Polygon Bridge ERC-20 Deposits Begin With Token Approval

For an ERC-20 deposit from Ethereum to Polygon PoS, a wallet usually needs to approve the bridge contract to spend the token before it can submit the deposit. The Polygon Bridge flow therefore has two distinct on-chain actions for many ERC-20s: granting an allowance, then depositing the tokens.

Approval is permission for a contract to move a specified token amount; it does not itself move funds across the bridge.

ERC-20 defines this through an allowance recorded by the token contract for an owner and a spender. In a bridge deposit, the owner is the wallet holding the tokens, and the spender is the contract that will handle the deposit. The Polygon Bridge deposit flow reflects this separation: the wallet first authorizes token use, then submits the bridge transaction. Each action is a transaction on Ethereum and requires its own confirmation.

What does an ERC-20 approval authorize?

An approval lets the designated spender transfer tokens from the owner’s address up to the approved amount. It does not give the spender general access to other tokens, and it does not transfer the approved tokens immediately. The deposit call is the separate step that asks the bridge to take the tokens and process the cross-chain transfer.

For a standard ERC-20 deposit, the bridge contract needs that allowance so it can call the token contract’s transfer function on the user’s behalf. If the wallet has already approved the same spender and the remaining allowance covers the deposit, a new approval may not be needed. Allowances belong to a particular token contract and spender, so approval for another token or another contract does not carry over.

Why are approval and deposit separate transactions?

The token contract maintains the allowance, while the bridge contract processes the deposit. Because those are separate contract actions, a standard ERC-20 flow generally needs one transaction to update the allowance and another to initiate the deposit. Some tokens and interfaces support signature-based permit flows that can combine authorization with another action, but users should not assume that option is available for every token or route.

Once the deposit is accepted, Polygon describes its native bridge as locking the asset on Ethereum and issuing a corresponding token on Polygon. The approval is only the authorization step before that process; it is not evidence that a deposit was submitted or completed. After approving, check that the wallet presents a separate deposit transaction and confirm its destination network and token before signing.

How much allowance should a depositor approve?

The amount should cover the intended deposit. Some interfaces request only that amount; others may request a larger or effectively unlimited allowance to avoid asking for approval again on later deposits. A larger allowance can be convenient, but it leaves more tokens available to that spender until the allowance is used or changed. For most users, approving the amount they plan to deposit is the narrower choice, especially when they do not expect repeated transfers through the same contract.

Before confirming, check the token, spender, network, and allowance amount shown in the wallet. If the spender or amount differs from what the deposit flow indicates, stop and verify the transaction details. After approval confirms, submit the deposit separately; then use the transaction status to distinguish an allowance change from a bridge transfer.