x402 micropayments are instant. That's the feature. But it's also the exposure. Instant means irreversible. An agent pays $0.01 for an API call. 👉 The endpoint is down. ❌ Payment is gone, no chargeback, no dispute window. Just a paid 5xx. Introducing Shield, the x402 plugin that refuses to take your money when the API is down. - Shield: runs a health check before returning a 402. Service unhealthy → buyer gets 503, not a payment request. - Preflight: probes the specific endpoint being called. One route can be down while the rest of the API is alive. Preflight catches exactly that. - Circuit Breaker: zero latency, no extra HTTP calls. Watches failure history and opens the circuit after repeated errors. Buyers never pay into a consistently failing service. - Refund: for the edge case where payment settles, then the endpoint errors anyway. Auto-credits the buyer on their next call. All four drop into Relai.protect() in a single line, whether your API is listed on or not. These are open SDK tools for any x402-compliant API. Trust between buyers and providers shouldn't depend on luck.