Bridge USDC from Base → Arc mainnet (chain 5042) through Circle Gateway. Arc has no official bridge; Gateway already routes to it as domain 26.
Live: https://base-to-arc-bridge.vercel.app
Verified with real funds: 1 USDC bridged Base → Arc, recipient received 0.9835 (0.010 Circle fee + 0.004989 Arc gas), both legs confirmed on-chain.
See SPEC.md for the protocol details, what was verified against the live API, and where the source thread was wrong.
cd arc-bridge
pnpm install
cp .env.example .env.local
pnpm devExplorer: arc-mainnet.cloud.blockscout.com (Blockscout).
An Arc RPC is baked into lib/chains.ts; override with NEXT_PUBLIC_ARC_RPC_URL.
Two different decimal counts, both correct. Arc's nativeCurrency is 18
decimals (native gas is wei), while the USDC token is 6 (USDC_DECIMALS). Use
USDC_DECIMALS for anything bridged. Confusing them misprices gas by 10^12 — see the
note in lib/chains.ts.
pnpm test # burn-intent construction + typed-data hashing
pnpm typecheck
pnpm buildArc's gas token is USDC. The final step, gatewayMint on Arc, needs Arc gas — which
a first-time bridger does not have. Set ARC_RELAYER_PRIVATE_KEY to a key funded with a
little Arc USDC and the server submits the mint for the user. Without it the app falls
back to asking the user's own wallet, which only works if they already hold Arc USDC.
Relaying is safe: gatewayMint pays destinationRecipient from the attestation, so the
relayer can spend gas but cannot redirect funds.
Step 3 is slow. Circle must index and finalize the Base deposit — roughly 20 minutes.
Until then the USDC sits in GatewayWallet, withdrawable. The UI polls and the flow is
resumable from any failed step, so closing the tab loses nothing but progress display.
Palette and type are taken from arc.io: their hero ramp
(#000B24 → #0B223E → #143453 → #326796 → #4197A1 → #E2D0AA → white) and the
Space Grotesk / Space Mono / DM Sans stack, with { … } mono eyebrow labels.
components/ArcBackdrop.tsx reproduces the "dawn horizon" as a viewport-fixed layer.
It stops at a warm glow instead of continuing to white — this is a dark card UI, and a
light lower half drops small muted copy to ~2.9:1. Every text run therefore sits on a
panel or a scrim, never straight on the gradient. Verified at 0/29 WCAG AA failures by
sampling real rendered pixels.
lib/gateway.ts domains, addresses, proxy client, fee extraction
lib/burn-intent.ts TransferSpec/BurnIntent + EIP-712 types ← the part that must be exact
lib/chains.ts Arc chain definition (USDC as native currency)
lib/abis.ts only the 5 functions this app calls
hooks/useBridge.ts 6-step state machine, resumable
app/api/gateway/… server proxy — adds X-ARC-PRIVATE-MAINNET-ENABLED
app/api/mint/ relayed gatewayMint on Arc
components/ BridgeCard, StepList, ConnectButton
USDC only, one direction, EOA signers. Other Gateway domains, Solana, smart-account signers, and burn-intent sets are deliberately out.