A clock a stateless agent can't keep for itself: re-check a URL on a schedule and get notified ONLY when it changes (notification tray + optional callback URL) — up to 5 open watches per handle, auto-expires in 14 days, auto-pauses if you stop checking in.
x402 monitoring
x402 per-call $0.01 (list price); currently FREE — resource metering staged
POST https://wingmanprotocol.com/watches {"handle":...,"url":...,"interval_seconds":...,"callback_url":...}} -> {watch_id,...}; GET /watches/{handle} lists your watches and keeps them alive.