# Hooks Reference

Hooks let a pool run extra logic around Vault operations, most relevantly computing a dynamic swap fee. A hook that changes the swap fee changes the quote, so an aggregator must model it. Every hook attached by an approved factory ships with its maths in the maths/open in new window package alongside the pool maths; the Vault class applies it when you pass the pool's hookType and a hookState.

For more details about Bush Hooks see Hooks Core Concepts and the Hooks API. To find which hook (if any) a pool has, call getHooksConfig(pool) on the Vault; hooksContract is address(0) for pools without one.

# Supported Hook Types

# Stable Surge Hook

• Uses the onComputeDynamicSwapFeePercentage hook to raise the fee on swaps that push a Stable pool further from balance, and charge the static fee otherwise.
• Pools with the StableSurge hook are deployed from a dedicated factory, StableSurgePoolFactoryopen in new window, which attaches the hook on registration. The pool itself is a standard Stable pool.
• Factory address: Deployment Addresses - see StableSurgePoolFactory.
• Contracts: StableSurgeHook.solopen in new window, SurgeHookCommon.solopen in new window
• Maths: TypeScriptopen in new window, Pythonopen in new window, Rustopen in new window
• Maths requires the configurable maxSurgeFeePercentage and surgeThresholdPercentage values for the pool, which can be fetched and tracked on the hook contract using the following functions and events:

function getMaxSurgeFeePercentage(address pool) external view returns (uint256);
function getSurgeThresholdPercentage(address pool) external view returns (uint256);

event MaxSurgeFeePercentageChanged(address indexed pool, uint256 newMaxSurgeFeePercentage);
event ThresholdSurgePercentageChanged(address indexed pool, uint256 newSurgeThresholdPercentage);

• In the maths package, pass the hook alongside the pool state:

const poolState: StableState = { poolType: 'STABLE', hookType: 'StableSurge', /* ...base + amp */ };

const hookState = {
  hookType: 'StableSurge',
  amp,                      // same raw amp as the pool state
  surgeThresholdPercentage, // getSurgeThresholdPercentage(pool)
  maxSurgeFeePercentage,    // getMaxSurgeFeePercentage(pool)
};

const amountOut = new Vault().swap(swapInput, poolState, hookState);

# Adding a hook

A new hook is contributed together with the factory that attaches it, and its maths is part of that contribution; see Contributing a Pool Factory.