Coverage Gate

A coverage gate fails a build when Code Coverage drops below a configured threshold. It is the standard mechanism for stopping coverage from eroding, and it is a direct instance of Goodhart's Law — a proxy turned into a target.

Its effects depend almost entirely on the accompanying convention. A gate paired with liberal exclusion annotations produces a number that means nothing, since the hard parts are simply excluded. A gate set at one hundred percent with no exclusions permitted produces something more interesting: because a branch cannot be annotated away, it has to be either covered or eliminated. That converts the gate into pressure on design, and much of the resulting work is genuinely good — removing guards that no input can reach, making an always-supplied optional required, modelling two fields that always travel together as one unit rather than two independent options.

The costs are also real. A strict gate makes any measurement artifact a hard block, so instrumentation quirks and flaky tests stop being annoyances and start blocking delivery. And a gate is only as good as its scope: one configured to run for changes targeting particular branches silently skips for changes targeting others, reporting neither pass nor fail. A skipped gate reads as a green build.