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
/buildverification. - User reports bug.
- Post-mortem after production incident.
/checkflags 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: TestV7_RefundIdempotent.
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 (
! holdover⊥ forbid). - References §I surface where it applies.
Bad: V8: code should be correct. Good: V8: ∀ pg_query ! params interpolated via driver, ⊥ string concat.
WHEN NOT TO ADD §V
- Bug was purely mechanical typo with no class (
i++vsi--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:
- §B entry (always).
- §V entry (usually).
- Test file (when §V added).
- Code fix.
- One commit.
No dashboards. No log files. SPEC.md + git is the full history.
