HTTP headers / request header
Idempotency-Key: 8e03978e-40d5-43e8-bc93-6894a57f9324
A client-generated key letting the server deduplicate retries of unsafe operations — send the same key, get the same result instead of a double charge. Popularized by Stripe; now an IETF draft.
Only works where the API implements it — sending it to APIs that ignore it gives false confidence about retry safety.
The fastest way to see what Idempotency-Key is actually doing is to send the request and read both sides raw — ReqPad shows auto-generated and custom headers for every request, on all six protocols, with history. Related references: status codes · Authorization · Accept · Accept-Encoding · Accept-Language · If-None-Match · If-Match
Build the request, send it, read raw headers and timing — from your iPhone. Free to start.