BIP 9 (Soft Fork Activation)

BIP 9 is the Bitcoin improvement proposal that introduced version bits, a mechanism for activating soft forks through miner signaling. It lets up to 29 upgrades be proposed in parallel, each assigned one bit in the block version field, and it activates a fork once 95 percent of blocks within a 2,016 block difficulty period signal readiness. It was used to activate CheckSequenceVerify in 2016 and segwit in 2017.

Why it matters

Coordinating a rule change across a decentralized network with no administrator is one of Bitcoin's hardest problems. BIP 9 was designed to make upgrades safe and observable: miners signal only when their software is ready to enforce the new rules, so activation happens when the hashrate overwhelmingly supports it, minimizing the risk of a chain split. The signaling data is public, which lets anyone track an upgrade's progress block by block.

The segwit fight exposed its weakness. Signaling stalled far below 95 percent for months, and the deadlock only broke under pressure from a user activated soft fork movement in 2017. That episode motivated BIP 8, which can force activation at a timeout height.

How it works

Each deployment has a name, a bit, a start time, and a timeout. It begins in a defined state, becomes started at the start time, and moves to locked in once a full retarget period meets the 95 percent threshold. One period later it is active. If the timeout passes first, the deployment fails and the bit is freed for reuse.