> ## 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: Add nanopayments to an x402 buyer > Add gas-free nanopayments to your x402 client alongside standard onchain payments Add gas-free nanopayments to an existing x402 client so it can pay without gas when a server supports it, and fall back to standard onchain payments when it does not. Gas-free payments require a one-time USDC deposit into a Gateway Wallet contract. After that, every subsequent payment is an offchain signature with zero gas cost. For the server-side counterpart, see the [x402 seller integration](/gateway-nanopayments/howtos/x402-seller). ## Prerequisites Before you begin, ensure that you've: * Set up an x402 client using `@x402/core`. * Installed [Node.js](https://nodejs.org/) v18+. * Generated an EVM wallet private key for signing. * Funded the wallet with testnet USDC from the [Circle Faucet](https://faucet.circle.com) so you can deposit into Gateway. ## Steps ```shell theme={null} npm install @circle-fin/x402-batching ``` If you plan to use `CompositeEvmScheme` (recommended for supporting both payment methods), also install: ```shell theme={null} npm install @x402/evm ``` Pick the approach that matches your setup: * **Composite (recommended)**: support both Gateway and onchain payments in one client, with automatic routing based on what each server offers. * **Existing client**: add Gateway to a client you've already configured, with minimal changes and standard onchain payments preserved. If you only need gas-free payments and don't need standard onchain support, use `GatewayClient` directly. See the [buyer quickstart](/gateway-nanopayments/quickstarts/buyer) for a full walkthrough. Use `CompositeEvmScheme` to route automatically based on the server's payment requirements. When the server offers a Gateway option, the client uses `BatchEvmScheme` for a gas-free payment. When only standard onchain options are available, it falls back to `ExactEvmScheme`: ```ts theme={null} import { ExactEvmScheme } from "@x402/evm/exact/client"; import { CompositeEvmScheme, BatchEvmScheme, } from "@circle-fin/x402-batching/client"; const composite = new CompositeEvmScheme( new BatchEvmScheme(signer), new ExactEvmScheme(signer), ); client.register("eip155:*", composite); ``` No per-request routing code is needed. `CompositeEvmScheme` checks each payment option's `extra.name` field and delegates to the correct scheme automatically. If you already have scheme registrations and want to add Gateway support with minimal changes while keeping standard onchain payments available: ```ts theme={null} import { ExactEvmScheme } from "@x402/evm/exact/client"; import { registerBatchScheme } from "@circle-fin/x402-batching/client"; registerBatchScheme(client, { signer: account, fallbackScheme: new ExactEvmScheme(account), }); ``` Before making gas-free payments, deposit USDC from your wallet into the Gateway Wallet contract. This is a one-time onchain transaction: ```ts theme={null} import { GatewayClient } from "@circle-fin/x402-batching/client"; const gatewayClient = new GatewayClient({ chain: "arcTestnet", privateKey: process.env.PRIVATE_KEY as `0x${string}`, }); const balances = await gatewayClient.getBalances(); if (balances.gateway.available < 1_000_000n) { await gatewayClient.deposit("1"); } ``` `getBalances()` calls the [Get Token Balances](/api-reference/gateway/all/get-token-balances) API endpoint. After the deposit confirms, your Gateway balance is available for gas-free payments. Deposit time depends on the blockchain. See [required block confirmations](/gateway/references/supported-blockchains#required-block-confirmations) for exact times per blockchain. [Unified Balance](https://docs.arc.io/app-kit/unified-balance) in Arc App Kits offers [fast deposits](https://docs.arc.io/app-kit/tutorials/unified-balance/use-fast-deposits), which credit your Gateway balance in seconds. Before attempting a gas-free payment, check whether the target server supports Gateway. The `supports()` method requests the target URL, checks for a `402` response, and inspects the `PAYMENT-REQUIRED` header for a compatible Gateway batching option: ```ts theme={null} const support = await gatewayClient.supports( "https://api.example.com/resource", ); if (!support.supported) { console.log("Server does not support Gateway -- falling back to onchain"); } ``` If you are using `CompositeEvmScheme`, this check is optional since the scheme handles fallback automatically. Withdraw USDC from Gateway back to your wallet at any time. Withdrawals are instant for both same-blockchain and crosschain destinations. The `withdraw()` method calls the [Create Transfer Attestation](/api-reference/gateway/all/create-transfer-attestation) API endpoint: ```ts theme={null} await gatewayClient.withdraw("5"); await gatewayClient.withdraw("5", { chain: "baseSepolia" }); ``` After paying for resources, you can look up individual transfers using the [Get x402 Transfer by ID](/api-reference/gateway-nanopayments/get-x402transfer-by-id) API endpoint, or search across transfer history using the [Search x402 Transfers](/api-reference/gateway-nanopayments/search-x402transfers) API endpoint: ```ts theme={null} const { transfers, pagination } = await gatewayClient.searchTransfers({ from: gatewayClient.address, status: "received", pageSize: 20, }); const transferUUID = transfers[0]?.id; if (transferUUID) { try { const transfer = await gatewayClient.getTransferById(transferUUID); console.log({ id: transfer.id, status: transfer.status, amount: transfer.amount, }); } catch (error) { console.error("Could not load transfer", error); } } if (pagination?.pageAfter) { const nextPage = await gatewayClient.searchTransfers({ from: gatewayClient.address, status: "received", pageSize: 20, pageAfter: pagination.pageAfter, }); } ``` `searchTransfers` supports filtering by sender, recipient, nonce, network, status, token, and date range. When you filter by `status`, also provide `from`, `to`, or `nonce` to narrow the search. This example uses the buyer's address as `from`. When `network` is omitted, the client defaults to the client's configured blockchain. See the [SDK reference](/sdks/gateway-nanopayments-sdk) for the full list of parameters.