TDD Implementation Skill

Enforce a strict test-driven development cycle: Red → Green → Refactor → Commit.

NEVER

NEVER write implementation before tests — this is the entire point; skip it and the skill has no value
NEVER skip mock bootstrap validation — write one mock round-trip test first and confirm it passes before writing 20 tests
NEVER skip the full test suite run — your file passing is not enough; regressions in unrelated files are your responsibility to notice
NEVER proceed if tests pass in Red phase — a test that passes before implementation is testing nothing; revise it
NEVER batch multiple features into one TDD cycle — one behavior change per red-green-refactor loop
NEVER treat a compile/import error as "red" — tests must fail for the right reason (assertion failure, not broken setup)

Before Writing Any Tests

If mockReset: true in test runner config, most internet mock examples silently fail — return values clear between tests.

grep -rn 'mockReset\|mockClear\|restoreMocks' vitest.config.* jest.config.*

If mockReset: true, you MUST reconfigure return values in beforeEach, not at module scope. Write ONE bootstrap test first to validate your mock wiring:

import { describe, it, expect, vi, beforeEach } from "vitest";

const mocks = vi.hoisted(() => ({
  myDep: vi.fn(),
}));

vi.mock("../path/to/dependency", () => ({
  myDep: (...args: unknown[]) => mocks.myDep(...args),
}));

describe("bootstrap", () => {
  beforeEach(() => {
    mocks.myDep.mockResolvedValue({ ok: true });
  });

  it("mock resolves correctly", async () => {
    const { myDep } = await import("../path/to/dependency");
    expect(await myDep()).toEqual({ ok: true });
  });
});

Run it, confirm it passes, then delete it and write the real tests using that same pattern.

Red Phase

Write tests covering: happy path, edge cases (empty/boundary inputs), error cases (invalid input, unauthorized, missing resource).

npx vitest run <test-file> --reporter=verbose

Checkpoint: All new tests MUST fail. If any pass → the behavior already exists or the test is wrong.

Green Phase

Write the minimum code to make tests pass. No untested features, no premature optimization, no untested error handling.

npx vitest run <test-file> --reporter=verbose

Checkpoint: All tests (new and existing) pass.

Full Suite

npx vitest run --reporter=verbose

If outside tests fail: determine if your change caused the regression. If yes → fix before proceeding. If pre-existing → note it and continue.

Refactor (Optional)

Only when tests are green. Extract helpers, improve naming, simplify conditionals. Re-run full suite after.

Quality Gates

npx eslint <changed-files>
npx tsc --noEmit

Fix before committing.

Mock Patterns Reference

Forwarding Pattern (survives mockReset)

const mockFn = vi.fn();
vi.mock("./dep", () => ({
  dep: (...args: unknown[]) => mockFn(...args),
}));
beforeEach(() => {
  mockFn.mockResolvedValue(defaultResult);
});

Counter-Based Sequencing (multi-query operations)

let callCount = 0;
mockExecute.mockImplementation(async () => {
  callCount++;
  if (callCount === 1) return insertResult;
  if (callCount === 2) return selectResult;
});

globalThis Registry (TDZ workaround for interdependent mocks)

vi.mock("./dep", () => {
  if (!(globalThis as any).__mocks) (globalThis as any).__mocks = {};
  const m = { dep: vi.fn() };
  (globalThis as any).__mocks.dep = m;
  return { dep: (...a: unknown[]) => m.dep(...a) };
});
// In tests: const mocks = (globalThis as any).__mocks;

Arguments

$ARGUMENTS: Optional description of what to implement via TDD
- Example: /tdd add rate limiting to the search endpoint
- If empty: ask the user what to implement

tdd