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backprop

backprop

github.com/juliusbrusseeTesting & quality1.2k2.6k installsUpdated 11w ago
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About backprop

backprop is a skill for turning a bug or failed verification into a code fix and corresponding specification change. It guides developers through tracing the cause, adding an invariant and test where appropriate, verifying the fix, and committing the changes together.

Key features

  1. 01Trace failures to a root cause
  2. 02Update the specification with a §B record
  3. 03Add behavioral invariants when needed
  4. 04Create a test for each new invariant
  5. 05Verify the fix and full test suite
  6. 06Commit spec, test, and code together

Use cases

  1. 01Address a bug reported by a user
  2. 02Respond to a failed build verification
  3. 03Record lessons from a production incident

Install

Copy the command and run it in a terminal.

$npx skills add https://github.com/juliusbrussee/cavekit --skill backprop

After installing, type/backpropin Claude Code to confirm it loaded.

Details

From the project 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

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:

§B row: B<next>|<date>|<root cause>|V<N>
§V line: V<next>: <testable rule that would have caught it>

Example:

§B row: B3|2026-04-20|refund job ran twice on retry|V7
§V line: V7: ∀ refund → idempotency key check before charge reversal
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 (! hold over ⊥ 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++ 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.

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