[BUG] A2AAgent returns empty AgentResult content when last event is TaskStatusUpdateEvent with no message

[yuu551]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.25.0

Python Version

3.12.0

Operating System

macOS 15.7

Installation Method

other

Steps to Reproduce

1. Create an A2A server:

# server.py
from strands import Agent
from strands.multiagent.a2a import A2AServer
from strands_tools.calculator import calculator

strands_agent = Agent(
    name="Calculator Agent",
    description="Calculator agent for arithmetic operations.",
    tools=[calculator],
    callback_handler=None,
)

a2a_server = A2AServer(agent=strands_agent, enable_a2a_compliant_streaming=True)
a2a_server.serve()

2. Call the agent synchronously or with invoke_async:

# client.py
from strands.agent.a2a_agent import A2AAgent

a2a_agent = A2AAgent(endpoint="http://localhost:9000")
result = a2a_agent("101 * 11 を計算して")
print(result.message)
#Result
# {'role': 'assistant', 'content': []}

Expected Behavior

AgentResult.message["content"] should contain the agent's response text, e.g.:

{'role': 'assistant', 'content': [{'text': '101 × 11 = 1111'}]}

Actual Behavior

AgentResult.message["content"] is an empty list:

{'role': 'assistant', 'content': []}

The text is correctly received in the stream — TaskArtifactUpdateEvents contain the expected text parts. Only the final AgentResult conversion produces empty content.

The issue is in convert_response_to_agent_result in strands/multiagent/a2a/_converters.py.

The A2A stream ends with the following sequence:

1. Multiple TaskArtifactUpdateEvents with text content (correct)
2. A final TaskArtifactUpdateEvent with lastChunk=True and empty parts
3. A final TaskStatusUpdateEvent with state=completed and message=None

stream_async selects the last complete event via _is_complete_event(). Since TaskStatusUpdateEvent with state=completed is considered complete, it becomes the last_complete_event — overwriting the previous TaskArtifactUpdateEvent with lastChunk=True.

When convert_response_to_agent_result receives (Task, TaskStatusUpdateEvent):

# Current code (lines 103-107)
elif isinstance(update_event, TaskStatusUpdateEvent):
    if update_event.status and hasattr(update_event.status, "message") and update_event.status.message:
        for part in update_event.status.message.parts:
            ...

update_event.status.message is None, so no content is extracted.

The fallback to task.artifacts (line 109) only runs when update_event is None:

elif update_event is None and task and hasattr(task, "artifacts") and task.artifacts is not None:
    for artifact in task.artifacts:
        ...

The Task object does contain the accumulated text in task.artifacts, but the converter never reaches this branch because update_event is a TaskStatusUpdateEvent, not None.

Possible Solution

Add a fallback to task.artifacts when TaskStatusUpdateEvent.status.message is None:

elif isinstance(update_event, TaskStatusUpdateEvent):
    if update_event.status and hasattr(update_event.status, "message") and update_event.status.message:
        for part in update_event.status.message.parts:
            if hasattr(part, "root") and hasattr(part.root, "text"):
                content.append({"text": part.root.text})
    # Fallback: extract from task.artifacts when status.message is None
    elif task and hasattr(task, "artifacts") and task.artifacts is not None:
        for artifact in task.artifacts:
            if hasattr(artifact, "parts"):
                for part in artifact.parts:
                    if hasattr(part, "root") and hasattr(part.root, "text"):
                        content.append({"text": part.root.text})

Workaround

Use stream_async and extract text directly from TaskArtifactUpdateEvent:

import asyncio
from strands.agent.a2a_agent import A2AAgent

async def main():
    a2a_agent = A2AAgent(endpoint="http://localhost:9000")
    async for event in a2a_agent.stream_async("101 * 11 を計算して"):
        if event.get("type") == "a2a_stream":
            a2a_event = event["event"]
            if isinstance(a2a_event, tuple):
                task, update = a2a_event
                if hasattr(update, "artifact") and update.artifact and update.artifact.parts:
                    for part in update.artifact.parts:
                        if hasattr(part, "root") and hasattr(part.root, "text"):
                            print(part.root.text, end="", flush=True)
        elif "result" in event:
            print()

asyncio.run(main())

Related Issues

• [BUG] parts of Artifact should not be empty in A2A #1640 — Server-side: lastChunk=True event sends empty parts array (violates A2A spec). This is the server-side counterpart; the current issue is about the client-side conversion logic.