state_override for multiple ERC-20s works; balance and allowance mapping slots are not 0 and 1 for WETH9

Goal: simulate a write function that pulls multiple ERC-20s from the caller, using eth_call with state_override to fake balances and allowances instead of actually holding tokens. It works for one token, then appears to break for two or more, so the assumption is that overriding several contracts at once is unsupported.

It is supported. state_override is a map keyed by contract address, and any number of entries may carry stateDiff simultaneously. The actual failure is that the slot indices for the balanceOf and allowance mappings were hardcoded as 0 and 1. That holds for a textbook OpenZeppelin ERC-20, but not for tokens with custom or historic layouts — the moment WETH9 or a rebasing/vault token enters the set, the computed slots point at unrelated storage and the override silently does nothing.

Slot keys are computed as keccak256(abi.encode(holder, balanceSlot)) for balances, and for allowances the nested mapping keccak256(abi.encode(spender, uint256(keccak256(abi.encode(owner, allowanceSlot))))). Only the base slot numbers change per token, so parameterise them:

TOKEN_SLOTS = { "0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2": (3, 4), # WETH9 "0xa3931d71877C0E7a3148CB7Eb4463524FEc27fbD": (2, 3), # sUSDS "0x9D39A5DE30e57443BfF2A8307A4256c8797A3497": (4, 5), # sUSDe "0xCd5fE23C85820F7B72D0926FC9b05b43E359b7ee": (101, 102), "0x4c9EDD5852cd905f086C759E8383e09bff1E68B3": (2, 3), } # default (0, 1)

then build one state_override[token] = {"stateDiff": {balance_slot: padded, allowance_slot: padded}} entry per token in a loop.

Diagnosis trick: verify a single token's override in isolation with a plain balanceOf call before simulating the real function. Also note that for a proxy token the slots belong to the implementation's layout but must be overridden on the proxy address, and stateDiff (not state) is what you want so the rest of storage is preserved.

Source: adapted from a Stack Exchange answer by FocusBTW. (question by FocusBTW.), https://ethereum.stackexchange.com/questions/167927/how-can-i-override-multiple-tokens-more-than-two-using-state-override-for-writ — CC BY-SA 4.0