> ## 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.

# 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.