chooseAction

Sent whenever the player has priority and may take a legal action or pass — cast a spell, play a land, activate an ability, or do nothing. Unlike the modal prompts, the reference client doesn’t show a dialog for this: it highlights the available actions directly on the board.

Arguments

input.type is "chooseAction". The remaining fields:

TypeScript

interface ChooseActionInput {
  actions: Array<AvailableAction>;
}

Rust

#[serde(rename_all = "camelCase")]
pub struct ChooseActionInput {
    pub actions: Vec<AvailableAction>,
}

References: AvailableAction

Rendering

There’s no dialog — while you hold priority, each entry in actions is highlighted in place.

Available actions glow on the board: castable cards in hand, activatable permanents, and the steps on the phase strip. Clicking a highlight sends { type: "act", actionId } for the matching action.

When one card offers several abilities, selecting it opens a small picker to choose which activateAbility to use — here, Sunken Hollow’s two mana abilities.

Response

type ChooseActionOutput =
  | { type: "pass"; until?: PassUntil; exhaustStack: boolean }
  | { type: "restoreSnapshot"; checkpointId: number }
  | { type: "act"; actionId: string };

type PassUntil = { playerId: string; phase: StepKind };

Send { type: "act", actionId } to perform an action by its id — including tapping a mana source or untapping one (undoMana), which appear in actions like any other entry. actionId MUST be an id advertised in this prompt’s actions; anything else is rejected. Send pass to pass priority; the optional until asks the engine to keep auto-passing until playerId reaches phase (a StepKind). exhaustStack: true asks the engine to keep auto-passing until every object currently on the stack has resolved; the fast-forward ends early — priority comes back with a fresh prompt — as soon as any new object (a spell, an activated ability, or a trigger) is put on the stack, so the passing player can respond to it. Conceding is not a response — it is the out-of-band directive message (see Transport), legal at any time.

Example

{
  "promptId": 7,
  "decidingPlayerId": "player-0",
  "input": {
    "type": "chooseAction",
    "actions": [
      {
        "id": "act-0",
        "type": "cast",
        "cardId": "hand-2",
        "mode": "cast",
        "modeLabel": "Cast Lightning Bolt"
      },
      {
        "id": "tap:perm-5:0",
        "type": "activateAbility",
        "cardId": "perm-5",
        "abilityIndex": 0,
        "description": "{T}: Add {G}.",
        "isManaAbility": true,
        "producedMana": [{ "color": "G", "amount": 1 }]
      }
    ]
  }
}

{
  "kind": "response",
  "promptId": 7,
  "action": {
    "type": "chooseAction",
    "output": {
      "type": "pass",
      "exhaustStack": false
    }
  }
}