microsoft/autogen · error · RuntimeError

Unexpected event type: {type(event)}

Error message

Unexpected event type: {type(event)}

What it means

While tool calls execute, AssistantAgent consumes an event stream from the internal tool-execution flow and requires every item to be a BaseAgentEvent or BaseChatMessage. This error means an object of some other type (raw string, Response, tool result, etc.) was emitted into that stream, breaking the agent-chat event protocol.

Source

Thrown at python/packages/autogen-agentchat/src/autogen_agentchat/agents/_assistant_agent.py:1228

                        for call in function_calls
                    ]
                )
                # Signal the end of streaming by putting None in the queue.
                stream_queue.put_nowait(None)
                return results

            task = asyncio.create_task(_execute_tool_calls(current_model_result.content, stream))

            while True:
                event = await stream.get()
                if event is None:
                    # End of streaming, break the loop.
                    break
                if isinstance(event, BaseAgentEvent) or isinstance(event, BaseChatMessage):
                    yield event
                    inner_messages.append(event)
                else:
                    raise RuntimeError(f"Unexpected event type: {type(event)}")

            # Wait for all tool calls to complete.
            executed_calls_and_results = await task
            exec_results = [result for _, result in executed_calls_and_results]

            # Yield ToolCallExecutionEvent
            tool_call_result_msg = ToolCallExecutionEvent(
                content=exec_results,
                source=agent_name,
            )
            event_logger.debug(tool_call_result_msg)
            await model_context.add_message(FunctionExecutionResultMessage(content=exec_results))
            inner_messages.append(tool_call_result_msg)
            yield tool_call_result_msg

            # STEP 4C: Check for handoff
            handoff_output = cls._check_and_handle_handoff(
                model_result=current_model_result,

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Make the custom tool-execution/reflection generator yield only BaseAgentEvent or BaseChatMessage instances (e.g. ToolCallExecutionEvent, ToolCallRequestEvent, TextMessage).
2. Print or log type(event) at the failure point to identify exactly which object is leaking into the stream.
3. Align versions: reinstall autogen-core, autogen-agentchat and autogen-ext from the same release so isinstance checks match.

from autogen_agentchat.base import Response
from autogen_agentchat.messages import BaseAgentEvent, BaseChatMessage

def is_stream_event(obj) -> bool:
    return isinstance(obj, (BaseAgentEvent, BaseChatMessage))

try:
    async for msg in agent.on_messages_stream(input_msgs, ct):
        handle(msg)
except RuntimeError as e:
    if "Unexpected event type" in str(e):
        log.error(f"Non-event object entered the tool-execution stream: {e}")
    raise

When it happens

Trigger: Overriding or customizing AssistantAgent's tool-execution/reflection path so it yields non-event objects; mixing autogen-core / autogen-agentchat / autogen-ext versions so the stream produces types the loop does not recognize.

Common situations: Subclassed agents or custom tool-call executors that yield strings or plain dicts; package version drift after partial upgrades where event class identities diverge.

Related errors

• Invalid chunk type: {type(chunk)}
• Please set OPENAI_API_KEY environment variable.
• Failed to deserialize response
• Value was null.
• Unable to convert "{value}" to enum {typeToConvert}.

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/cef3a8d065689bd3. Report an issue: GitHub.