The four Effects

An Effect is something a Script asks the Shelf to do. Every one of them is performed by the Shelf, in the order you listed them.

Effect	Does	Worth knowing
`Open(app:)`	Brings an application to the front, launching it if needed	Takes the displayed name or the bundle name: `Calculatrice` or `Calculator` both work
`Kill(app:)`	Terminates it immediately	Never asks, never lets it save. Same two spellings accepted
`Paste(text:)`	Puts the text on the clipboard, restores focus to the application you came from, and synthesises ⌘V	The text stays on the clipboard afterwards, so it can be pasted again by hand
`Notify(message:)`	Shows a message in the bar while it is still on screen	Not a system notification, and the only way a Script reports anything, failure included

Paste is the one that needs Accessibility, because synthesising ⌘V does. Nothing else does, and the permission is requested the first time a Script uses it.

Order is the semantics

The list goes out only once run has returned the whole of it. Once it has, the Shelf performs the Effects in order and there is no way back — nothing comes back to the Script, so no Script can stop partway and offer you a list to choose from. clean kills its whole list or none of it.

That also means a Script must survive its own run. The seeded clean keeps Starkit on an untouchable list for exactly this reason: a Kill aimed at Starkit would end the process partway down its own list.

/// Never Killed, whatever `keep` says.
const untouchable = ["Starkit"]

Returning nothing

[] is a legitimate answer. It performs nothing and says nothing — the seeded work.gleam ships that way until you fill it in.

Failure is a Notify

There is no error channel. A Script that cannot do what it was asked says so:

case video_id(input) {
  Error(_) -> [Notify("Starkit could not read that as a YouTube link.")]
  Ok(id) -> [Paste(markdown(id))]
}

panic, todo and an unhandled crash all become a Refusal naming the Script, with the stack trace as the detail, and no Effect is performed. That is a worse report than a sentence you wrote, so write the sentence.

There is no fifth Effect

And no escape hatch standing in for one. A Script that needs a new capability gets a new word in the Vocabulary, which is a design decision — see limits.