Definition
The first-seen rule, sometimes called first-seen-safe, is the default mempool policy in Bitcoin Core: when a node receives two transactions that spend the same coins, it keeps the one it saw first and refuses to relay or accept the conflicting later one. This policy is the foundation that makes zero-confirmation acceptance even plausible, because honest nodes will not propagate a competing double-spend once they have committed to the original.
A policy, not a consensus rule
It is essential to understand that first-seen is a node-relay policy, not a Bitcoin consensus rule. Nothing in the protocol forces a miner to honor the order in which transactions arrived. A miner is free to mine whichever conflicting transaction pays them more, and Replace-by-Fee deliberately relaxes first-seen so that flagged transactions can be superseded by higher-fee versions. Different nodes can also legitimately see different transactions first due to propagation timing.
Implications for security
Because the rule is voluntary, the safety of an unconfirmed payment ultimately rests on the assumption that most relaying nodes and the next miner behave honestly. For low-value, in-person sales this is often acceptable; for anything material, waiting for confirmations is the only guarantee that does not depend on others' cooperation.
See zero-confirmation transaction and double-spend relay policy for related behavior.
In Simple Terms
The first-seen rule, sometimes called first-seen-safe, is the default mempool policy in Bitcoin Core: when a node receives two transactions that spend the same coins,…