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x-idempotency-key

de facto — unregistered request reached by regulation 2 spellings

Declared by 45 providers across 386 published specification files in the APIs.io network.

What this header is for, and how to use it →

A count here is providers whose published contract declares this header — not providers who send it. Response headers set at the edge rarely appear in an OpenAPI at all, so a low number can measure documentation practice rather than deployment.

Spelled 2 ways

HTTP field names are case-insensitive (RFC 9110 §5.1), so every spelling below is the same header on the wire. A contract is not the wire: generated clients key off the string, and a developer reading two of these sees two different headers.

x-idempotency-keyX-Idempotency-Key

Reached by regulation

Basis: mandated. The law, or a technical standard the law makes binding, names this header. Only a credentialed caller can observe it in flight — the catalog can see a contract declare it, not a deployment honour it.

cma-open-banking-order

Carried by the uk-open-banking-standard standard.

Providers declaring it (45)

aldermore allica-bank appdirect ardent bank-of-ireland-uk bank-of-scotland c-hoare-and-co cashfree cashplus chase-uk commerce-coinbase coutts coventry-building-society cover-genius crossmint cynergy-bank first-direct flutterwave gb-bank hampden-and-co investec kroo light lloyds-banking-group monument-bank nala natwest onsi open-banking-uk paragon-bank primer recognise-bank revolut runa santander-uk secure-trust-bank shawbrook-bank slash socotra tandem-bank vanquis-banking-group versusgame vital-io yorkshire-building-society zopa

Explore

All headers Idempotency & Reliability Guidance on headers.apievangelist.com