What is Nonce?
A nonce is a one-time value included in a 402 challenge. It binds a payment to a single request so the same proof can’t be replayed to unlock access twice. The caller echoes it back when retrying with payment.
Nonces are what make a pay-per-call protocol safe against replay: each challenge is unique, and a payment proof is only valid for the request that issued the matching nonce.
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