Agent skill · software engineering · juliusbrussee
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.
Why this skill is useful
Adds a structured protocol for tracing and documenting bugs that the AI wouldn't reliably generate on its own.
What it needs
About 2k tokens when loaded. Last updated 2026-06-18. 1,134 stars on the source repository.
What this skill does
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 1. TRACE Read failure output / bug report. Find exact file:line of wrong behavior. Name root cause in one caveman sentence. 2. 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) 3. PROPOSE Draft the spec change. Never skip §B; §V/§I/§T are case-by-case. Template: Example: 4. GENERATE TEST New invariant without test = lie. Add failing test first. Name test so it cites the invariant: TestV7RefundIdempotent. 5. VERIFY Fix code. Run test. Must pass. Run full suite. Must not regress. 6. LOG Commit spec edit + test + code fix together. Commit msg: backprop §B.<n> + §V.<N>: <one-line cause>. WHAT MAKES A GOOD INVARIANT Testable in code (grep-able or assert-able). Scoped to a behavior, not a file. Stated positively when possible (! hold over ⊥ forbid). References §I surface where it applies. Bad: V8: code should be correct. Good: V8: ∀ pgquery ! params interpolated via driver, ⊥ string concat. WHEN NOT TO ADD §V Bug was purely mechanical typo with no class (i++ vs i-- in throwaway). Fix is a one-time migration. Root cause is external dep (upgrade deps instead, note in §C). Still append §B entry — record that this failure mode was considered. Future bug with same smell → §B search shows precedent. OUTPUT SHAPE Every backprop run produces: 1. §B entry (always). 2. §V entry (usually). 3. Test file (when §V added). 4. Code fix. 5. One commit. No dashboards. No log files. SPEC.md + git is the full history.
How to use it
Reference it in AdaL, Claude Code, Cursor or any coding agent — nothing to install:
@skills juliusbrussee/backprop