Hooks: Getting Started

A step-by-step walkthrough of creating and testing your first CoW Hook using the CoW Swap UI

This tutorial walks you through creating your first CoW Hook using the CoW Swap interface. You’ll need to encode your contract call beforehand, but the rest happens entirely through the UI. By the end, you’ll have placed an order with a pre-hook or post-hook that executes an arbitrary on-chain call alongside your swap.

Prerequisites

• A wallet with some tokens on a supported network (Ethereum, Gnosis Chain, Arbitrum, or Base)
• Access to CoW Swap

Step 1: Decide what call to make

A hook is defined by three fields that model an Ethereum call:

Field	Description
target	The contract address to call
callData	The encoded function call data
gasLimit	Gas needed to execute the call (paid in sell token as part of solver fees)

For example, if you want to claim GNO from Gnosis Chain validators before a swap, you’d encode the claim function call for the validator deposit contract.

If you’re not sure how to encode callData, you can use tools like Foundry’s cast, ethers.js, or viem to generate it from an ABI and function parameters.

Step 2: Add the hook in CoW Swap

1. Navigate to CoW Swap and connect your wallet
2. Open the Hook section — look for the pre/post hook controls near your order form
3. Decide whether you want the call to run before (pre-hook) or after (post-hook) your swap
4. Open the hook builder (the “Build your own hooks” dApp)
5. Enter the three fields from Step 1: target, callData, and gasLimit
6. Click “Add Pre-Hook” or “Add Post-Hook”

CoW Swap will simulate the call on-chain so you can verify it does what you expect before placing the order.

The call executes in the context of the HooksTrampoline contract. If your hook needs to act on tokens the user holds (e.g., staking received tokens), you’ll need CoW Shed — see Post-Hook Examples for patterns.

Step 3: Place the order

Once you’re happy with the simulation:

1. Select your sell token, buy token, and amount as you normally would
2. Review the order — the execution fee will include the gas cost of your hook
3. Place the order

Your order will appear in CoW Explorer. In the order details, scroll to the appData field and click “Show more” to see the hook data attached to your order.

Step 4: Verify the settlement

When your order is filled (usually within a few seconds to minutes):

1. Find the transaction hash in the order details on CoW Explorer
2. Open the transaction in Etherscan or Tenderly to inspect the internal calls
3. Verify that your hook executed as expected — you should see the HooksTrampoline contract making the call you specified

For example, a pre-hook that claims GNO would show the trampoline contract calling the claim function before the swap executes, giving your order additional GNO balance to trade with.

How solvers handle hooks

Solvers execute your hooks as part of the settlement transaction. When a solver posts a batch solution, it includes all pre- and post-hooks for the orders in that batch. This means:

• Gas-less for users — hook gas costs are rolled into the solver fees, paid from your sell token
• Atomic execution — if a hook fails, the entire settlement reverts (your swap doesn’t execute either)
• Batch-level — hooks execute within the same transaction as the swap

What’s next?

Hook Store

Browse and use pre-built hooks from the Hook Store

Build a Hook dApp

Create your own hook dApp with a custom UI

Post-Hook Examples

Swap → stake, swap → bridge, and swap → vault patterns

Hooks SDK Guide

Programmatic hooks with the Trading SDK