> For clean Markdown of any page, append .md to the page URL. > For a complete documentation index, see https://docs.0x.org/llms.txt. > For AI client integration (Claude Code, Cursor, etc.), connect to the MCP server at https://docs.0x.org/_mcp/server. # FAQ > Common issues and troubleshooting tips for 0x APIs ## CORS Errors #### Why am I getting a CORS error when using the 0x API? TL;DR CORS errors happen because the 0x API enforces strict security policies. Avoid making API calls directly from the browser to prevent CORS issues and exposing your API keys. Instead, use a backend server, serverless functions, or a full-stack framework. **Error Message** You might see an error like this: ```curl http://localhost:3000 has been blocked by CORS policy: Request header field 0x-version is not allowed by Access-Control-Allow-Headers in preflight response. ``` Explanation
This error occurs because browsers block requests with custom headers (e.g., 0x-version) unless explicitly allowed by the server. Making API calls from a browser is not recommended because it can expose your API keys. **Recommended Solutions**
  1. Use a Backend Server — Proxy API calls through a backend server and handle API keys securely (e.g., axios in Node.js or requests in Python).
  2. Serverless Functions — Use AWS Lambda, Vercel Functions, or Netlify Functions.
  3. Full-Stack Framework — Implement backend logic in Next.js or Remix.
## Troubleshooting #### I received an API issue / error code. Help! See Handling API issues and error codes for a full list of common 0x issue types and error codes and how to resolve them. #### Why does my 0x transaction revert? If your 0x quote is reverting, besides the standard revert issues related to ETH transactions, check the following: For more details, see FAQ. #### Did my order revert because the RFQ quote expired? RFQ quotes from Market Makers are only valid for a short period (roughly 60s on mainnet). Two ways to check: Best practice: refresh quotes about every 30s to prevent RFQ expiry. See Matcha.xyz for an example. {" "} #### 0x orders are reverting but my transaction is fine, what is happening? Some subset of 0x orders may revert (not fill) while the overall transaction succeeds. This is expected: due to timing/pricing, some orders may be filled or expired before a user attempts to fill. The protocol uses fallback orders to compensate, resulting in a successful transaction even though internal order reverts occurred. #### How does including \`taker\` in the API call help with catching issues? Passing a taker lets 0x API provide a bespoke quote and catch revert issues: TL;DR Pass taker to validate quotes and avoid many revert cases. {" "} #### I received an \`INPUT\_INVALID\` error. Help! Verify that the API request is formatted properly. If the issue persists,{" "} contact support . {" "} #### I received \`BUY\_TOKEN\_NOT\_AUTHORIZED\_FOR\_TRADE\` or \`SELL\_TOKEN\_NOT\_AUTHORIZED\_FOR\_TRADE\`. Help! The buy token is not authorized for trade due to legal restrictions. #### I received an \`INTERNAL\_SERVER\_ERROR\` error. Help! An internal server error occurred. #### How will I know if the trade is not possible due to insufficient asset liquidity? The API will return liquidityAvailable=false and omit other response params if there isn’t enough liquidity available for the requested quote.

liquidityAvailable parameter #### What does MEV protection mean? Use the getSources endpoint. *** ## EVM Swap API ### About EVM Swap API #### How do I get a list of all the sources the API is sourcing from? Use the getSources endpoint. {" "} #### How do I find a list of supported chains for Swap? Use the getChains endpoint to get a list of supported chains for Swap. #### Is there a fee to use Swap API? We offer two transparent, flexible tiers for Web3 businesses of all sizes. You can get started here. If you are a high-volume app or have a unique business model, please contact us to discuss a custom plan. 0x takes an on-chain fee on swaps for select tokens for teams on the Standard tier. This fee is charged on-chain to your users during the transaction. In cases where we charge a fee, we return the value in the API response as zeroExFee. See the Swap API reference for details. #### How does Swap API select the best orders for me? Swap API samples each liquidity source and adjusts for gas consumption (using your specified gas price, or a fast default) and any associated fees. It evaluates different compositions of sources to select the best set of orders, and also creates fallback orders to ensure executability. Example: Curve’s nominal price may look best, but after factoring gas \* gasPrice and fees, it may not be optimal. {" "} #### How can I find the tokens that 0x supports for trading? 0x supports all tokens by default except those blocked for compliance reasons. {" "} #### Is it possible to use the Swap API to trade custom ERC-20 tokens or altcoins? If you’d like to trade a custom token, you’ll need to create liquidity by deploying a pool on an AMM the API sources (e.g., Uniswap, SushiSwap, Curve). #### What does MEV protection mean? MEV protection applies to RFQ orders, which are filled privately and atomically to prevent sandwiching/front-running. It does not apply to AMM trades, which execute on public mempools and are subject to typical MEV risks. ### Monetizing your EVM Swap Integration #### I am building a DEX app using Swap API. Can I charge my users a trading fee/commission? You can collect affiliate fees or trade surplus. Read the full guide: monetizing your swap integration. #### How is the trading fee/commission I charge returned by Swap API? If swapFeeRecipient, swapFeeBps, and swapFeeToken are set, the response includes fees.integratorFee.amount which you can display to users. ```text amount = (swapFeeBps / 10000) * sellAmount // in sellToken base units ``` Example for 1% of 100 USDC: Fee amount = 1000000 = 1 USDC. See the guide: monetization. In API v2, you can take fees on buyToken or sellToken. #### Can I collect trade surplus (positive slippage)? Available to select integrators on a custom pricing plan. Contact support. See the monetization guide. ### AllowanceHolder and Permit2 #### What is the difference between using AllowanceHolder and Permit2 for Swap API? 0x Swap API supports two allowance methods: AllowanceHolder (recommended) and Permit2 (advanced). AllowanceHolder (Recommended)
Better UX, lower gas, simpler integration, and safe defaults. Great for aggregators, teams upgrading from Swap v1, and advanced wallets (multisigs/smart accounts). Endpoints: Allowance target is returned in issues.allowance.spender or allowanceTarget. Chain-specific AllowanceHolder addresses (verify with official docs before hardcoding): Permit2 (Advanced Only)
Enables time-limited/granular approvals but requires a double-signature flow. If you use Permit2, set allowances on Permit2 only — never on the Settler (funds may be lost). Endpoints: Permit2 address (all chains): 0x000000000022D473030F116dDEE9F6B43aC78BA3 Read more about the contracts [here](/docs/core-concepts/contracts). {" "} #### Should I use AllowanceHolder or Permit2 for my meta-aggregator project? Depends on UX and integration type: {" "} #### If an allowance is needed when using AllowanceHolder, which contract will \`issues.allowance.spender\` or \`allowanceTarget\` return? The AllowanceHolder contract. #### How should the signed Permit2 EIP-712 (\`permit2.eip712\`) message look? Ethereum signatures: r, s, v, ordered as r, s, v, where: Pack to 65 bytes: bytes 0–31 = r, 32–63 = s, 64 = v. ### Parameter Questions #### What is gas? Gas is the fee required to conduct a transaction on Ethereum, paid in ETH and denominated in Gwei (1 ETH = 1,000,000,000 Gwei). #### What is the significance of \`0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE\`? It’s a standardized representation of native tokens (e.g., ETH, BNB, POL) which don’t have contract addresses like ERC-20s.
On Mantle, \$MNT is an exception. It has the contract address 0xdeaddeaddeaddeaddeaddeaddeaddeaddead0000. Note this is not applicable to wrapped/bridged MNT. See Handling native tokens with 0x Swap API for additional information. #### Does the 0x API return \`buyAmount\` in base units? Yes. All buyAmount values are in the token’s base units. Useful helpers: {" "} #### Is it possible to quote by the \`buyAmount\`? Yes — buyAmount is supported. You can specify the exact amount of the buy token you want to receive, and the API will calculate the required sell amount. See the{" "} API reference {" "} for details. {" "} #### How does Exact Buy handle token allowances and trade surplus? Allowances: The taker approves up to{" "} maxSellAmount — a ceiling, not a fixed spend. The swap will never pull more than this amount from the taker's wallet. Trade surplus: Any trade surplus comes from the sell side, not the buy side. If the on-chain fill settles for less than the estimated sell amount, the difference is returned to the taker in the same transaction as excess sell token. The buyAmount is an exact output — the recipient always receives precisely that amount, no more. See the{" "} Exact Buy guide for details. {" "} #### Can I skip validation for \`/quote\` by leaving the \`taker\` parameter blank? No — taker is required in v2 for /quote. It enables us to return calldata and identify issues that would cause failures. #### Why does the value of the \`to\` field in \`/swap/quote\` vary? The Settler contract can be redeployed; the to target address will therefore change. See: current/future Settler addresses. ### Best Practices #### What is the best way to query swap prices for many asset pairs without exceeding the rate limit? Contact 0x to discuss a solution that best suits your use case. {" "} #### How can I display the 0x Swap Fee to my end users? When applicable, the fee is returned as zeroExFee in quotes. You are responsible for showing the amount and token to users. See the Pricing Page and{" "} /quote response . {" "} #### Is there a way to sell assets via Swap API if the exact sellToken amount is not known before the transaction is executed? Not currently, but we’re exploring this feature. {" "} #### How can I get the ABI for an ERC-20 contract via TypeScript? Options: #### What’s the best way to access 0x trade data? Depends on your use case: See the 0x transaction data guide. #### Can I detect whether a transaction originated from the 0x API by parsing transaction receipts? Not directly via onchain data alone. While all 0x API trades settle through the Settler contract, transactions don’t include a unique tag that explicitly ties them to the API.

Tracking your own app’s activity: Detecting any/all API-routed transactions: *** ## EVM Gasless API ### About EVM Gasless API #### How do I get a list of sources the API is sourcing from? Use the getSources{" "} endpoint. #### What chains does Gasless API support? See the [full list of supported chains](/docs/core-concepts/0x-cheat-sheet#-chain-support) #### Which tokens are supported by Gasless API? **Short answer:** Gasless API provides a gasless experience at two points of the trade flow where users normally pay gas: **What doesn’t work:** You can’t use Gasless API to sell a chain’s native coin (e.g., ETH on Mainnet, POL on Polygon). Native coins aren’t ERC-20s, so they lack transferFrom which the relay system uses. Use the Swap API instead (user pays gas in the native token) or have users wrap the coin (e.g., ETH → WETH). **Work-arounds for tokens not supported by Gasless API:**
  1. Use the regular Swap API — the user pays gas in the native coin.
  2. Wrap the coin first (e.g., ETH → WETH), then use Gasless API.
#### What if my user wants to sell a native token (e.g., swap ETH for USDC) on Mainnet? Use the{" "} Swap API{" "} (user pays gas in the native token), or have users wrap the native coin first (e.g., ETH → WETH), then use Gasless API. #### What is the minimum amount users can trade with Gasless API? It varies by chain, size, and gas conditions. If the amount is too small, the API returns an error with the estimated minimum. As a guideline, set a minimum of about $10 on Mainnet and $1 on other chains. #### What’s the difference between Permit (EIP-2612) and Permit2? Both enable gasless-like UX, but they serve different purposes.

In short: Permit (EIP-2612) is an ERC-20 extension enabling gasless approvals via an EIP-712 signature —{" "} no on-chain tx — but only for tokens that implement EIP-2612. Many older tokens (e.g., WETH) don’t, so they still need a regular{" "} approve().

Permit2 (by Uniswap Labs) is a universal approval contract for any ERC-20. Users first do a standard gasful approve() to Permit2, then future moves can be authorized via EIP-712 signatures (gasless for subsequent swaps). This is useful when tokens don’t support{" "} permit() but you want a consistent meta-tx flow. Permit2 is deployed at 0x000000000022D473030F116dDEE9F6B43aC78BA3 on major chains. ### Gas #### Who pays for the gas fees to allow those swaps to happen? 0x covers the gas fee up front. The cost is wrapped into the trade and paid in the token the user is trading. Applications may choose to sponsor transactions; in that case they pay 0x directly and users are not billed on chain. ### Monetizing Gasless API #### Can I monetize using Gasless API? You have full flexibility on the fees you collect on trades.

Include these parameters when making a Gasless API request: ```bash https://api.0x.org/gasless/quote # Request a firm quote ?chainId=1 # Ethereum Mainnet &sellToken=0x6B175474E89094C44Da98b954EedeAC495271d0F # Sell DAI &sellAmount=4000000000000000000000 # Sell amount: 4000 (18 decimals) &buyToken=0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE # Buy ETH &taker=$USER_TAKER_ADDRESS # Trader address &swapFeeRecipient=$INTEGRATOR_WALLET_ADDRESS # Recipient of affiliate fees &swapFeeBps=100 # 1% fee (100 bps) &swapFeeToken=0x6B175474E89094C44Da98b954EedeAC495271d0F # Take fee in sellToken (DAI) --header '0x-api-key: [API_KEY]' # Your API key --header '0x-version: v2' # API version ``` ### Gasless Approvals #### What tokens work with gasless approvals? Gasless approvals work only for tokens that implement [EIP-2612](https://eips.ethereum.org/EIPS/eip-2612) (`permit`). Check the `approval` object in the [quote](https://docs.0x.org/api-reference/evm-ap-is/gasless/getquote) — if it's not `null`, the token supports gasless approval. #### How do I know if an approval is required? Check the response from [/gasless/quote](https://docs.0x.org/api-reference/evm-ap-is/gasless/getquote), #### My user needs an approval before swapping — are these separate transactions? Do I need 2 signatures? Gasless approvals and gasless swap are bundled in the same{" "} transaction, but each requires a signature for its EIP-712 object. You can design the UI so it feels like a single step to the user. #### What does a gasless approve + swap happy path look like? See the flow charts{" "} here . ### Troubleshooting #### I received one of these errors: INPUT\_INVALID, BUY\_TOKEN\_NOT\_AUTHORIZED\_FOR\_TRADE, INTERNAL\_SERVER\_ERROR. Help! Read more about{" "} Status Codes . *** ## Solana Swap API #### Does 0x create ATAs if we don't have one, specifically for our fee address? **For the user/taker:** Yes. The setup instructions 0x returns automatically create associated token accounts (ATAs) for the taker as needed, using `createAssociatedTokenAccountIdempotent` — so these won't fail if an ATA already exists. See [what's included in the instructions](/svm/solana-swap-api/guides/important-integration-notes#instructions-grouping) for the full list. **For an integrator fee address:** No. You should assume the fee recipient ATA must already exist. If it may not exist, add a `createAssociatedTokenAccountIdempotent` instruction for the fee recipient + fee token mint yourself, **before** the swap/fee transfer path. See the [Integrator Byte Costs](/svm/solana-swap-api/guides/integrator-byte-costs) guide for how to account for the extra bytes this adds. The relevant field is [`swap_fee_recipient`](/api-reference/solana-swap-ap-is/swap/instructions#swap_fee_recipient) — it accepts either a wallet address (for native SOL output) or a valid token account address (for token outputs). *** ## Trade Analytics API #### What is the 0x Trade Analytics API? The Trade Analytics API gives integrators easy access to the history of trades initiated through 0x APIs and settled on 0x Smart Contracts. This API offers comprehensive transaction records, providing useful insights into user trading behavior. {" "} #### Whose data is returned? The Trade Analytics API will only return data for the app associated with the API key that makes the request. While all trading data is publicly available onchain, the Trade Analytics API provides a streamlined, well formatted data for the specific app making the request. #### What is data finality in the context of the Trade Analytics API? The data accessed through the API is deemed final 48 hours after a trade is completed. This delay allows time for the data pipeline to capture all relevant information, including fees from proxy contracts and updated USD prices from Coingecko. To ensure that your trading data is up to date, we recommend that you fetch and update data from the preceding 2 days in addition to the current day. #### How quickly will a new trade show up in the Trade Analytics API? Data is updated roughly every 15 minutes. While generally reliable, data from these updates can sometimes be missing or incomplete, especially if they have a dependency on external sources such as token USD price providers. Each trade data is considered final 48 hours after it’s mined onchain – this enables us to address any gaps from earlier updates, ensuring data accuracy. We don’t recommend using this API for use cases that require real time updates. If you’re interested in such data, please [submit a feature request](https://0x.canny.io/request-features). #### Can I detect whether a transaction originated from the 0x API by parsing transaction receipts? It's a common question — and the short answer is: **not directly via onchain data alone.** While all 0x API trades are settled through the 0x Settler smart contract, these transactions don’t include a unique tag or flag that explicitly ties them to the API. That said, there are a few ways to approach attribution, depending on what you're trying to track: **🔍 If you're trying to track your own app's activity** **🌍 If you're trying to detect any and all transactions routed via the 0x API** We understand attribution is an important use case, whether you're tracking your own usage or seeking broader ecosystem insights. Don’t hesitate to contact us to discuss how we can best support you. #### What is the difference between using the Trade Analytics API vs the 0x-parser? Trade Analytics API is best for historical analysis and business intelligence. Provides comprehensive trade data including volumes, fees, and USD values. 0x-parser is a best for real-time transaction data and displaying final swap amounts immediately after settlement. Read more about [accessing 0x transaction data here](/docs/core-concepts/transaction-data). *** ## 0x Dashboard #### Does the 0x Dashboard support having multiple user accounts for our team? For now we only support one user per team account, but we will add support for multiple users in the coming weeks. #### What is an App? An app is a self-contained unit for each application that you’re building. You can set up multiple apps, each with unique API keys and configurations, on the 0x Dashboard. *** ## Smart Contracts #### Are the smart contracts audited? Yes. See{" "} {" "} audit reports{" "} . Check out our{" "} bounty program. *** ## Building with 0x #### My project would like to integrate 0x. How can I contact the 0x team? Create an account on the 0x Dashboard to get an API key and start building for free. You can also review plans and contact our team for custom needs. #### 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. Please refer to the ["Liquidity Integrations"](/liquidity-integration/liquidity-integrations) tab.