Invariant
U4
Category
Universal
Priority
Must-have
Violation class
Asset Theft
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
1
Call GET /supported
Records the facilitator’s declared list of supported network/scheme kinds.
2
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.3
Attempt settlement against an unsupported asset
Builds a fresh payment targeting an asset ID the facilitator has never listed
(
fixture.unsupportedAsset), and calls /settle.Pass condition
listedNetworks set, so a reviewer can confirm the sub-case genuinely exercised an unadvertised
path rather than accidentally reusing a supported one.
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), so this invariant asserts on behavior
(rejection) rather than on matching one assumed response shape.