microsoft/semantic-kernel · error · ValueError

The task must be a ChatMessageContent object.

Error message

The task must be a ChatMessageContent object.

What it means

MagenticOne only supports a single ChatMessageContent as the task because the start message carries that body into the MagenticContext.task field. Passing a plain str or a list of messages would break the context contract, so _start refuses non-ChatMessageContent input.

Source

Thrown at python/semantic_kernel/agents/orchestration/magentic.py:820

            description=description,
            input_transform=input_transform,
            output_transform=output_transform,
            agent_response_callback=agent_response_callback,
            streaming_agent_response_callback=streaming_agent_response_callback,
        )

    @override
    async def _start(
        self,
        task: DefaultTypeAlias,
        runtime: CoreRuntime,
        internal_topic_type: str,
        cancellation_token: CancellationToken,
    ) -> None:
        """Start the Magentic pattern."""
        if not isinstance(task, ChatMessageContent):
            # Magentic One only supports ChatMessageContent as input.
            raise ValueError("The task must be a ChatMessageContent object.")

        target_actor_id = await runtime.get(self._get_manager_actor_type(internal_topic_type))
        await runtime.send_message(
            MagenticStartMessage(body=task),
            target_actor_id,
            cancellation_token=cancellation_token,
        )

    @override
    async def _prepare(
        self,
        runtime: CoreRuntime,
        internal_topic_type: str,
        exception_callback: Callable[[BaseException], None],
        result_callback: Callable[[DefaultTypeAlias], Awaitable[None]],
    ) -> None:
        """Register the actors and orchestrations with the runtime and add the required subscriptions."""
        await self._register_members(runtime, internal_topic_type, exception_callback)

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Wrap your task in a ChatMessageContent before invoking: ChatMessageContent(role=AuthorRole.USER, content=...).
2. If you have a str, convert it: task = ChatMessageContent(role=AuthorRole.USER, content=my_str).
3. Use the input_transform only for transforming already-valid input, not to bypass the type requirement.

Example fix

// before
result = await orch.invoke("Summarize the quarterly report", runtime)  # raises

// after
from semantic_kernel.contents.chat_message_content import ChatMessageContent
from semantic_kernel.contents.utils.author_role import AuthorRole

task = ChatMessageContent(role=AuthorRole.USER, content="Summarize the quarterly report")
result = await orch.invoke(task, runtime)

# Coerce input to ChatMessageContent before invoking Magentic
task_msg = task if isinstance(task, ChatMessageContent) else ChatMessageContent(
    role=AuthorRole.USER, content=str(task)
)
result = await orch.invoke(task_msg, runtime)

from semantic_kernel.contents.chat_message_content import ChatMessageContent

def is_magentic_task(task) -> bool:
    return isinstance(task, ChatMessageContent)

try:
    result = await orch.invoke(task, runtime)
except ValueError as e:
    if "must be a ChatMessageContent" in str(e):
        result = await orch.invoke(
            ChatMessageContent(role=AuthorRole.USER, content=str(task)), runtime
        )
    else:
        raise

When it happens

Trigger: Calling `await orchestration.invoke(task, runtime)` where task is a str, a list[ChatMessageContent], or any non-ChatMessageContent value. (Other orchestrations accept str/list, but Magentic requires exactly one ChatMessageContent.)

Common situations: Porting code from GroupChatOrchestration or SequentialOrchestration which accept str or list inputs. Passing a raw string prompt expecting it to be wrapped automatically.

Related errors

• All members must have a description.
• Last message in chat history was null or whitespace.
• At least one action must be provided.
• All declarative agents must have an Id or a Name assigned.
• AgentDefinition Id must be set

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/ec297a700a7fc37f. Report an issue: GitHub.