---
title: "Guards and confirm · Affordances"
description: "A guard is the precondition an action checks before it runs. Agents see it before they call, and get its reason when it fails."
url: "https://affordances.vercel.app/docs/concepts/guards"
image: "https://affordances.vercel.app/og/docs/concepts/guards"
lang: "en"
---

# Guards and confirm

A guard is the precondition an action checks before it runs. Agents see it before they call, and get its reason when it fails.

A guard is a function that returns `true` when the action may run, or a string saying why it can't:

```ts
defineAction({
  name: 'addToCart',
  // …
  requires: 'The variant is in stock',
  guard: ({ input, state }) => {
    if (!input) return state.product.variants.some((v) => v.stock > 0) || 'Sold out'
    const variant = state.product.variants.find((v) => v.sku === input.sku)
    return (variant && variant.stock > 0) || `${input.sku} is out of stock`
  },
})
```

`requires` is the human-readable version, shown in the graph as the descriptor's `guard`. The function is what's enforced.

## Two calls

The handler calls the guard twice in an action's life:

- **Building the graph**, with `input` undefined. The result becomes `available` and `reason`, so an agent knows what it can do before it tries. Return `true` here unless the action is unavailable whatever the input.
- **Handling a call**, with the validated input and freshly loaded state. A failure answers `unavailable`, with the guard's string as the message.

A guard may also return `false`, which fails without a reason. Prefer a sentence the agent can pass on.

## Order

Every agent call goes through the same steps, and stops at the first failure:

1. Find the action on the page's route: `not_found`.
2. Check the Origin of cookie-authenticated calls, and `expose`: `unauthorized`.
3. Apply the rate limit: `rate_limited`.
4. Validate the input: `invalid_input`, with the schema's issues.
5. Run the guard: `unavailable`.
6. For `confirm` actions without `confirm: true`: `confirm_required`, with the summary.
7. Run the action, reload state, and answer with a delta.

Input is always valid by the time the guard sees it, and the action never runs before the guard passes.

## The UI path

When your own components call the server action, it validates the input and runs. It doesn't evaluate `guard` or check `expose`: those protect the agent path, and your UI already decides what to render. If a check must hold for people too, such as stock at the moment of purchase, enforce it inside `run` as well.