fix(checkout): price gas units before using them as a balance requirement

[allan-almeida-imtbl]

Summary

Smart Checkout's gas affordability check has never been able to fail: it compares gas units against a wei balance. The gas estimate is now priced at the current gas price before it becomes a balance requirement.

Linear: BLOC-683 (parent BLOC-678, Fee model vector, zkEVM → OP Stack migration).

The bug, by example

A wallet holding 1.001 IMX buys a 1.0 IMX item. The purchase needs one approval plus the fulfilment. The node estimates 50,000 + 150,000 = 200,000 gas units, and the gas price is 10 gwei.

	Today's code	Real cost
Item	1.0 IMX	1.0 IMX
Gas	200,000 wei = 0.0000000000002 IMX	200,000 × 10 gwei = 0.002 IMX
Total needed	1.0000000000002 IMX	1.002 IMX
Wallet	1.001 IMX	1.001 IMX
Check says	sufficient	insufficient

Today the check passes, the widget proceeds to signing, and the transaction is rejected at submission with an out-of-funds error. Smart Checkout never gets the chance to route the user to a top-up. The gas figure it adds is ten billion times too small to change the outcome, so in effect the check only tests the item price.

With this change the same wallet is told it is 0.001 IMX short and offered the insufficient-funds routes (on-ramp / swap) before anything is signed.

Detail and impact of the change

Fixed

gasCalculator (packages/checkout/sdk/src/smartCheckout/gas/gasCalculator.ts) summed the estimateGas results for the approvals and the fulfilment transaction and returned that sum as a native ItemRequirement with isFee: true. estimateGas answers in gas units. The sum is now multiplied by getGasPriceInWei(feeData), fetched once alongside the estimates. The caller-supplied gas-limit branch (TransactionOrGasType.GAS) already did this multiplication; both branches now use the one price. If the node returns no usable fee data the calculator returns null (no gas requirement), which is what the gas-limit branch already did in that case.

Changed

Visible to integrators using external wallets: a wallet that cannot cover gas now fails the affordability check and gets the insufficient-funds routes instead of a submission failure, and the routes' top-up amounts include real gas.

Passport wallets (review feedback). A Passport wallet does not pay gas from its native balance; the relayer prices and collects the fee. With the requirement now correctly sized, buy would have asked a Passport user holding 50 USDC and no IMX to top up 0.002 IMX for gas they are never charged. buy now runs Smart Checkout for Passport without a transaction or gas limit, which skips only the gas requirement: the item-price check and the top-up routes stay. This differs from sell, which skips Smart Checkout entirely for Passport, because listing needs no funds and buying does. Test added: a Passport provider reaches smartCheckout with undefined as the gas argument.

Anything else worth calling out?

• Why it never surfaced. The wrong number is a valid amount, so nothing throws. Gas on zkEVM today is a fraction of a cent, so the band of wallets that pass and then fail is very thin, and when it happens the error appears at the wallet as "insufficient funds", which reads as a user problem. After the migration the L1 data fee makes gas material on cheap items and that band widens, and the L1 fee is added to this same requirement by BLOC-697; a correct L1 fee on a wrong base would still be wrong, so the base lands first.
• Tests. The existing cases used a 1 wei gas price, where units and wei are the same number, which is why they agreed with the bug. They now supply fee data explicitly. New cases price the estimates and a caller-supplied limit at 10 gwei and cover the no-fee-data path.
• Out of scope, noted for later. For an ERC-20 gas token the requirement amount is still the native-wei cost assigned to the ERC-20. That predates this change and is unrelated to the units bug.

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☁️ Nx Cloud last updated this comment at 2026-09-30 00:24:22 UTC