Curve swap is a Wallet Approval Failure Recovery Workflow

Two signatures and the actual transaction cost

Two on-chain transactions cover a fresh ERC-20 approval and the Curve swap, while a sufficient existing allowance reduces the sequence to one.

The approval writes an allowance for the prepared spender; the swap then calls that spender with the quoted route and minimum output. Both consume the network’s native gas token. Ethereum, Arbitrum One, OP Mainnet and Base charge ETH, while Polygon PoS charges POL. Those network costs sit beside the pool fee that the quote already incorporates into expected output.

One hypothetical worked example labels every changing input: the approval uses 50,000 gas, the swap uses 180,000 gas and both use a 20 gwei gas price. The combined gas is (50,000 + 180,000) × 20 gwei = 4,600,000 gwei, which equals 0.0046 ETH. The arithmetic shows the method; it isn’t a live fee quote.

If the approval reverts, the swap never broadcasts. If approval succeeds and the later swap reverts, the allowance remains attached to 1 approved spender.

Allowance boundaries before another attempt

Three values define every ERC-20 allowance used by a Curve swap: the token owner, the spender contract and the approved amount.

ERC-20 represents an approval amount as uint256, a 256-bit unsigned integer whose maximum value is 2^256 − 1. An exact-amount approval caps the available allowance at the planned input, while a maximum approval avoids another approval for later swaps through the same spender. Calling approve again replaces the recorded value for that owner-spender pair.

Ethereum USDT adds a specific recovery branch because its token contract rejects a nonzero approval when a nonzero allowance already exists. Set the allowance to 0 first, let that transaction confirm and then submit the intended nonzero value. This creates 2 approval calls before the swap. A changed Curve route can also name a different spender, so an old allowance won’t satisfy the new call. Match the spender address to the prepared transaction in MetaMask or Rabby, which is discussed in Using Curve swap.

Why does the wallet approval fail?

Four checks resolve most Curve approval failures: active chain, native gas balance, token allowance state and the spender encoded by the prepared transaction.

A Curve wallet approval must match the network that supplied the quote. Ethereum mainnet uses chain ID 1, Arbitrum One uses 42161, OP Mainnet uses 10, Base uses 8453 and Polygon PoS uses 137. A ticker alone can’t prove that match because USDC, USDT and DAI exist as separate contracts across networks. Compare the active chain, token contract and account with the prepared call. MetaMask and WalletConnect present the request; they don’t rewrite its chain, token or spender.

Rejecting the prompt before broadcast creates no transaction hash and consumes no gas. Correct the displayed account or network, then request a fresh approval.

Low native-token balance can stop simulation or submission before the chain sees the call. Reserve enough ETH or POL for the wallet’s displayed limit, without subtracting gas from an ERC-20 input such as USDC.

An earlier pending transaction from the same account can also block the approval. Replace or cancel that earlier transaction with its original nonce, wait for 1 receipt and only then resubmit. A mined approval carrying receipt status 0 failed on-chain. Its decoded revert data should lead to exactly 1 corrected approval.

Route selection and minimum output

Five swap steps fit inside Curve Router v1.2, which executes an off-chain selected route as 1 on-chain transaction with a final minimum output.

Curve Router encodes the path in 11 address slots: input token, pool or zap, output token and the same alternating pattern for later steps. Its swap-parameter matrix contains 5 rows of 5 values covering coin indexes, swap type, pool type and coin count. The off-chain quote estimates final output, while the transaction supplies 1 minimum-output value. If execution would return less, the call reverts. Re-quoting can change the route or bound without changing an allowance already granted to the same token and spender. Curve Router v1.2 stops after 5 swaps.

Pending, replaced, reverted and confirmed states

Four transaction states matter during Curve swap recovery: pending, replaced, reverted and confirmed, each requiring a different next action from the connected wallet.

A pending Ethereum type 2 transaction has 2 fee controls, max fee per gas and max priority fee per gas, plus 1 account nonce.

• Pending means no receipt exists yet; monitor the original hash and avoid creating a second nonce problem.
• Replaced means another transaction used the same nonce; follow the replacement hash shown by MetaMask, Rabby or the chain explorer.
• Reverted means the chain included the call with receipt status 0; the account pays gas and the swap’s state changes roll back.
• Confirmed means the receipt reports status 1; next inspect the destination, output token and transferred amount.

Nonce order matters because an account’s later transaction can’t settle before an earlier pending nonce. A wallet cancellation creates a replacement transaction from the same account with that nonce, commonly transferring 0 native units back to the sender. When a swap reverts after a separate approval confirmed, the prior allowance remains on-chain because the failed swap can’t undo an earlier receipt. Track both hashes.

In day-to-day use, Curve recovery finishes when the output check agrees with receipt status 1.

Recovery discipline for active DeFi traders

Three records close a Curve swap recovery cleanly: the transaction receipt, the output token’s Transfer event and the wallet’s final token balance.

ERC-20’s Transfer event contains 3 arguments: sender, recipient and amount; the first 2 addresses are indexed. Compare the recipient with your wallet and the token contract with the quoted output asset. Explorers such as Etherscan, Arbiscan and BaseScan expose the receipt and logs, while the wallet balance provides a separate display check. If the token doesn’t appear in MetaMask, adding its correct contract to the asset list changes only the display. The on-chain balance already exists; that display action doesn’t create, reverse or repeat the Curve transaction.

Token decimals explain many apparent output mismatches. Ethereum USDC and USDT use 6 decimals, while DAI, WETH and CRV use 18. The receipt records base units; the interface formats them for reading. This recovery routine suits active DeFi traders who separate approval state, transaction state and balance state before trying again. Ledger and Safe add signing steps, yet the same 3 records close the workflow for 18-decimal CRV.