Paying for Gas in Tokens
How to pay Celo gas fees using whitelisted ERC20 tokens.
Transaction Fees
As in Ethereum, transaction fees play a critical role in the Celo protocol as a safeguard against denial-of-service attacks. In order to simplify the process of sending funds, these fees can be paid in ERC20 tokens, and not just the native token of the protocol, CELO. This means that a user sending Celo Dollars to friends or family will be able to pay their transaction fee out of their Celo Dollar balance, and do not need to hold a separate balance of CELO in order to make transactions.
Fee Currency Field
The protocol maintains a governable whitelist of smart contract addresses which can be used to pay for transaction fees. These smart contracts implement an extension of the ERC20 interface, with additional functions that allow the protocol to debit and credit transaction fees. When creating a transaction, users can specify the address of the currency they would like to use to pay for gas via the feeCurrency field. Leaving this field empty will result in the native currency, CELO, being used. Note that transactions that specify non-CELO gas currencies will cost approximately 50k additional gas.
Whitelisted Gas Fee Addresses
To obtain a list of the gas fee addresses that have been whitelisted using Celo's Governance Process, you can run the getWhitelist method on the FeeCurrencyWhitelist contract. All other notable mainnet core smart contracts are listed here.
Tokens with Adapters
After Contract Release 11, addresses in the whitelist are no longer guaranteed to be full ERC20 tokens and can now also be adapters. Adapters are whitelisted in-lieu of tokens in the scenario that a ERC20 token has decimals other than 18.
The Celo Blockchain natively works with 18 decimals when calculating gas pricing, so adapters are needed to normalize the decimals for tokens that use a different one. Some stablecoins use 6 decimals as a standard.
Transactions with those ERC20 tokens are performed as usual (using the token address), but when paying gas currency with those ERC20 tokens, the adapter address should be used. This adapter address is also the one that should be used when querying Gas Price Minimum.
Adapters can also be used to query balanceOf(address) of an account, but it will return the balance as if the token had 18 decimals and not the native ones. This is useful to calculate if an account has enough balance to cover gas after multiplying gasPrice * estimatedGas without having to convert back to the token's native decimals.
Adapters by network
Mainnet
| Name | Token | Adapter |
|---|---|---|
USDC | 0xcebA9300f2b948710d2653dD7B07f33A8B32118C | 0x2F25deB3848C207fc8E0c34035B3Ba7fC157602B |
USDT | 0x48065fbbe25f71c9282ddf5e1cd6d6a887483d5e | 0x0e2a3e05bc9a16f5292a6170456a710cb89c6f72 |
Alfajores (testnet)
| Name | Token | Adapter |
|---|---|---|
USDC | 0x2F25deB3848C207fc8E0c34035B3Ba7fC157602B | 0x4822e58de6f5e485eF90df51C41CE01721331dC0 |
Baklava (testnet)
N/A
Enabling Transactions with ERC20 Token as Fee Currency in a wallet
We recommend using the viem library as it has support for the feeCurrency field in the transaction required for sending transaction where the gas fees will be paid in ERC20 tokens.
Estimating gas price
To estimate gas price use the token address (in case of cUSD, cEUR and cREAL) or the adapter address (in case of USDC and USDT) as the value for feeCurrency field in the transaction.
The Gas Price Minimum value returned from the RPC has to be interpreted in 18 decimals.
Preparing a transaction
When preparing a transaction that uses ERC20 token for gas fees, use the token address (in case of cUSD, cEUR and cREAL) or the adapter address (in case of USDC and USDT) as the value for feeCurrency field in the transaction.
The recommended transaction type is 123, which is a CIP-64 compliant transaction read more about it here.
Here is how a transaction would look like when using USDC as a medium to pay for gas fees.
let tx = {
// ... other transaction fields
feeCurrency: "0x2f25deb3848c207fc8e0c34035b3ba7fc157602b", // USDC Adapter address
type: "0x7b",
};
To get details about the underlying token of the adapter you can call adpatedToken function on the adapter address, which will return the underlying token address.