Swap Integration

Integrate Uniswap swaps into frontends, backends, and smart contracts.

Prerequisites

This skill assumes familiarity with viem basics (client setup, account management, contract interactions, transaction signing). Install the uniswap-viem plugin for comprehensive viem/wagmi guidance: claude plugin add @uniswap/uniswap-viem

Quick Decision Guide

Building...	Use This Method
Frontend with React/Next.js	Trading API
Backend script or bot	Trading API
Smart contract integration	Universal Router direct calls
Need full control over routing	Universal Router SDK

Routing Types Quick Reference

Type	Description	Chains
CLASSIC	Standard AMM swap through Uniswap pools	All supported chains
DUTCH_V2	UniswapX Dutch auction V2	Ethereum, Arbitrum, Base, Unichain
PRIORITY	MEV-protected priority order	Base, Unichain
WRAP	ETH to WETH conversion	All
UNWRAP	WETH to ETH conversion	All

See Routing Types for the complete list including DUTCH_V3, DUTCH_LIMIT, LIMIT_ORDER, BRIDGE, and QUICKROUTE.

Integration Methods

1. Trading API (Recommended)

Best for: Frontends, backends, scripts. Handles routing optimization automatically.

Base URL: https://trade-api.gateway.uniswap.org/v1

Authentication: x-api-key: <your-api-key> header required

Getting an API Key: The Trading API requires an API key for authentication. Visit the Uniswap Developer Portal to register and obtain your API key. Keys are typically available for immediate use after registration. Include it as an x-api-key header in all API requests.

Required Headers — Include these in ALL Trading API requests:

Content-Type: application/json
x-api-key: <your-api-key>
x-universal-router-version: 2.0

3-Step Flow:

1. POST /check_approval  -> Check if token is approved
2. POST /quote           -> Get executable quote with routing
3. POST /swap            -> Get transaction to sign and submit

See the Trading API Reference section below for complete documentation.

2. Universal Router SDK

Best for: Direct control over transaction construction.

Installation:

npm install @uniswap/universal-router-sdk @uniswap/sdk-core @uniswap/v3-sdk

Key Pattern:

import { SwapRouter } from '@uniswap/universal-router-sdk';

const { calldata, value } = SwapRouter.swapCallParameters(trade, options);

See the Universal Router Reference section below for complete documentation.

3. Smart Contract Integration

Best for: On-chain integrations, DeFi composability.

Interface: Call execute() on Universal Router with encoded commands.

See the Universal Router Reference section below for command encoding.

---

Trading API Reference

Step 1: Check Token Approval

POST /check_approval

Request:

{
  "walletAddress": "0x...",
  "token": "0x...",
  "amount": "1000000000",
  "chainId": 1
}

Response:

{
  "approval": {
    "to": "0x...",
    "from": "0x...",
    "data": "0x...",
    "value": "0",
    "chainId": 1
  }
}

If approval is null, token is already approved.

Step 2: Get Quote

POST /quote

Request:

{
  "swapper": "0x...",
  "tokenIn": "0x...",
  "tokenOut": "0x...",
  "tokenInChainId": "1",
  "tokenOutChainId": "1",
  "amount": "1000000000000000000",
  "type": "EXACT_INPUT",
  "slippageTolerance": 0.5,
  "routingPreference": "BEST_PRICE"
}

Note: tokenInChainId and tokenOutChainId must be strings (e.g., "1"), not numbers.

Key Parameters:

Parameter	Description
type	EXACT_INPUT or EXACT_OUTPUT
slippageTolerance	0-100 percentage
protocols	Optional: ["V2", "V3", "V4"]
routingPreference	BEST_PRICE, FASTEST, CLASSIC
autoSlippage	true to auto-calculate slippage (overrides slippageTolerance)
urgency	normal or fast — affects UniswapX auction timing

Response:

{
  "routing": "CLASSIC",
  "quote": {
    "input": { "token": "0x...", "amount": "1000000000000000000" },
    "output": { "token": "0x...", "amount": "999000000" },
    "slippage": 0.5,
    "route": [],
    "gasFee": "5000000000000000",
    "gasFeeUSD": "0.01",
    "gasUseEstimate": "150000"
  },
  "permitData": {}
}

Display tip: Use gasFeeUSD (a string with the USD value) for gas cost display. Do not manually convert gasFee (wei) using a hardcoded ETH price — this leads to wildly inaccurate estimates (e.g., ~$87 instead of ~$0.01).

Step 3: Execute Swap

POST /swap

Request - Spread the quote response directly into the body:

// CORRECT: Spread the quote response, strip null fields
const quoteResponse = await fetchQuote(params);

// Remove null permitData/permitTransaction (API rejects null values)
const { permitData, permitTransaction, ...cleanQuote } = quoteResponse;

const swapRequest = {
  ...cleanQuote,
  // Only include permitData if it's a valid object (not null)
  ...(permitData && { permitData }),
};

// If using Permit2 signature, include BOTH signature and permitData
if (permit2Signature && permitData) {
  swapRequest.signature = permit2Signature;
  swapRequest.permitData = permitData;
}

Critical: Do NOT wrap the quote in {quote: quoteResponse}. The API expects the quote response fields spread into the request body.

Permit2 Rules:

• signature and permitData must BOTH be present, or BOTH be absent
• Never set permitData: null - omit the field entirely
• The quote response often includes permitData: null - strip this before sending

Response (ready-to-sign transaction):

{
  "swap": {
    "to": "0x...",
    "from": "0x...",
    "data": "0x...",
    "value": "0",
    "chainId": 1,
    "gasLimit": "250000"
  }
}

Response Validation - Always validate before broadcasting:

function validateSwapResponse(response: SwapResponse): void {
  if (!response.swap?.data || response.swap.data === '' || response.swap.data === '0x') {
    throw new Error('swap.data is empty - quote may have expired');
  }
  if (!isAddress(response.swap.to) || !isAddress(response.swap.from)) {
    throw new Error('Invalid address in swap response');
  }
}

Supported Chains

ID	Chain	ID	Chain
1	Ethereum	8453	Base
10	Optimism	42161	Arbitrum
56	BNB	42220	Celo
130	Unichain	43114	Avalanche
137	Polygon	81457	Blast
196	X Layer	7777777	Zora
324	zkSync	480	World Chain
1868	Soneium	143	Monad

Routing Types

Type	Description
CLASSIC	Standard AMM swap through Uniswap pools
DUTCH_V2	UniswapX Dutch auction V2
DUTCH_V3	UniswapX Dutch auction V3
PRIORITY	MEV-protected priority order (Base, Unichain)
DUTCH_LIMIT	UniswapX Dutch limit order
LIMIT_ORDER	Limit order
WRAP	ETH to WETH conversion
UNWRAP	WETH to ETH conversion
BRIDGE	Cross-chain bridge
QUICKROUTE	Fast approximation quote

UniswapX availability: UniswapX V2 orders are supported on Ethereum (1), Arbitrum (42161), Base (8453), and Unichain (130). The auction mechanism varies by chain — see UniswapX Auction Types below.

---

Critical Implementation Notes

These are common pitfalls discovered during real-world Trading API integration. Follow these rules to avoid on-chain reverts and API errors.

1. Swap Request Body Format

The /swap endpoint expects the quote response spread into the request body, not wrapped in a quote field.

// WRONG - causes "quote does not match any of the allowed types"
const badRequest = {
  quote: quoteResponse, // Don't wrap!
  signature: '0x...',
};

// CORRECT - spread the quote response
const goodRequest = {
  ...quoteResponse,
  signature: '0x...', // Only if using Permit2
};

2. Null Field Handling

The API rejects permitData: null. Always strip null fields before sending:

function prepareSwapRequest(quoteResponse: QuoteResponse, signature?: string): object {
  // Strip null values that the API rejects
  const { permitData, permitTransaction, ...cleanQuote } = quoteResponse;

  const request: Record<string, unknown> = { ...cleanQuote };

  // Only include permitData if it's a valid object AND we have a signature
  if (signature && permitData && typeof permitData === 'object') {
    request.signature = signature;
    request.permitData = permitData;
  }

  return request;
}

3. Permit2 Field Rules

When using Permit2 for gasless approvals:

Scenario	signature	permitData
Standard swap (no Permit2)	Omit	Omit
Permit2 swap	Required	Required
Invalid	Present	Missing
Invalid	Missing	Present
Invalid (API error)	Any	null

4. Pre-Broadcast Validation

Always validate the swap response before sending to the blockchain:

import { isAddress, isHex } from 'viem';

function validateSwapBeforeBroadcast(swap: SwapTransaction): void {
  // 1. data must be non-empty hex
  if (!swap.data || swap.data === '' || swap.data === '0x') {
    throw new Error('swap.data is empty - this will revert on-chain. Re-fetch the quote.');
  }

  if (!isHex(swap.data)) {
    throw new Error('swap.data is not valid hex');
  }

  // 2. Addresses must be valid
  if (!isAddress(swap.to)) {
    throw new Error('swap.to is not a valid address');
  }

  if (!isAddress(swap.from)) {
    throw new Error('swap.from is not a valid address');
  }

  // 3. Value must be present (can be "0" for non-ETH swaps)
  if (swap.value === undefined || swap.value === null) {
    throw new Error('swap.value is missing');
  }
}

5. Browser Environment Setup

When using viem/wagmi in browser environments, you need Node.js polyfills:

Install buffer polyfill:

npm install buffer

Add to your entry file (before other imports):

// src/main.tsx or src/index.tsx
import { Buffer } from 'buffer';
globalThis.Buffer = Buffer;

// Then your other imports
import React from 'react';
import { WagmiProvider } from 'wagmi';
// ...

Vite configuration (vite.config.ts):

export default defineConfig({
  define: {
    global: 'globalThis',
  },
  optimizeDeps: {
    include: ['buffer'],
  },
  resolve: {
    alias: {
      buffer: 'buffer',
    },
  },
});

Without this setup, you'll see: ReferenceError: Buffer is not defined

CORS Proxy Configuration

The Trading API does not support browser CORS preflight requests — OPTIONS requests return 415 Unsupported Media Type. Direct fetch() calls from a browser will always fail. You must proxy API requests through your own server or dev server.

Vite dev proxy (merge into the same vite.config.ts used for the Buffer polyfill above):

export default defineConfig({
  server: {
    proxy: {
      '/api/uniswap': {
        target: 'https://trade-api.gateway.uniswap.org/v1',
        changeOrigin: true,
        rewrite: (path) => path.replace(/^\/api\/uniswap/, ''),
      },
    },
  },
});

Then use /api/uniswap/quote instead of the full URL in your frontend code.

Vercel production proxy (vercel.json):

{
  "rewrites": [
    {
      "source": "/api/uniswap/:path*",
      "destination": "https://trade-api.gateway.uniswap.org/v1/:path*"
    }
  ]
}

Cloudflare Pages (public/_redirects):

/api/uniswap/* https://trade-api.gateway.uniswap.org/v1/:splat 200

Next.js (next.config.js):

module.exports = {
  async rewrites() {
    return [
      {
        source: '/api/uniswap/:path*',
        destination: 'https://trade-api.gateway.uniswap.org/v1/:path*',
      },
    ];
  },
};

Without a proxy, you'll see: 415 Unsupported Media Type on preflight or CORS errors in the browser console.

6. Quote Freshness

• Quotes expire quickly (typically 30 seconds)
• Always re-fetch if the user takes time to review
• Use the deadline parameter to prevent stale execution
• If /swap returns empty data, the quote likely expired

---

Universal Router Reference

The Universal Router is a unified interface for swapping across Uniswap v2, v3, and v4.

Core Function

function execute(
    bytes calldata commands,
    bytes[] calldata inputs,
    uint256 deadline
) external payable;

Command Encoding

Each command is a single byte:

Bits	Name	Purpose
0	flag	Allow revert (1 = continue on fail)
1-2	reserved	Use 0
3-7	command	Operation identifier

Swap Commands

Code	Command	Description
0x00	V3_SWAP_EXACT_IN	v3 swap with exact input
0x01	V3_SWAP_EXACT_OUT	v3 swap with exact output
0x08	V2_SWAP_EXACT_IN	v2 swap with exact input
0x09	V2_SWAP_EXACT_OUT	v2 swap with exact output
0x10	V4_SWAP	v4 swap

Token Operations

Code	Command	Description
0x04	SWEEP	Clear router token balance
0x05	TRANSFER	Send specific amount
0x0b	WRAP_ETH	ETH to WETH
0x0c	UNWRAP_WETH	WETH to ETH

Permit2 Commands

Code	Command	Description
0x02	PERMIT2_TRANSFER_FROM	Single token transfer
0x03	PERMIT2_PERMIT_BATCH	Batch approval
0x0a	PERMIT2_PERMIT	Single approval

SDK Usage

import { SwapRouter, UniswapTrade } from '@uniswap/universal-router-sdk'
import { TradeType } from '@uniswap/sdk-core'

// Build trade using v3-sdk or router-sdk
const trade = new RouterTrade({
  v3Routes: [...],
  tradeType: TradeType.EXACT_INPUT
})

// Get calldata for Universal Router
const { calldata, value } = SwapRouter.swapCallParameters(trade, {
  slippageTolerance: new Percent(50, 10000), // 0.5%
  recipient: walletAddress,
  deadline: Math.floor(Date.now() / 1000) + 1200 // 20 min
})

// Send transaction
const tx = await wallet.sendTransaction({
  to: UNIVERSAL_ROUTER_ADDRESS,
  data: calldata,
  value
})

---

Permit2 Integration

Permit2 enables signature-based token approvals instead of on-chain approve() calls.

Approval Target: Permit2 vs Legacy (Direct to Router)

There are two approval paths. Choose based on your integration type:

Approach	Approve To	Per-Swap Auth	Best For
Permit2 (recommended)	Permit2 contract	EIP-712 signature	Frontends with user interaction
Legacy (direct approve)	Universal Router	None (pre-approved)	Backend services, smart accounts

Permit2 flow (frontend with user signing):

1. User approves token to Permit2 contract (one-time)
2. Each swap: user signs an EIP-712 permit message
3. Universal Router uses the signature to transfer tokens via Permit2

Legacy flow (backend services, ERC-4337 smart accounts):

1. Approve token directly to the Universal Router address (one-time)
2. Each swap: no additional authorization needed
3. Simpler for automated systems that cannot sign EIP-712 messages

Use the Trading API's /check_approval endpoint — it returns the correct approval target based on the routing type.

How It Works

1. User approves Permit2 contract once (infinite approval)
2. For each swap, user signs a message authorizing the transfer
3. Universal Router uses signature to transfer tokens via Permit2

Two Modes

Mode	Description
SignatureTransfer	One-time signature, no on-chain state
AllowanceTransfer	Time-limited allowance with on-chain state

Integration Pattern

import { getContract, maxUint256, type Address } from 'viem';

const PERMIT2_ADDRESS = '0x000000000022D473030F116dDEE9F6B43aC78BA3' as const;

// Check if Permit2 approval exists
const allowance = await publicClient.readContract({
  address: PERMIT2_ADDRESS,
  abi: permit2Abi,
  functionName: 'allowance',
  args: [userAddress, tokenAddress, spenderAddress],
});

// If not approved, user must approve Permit2 first
if (allowance.amount < requiredAmount) {
  const hash = await walletClient.writeContract({
    address: tokenAddress,
    abi: erc20Abi,
    functionName: 'approve',
    args: [PERMIT2_ADDRESS, maxUint256],
  });
  await publicClient.waitForTransactionReceipt({ hash });
}

// Then sign permit for the swap
const permitSignature = await signPermit(...);

---

UniswapX Auction Types

UniswapX routes swaps through off-chain fillers who compete to execute orders at better prices than on-chain AMMs. The auction mechanism varies by chain.

Exclusive Dutch Auction (Ethereum)

• Starts with an RFQ (Request for Quote) phase where permissioned quoters compete
• Winning quoter receives exclusive filling rights for a set period
• If the exclusive filler doesn't execute, falls back to an open Dutch auction where the price decays each block
• Best for large swaps where MEV protection matters most

Trading API routing type: DUTCH_V2 or DUTCH_V3

Open Dutch Auction (Arbitrum)

• Direct open auction without an RFQ phase
• Fillers compete on-chain through a descending price mechanism
• Leverages Arbitrum's fast 0.25-second block times for rapid price discovery
• The Unimind algorithm sets auction parameters based on historical pair performance

Trading API routing type: DUTCH_V2

Priority Gas Auction (Base, Unichain)

• Fillers bid by submitting transactions with varying priority fees at a target block
• Highest priority fee wins the right to fill the order
• Exploits OP Stack's priority ordering mechanism
• Effective on chains where block builders respect priority ordering

Trading API routing type: PRIORITY

Key Properties (All Auction Types)

• Gasless for users — fillers pay gas fees, incorporated into final pricing
• No cost on failure — if a swap doesn't fill, the user pays nothing
• MEV protection — auction mechanics prevent frontrunning and sandwich attacks
• UniswapX V2 is currently supported on Ethereum (1), Arbitrum (42161), Base (8453), and Unichain (130)

For more detail, see the UniswapX Auction Types documentation.

---

Direct Universal Router Integration (SDK)

For direct Universal Router integration without the Trading API, use the SDK's high-level API.

Installation

npm install @uniswap/universal-router-sdk @uniswap/router-sdk @uniswap/sdk-core @uniswap/v3-sdk viem

High-Level Approach (Recommended)

Use RouterTrade + SwapRouter.swapCallParameters() for automatic command building:

import { SwapRouter } from '@uniswap/universal-router-sdk';
import { Trade as RouterTrade } from '@uniswap/router-sdk';
import { TradeType, Percent } from '@uniswap/sdk-core';
import { Route as V3Route, Pool } from '@uniswap/v3-sdk';

// 1. Fetch pool data (required to construct routes)
// Using viem to read on-chain pool state:
const slot0 = await publicClient.readContract({
  address: poolAddress,
  abi: [
    {
      name: 'slot0',
      type: 'function',
      stateMutability: 'view',
      inputs: [],
      outputs: [
        { name: 'sqrtPriceX96', type: 'uint160' },
        { name: 'tick', type: 'int24' },
        { name: 'observationIndex', type: 'uint16' },
        { name: 'observationCardinality', type: 'uint16' },
        { name: 'observationCardinalityNext', type: 'uint16' },
        { name: 'feeProtocol', type: 'uint8' },
        { name: 'unlocked', type: 'bool' },
      ],
    },
  ],
  functionName: 'slot0',
});
const liquidity = await publicClient.readContract({
  address: poolAddress,
  abi: [
    {
      name: 'liquidity',
      type: 'function',
      stateMutability: 'view',
      inputs: [],
      outputs: [{ type: 'uint128' }],
    },
  ],
  functionName: 'liquidity',
});

const pool = new Pool(tokenIn, tokenOut, fee, slot0[0].toString(), liquidity.toString(), slot0[1]);

// 2. Build route and trade
const route = new V3Route([pool], tokenIn, tokenOut);
const trade = RouterTrade.createUncheckedTrade({
  route,
  inputAmount: amountIn,
  outputAmount: expectedOut,
  tradeType: TradeType.EXACT_INPUT,
});

// 3. Get calldata
const { calldata, value } = SwapRouter.swapCallParameters(trade, {
  slippageTolerance: new Percent(50, 10000), // 0.5%
  recipient: walletAddress,
  deadline: Math.floor(Date.now() / 1000) + 1800,
});

// 4. Execute with viem
const hash = await walletClient.sendTransaction({
  to: UNIVERSAL_ROUTER_ADDRESS,
  data: calldata,
  value: BigInt(value),
});

Low-Level Approach (Manual Commands)

For custom flows (fee collection, complex routing), use RoutePlanner directly:

import { RoutePlanner, CommandType, ROUTER_AS_RECIPIENT } from '@uniswap/universal-router-sdk';
import { encodeRouteToPath } from '@uniswap/v3-sdk';

// Special addresses
const MSG_SENDER = '0x0000000000000000000000000000000000000001';
const ADDRESS_THIS = '0x0000000000000000000000000000000000000002';

Example: V3 Swap with Manual Commands

import { RoutePlanner, CommandType } from '@uniswap/universal-router-sdk';
import { encodeRouteToPath, Route } from '@uniswap/v3-sdk';

async function swapV3Manual(route: Route, amountIn: bigint, amountOutMin: bigint) {
  const planner = new RoutePlanner();

  // Encode V3 path from route
  const path = encodeRouteToPath(route, false); // false = exactInput

  planner.addCommand(CommandType.V3_SWAP_EXACT_IN, [
    MSG_SENDER, // recipient
    amountIn, // amountIn
    amountOutMin, // amountOutMin
    path, // encoded path
    true, // payerIsUser
  ]);

  return executeRoute(planner);
}

Example: ETH to Token (Wrap + Swap)

async function swapEthToToken(route: Route, amountIn: bigint, amountOutMin: bigint) {
  const planner = new RoutePlanner();
  const path = encodeRouteToPath(route, false);

  // 1. Wrap ETH to WETH (keep in router)
  planner.addCommand(CommandType.WRAP_ETH, [ADDRESS_THIS, amountIn]);

  // 2. Swap WETH → Token (payerIsUser = false since using router's WETH)
  planner.addCommand(CommandType.V3_SWAP_EXACT_IN, [
    MSG_SENDER,
    amountIn,
    amountOutMin,
    path,
    false,
  ]);

  return executeRoute(planner, { value: amountIn });
}

Example: Token to ETH (Swap + Unwrap)

async function swapTokenToEth(route: Route, amountIn: bigint, amountOutMin: bigint) {
  const planner = new RoutePlanner();
  const path = encodeRouteToPath(route, false);

  // 1. Swap Token → WETH (output to router)
  planner.addCommand(CommandType.V3_SWAP_EXACT_IN, [
    ADDRESS_THIS,
    amountIn,
    amountOutMin,
    path,
    true,
  ]);

  // 2. Unwrap WETH to ETH
  planner.addCommand(CommandType.UNWRAP_WETH, [MSG_SENDER, amountOutMin]);

  return executeRoute(planner);
}

Example: Fee Collection with PAY_PORTION

async function swapWithFee(route: Route, amountIn: bigint, feeRecipient: Address, feeBips: number) {
  const planner = new RoutePlanner();
  const path = encodeRouteToPath(route, false);
  const outputToken = route.output.wrapped.address;

  // Swap to router (ADDRESS_THIS)
  planner.addCommand(CommandType.V3_SWAP_EXACT_IN, [ADDRESS_THIS, amountIn, 0n, path, true]);

  // Pay fee portion (e.g., 30 bips = 0.3%)
  planner.addCommand(CommandType.PAY_PORTION, [outputToken, feeRecipient, feeBips]);

  // Sweep remainder to user
  planner.addCommand(CommandType.SWEEP, [outputToken, MSG_SENDER, 0n]);

  return executeRoute(planner);
}

Execute Route Helper

import { UNIVERSAL_ROUTER_ADDRESS } from '@uniswap/universal-router-sdk';

const ROUTER_ABI = [
  {
    name: 'execute',
    type: 'function',
    stateMutability: 'payable',
    inputs: [
      { name: 'commands', type: 'bytes' },
      { name: 'inputs', type: 'bytes[]' },
      { name: 'deadline', type: 'uint256' },
    ],
    outputs: [],
  },
] as const;

async function executeRoute(planner: RoutePlanner, options?: { value?: bigint }) {
  const deadline = BigInt(Math.floor(Date.now() / 1000) + 1800);
  const routerAddress = UNIVERSAL_ROUTER_ADDRESS('2.0', 1); // version, chainId

  const { request } = await publicClient.simulateContract({
    address: routerAddress,
    abi: ROUTER_ABI,
    functionName: 'execute',
    args: [planner.commands, planner.inputs, deadline],
    account,
    value: options?.value ?? 0n,
  });

  return walletClient.writeContract(request);
}

Command Cheat Sheet

Command	Parameters
V3_SWAP_EXACT_IN	(recipient, amountIn, amountOutMin, path, payerIsUser)
V3_SWAP_EXACT_OUT	(recipient, amountOut, amountInMax, path, payerIsUser)
V2_SWAP_EXACT_IN	(recipient, amountIn, amountOutMin, path[], payerIsUser)
V2_SWAP_EXACT_OUT	(recipient, amountOut, amountInMax, path[], payerIsUser)
WRAP_ETH	(recipient, amount)
UNWRAP_WETH	(recipient, amountMin)
SWEEP	(token, recipient, amountMin)
TRANSFER	(token, recipient, amount)
PAY_PORTION	(token, recipient, bips)

Fee Tiers

Tier	Value	Percentage
LOWEST	100	0.01%
LOW	500	0.05%
MEDIUM	3000	0.30%
HIGH	10000	1.00%

---

Common Integration Patterns

Frontend Swap Hook (React)

Note: Ensure you've set up the Buffer polyfill and CORS proxy (see Critical Implementation Notes). For wagmi v2 useWalletClient() pitfalls, see wagmi v2 Integration Pitfalls below.

import { isAddress, isHex } from 'viem';
import { useWalletClient } from 'wagmi';

// In browser apps, use your CORS proxy path instead (see CORS Proxy Configuration)
// e.g., const API_URL = '/api/uniswap';
const API_URL = 'https://trade-api.gateway.uniswap.org/v1';

function useSwap() {
  const { data: walletClient } = useWalletClient();
  const [quoteResponse, setQuoteResponse] = useState(null);
  const [loading, setLoading] = useState(false);
  const [error, setError] = useState(null);

  const getQuote = async (params) => {
    setLoading(true);
    setError(null);
    try {
      const response = await fetch(`${API_URL}/quote`, {
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
          'x-api-key': API_KEY,
          'x-universal-router-version': '2.0',
        },
        body: JSON.stringify(params),
      });
      const data = await response.json();
      if (!response.ok) throw new Error(data.detail || 'Quote failed');
      setQuoteResponse(data); // Store the FULL response, not just data.quote
    } catch (err) {
      setError(err.message);
    } finally {
      setLoading(false);
    }
  };

  const executeSwap = async (permit2Signature?: string) => {
    if (!quoteResponse) throw new Error('No quote available');

    // CRITICAL: Strip null fields and spread quote response into body
    const { permitData, permitTransaction, ...cleanQuote } = quoteResponse;

    const swapRequest: Record<string, unknown> = {
      ...cleanQuote,
    };

    // CRITICAL: Only include permitData if we have BOTH signature and permitData
    // The API requires both fields to be present or both to be absent
    if (permit2Signature && permitData && typeof permitData === 'object') {
      swapRequest.signature = permit2Signature;
      swapRequest.permitData = permitData;
    }

    const swapResponse = await fetch(`${API_URL}/swap`, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'x-api-key': API_KEY,
        'x-universal-router-version': '2.0',
      },
      body: JSON.stringify(swapRequest),
    });
    const data = await swapResponse.json();
    if (!swapResponse.ok) throw new Error(data.detail || 'Swap failed');

    // CRITICAL: Validate response before broadcasting
    if (!data.swap?.data || data.swap.data === '' || data.swap.data === '0x') {
      throw new Error('Empty swap data - quote may have expired. Please refresh.');
    }

    // Send transaction via wallet (walletClient from useWalletClient())
    if (!walletClient) throw new Error('Wallet not connected');
    const tx = await walletClient.sendTransaction(data.swap);
    return tx;
  };

  return { quote: quoteResponse?.quote, loading, error, getQuote, executeSwap };
}

wagmi v2 Integration Pitfalls

The useWalletClient() hook from wagmi v2 can return undefined even when the wallet is connected — it resolves asynchronously. This causes "wallet not connected" errors at swap time. Additionally, the returned client needs a chain for sendTransaction() to work.

Recommended pattern — use @wagmi/core action functions at swap time instead of hooks:

import { getWalletClient, getPublicClient, switchChain } from '@wagmi/core';
import type { Config } from 'wagmi';

async function executeSwapTransaction(
  config: Config,
  chainId: number,
  swapTx: { to: string; data: string; value: string }
) {
  // 1. Ensure the wallet is on the correct chain
  await switchChain(config, { chainId });

  // 2. Get wallet client with explicit chainId — avoids undefined and missing chain
  const walletClient = await getWalletClient(config, { chainId });

  // 3. Execute the swap
  const hash = await walletClient.sendTransaction({
    to: swapTx.to as `0x${string}`,
    data: swapTx.data as `0x${string}`,
    value: BigInt(swapTx.value || '0'),
  });

  // 4. Wait for confirmation
  const publicClient = getPublicClient(config, { chainId });
  if (!publicClient) throw new Error(`No public client configured for chainId ${chainId}`);
  return publicClient.waitForTransactionReceipt({ hash });
}

Why this matters:

• useWalletClient() hook returns { data: undefined } during async resolution, even after useAccount() shows connected
• getWalletClient(config, { chainId }) is a promise that resolves only when the client is ready, and includes the chain
• switchChain() prevents "chain mismatch" errors when the wallet is on a different network than the swap

Backend Swap Script (Node.js)

import { createWalletClient, createPublicClient, http, isAddress, isHex, type Address } from 'viem';
import { privateKeyToAccount } from 'viem/accounts';
import { mainnet } from 'viem/chains';

const API_URL = 'https://trade-api.gateway.uniswap.org/v1';
const API_KEY = process.env.UNISWAP_API_KEY!;

const account = privateKeyToAccount(process.env.PRIVATE_KEY as `0x${string}`);
const publicClient = createPublicClient({ chain: mainnet, transport: http() });
const walletClient = createWalletClient({ account, chain: mainnet, transport: http() });

// Helper to strip null fields from quote response
function prepareSwapRequest(quoteResponse: Record<string, unknown>, signature?: string): object {
  const { permitData, permitTransaction, ...cleanQuote } = quoteResponse;

  const request: Record<string, unknown> = { ...cleanQuote };

  // CRITICAL: Only include permitData if we have BOTH signature and permitData
  // The API requires both fields to be present or both to be absent
  if (signature && permitData && typeof permitData === 'object') {
    request.signature = signature;
    request.permitData = permitData;
  }

  return request;
}

// Validate swap response before broadcasting
function validateSwap(swap: { data?: string; to?: string; from?: string }): void {
  if (!swap?.data || swap.data === '' || swap.data === '0x') {
    throw new Error('swap.data is empty - quote may have expired');
  }
  if (!isHex(swap.data)) {
    throw new Error('swap.data is not valid hex');
  }
  if (!swap.to || !isAddress(swap.to) || !swap.from || !isAddress(swap.from)) {
    throw new Error('Invalid address in swap response');
  }
}

async function executeSwap(tokenIn: Address, tokenOut: Address, amount: string, chainId: number) {
  const ETH_ADDRESS = '0x0000000000000000000000000000000000000000';

  // 1. Check approval (for ERC20 tokens, not native ETH)
  if (tokenIn !== ETH_ADDRESS) {
    const approvalRes = await fetch(`${API_URL}/check_approval`, {
      method: 'POST',
      headers: {
        'x-api-key': API_KEY,
        'Content-Type': 'application/json',
        'x-universal-router-version': '2.0',
      },
      body: JSON.stringify({
        walletAddress: account.address,
        token: tokenIn,
        amount,
        chainId,
      }),
    });
    const approvalData = await approvalRes.json();

    if (approvalData.approval) {
      const hash = await walletClient.sendTransaction({
        to: approvalData.approval.to,
        data: approvalData.approval.data,
        value: BigInt(approvalData.approval.value || '0'),
      });
      await publicClient.waitForTransactionReceipt({ hash });
    }
  }

  // 2. Get quote
  const quoteRes = await fetch(`${API_URL}/quote`, {
    method: 'POST',
    headers: {
      'x-api-key': API_KEY,
      'Content-Type': 'application/json',
      'x-universal-router-version': '2.0',
    },
    body: JSON.stringify({
      swapper: account.address,
      tokenIn,
      tokenOut,
      tokenInChainId: String(chainId),
      tokenOutChainId: String(chainId),
      amount,
      type: 'EXACT_INPUT',
      slippageTolerance: 0.5,
    }),
  });
  const quoteResponse = await quoteRes.json(); // Store FULL response

  if (!quoteRes.ok) {
    throw new Error(quoteResponse.detail || 'Quote failed');
  }

  // 3. Execute swap - CRITICAL: spread quote response, strip null fields
  const swapRequest = prepareSwapRequest(quoteResponse);

  const swapRes = await fetch(`${API_URL}/swap`, {
    method: 'POST',
    headers: {
      'x-api-key': API_KEY,
      'Content-Type': 'application/json',
      'x-universal-router-version': '2.0',
    },
    body: JSON.stringify(swapRequest),
  });
  const swapData = await swapRes.json();

  if (!swapRes.ok) {
    throw new Error(swapData.detail || 'Swap request failed');
  }

  // 4. Validate before broadcasting
  validateSwap(swapData.swap);

  const hash = await walletClient.sendTransaction({
    to: swapData.swap.to,
    data: swapData.swap.data,
    value: BigInt(swapData.swap.value || '0'),
  });
  return publicClient.waitForTransactionReceipt({ hash });
}

Smart Contract Integration (Solidity)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface IUniversalRouter {
    function execute(
        bytes calldata commands,
        bytes[] calldata inputs,
        uint256 deadline
    ) external payable;
}

interface IERC20 {
    function approve(address spender, uint256 amount) external returns (bool);
}

contract SwapIntegration {
    IUniversalRouter public immutable router;
    address public constant PERMIT2 = 0x000000000022D473030F116dDEE9F6B43aC78BA3;

    constructor(address _router) {
        router = IUniversalRouter(_router);
    }

    function swap(
        bytes calldata commands,
        bytes[] calldata inputs,
        uint256 deadline
    ) external payable {
        router.execute{value: msg.value}(commands, inputs, deadline);
    }

    // Approve token for Permit2 (one-time setup)
    function approveToken(address token) external {
        IERC20(token).approve(PERMIT2, type(uint256).max);
    }
}

---

Advanced Patterns

Smart Account Integration (ERC-4337)

Execute Trading API swaps through ERC-4337 smart accounts with delegation. The pattern:

1. Get swap calldata from Trading API (standard 3-step flow)
2. Wrap the calldata in a delegation redemption execution
3. Submit via bundler as a UserOperation

// After getting swap calldata from Trading API:
const { to, data, value } = swapResponse.swap;

// Wrap in delegation execution
const execution = {
  target: to, // Universal Router
  callData: data,
  value: BigInt(value),
};

// Submit via bundler
const userOpHash = await bundlerClient.sendUserOperation({
  account: delegateSmartAccount,
  calls: [
    {
      to: delegationManagerAddress,
      data: encodeFunctionData({
        abi: delegationManagerAbi,
        functionName: 'redeemDelegations',
        args: [[[signedDelegation]], [0], [[execution]]],
      }),
      value: execution.value,
    },
  ],
});

Key considerations:

• Use legacy approvals (direct to Universal Router) instead of Permit2 for smart accounts — see Approval Target
• Add 20-30% gas buffer for bundler gas estimation
• Handle bundler-specific error codes separately from standard transaction errors

See Advanced Patterns Reference for the complete implementation with types and error handling.

WETH Handling on L2s

On L2 chains (Base, Optimism, Arbitrum), swaps outputting ETH may deliver WETH instead of native ETH. Always check and unwrap after swaps:

import { parseAbi, type Address } from 'viem';

const WETH_ABI = parseAbi([
  'function balanceOf(address) view returns (uint256)',
  'function withdraw(uint256)',
]);

const WETH_ADDRESSES: Record<number, Address> = {
  1: '0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2',
  10: '0x4200000000000000000000000000000000000006',
  8453: '0x4200000000000000000000000000000000000006',
  42161: '0x82aF49447D8a07e3bd95BD0d56f35241523fBab1',
};

// After swap completes on an L2:
const wethAddress = WETH_ADDRESSES[chainId];
if (wethAddress) {
  const wethBalance = await publicClient.readContract({
    address: wethAddress,
    abi: WETH_ABI,
    functionName: 'balanceOf',
    args: [accountAddress],
  });

  if (wethBalance > 0n) {
    const hash = await walletClient.writeContract({
      address: wethAddress,
      abi: WETH_ABI,
      functionName: 'withdraw',
      args: [wethBalance],
    });
    await publicClient.waitForTransactionReceipt({ hash });
  }
}

See Advanced Patterns Reference for chain-specific WETH addresses and integration details.

Rate Limiting

The Trading API enforces rate limits (~10 requests/second per endpoint). For batch operations:

• Add 100-200ms delays between sequential API calls
• Implement exponential backoff with jitter on 429 responses
• Cache approval results — approvals rarely change between calls

// Exponential backoff for 429 responses
async function fetchWithRetry(url: string, init: RequestInit, maxRetries = 5): Promise<Response> {
  for (let attempt = 0; attempt <= maxRetries; attempt++) {
    const response = await fetch(url, init);
    if (response.status !== 429 && response.status < 500) return response;
    if (attempt === maxRetries) throw new Error(`Failed after ${maxRetries} retries`);

    const delay = Math.min(200 * Math.pow(2, attempt) + Math.random() * 100, 10000);
    await new Promise((resolve) => setTimeout(resolve, delay));
  }
  throw new Error('Unreachable');
}

See Advanced Patterns Reference for batch operation patterns and full retry implementation.

---

Key Contract Addresses

Universal Router (v4)

Addresses are per-chain. The legacy v1 address 0x3fC91A3afd70395Cd496C647d5a6CC9D4B2b7FAD is deprecated.

Chain	ID	Address
Ethereum	1	0x66a9893cc07d91d95644aedd05d03f95e1dba8af
Unichain	130	0xef740bf23acae26f6492b10de645d6b98dc8eaf3
Optimism	10	0x851116d9223fabed8e56c0e6b8ad0c31d98b3507
Base	8453	0x6ff5693b99212da76ad316178a184ab56d299b43
Arbitrum	42161	0xa51afafe0263b40edaef0df8781ea9aa03e381a3
Polygon	137	0x1095692a6237d83c6a72f3f5efedb9a670c49223
Blast	81457	0xeabbcb3e8e415306207ef514f660a3f820025be3
BNB	56	0x1906c1d672b88cd1b9ac7593301ca990f94eae07
Zora	7777777	0x3315ef7ca28db74abadc6c44570efdf06b04b020
World Chain	480	0x8ac7bee993bb44dab564ea4bc9ea67bf9eb5e743
Avalanche	43114	0x94b75331ae8d42c1b61065089b7d48fe14aa73b7
Celo	42220	0xcb695bc5d3aa22cad1e6df07801b061a05a0233a
Soneium	1868	0x4cded7edf52c8aa5259a54ec6a3ce7c6d2a455df
Ink	57073	0x112908dac86e20e7241b0927479ea3bf935d1fa0
Monad	143	0x0d97dc33264bfc1c226207428a79b26757fb9dc3

For testnet addresses, see Uniswap v4 Deployments.

Permit2

Chain	Address
All chains	0x000000000022D473030F116dDEE9F6B43aC78BA3

---

Troubleshooting

Common Issues

Issue	Solution
"Insufficient allowance"	Call /check_approval first and submit approval tx
"Quote expired"	Increase deadline or re-fetch quote
"Slippage exceeded"	Increase slippageTolerance or retry
"Insufficient liquidity"	Try smaller amount or different route
"Buffer is not defined"	Add Buffer polyfill (see Critical Implementation Notes)
On-chain revert with empty data	Validate swap.data is non-empty hex before broadcasting
"permitData must be of type object"	Strip permitData: null from request - omit field entirely
"quote does not match any of the allowed types"	Don't wrap quote in {quote: ...} - spread it into request body
Received WETH instead of ETH on L2	Check and unwrap WETH after swap (see WETH Handling on L2s)
429 Too Many Requests	Implement exponential backoff and add delays between batch requests (see Rate Limiting)
415 on OPTIONS preflight / CORS error	Set up a CORS proxy (see CORS Proxy Configuration in Browser Environment Setup)
walletClient is undefined when wallet is connected	Use getWalletClient() from @wagmi/core instead of the useWalletClient() hook (see wagmi v2 Integration Pitfalls)
"Please provide a chain with the chain argument"	Pass chainId to getWalletClient(config, { chainId })
Chain mismatch error on swap	Call switchChain() before getWalletClient() (see wagmi v2 Integration Pitfalls)

API Validation Errors (400)

Error Message	Cause	Fix
"permitData" must be of type object	Sending permitData: null	Omit the field entirely when null
"quote" does not match any of the allowed types	Wrapping quote in {quote: quoteResponse}	Spread quote response: {...quoteResponse}
signature and permitData must both be present	Including only one Permit2 field	Include both or neither

API Error Codes

Code	Meaning
400	Invalid request parameters (see validation errors above)
401	Invalid or missing API key
404	No route found for pair
429	Rate limit exceeded
500	API error - implement exponential backoff retry

Pre-Broadcast Checklist

Before sending a swap transaction to the blockchain:

1. Verify swap.data is non-empty hex (not '', not '0x')
2. Verify addresses - swap.to and swap.from are valid
3. Check quote freshness - Re-fetch if older than 30 seconds
4. Validate gas - Apply 10-20% buffer to estimates
5. Confirm balance - User has sufficient token balance

---

Additional Resources

• Universal Router GitHub
• Uniswap Docs
• SDK Monorepo
• Permit2 Patterns