Short answer: not with a flow hook. In Prefect 3.x, there is no

on_late

argument on

@flow

. The supported flow hooks are: -

on_completion

-

on_failure

-

on_cancellation

-

on_crashed

-

on_running

So if you're looking for something parallel to:

python
@flow(on_cancellation=[...])

there isn't an equivalent:

python
@flow(on_late=[...])  # not supported

Why:

Late

is emitted as a flow-run state/event, but Prefect does not expose it as a flow hook. Verified in the docs/source for state change hooks and flow-run events: - State change hooks - Flow run events Recommended approach: use an Automation that listens for the

prefect.flow-run.Late

event. That’s the robust/server-side way to react to late runs: - Create automations - Event triggers - Automations Conceptually, you want: - trigger on

prefect.flow-run.Late

- then run an action like a notification, webhook, or triggered deployment A minimal example pattern is:

json
{
  "trigger": {
    "type": "event",
    "posture": "Reactive",
    "expect": ["prefect.flow-run.Late"]
  }
}

One important note from Prefect docs: hooks are client-side / best-effort, while automations are the preferred mechanism for reliable reactions to flow-run state changes. If you want, I can give you either: 1. a Prefect Cloud automation example for Late runs, or 2. a Python/CLI example to create one.