x402

Payment-Identifier (x402 extension)

An x402 extension that lets buyers attach a stable identifier to retries so vendors can deduplicate payment processing.

The Payment-Identifier extension to x402 lets a buyer attach a stable string identifier to a paid request via the X-PAYMENT-IDENTIFIER header. If the buyer retries the same request with the same identifier, the vendor returns the cached response without charging again.

Mission Control sets the identifier as pay_<mission_id>_<step_id> for every step. Combined with a UNIQUE constraint on (payment_intent_id, idempotency_key) in the database, this gives two layers of duplicate-payment protection: one at the vendor wire level (x402 dedupe), one at our local persistence layer (DB unique).

x402— An open standard for HTTP-native programmatic payments using stablecoins, hosted by the x402 Foundation under the Linux Foundation.
Idempotency (in payments)— A guarantee that retrying the same paid call with the same identifier results in at most one charge.