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Blockradar’s core APIs cover the most common fintech flows: stablecoin deposits, payouts, swaps, and settlements. But what if you need to go beyond what’s natively supported? What if you need to interact with a DeFi protocol, your own token contract, or an onchain service unique to your business? That’s where the Smart Contract API comes in. It lets you interact with any smart contract directly from your Blockradar wallet without managing RPC endpoints, signing flows, or node infrastructure yourself. With it, you can:
  • Connect to DeFi protocols for yield or liquidity management
  • Automate stablecoin lending, staking, or treasury operations
  • Integrate tokenized real-world assets (RWAs) into your product flows
  • Power programmable settlements, payouts, or compliance checks using onchain logic
  • Extend your platform with custom tokens, reward systems, or fee rules
The Smart Contract API gives you three capabilities: read contract state, write to contracts, and estimate gas before execution.

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Prerequisites

1

API Key

Get your API key from the Blockradar Dashboard. Navigate to Developers to generate one.
2

Wallet Created

You’ll need a walletId for all smart contract operations.
3

Contract ABI

You need the ABI (Application Binary Interface) for the contract function you want to call. This is publicly available for verified contracts on block explorers like Etherscan.
4

Contract Address

The onchain address of the smart contract you want to interact with.

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How It Works

In this guide, we’ll walk through a real example: swapping an unsupported token into USDT using Uniswap V2. The same pattern applies to any smart contract interaction. The flow:
  1. Read the token balance from the contract
  2. Approve Uniswap to spend the token
  3. Estimate the network fee for the swap
  4. Execute the swap
  5. Confirm via webhook

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Step 1: Understand the Contract

We’ll use Uniswap V2 Router’s swapExactTokensForTokens method. Method signature: 
function swapExactTokensForTokens(
  uint amountIn,
  uint amountOutMin,
  address[] calldata path,
  address to,
  uint deadline
) external returns (uint[] memory amounts);
ParameterDescription
amountInAmount of the token you want to sell
amountOutMinMinimum amount of output token you’ll accept (slippage protection)
pathToken addresses for the swap route
toYour Blockradar wallet address (receives the output token)
deadlineUnix timestamp after which the transaction expires

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Step 2: Read the Token Balance

Before swapping, check how much of the token your wallet holds using contracts/read. 
const res = await fetch(
  'https://api.blockradar.co/v1/wallets/{walletId}/contracts/read',
  {
    method: 'POST',
    headers: {
      'x-api-key': '<api-key>',
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({
      abi: [{
        "constant": true,
        "inputs": [{ "name": "_owner", "type": "address" }],
        "name": "balanceOf",
        "outputs": [{ "name": "balance", "type": "uint256" }],
        "stateMutability": "view",
        "type": "function"
      }],
      address: "0xUnsupportedTokenAddress",
      method: "balanceOf",
      parameters: ["0xYourWalletAddress"]
    })
  }
).then(r => r.json());

// Response:
// { "data": "1000000000000000000", "message": "Contract read successfully" }
ParameterDescription
abiThe ABI of the contract function you’re calling
addressThe token’s smart contract address
methodThe function name to call
parametersArguments for the function
Use the balance returned here as the amountIn value for your swap call.

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Step 3: Approve the Contract to Spend Your Token

Before Uniswap can move your token, you need to grant it permission using the token contract’s approve function. 
await fetch(
  'https://api.blockradar.co/v1/wallets/{walletId}/contracts/write',
  {
    method: 'POST',
    headers: {
      'x-api-key': '<api-key>',
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({
      abi: [{
        "inputs": [
          { "name": "_spender", "type": "address" },
          { "name": "_value", "type": "uint256" }
        ],
        "name": "approve",
        "outputs": [],
        "stateMutability": "nonpayable",
        "type": "function"
      }],
      address: "0xUnsupportedTokenAddress",
      method: "approve",
      parameters: ["0xUniswapRouterAddress", "1000000000000000000"]
    })
  }
);
Do not proceed to the swap immediately after submitting the approval. Wait for the custom-smart-contract.success webhook confirming the approval transaction was mined before moving to the next step.
Approval webhook payload: 
{
  "event": "custom-smart-contract.success",
  "data": {
    "status": "SUCCESS",
    "type": "CUSTOM_SMART_CONTRACT",
    "hash": "0xfedcba098765...",
    "tokenAddress": "0xUnsupportedTokenAddress"
  }
}

​
Step 4: Estimate the Network Fee

Before executing the swap, estimate the gas cost to ensure your wallet has enough native token to cover it. 
const fee = await fetch(
  'https://api.blockradar.co/v1/wallets/{walletId}/contracts/network-fee',
  {
    method: 'POST',
    headers: {
      'x-api-key': '<api-key>',
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({
      abi: [ /* ABI of swapExactTokensForTokens */ ],
      address: "0xUniswapRouterAddress",
      method: "swapExactTokensForTokens",
      parameters: [
        "1000000000000000000",
        "990000",
        ["0xUnsupportedTokenAddress", "0xUSDTAddress"],
        "0xYourWalletAddress",
        Math.floor(Date.now() / 1000) + 60 * 20
      ]
    })
  }
).then(r => r.json());

// Response:
// { "data": { "balance": "1.0123", "fee": "0.00422" } }

​
Step 5: Execute the Swap

Once the approval is confirmed and you’ve reviewed the fee, execute the swap. 
const swap = await fetch(
  'https://api.blockradar.co/v1/wallets/{walletId}/contracts/write',
  {
    method: 'POST',
    headers: {
      'x-api-key': '<api-key>',
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({
      abi: [ /* ABI of swapExactTokensForTokens */ ],
      address: "0xUniswapRouterAddress",
      method: "swapExactTokensForTokens",
      parameters: [
        "1000000000000000000",
        "990000",
        ["0xUnsupportedTokenAddress", "0xUSDTAddress"],
        "0xYourWalletAddress",
        Math.floor(Date.now() / 1000) + 60 * 20
      ]
    })
  }
).then(r => r.json());

// Response:
// { "message": "Contract write initiated successfully", "data": { "status": "PENDING" } }
Listen for the webhook to confirm completion: 
{
  "event": "custom-smart-contract.success",
  "data": {
    "status": "SUCCESS",
    "type": "CUSTOM_SMART_CONTRACT",
    "hash": "0xfedcba098765...",
    "tokenAddress": "0xUniswapRouterAddress"
  }
}

​
Webhook Events

EventDescription
custom-smart-contract.successContract write confirmed on-chain
custom-smart-contract.failedContract write failed

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Other Endpoints

EndpointDescription
POST /v1/wallets/{walletId}/contracts/readRead state from any contract
POST /v1/wallets/{walletId}/contracts/writeExecute any contract function
POST /v1/wallets/{walletId}/contracts/network-feeEstimate gas before execution
POST /v1/wallets/{walletId}/contracts/write/signSign a transaction without broadcasting

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Security Notes

  • All transactions are signed and broadcast through Blockradar’s infrastructure — no need to manage private keys or RPC nodes
  • Native AML and compliance checks apply where possible
  • Every contract interaction is a real on-chain transaction and requires gas in the native token of the chain

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API Reference

EndpointDescription
Master Wallet ReadRead contract state from master wallet
Master Wallet WriteExecute contract function from master wallet
Master Wallet Network FeeEstimate gas from master wallet
Child Address ReadRead contract state from child address
Child Address WriteExecute contract function from child address
Child Address Network FeeEstimate gas from child address