What actually changes for builders when Arc moves from testnet to mainnet

*Updated after Arc mainnet launched on 16 September 2026.*

Circle published the Arc mainnet RPC (https://rpc.mainnet.arc.io) and block explorer (https://explorer.arc.io) at docs.arc.io on 16 September 2026 — the two values every application, wallet config and indexer needs. Everything else that changed is documented and can be inventoried: the chain ID moves from 5042002 to 5042, every testnet contract address (including EURC) has a published mainnet counterpart, and faucet USDC becomes real USDC with real funding and monitoring obligations. This is a change inventory, not a setup guide — for the mechanics of connecting to either network, see docs.arc.io.

*The Editor · 9 min read · Updated 16 Sep 2026*

Arc's public testnet has been live since 28 Oct 2025, long enough that most of a team's integration work — RPC calls, contract deployment scripts, indexer configs — has been written against testnet values. Mainnet is 16 September 2026. As of 2 September, Circle's own connection reference documents testnet endpoints only; it explicitly does not list a mainnet RPC URL or explorer. That is not an oversight in Built on Arc's research — it is the actual state of Circle's published documentation, and it means any code, config file, or environment variable currently pointing at production values for Arc is pointing at nothing, because those values have not been issued.

Chain ID: 5042002 → 5042

Per docs.arc.io, Arc testnet's chain ID is 5042002 (hex 0x4cef52); mainnet's is 5042 (hex 0x13b2). The 1243 "ARC Mainnet" entry on ChainList is Archie Chain, an unrelated project. Any place a chain ID is hardcoded — wallet network configs, contract deployment scripts, cross-chain messaging setups, front-end network switchers — is a place that needs a mainnet branch, not a value swap, since testnet keeps running after mainnet for continued development and staging.

RPC and explorer URLs: published 16 Sep 2026

Testnet's values: RPC https://rpc.testnet.arc.io, explorer https://testnet.arcscan.app. Circle published the mainnet equivalents on 16 September 2026: RPC https://rpc.mainnet.arc.io, explorer https://explorer.arc.io, per docs.arc.io, which also lists mainnet endpoints for Alchemy, Blockdaemon, dRPC and QuickNode. Take both from docs.arc.io rather than from any URL pattern you infer.

Contract addresses: reissued, not migrated

Circle published Arc mainnet contract addresses at docs.arc.io/arc/references/contract-addresses on 16 September 2026: USDC 0x3600000000000000000000000000000000000000 (the same address on testnet), EURC 0xbEf5f6d51CB62b58e6A8f77868681825C6fe21c1 (testnet 0x89B50855Aa3bE2F677cD6303Cec089B5F319D72a), USYC 0x8a5D989Bbb96929F689B0200f435f53dA42bF490, CCTP (Circle's Cross-Chain Transfer Protocol, which moves USDC between chains by burning it on one and minting it on the other) V2 TokenMessengerV2 0x28b5a0e9C621a5BadaA536219b3a228C8168cf5d, MessageTransmitterV2 0x81D40F21F12A8F0E3252Bccb954D722d4c464B64, TokenMinterV2 0xfd78EE919681417d192449715b2594ab58f5D002, Gateway Wallet 0x77777777Dcc4d5A8B6E418Fd04D8997ef11000eE, Gateway Minter 0x2222222d7164433c4C09B0b0D809a9b52C04C205, StableFX FxEscrow 0xe2E5F173576B513d994073CCbDaCBE027d43DFe6, Multicall3 0xcA11bde05977b3631167028862bE2a173976CA11, Permit2 0x000000000022D473030F116dDEE9F6B43aC78BA3, CREATE2 factory 0x4e59b44847b379578588920cA78FbF26c0B4956C. Arc's CCTP and Gateway domain is 26. Any application with a testnet contract address baked into source, a subgraph manifest, or an indexer config has a hardcoded value that does not carry over — swap in the mainnet address, not the same address on a new chain ID.

Faucet USDC vs. treasury USDC

Testnet USDC comes from Circle's faucet (faucet.circle.com) and has no value — it exists purely to pay testnet gas and move testnet balances. Mainnet gas is paid in real USDC, which converts "get testnet tokens" from a five-minute faucet claim into an actual treasury operation: funding gas wallets, monitoring balances so a hot wallet with zero USDC doesn't simply stop being able to transact, and accounting for gas as a real operating cost rather than a free testnet input. Built on Arc's guide on gas budgeting covers the operational side of that shift: /guides/what-usdc-gas-means-for-builder-budgets.

What else is documented to change at the protocol level

Circle's EVM-differences reference for Arc testnet notes several behaviors that diverge from standard Ethereum — no EIP-1559 base-fee burn (both base fee and priority fee go to the block proposer), PREVRANDAO always returns zero, reverts on value transfers to already-destructed contracts or to precompile addresses, and native USDC transfers emitting Transfer logs from a system address that indexers need to de-duplicate against ERC-20 contract events. Circle's own documentation flags that testnet fee-bound parameters specifically — the minimum and maximum base fee, and throughput limits — "may change before mainnet launch." A team should treat its testnet-derived fee assumptions as provisional, not final, and re-verify them against docs.arc.io once mainnet parameters are published rather than carrying testnet numbers forward by default.

Which providers have committed to mainnet vs. testnet only

This is the most useful thing to actually track between now and 16 September, and it is unevenly documented. Circle's own App Kit, Circle Wallets, and CCTP are Circle products and will presumably move with Arc itself, though Circle has not published a mainnet-specific statement for each as of 2 Sep 2026. Among named integration partners in Circle's 5 Aug 2026 founding-validator release — BNY, DTCC, Rain, Thunes, Wirex, Binance Wallet, Chainlink, Fireblocks, Kraken, Ledger, MetaMask, Uniswap Labs, Upbit, and others — the release itself is framed around the mainnet launch, which is the strongest public signal that those integrations are mainnet-directed rather than testnet-only exercises. Third-party infrastructure vendors (RPC providers, indexers, AA providers) documented on docs.arc.io are, without exception, currently described only against testnet in their own materials; none Built on Arc reviewed carries a mainnet-specific claim as of 2 Sep 2026. Track ongoing mainnet-launch coverage at Radian.

What this page deliberately does not do

This is an inventory of what changes, not instructions for how to change it — Arc testnet setup, RPC configuration, and faucet mechanics are Circle's documentation to own; see docs.arc.io for the actual steps. Built on Arc's role is telling a technical decision-maker what to re-verify before mainnet, not walking through the commands.

Sources: - Arc connection reference (RPC, chain ID, explorer) - Arc contract addresses - Arc gas and fees reference - Arc EVM differences reference - Circle: Connect to Arc - Circle: founding validator cohort press release, 5 Aug 2026 - Circle: Arc Public Testnet launch, 28 Oct 2025

*Built on Arc is an independent directory. Arc is a Circle product; we are not affiliated with, endorsed by, or operated by Circle.*