeError is raised. It signals a protocol-shape mismatch between the agent and litellm's conversions, not a transport failure.

Solutions

  1. Align a2a-sdk versions on both sides (litellm pins a2a-sdk>=1.1.0,<2.0) and retest
  2. Upgrade litellm — compat-conversion fixes for new a2a-sdk releases land frequently
  3. On the agent side, ensure a non-streaming message/send terminates with a Message or Task result, not a bare status-update event
Defensive patterns

Strategy: try-catch

Validate before calling

# On the agent side: guarantee a non-streaming message/send ends with
# a terminal result. In an a2a-sdk agent handler, return the Task/Message:
async def handle(self, ctx, msg):
    task = self._tasks[msg.context_id]  # ...do work...
    return Task(state=TaskState.completed, ...)  # NOT a bare status-update event

Try / catch

try:
    resp = await asend_message(a2a_client=client, request=req)
except RuntimeError as e:
    if 'expects the agent' in str(e) and 'Message or Task' in str(e):
        # protocol-shape mismatch: align SDK versions, then retry once
        _align_and_warn_sdk_versions()
        resp = await asend_message(a2a_client=client, request=req)
    else:
        raise

Prevention

When it happens

Trigger: An agent whose last emitted event for a non-streaming send is a TaskStatusUpdateEvent or custom artifact instead of a completed Task/Message; version skew between the agent's a2a-sdk and the one litellm converts through (a2a.compat.v0_3).

Common situations: Agent frameworks emitting streaming-style terminal events for non-streaming sends; upgrading the agent SDK but not litellm (or vice versa); draft/0.3-era agents talking to the 1.x code path.

Related errors

AI-assisted analysis of BerriAI/litellm@77b7c6c40c (2026-08-18). Data as JSON: /api/errors/a26fbe94396ea27f. Report an issue: GitHub.

Appendix: source

Thrown at litellm/a2a_protocol/main.py:262

async def _send_message(a2a_client: "A2AClientType", request: "SendMessageRequest") -> "SendMessageResponse":
    """Send a non-streaming message via a2a-sdk 1.x and return JSON-RPC response."""
    if _a2a_conversions is None:
        raise ImportError(
            "The 'a2a' package is required for A2A agent invocation. Install it with: pip install a2a-sdk"
        )

    pb_request: Final = _to_core_send_message_request(request)
    last_event = None
    async for event in a2a_client.send_message(pb_request, context=_get_a2a_call_context(a2a_client)):
        last_event = event
    if last_event is None:
        raise RuntimeError("A2A send_message failed: no response received from agent.")

    stream_compat: Final = _to_compat_stream_response(last_event, request_id=request.id)
    result: Final = stream_compat.result
    if not isinstance(result, (Message, Task)):
        raise RuntimeError(
            "A2A send_message failed: non-streaming message/send expects the "
            "agent's final event to be a Message or Task result."
        )
    return SendMessageResponse(
        root=SendMessageSuccessResponse(
            id=request.id,
            result=result,
        )
    )


async def _execute_a2a_send_with_retry(
    a2a_client: "A2AClientType",
    request: "SendMessageRequest",
    agent_card: Optional["AgentCard"],
    card_url: str | None,
    api_base: str | None,
    agent_name: str | None,

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