Why is a diff not enough without a comprehension delta?#
A diff is complete and says almost nothing about meaning. It lists every line that moved and leaves the reader to work out what is now true that was not before. When pull requests arrive faster than reviewers can read them, that reconstruction is the step that gets skipped. A generic AI summary answers a different question: it describes what the change says without telling the reader what they now have to believe.
Why does a comprehension delta say what did not change?#
Because a reviewer who cannot see what stayed the same has to check everything again. Stating the invariants, such as an unchanged public API or data model, lets the reviewer spend their attention on the part that moved.
A comprehension delta is also personal. It should be computed from what this reader last knew, the checkpoint a re-entry brief builds on, so two people returning to the same work get different deltas.