Reference
Idempotency
One header that makes retrying a send safe.
Send an Idempotency-Key header on any POST that creates something. Repeating a key returns the
original result instead of acting twice — safe for retrying a request whose response you never saw.
Keys are remembered for 24 hours, scoped to your organisation.
Use it on every send
An invoice number, once issued, is permanently claimed by that taxpayer. A retried send without an
idempotency key either duplicates the invoice or fails with DUPLICATE_INVOICE_NUMBER —
and neither is cleanly recoverable.