Actually, for that matter, look through the examples listed of entities already using a header named Idempotency-Key, and all of them use unquoted values. Yande

Actually, for that matter, look through the examples listed of entities already using a header named Idempotency-Key, and all of them use unquoted values. Yandex Cloud and Worldpay say it must be a UUID (with their examples showing canonical textual representation), Adyen says it must not exceed 64 characters, the others don’t say anything about the value. All show UUIDs in canonical textual representation as their examples (well, Adyen actually prefixes it with UNIQUE-ID-, showing practically that it doesn’t have to be a UUID).