Soft fork signaling thresholds

Bitcoin's soft forks have historically activated through miner-signaling thresholds, and the threshold rose with experience. P2SH (BIP-16) activated in April 2012 after a miner poll with a roughly 55 percent threshold — low enough that non-upgraded miners went on producing invalid blocks for a period after activation. That experience pushed later deployments to a 95 percent threshold: BIP-66 (strict DER, July 2015) and BIP-65 (CheckLockTimeVerify, December 2015) activated at 95 percent under the block-version method, and BIP-9 formalized the approach as version bits — signal within a 2,016-block retarget window, lock in at 95 percent, activate after a further grace period — used by the CSV soft forks (BIPs 68/112/113, 2016) and SegWit. Signaling measures miner readiness, not enforcement: in July 2015, immediately after BIP-66 activation, miners who had signaled the new rules but were not fully validating built on an invalid block, causing a brief chain split of several blocks.

Confidence is medium: the sequence and thresholds are corroborated across the corpus's verified BIP entries (bip-0016, bip-0066, bip-0009) and the owner-research notes, but the canonical BIP texts and contemporaneous incident reports were not re-fetched during this verification, and the specific pre-BIP-9 percentages rest on the secondary account.

Nuance: The 95 percent figure is a chain-split safety parameter, not a constant of the protocol — later deployments varied it (Taproot's Speedy Trial used 90 percent within a single retarget period; see taproot-activation). Whether threshold signaling amounts to a miner vote over the rules is a governance claim, not part of this fact — see miner-signaling-is-a-vote-on-bitcoins-rules for that dispute and segwit-activation for the one deployment where the question turned hot.

Common misstatements:

• "95% has always been the rule." — P2SH activated at roughly 55%, and post-2017 deployments have used other values (90% for Taproot).
• "A miner that signals is enforcing the new rules." — The July 2015 split happened precisely because signaling outran validation.
• "The threshold is how Bitcoin votes on changes." — The mechanism coordinates a safe transition; rule adoption rests with the nodes that enforce validity (see nodes-enforce-rules).