BIP 125 — Opt-in Full Replace-by-Fee Signaling

BIP 125 (Opt-in Full Replace-by-Fee Signaling) defines how an unconfirmed transaction signals replaceability via nSequence, letting the sender broadcast a higher-fee replacement. It is relay/mempool policy, not consensus. Node policy has since trended toward accepting replacements regardless of signaling ('full RBF'). Repository status: Deployed.

Debate relevance: Central to fee-management and zero-confirmation debates: merchants accepting unconfirmed payments, fee-bumping stuck transactions, and pinning discussions all cite it. Common misstatements: "RBF lets people double-spend" — replacement applies only to unconfirmed transactions, which were never final; confirmed transactions are unaffected. Also, treating BIP 125 signaling as a consensus rule rather than relay policy.