A swap surplus is the result of a unique market dynamic known as "positive slippage". In other words, if a token pair's rate changes positively from the time of submitting a transaction to the time of its confirmation, an excess aka "surplus" amount of tokens is returned (relative to the guaranteed amount entered in the swap window).

***Please note: A swap surplus is not a trading fee, and does not occur with every swap.
​

**** With the 1inch router v4 and v5 update, optimized routers such as unoswap, uniswapV3Swap and clipperSwap along with any fillOrder function will never take a surplus, only the swap function.

With [1IP-28] Discontinue Swap Surplus Collection, the community has decided to disconnect the collection of the swap surplus by the 1inch DAO. The primary purpose was to ensure the stable and consistent growth of the 1inch Network. 1inch Network Interface is dependent on the third-party API providers, which manage the infrastructure behind the swap functionality. Following the decision of the DAO to discontinue swap surplus collection, the operational set up of such third-party API providers imposed the collection of slippage as a fee for swap services provision and maintenance. Currently, the surplus amount is retained by these providers as a service fee.

Want to participate in 1inch Network governance? Check out this guide.