If you hold tokens only on the source chain, cBridge needs that chain’s gas token for approval and sending, but you do not need destination gas for delivery. The cBridge base fee pays for the destination transaction that releases or mints your tokens. You need destination gas when you later move or swap them.
cBridge Source-Chain Gas Pays for Approval and Sending
On the source chain, your wallet pays gas for each transaction it confirms before the transfer can leave. Suppose the selected route sends USDC from Ethereum to Optimism. You hold USDC and some ETH on Ethereum, but nothing on Optimism. Ethereum ETH pays for the steps you sign there; USDC is the asset you send. ETH on Ethereum and ETH on Optimism are separate balances, even at the same wallet address.
- Token approval: For an ERC-20 token such as USDC, the bridge contract may first need permission to spend your chosen amount. That approval uses Ethereum gas. An existing approval that still covers the amount can eliminate this step.
- Transfer submission: The send transaction commits the USDC to the bridge flow. It uses Ethereum gas separately from any fee deducted from the transferred USDC.
Read the wallet’s gas estimate for each confirmation before setting the transfer amount. If you have USDC but no ETH on Ethereum, you cannot submit either transaction. If you send ETH itself, the amount sent plus the send transaction’s gas cost must fit your Ethereum balance. Sending your full gas-token balance leaves nothing to pay for that transaction or a later refund.
The Base Fee Covers Destination Release or Minting
Celer cBridge charges a base fee in the token being transferred to pay for delivery. In a pool-based transfer, the destination side releases tokens from liquidity; in a canonical token transfer, it may mint corresponding tokens. Either destination transaction consumes that network’s native gas token. The base fee compensates the party carrying it out, so your empty Optimism wallet does not need to sign or fund delivery.
The amount received can differ from the amount sent because the quote accounts for the base fee and any applicable protocol fee or bridge rate. The base fee is a transfer cost, not ETH deposited into your Optimism wallet. Review the quoted receive amount for the exact token and route selected. On a small transfer, a largely fixed delivery cost takes a larger share of the tokens.
After the Ethereum send transaction confirms, lack of ETH in your Optimism wallet alone should not stall delivery. The bridge coordinates the destination transaction. If the transfer remains pending, its status and the source transaction confirmation identify which stage is outstanding; topping up the receiving wallet does not itself finish a pending release.
Destination Gas Determines Whether Arrived Tokens Are Usable
A completed USDC transfer gives you USDC on Optimism, but zero ETH there still prevents your wallet from sending it onward. Approving USDC for a swap, swapping it, or transferring it to another address each requires an Optimism transaction paid in ETH on Optimism. The tokens can sit at the address while you arrange gas; arrival and use are separate events.
Plan an Optimism ETH balance before your first action with the USDC. You could receive native ETH through a separate transfer or another source that deposits it directly on Optimism. The amount needed depends on that action’s gas estimate and network conditions when you submit it. ETH left on Ethereum cannot pay for an Optimism transaction.
The boundary changes when a cBridge route delivers the destination’s native gas token: that arrival can fund your next transaction. If the quote instead shows wrapped ETH, or WETH, the received balance cannot directly pay gas. WETH is a token contract balance, and unwrapping it on Optimism requires an on-chain transaction that itself needs ETH. The token shown in the receive quote decides whether the transfer also solves the next-step gas problem.
Refunds and LP Withdrawals Need Gas Where Confirmed
Refunds and liquidity provider withdrawals each need gas on the chain where you confirm an on-chain transaction. A failed transfer that becomes eligible for a refund requires a new transaction on the source chain. In the USDC example, the refund returns assets through an Ethereum contract, so your wallet needs Ethereum ETH to claim them. Keep source gas after sending. A zero ETH balance can leave refundable USDC waiting for a transaction you cannot submit.
A liquidity provider (LP) deposits tokens into bridge pools for other users’ transfers. Withdrawing that liquidity is separate from receiving an ordinary transfer. If you later withdraw an LP position, an on-chain confirmation needs the native gas token of the chain where you confirm it. A wallet signature that authorizes a request can be free; the transaction that completes a withdrawal still costs gas.
For the example transfer, retaining ETH on Ethereum preserves the refund option. ETH on Optimism then makes the delivered USDC movable or swappable.
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