> ## 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: Build a hook receiver > Implement executeWithCrossChainToken so logic runs atomically when a CCTP for non-USDC transfer arrives. A hook receiver is a destination-side contract that receives CCTP for non-USDC tokens and runs custom logic in the same transaction that delivers them. The receiver must implement `executeWithCrossChainToken` and return [`EXECUTE_SUCCESS`](/cctp/expanded-assets/references/contract-reference#execute_success). For background, see [Hooks](/cctp/expanded-assets/concepts/hooks). ## Prerequisites Before you begin, ensure that you've: * Set up a Solidity development environment (Foundry, Hardhat, or similar) * Located the local [`CrossChainTokenService` address](/cctp/expanded-assets/references/contract-addresses) ## Steps Create a contract that exposes the hook entrypoint the destination service calls. Restrict the caller to the local `CrossChainTokenService`, run your logic, and return `EXECUTE_SUCCESS`. ```solidity Solidity theme={null} import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol"; contract VaultDepositReceiver { bytes32 public constant EXECUTE_SUCCESS = keccak256("circle-cctpx-execute-success"); address public immutable service; address public immutable vault; error OnlyService(); constructor(address _service, address _vault) { service = _service; vault = _vault; } function executeWithCrossChainToken( uint32 /* sourceDomain */, bytes calldata /* sourceAddress */, bytes32 /* tokenId */, address token, uint256 amount, uint32 /* finalityThresholdExecuted */, bytes calldata hookData ) external returns (bytes32) { if (msg.sender != service) revert OnlyService(); address recipient = abi.decode(hookData, (address)); // Example app logic only — not a CCTP requirement. Replace with your own. IERC20(token).approve(vault, amount); IVault(vault).depositFor(recipient, token, amount); return EXECUTE_SUCCESS; } } ``` Reverting inside `executeWithCrossChainToken` rolls back destination delivery (including the mint or unlock) in that `receiveMessage` transaction. The source burn or lock has already happened and is not undone. Delivery can be retried later. Use `autoExecuteHookData: false` if you want tokens delivered without running hook logic. Deploy the receiver on every blockchain where transfers will arrive with hook data. Each deployment must bind to the local `CrossChainTokenService`. On the source blockchain, encode the hook payload and call [`crossChainTransfer`](/cctp/expanded-assets/references/contract-reference) with `destinationAddress` set to the receiver contract and `autoExecuteHookData` set to `true`. ```typescript TypeScript theme={null} import { encodeAbiParameters, encodePacked } from "viem"; // hookData is an application payload — ABI encoding is fine here. const hookData = encodeAbiParameters([{ type: "address" }], [recipient]); // destinationAddress must be raw packed address bytes (20 bytes on EVM). // Do not use encodeAbiParameters — that pads to 32 bytes and breaks delivery. const destinationAddress = encodePacked(["address"], [receiverAddress]); await walletClient.writeContract({ address: serviceAddress, abi: serviceAbi, functionName: "crossChainTransfer", args: [ tokenId, 100n * 10n ** 18n, destinationDomain, destinationAddress, "0x0000000000000000000000000000000000000000000000000000000000000000", 2000, // standard finality; prefer this for hooks claim, true, // autoExecuteHookData hookData, ], value: feeTotalAmount, }); ``` The `claim` and `feeTotalAmount` values are returned by the Iris fee-quote endpoint. Use a standard quote body (`amount` + `feeToken` only) unless your token is enabled for fast transfer. Do not include `FORWARD` when `autoExecuteHookData` is `true`—hook transfers require self-relay. See [`QuoteClaim`](/cctp/expanded-assets/references/contract-reference). Before calling `crossChainTransfer` on the service, approve the source `TokenManager` (not the service) for the transfer amount. This applies to `NATIVE_CROSSCHAIN_TOKEN`, `BURN_MINT`, and `LOCK_UNLOCK`. Iris does not relay transfers that carry hook data with `autoExecuteHookData` set to `true`. After Iris attests the source blockchain message, you or your own relayer must call `receiveMessage` on the destination `MessageTransmitterV2` to deliver the transfer and trigger the hook. See [Self-relay required for hook transfers](/cctp/expanded-assets/concepts/hooks#self-relay-required-for-hook-transfers). When the destination service receives the message, it delivers the tokens and calls your receiver's hook in the same transaction.