Replace-by-fee (RBF)

When a transaction sits unconfirmed because its fee is too low, RBF allows the sender to create a new version spending the same inputs with a higher fee. Nodes that follow replacement rules swap the old version for the new one in their mempools, and miners prefer it because it pays more. Most wallets offer this as a speed-up or bump fee button.

Originally, BIP-125 made replacement opt-in: a transaction had to signal that it could be replaced. Over time many miners accepted replacements regardless, and Bitcoin Core 28, released in 2024, enabled full RBF by default, so any unconfirmed transaction can be replaced. RBF can also be used to cancel a payment by sending the same coins back to oneself with a higher fee, as long as the original has not confirmed.

That flexibility means unconfirmed Bitcoin payments are not reliable: a merchant accepting zero-confirmation payments risks having them replaced. The replacement must pay a higher absolute fee and fee rate than the original, and complex rules around package replacement have been exploited in theoretical attacks on Lightning. Account-based chains offer a similar feature by reusing the same nonce.