💡 The Plain-English Definition
Fee sniping is a theoretical attack where miners deliberately reorganise recent blocks — essentially rewriting the last few blocks of the chain — to grab the high transaction fees in those blocks for themselves, instead of honestly building on the chain tip, the newest block.
🤔 But Why Though?
At any moment a miner has two options. One: honestly build a new block on top of the current chain tip and earn whatever fees are sitting in the mempool, Bitcoin’s waiting room for unconfirmed transactions. Two: secretly mine an alternative chain that rewrites recent blocks and claims the fees already collected in them. Right now, option one is overwhelmingly the rational choice. The block subsidy — currently 3.125 BTC per block — makes the fees in recent blocks a relatively small prize, and secretly mining enough blocks to overtake the honest chain is enormously hard. When the subsidy is large, fee sniping simply doesn’t pay.
The concern grows as the subsidy halves toward zero. If a block holds £500,000 of transaction fees but the subsidy is worth only £10,000, the maths shifts. A miner with significant hash rate — a large share of the total computing power aimed at mining — might rationally try to rewrite the last block and claim those fees, especially if several miners started doing it at once, in a chaotic scramble for high-fee blocks instead of honest building. Bitcoin Core, the main node software, partly defends against this with locktime: wallets can set a transaction to be valid only in the current or a future block, which makes it worthless inside a rewritten past block. That makes fee sniping less rewarding, though it doesn’t remove the theoretical incentive entirely.
🌍 The Real-World Analogy
Imagine a poker game where players earn tokens from two sources: a fixed hourly stipend from the house, and the pot of chips built up during each hand. While the stipend is large relative to a typical pot, everyone plays honestly — the stipend is the reliable income. But if the stipend shrinks to almost nothing and a single pot becomes worth ten times more than it, some players might start cheating to steal that pot rather than play the next hand straight. Fee sniping is that same calculation applied to mining: when the “pot” (transaction fees) vastly exceeds the “stipend” (the block subsidy), the pull to cheat rather than play honestly grows.
⚡ So What?
Fee sniping isn’t a practical concern today — the economics strongly favour honest mining. It’s a long-horizon risk, mostly relevant for thinking about Bitcoin’s security in the 2060s, 2080s, and beyond, as the subsidy keeps shrinking. It’s worth understanding because it lets you weigh the “Bitcoin works because miners are paid to be honest” argument honestly: that argument holds now, but staying true over the long term depends on the fee market growing.
