Replace-by-Fee (RBF)

Overview

Replace-by-Fee (RBF) allows a sender to publish a replacement transaction that conflicts with one or more unconfirmed transactions and offers stronger fee incentive. BIP 125 documented opt-in signaling and relay rules used by Bitcoin nodes.

A typical replacement spends at least one of the same inputs, pays a higher absolute fee, and satisfies node policy designed to limit abuse. Changing the fee also changes the signed transaction and usually produces a new transaction identifier. The old version remains part of the operational history.

Wallets use RBF when a transaction’s Miner Fee is too low for current Blockspace Demand. The replacement can preserve the recipient and amount while reducing change, or it can alter outputs under the sender’s control. Receivers should verify the actual replacement details rather than assume only the fee changed.

RBF reinforces why unconfirmed payments carry double-spend risk. A merchant can see the original transaction, then observe a replacement that redirects value elsewhere. Zero-confirmation acceptance requires a separate risk policy and should never be described as irreversible.

Payment systems should link every replacement hash, mark superseded transactions, and continue tracking the version that confirms. Repeated Broadcast or Rebroadcast of identical bytes is not RBF because the transaction does not change. Accounting should recognize one intended spend, not count each version as a separate payment.

Replacement policy can evolve across node software versions, so wallet and merchant behavior should be tested against current relay rules. A transaction that did not explicitly signal older opt-in behavior may still be replaceable under broader mempool policy, reinforcing the need to treat all unconfirmed spends cautiously.