​
Overview

Aave V3 is a decentralized liquidity protocol where users can supply assets to earn yield and withdraw them at any time. In this guide, we’ll walk through using Turnkey to sign common Aave transactions on Base Mainnet with USDC: approving the Aave Pool to spend USDC, supplying USDC, reading the resulting aUSDC balance, and withdrawing USDC back to the wallet. We’ll also demonstrate Turnkey’s policy engine by restricting signing operations to the specific Aave Pool and USDC contracts on Base. A working example can be found here.

​
Getting started

The first step is to set up your Turnkey organization and account. By following the Quickstart guide, you should have:
  • A root user with a public/private API key pair within the Turnkey parent organization
  • An organization ID
The next step is to create another non-root user within the organization with a different API key and exclude it from the root quorum. If you would like to remove a root user from root quorum, you can do this from the Turnkey dashboard or API. Here’s a simple script that shows how to update the root quorum using @turnkey/sdk-server. Finally, make sure you have a wallet with an Ethereum account created within this organization and have it funded with some ETH and USDC on Base Mainnet.

​
Setting up the policy for the non-root user

Now we’ll use a non-root Turnkey user to sign Aave transactions, while ensuring it can only interact with the USDC token and the Aave Pool contracts on Base mainnet. We’ll define a new API client that would use the organization root user to create the required policy: 
import { Turnkey } from "@turnkey/sdk-server";
import { AaveV3Base } from "@bgd-labs/aave-address-book";

const turnkeyClient = new Turnkey({
    apiBaseUrl: "https://api.turnkey.com",
    apiPrivateKey: process.env.TURNKEY_API_PRIVATE_KEY!,
    apiPublicKey: process.env.TURNKEY_API_PUBLIC_KEY!,
    defaultOrganizationId: process.env.TURNKEY_ORGANIZATION_ID!,
}).apiClient();

const userId = "<The id of the non-root user that you'll be using to sign the Aave related transactions>";

// Pull addresses from Aave Address Book (Base)
const USDC_ADDRESS = AaveV3Base.ASSETS.USDC.UNDERLYING;
const AAVE_POOL = AaveV3Base.POOL;

const policyName =
    "Allow API key user to only sign txs to Aave Pool and USDC";
    const effect = "EFFECT_ALLOW";
    const consensus = `approvers.any(user, user.id == '${userId}')`;
    const condition = `eth.tx.to in ['${USDC_ADDRESS}', '${AAVE_POOL}']`;
    const notes = "";

const { policyId } = await turnkeyClient.createPolicy({
    policyName,
    condition,
    consensus,
    effect,
    notes,
});

​
Set up the Turnkey signer

We’ll be using @turnkey/viem to create a Turnkey custom signer which implements the signing APIs expected by the viem client. Notice that the Turnkey API client is going to use the non-root user API key now as using a root user will bypass the policy engine evaluation. 
import { base } from "viem/chains";
import { createAccount } from "@turnkey/viem";
import { Turnkey as TurnkeyServerSDK } from "@turnkey/sdk-server";
import {
    parseAbi,
    erc20Abi,
    parseUnits,
    createWalletClient,
    http,
    createPublicClient,
    type Account,
} from "viem";

const turnkeyClient = new TurnkeyServerSDK({
    apiBaseUrl: process.env.TURNKEY_BASE_URL!,
    apiPrivateKey: process.env.NONROOT_API_PRIVATE_KEY!,
    apiPublicKey: process.env.NONROOT_API_PUBLIC_KEY!,
    defaultOrganizationId: process.env.TURNKEY_ORGANIZATION_ID!,
});

const turnkeyAccount = await createAccount({
    client: turnkeyClient.apiClient(),
    organizationId: process.env.TURNKEY_ORGANIZATION_ID!,
    signWith: process.env.SIGN_WITH!,
});

const client = createWalletClient({
    account: turnkeyAccount as Account,
    chain: base,
    transport: http(
      `https://base-mainnet.infura.io/v3/${process.env.INFURA_API_KEY!}`,
    ),
});

// Use the standard viem client for non-signing operations
const publicClient = createPublicClient({
    transport: http(
      `https://base-mainnet.infura.io/v3/${process.env.INFURA_API_KEY!}`,
    ),
    chain: base,
});

​
Approve the Aave Pool contract to spend USDC

import { AaveV3Base } from "@bgd-labs/aave-address-book";

// Pull addresses from Aave Address Book (Base)
const USDC_ADDRESS = AaveV3Base.ASSETS.USDC.UNDERLYING;
const AAVE_POOL = AaveV3Base.POOL;

// Approve Pool to spend 10 USDC
const { request: approveReq } = await publicClient.simulateContract({
    address: USDC_ADDRESS as `0x${string}`,
    abi: erc20Abi,
    functionName: "approve",
    args: [AAVE_POOL as `0x${string}`, parseUnits("10", 6)], // USDC has 6 decimals
    account: walletClient.account,
});

const approveHash = await walletClient.writeContract(approveReq);
const receiptApprove = await publicClient.waitForTransactionReceipt({
    hash: approveHash,
});

console.log("Approve transaction:", `https://basescan.org/tx/${approveHash}`, receiptApprove.status);

​
Deposit USDC into Aave Pool

// Supply 0.5 USDC
const poolAbi = parseAbi([
    "function supply(address asset,uint256 amount,address onBehalfOf,uint16 referralCode)",
]);

const { request: supplyReq } = await publicClient.simulateContract({
    address: AAVE_POOL as `0x${string}`,
    abi: poolAbi,
    functionName: "supply",
    args: [
      USDC_ADDRESS,
      parseUnits("0.5", 6),
      (walletClient.account as Account).address,
      0,
    ],
    account: walletClient.account as Account,
});

// The USDC transfer path & proxy layers sometimes make estimateGas under-shoot
// Adding a buffer to avoid occasional out-of-gas errors
const gas = await publicClient.estimateContractGas({
    address: AAVE_POOL as `0x${string}`,
    abi: poolAbi,
    functionName: "supply",
    args: [
      USDC_ADDRESS,
      parseUnits("0.5", 6),
      (walletClient.account as Account).address,
      0,
    ],
    account: walletClient.account as Account,
});

const gasWithBuffer = (gas * 130n) / 100n;

const supplyHash = await walletClient.writeContract({
    ...supplyReq,
    gas: gasWithBuffer,
});
const receiptSupply = await publicClient.waitForTransactionReceipt({
    hash: supplyHash,
});

console.log("Supply transaction:", `https://basescan.org/tx/${supplyHash}`, receiptSupply.status);

​
Read the aUSDC balance

When you supply USDC into Aave, you receive aUSDC (an ERC-20 “receipt” token), aUSDC.balanceOf(user) already shows your live underlying balance (principal + interest), because it’s auto-scaled by Aave’s liquidity index. 
// Pull aUSDC address from Aave Address Book (Base)
const aUSDC = AaveV3Base.ASSETS.USDC.A_TOKEN;

// Read aUSDC balance which tracks your supplied principal + interest
const erc20ReadAbi = parseAbi([
    "function balanceOf(address) view returns (uint256)",
    "function decimals() view returns (uint8)",
]);

const [decimals, rawBal] = await Promise.all([
    publicClient.readContract({
        address: aUSDC as `0x${string}`,
        abi: erc20ReadAbi,
        functionName: "decimals",
}),
    publicClient.readContract({
        address: aUSDC as `0x${string}`,
        abi: erc20ReadAbi,
        functionName: "balanceOf",
        args: [(walletClient.account as Account).address],
    }),
]);

console.log("aUSDC balance:", formatUnits(rawBal, Number(decimals)));

​
Withdraw USDC from the Aave Pool

On withdraw, Aave burns your aUSDC and sends you back USDC: 
// Pull addresses from Aave Address Book (Base)
const USDC_ADDRESS = AaveV3Base.ASSETS.USDC.UNDERLYING;
const AAVE_POOL = AaveV3Base.POOL;

const poolReadWriteAbi = parseAbi([
    "function withdraw(address asset,uint256 amount,address to) returns (uint256)",
]);

// withdraw 0.1 USDC
const { request: withdrawReq } = await publicClient.simulateContract({
    address: AAVE_POOL as `0x${string}`,
    abi: poolReadWriteAbi,
    functionName: "withdraw",
    args: [
      USDC_ADDRESS,
      parseUnits("0.1", 6),
      (walletClient.account as Account).address,
    ],
    account: walletClient.account as Account,
});

// the USDC transfer path & proxy layers sometimes make estimateGas under-shoot
// adding a buffer to avoid occasional out-of-gas errors
const gas = await publicClient.estimateContractGas({
    address: AAVE_POOL as `0x${string}`,
    abi: poolReadWriteAbi,
    functionName: "withdraw",
    args: [
      USDC_ADDRESS,
      parseUnits("0.1", 6),
      (walletClient.account as Account).address,
    ],
    account: walletClient.account as Account,
});

const gasWithBuffer = (gas * 130n) / 100n;

const withdrawHash = await walletClient.writeContract({
    ...withdrawReq,
    gas: gasWithBuffer,
});
const receiptWithdraw = await publicClient.waitForTransactionReceipt({
    hash: withdrawHash,
});

console.log("Withdraw transaction:", `https://basescan.org/tx/${withdrawHash}`, receiptWithdraw.status);