Check an x402 payment before your agent pays it. Guardian requests the paid endpoint once without paying, reads the 402 challenge and compares it with what the marketplace listing promised (price, token, endpoint), with the payee you expect, and with the token contract itself (EIP-712 domain). Nothing is signed or paid.
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Caught a real attack: the 16 Sep scan flagged a live listing (Market Signal API, sid 39876) hiding a shell payload on 0m.ar — DENY. It has since left the marketplace; see it and the full dated history at /trust.
MCP server: https://guardian-mcp-rho.vercel.app/mcp (tools/list, check_payment, probe_payment, verify_settlement, check_listing, guard paid over x402) · API: POST /probe-payment (URL in, verdict out) · POST /check-payment (challenge in, verdict out) · /demo/x402 · /trust (marketplace scan) · /agent-runs · /rules · CLI for Onchain OS: node scripts/safe-pay.js --sid <sid> · source