All transactions on the blockchain require a network cost to be processed. On UniswapX swaps, the filler pays most network costs on the swapper's behalf.
Fillers factor the network cost into the swap price and can lower transaction costs by batching multiple orders or filling orders from their own private holdings.
Since swappers don’t have to pay network costs, they don’t need to own/hold the network's native token (ETH, MATIC, etc.) to swap and they don’t have to pay anything for a failed swap.
Swappers still need to pay the network costs under specific circumstances:
- Initial onchain token approvals incur a network cost for the swapper
- Native network tokens (ETH, MATIC, etc.) must be wrapped before they can be swapped on UniswapX which incurs a network cost for the swapper.