FirstSwapOfDayHook is built and its tests pass against a real v4 PoolManager. With the network off, forge build and forge test succeed (40 tests, 0 failures) and forge fmt --check is clean.

I also ran the two protected files. The three hook checks in Hook.protected.t.sol pass, using a scratch copy fed with the compiled creation code. I only ran that copy with baseFee 3000, the 0x1080 flags, and a placeholder manager address. The Token.protected.t.sol suite is skipped when no token creation code is supplied, and this task has no token. I did not run it, so it is unchecked.

Design

• Permissions: the hook uses only afterInitialize and beforeSwap, so its address needs the low bits 0x1080.
• Fees: the first swap in a pool on a UTC day gets a zero LP fee through the fee override, and later swaps that day get baseFee.
• State: a pool's last free day is stored per PoolId, so pools that share the hook don't affect each other.
• Validation: afterInitialize reverts unless key.fee == DYNAMIC_FEE_FLAG.
• Authentication: both callbacks revert NotPoolManager unless the caller is the immutable poolManager. sender and hookData are ignored, so the free swap belongs to the pool, not to an account.
• No admin or exits blocked: there is no owner, no upgrade path, and no liquidity callbacks, so LP exits are never blocked. The runtime code has no DELEGATECALL, CALLCODE or SELFDESTRUCT.
• Views and event: currentDay, freeSwapAvailable, nextSwapFee and lastFreeSwapDay, plus a FreeSwapGranted event.

Tests test/FirstSwapOfDayHook.t.sol initializes pools, adds liquidity, swaps through PoolSwapTest and removes liquidity through PoolModifyLiquidityTest. It measures the fee charged from the pool's fee-growth accumulators rather than assuming it. It covers:

• Fees and time: the day rollover (one second before and exactly at midnight, multi-day gaps) and a second swap in the same day, in both directions and for exact-in and exact-out.
• Failure paths: static-fee, zero-fee and max-static-fee pools reverting at initialization, direct callers being refused, and reverted swaps not consuming the free swap.
• Isolation and exits: per-pool isolation, different routers and arbitrary hookData not changing the outcome, and LPs being able to add and remove liquidity.
• Fuzzing: over base fee, swap size and timestamps.

Things to know

• Vendored dependencies: lib/ holds v4-core, solmate and forge-std as ordinary files, with no submodules. I pinned v4-core to 46c6834, not the v4.0.0 tag, because only that commit has types/PoolOperation.sol, which the protected test imports. The pins and licenses are in the README.
• Support files the protected tests import: I added src/HookFlags.sol and test/mocks/MockERC20.sol.
• Gaming is inherent to the design: anyone can take the day's free swap at any size, and a dust swap can burn it so everyone else pays baseFee that day. The README documents this.
• Deployer choices: the poolManager address, baseFee and address mining are the deployer's job and are listed in the README. The lifecycle needs a fork rehearsal against the real manager, which I did not run.
• Not audited: this is not an audit. The README says an independent review is needed before real liquidity.
• Nothing committed: I left everything uncommitted, since you didn't ask for a commit.

Review complete. Wrote two ranked, reproducible findings to .imd-findings.json. JSON validation passed; no project files were modified.