{"product":"change-check","priceUsd":0.002,"network":"Base mainnet USDC via x402","method":"POST","description":"Cheap agent-ready change check for one HTTPS page. Supply 1-5 watched field baselines; returns unchanged only when every enrolled baseline is still provably present, otherwise requires_fallback without falsely claiming a change.","input":{"url":"https://example.com/pricing","fields":[{"field":"price","baselineText":"$20"}]}}