An API change announced ahead of its effective date.

Every queued webhook delivery, test pings included, is this envelope POSTed to your endpoint. Headers: X-TrackingMCP-Event (the type), X-TrackingMCP-Delivery (equals body id), X-TrackingMCP-Signature: sha256=hex(HMAC-SHA256(secret, raw body)) where secret is the one returned ONCE at registration. Verify the signature before trusting the payload. Answer any 2xx; a non-2xx or 10-second timeout is retried after 10s, 1m, 5m, 30m and 2h, then marked dead after the sixth failed attempt (about a 2h36m retry window). Delivery is at-least-once, duplicates are possible, and ordering is not guaranteed; deduplicate by delivery id and fetch canonical shipment state. There is no replay endpoint. Enqueueing shipment_updated is currently best-effort, so REST reconciliation/polling remains required after receiver or platform outages. After 20 consecutive failed deliveries the endpoint is disabled automatically; one success resets the counter. Alert events can be suppressed for 24 hours by their alert policy; shipment_updated is emitted only for a newly persisted material change.