About
The Defense-in-Depth skill provides a robust framework for hardening codebases against failures caused by invalid data. Instead of relying on a single point of failure for validation, this skill guides developers through implementing checks across four critical layers: Entry Point, Business Logic, Environment Guards, and Debug Instrumentation. By mapping data flows and enforcing constraints at every checkpoint, it ensures that bugs cannot bypass guards via refactoring, mocks, or alternative code paths, ultimately making critical system failures structurally impossible.