> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cow.bleu.builders/llms.txt
> Use this file to discover all available pages before exploring further.

# GPv2Interaction API

> Interaction library API for executing arbitrary contract calls during settlements

# GPv2Interaction API

The `GPv2Interaction` library enables settlement contracts to execute arbitrary external contract calls during trading settlements, supporting complex strategies like liquidity sourcing and protocol integrations.

## Data Structure

```solidity theme={null}
struct Data {
    address target;
    uint256 value;
    bytes callData;
}
```

| Field      | Type      | Description                     |
| ---------- | --------- | ------------------------------- |
| `target`   | `address` | Contract address to call        |
| `value`    | `uint256` | ETH value to send with the call |
| `callData` | `bytes`   | Encoded function call data      |

## Functions

### execute

Performs the actual contract interaction using inline assembly for gas optimization. Propagates revert reasons if calls fail.

```solidity theme={null}
function execute(Data calldata interaction) internal {
    address target = interaction.target;
    uint256 value = interaction.value;
    bytes calldata callData = interaction.callData;

    // solhint-disable-next-line no-inline-assembly
    assembly {
        let freeMemoryPointer := mload(0x40)
        calldatacopy(freeMemoryPointer, callData.offset, callData.length)
        if iszero(
            call(gas(), target, value, freeMemoryPointer, callData.length, 0, 0)
        ) {
            returndatacopy(0, 0, returndatasize())
            revert(0, returndatasize())
        }
    }
}
```

### selector

Extracts the 4-byte function selector from calldata for event logging.

```solidity theme={null}
function selector(Data calldata interaction) internal pure returns (bytes4);
```

## Settlement Integration

Interactions run at three distinct phases:

| Phase            | Stage             | Common Use Cases                   |
| ---------------- | ----------------- | ---------------------------------- |
| Pre-settlement   | `interactions[0]` | Approvals, permits, token wrapping |
| Intra-settlement | `interactions[1]` | DEX swaps, liquidity sourcing      |
| Post-settlement  | `interactions[2]` | Token unwrapping, fee distribution |

## Common Use Cases

* **DEX swaps** for liquidity sourcing (Uniswap, Balancer, Curve)
* **Token wrapping/unwrapping** (WETH)
* **Approval grants** to external protocols
* **Trade routing** through aggregators like 1inch
* **EIP-2612 permit calls** for gasless approvals

## Security

<Warning>
  Interactions cannot target the `vaultRelayer` contract. Only authorized solvers may submit settlements with interactions. Reentrancy guards prevent attack vectors. Misbehaving solvers face financial penalties.
</Warning>

## Best Practices

* Order interactions by settlement phase carefully
* Ensure all calls succeed (failures cause the entire settlement to revert)
* Use type-safe encoding methods (e.g., `abi.encodeWithSelector`) rather than manual byte manipulation