Bug → spec protocol. When a bug is found or a test fails, trace the cause, decide whether a new §V invariant would catch recurrence, append to §B. This is the one non-obvious thing SDD does that plan-then-execute doesn't. Triggers on test failure, bug report, post-mortem, or explicit user ask.
Install
npx skills add JuliusBrussee/cavekit --skill backpropnpx skills add JuliusBrussee/cavekit --skill backprop -a claude-code -gnpx skills add JuliusBrussee/cavekit --skill backprop -a codex -ggemini skills install https://github.com/JuliusBrussee/cavekit.git --path skills/backpropgh skill install JuliusBrussee/cavekit backpropgit clone --depth 1 https://github.com/JuliusBrussee/cavekit /tmp/JuliusBrussee__cavekit
mkdir -p ~/.claude/skills ~/.agents/skills
cp -R /tmp/JuliusBrussee__cavekit/skills/backprop ~/.claude/skills/backprop # Claude Code
cp -R /tmp/JuliusBrussee__cavekit/skills/backprop ~/.agents/skills/backprop # Codex, Cursor, Gemini CLI, Copilot, OpenCode