BIP 125 (Replace-by-Fee)

BIP 125 specifies opt-in Replace-by-Fee (RBF): a transaction signals it can be replaced by setting at least one input's nSequence to a value less than 0xfffffffe. Wallets and miners that honor the signal then accept a replacement transaction that meets specific policy rules.

The five "BIP 125 conditions" a replacement must satisfy:

1. The replacement pays a higher absolute fee than the original.
2. The replacement pays a higher feerate (sats per vbyte).
3. The replacement pays at least the minimum incremental relay fee.
4. The replacement doesn't add inputs that weren't visible (unconfirmed) to the original.
5. The replacement evicts at most 100 transactions total (including descendants).

This was opt-in RBF, the default behavior in Bitcoin Core 0.12 (2016) through 23.x. The opt-in compromise was a 2016 concession: merchants who wanted to keep accepting zero-conf payments could refuse to honor RBF-flagged transactions, while users who wanted fee-bumping ability could opt in.

In 2024, Bitcoin Core 28.0 changed the default to full RBF: every unconfirmed transaction is replaceable regardless of the nSequence signal. The reasoning that finally won: zero-conf was never actually secure (see race attack), so the opt-in compromise was protecting a security model that didn't exist. Merchants who relied on zero-conf migrated to Lightning or started requiring confirmations.

In practice now: every modern wallet supports RBF, every modern node will relay full-RBF replacements, and "I forgot to set a high enough fee" is no longer a stuck transaction. It's a one-click fee bump.

Spec: BIP-125.