# Wrapper Pools (/docs/protocols/uniswap-labs-hooks/erc4626-wrapper/concepts/wrapper-pools)

Learn how an ERC-4626 wrapper pool works, which hook permissions it enables, and how the wrapper hook contracts are organized.

A wrapper pool is a v4 pool whose two currencies are a vault share token and its underlying, and whose behavior is defined entirely by the hook rather than by a liquidity curve:

* **No liquidity.** `beforeAddLiquidity` unconditionally reverts with `LiquidityNotAllowed`. The pool never holds reserves of either token between transactions.
* **No fee.** Initialization requires `poolKey.fee == 0` (`InvalidPoolFee`), and `beforeSwap` returns a zero fee override.
* **Fixed token pair.** Initialization requires the pool's two currencies to be exactly the wrapper and underlying currencies (`InvalidPoolToken`).

Because the pool holds no reserves, the hook sources tokens through v4 flash accounting during the swap itself (see [Swap Mechanics](/docs/protocols/uniswap-labs-hooks/erc4626-wrapper/concepts/swap-mechanics)) rather than from a standing balance.

## Hook Permissions
`getHookPermissions` enables exactly four flags:

| Permission              | Purpose                                                                    |
| :---------------------- | :------------------------------------------------------------------------- |
| `beforeInitialize`      | Validate the token pair and enforce a zero fee.                            |
| `beforeAddLiquidity`    | Block all liquidity provision.                                             |
| `beforeSwap`            | Perform the wrap or unwrap and return the swap delta.                      |
| `beforeSwapReturnDelta` | Allow `beforeSwap` to return a `BeforeSwapDelta` and take over accounting. |

No `after*` callbacks are used. The hook's deployed address must encode these flags in its low bits, per standard v4 hook address mining.

## Contract Hierarchy
```text
BaseHook, DeltaResolver
  └─ BaseTokenWrapperHook              abstract: pool rules, beforeSwap dispatch, delta settlement
       └─ ERC4626WrapperHook           vault deposit/redeem and preview math
            ├─ ERC4626RoutingHook                    quoting twin (overrides _deposit to a preview)
            │    └─ DeallocatingERC4626RoutingHook   deallocating quoting twin
            └─ DeallocatingERC4626WrapperHook        Spark variant (overrides the unwrap source)
```

`BaseTokenWrapperHook` is shared by every wrapper hook, including the WETH and wstETH wrappers. It owns the generic machinery: pool validation, the `beforeSwap` dispatch between wrap and unwrap, liquidity blocking, and the `DeltaResolver`-based settlement helpers. `ERC4626WrapperHook` fills in the vault-specific seams: `_deposit`, `_withdraw` (which delegates to the overridable `_redeemToUnderlying`), the `previewMint` and `previewWithdraw` helpers, and the exact-output flag.