The relevant draft spec for a standard way of signalling this: https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-idempotency-key-header-04 https://datatr

The relevant draft spec for a standard way of signalling this: https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-idempotency-key-header-04 https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-ide... The diagram in the article shows “Header Idempotency Key : "A"” (honestly not sure whether the second, third and fourth of those spaces exist: the diagram’s text kerning is atrocious). This should be the header field `Idempotency-Key: "A"`, though “A” would be a bad string to use (see Security Considerations).