How a 'Blast-Radius Card' Can Prevent Shared Code Changes From Breaking Releases

A contributor's timeout fix to a shared helper imported by twelve modules appeared complete because unit tests passed, but a later release candidate failed when a worker called the old function signature through a separate package. Maintainers had to manually reconstruct the call graph, delaying the release and consuming valuable review time. The proposed solution is a structured markdown file called BLAST_RADIUS.md, kept in the branch and treated as a merge gate, which documents every call site, signature change, and affected test before any code is merged. The card is populated using repeatable terminal commands — such as ripgrep and git grep — so any reviewer can independently verify the full impact of a change without relying on tribal knowledge. Three rules enforce honesty: every call site must be traceable to a rerunnable command, every exclusion must cite a clear import boundary, and every intended behavior change must be described in a single sentence.