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How to Set Your Token Allowances

Some interactions with 0x require or are improved by setting token allowances, or in other words, giving 0x's smart contracts permission to move certain tokens on your behalf. Some situations of when you would need to set a token allowance include:

  • When submitting a 0x API quote selling ERC20 tokens, you will need to give an allowance to the allowanceTarget specified in the quote response
  • When trading ERC20 tokens using the Exchange contract, you will have to give an allowance to the ERC20Proxy contract

Setting Allowances for 0x API quotes​

0x API uses an advanced architecture to minimize the transaction costs for its users, as a result of this the target for allowances has changed for the v1 endpoints. For users of the swap/v1/* swap endpoints the ERC20 allowance target has changed and a new field allowanceTarget is introduced in the /price and /quote responses. The allowanceTarget field is the contract address that the user needs to set an ERC20 allowance for in order to be able to perform the trade. Note that allowanceTarget is only relevant when selling an ERC20 token.

tip

When setting the token allowance, make sure to provide enough allowance for the buy or sell amount as well as the gas; otherwise, you may receive a 'Gas estimation failed' error.

Setting Allowances for a quote with Web3.js​

All code snippets provided are designed to work in a browser environment with an injected web3 instance (like Metamask). You can use the npm web3 module and modify these snippets to run them in a node environment.

import { ERC20TokenContract } from '@0x/contract-wrappers';
import { BigNumber } from '@0x/utils';

(async () => {
    const web3 = window.web3;

    // Get a quote from 0x API which contains `allowanceTarget`
    // This is the contract that the user needs to set an ERC20 allowance for
    const res = await fetch(`https://api.0x.org/swap/v1/quote?${qs.stringify(params)}`);
    const quote = await res.json();

    // Set up approval
    const USDCaddress = '0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48';
    const USDCcontract = new ERC20TokenContract(USDCaddress, web3.eth.currentProvider);
    const maxApproval = new BigNumber(2).pow(256).minus(1);

    // Send the approval to the allowance target smart contract
    const chainId = 1;
    const approvalTxData = USDCcontract
        .approve(quote.allowanceTarget, maxApproval)
        .getABIEncodedTransactionData();
    await web3.eth.sendTransaction(approvalTxData);
})();

Setting Allowances on Etherscan​

There are a lot of dApps that let you set your allowances in Metamask (including RadarRelay), but for this example we will be using Etherscan. To set your WETH allowance for the ERC20Proxy contract you can navigate to the Dapp view for the WETH ERC20 contract. Here you can use Metamask to call the approve method to approve the ERC20Proxy for the max uint256 amount which is 115792089237316195423570985008687907853269984665640564039457584007913129639935. Again, you can find the address of the ERC20Proxy in the 0x cheat sheet.

You can give a WETH allowance to any smart contract this way. To set your allowance for a different token, you'll have to navigate to the smart contract interface for that token.