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ERROR is a reactive reference to $error — the error message string available inside on_error callbacks. When an action fails, the framework catches the error and makes its message available through this variable. 
from prefab_ui.components import Button
from prefab_ui.actions import ShowToast
from prefab_ui.actions.mcp import CallTool
from prefab_ui.rx import ERROR

Button(
    "Save",
    on_click=CallTool(
        "save_data",
        on_error=ShowToast(
            f"Failed: {ERROR}",
            variant="error",
        ),
    ),
)

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Where It’s Available

ERROR is only meaningful inside on_error handlers. Outside that context, $error is undefined. Every action that supports callbacks (on_success / on_error) makes this variable available when the action fails. Typical use: show a toast with the error message, or write it to state so it can be displayed in the UI: 
from prefab_ui.actions import SetState

CallTool(
    "fetch_data",
    on_error=SetState("error_message", ERROR),
)

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Import

from prefab_ui.rx import ERROR