CONCEPT
The Diagnostic Gap
The distance between what a practitioner understands about a system and what the system requires her to understand when it fails — a gap that abstraction widens invisibly, that AI-generated code has made the widest in computing history, and that is only measured at the moment when the measurement matters most.
The diagnostic gap is Spolsky's framework's most consequential extension: the structural distance between a practitioner's competence at the abstraction level and her competence at the level where failures actually live. Every abstraction produces a gap by hiding layers from daily cognition. When the abstraction works, the gap is invisible — the developer operates confidently at the surface, and the confidence is justified by the abstraction's reliability. When the abstraction leaks, the confidence becomes a liability: the developer discovers she understood the abstraction, not the system, and the system is what is breaking. The gap is self-concealing because the developer who lacks diagnostic capability does not know she lacks it — the abstraction has never required her to exercise it. Its width is measured only at the moment of leak, under exactly the conditions that make the measurement most expensive.
