UTXO consolidation: the fee and privacy trade-off

The privacy cost is the mirror image of the fee benefit. A consolidation spends every output it touches in one transaction, so the common-input heuristic merges all of them into a single cluster. If those outputs came from unrelated sources — payments from different counterparties, coins bought at different times, funds received at addresses the owner had kept apart — the consolidation publishes that they share an owner. The link is permanent and it is transitive: anything those outputs were connected to is now connected to everything else.

This is why consolidation is dangerous when it is automatic. A wallet that sweeps all available outputs whenever fees are low will merge groups the owner never intended to merge, and it will do so without asking. The damage is not visible at the time, because the transaction looks like an ordinary self-transfer. It becomes visible later, when an analyst reads the cluster and finds that a payment received from one source is now attributed to the same party as a payment received from another.

The safe version of consolidation is selective. Merge outputs that already belong together — coins received at addresses from the same wallet, funds from the same source, change from the same group — and leave everything else alone. That requires the owner to know where each output came from, which is the same labelling discipline that coin control requires. The coin control page covers the mechanism.

The first question is whether the outputs are already linked. If they were all received at addresses from one wallet and have been spent together before, consolidating them discloses nothing new and the fee saving is free. If they came from different sources and have never been combined, consolidating them creates a link that did not exist, and the fee saving is paid for in privacy.

The second question is whether the outputs are worth consolidating at all. An output whose value is close to the cost of spending it is dust, and including it in a consolidation is exactly the outcome a dust attacker is hoping for. The dust page covers how to identify those outputs and why they should be left alone or disposed of separately.

The third question is timing. Consolidation is cheapest when fees are low, and the mempool is quietest at predictable times. But a consolidation broadcast at an unusual hour is itself a signal, and an owner who consolidates on a fixed schedule publishes that schedule. The honest position is that consolidation is a tool with a real cost on both sides, and that the right answer depends on whether the outputs being merged are already connected.