BIP 42 — A finite monetary supply for Bitcoin, Pieter Wuille

BIP 42 (A finite monetary supply for Bitcoin, Pieter Wuille) is a consensus change fixing a subtle flaw in the original subsidy code: the halving calculation relied on an integer shift whose behavior would have become undefined far in the future (around the 64th halving, roughly the 23rd century), theoretically breaking the 21 million cap. The fix caps the shift so the subsidy cleanly reaches zero. It was published on April 1, 2014, with deliberately humorous prose, but the code change is real and deployed. Repository status: Deployed.

Debate relevance: Comes up in monetary-policy debates as the precise footnote to '21 million is guaranteed by the code': the cap holds because the code was corrected, and because participants chose to keep it. Common misstatements: "BIP 42 is just an April Fools joke" — the framing was humorous; the consensus fix is real. Conversely, "Bitcoin's supply cap was mathematically perfect from day one" overstates the original code.