The value of DEFAULT_CHECKPOINTS_ENABLED is true and changing it will allow nodes using the default settings to accept blocks that would be currently rejected during a potential reorg. Is this change considered a hard-fork, or only changes to clearly enforced consensus rules can qualify as such?


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A hard fork is defined as a relaxation of the consensus rules. (In contrast a soft fork is defined as a restriction of the consensus rules.) Changing the consensus rules to ignore checkpoints by default (assuming the reference client Bitcoin Core defines the consensus rules) would be a relaxation of the consensus rules and hence theoretically a hard fork. However, in reality the checkpoints that have been defined in Bitcoin Core are many years deep and the probability of a re-org back to these checkpoints is infinitesimally small (zero to a huge number of decimal places). So practically it wouldn't have any impact.

Checkpoints and assumedvalid was discussed at this London BitDevs Socratic Seminar in November 2021 on AssumeUTXO.

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