In addition to Real x402 on Solana, Leash supports MPP (Machine Payments Protocol) paywalls on Solana: the seller advertises a challenge as application/problem+json on 402, the buyer attaches a crypto credential (challenge-bound signed transaction), and a facilitator broadcasts settlement and returns a signature the seller records on the receipt. Use MPP when you want RFC 7807-shaped errors, body-carried challenges (instead of x402 PAYMENT-* headers), or a separate settle round-trip to your facilitator.

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How Leash detects the rail

@leashmarket/core classifies the unpaid 402 via detectProtocol: x402 carries the offer in PAYMENT-REQUIRED; MPP carries it in application/problem+json with type: "https://paymentauth.org/problems/payment-required" and a challengeId. @leashmarket/buyer-kit runs this automatically — a single buyer.fetch(url, init) can settle either rail.

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Seller surfaces

Surfacex402MPP
@leashmarket/seller-kitcreateSeller + @x402/honocreateMppSeller (Hono), challenge issuance + credential verify + facilitator client
Hosted payment linksDefault protocol: "x402" on POST /v1/payment-linksSet "protocol": "mpp" — see Payment links
Runner endpointsEndpointV1 default payment_protocol: "x402"payment_protocol: "mpp" on the stored descriptor
MPP routes still price in Leash-known stables and settle to the seller agent’s Asset Signer PDA; the difference is the HTTP challenge envelope and the POST /mpp/settle facilitator step instead of wiring through an x402 PAYMENT-RESPONSE header.

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Receipts

MPP settlements produce ReceiptV02 with v: "0.2" and protocol: "mpp" (see Receipt V2 schema): fields such as mpp_challenge_id, mpp_settlement_tx, and optional tx_sig mirror the on-chain proof. Legacy ReceiptV1 remains valid for historical x402-only traffic — use parseReceiptAny / receiptProtocol in @leashmarket/schemas when ingesting mixed feeds.

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Facilitators

Your facilitator must expose POST /mpp/settle compatible with Leash’s MPP client. x402 HTTPS facilitators are not drop-in substitutes for MPP settle — configure both when you run dual-protocol traffic. See Run a facilitator.

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See also