regression-guard

name	regression-guard
description	Post-fix test-coverage analysis. Reviews a just-landed fix, generates targeted unit and regression tests, audits how the bug could have been caught earlier. Use after a fix is merged, when asked to 'add regression tests', 'pay off test debt', 'guard against regression', or 'ensure this doesn't happen again'.
version	1.0.0
user_invocable	true

Regression Guard

Post-fix test-debt payoff loop. Takes the git diff of a recent fix as input and generates targeted tests to prevent the bug from returning.

Usage

/regression-guard [commit-range]

If no commit range is specified, uses HEAD~3..HEAD.

Step 1 — Review What Was Fixed

Build a structured summary from git:

git log --oneline -10
git diff HEAD~3..HEAD
git log -1 --format=%B

Produce:

• What broke — the failure mode
• What changed — specific code paths
• Files and functions touched
• Issue/PR reference

Step 2 — Generate Unit Tests

Write the smallest set of unit tests that lock in the correct behavior:

tests/unit/<area>/<fix-slug>.test.<ext>

Rules:

• Each test must be red when reverted against the pre-fix state
• Cover the happy path AND the corrected edge cases
• State which assertion would be red if reverted

Step 3 — Generate Regression Tests

Write regression tests that construct the exact pre-fix state:

tests/regression/<area>/<fix-slug>-regression.test.<ext>

Rules:

• Include a top-of-file comment identifying the pre-fix state and symptom
• The test must pass the full check suite with no manual edits

Step 4 — Audit: Could This Have Been Caught Earlier?

Answer honestly. Pick the most relevant gap category:

• Missing boundary test — contract assertion at a module seam
• Missing negative test — error path never exercised
• Non-hermetic test harness — shared state leaked between tests
• Missing integration coverage — only reproduced under production shape
• Contract drift — upstream contract widened/narrowed without downstream rebase

For each gap, write a concrete, actionable suggestion. No generic "add more tests."

Step 5 — Recommend Integration Testing Additions

Name:

1. The specific test command to extend
2. The specific file to add a test into (or create)
3. A concrete test stub that would have caught the bug

Output Report

Write to docs/investigations/<date>-regression-guard-<fix-slug>.md:

1. Fix summary
2. Generated unit tests (file paths + descriptions)
3. Generated regression tests (file paths + pre-fix state each reconstructs)
4. "Could this have been caught earlier?" diagnosis
5. Integration testing recommendations
6. Baseline comparison table

Ship as a Draft PR

1. Create a new branch
2. Commit the generated test files and output report
3. Open a draft PR against main
4. Never push directly to main