ERC-8121 introduces hooks, a specification for cross-chain function calls. A hook fully specifies what function to call, with what parameters, on which contract, on which chain.
A hook combines a function selector, function signature, human-readable function call, return type, and ERC-7930 interoperable address:
Hooks are useful for redirecting to trusted credential registries (PoP, KYC, KYA for AI agents), cross-chain metadata resolution, and singleton registries with known security properties.