Track check — this skill is game-framed. Resolve production/track.txt (or the session's PROJECT_TYPE line) before Phase 0.

• game — run as written.
• product (web / mobile / service) — substitute as you read: player becomes user, game becomes product, GDD becomes PRD (design/gdd/ → product/prd/), engine becomes the stack pinned in .codex/studio/technical-preferences.md. Map coverage to the PRD's critical user journeys, not GDD critical paths.
• Unresolved — ask which track this is before doing anything. A greenfield project has no signal either way; do not infer one from the repository contents.

Regression Suite

This skill ensures that every bug fix is backed by a test that would have caught the original bug — and that the regression suite stays current as the game evolves. It also detects when new features have been added without corresponding regression coverage.

A regression suite is not a new test category — it is a curated list of tests already in tests/ that collectively cover the game's critical paths and known failure points. This skill maintains that list.

Output: tests/regression-suite.md

When to run:

• After fixing a bug (confirm a regression test was written or identify gap)
• Before a release gate ($gate-check polish requires regression suite exists)
• As part of sprint close to detect coverage drift

1. Parse Arguments

Modes:

• $regression-suite update — scan new bug fixes this sprint and check for regression test presence; add new tests to the suite manifest
• $regression-suite audit — full audit of all GDD critical paths vs. existing test coverage; flag paths with no regression test
• $regression-suite report — read-only status report (no writes); suitable for sprint reviews
• No argument — run update if a sprint is active, else audit

2. Load Context

Step 2a — Load existing regression suite

Read tests/regression-suite.md if it exists. Extract:

• Total registered regression tests
• Last updated date
• Any tests flagged as STALE or QUARANTINED

If it does not exist: note "No regression suite found — will create one."

Step 2b — Load test inventory

Glob all test files:

tests/unit/**/*_test.*
tests/integration/**/*_test.*
tests/regression/**/*

For each file, note the system (from directory path) and file name. Do not read test file contents unless needed for name-to-test mapping.

Step 2c — Load GDD critical paths

For audit mode: read design/gdd/systems-index.md to get all systems. For each MVP-tier system, read its GDD and extract:

• Acceptance Criteria (these define the critical paths)
• Formulas section (formulas must have regression tests)
• Edge Cases section (known edge cases should have regression tests)

For update mode: skip full GDD scan. Instead read the current sprint plan and story files to find stories with Status: Complete this sprint.

Step 2d — Load closed bugs

Glob production/qa/bugs/*.md and filter for bugs with a Status: Closed or Status: Fixed field. Note:

• Which story or system the bug was in
• Whether a regression test was mentioned in the fix description

3. Map Coverage — Critical Paths

For audit mode only:

For each GDD acceptance criterion, determine whether a test exists:

1. Grep tests/unit/[system]/ and tests/integration/[system]/ for file names and function names related to the criterion's key noun/verb
2. Assign coverage:

3. Elevate MISSING items that correspond to formulas or state machines to HIGH PRIORITY gap — these are the most likely regression sources.

4. Map Coverage — Fixed Bugs

For each closed bug:

1. Extract the system slug from the bug's metadata
2. Grep tests/unit/[system]/ and tests/integration/[system]/ for a test that references the bug ID or the specific failure scenario
3. Assign:
   • HAS REGRESSION TEST — a test was found that would catch this bug
   • MISSING REGRESSION TEST — bug was fixed but no test guards against recurrence

For MISSING REGRESSION TEST items:

• Flag them as regression gaps
• Suggest the test file path: tests/unit/[system]/[bug-slug]_regression_test.[ext]
• Note: "Without this test, this bug can silently return in a future sprint."

5. Detect Coverage Drift

Coverage drift occurs when the game grows but the regression suite doesn't.

Check for drift indicators:

• Stories completed this sprint with no corresponding test files in tests/
• New systems added to systems-index.md since the last regression-suite update
• GDD sections added or revised since the regression suite was last updated (use Grep on GDD file modification hints if available, or ask the user)
• tests/regression-suite.md last-updated date vs. current date — if gap > 2 sprints, flag as likely stale

6. Generate Report and Suite Manifest

Report format (in conversation)

## Regression Suite Status

**Mode**: [update | audit | report]
**Existing registered tests**: [N]
**Test files scanned**: [N]

### Critical Path Coverage (audit mode only)
| System | Total ACs | Covered | Partial | Missing | Exempt |
|--------|-----------|---------|---------|---------|--------|
| [name] | [N] | [N] | [N] | [N] | [N] |

**Coverage rate (non-exempt)**: [N]%

### Bug Regression Coverage
| Bug ID | System | Severity | Has Regression Test? |
|--------|--------|----------|----------------------|
| BUG-NNN | [system] | S[N] | YES / NO ⚠ |

**Bugs without regression tests**: [N]

### Coverage Drift Indicators
[List new systems or stories with no test coverage, or "None detected."]

### Recommended New Regression Tests
| Priority | System | Suggested Test File | Covers |
|----------|--------|---------------------|--------|
| HIGH | [system] | `tests/unit/[system]/[slug]_regression_test.[ext]` | BUG-NNN / AC-[N] |
| MEDIUM | [system] | `tests/unit/[system]/[slug]_test.[ext]` | [criterion] |

Suite manifest format (tests/regression-suite.md)

The manifest is a curated index — not the tests themselves, but a registry of which tests should always pass before a release:

# Regression Suite Manifest

> Last Updated: [date]
> Total registered tests: [N]
> Coverage: [N]% of GDD critical paths

## How to run

[Engine-specific command to run all regression tests]

## Registered Regression Tests

### [System Name]

| Test File | Test Function (if known) | Covers | Added |
|-----------|--------------------------|--------|-------|
| `tests/unit/[system]/[file]_test.[ext]` | `test_[scenario]` | AC-N / BUG-NNN | [date] |

## Known Gaps

Tests that should exist but don't yet:

| Priority | System | Suggested Path | Covers | Reason Not Yet Written |
|----------|--------|----------------|--------|------------------------|
| HIGH | [system] | `tests/unit/[system]/[path]` | BUG-NNN | Bug fixed without test |

## Quarantined Tests

Tests that are flaky or disabled (do not run in CI):

| Test File | Function | Reason | Quarantined Since |
|-----------|----------|--------|-------------------|
| (none) | | | |

7. Write Output

Ask: "May I write/update tests/regression-suite.md with the current regression suite manifest?"

For update mode: append new entries; never remove existing entries (use Edit with targeted insertions). For audit mode: rewrite the full manifest with updated coverage data. For report mode: do not write anything.

After writing (if approved):

• For each HIGH priority gap: "Consider creating the missing regression test before the next sprint. Run $test-helpers to scaffold the test file."
• If bug regression gaps > 0: "These bugs can silently return without regression tests. The next sprint should include a story to write the missing tests."
• If coverage drift detected: "Regression suite may be drifting. Consider running $regression-suite audit at the next sprint boundary."

Verdict: COMPLETE — regression suite updated. (If user declined write: Verdict: BLOCKED.)

Collaborative Protocol

• Never remove existing regression tests from the manifest without explicit user approval — removing a test that was deliberately written is a regression risk itself
• Gaps are advisory, not blocking — surface them clearly but do not prevent other work from proceeding (except at release gate where regression suite is required)
• Quarantine is not deletion — tests with intermittent failures should be quarantined (noted in manifest) but not removed; they should be fixed by $test-flakiness
• Ask before writing — always confirm before creating or updating the manifest