Crypto glossary
Preconfirmation
A preconfirmation is a fast promise, given before a block is produced or finalised, that a transaction will be included with a specific result.
A preconfirmation, or preconf, is a commitment from whoever will build the next block that a transaction will be included, sometimes with a guaranteed execution result. On centralised rollups, the sequencer's instant response is effectively a preconfirmation, and some L2s now stream partial blocks every few hundred milliseconds to make that feedback even faster.
Preconfirmations matter for user experience and for based rollups, which would otherwise feel as slow as Ethereum's 12-second slots. In that setting, upcoming Ethereum proposers opt in to issue signed promises, often through restaking or collateral so they can be penalised for breaking them. Traders, games and payment apps rely on this speed to show results immediately.
A preconfirmation is not finality. It is only as reliable as the party that issues it and the penalty it faces if it lies, and a reorganisation or sequencer failure can still undo it. Wallets and exchanges that credit funds on a preconfirmation take that risk, so large transfers usually wait for L1 settlement.
Example
A trader on an L2 sees their swap marked as done within a quarter of a second thanks to a preconfirmation, while the batch containing it reaches Ethereum a few minutes later.