> ## 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.

# Hooks

> Atomically execute logic on the destination blockchain when a CCTP for non-USDC transfer arrives.

CCTP for non-USDC transfers can carry arbitrary `hookData` that is delivered
together with the tokens on the destination blockchain. When
`autoExecuteHookData` is `true` and the recipient implements
`executeWithCrossChainToken`, CCTP for non-USDC invokes that entrypoint in the
same transaction that delivers the tokens.

## Enabling hook execution

A sender provides three parameters on
[`crossChainTransfer`](/cctp/expanded-assets/references/contract-reference) to
control hook behavior:

| Parameter | Purpose |
| - | - |
| `destinationAddress` | The contract that receives the tokens and the hook callback |
| `hookData` | Arbitrary `bytes` payload passed to the recipient's hook |
| `autoExecuteHookData` | When `true` and `hookData` is non-empty, the destination `CrossChainTokenService` executes the hook |

If `autoExecuteHookData` is `false`, the tokens are delivered but the hook is
not called. If `hookData` is empty, the hook is not called even when
`autoExecuteHookData` is `true`.

<Warning>
  Hook receivers execute arbitrary logic in the same transaction that delivers
  tokens. A vulnerable hook can lock or redirect funds permanently. Thoroughly
  audit any hook contract before deploying to mainnet—incorrect access control,
  unchecked external calls, and missing input validation are common failure modes.
</Warning>

## Receiver contract requirement

When `autoExecuteHookData` is `true` and `hookData` is non-empty, the
`destinationAddress` **must** be a contract that implements
`executeWithCrossChainToken` and returns the
[`EXECUTE_SUCCESS`](/cctp/expanded-assets/references/contract-reference#execute_success)
sentinel. Restrict the caller to the local `CrossChainTokenService`.

```solidity Solidity theme={null}
function executeWithCrossChainToken(
    uint32 sourceDomain,
    bytes calldata sourceAddress,
    bytes32 tokenId,
    address token,
    uint256 amount,
    uint32 finalityThresholdExecuted,
    bytes calldata hookData
) external returns (bytes32);
```

Return `keccak256("circle-cctpx-execute-success")`. See
[Build a hook receiver](/cctp/expanded-assets/howtos/build-hook-receiver) for a
full example.

If the destination is an externally owned account, doesn't implement the
interface, or returns a value other than `EXECUTE_SUCCESS`, the destination
service reverts with `HookDataExecutionFailed`. Sending hook data to an address
that can't satisfy the interface will fail the transfer at delivery.

## Atomicity

The hook executes in the same transaction as token delivery. If the hook
reverts, the entire transfer reverts. Tokens are not delivered and the
source-domain transfer remains intact. Designing hooks that consider their own
failure modes is the receiver's responsibility.

## Hook parameters

The `sourceAddress` is encoded as `bytes` rather than `address` to support
source blockchains where addresses do not fit in 20 bytes.

`finalityThresholdExecuted` is the finality threshold at which the inbound
message was attested: `2000` for finalized transfers, `1000` for fast
(pre-finalized) transfers. Receivers can branch on this value to decide whether
to perform side-effects immediately or defer them.

## Finality threshold and rollback

Hooks may execute on pre-finalized messages when the sender opts into
`minFinalityThreshold < 2000`. At that finality, the source blockchain
transaction can still be reorged, which would invalidate the delivery.
Integrators using `autoExecuteHookData` with `minFinalityThreshold < 2000` must
treat hook execution on unfinalized messages as provisional and design their
hook contracts to tolerate potential rollback—for example, by deferring
irreversible side-effects until `finalityThresholdExecuted` equals `2000` or
until finality is otherwise confirmed.

Prefer standard finality (`2000`) for hook transfers unless you have explicitly
designed for provisional execution.

## Self-relay required for hook transfers

Iris does not relay transfers that carry hook data with
`autoExecuteHookData = true`. Do not include `FORWARD` in the fee quote for
these transfers. You or your own relayer infrastructure must call
`receiveMessage` on the destination `MessageTransmitterV2` to complete the
transfer. Plan for this when designing a hook-based integration: every hook
transfer requires the sender (or a relayer the sender operates) to drive the
destination call.