Supply cap enforcement

The supply schedule is enforced by every validating node, not by any central table of issued coins. Bitcoin Core's GetBlockSubsidy computes the permitted subsidy for a height as 50 * COIN right-shifted once per completed 210,000-block interval (a halving), with the reward forced to zero after 64 halvings. A block whose coinbase output exceeds subsidy plus collected fees fails validation and is ignored by the network regardless of how much proof-of-work it carries.

Nuance: Enforcement is code, and code can have bugs: in August 2010 an integer-overflow bug briefly allowed a transaction creating ~184 billion BTC (see value-overflow-incident). It was patched and reorganized out within hours — a useful case study that the cap is a continuously enforced social-technical rule, not a law of physics. The honest framing is "every economically relevant node checks every coinbase," not "the cap cannot ever be violated by any bug."

Common misstatements:

• "The 21M cap is mathematically impossible to break." — It is enforced by validating software and the incentive to run it; the 2010 overflow shows enforcement is active, not automatic.
• "The subsidy divides by two forever." — Integer right-shifts truncate; the subsidy hits 1 sat and then 0, ending issuance entirely (around 2140).