Token Approvals Before a Decentralized Swap Explained

A token approval is an on-chain permission that lets a smart contract spend a specified amount of an ERC-20 token from your wallet. If the swap contract does not already have enough allowance, you generally need to approve it before the swap can execute. That extra step matters when you compare the time and network cost of different ways to trade.

An approval grants a spending allowance

Suppose you want to exchange cbBTC for USDT on Ethereum. The swap contract needs permission to take the cbBTC from your wallet; it cannot spend your tokens just because you connected a wallet or requested a quote. With Fermi swap, the service is a concrete example of exchanging tokens from your wallet, with trades filled from its own token inventory; the same approval concept applies to decentralized swaps generally. The Fermi swap service is one way to make that kind of exchange.

For a standard ERC-20 token, the approval transaction calls approve on the token contract and records an allowance for a particular spender address. It does not transfer cbBTC or execute the swap. Once the allowance is recorded, the swap contract can call transferFrom during the swap to take the approved input tokens and deliver the output.

The process is usually:

• Approve the swap contract to spend some amount of cbBTC.
• Wait for that transaction to be confirmed and the allowance to update.
• Submit the swap; the contract uses the allowance to take the cbBTC.

A first approval can mean two transactions and two gas charges

If your existing allowance is zero or smaller than the input amount, the first swap usually involves two on-chain transactions: the approval, then the swap. Each consumes gas, paid in ETH on Ethereum, and the amount depends on how much computation the transaction uses and the network’s fee market at that time. The approval cost is separate from the swap amount and does not grow in proportion to how many cbBTC you intend to exchange.

For example, if you approve 0.01 cbBTC and swap 0.01 cbBTC, you have authorized enough for that trade. If you approve 1 cbBTC but trade only 0.01, the remaining allowance may still be available to that spender. On a later swap, a sufficient allowance can avoid another approval transaction, though the swap itself still requires gas.

There are exceptions. Some tokens and swap contracts support permit, which lets a wallet sign approval data that a contract can use during a transaction; when supported, this may combine approval and swap into one on-chain transaction. Some tokens also require an allowance to be reset to zero before setting a new nonzero amount, which can add another transaction. Check the spender address and allowance amount before signing, especially when a wallet offers an unlimited allowance.

Choose an allowance based on how often you trade

A specific allowance limits the amount the spender can take under that approval, so it suits a one-off trade or a cautious first use. It may mean another approval transaction when you trade again. A large or unlimited allowance can reduce repeat setup, but it leaves that spender with broader permission until the allowance is spent or revoked.

In practice, I’d compare the extra approval cost with how often I expect to use the same spender, and choose the smallest allowance that fits that pattern. For a single cbBTC-to-USDT trade, approving only the intended input is straightforward; for repeated swaps through the same contract, an existing allowance may save a transaction. The key is that allowance belongs to a specific token and spender, so approval for one swap contract does not automatically authorize another.

Token approval is the permission step that can precede a decentralized swap: it records what a contract may spend, while the swap performs the exchange. Check whether an adequate allowance already exists, consider the extra gas and any ongoing permission, then approve and swap if the trade still suits your comparison.