wagmi write() undefined because usePrepareContractWrite reverted on ERC20 allowance — read its error and refetch after approve

Symptom: a wagmi contract-write button does nothing. write from useContractWrite(config) is undefined, no transaction is requested, and no error surfaces in the UI.

Cause: usePrepareContractWrite performs an eth_call simulation and only produces a config if it succeeds. When the call reverts, write is never created. The revert reason exists — but only on that hook's error, which is easy to leave unread:

ContractFunctionExecutionError: The contract function "makePayment" reverted
with the following reason: ERC20: transfer amount exceeds allowance

So the real bug here was a missing token approval, reported in a place nobody was looking.

Fix, two parts: 1. Always destructure and surface error/isError from the prepare hook, and gate the button on !write so a disabled control shows the cause instead of a dead click. 2. For approve-then-spend flows, the simulation was run before the allowance existed and will not re-run on its own. Take refetch from the prepare hook and call it once the approve transaction is confirmed, then call write:

const { config, refetch } = usePrepareContractWrite({ ... });
const { write } = useContractWrite(config);
// after approve receipt: await refetch(); write?.();

Version note: the hooks shown are wagmi v1. In wagmi v2 they were replaced by useSimulateContract and useWriteContract — the failure mode is identical (simulateData?.request is undefined when the simulation reverts, so writeContract(simulateData!.request) cannot run), and the fix is the same: read the simulate hook's error and refetch it after the allowance changes.

Source: adapted from a Stack Exchange answer by Agustin Gonzalez Ribas (question by Agustin Gonzalez Ribas), https://ethereum.stackexchange.com/questions/159091/wagmi-error-write-undefined — CC BY-SA 4.0