Fee Bumping

Fee bumping is what you do when your transaction is stuck. The mempool has more transactions than will fit in the next several blocks, miners are picking the higher-fee ones first, and yours is sitting too far down the priority list. Two mechanisms let you raise the effective fee after broadcast:

Replace-by-Fee (RBF). Rebroadcast the same transaction with a higher fee, replacing the original in mempools. Defined in BIP 125, where the original transaction signals that it can be replaced. Since Bitcoin Core 28.0 (October 2024), Core nodes on default settings run full RBF and accept the replacement whether or not the original signaled. A wallet may still refuse to bump a transaction that didn't signal; Bitcoin Core's own wallet did until 30.0 (October 2025).

Child-Pays-for-Parent (CPFP). Don't touch the stuck transaction. Instead, spend one of its unconfirmed outputs in a new "child" transaction with a fee large enough to cover both itself and the parent's shortfall. Miners are economically incentivized to include both - the child only confirms if the parent does. Useful when you can't RBF (your wallet won't replace a transaction that didn't signal, or you don't control all the inputs) but you do control an output.

Most modern wallets handle this for you with a "bump fee" button. The actual decision tree:

• You're the sender, RBF was signaled -> RBF (cleaner, replaces in place).
• You're the receiver of a stuck incoming transaction -> CPFP, spending the unconfirmed output to yourself.
• Sender, RBF wasn't signaled -> RBF still works on Bitcoin Core 28.0+ nodes if your wallet will build the replacement; otherwise CPFP if you have a spendable output, or wait.

See Fee Estimation to avoid needing this in the first place.