The Tool Desk
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How does a flash loan work?
A flash loan is not an ordinary loan that remains open for hours or days. Borrowing, using the funds, and repaying them are linked in one transaction or flash call. ERC-3156 describes a lender contract lending assets to a borrower contract on the condition that the assets and any optional fee are returned before the transaction ends. ERC-7399 similarly describes lending for the duration of a lender’s flash call, with repayment before that call concludes. These are distinct interfaces, not a guarantee that every protocol implements flash loans the same way.
In a typical implementation, the sequence is:
- Request: A borrower contract asks a lending contract for an asset and amount.
- Transfer and callback: The lender transfers the assets and invokes the borrower’s callback or corresponding execution logic.
- Use the funds: The borrower contract carries out its planned operations, such as trading between venues or changing a lending position.
- Check repayment: Before execution ends, the lender checks for the principal and any applicable fee.
- Complete or revert: If the repayment condition is met, the transaction completes. If it is not, execution reverts; Ethereum.org explains this as the transaction being reversed as if it had not happened.
Liquidity commonly comes from pools, but the assets available and the rules for borrowing depend on the protocol and its implementation. “Uncollateralized” therefore means that the borrower does not need to lock up collateral for a conventional loan term; it does not mean the assets can be kept without repayment.
What are flash loans used for?
Arbitrage across markets
A trader may borrow an asset, buy it on one venue and sell it on another where the price differs, then repay the loan within the same transaction. The strategy must account for the loan fee, trading costs, slippage, and whether both trades can execute at usable prices. A price difference alone does not guarantee a profit.
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Refinancing or changing collateral
Flash-loan integrations can help combine position changes that would otherwise require separate steps. Aave identifies operations such as collateral swaps and debt-related position changes among the uses of its integrations. The exact options depend on the protocol, supported assets, and implementation.
Liquidations
Flash loans can supply funds for a liquidation operation, where a position that falls below a protocol’s requirements is closed or partially closed under that protocol’s rules. Whether a particular operation is supported—and whether it is economically worthwhile—depends on the protocol and the transaction’s costs and conditions.
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Flash loan vs. Uniswap flash swap
A Uniswap v2 flash swap is a related, protocol-specific way to withdraw assets temporarily from a pool and execute other logic before the transaction finishes. The caller must satisfy Uniswap’s final settlement conditions, which can involve repayment or the relevant swap outcome. Both mechanisms let a transaction use assets temporarily, but a “flash swap” refers to Uniswap’s mechanism; it is not a universal synonym for every flash-loan interface.
Advantages and limitations
| Aspect | Potential benefit | Limitation or risk |
|---|---|---|
| Collateral | No collateral is needed for the brief loan when the contract’s conditions are met. | The borrower must still return principal and any applicable fee before the transaction ends. |
| Liquidity | Funds can support a multi-step operation that might otherwise require capital to be held in advance. | Available assets, liquidity, and protocol rules constrain the operation. |
| Execution | Trades or position changes can be combined atomically. | The entire strategy must be encoded and execute successfully in one transaction; a failed repayment condition reverts it. |
| Lender exposure | Contract-enforced repayment prevents an under-repaid transaction from completing as a successful loan. | This does not establish that the borrower’s strategy, other contracts, or the wider protocol are safe. |
Are flash loans safe?
Atomic repayment limits a lender’s credit risk from a borrower completing a transaction without returning the required amount. It does not make the overall strategy risk-free. A smart-contract vulnerability, an unreliable price or oracle design, slippage, execution costs, or another protocol’s failure can make an operation fail or cause harm elsewhere. Aave’s risk guidance discusses oracle risks and mitigations, and its FAQ states, “No protocol can be considered entirely risk free.” That is a general warning, not a finding that every flash loan is unsafe.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What determines fees and available liquidity?
Fees, supported assets, network availability, and maximum usable liquidity are implementation-specific and can change. ERC-3156 and ERC-7399 define different interfaces; they do not set one universal flash-loan fee or asset list. Aave’s FAQ reports fee percentages for particular Aave V2 and V3 contexts, but those version-specific values should not be treated as universal or necessarily current rates. For an actual transaction, check the relevant protocol’s live documentation or configuration for the network and version you intend to use.
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