> For the complete documentation index, see [llms.txt](https://docs.bsc.lista.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.bsc.lista.org/for-developer/collateral-debt-position/flash-loan.md).

# Flash Loan

> **The lisUSD CDP is being wound down** — see [Mechanics](/for-developer/collateral-debt-position/mechanics.md) for the live gates. Flash minting still works, so an existing integration keeps running. New integrations should use [Lista Lending](/for-developer/lista-lending.md), whose `Moolah.flashLoan` is in the [contract reference](/for-developer/lista-lending/contract-reference.md).

`flash.sol` is a standard **ERC-3156** flash *minter*: it mints lisUSD to your contract, calls you back, and burns it again at the end of the same transaction. There is no collateral and no position — if the loan plus fee is not returned before the call ends, everything reverts.

**Mainnet lender:** [`0x64d94e715B6c03A5D8ebc6B2144fcef278EC6aAa`](https://bscscan.com/address/0x64d94e715B6c03A5D8ebc6B2144fcef278EC6aAa) (`FlashMinter` on the [Smart Contract](/for-developer/collateral-debt-position/smart-contract.md) page)

## The interface

```solidity
function maxFlashLoan(address token) external view returns (uint256);
function flashFee(address token, uint256 amount) external view returns (uint256);
function flashLoan(IERC3156FlashBorrower receiver, address token, uint256 amount, bytes calldata data) external returns (bool);
```

* **`token` must be lisUSD.** Anything else reverts `Flash/token-unsupported`; `maxFlashLoan` returns `0` for it.
* `flashFee` applies the `toll` rate and returns an 18-decimal wad.
* `data` is forwarded verbatim to your callback — it is the only channel for passing state in.

## Writing the borrower

Implement [`IERC3156FlashBorrower`](https://github.com/lista-dao/lista-dao-contracts/blob/master/contracts/interfaces/IERC3156FlashBorrower.sol) and do three things in `onFlashLoan`: reject callers that are not the lender, run your logic, and approve the lender for `amount + fee` before returning `keccak256("ERC3156FlashBorrower.onFlashLoan")`.

```solidity
pragma solidity ^0.8.10;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "../interfaces/IERC3156FlashBorrower.sol";
import "../interfaces/IERC3156FlashLender.sol";

contract FlashBorrower is IERC3156FlashBorrower {
    IERC3156FlashLender lender;

    constructor(IERC3156FlashLender lender_) {
        lender = lender_;
    }

    function flashBorrow(address token, uint256 amount) public {
        bytes memory data = abi.encode(/* your parameters */);
        lender.flashLoan(this, token, amount, data);
    }

    function onFlashLoan(
        address initiator,
        address token,
        uint256 amount,
        uint256 fee,
        bytes calldata data
    ) external override returns (bytes32) {
        require(msg.sender == address(lender), "untrusted lender");
        require(initiator == address(this), "untrusted initiator");

        // ... your logic. It must leave this contract holding `amount + fee`.

        IERC20(token).approve(address(lender), amount + fee);
        return keccak256("ERC3156FlashBorrower.onFlashLoan");
    }
}
```

Return anything other than that hash and the call reverts `Flash/callback-failed`. Skip the approve, or end the callback short of `amount + fee`, and the lender's `transferFrom` reverts `LisUSD/insufficient-allowance` or `LisUSD/insufficient-balance`.

> **Flash loans cannot bid on CDP liquidations.** This fork makes the auction buy side permissioned — `Clipper.take` / `redo` are `auth` and `Interaction.buyFromAuction` is whitelisted. The liquidation surface that *is* open to third parties is on Lista Lending: see [Liquidator Integration](/for-developer/lista-lending/liquidator-integration.md).

## Source

* [`flash.sol`](https://github.com/lista-dao/lista-dao-contracts/blob/master/contracts/flash.sol) — the lender, including the `max` ceiling and `toll` rate.
* [`flashBorrower.sol`](https://github.com/lista-dao/lista-dao-contracts/blob/master/contracts/mock/flashBorrower.sol) — a fuller borrower, with branching on the decoded `data`.
* [`flash.test.js`](https://github.com/lista-dao/lista-dao-contracts/blob/master/test/flash.test.js) — a worked end-to-end example.


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