The assumption breaks for collaborative transactions such as CoinJoin, where several users deliberately combine inputs, and for some payment protocols. Good clustering detects those patterns and excludes them rather than merging unrelated users into one oversized cluster.
Back to glossary
Glossary term
What is Co-spend heuristic?
The co-spend heuristic, also called the common-input-ownership heuristic, assumes that all input addresses spent together in one Bitcoin-style transaction are controlled by the same owner, because signing requires each input's private key. It is the foundational technique for clustering addresses on UTXO blockchains into wallets and entities.