> ## Documentation Index > Fetch the complete documentation index at: https://docs.hyperlane.xyz/llms.txt > Use this file to discover all available pages before exploring further. # Post-Dispatch Hooks Overview > Configure additional origin chain behavior for messages dispatched via the Mailbox using post-dispatch hooks Post-dispatch hooks allow developers to configure additional origin chain behavior with message content dispatched via the Mailbox. ```mermaid theme={null} flowchart TB subgraph Origin Sender M_O[(Mailbox)] Hook[IPostDispatchHook] Sender -- "dispatch(..., metadata, hook)
{value}" --> M_O M_O -- "postDispatch(message, metadata)
{value}" --> Hook end ``` This allows developers to integrate third party/native bridges, make additional chain commitments, or require custom fees all while maintaining a consistent single-call Mailbox interface. ```solidity Solidity theme={null} // SPDX-License-Identifier: MIT OR Apache-2.0 pragma solidity >=0.8.0; /*@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@@@@@@@@@@@@@@@@@ @@@@@ HYPERLANE @@@@@@@ @@@@@@@@@@@@@@@@@@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@@ @@@@@@@@*/ interface IPostDispatchHook { enum Types { UNUSED, ROUTING, AGGREGATION, MERKLE_TREE, INTERCHAIN_GAS_PAYMASTER, FALLBACK_ROUTING, ID_AUTH_ISM, PAUSABLE, PROTOCOL_FEE, LAYER_ZERO_V1, RATE_LIMITED, ARB_L2_TO_L1, OP_L2_TO_L1 } /** * @notice Returns an enum that represents the type of hook */ function hookType() external view returns (uint8); /** * @notice Returns whether the hook supports metadata * @param metadata metadata * @return Whether the hook supports metadata */ function supportsMetadata( bytes calldata metadata ) external view returns (bool); /** * @notice Post action after a message is dispatched via the Mailbox * @param metadata The metadata required for the hook * @param message The message passed from the Mailbox.dispatch() call */ function postDispatch( bytes calldata metadata, bytes calldata message ) external payable; /** * @notice Compute the payment required by the postDispatch call * @param metadata The metadata required for the hook * @param message The message passed from the Mailbox.dispatch() call * @return Quoted payment for the postDispatch call */ function quoteDispatch( bytes calldata metadata, bytes calldata message ) external view returns (uint256); } ``` ## Post Dispatch In addition to the `message` dispatched via the Mailbox, the `postDispatch` function receives a `metadata` parameter. The `metadata` parameter is passed from the `dispatch` call through the Mailbox unmodified. This allows developers to pass any context they wish through to the hook. ```solidity Solidity theme={null} function postDispatch( bytes calldata metadata, bytes calldata message ) external payable; ``` If the `postDispatch` function receives insufficient payment, it may revert. Post-Dispatch Hooks may be replayable. Developers creating custom hooks should implement safe checks to prevent this behavior. [Here](https://github.com/hyperlane-xyz/hyperlane-monorepo/blob/b69bc23239ecfc8f8a6277bb0f9bc248cffea234/solidity/contracts/hooks/warp-route/RateLimitedHook.sol#L16) is an example implementation. ### Quote Dispatch (Fees) Fees are often charged in `postDispatch` to cover costs such as destination chain transaction submission and security provisioning. To receive a quote for a corresponding `postDispatch` call, you can query the `quoteDispatch` function. ```solidity Solidity theme={null} function quoteDispatch( bytes calldata metadata, bytes calldata message ) external view returns (uint256); ``` The Mailbox has a `quoteDispatch` function that returns the aggregate fee required for a `dispatch` call to be successful. ```mermaid theme={null} flowchart TB subgraph Origin Sender M_O[(Mailbox)] R_H[RequiredHook] Hook[Hook] Sender -- "quoteDispatch(..., metadata, hook)" --> M_O M_O -- "required = quoteDispatch(message, metadata)" --> R_H M_O -- "fee = hook.quoteDispatch(message, metadata)" --> Hook M_O -- "required + fee" --> Sender end ``` Move to implementing a hook guide [here.](/docs/protocol/ISM/custom-ISM) ### Overriding default Hook Metadata To override the default metadata, there is a `dispatch` overload that takes an optional `metadata` parameter. Hooks currently expect metadata to be formatted with the [`StandardHookMetadata` library](/docs/reference/developer-tools/libraries/hookmetadata). ```solidity Solidity theme={null} function dispatch( uint32 destinationDomain, bytes32 recipientAddress, bytes calldata body, bytes calldata defaultHookMetadata ) external payable returns (bytes32 messageId); ``` The custom `metadata` will be passed to the required hook's `quoteDispatch` and `postDispatch` functions, before being passed to the default hook's `postDispatch` function. ```mermaid theme={null} flowchart LR subgraph Origin Chain Sender M_O[(Mailbox)] R_H[RequiredHook] D_H[DefaultHook] Sender -- "dispatch(..., metadata){value}" --> M_O M_O -. "fee = quoteDispatch(...)" .- R_H M_O -- "postDispatch(metadata, ...)
{fee}" --> R_H M_O -. "postDispatch(metadata, ...)
{value - fee}" ..-> D_H end ``` #### Examples Checkout the: [`StandardHookMetadata` Library](https://github.com/hyperlane-xyz/hyperlane-monorepo/blob/b69bc23239ecfc8f8a6277bb0f9bc248cffea234/solidity/contracts/hooks/libs/StandardHookMetadata.sol) ```solidity Solidity theme={null} // send message from originChain to destinationChain TestRecipient IMailbox mailbox = IMailbox("mailboxAddress"); mailbox.dispatch{value: msg.value}( destinationDomain, "paddedRecipient", // Convert recipient address to padded bytes32 format bytes("messageBody"), StandardHookMetadata.overrideGasLimit(200000) ); ``` ### Custom hook and metadata After implementing the above interfaces, you can override default hook along the hook metadata by using the overloaded `dispatch` call in our mailbox: ```solidity Solidity theme={null} function dispatch( uint32 destinationDomain, bytes32 recipientAddress, bytes calldata body, bytes calldata customHookMetadata, IPostDispatchHook customHook ) external payable returns (bytes32 messageId); ``` #### Examples ```solidity Example theme={null} // send message from originChain to destinationChain TestRecipient IMailbox mailbox = IMailbox("mailboxAddress"); IPostDispatchHook merkleTree = IPostDispatchHook("merkleTreeHookAddress"); mailbox.dispatch( destinationDomain, "paddedRecipient", // Convert recipient address to padded bytes32 format bytes("messageBody"), "0x", // empty metadata merkleTree ); ```