Anchor Output

An anchor output is a small, immediately spendable output added to a Lightning Network commitment transaction so that either channel party can raise its effective feerate after the fact. Lightning commitment transactions are signed long before they are broadcast, and the fee chosen at signing time may be far too low once they actually hit the network. Because such a transaction often gates time-sensitive funds, a too-low fee that delays confirmation past a timelock can let a counterparty steal funds.

How Anchors Enable Fee Bumping

Rather than pre-committing to a fee, the anchor design adds two outputs — one per party — and encumbers all other commitment outputs with a 1 OP_CHECKSEQUENCEVERIFY (CSV) delay so they cannot be spent for at least one block. A party who needs the commitment confirmed spends its own anchor in a high-fee child transaction, dragging the parent along via Child-Pays-For-Parent (CPFP). This decouples the fee decision from the moment of signing.

Carve-Out and Pinning Resistance

Earlier fee-bumping via replace-by-fee proved vulnerable to transaction pinning. Anchors instead rely on the CPFP carve-out rule, which grants a small exemption to descendant limits so a counterparty cannot saturate them and block your fee bump. Full effectiveness depends on package relay, with higher commitment feerates as an interim fallback. Later "ephemeral anchor" designs refine this further.

Anchor outputs are a direct response to weaknesses in older fee-bumping schemes. For background see Fee Estimation and the attack they defend against, Transaction Pinning.