Integrating Furlpay with Arbitrum: Payments, x402 and Contracts on the Largest L2

Arbitrum One has quietly become the institutional settlement layer of Ethereum. The network has processed over 2.1 billion lifetime transactions, secures roughly $20B in value, and holds the deepest stablecoin liquidity of any L2 — which is why Mastercard chose it for 24/7 stablecoin settlement and why Robinhood built its tokenized-stock chain on Arbitrum Orbit. January's ArbOS "Dia" upgrade added predictable gas pricing on top. For a payments company whose pitch is "0.5% USDC rails instead of 3.5% card interchange," this is the rail you settle on.

This week we made Furlpay Arbitrum-first. This post is the engineering log: what shipped, how it fits together, and what is still in front of us before the Arbitrum Open House in London (July 10-12).

What shipped: the API layer

• x402 on Arbitrum — our facilitator now verifies and settles agentic payments on arbitrum and arbitrum-sepolia alongside Base and Solana, and quotes default to native USDC on Arbitrum One (0xaf88...5831). The hardening we published in "Five Ways to Rob an AI Agent" carries over unchanged: HMAC-bound quotes, single-use nonces, strict web-layer parsing.
• A unified payments API — POST /api/payments/create returns a 0.5% fee quote plus EIP-712 typed data; /execute verifies the payer's signature, screens both parties against sanctions lists, and settles on Arbitrum One. Receipts link straight to Arbiscan. Batch, refund, history and a public fee estimator round out the surface.
• A chain registry — /api/chain/status, /gas-price, /contracts, /usdc/supply and /tx lookups across Arbitrum One (42161), Arbitrum Sepolia (421614), Robinhood Chain (4663), Base and Ethereum. Mock-first, so the whole stack runs with zero credentials; flip INTEGRATION_MODE=live for real JSON-RPC.
• An x402 resource catalog — agents can now discover what Furlpay sells (GET /api/x402/resources), quote any resource without triggering a 402 (/price), and confirm settlements (/settlement/:tx). Third-party servers onboard their own priced endpoints via /register.

What shipped: the contracts

Four Solidity contracts, OpenZeppelin 5 throughout, compiled for the Cancun EVM target Arbitrum supports, with a 17-test suite covering the full lifecycle of each — payments, replays, expiry, wrong-signer rejection, fee ceilings, escrow release and refund paths, and EIP-3009 settlement.

apps/web (Next.js API)                    packages/contracts (Arbitrum One)
POST /api/payments/create  ── EIP-712 ──>  FurlPayRouter.pay()      0.5% fee, nonce replay guard
POST /api/x402/facilitator/settle ──────>  X402Facilitator.settle() EIP-3009, resource-bound log
POST /api/travel/bookings  ─── USDC ───>   FurlPayEscrow            oracle release / timed refund
                                     └──>  BookingReceipt (ERC-721) minted to the traveler

• FurlPayRouter — a payer signs a Payment authorization off-chain (the exact typed data our API already emits); a relayer submits it. The merchant receives amount minus fee, the treasury accrues the fee, and the contract enforces a hard 1% fee ceiling on-chain regardless of what any API quotes. Nonces are single-use; authorizations expire.
• X402Facilitator — forwards EIP-3009 transferWithAuthorization to USDC, so the facilitator never holds funds or keys, then writes an indexed settlement record binding the payment to a hash of the resource it bought. Coinbase's hosted facilitator and PayAI already settle x402 on Arbitrum; ours differs by indexing settlements on-chain and binding them to resources, which is what /api/x402/settlement serves back to agents.
• FurlPayEscrow — travel bookings fund in USDC and release on oracle-confirmed check-in, refund inside the cancellation window, or release permissionlessly after a 7-day grace period so providers are never hostage to a dead oracle.
• BookingReceipt — an ERC-721 minted on funding whose tokenId is the booking id, so proof-of-purchase is verifiable straight from the booking reference.

Design decisions worth stealing

• Sign off-chain, settle on-chain. Users and agents never pay gas to authorize; a relayer batches submissions. Arbitrum's post-Dia fee floor makes the relayer's cost per settlement fractions of a cent.
• Compliance before the chain. Both legs of every payment pass sanctions screening server-side before any transaction is submitted — the same pattern institutional settlers use, and the reason the contract layer can stay simple.
• Fee ceilings belong on-chain. The API quotes 0.5%, but the router refuses anything above 1% cryptographically. If our API is ever compromised, the blast radius is bounded by the contract.
• Mock-first infrastructure. Every endpoint runs deterministically with zero credentials and switches to live RPC with one environment variable. Demos never depend on testnet faucets cooperating.

What is left

• Deploy to Arbitrum Sepolia, then One — the deploy script already prints the FURLPAY_* environment variables the web app reads; the contract directory flips from "pending deployment" to live addresses the moment they exist.
• The relayer — a small service that takes executed API payments and submits them to FurlPayRouter, completing the off-chain-to-on-chain loop.
• Merchant API and compliance endpoints — checkout sessions, payouts, webhooks, and public sanctions-screening routes on top of the screening engine that already guards /execute.
• ERC-4337 passkey wallets — our WebAuthn authentication already exists; wiring it to a smart account with a USDC paymaster removes the last piece of gas UX entirely.
• The Orbit question — Robinhood validated launch-on-One, migrate-to-Orbit. If Furlpay's settlement volume justifies a dedicated chain with compliance hooks at the sequencer level, the Nitro stack is the obvious path.

The pattern behind all of it: stablecoin settlement is consolidating on Arbitrum, agentic payments are consolidating on x402, and Furlpay sits at the intersection — the payment rail where both humans and machines transact, with compliance built in rather than bolted on. If you are at the Open House in London, come find us; the contracts will be on Sepolia by then.