microsoft/autogen · error · ValueError

Task must be a string, a BaseChatMessage, or a list of BaseC

Error message

Task must be a string, a BaseChatMessage, or a list of BaseChatMessage.

What it means

The task parameter of run()/run_stream() accepts exactly three shapes: a str, a single BaseChatMessage, or a list of BaseChatMessage. Any other type (int, dict, tuple, an agent event, None-like sentinels) hits the final else branch.

Source

Thrown at python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_base_group_chat.py:472

        """
        # Create the messages list if the task is a string or a chat message.
        messages: List[BaseChatMessage] | None = None
        if task is None:
            pass
        elif isinstance(task, str):
            messages = [TextMessage(content=task, source="user")]
        elif isinstance(task, BaseChatMessage):
            messages = [task]
        elif isinstance(task, list):
            if not task:
                raise ValueError("Task list cannot be empty.")
            messages = []
            for msg in task:
                if not isinstance(msg, BaseChatMessage):
                    raise ValueError("All messages in task list must be valid BaseChatMessage types")
                messages.append(msg)
        else:
            raise ValueError("Task must be a string, a BaseChatMessage, or a list of BaseChatMessage.")
        # Check if the messages types are registered with the message factory.
        if messages is not None:
            for msg in messages:
                if not self._message_factory.is_registered(msg.__class__):
                    raise ValueError(
                        f"Message type {msg.__class__} is not registered with the message factory. "
                        "Please register it with the message factory by adding it to the "
                        "custom_message_types list when creating the team."
                    )

        if self._is_running:
            raise ValueError("The team is already running, it cannot run again until it is stopped.")
        self._is_running = True

        if self._embedded_runtime:
            # Start the embedded runtime.
            assert isinstance(self._runtime, SingleThreadedAgentRuntime)
            self._runtime.start()

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Convert the value: strings pass through, dicts go through MessageFactory.create(), otherwise build TextMessage(content=str(value), source="user").
2. Check the variable you are forwarding — often an upstream function returns a dict or object instead of the expected str/message.
3. Pass None explicitly when there is no task.

Example fix

# before
await team.run(task=result.json())  # dict -> ValueError

# after
await team.run(task=str(result.json()))  # or TextMessage(content=..., source="user")

from autogen_agentchat.messages import BaseChatMessage, TextMessage

def normalize_task(task):
    if isinstance(task, str):
        return [TextMessage(content=task, source="user")]
    if isinstance(task, BaseChatMessage):
        return [task]
    if isinstance(task, list):
        return [m if isinstance(m, BaseChatMessage) else TextMessage(content=str(m), source="user") for m in task]
    return [TextMessage(content=str(task), source="user")]

def is_valid_task(task) -> bool:
    return task is None or isinstance(task, (str, BaseChatMessage)) or (
        isinstance(task, list) and task and all(isinstance(m, BaseChatMessage) for m in task)
    )

When it happens

Trigger: Calling team.run(task=42), team.run(task={'prompt': 'hi'}), or passing a BaseAgentEvent instance. Note task=None is valid and means 'no new task'.

Common situations: Passing a dict expecting it to be interpreted as a message; passing a variable whose type changed upstream; passing a ChatCompletion result object directly.

Related errors

• All messages in task list must be valid BaseChatMessage type
• Expected Memory, List[Memory], or None, got {type(memory)}
• Unsupported tool type: {type(tool)}
• Unsupported handoff type: {type(handoff)}
• Message type must be a string, got {type(message_type)}

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/2f357c042f6fc78d. Report an issue: GitHub.