> ## Documentation Index > Fetch the complete documentation index at: https://developers.circle.com/llms.txt > Use this file to discover all available pages before exploring further. > ## Agent Instructions > Before reading these docs, install the relevant Circle Skill to get pre-loaded context — skills are LLM-optimized and cover architecture decisions, correct flows, and common pitfalls. In Claude Code: run /plugin marketplace add circlefin/skills then /plugin install circle-skills@circle. With Vercel Skills CLI: run npx skills add circlefin/skills. Available skills: use-usdc https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-usdc/SKILL.md (balances, transfers, approvals on EVM and Solana); EURC has no skill — see https://developers.circle.com/stablecoins/what-is-eurc.md; bridge-stablecoin https://github.com/circlefin/skills/blob/master/plugins/circle/skills/bridge-stablecoin/SKILL.md (crosschain transfers via CCTP, USDC, EURC, and third-party assets); use-arc https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-arc/SKILL.md (Arc chain config, contract deployment, USDC bridging — Arc docs: https://docs.arc.io/llms.txt); use-circle-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-circle-wallets/SKILL.md (choose wallet type); use-developer-controlled-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-developer-controlled-wallets/SKILL.md (custodial wallets for payouts and treasury); use-user-controlled-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-user-controlled-wallets/SKILL.md (embedded wallets with Web2 login); use-modular-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-modular-wallets/SKILL.md (smart contract wallets with passkeys and ERC-4337); use-gateway https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-gateway/SKILL.md (unified USDC balance, nanopayments); use-smart-contract-platform https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-smart-contract-platform/SKILL.md (deploy, interact, monitor contracts). Pair any skill with the Circle MCP server for live SDK signatures, contract addresses, and chain IDs: {"mcpServers":{"circle":{"url":"https://api.circle.com/v1/codegen/mcp"}}}. > Read the product overview first. Each product has an overview explaining when and why to use it. Read it before quickstarts. > Pick the right wallet type. Developer-controlled (you hold keys), user-controlled (end user holds keys via PIN/social login), or modular (smart contract wallets with passkeys). See https://developers.circle.com/wallets.md. > Use CCTP V2 for crosschain transfers. CCTP natively transfers USDC and EURC via burn-and-mint, and supports permissionless wrapping of third-party assets via lock-and-mint. V1 is legacy; always use V2 unless V1 is specified or the chain requires it (Noble, Sui, and Aptos still require V1). Migration guide: https://developers.circle.com/cctp/migration-from-v1-to-v2.md. > Use Bridge Kit for frontend bridging. Wraps CCTP with a simpler SDK for user-facing apps. Use CCTP directly for backend transfers. > Gas Station vs Paymaster. Gas Station sponsors gas for Circle Wallet transactions. Paymaster lets users pay gas in USDC. Different use cases — read both overviews. > Gateway Nanopayments for sub-cent payments. Gasless USDC micropayments down to $0.000001 via x402 and batched settlement. For pay-per-request APIs, AI agent payments, streaming. > Look up USDC addresses per chain. Never hardcode — use https://developers.circle.com/stablecoins/usdc-contract-addresses.md. > Prefer SDKs over raw API calls. Node.js and Python SDKs handle auth, retries, and errors. > API key required. Bearer token in Authorization header. Testnet and mainnet use separate keys and may use different base URLs depending on the product. > Set up webhooks when available. Most operations are async. Webhooks deliver transaction confirmations and state changes. > When calling list endpoints, paginate using pageSize and pageAfter until no nextPageAfter cursor is returned—stopping at the first page silently misses records. > Building an AI agent? Start with the Agent Stack—Circle CLI, agent wallets, and nanopayments built for autonomous use cases: https://developers.circle.com/agent-stack.md. # How-to: Set up a Permit2 allowance > Grant the Permit2 contract an allowance on the buyer's USDC so Facilitator Service can settle usage-based payments Usage-based [`upto`](/facilitator-service/how-it-works#usage-based-payments) payments move USDC through the [Permit2](https://github.com/Uniswap/permit2) contract, so the buyer's USDC allowance for Permit2 must cover each payment. There are two ways to grant the allowance: * **Onchain approval**: one `approve` transaction grants Permit2 spending power indefinitely. Best when the buyer transacts repeatedly and can hold the gas asset. * **Gasless permit**: the buyer signs an EIP-2612 permit for each payment, which you, as the seller, attach to the Facilitator Service call. Best when the buyer can't hold the gas asset or integrates purely offchain, since Circle pays settlement gas when the permit is applied. ## Prerequisites Before you begin, ensure that you've: * Installed [Node.js v22.6+](https://nodejs.org/) * Installed [viem](https://viem.sh/) * Funded the buyer wallet with Arc testnet USDC from the [Circle Faucet](https://faucet.circle.com) ## Grant Permit2 spending power Pick the tab for the way you want to grant the allowance. Send one `approve` from the buyer, with Permit2 as the spender. The example targets Arc testnet. ```typescript approve-permit2.ts theme={null} import { createWalletClient, http, erc20Abi, maxUint256 } from "viem"; import { privateKeyToAccount } from "viem/accounts"; import { arcTestnet } from "viem/chains"; const buyer = privateKeyToAccount( process.env.BUYER_PRIVATE_KEY as `0x${string}`, ); const client = createWalletClient({ account: buyer, chain: arcTestnet, transport: http(), }); const hash: `0x${string}` = await client.writeContract({ address: "0x3600000000000000000000000000000000000000", // USDC on Arc testnet abi: erc20Abi, functionName: "approve", args: ["0x000000000022D473030F116dDEE9F6B43aC78BA3", maxUint256], // Permit2 }); ``` Have the buyer sign the permit under the USDC contract's own EIP-712 domain, not the Permit2 domain. Use the `name` and `version` that USDC reports on that network: `USDC` on Arc and `USD Coin` on Base and Polygon PoS, with version `2`. ```typescript sign-permit.ts theme={null} import { createPublicClient, http, parseAbi } from "viem"; import { privateKeyToAccount } from "viem/accounts"; import { arcTestnet } from "viem/chains"; const usdc = "0x3600000000000000000000000000000000000000"; const permit2 = "0x000000000022D473030F116dDEE9F6B43aC78BA3"; const buyer = privateKeyToAccount( process.env.BUYER_PRIVATE_KEY as `0x${string}`, ); const publicClient = createPublicClient({ chain: arcTestnet, transport: http(), }); const nonce: bigint = await publicClient.readContract({ address: usdc, abi: parseAbi(["function nonces(address owner) view returns (uint256)"]), functionName: "nonces", args: [buyer.address], }); const deadline: bigint = BigInt(Math.floor(Date.now() / 1000) + 900); const value: bigint = 5000000n; // must equal the Permit2 maximum const signature: `0x${string}` = await buyer.signTypedData({ domain: { name: "USDC", version: "2", chainId: 5042002, verifyingContract: usdc, }, types: { Permit: [ { name: "owner", type: "address" }, { name: "spender", type: "address" }, { name: "value", type: "uint256" }, { name: "nonce", type: "uint256" }, { name: "deadline", type: "uint256" }, ], }, primaryType: "Permit", message: { owner: buyer.address, spender: permit2, value, nonce, deadline }, }); ``` The buyer sends the signature and permit fields to you to attach to each Facilitator Service call. If a permit is present but invalid, Facilitator Service returns one of the [permit reasons](/api-reference/facilitator-service/errors#verify-and-settle-reasons) on `/verify` or `/settle`.