Introduction

Assay is a Uniswap v4 hook that prices adverse selection per swap rather than per pool. It is a pricing mechanism, not an access-control mechanism: it never blocks, never censors, and never takes custody of swap principal.

The problem

An automated market maker makes a standing offer at a price that only updates when someone trades against it. When the external market moves, the pool is stale, and an arbitrageur takes the difference. That bleed has a name in the literature — loss-versus-rebalancing — and it is the dominant cost of providing liquidity.

Dynamic-fee hooks respond by raising the fee when volatility rises. But volatility is a property of the block, not of the order. Within one block, a retail swap and a top-of-block arbitrage pay the same rate — though one is the liquidity provider's entire revenue and the other is their entire loss.

Assay prices the order. It measures how far the pool sits from a cached reference price, signs that distance by the direction the swap trades, and charges a share of what the swap actually captures. Two swaps in the same block, against the same drift, in opposite directions, are quoted differently. That single sign flip is the whole mechanism.

Deployment