Greyline: Endpoint Monitor

x402 endpoints fail in specific ways: the payment amount changes unexpectedly after a contract update, the response schema shifts when a dependency upgrades, call volume drops to zero and no one notices for two days. These are not hypothetical edge cases. They happen in production, and without active monitoring, you find out when a payer reports a problem.

Greyline: Endpoint Monitor gives your agent a structured protocol to register x402 endpoints with their expected parameters, poll them on a schedule, and log anomalies against four categories: price drift (payment amount changed from baseline), schema drift (response structure changed), availability drop (success rate fell below threshold), and endpoint dark (no responses within the inactivity window). The persistent registry lives in ENDPOINT_MONITOR.md alongside your other agent memory files. Every check, clean or anomalous, is logged.

This skill sits between x402 Revenue Dashboard ($29) and Greyline: Agent Security ($49). Revenue Dashboard is passive reporting from transaction logs. Agent Security is runtime protection against external threats. This skill fills the gap: active outbound polling that alerts before the problem shows up in revenue data.