blast-radius · cursor — Claude Code skill: what a change breaks

What blast-radius does

The skill reads the diff: symbols added, changed and removed, plus the behaviour that shifted but never made it into the diff itself. From there it hunts for the single fact the change's safety rests on. Time goes there, not into a long list of maybes — if that fact holds, most of the scary cases clear at once.

Then comes the part grep cannot reach. Library source, its pinned version and any local patch. Timing: microtasks, unmount and teardown. Things a symbol search walks past — a JSON response, a DB column, a wire format, a feature flag, code three hops downstream. Every risk gets a real likelihood and a real cost, and confirmed risks are listed apart from the ones checked and cleared.

The hard requirement is proof by execution. A script or test calls the actual code and fails loudly when the conclusion is wrong. No such proof, and the fact is written down as unproven — which is your signal not to merge on faith. For a wide change the skill suggests running it as an arena: several models on the same question, since they catch different real bugs.