BIP 91

BIP-91 is the miner-coordinated mechanism that activated SegWit in August 2017, defusing what was about to be a chain-splitting confrontation with BIP-148 (UASF).

The deadlock context: SegWit had been ready for a year. The BIP-9 signaling threshold (95%) couldn't be reached because a minority of miners were holding out. The UASF crowd was preparing to enforce SegWit unilaterally starting August 1, 2017, which would have orphaned non-signaling blocks and potentially split the chain.

BIP-91 was the compromise. Authored by James Hilliard and adopted by miners as part of the "SegWit2x" / New York Agreement effort, it lowered the activation threshold and forced rapid coordination:

1. 80% threshold (lower than BIP-9's 95%) for SegWit signaling.
2. Mandatory. Once 80% was reached in a 336-block window, all miners had to signal for SegWit going forward, or their blocks would be orphaned.
3. Quick deployment. Locked in mid-July 2017, activated in August.

What this actually achieved:

• SegWit activated cleanly about 10 days before BIP-148's August 1 deadline. No chain split, no orphaned blocks at scale.
• Demonstrated miner responsiveness to user pressure. The credible threat of UASF moved miners who had been refusing for over a year.
• The "SegWit2x" hard-fork half of the agreement was abandoned later in 2017, when it became clear users had no appetite for the 2MB block-size hard fork that miners had wanted. The "2x" never happened.

BIP-91 is a small technical document with outsized historical importance. It's the bridge between a stalemate and a successful upgrade, and the moment that established the modern "users hold the leverage, not miners" framing in Bitcoin governance.

See The Block Size War for the standoff BIP-91 defused.