---
title: ERROR
sidebarTitle: $error
description: The error message available in on_error callbacks.
icon: triangle-exclamation
---

`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.

```python
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",
        ),
    ),
)
```

## 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:

```python
from prefab_ui.actions import SetState

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

## Import

```python
from prefab_ui.rx import ERROR
```
