> ## Documentation Index
> Fetch the complete documentation index at: https://docs.barker.money/llms.txt
> Use this file to discover all available pages before exploring further.

# Smart Contracts

> BarkerEngine ABI for headless integrations. ERC-4626 standard.

If you're integrating headless (your own UI calling the engine directly), this page is the on-chain surface you'll need. Embed integrations don't need this page — the iframe handles everything.

## BarkerEngine is ERC-4626

`BarkerEngine` is a **standard ERC-4626 vault**. If your stack already has ERC-4626 helpers (Wagmi / viem / OpenZeppelin SDK), they work out of the box — `engine.balanceOf(user)` returns shares, `engine.convertToAssets(shares)` returns asset value.

The engine address is **per-partner**, returned by `GET /api/partner/products` as `engine_contract_address`. There is no static "Barker engine per chain" — every partner has their own.

## Functions you'll call

```solidity theme={null}
// Deposit `assets`, mint shares to `receiver`. Returns shares minted.
function deposit(uint256 assets, address receiver) external returns (uint256 shares);

// Burn `shares` from `owner`, send assets to `receiver`.
// owner ≠ caller requires ERC-20 allowance.
function redeem(uint256 shares, address receiver, address owner) external returns (uint256 assets);

// Burn enough shares from `owner` to send exactly `assets` to `receiver`.
function withdraw(uint256 assets, address receiver, address owner) external returns (uint256 shares);

// Show "you'll receive" copy before signing
function previewDeposit(uint256 assets) external view returns (uint256 shares);
function previewRedeem(uint256 shares) external view returns (uint256 assets);

// User's position (in shares + asset-denominated value)
function balanceOf(address user) external view returns (uint256 shares);
function convertToAssets(uint256 shares) external view returns (uint256 assets);

// Underlying asset (e.g. USDC) the engine wraps
function asset() external view returns (address);
```

### Minimal ABI fragment (copy-paste)

```json theme={null}
[
  { "type":"function","name":"deposit","stateMutability":"nonpayable","inputs":[{"name":"assets","type":"uint256"},{"name":"receiver","type":"address"}],"outputs":[{"type":"uint256"}] },
  { "type":"function","name":"redeem","stateMutability":"nonpayable","inputs":[{"name":"shares","type":"uint256"},{"name":"receiver","type":"address"},{"name":"owner","type":"address"}],"outputs":[{"type":"uint256"}] },
  { "type":"function","name":"withdraw","stateMutability":"nonpayable","inputs":[{"name":"assets","type":"uint256"},{"name":"receiver","type":"address"},{"name":"owner","type":"address"}],"outputs":[{"type":"uint256"}] },
  { "type":"function","name":"balanceOf","stateMutability":"view","inputs":[{"name":"account","type":"address"}],"outputs":[{"type":"uint256"}] },
  { "type":"function","name":"convertToAssets","stateMutability":"view","inputs":[{"name":"shares","type":"uint256"}],"outputs":[{"type":"uint256"}] },
  { "type":"function","name":"previewDeposit","stateMutability":"view","inputs":[{"name":"assets","type":"uint256"}],"outputs":[{"type":"uint256"}] },
  { "type":"function","name":"previewRedeem","stateMutability":"view","inputs":[{"name":"shares","type":"uint256"}],"outputs":[{"type":"uint256"}] },
  { "type":"function","name":"asset","stateMutability":"view","inputs":[],"outputs":[{"type":"address"}] }
]
```

For the full ABI, pull the verified contract from the chain explorer using `engine_contract_address`.

## Permit (optional, saves one tx)

If the underlying token (USDC, USDT, DAI…) supports **EIP-2612 `permit`**, you can swap the `approve` transaction for a signed message — one tx instead of two.

```ts theme={null}
const signature = await wallet.signTypedData({
  domain: { name: "USD Coin", version: "2", chainId, verifyingContract: usdcAddr },
  types: {
    Permit: [
      { name: "owner", type: "address" },
      { name: "spender", type: "address" },
      { name: "value", type: "uint256" },
      { name: "nonce", type: "uint256" },
      { name: "deadline", type: "uint256" },
    ],
  },
  primaryType: "Permit",
  message: { owner: user, spender: engineAddr, value: amount, nonce, deadline },
});
// then submit one tx that calls permit() + deposit() atomically
// (use a multicall helper from your wallet stack)
```

BarkerEngine doesn't bundle `permit + deposit` in one function. Use multicall (Wagmi/viem) or a thin partner-owned router contract.

## Safety properties

* **Non-upgradeable** per deployment — once deployed, bytecode never changes.
* **No admin withdrawal path** — funds can only exit via the user's `redeem` / `withdraw`.

## What's next

* [Headless API integration](/integration-api) — end-to-end deposit/withdraw flow
* [Compliance & risk](/compliance) — what to disclose to your users