regression-test

name	regression-test
description	Create regression tests for reported bugs to prevent them from recurring. Use when (1) user reports a bug and asks to write tests, (2) user says "write regression test", "add test for this bug", "make sure this never happens again", "test this fix", (3) after fixing a bug to ensure it stays fixed, (4) user describes a scenario that broke and wants test coverage.

Regression Test Creator

Create focused regression tests that reproduce reported bugs and verify fixes.

Process

1. Understand the bug - Identify the exact failure scenario, inputs, and expected vs actual behavior
2. Locate the failing code - Find the source file(s) where the bug occurs
3. Find existing tests - Check for existing test files in the same package/feature using **/*.test.ts glob
4. Write the test - Create a test that:
   • Reproduces the exact bug scenario (the test would FAIL before the fix)
   • Verifies the correct behavior after the fix
   • Covers edge cases related to the same bug class
5. Run the test - Execute with pnpm run test or package-specific pnpm --filter <package> test
6. Verify the fix - Confirm all tests pass

Test Structure

Follow existing test patterns in the codebase:

• Use vitest (describe, it, expect, vi)
• Co-locate test files next to the source: my-module.ts → my-module.test.ts
• Use vi.mock() for external dependencies
• Use afterEach(() => vi.clearAllMocks()) for mock isolation
• Name tests descriptively: it("does not crash when timestamp is zero")

Test Naming Convention

Name describe blocks after the class/function being tested. Name test cases after the specific bug scenario:

describe("ComponentName", () => {
  describe("bug scenario description", () => {
    it("specific regression case", () => { ... });
  });
});

Key Principles

• Each regression test must target the exact scenario that caused the bug
• Include edge cases from the same bug class (e.g., if 0 caused a crash, also test NaN, Infinity, null)
• Mock external dependencies to isolate the unit under test
• Keep tests focused - one bug scenario per test case
• Test the fix, not the implementation details