langchain-ai/langgraph · error · ValueError

Tools can provide a list of messages in Command.update only

Error message

Tools can provide a list of messages in Command.update only when using list of messages as ToolNode input, got: {command.update} for tool '{call['name']}'

What it means

Error "Tools can provide a list of messages in Command.update only when using list of messages as ToolNode input, got: {command.update} for tool '{call['name']}'" thrown in langchain-ai/langgraph.

Source

Thrown at libs/prebuilt/langgraph/prebuilt/tool_node.py:1534

                    "Tools can provide a dict in Command.update only when using dict "
                    f"with '{self._messages_key}' key as ToolNode input, "
                    f"got: {command.update} for tool '{call['name']}'"
                )
                raise ValueError(msg)

            updated_command = deepcopy(command)
            state_update = cast("dict[str, Any]", updated_command.update) or {}
            messages_update = state_update.get(self._messages_key, [])
        elif isinstance(command.update, list):
            # Input type is list when ToolNode is invoked with a list input
            # (e.g. [AIMessage(..., tool_calls=[...])])
            if input_type != "list":
                msg = (
                    "Tools can provide a list of messages in Command.update "
                    "only when using list of messages as ToolNode input, "
                    f"got: {command.update} for tool '{call['name']}'"
                )
                raise ValueError(msg)

            updated_command = deepcopy(command)
            messages_update = updated_command.update
        else:
            return command

        # convert to message objects if updates are in a dict format
        messages_update = convert_to_messages(messages_update)

        # no validation needed if all messages are being removed
        if messages_update == [RemoveMessage(id=REMOVE_ALL_MESSAGES)]:
            return updated_command

        has_matching_tool_message = False
        for message in messages_update:
            if not isinstance(message, ToolMessage):
                continue

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/prebuilt/langgraph/prebuilt/tool_node.py:1534 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of langchain-ai/langgraph@38031739e5 (2026-08-26). Data as JSON: /api/errors/41ed682c5779b3bf. Report an issue: GitHub.