Actions

An Action is a capability a consumer may invoke by name with data: an agent's tool, a page Block's button. It ends in one of the application's own Messages, so update stays the only place a Message has effects:

import { Action } from 'foldkit-surface'

const AddToCart = Action.define({
  name: 'addToCart',
  description: 'Add a product to the cart',
  input: Schema.Struct({ productId: ProductId }),
  toMessage: input => Message.AddedToCart(input),
})

Action.run(AddToCart, { productId: 'p1' }) // Result: the Message, or why the input was refused

Action.run decodes the data as the Action's input before toMessage sees it, so data from a Document or a tool call never executes. The Action is declared once: Agent.action(AddToCart) exposes it to an agent, and a page's Catalog lists it for Blocks to reference by name.