> ## Documentation Index
> Fetch the complete documentation index at: https://docs.algoward.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# Allowlist Enforcement

> GET /supported must be authoritative over what /settle will actually move.

<CardGroup cols={4}>
  <Card title="Invariant" icon="tag">U4</Card>
  <Card title="Category" icon="globe">Universal</Card>
  <Card title="Priority" icon="circle-exclamation">Must-have</Card>
  <Card title="Violation class" icon="triangle-exclamation">Asset Theft</Card>
</CardGroup>

## What it proves

`GET /supported` is a facilitator's public declaration of exactly which networks, schemes, and
assets it's willing to move value on. A resource server (and its client) trusts that list when
deciding to use a facilitator at all. If `/settle` will actually process a network or asset that
`/supported` never advertised, that declaration is decorative — a seller or attacker could push
value through a path nobody agreed the facilitator would touch.

**Source:** x402 protocol spec — `GET /supported` must be authoritative over what `/settle` will
actually move.

## How it works

<Steps>
  <Step title="Call GET /supported">
    Records the facilitator's declared list of supported network/scheme kinds.
  </Step>

  <Step title="Attempt settlement against an unsupported network">
    Builds a valid payment, then mutates its network to a syntactically valid but deliberately
    unregistered CAIP-2 identifier (`fixture.unsupportedNetwork`), and calls `/settle`.
  </Step>

  <Step title="Attempt settlement against an unsupported asset">
    Builds a fresh payment targeting an asset ID the facilitator has never listed
    (`fixture.unsupportedAsset`), and calls `/settle`.
  </Step>
</Steps>

## Pass condition

```ts theme={null}
const passed = networkRejected && assetRejected;
```

Both the unsupported-network and unsupported-asset settlement attempts must be rejected. The
result also records whether the tested network was actually absent from the declared
`listedNetworks` set, so a reviewer can confirm the sub-case genuinely exercised an unadvertised
path rather than accidentally reusing a supported one.

<Note>
  `U4`'s parsing of `/supported` is intentionally permissive — the exact key/shape names for the
  by-network signer map weren't fully pinned down from documentation alone (see
  [D3](/reference/decision-log#d3-api-shapes)), so this invariant asserts on **behavior**
  (rejection) rather than on matching one assumed response shape.
</Note>

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