0x Documentation
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API FAQs

Why does the value of the to field in the /swap/quote response vary?

0x API selects the best 0x protocol interface to interact with based on the provided parameters and the smart order routing logic.
There are two main interfaces that 0x API will provide quotes for:
​Exchange contract - the main entry-point of the 0x protocol, all swap quotes are settled by the exchange contract.
​Forwarder extension contract - a payable interface allowing swaps between native ETH (as a sellToken) and another ERC20 asset.
Find the currently deployed contracts here.

How do I query what tokens are available to be swapped through the API?

All ERC20 tokens are supported with the caveat that if the token is fee-on-transfer or has some other special logic there is no way for the API to detect this and their requests may fail/transactions revert/result in some wonky swaps. We recommend referring to tokenlist.org, specifically the CoinGecko tokenlist for a list of all available ERC20 tokens.

What is the best way to query swap prices for many asset pairs without exceeding the rate limit?

Our rate limits exists because we want to encourage anyone using our infra to actually swap, not just use our API as a price oracle. If you would like to query for token prices, we would recommend either setting up your own 0x API instance via the repo README instructions or query a 3rd party service like coingecko.

How does the 0x API decide between the Forwarder and the Exchange?

The current 0x API logic for selecting between forwarder or exchange is deterministic and enable developers to easily ensure which interface is used for a quote:
  • If sellToken is not set to ETH (say WETH), 0x API will always provide a quote that interfaces with the main exchange contract.
  • If sellToken is set to ETH, 0x API will always provide a quote that interfaces with forwarder contract.
TLDR sellToken=ETH uses Forwarder, sellToken=WETH uses Exchange.

Are the smart contracts audited?

Yes, 0x protocol’s contracts are open source and extensively tested by 0x core team’s internal protocol team and by external auditors (Consensys dilligence, Trail of Bits).
The Exchange contract, because they are the main entry point of the 0x protocol, is versioned, and governed by ZRX token holders. Because of its critical place in 0x infrastructure, the 0x core team employs a number of external audits to ensure the safe and intended usage of the 0x contract.
The Forwarder contract is internally audited and constantly improved upon by the 0x core team. Forwarder, an optional extension contract that users can opt in/out of using when swapping, has a minimized attack surface because no approval of user funds is needed.

What is the difference between price and guaranteedPrice?

The price field provides developers a sense of what the BEST price they could receive, denominated in sellToken for one buyToken. The gauranteedPrice is the price that developers can expect in a WORST case scenario where all the fallback orders are utilized over the better priced orders.
Say you found a swap for ETH to DAI at 220 DAI while the market price for ETH is 200 DAI. Obviously this is a great price and will not last forever, a β€œrace” of sorts occurs as users compete to settle the swap of ETH for 220 DAI by selecting a competitive gasPrice that would result in their transaction being mined over others.
When such a race happens, only one user gets to receive the swap of ETH for 220 DAI, the other users will see their transactions revert.
To ensure that users will always have their swap executed within a reasonable price, 0x API quoting logic provides a set of fallback orders, created with on-chain sources (kyber, uniswap, PLP, curve) that will be filled at a slightly worse rate if the more aggressively priced orders expires or are filled by somebody else.
TLDR expect the actual settled price of a 0x quote to be somewhere between price and gauranteedPrice.

What is slippagePercentage?

Developers can influence how much β€œworse” the guaranteedPrice is through the slippagePercentage parameter. With on-chain sources, prices can vary between the quote being made and settlement. The slippagePercentage provides a "upper bound" to how much the price provided by these on-chain sources can slip and remain desirable by the developer.
TLDR slippagePercentage controls how much worse the price can be for the fallback orders provided in a 0x API quote which influences the guaranteedPrice.

What are the differences when quoting by sellAmount and buyAmount?

  • If sellToken is utilized, then any unused sellToken will be refunded to the user.
  • When buyAmount is used, the only guarantee is that at least the amount specified is bought. 0xAPI will not terminate early in the case where one order fills at a better price, so the user can in effect over buy the specified amount. This is somewhat amplified by usage of slippagePercentage which underestimates the on-chain price by a percentage.
Also, some liquidity sources do not enable querying by buyAmount (i.e Kyber), these sources are ignored when quoting for buyAmounts
TLDR Whenever possible, use sellAmount over buyAmount to get more deterministic behavior.

How does 0x API select the best orders for me?

Beyond simply sampling each liquidity source for their respective prices, 0x API adjusts for the gas consumption of each liquidity source with the specified gas price (if none provided 0x API will use ethGasStation's fast amount of gwei) and any associated fees with the specific liquidity source. By sampling through varying compositions of liquidity sources, 0x API selects the best set of orders to give you the best price. 0x API also creates another set of fallback orders to ensure that the quote can be executed by users.
Ex: 0x API will adjust the price potentially received from Curve Finance by gas * gasPrice and its fees. Because of Curve Finance’s costly gas consumption, its nominal price may not be the best price when settled.

What is the protocolFee?

The community voted to remove protocol fees in ZEIP-91 which decreased the protocol fee multiplier from the current value (70,000) to zero (0) for v3 onward.
The following is kept for historical reference but no longer applies
A protocol fee was paid by takers and ultimately rebated to market makers when their orders are filled.
Protocol fees were calculated per order using the gas price multiplied by a constant (currently 70,000). 0x API calculated the required protocol fees to be paid and returns this in the value field. Since we recommended using a high gas price, in the event where the taker fills at a lower gas price, the excess was returned.
It was possible for the protocol fees to be paid in WETH rather than sent in the transaction as ETH. This could be achieved by the taker having a WETH balance and setting a WETH allowance to the Protocol fee collector address. This address can be read on the Exchange contract: Exchange.protocolFeeCollector.
TLDR 0x API handled the heavy lifting related to protocol fees and provides a value field.

Why does my 0x transaction revert?

Aw shucks, are 0x quotes reverting? There are a number of low hanging fruits to check for when attempting to execute a quote beyond the obvious revert issues related to eth transactions:
  • Are allowances properly set for the user to trade the sellToken?
  • Does the user have enough sellToken balance to execute the swap?
  • Do users have enough ETH or WETH to pay the protocolFee?

How does takerAddress help with catching issues?

By passing a takerAddress parameter, 0x API can provide a more bespoke quote and help catch revert issues:
  • 0x API will estimate the gas cost for takerAddress to execute the provided quote.
  • If successfully called, the gas parameter in the quote will be an accurate amount of gas needed to execute the swap.
  • If unsuccessful for revert reasons suggested above, then 0x API will throw a gas cost estimation error, alluding to an issue with the takerAddress executing the quote.
TLDR Pass takerAddress to get the quote validated before provided to you, assuring that a number of revert cases will not occur.

0x orders are reverting but my transaction is fine, what is happening?

Developers may note when analyzing their transactions that some subset of 0x orders may revert (not filled) but the whole transaction is successful. This is expected behavior as implied earlier, some orders due to timing, and the pricing may be filled or expired before a users attempt to fill the order. This would result in a revert and 0x protocol will utilize fallback orders to compensate for the reverted order. This will result in a successful transaction even though reverts occur within the transactions.

How does gas price influence my transaction?

It is important to set a higher gas price as a default rather than to set it low and rely on the user to increase this in their wallet. The gas price directly influences the 0x Protocol fee paid to market makers. The 0x API calculates this protocol fee for you (and returns it in the value field) based on the supplied gasPrice (defaulted to fast). But if a gas price of 1 gwei was supplied and the user changes this to any higher amount, the transaction will fail as the value has not been updated to reflect the new gas price for the protocol fee.
It is best to understand your users and what gas price they typically fill at, if the user lowers their gas price the additional funds are returned.
Finding the best quote is also influenced by how expensive it is to fill a particular source. As an example, Kyber and Curve are relatively expensive steps in a route to fill as they consume a large amount of gas. When this is multiplied by the gas price some routes, although offer a better price on paper, result in a greater cost to the user. 0x API routes based on the gasPrice parameter provided, or defaults to a fast amount of gwei. When changing the gas price parameter from 1 to 8 gwei one can observe the change in routing.
TLDR Inform users to NOT increase the gas price beyond the quoted amount as the transaction will fail.
From our data, this is one of the largest reasons of why 0x quotes revert,** do your part in lowering the revert rates**!

What is the return of executing the provided calldata? fillResults?

0x protocol returns a FillResults object that returns the result of executing a 0x swap:
struct FillResults {
uint256 makerAssetFilledAmount; // Total amount of buyToken filled.
uint256 takerAssetFilledAmount; // Total amount of sellToken filled.
uint256 protocolFeePaid; // Total amount of protocolFee pair in WETH or ETH
...
}
On-chain, easily decode the result of executing the calldata like so:
import "@0x/contracts-exchange-libs/contracts/src/LibFillResults.sol";
​
...
​
(bool success, bytes memory data) = address(exchange).call.value(quote.protocolFee)(quote.calldataHex);
require(success, "Swap not filled");
fillResults = abi.decode(data, (LibFillResults.FillResults));

How can I find the 0x API liquidity sources for each chain?

Use the API endpoint /swap/v1/sources to get the liquidity sources per chain. You will need to specify the root-endpoint for the chain you are interested in, for example, https://polygon.api.0x.org/swap/v1/sources for the Polygon Network or https://api.0x.org/swap/v1/sources for Ethereum Mainnet. See the 0x API SwapIntroductionfor a full list of endpoints we support.

My project is interested to apply as a liquidity source in 0x ecosystem. How can I contact the 0x team?

Thank you for your interest in providing liquidity to the 0x ecosystem via our RFQ suite of products. Please fill out this form to help us learn more about your firm, and determine whether 0x is a good fit for you. Our team will review and reach out to you.

My project would like to integrate the 0x API. How can I contact the 0x team?

We appreciate your interest in consuming liquidity from the 0x API. Please fill out this form for us to to learn more about application and how you use the 0x API. Our team will review and reach out to you.

I am building an DEX app using 0x API. Can I charge a trading fee/commission fee/transaction fee when using the 0x API?

Yes, this can be done by setting the feeRecipient and buyTokenPercentageFee parameters in a /swapAPI request. Set a buyTokenPercentageFee on your DEX trades which represents the percentage (between 0 - 1.0) of the buyAmount (tokens being received) that should be attributed to feeRecipient (your wallet) as an affiliate fee. Details about these parameters can be found in GET /swap/v1/quote.
TL;DR: you have full flexibility on the fees you collect on your trades

Is there a fee to use the 0x API?

No, the 0x API is free to use and is intended for public use. If you are an integrator who needs access higher rate limits, please fill out 0x API Taker Integration Request Form to apply for a custom enterprise API endpoint.

What is Slippage Protection ?

Slippage Protection is a feature of the 0x API that finds the best routes for decentralized exchange (DEX) trades while avoiding slippage and MEV attacks.
Slippage Protection incorporates slippage forecasts into 0x API’s smart order routing algorithm to deliver the optimal trade route. With Slippage Protection activated, 0x API will enable developers to surface more reliable quotes and consistently deliver the best executed price to users. Slippage Protection is currently supported on Ethereum for the most active trading pairs (ETH-USDC, ETH-DAI, ETH-USDT, ETH-WBTC, WETH-USDC, WETH-DAI, WETH-USDT, WETH-WBTC)
Slippage Protection is an auto-enabled feature of the 0x API, and no additional action is required to enable to implement it in your API request. Read here for the full details of Slippage Protection.

If the difference between the quoted price and the executed price is positive what happens to the funds? Is positive slippage returned to the user?

In cases of positive slippage (i.e. when the quoted price is more than the executed price) the 0x protocol returns that difference back to the user.

Is it possible to use the 0x API to trade custom ERC20 tokens or altcoins?

If you would like to trade a custom token, you will need to create the liquidity either by using 0x limit orders or by creating a Liquidity Pool for your token on one of the various AMM sources that the API sources from, such as Uniswap, SushiSwap, or Curve. Learn more about creating limit order: https://docs.0x.org/protocol/docs/exchange-proxy/features/nativeorders#limit-orders​