Definition
SPV mining, also called validationless mining, is the practice of beginning work on the next block using only the previous block's block header, before the full block has been downloaded and validated. The name borrows from "simplified payment verification" because, like an SPV client, the miner trusts the header without checking every transaction beneath it.
Why miners do it
The motive is speed. When a competing pool finds a block, every other miner faces a costly delay while the full block propagates and validates. By extracting just the new header — often relayed faster than the complete block — a pool can immediately start hashing the next candidate. Because it has not yet verified the transaction set, it typically mines an empty block during that gap, then switches to a full block once validation completes. Shaving these seconds reduces the pool's orphan risk during block propagation.
The danger it introduces
Validationless mining carries a real hazard: if the header belongs to a block that is actually invalid, the miner can extend an invalid chain and produce blocks that the network rejects. This is not theoretical — during the July 2015 BIP 66 soft-fork activation, several pools relying on SPV mining built on an invalid chain, briefly causing a fork and temporarily showing confirmations that did not hold. The episode is the standard cautionary example of trusting a header without verifying its contents.
The safe alternative is always to mine on a fully validated tip; doing otherwise increases exposure to stale blocks and consensus faults when soft forks activate.
In Simple Terms
SPV mining, also called validationless mining, is the practice of beginning work on the next block using only the previous block’s block header, before the…