Build-an-X for webhook delivery + receiver tests per Standard Webhooks (standardwebhooks.com) - HMAC-SHA256 signature verification, retry semantics with exponential backoff + jitter, replay-window check via timestamp tolerance, ordering guarantees, dead-letter handling for permanent failures, content-type + body-encoding fidelity. Use when authoring tests for webhook senders OR receivers in any system (Stripe / Twilio / SendGrid / GitHub / GitLab outbound webhooks; SaaS app inbound webhooks).