Balancer V3 Router

The Router is the user-facing entry point for all Balancer V3 operations. It handles ETH wrapping, Permit2, and the unlock callback pattern.

Architecture

code

┌─────────────────────────────────────────────────────────────────┐
│                        USER                                     │
│                          │                                      │
│                          ▼                                      │
│ ┌──────────────────────────────────────────────────────────┐   │
│ │                      ROUTER                               │   │
│ │  • ETH handling (wrap/unwrap WETH)                       │   │
│ │  • Permit2 integration                                   │   │
│ │  • Deadline checks                                       │   │
│ │  • User-friendly interfaces                              │   │
│ └────────────────────────────┬─────────────────────────────┘   │
│                              │                                  │
│                              ▼                                  │
│ ┌──────────────────────────────────────────────────────────┐   │
│ │                      VAULT                                │   │
│ │  • unlock() callback                                     │   │
│ │  • Core swap/liquidity logic                             │   │
│ │  • Token accounting                                      │   │
│ └──────────────────────────────────────────────────────────┘   │
└─────────────────────────────────────────────────────────────────┘

Router Contract

solidity

contract Router is IRouter, RouterHooks {
    constructor(
        IVault vault,
        IWETH weth,
        IPermit2 permit2,
        string memory routerVersion
    ) RouterHooks(vault, weth, permit2, routerVersion);
}

Core Swap Functions

swapSingleTokenExactIn

solidity

function swapSingleTokenExactIn(
    address pool,
    IERC20 tokenIn,
    IERC20 tokenOut,
    uint256 exactAmountIn,
    uint256 minAmountOut,
    uint256 deadline,
    bool wethIsEth,
    bytes calldata userData
) external payable returns (uint256 amountOut);

swapSingleTokenExactOut

solidity

function swapSingleTokenExactOut(
    address pool,
    IERC20 tokenIn,
    IERC20 tokenOut,
    uint256 exactAmountOut,
    uint256 maxAmountIn,
    uint256 deadline,
    bool wethIsEth,
    bytes calldata userData
) external payable returns (uint256 amountIn);

Query Functions (Read-Only)

solidity

// Preview swap without executing
function querySwapSingleTokenExactIn(
    address pool,
    IERC20 tokenIn,
    IERC20 tokenOut,
    uint256 exactAmountIn,
    address sender,
    bytes memory userData
) external returns (uint256 amountCalculated);

function querySwapSingleTokenExactOut(
    address pool,
    IERC20 tokenIn,
    IERC20 tokenOut,
    uint256 exactAmountOut,
    address sender,
    bytes memory userData
) external returns (uint256 amountCalculated);

Add Liquidity Functions

Proportional

solidity

function addLiquidityProportional(
    address pool,
    uint256[] memory maxAmountsIn,
    uint256 exactBptAmountOut,
    bool wethIsEth,
    bytes memory userData
) external payable returns (uint256[] memory amountsIn);

Unbalanced

solidity

function addLiquidityUnbalanced(
    address pool,
    uint256[] memory exactAmountsIn,
    uint256 minBptAmountOut,
    bool wethIsEth,
    bytes memory userData
) external payable returns (uint256 bptAmountOut);

Single Token Exact Out

solidity

function addLiquiditySingleTokenExactOut(
    address pool,
    IERC20 tokenIn,
    uint256 maxAmountIn,
    uint256 exactBptAmountOut,
    bool wethIsEth,
    bytes memory userData
) external payable returns (uint256 amountIn);

Donation

solidity

function donate(
    address pool,
    uint256[] memory amountsIn,
    bool wethIsEth,
    bytes memory userData
) external payable;

Custom

solidity

function addLiquidityCustom(
    address pool,
    uint256[] memory maxAmountsIn,
    uint256 minBptAmountOut,
    bool wethIsEth,
    bytes memory userData
) external payable returns (uint256[] memory amountsIn, uint256 bptAmountOut, bytes memory returnData);

Remove Liquidity Functions

Proportional

solidity

function removeLiquidityProportional(
    address pool,
    uint256 exactBptAmountIn,
    uint256[] memory minAmountsOut,
    bool wethIsEth,
    bytes memory userData
) external payable returns (uint256[] memory amountsOut);

Single Token Exact In

solidity

function removeLiquiditySingleTokenExactIn(
    address pool,
    uint256 exactBptAmountIn,
    IERC20 tokenOut,
    uint256 minAmountOut,
    bool wethIsEth,
    bytes memory userData
) external payable returns (uint256 amountOut);

Single Token Exact Out

solidity

function removeLiquiditySingleTokenExactOut(
    address pool,
    uint256 maxBptAmountIn,
    IERC20 tokenOut,
    uint256 exactAmountOut,
    bool wethIsEth,
    bytes memory userData
) external payable returns (uint256 bptAmountIn);

Recovery Mode

solidity

// Works even when pool is paused
function removeLiquidityRecovery(
    address pool,
    uint256 exactBptAmountIn,
    uint256[] memory minAmountsOut
) external payable returns (uint256[] memory amountsOut);

Pool Initialization

solidity

function initialize(
    address pool,
    IERC20[] memory tokens,
    uint256[] memory exactAmountsIn,
    uint256 minBptAmountOut,
    bool wethIsEth,
    bytes memory userData
) external payable returns (uint256 bptAmountOut);

The Unlock Callback Pattern

All operations use the unlock pattern:

solidity

// Example: swapSingleTokenExactIn internal flow
function swapSingleTokenExactIn(...) external payable saveSender(msg.sender) returns (uint256) {
    return abi.decode(
        _vault.unlock(
            abi.encodeCall(
                RouterHooks.swapSingleTokenHook,
                SwapSingleTokenHookParams({
                    sender: msg.sender,
                    kind: SwapKind.EXACT_IN,
                    pool: pool,
                    tokenIn: tokenIn,
                    tokenOut: tokenOut,
                    amountGiven: exactAmountIn,
                    limit: minAmountOut,
                    deadline: deadline,
                    wethIsEth: wethIsEth,
                    userData: userData
                })
            )
        ),
        (uint256)
    );
}

RouterHooks

Implements the actual logic called by the Vault:

solidity

abstract contract RouterHooks is RouterCommon {
    // Called by Vault during unlock
    function swapSingleTokenHook(SwapSingleTokenHookParams calldata params) external returns (uint256);
    function addLiquidityHook(AddLiquidityHookParams calldata params) external returns (...);
    function removeLiquidityHook(RemoveLiquidityHookParams calldata params) external returns (...);
    function initializeHook(InitializeHookParams calldata params) external returns (uint256);
}

ETH Handling

solidity

// When wethIsEth = true:
// - For input: Router wraps ETH to WETH
// - For output: Router unwraps WETH to ETH

// Receive ETH (used for unwrapping)
receive() external payable {
    // Only accept from WETH contract
}

// Return unused ETH
function _returnEth(address sender) internal {
    uint256 balance = address(this).balance;
    if (balance > 0) {
        payable(sender).sendValue(balance);
    }
}

Permit2 Integration

solidity

// Take tokens from user via Permit2
function _takeTokenIn(
    address sender,
    IERC20 token,
    uint256 amountIn,
    bool wethIsEth
) internal {
    if (wethIsEth && address(token) == address(_weth)) {
        _wrapEth(sender, amountIn);
    } else {
        permit2.transferFrom(sender, address(_vault), uint160(amountIn), address(token));
    }
}

saveSender Modifier

solidity

// Stores msg.sender in transient storage for hook callbacks
modifier saveSender(address sender) {
    _currentSwapTokensInSlot().tstore(0);
    _currentSwapTokensOutSlot().tstore(0);
    _currentSwapTokenInAmounts().tstore(0, 0);
    _currentSwapTokenOutAmounts().tstore(0, 0);
    _sender().tstore(sender);
    _;
}

BatchRouter

For multi-hop swaps and batch operations:

solidity

contract BatchRouter is IBatchRouter, BatchRouterHooks {
    // Multi-hop swaps
    function swapExactIn(
        SwapPathExactAmountIn[] memory paths,
        uint256 deadline,
        bool wethIsEth,
        bytes calldata userData
    ) external payable returns (uint256[] memory pathAmountsOut, ...);

    function swapExactOut(
        SwapPathExactAmountOut[] memory paths,
        uint256 deadline,
        bool wethIsEth,
        bytes calldata userData
    ) external payable returns (uint256[] memory pathAmountsIn, ...);
}

CompositeLiquidityRouter

For nested pool operations (pools containing BPT):

solidity

contract CompositeLiquidityRouter is ICompositeLiquidityRouter, CompositeLiquidityRouterHooks {
    // Add liquidity to nested pools in one transaction
    // Remove liquidity from nested pools in one transaction
}

BufferRouter

For ERC4626 buffer operations:

solidity

contract BufferRouter is IBufferRouter {
    // Initialize buffers
    // Add/remove buffer liquidity
    // Direct wrap/unwrap through buffers
}

Security Considerations

•Deadline: All user-facing functions have deadline checks
•Slippage: Limit parameters for min/max amounts
•Sender Storage: Uses transient storage to track original sender
•Reentrancy: Protected via Vault's transient modifier

Reference Files

•pkg/vault/contracts/Router.sol - Main Router implementation
•pkg/vault/contracts/RouterHooks.sol - Hook implementations
•pkg/vault/contracts/RouterCommon.sol - Shared utilities
•pkg/vault/contracts/BatchRouter.sol - Multi-hop operations
•pkg/vault/contracts/CompositeLiquidityRouter.sol - Nested pools