BaseFeeAdjuster

BaseFeeAdjuster is a SwapVM (the Aqua swap engine) price-modifier instruction. It adjusts swap amounts based on the current network gas price: when block.basefee exceeds the maker's configured baseline, the maker improves the effective price to compensate the taker for elevated transaction costs.

Source: src/instructions/BaseFeeAdjuster.sol

Compatible with 1→0 swaps (token1 → token0) only.

Router scope: BaseFeeAdjuster is not part of the shipped Aqua router opcode set (Controls, XYCSwap, XYCConcentrate, Decay, Fee, PeggedSwap, Extruction). It is a SwapVM price-modifier instruction offered on the relevant router, alongside the other post-swap modifiers listed under Related (for example OraclePriceAdjuster), rather than on the Aqua router.

Args encoding

Field Offset Size Type Description
baseGasPrice 0 8 bytes uint64 Baseline gas price (wei); adjustment triggers when block.basefee > baseGasPrice
ethToToken1Price 8 12 bytes uint96 ETH price in token1 units (1e18 scale, e.g. 3000e18 for 1 ETH = 3000 USDC)
gasAmount 20 3 bytes uint24 Estimated gas units to compensate (e.g. 150000)
maxPriceDecay 23 8 bytes uint64 Maximum allowed price adjustment coefficient in 1e18 scale

Total: 31 bytes. Build with BaseFeeAdjusterArgsBuilder.build(uint64 baseGasPrice, uint96 ethToToken1Price, uint24 gasAmount, uint64 maxPriceDecay).

Instructions

_baseFeeAdjuster1D

Solidity
1
function _baseFeeAdjuster1D(Context memory ctx, bytes calldata args) internal view

Must be placed after a swap instruction so that both amountIn and amountOut are already computed.

Behavior

When block.basefee > baseGasPrice:

  1. Calculates extra cost: extraGasCost = (block.basefee − baseGasPrice) × gasAmount
  2. Converts to token1: extraCostInToken1 = extraGasCost × ethToToken1Price / 1e18
  3. Computes a price improvement ratio capped by maxPriceDecay:
  • exactIn: increases amountOut (taker receives more token0)
  • exactOut: decreases amountIn (taker pays less token1)

When block.basefee <= baseGasPrice: no adjustment is made.

Errors: BaseFeeAdjusterShouldBeAppliedAfterSwap() if amountIn == 0 || amountOut == 0.

  • LimitSwap: typical predecessor that sets the base amounts.
  • OraclePriceAdjuster: oracle-based price adjustment, also applied post-swap.

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