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

The single most important fact on this page: as of 2 September 2026, Circle has not published official Arc mainnet RPC or block-explorer URLs — the two values every application, wallet config, and indexer needs before the 16 September mainnet launch. Everything else that changes is documented and can be inventoried now: the chain ID moves from 5042002 to 1243, every testnet contract address (including EURC) needs a mainnet counterpart that does not exist yet, 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.

Why a gap this basic still exists two weeks out

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 → 1243

Per ChainList, Arc testnet's chain ID is 5042002; mainnet's is 1243 (hex 0x4db). 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 will presumably keep running after mainnet launches for continued development and staging.

RPC and explorer URLs: unresolved as of 2 Sep 2026

Testnet's values are well documented: RPC https://rpc.testnet.arc.io, explorer https://testnet.arcscan.app. Circle has not published mainnet equivalents. Every RPC provider that documents Arc support today — Alchemy, Blockdaemon, dRPC, QuickNode, GetBlock — documents testnet endpoints only, which follows directly from Circle not having issued mainnet values yet. A team's mainnet cutover plan cannot be finalized until Circle publishes these; watching docs.arc.io and the provider dashboards in the run-up to 16 September is the only verifiable path here, and Built on Arc will not guess at a URL pattern.

Contract addresses: reissued, not migrated

Circle's published contract-addresses reference is explicit that its listed values — USDC, EURC, USYC, the CCTP TokenMessengerV2 contract, and the GatewayWallet/GatewayMinter pair — are for Arc Testnet only, with mainnet addresses "not yet available." EURC in particular has a testnet-only address published on docs.arc.io; no mainnet EURC address exists as of 2 Sep 2026. Any application with a testnet contract address baked into source, a subgraph manifest, or an indexer config has a hardcoded value that will not carry over — it will need the mainnet address once Circle publishes it, not a redeploy of the same address to 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 - ChainList: Arc entries - 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.*