v4 constraints

Uniswap v4 shapes this design more than the economics do. Three constraints in particular decide what the hook can and cannot be.

Permissions

A v4 hook's permissions are encoded in the low bits of its own address, so they are fixed at deployment and cannot be changed. Assay claims exactly five — beforeInitialize, afterInitialize, beforeSwap, afterSwap, and afterSwapReturnDelta — giving mask 0x30C4. The address is CREATE2-mined to carry it.

It claims no liquidity permissions at all. A liquidity provider can always withdraw; the hook is not in that path and structurally cannot block it.

Dynamic fee handshake

A pool must be created with the dynamic-fee flag for a hook to override its fee. beforeInitialize rejects any pool that was not — one of only two places the hook ever reverts, and it runs before any liquidity exists.

The other rejection is the currency binding. v4 hooks are permissionless, so anyone can create a pool naming this one. The reference oracle describes exactly one token pair, so a pool of unrelated assets would be priced against a reference for something else entirely, while the hook reported that reading as fresh. Refusing at creation is the only point where refusal costs nothing.

Hot path

beforeSwap must never revert. A revert there is a denial of service against every liquidity provider in the pool, so every failure path degrades to a quoted fee instead — a stale reference quotes the ceiling rather than erroring.

Reading a Chainlink feed costs roughly 20,774 gas, which is why the reference is cached in a single packed storage slot and refreshed at most once per block. The common path makes no external call at all.