Harvest Function

A harvest function is a smart contract method, typically named harvest() or claimRewards(), that automates the process of collecting accrued yield, fees, or other incentives from a liquidity pool or staking position. When called, it triggers the protocol to calculate and transfer the caller's accumulated rewards, often in the form of governance tokens or a share of trading fees. This function is a core component of yield farming and automated vault strategies, enabling the efficient compounding of returns. The execution of a harvest function often requires paying a gas fee on the underlying blockchain network.

The primary purpose of a harvest function is to convert illiquid, pending rewards into liquid, transferable assets that can be reinvested or withdrawn. In complex DeFi strategies, such as those used by yield aggregators like Yearn Finance, the function not only claims rewards but also automatically swaps them for more of the base asset and redeposits them into the protocol. This automated reinvestment, or compounding, is critical for maximizing Annual Percentage Yield (APY). The frequency of calling the harvest function can significantly impact overall returns due to compounding effects and gas cost considerations.

From a technical perspective, a harvest function call initiates a state-changing transaction on-chain. It interacts with the reward distribution logic of a master chef contract, gauge system, or liquidity mining program. The function must often handle multiple token transfers and potentially complex swap operations via decentralized exchanges. For users, the decision of when to harvest involves a cost-benefit analysis: the value of the claimed rewards must outweigh the gas cost of the transaction. This has led to the development of gas-efficient harvesting strategies and keeper networks that batch transactions for optimal efficiency.

Harvest functions are integral to the security and economic design of DeFi protocols. They are a common attack vector, as malicious actors may attempt to manipulate reward calculations through flash loans or other means in so-called "harvest" attacks. Furthermore, the function's logic determines the fairness and timeliness of reward distribution. Protocols must carefully design these functions to prevent front-running and ensure that rewards are accurately accounted for between the last harvest and the current block timestamp, a concept known as reward accrual.

A harvest function is a smart contract method that automates the process of claiming accrued rewards—such as governance tokens, trading fees, or liquidity provider incentives—and reinvesting them to compound returns. In decentralized finance (DeFi) protocols, particularly in yield farming vaults and automated market makers (AMMs), this function is triggered to collect pending earnings from a liquidity pool or staking contract. The core action involves calling an external protocol's reward distribution contract, receiving the tokens, and then often swapping a portion for the underlying assets to redeposit, increasing the user's principal stake. This automation is crucial for maximizing annual percentage yield (APY) by frequently compounding earnings.

The function's execution typically involves several key steps managed within a single transaction. First, it checks for any unclaimed rewards using a function like pendingRewards(). It then calls the reward contract's claim function, such as getReward(). Following the claim, the function often performs a token swap via a decentralized exchange to convert rewards into the base assets of the vault. Finally, it redeposits the newly acquired liquidity, minting additional LP tokens or increasing the user's share in the vault. This entire sequence is often permissionless, allowing any user or keeper network to trigger it, with the caller usually receiving a small portion of the harvested rewards as a gas fee incentive.

Harvest functions are central to the architecture of yield aggregators like Yearn Finance and Beefy Finance. These protocols use sophisticated strategies where the harvest function is part of a larger strategy contract. The strategy's logic determines optimal timing for harvesting (e.g., based on reward thresholds or gas costs) and the best routing for token swaps. Because calling the function requires paying gas fees on the underlying blockchain, protocols must balance harvest frequency against transaction costs to ensure net profitability for users. Inefficient or poorly secured harvest functions can be vulnerable to MEV (Maximal Extractable Value) exploits, such as sandwich attacks during the swap phase.

For developers and auditors, understanding the harvest flow is critical. The function must handle slippage during swaps, manage multiple token approvals securely, and correctly account for fees. A common pattern is the use of a harvest caller reward, a small percentage of the harvested tokens paid to the Ethereum transaction sender (the "harvester") to incentivize timely execution. This creates a decentralized keeper economy. The function's efficiency directly impacts the vault's advertised APY, as delays or high gas costs can lead to yield leakage. Consequently, optimizing gas usage and securing the swap path are primary concerns in strategy design.

The harvest function is a public or external method in a DeFi vault or strategy contract that triggers the conversion of accrued, non-liquid rewards into a withdrawable asset, typically the vault's underlying token. Its primary purpose is to realize yield by claiming incentives from external protocols (like staking rewards, trading fees, or liquidity provider tokens) and swapping them into the base asset, thereby increasing the value of each user's share. A well-optimized harvest() is critical for capital efficiency, as it manages gas costs and minimizes slippage during asset conversion.

A typical harvest() implementation involves several key steps executed in a single transaction. First, it calls a reward claim function on an external protocol (e.g., MasterChef.deposit(0)). Next, it sells the received reward tokens for the desired base asset via a decentralized exchange router. It then takes a performance fee, often sent to a treasury or fee recipient, before minting new vault shares or reinvesting the remaining proceeds. Developers must incorporate access control (like onlyOwner or onlyKeeper) and profit threshold checks to prevent economically inefficient harvests that cost more in gas than the value they generate.

Common Code Patterns

Key Solidity patterns include using a harvestOnDeposit boolean flag for automatic compounding, emitting a Harvest event for off-chain monitoring, and employing keeper networks like Chainlink Automation or Gelato to trigger the function when gas is low and profit is sufficient. The function must also handle multiple reward tokens, often looping through an array of reward contracts, and robustly manage the approval and swapping logic via interfaces to DEXes like Uniswap or 1inch.

Security considerations for the harvest function are paramount. It is a frequent attack vector for reentrancy and flash loan manipulation, especially during the swap step. Mitigations include using the checks-effects-interactions pattern, implementing slippage protection with minimum output parameters, and shielding the function from being called by arbitrary external actors. Audits often focus on the harvest flow's resilience to price oracle manipulation and its interaction with external, non-upgradeable contracts.

From a user's perspective, a successful harvest increases the price per share of the vault token. The function's efficiency directly impacts the Annual Percentage Yield (APY) displayed by the protocol. Transparent vaults will expose harvest history and fee structures on-chain, allowing analysts to verify performance. The gas cost of harvests, often borne by the keeper or protocol treasury, is a key operational metric for evaluating a strategy's long-term viability.