Minter

The Minter is an abstract contract inherited by the Cap vault that handles fee calculation and routing for minting, burning, and redeeming cUSD. Dynamic fees are applied based on each backing asset's current allocation relative to its optimal ratio, incentivising deposits that balance the reserve and penalising those that over-concentrate it. Whitelisted addresses bypass all dynamic fees, and a flat redeem fee applies when burning cUSD proportionally across all backing assets.

Mechanics

Dynamic Mint/Burn Fees

Every cUSD mint or burn is priced using oracle-based valuations of the input asset and the cUSD token itself. After computing a pre-fee output amount, the contract calculates the new ratio — the asset's share of the total basket value after the swap — and runs it through a kinked piecewise-linear fee schedule.

Mint fee zones (based on post-mint asset ratio):

1. Below or at optimalRatio: Only minMintFee applies (a flat floor, capped at 5%, typically 10 bps).

2. Between optimalRatio and mintKinkRatio: Fee rises linearly from minMintFee toward minMintFee + slope0.

3. Above mintKinkRatio: Fee steepens further, adding slope1 proportional to the excess above the kink.

Burn fee zones (based on post-burn asset ratio):

1. At or above optimalRatio: Zero fee — burning a well-allocated asset is always free.

2. Between burnKinkRatio and optimalRatio: Fee rises linearly using slope0.

3. Below burnKinkRatio: Fee steepens using slope1 proportional to the shortfall below the kink.

All rates, ratios, and fees are expressed in ray format where 1e27 = 100%. For example, a minMintFee of 1e24 is 0.1% (10 bps).

Mint/Burn Process Flow

1. Pre-fee amount: _amountOutBeforeFee fetches oracle prices for the asset and cUSD, computes the raw output amount, and derives the post-swap allocation ratio.

2. Whitelist check: If $.whitelist[user] is true, the pre-fee amount is returned unchanged with zero fee.

3. Fee application: For non-whitelisted users, _applyFeeSlopes selects the correct zone from the kinked curve, computes the fee in ray arithmetic, and deducts it from the output.

4. Return: (amountOut, fee) is returned to the calling vault for execution.

Redeem Process Flow

Redeeming burns cUSD in exchange for a proportional slice of every backing asset in the vault, at the current basket weights. This path avoids dynamic mint/burn fees entirely but applies a flat redeemFee.

1. A shares ratio is computed: cUSD burned / total cUSD supply, scaled to 1e33 precision.

2. For each asset in the vault, the user's proportional entitlement is totalSupplies(asset) × shares / 1e33.

3. If the user is whitelisted, redeemFee is set to zero; otherwise the flat fee is deducted from each asset amount.

4. Arrays of amountsOut and fees (one entry per asset) are returned.

Deposit Caps

Each backing asset has an independent deposit cap (depositCap[asset]). Deposits that would push total supply beyond this cap revert. A cap of 0 means unlimited.

Whitelisting

Whitelisted addresses receive fee-free minting, burning, and redeeming. This is intended for protocol-controlled addresses (e.g. the insurance fund, authorised integrators). The whitelist is managed by the access-controlled setWhitelist function.

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Architecture

Core Functions

getMintAmount(address _asset, uint256 _amountIn)

function getMintAmount(address _asset, uint256 _amountIn)
    external
    view
    returns (uint256 amountOut, uint256 fee);

Simulates a mint for msg.sender. Returns the cUSD amount received and the fee charged given the current basket state.

Parameter	Type	Description
_asset	address	The backing asset being deposited
_amountIn	uint256	Amount of _asset to deposit

Returns: amountOut — cUSD minted after fees; fee — cUSD equivalent of the fee deducted.

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getMintAmount(address _user, address _asset, uint256 _amountIn)

function getMintAmount(address _user, address _asset, uint256 _amountIn)
    external
    view
    returns (uint256 amountOut, uint256 fee);

Same as above but simulates for an arbitrary _user address. Use this overload when quoting on behalf of another address (e.g. checking whether a specific wallet is whitelisted).

Parameter	Type	Description
_user	address	Address whose whitelist status is applied
_asset	address	The backing asset being deposited
_amountIn	uint256	Amount of _asset to deposit

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getBurnAmount(address _asset, uint256 _amountIn)

function getBurnAmount(address _asset, uint256 _amountIn)
    external
    view
    returns (uint256 amountOut, uint256 fee);

Simulates burning _amountIn cUSD to receive _asset. Uses msg.sender for the whitelist check.

Parameter	Type	Description
_asset	address	The backing asset to receive
_amountIn	uint256	Amount of cUSD to burn

Returns: amountOut — asset amount received after fees; fee — fee in cUSD terms.

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getBurnAmount(address _user, address _asset, uint256 _amountIn)

function getBurnAmount(address _user, address _asset, uint256 _amountIn)
    external
    view
    returns (uint256 amountOut, uint256 fee);

Same as above but simulates for an arbitrary _user.

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getRedeemAmount(uint256 _amountIn)

function getRedeemAmount(uint256 _amountIn)
    external
    view
    returns (uint256[] memory amountsOut, uint256[] memory redeemFees);

Simulates redeeming _amountIn cUSD for a proportional share of every backing asset. Uses msg.sender for the whitelist check.

Parameter	Type	Description
_amountIn	uint256	Amount of cUSD to redeem

Returns: amountsOut — array of asset amounts (one per vault asset, in vault asset order); redeemFees — flat fee deducted per asset.

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getRedeemAmount(address _user, uint256 _amountIn)

function getRedeemAmount(address _user, uint256 _amountIn)
    external
    view
    returns (uint256[] memory amountsOut, uint256[] memory redeemFees);

Same as above but simulates for an arbitrary _user.

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getFeeData(address _asset)

function getFeeData(address _asset) external view returns (FeeData memory feeData);

Returns the current fee schedule for _asset.

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setFeeData(address _asset, FeeData calldata _feeData)

function setFeeData(address _asset, FeeData calldata _feeData) external;

Updates the fee schedule for _asset. Access-controlled. Reverts with:

• InvalidMinMintFee if minMintFee >= 0.05e27 (>= 5%)

• InvalidMintKinkRatio if mintKinkRatio == 0 or >= 1e27

• InvalidBurnKinkRatio if burnKinkRatio == 0 or >= 1e27

• InvalidOptimalRatio if optimalRatio == 0, >= 1e27, or equals either kink

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getRedeemFee()

function getRedeemFee() external view returns (uint256 redeemFee);

Returns the current flat redeem fee in ray format (1e27 = 100%).

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setRedeemFee(uint256 _redeemFee)

function setRedeemFee(uint256 _redeemFee) external;

Sets the flat redeem fee. Access-controlled. Emits SetRedeemFee.

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setWhitelist(address _user, bool _whitelisted)

function setWhitelist(address _user, bool _whitelisted) external;

Grants or revokes fee-free access for _user. Access-controlled. Emits SetWhitelist.

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setDepositCap(address _asset, uint256 _cap)

function setDepositCap(address _asset, uint256 _cap) external;

Sets the maximum total deposit allowed for _asset. _cap = 0 means no limit. Access-controlled. Emits SetDepositCap.

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whitelisted(address _user)

function whitelisted(address _user) external view returns (bool isWhitelisted);

Returns true if _user is whitelisted (fee-exempt).

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depositCap(address _asset)

function depositCap(address _asset) external view returns (uint256 cap);

Returns the deposit cap for _asset in asset-native units.

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Data Structures

MinterStorage

Internal ERC-7201 namespaced storage layout for the Minter module.

struct MinterStorage {
    address oracle;
    uint256 redeemFee;
    mapping(address => FeeData) fees;
    mapping(address => bool) whitelist;
    mapping(address => uint256) depositCap;
}

Field	Type	Description
oracle	address	Address of the price oracle used to value assets and cUSD
redeemFee	uint256	Flat fee applied on redeem, in ray format (1e27 = 100%)
fees	mapping(address => FeeData)	Fee schedule per backing asset
whitelist	mapping(address => bool)	Fee-exempt addresses
depositCap	mapping(address => uint256)	Maximum asset deposit per backing asset, in asset-native units

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FeeData

Fee schedule for a single backing asset. All ratio and rate fields are in ray format (1e27 = 100%).

struct FeeData {
    uint256 minMintFee;
    uint256 slope0;
    uint256 slope1;
    uint256 mintKinkRatio;
    uint256 burnKinkRatio;
    uint256 optimalRatio;
}

Field	Type	Description
minMintFee	uint256	Floor mint fee, always applied regardless of ratio. Max 0.05e27 (5%). Typical value: 1e24 (0.1%).
slope0	uint256	Rate of fee increase between optimalRatio and the kink ratio.
slope1	uint256	Rate of fee increase beyond the kink ratio — steeper than slope0.
mintKinkRatio	uint256	Ratio above which slope1 activates for mints. Must be in (0, 1e27).
burnKinkRatio	uint256	Ratio below which slope1 activates for burns. Must be in (0, 1e27).
optimalRatio	uint256	Target allocation ratio for the asset in the basket. Minting below this is cheap; burning above is free.

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Events

Event	Parameters	Emitted when
SetFeeData	address asset, FeeData feeData	Fee schedule updated for an asset
SetRedeemFee	uint256 redeemFee	Flat redeem fee updated
SetWhitelist	address user, bool whitelisted	Whitelist status changed for an address
SetDepositCap	address asset, uint256 cap	Deposit cap updated for an asset

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Errors

Error	Condition
InvalidMinMintFee	minMintFee >= 0.05e27 (5%)
InvalidMintKinkRatio	mintKinkRatio == 0 or >= 1e27
InvalidBurnKinkRatio	burnKinkRatio == 0 or >= 1e27
InvalidOptimalRatio	optimalRatio == 0, >= 1e27, or equals mintKinkRatio/burnKinkRatio

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Usage Examples

1. Quoting a mint before transacting

interface IMinter {
    function getMintAmount(address _user, address _asset, uint256 _amountIn)
        external
        view
        returns (uint256 amountOut, uint256 fee);
}

address vault    = 0x...; // Cap vault proxy
address usdc     = 0x...;
address alice    = 0x...;
uint256 amountIn = 1000e6; // 1,000 USDC (6 decimals)

(uint256 cusdOut, uint256 feePaid) = IMinter(vault).getMintAmount(alice, usdc, amountIn);
// cusdOut  — cUSD minted after dynamic fee
// feePaid  — cUSD equivalent of the fee charged (goes to insurance fund)

2. Checking whitelist status before routing

interface IMinter {
    function whitelisted(address _user) external view returns (bool);
    function getMintAmount(address _user, address _asset, uint256 _amountIn)
        external
        view
        returns (uint256 amountOut, uint256 fee);
}

function quoteMint(address vault, address user, address asset, uint256 amountIn)
    external
    view
    returns (uint256 amountOut, uint256 fee, bool isFeeExempt)
{
    isFeeExempt = IMinter(vault).whitelisted(user);
    (amountOut, fee) = IMinter(vault).getMintAmount(user, asset, amountIn);
}

3. Simulating a redeem across all basket assets

interface IMinter {
    function getRedeemAmount(address _user, uint256 _amountIn)
        external
        view
        returns (uint256[] memory amountsOut, uint256[] memory redeemFees);
}

interface IVault {
    function assets() external view returns (address[] memory);
}

function previewRedeem(address vault, address user, uint256 cusdAmount)
    external
    view
    returns (address[] memory assetAddresses, uint256[] memory amounts, uint256[] memory fees)
{
    assetAddresses = IVault(vault).assets();
    (amounts, fees) = IMinter(vault).getRedeemAmount(user, cusdAmount);
    // amounts[i] and fees[i] correspond to assetAddresses[i]
}