on "event" {} handler

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# on "event" {} handler

_WCL block_

Binds a wscript file to a lifecycle event; handler failures are logged, never fatal.

An `on "<event>" {}` block reacts to a lifecycle event by running a wscript file (`fn handle(event, lab)`).

```wcl
on "vm.crashed"    { run = "scripts/collect-dumps.ws" }
on "host.disk_low" { run = "scripts/alert.ws" }
```

**Handler failures are logged, never fatal.** See [the event list](../references/fact_events.md) for every event name and [the Event type](../references/entity_event_type.md) for the handler payload.

## Related

- [Provisions & event handlers](../references/concept_provisions.md)

- [provision {} block](../references/entity_provision_block.md)

- [Lifecycle events](../references/fact_events.md)

- [Event](../references/entity_event_type.md)

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