backprop
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
Install this Claude and Codex skill with the command below. The command stays visible even when copy support is unavailable.
git clone https://github.com/JuliusBrussee/cavekit /tmp/cavekit && ln -s /tmp/cavekit/skills/backprop ~/.claude/skills/backpropFrom README
backprop — bug → spec Plan-then-execute fixes the code & forgets. SDD fixes the code AND edits spec so recurrence is impossible. That edit is backprop. WHEN TO BACKPROP Test failed at /build verification. User reports bug. Post-mortem after production incident. /check flags VIOLATE with root cause found. SIX STEPS TRACE Read failure output / bug report. Find exact file:line of wrong behavior. Name root cause in one caveman sentence. ANALYZE Ask three questions: Would a new §V invariant catch this class of bug? (most common: yes) Is §I wrong — did spec claim shape the code cannot deliver? (sometimes) Is §T wrong — did we build the wrong thing? (rare but real) PROPOSE Draft the spec change. Never skip §B; §V/§I/§T are case-by-case. Template: Example: GENERATE TEST New invariant without test = lie. Add failing test first. Name test so it cites the invariant: TestV7RefundIdempotent. VERIFY Fix code. Run test. Must pass. Run full suite. Must not regress.