Why bridges deduct gas from the tokens you receive

A bridge deducts gas from the tokens you receive when a relayer pays to complete a transaction on the destination chain and then recovers that cost from your transfer. The arrangement lets a recipient get usable tokens without first holding the destination chain’s native gas token, but it reduces the amount delivered. The deduction is a payment mechanism, not a blockchain rule: bridges can instead ask you to pay a separate fee or complete the destination step yourself.

What does the deducted fee pay for?

Usually, it reimburses the party that submits a transaction on the destination chain. Moving a message or asset across chains is not the same as paying for every action needed after arrival: the destination chain charges for its own transaction, and that charge is paid in its native gas token. A relayer can supply that token, execute the transaction for you, and collect reimbursement from the bridged amount.

BTTC’s description of its Auto Receive mode illustrates this model: a relayer pays the destination gas and a service fee is deducted from the transfer. Symbiosis’s documentation describes a similar sequence, in which relayers cover destination gas first and the bridge later deducts an approved amount from the released tokens. The exact contract differs, but the trade is the same: the user receives less of the asset in exchange for having the destination transaction handled for them.

This can be useful when you are bridging into a wallet with no ETH, MATIC or other destination gas token. The bridge may be able to deliver the asset and perform a follow-on action, such as a swap, in one flow. For an overview of that process, see Superbridge’s guide to completing a cross-chain swap. The convenience comes with a cost that should be visible in the quote.

Is the whole deduction actually gas?

Not necessarily: “gas” on a quote can refer to a bundled charge rather than the exact network fee. A relayer may add compensation for providing the service, and the route may include liquidity, protocol or other bridge fees. Across’s fee documentation, for example, separates relayer and liquidity-provider costs within the difference between the amount deposited and the amount received. 0x’s cross-chain documentation also notes that provider fees can be reflected in the quoted output without appearing as a separate fee line.

The destination gas component can change with network conditions and the work the transaction performs. A straightforward delivery and a delivery followed by a token swap require different execution. The destination chain’s gas price can also move between the quote and execution. Some bridges estimate and collect in advance; the relayer may bear some difference if the actual cost changes, while other designs can use a fixed or capped amount. Check the route’s terms and quote rather than assuming every deduction is a direct pass-through of the final gas bill.

It also helps to separate three costs that an interface may show together:

• Origin gas: the fee for submitting your transaction on the chain where the assets start, typically paid from your wallet in that chain’s native token.
• Destination execution: gas for a relayer’s transaction on the receiving chain, which may be reimbursed from the output.
• Bridge or route fees: charges for relaying, liquidity or protocol services, which may be included in the quoted output rather than itemised.

A low displayed fee can therefore be misleading if it excludes origin gas or quotes only a part of the route. Compare the amount you will receive and any separate native-token charge, not just a single line labelled “gas.”

What are the alternatives to a token deduction?

A bridge can require the user to pay destination gas separately, or ask the user to claim or receive the funds manually. Superbridge’s help page on destination gas explains that funds can arrive even when the wallet lacks gas, but the user may be unable to send or swap them until they obtain the chain’s gas token. That route avoids a relayer-funded delivery, but shifts the extra step and the need for a separate token balance to the user.

Another design charges the fee in native currency up front. 0x’s documentation describes bridge-native fees that are added to the transaction’s value instead of being deducted from the sell amount; whether a route uses this model depends on its provider. This makes the payment source clearer, but requires the user to hold the relevant native token before starting. A deduction from the output is easier when the user has only the asset being bridged; paying separately can make the net token amount easier to predict.

For most users who want an asset ready to use immediately, a relayer-assisted route is simpler if its quoted net output is acceptable. Before signing, check the source and destination chains, the token you will receive, the minimum output, any separate native fee, and whether the route includes a swap or automatic receive. If you prefer to manage gas yourself and already hold the destination token, compare the manual option’s full cost and steps.

What should you watch before confirming?

Watch the quote’s net amount, its expiry or refresh behaviour, and the fee breakdown. If the deduction changes sharply between quotes, the destination gas estimate or route conditions may have moved; if the fee is bundled, the interface may not show how much goes to gas versus other services. After bridging, verify whether the transfer is still processing, completed, or awaiting a manual claim before retrying it.

The useful comparison is not “fee versus no fee,” but a smaller automatic receipt versus a larger receipt that may require a separate gas purchase and transaction. The next signals are the destination gas estimate, the quoted minimum output, whether the relayer completes the final action, and any separate native-token amount requested at confirmation.