Arc Swap — token swap + gasless cross-asset payments DApp on Arc Testnet

[osr21]

What I built

Arc Swap is a full-stack DeFi application on Arc Testnet (Chain ID 5042002) with three features:

1. Token swap — USDC ↔ EURC ↔ cirBTC with a smart routing stack
2. Gasless cross-asset payment — send any token, recipient gets any token, sender pays zero gas using ERC-2612 permit
3. Liquidity pool — a self-deployed Uniswap V2 USDC/EURC pool with add/remove liquidity UI

Live app: https://arc-swap-dapp.replit.app

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Architecture

Stack: React + Vite + TypeScript frontend · Express 5 + Node.js 24 backend · PostgreSQL + Drizzle ORM · Wagmi v2 + viem

Swap routing (three-tier fallback):

Uniswap V2 pool (on-chain, user pays gas)
→ only used if pool price is within 5% of live Frankfurter forex rate
Circle App Kit (LiFi Diamond → Curve StableSwap)
→ works for USDC→EURC; fails for EURC→USDC on testnet (no reverse route)
Backend peer-to-peer transfer at live market rate
→ always available, zero gas for user

Exchange rates come from real-world APIs — testnet pool prices are artificial:

• USDC ↔ EURC: Frankfurter (ECB forex, free, no auth)
• cirBTC pairs: CoinGecko free tier
• 5-minute server-side cache

Gasless payment flow (ERC-2612 permit):

User signs permit off-chain (zero gas)
→ Backend submits permit tx
→ Backend calls transferFrom (pulls tokenIn from sender)
→ Backend sends tokenOut to recipient at live market rate
→ Wrapped in Arc v0.7.2 Memo precompile for on-chain metadata

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Deployed contracts (Arc Testnet)

Contract	Address
Uniswap V2 Factory	0x7483847d46db2920dd64efa676cf72dcf765814f
Uniswap V2 Router02	0xe27d5d256b370604f1ff060fb489c6a8e3f8a6d9
USDC/EURC Pair	0xb3685D16AAa06361ED28377b1319136650Fa9A13
Memo Precompile (Arc v0.7.2)	0x5294E9927c3306DcBaDb03fe70b92e01cCede505

Live pool: ~$144 TVL · 72.4 USDC / 53.9 EURC reserves

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Arc-specific learnings

1. eth_estimateGas is broken

Every writeContract call needs an explicit gas: override. Without it, transactions revert silently or fail to submit entirely.

// Wrong — gas estimation returns garbage on Arc Testnet
await walletClient.writeContract({ address, abi, functionName: "createPair", args });

// Correct
await walletClient.writeContract({
  address, abi, functionName: "createPair", args,
  gas: 5_000_000n  // CREATE2 internally — needs 5M, not the usual 500k
});

Values that work reliably:

approve / transfer: 100_000n
swapExactTokensForTokens: 250_000n
addLiquidity (first time on new pair): 3_000_000n
createPair: 5_000_000n
2. waitForTransactionReceipt does not throw on revert
With viem on Arc Testnet, a reverted transaction returns receipt.status === "reverted" but does not throw. If you don't check this manually, execution continues as if the transaction succeeded — which caused a critical silent failure: the inbound transferFrom was reverting but the backend kept sending tokens out.

const receipt = await publicClient.waitForTransactionReceipt({ hash });
if (receipt.status === "reverted") {
  throw new Error("Transaction reverted");
}

. Circle App Kit works for USDC→EURC but not EURC→USDC
Confirmed via on-chain analysis: App Kit routes USDC→EURC through LiFi Diamond (0x20f6ee51340adeed01a59b0e65cb3703f3dc860c) → Curve StableSwap (0x86e56b2445287b1dc27f0a3594c5ac64b49c2474). The reverse direction consistently hits ONCHAIN_SIMULATION_FAILED — no liquidity for EURC→USDC in LiFi's routing on Arc Testnet.

Example confirmed Kit swap: [0xe67fc4a...](https://testnet.arcscan.app/tx/0xe67fc4a6469cd28e79c0d47d65bcf912b398183f583b8fc1f3229c5e46a0b1bc) — 1.994 USDC → 2.314 EURC, 687k gas.

Also: kit.estimateSwap() returns wildly incorrect rates on Arc Testnet (e.g. 17 EURC/USDC). Replace with Frankfurter/CoinGecko.

4. USDC has dual existence — native currency + ERC20
On Arc Testnet, USDC is both the native gas token and an ERC20 at 0x3600000000000000000000000000000000000000. Every ERC20 USDC transfer emits two Transfer events: one from 0xffffffffffffffffffffffffffffffffffffffff (the native mirror) and one from 0x3600.... Only the 0x3600... log is the canonical ERC20 event.

5. Arc v0.7.2 Memo precompile
The Memo precompile at 0x5294E9927c3306DcBaDb03fe70b92e01cCede505 lets you attach human-readable metadata to any transaction. Arc Swap uses it two ways:

Swaps (fire-and-forget, non-fatal): memo(bytes data) attaches a label like "Arc Swap: USDC → EURC | 10 in → 8.72 out" to the outbound transfer
Payments (atomic): memo(address target, bytes data, bytes32 memoId, bytes memoData) wraps the outbound ERC20 transfer so the payment and its metadata land in one atomic on-chain event with a unique memoId
Issues filed against circlefin/arc-node while building
#88 · #93 · #101 · #107 · #110 · #111 · #153 · #164

Happy to answer questions about deploying Uniswap V2 on Arc, the permit flow, or the Memo precompile integration.

Link to dapp live on Arc Testnet https://arc-swap-dapp.replit.app/