BNB Smart Chain Migration Guide
Last updated
Last updated
Across is expanding support to BNB Smart Chain, enabling seamless bridging between Ethereum Mainnet, other supported chains, and BNB Smart Chain.
The migration is scheduled to go live in early May 2025. All integrators and relayers are encouraged to complete the necessary updates before the migration happens to prevent disruptions.
The changes detailed in this guide are limited to updates within the API to support bridging to and from BNB Smart Chain. This guide is aimed at Developers, Integrators, and Relayers using the Across API. Here are some details:
Developers and Integrators: Updates to API responses, including a new field and optional query parameter behavior.
Relayers and Indexers: No event changes; only /suggested-fees API updates need attention.
With BNB Smart Chain support, Across will enable bridging for:
USDC, USDT, ETH → Ethereum Mainnet ↔ Supported L2s ↔ BNB Smart Chain
Expanding support to BNB Smart Chain enhances Across's crosschain interoperability, enabling efficient, cheap and quick bridging across a broader set of EVM-compatible networks.
The SpokePool contract deployed on BNB Smart Chain supports bridging between Ethereum Mainnet, supported L2s, and BNB Smart Chain. Integrators must continue using the DepositV3 function for BNB routes.
BNB Smart Chain will also be available in the Across .
Across Protocol will use 56 as the chain ID for BNB Smart Chain Mainnet.
BNB Smart Chain testnet is not currently supported. Developers and integrators should test their integrations directly on BNB Smart Chain mainnet, using small token amounts.
For all BNB Smart Chain routes (to and from BSC), integrators must not set the outputToken parameter to 0x0.
If you currently use outputToken=0x0 for existing non-BSC routes, you may continue to do so for the time being. However, this behavior is deprecated and will be disallowed on all chains in approximately two weeks (June 19th, 2025). At that time, Across Protocol contracts will include a check to prevent transactions with outputToken=0x0.
We strongly encourage all integrators to update their integrations to explicitly define outputToken for all routes.
The /suggested-fees API has two critical updates for BNB Smart Chain support:
outputAmount field: Each response now includes the outputAmount to be received after bridging.
Decimal enforcement: An optional allowUnmatchedDecimals query parameter has been added.
If not set to true, the API will return a 4XX error when the inputToken and outputToken have different decimals.
This ensures integrators explicitly handle cases where the same token (e.g., USDC) has different decimal places across chains (e.g., 6 vs 18).
We strongly encourage developers to test these changes in production environments using small token amounts to verify behavior before going live.
There are no changes to the structure of other fields in the /suggested-fees API response.
When calling the DepositV3 function for transactions involving BNB Smart Chain (as either the source or destination chain), integrators must always specify a valid outputToken address. Setting outputToken to 0x0 is not supported for BSC routes and will lead to transaction failure.
For existing integrations on other supported chains that currently utilize outputToken=0x0, this behavior is now deprecated. While it will continue to function for approximately 2 weeks (June 19th, 2025), we strongly advise updating your integration to provide an explicit outputToken address for all routes.
After this deprecation period, Across Protocol contracts will disallow the use of outputToken=0x0.
Relayers must ensure their configurations are updated to recognize chain ID 56 for BNB Smart Chain when handling deposits and repayments.
Relayers must not fill any deposits where the outputToken is set to 0x0 and the transaction involves BNB Smart Chain (either as the originChainId or destinationChainId). Such deposits are invalid for BSC routes and should not be processed.
Please ensure your relaying infrastructure is configured to identify and reject deposits with outputToken=0x0 for all BNB Smart Chain transactions. While outputToken=0x0 might still appear in some non-BSC routes for a limited period, relayers should prepare for its complete removal across the protocol in two weeks (June 19th, 2025).
We will notify you as soon as the BNB routes are live for testing. Please join our to be notified at the earliest.
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