Gasless architecture

Gasless swaps let a user trade without holding gas: the user signs once, an allowlisted relayer submits the transaction and pays the gas, and the AchSponsoredExecutorV3 contract executes exactly what was signed.

Components

Component	Role
Wallet	Signs one Permit2 PermitWitnessTransferFrom. The swap is its witness.
AchSwap frontend	Builds the route calldata (KyberSwap build, LI.FI transaction, or AchSwap aggregator call), the order, and the signature request.
Relay (/api/relay)	Validates the order and signature off chain, simulates, and broadcasts from an allowlisted relayer wallet.
AchSponsoredExecutorV3	Pulls the input through Permit2, calls the allowlisted target, and enforces the minimum output on the recipient's balance.
Permit2	Verifies the signature (token, amount, spender, nonce, deadline, chain, witness) and moves the input.
Targets	KyberSwap router, LI.FI diamond, AchSwap execution router.

Flow

1. The frontend gets a quote. Gasless applies to exact-input swaps from USDC, EURC or cirBTC through KyberSwap, LI.FI or the AchSwap aggregator. The input must be above the USD floor.
2. If the input token has no Permit2 allowance, the user sends a one-time ERC-20 approval to Permit2.
3. The frontend builds the target calldata with the executor as sender and the user as recipient, then the order: (user, tokenIn, amountIn, nonce, deadline, target, spender, tokenOut, minAmountOut, recipient, value, callData).
4. The user signs once. The frontend posts { mode: "sponsored", order, signature } to /api/relay.
5. The relay checks the order and recovers the signer. It simulates execute(order, signature), refuses anything above its gas cap, then signs locally and broadcasts. It never attaches native value.
6. On chain the executor:
   • checks the relayer, the input/target/spender allowlists, the value rules and the deadline;
   • calls Permit2.permitWitnessTransferFrom with the witness it recomputes from the order;
   • approves the exact input to the spender and calls the target;
   • resets the approval and refunds any unused input to the user;
   • forwards output left on the executor, then requires the recipient's balance increase to be at least minAmountOut.

The signature

Domain: { name: "Permit2", chainId: 5042, verifyingContract: 0x000000000022D473030F116dDEE9F6B43aC78BA3 }.

PermitWitnessTransferFrom(TokenPermissions permitted,address spender,uint256 nonce,uint256 deadline,SponsoredSwap witness)
SponsoredSwap(address target,address spender,address tokenOut,uint256 minAmountOut,address recipient,uint256 value,bytes32 callDataHash)
TokenPermissions(address token,uint256 amount)

spender in the permit is the executor. callDataHash is keccak256(callData). The executor recomputes the witness from the order it receives, so a relayer that changes any field makes Permit2 reject the signature with InvalidSigner. The Permit2 nonce is the only nonce: a signature executes at most once, and a user can cancel it with invalidateUnorderedNonces. executor.signingDigest(order) returns the exact digest a wallet signs.

Arc USDC rules

Arc has one USDC balance with two views: the 6-decimal ERC-20 at 0x3600000000000000000000000000000000000000 and the 18-decimal native currency.

• USDC input is always tokenIn = 0x3600… with a 6-decimal amount.
  • Allowance mode (KyberSwap, LI.FI): spender = router, value = 0.
  • Native-forward mode (AchSwap router): spender = 0x0, value = amountIn × 10^12. The executor forwards the pulled USDC as native value.
• USDC output is signed as tokenOut = 0x0000000000000000000000000000000000000000 with minAmountOut in 18-decimal wei. The executor then measures the native balance. Signing 0x3600… as output reverts with InvalidOutputToken().
• Raw native input (address(0)) is not accepted, and execute is not payable.

Security properties

• Only the user chooses the trade. Every field is bound by the one signature. The relayer can only submit it unchanged, or not at all.
• Restricted inputs, open outputs. Inputs, targets, spenders and relayers are owner-curated allowlists. Output tokens are not allowlisted: the user signs tokenOut, and the executor holds no funds between transactions. An unusual output token can only affect the user who chose it.
• Exact amounts. Permit2 pulls exactly amountIn. Taxed or short input deliveries revert. Approvals are exact and reset to zero in the same transaction.
• Minimum on actual receipt. minAmountOut is checked against the recipient's balance increase, not against the router's return value.
• Wallet stays the recipient. The frontend and the relay both refuse a recipient other than the signer.
• No delegatecall, reentrancy guarded, pausable.

Configuration

On chain (owner only): addInputToken / removeInputToken, addTarget / removeTarget, addApprovalTarget / removeApprovalTarget, addRelayer / removeRelayer, setRelayerEnforcement, pause / unpause.

List	Current entries
Input tokens	EURC 0xbEf5f6d51CB62b58e6A8f77868681825C6fe21c1, cirBTC 0x171A4217b86A807A64eB94757Db6849fb4bDbAA0, USDC 0x3600000000000000000000000000000000000000
Targets and approval spenders	KyberSwap 0x6131B5fae19EA4f9D964eAc0408E4408b66337b5, LI.FI 0xA4072583658Fae592A3506A42431cb6316a8d40b, AchExecutionRouter 0xb1C3B6B8B371aFd9c938AC064f7aeD33d6BC26cA
Relayers	0x8bbB0990B9Ba9DeFDb10389e59955886F79B19cC, 0x6e0df2d65d309b55B217B5237657302386E75584
Relayer enforcement	on

The app's executor address, allowlist mirror and EIP-712 types live in shared/deployment-config.js. Server-side operator settings:

Setting	Meaning
FEATURE_GASLESS	Must be exactly true to enable the relay and the gasless toggle.
RELAYER_PRIVATE_KEY / RELAYER_PRIVATE_KEYS	Funded relayer wallets. Each must be on the executor's relayer list.
MINIUM_USDC, MINIUM_EURC, MINIUM_cirBTC	USD floor per input token, read on every request (default $5).
GASLESS_MAX_GAS	Simulated gas cap per sponsored swap (default 3,000,000).

Deployment

Item	Value
AchSponsoredExecutorV3	0x5D5B486D032Da0B9d02651375CEe1107D40bef0b
Deployment transaction	0xbfb020e4a99a2bf7d27ce7bf965486344b978e83cb96ef4f561180ebfabfde26 (block 23434238)
Constructor argument	Permit2 0x000000000022D473030F116dDEE9F6B43aC78BA3
Compiler	solc 0.8.24, via-IR, optimizer 200 runs, EVM cancun
Runtime code hash	0x105b9382ca96cddf71c425ede9436a695987dc2a6ca92db621fe91d5d626e0b3

The deployment transaction's input matches the compiled creation bytecode byte for byte. The live runtime code matches the compiled runtime once the Permit2 immutable is filled in.

Superseded executors, not used by the app: v2 0x114FFB915eF00173D1c986c6FF5d445175622442 (two signatures), v1 0x1bC5c96ce21bc721e97DcBeABe787B9d83Dc8b1d.

See the user guide and contract addresses.