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x402 is the HTTP 402 payment standard that lets an agent pay for a request with a signed on-chain authorization instead of an API key. Utter builds on it but needs two things the standard exact scheme cannot do: meter the price against the response and settle only after the response passes validation. So Utter ships its own scheme, utter-escrow, a superset of x402 v2. Everything an agent reads in a 402 and everything it sends back in the X-PAYMENT header follows the x402 shape, so a standard client understands the envelope even though the settlement is an Utter extension.

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Two schemes, one envelope

The standard x402 scheme, backed by EIP-3009 TransferWithAuthorization. The buyer signs a transfer for an exact value and it moves, full stop. Utter supports it as a flat-only fallback: no gate, no metering. Use it for a fixed-price endpoint where the price never depends on the response. It reuses circlefin/arc-nanopayments.
Utter’s own scheme. The buyer signs a capped authorization, the facilitator reserves the cap, the handler runs, the response is classified, and only a validated success is debited for min(computed, cap). This is the scheme that makes the escrow response gate work. Permit2 is named in the spec as a no-deposit metered alternative.
The standard exact scheme transfers a fixed value before the handler runs, so it can neither meter nor gate. Never regress a metered or gated endpoint to bare exact. See why a bare charge does not work.

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The 402 response

A call without an X-PAYMENT header returns HTTP 402 with a JSON body. The accepts array lists the schemes the endpoint takes; the escrow entry carries everything an agent needs to sign.
The network string is always eip155:5042002 (Arc Testnet). The asset is USDC, which on Arc is both the 6-decimal ERC-20 and the 18-decimal native gas token, so always read decimals() at runtime and never mix the two lenses. maxAmountRequired is the cap, a ceiling and not the final price. The extra.eip712 block is the domain the buyer signs under.

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What the buyer signs

The buyer signs a DebitAuthorization with EIP-712, under the UtterEscrow domain (version 1). The typed fields are in this locked order:
The field order is part of the EIP-712 type hash. Signing the same values in a different order produces a different hash and the facilitator will not recover the expected signer. Keep the order exactly buyer, resourceId, maxAmount, nonce, validBefore.
The nonce doubles as the idempotency key across the whole money path. A retry reuses the same nonce so it can never double charge or re-sign. See exactly-once settlement.

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The X-PAYMENT header

The signed authorization goes back as a base64-encoded JSON PaymentPayload in the X-PAYMENT header:
The gate decodes the header, hands the payload to the facilitator’s /verify, and only reserves the cap if the signer, balance, nonce, and expiry all check out. From there the escrow response gate takes over: run the handler, classify, then settle, release, or fail closed.
The reference buyer SDK builds this payload for you: it reads the 402, signs the DebitAuthorization, and retries with the X-PAYMENT header. You do not hand-roll EIP-712 unless you want to.

The escrow response gate

The full flow from a 402 to a gated settlement.

Metering and classification

How the final price is computed and how a response is graded.

Exactly-once settlement

Why the nonce is the idempotency key and how retries stay safe.

Call endpoint reference

The full 402 body and header formats, field by field.