Which agents registered on Arc actually answer. Every endpoint every agent lists in its ERC-8004 registration file (web, A2A, MCP, x402, the file itself) is called once an hour through an address-checked fetcher: A2A cards are read, MCP servers get the initialize handshake, x402 paywalls are parsed the way a standard client would and checked against the on-chain EIP-712 domains of USDC and EURC on Arc (a wrong domain means every signature fails - we name it).

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