400
providers.event_id_requiredprovidersprovider_event_id is required
Returned by the implementation; no specification rule declares it yet.
Provider event de-duplication is keyed on the provider's own event id, so an event without one cannot be recorded idempotently.
Why the API refuses
provider_event_id is required
What to do
Include the provider's event identifier. Without it, a redelivered notification would be applied twice, which is the one failure this field exists to prevent.