Regression Test Writer

Adds a clean, automated regression test to the existing test suite. Ensures fixed bugs never return undetected.

1. Purpose

Lock in a bug fix with an automated test case. A "Regression Test" tests the exact boundary condition that previously caused a failure, running in CI alongside existing tests.

2. When to Use

Use this skill when:

• A bug fix has been prepared or applied.
• Adding test coverage for newly discovered edge cases.
• Preventing regressions during ongoing refactors.

3. When Not to Use

• Do not use as a throwaway manual repro script; use bug-reproducer.
• Do not use to review diffs; use diff-reviewer.

4. Required Inputs

• bug_description: What failed and under what input.
• target_test_file: Existing test file or new test path.
• test_runner: The project test command (e.g. npm test, pytest).

5. Missing-Input Behavior

1. If test_runner is unknown: inspect package.json, Makefile, or pyproject.toml to find it.
2. If target_test_file is not specified: place next to existing tests matching test_*.py or *.test.ts.
3. Do not invent new testing libraries; use whatever framework is already installed.

6. Procedure

Follow these numbered actions:

1. Action 1: Match Existing Test Style
   • Read 1 existing test in the target suite to mimic its assertion style, naming conventions, and imports.
2. Action 2: Author Focused Test Case
   • Name the test descriptively (e.g., test_divide_by_zero_raises_error).
   • Keep test execution fast (< 100ms) with no external network dependencies.
3. Action 3: Run Test Suite
   • Execute the targeted test using project runner.
4. Action 4: Emit Test Documentation
   • Format using Section 8.

7. Decision Rules

• If the test requires database access: use existing test database fixtures or SQLite in-memory mocks.
• Never write assert True or vacuous tests that pass regardless of logic.

8. Output Format

Emit the test summary using this template:

### Regression Test: [Test Name]

- **Test File**: `[path to test file]`
- **Framework**: [e.g., Jest, Vitest, Pytest, Go testing]
- **Execution Command**: `[command to run just this test]`

#### Test Implementation
```[language]
[Test function code]

Execution Evidence

[Terminal test output confirming pass]

## 9. Verification Checklist
- [ ] Test code adheres to existing project testing conventions.
- [ ] Test has explicit assertions that would fail if bug was reintroduced.
- [ ] Test executes deterministically without network calls.
- [ ] Test suite passes cleanly.

## 10. Common Failures and Recovery
| Failure Symptom | Cause | Recovery Action |
| :--- | :--- | :--- |
| Test passes even on broken code | Asserting on wrong value | Re-verify that assertion checks the specific fixed property. |
| Test breaks CI due to missing mock | Calling real external API | Mock the network request using project's mock library. |

## 11. Stop or Escalation Conditions
Stop and ask the user when:
- No test framework is configured in the repository.

## 12. Examples

### Example 1: Successful Execution
**Input:**
```text
Bug: Empty string in parseCSV() caused unhandled crash. Fixed in src/csv.ts.
Framework: Vitest
File: tests/csv.test.ts

Output:

### Regression Test: parseCSV handles empty string cleanly

- **Test File**: `tests/csv.test.ts`
- **Framework**: Vitest
- **Execution Command**: `npx vitest run tests/csv.test.ts`

#### Test Implementation
```typescript
it('returns empty array when input is empty string', () => {
    const result = parseCSV('');
    expect(result).toEqual([]);
});

Execution Evidence

 ✓ tests/csv.test.ts (1)
   ✓ returns empty array when input is empty string

Test Files  1 passed (1)
     Tests  1 passed (1)

### Example 2: Ambiguous or Missing Input
**Input:**
```text
Add a test.

Output:

### Regression Test: Halted - Missing Target

- **Status**: Halted

Please specify:
1. The bug or behavior to test.
2. The target test file or module name.

Example 3: Expected Final Output

Output:

### Regression Test: User age negative boundary

- **Test File**: `tests/test_validation.py`
- **Framework**: Pytest
- **Execution Command**: `pytest tests/test_validation.py -k test_negative_age`

#### Test Implementation
```python
def test_negative_age_raises_validation_error():
    with pytest.raises(ValidationError, match="Age must be positive"):
        validate_user_profile({"name": "Test", "age": -5})

Execution Evidence

tests/test_validation.py::test_negative_age_raises_validation_error PASSED [100%]
1 passed in 0.04s