Followers milestone
A tracked account passed a follower milestone (1,000, 10,000, 50,000, 100,000, 500,000, 1,000,000, 5,000,000, 10,000,000), as the synced data shows it. One event per milestone; an account that falls below a milestone and passes it again more than 30 days later reports it again. Accounts first seen above a milestone never report it. Opt-in: endpoints subscribed to all events (*) do not receive it; subscribe to it by name.
Headers
The event id; the same on every retry of this event. Dedupe on it.
Unix seconds of this delivery attempt (re-signed on every retry). Reject values more than 5 minutes off.
Space-separated v1,<base64 HMAC-SHA256 of "<webhook-id>.<webhook-timestamp>.<body>"> signatures, one per active signing secret (two during a rotation). Verify with any Standard Webhooks library.
Payload
Response
Any 2xx within 15 seconds acknowledges the event. Anything else, a timeout or a redirect is retried (10 attempts over about 3 days); 410 disables the endpoint.
Changes
Webhooks have no change history: a webhook is keyed by its event name rather than a path and method, so no recorded change can name it.