test-driven-development

Execute

→ Implementing a feature or bugfix? → No production code without a failing test first. Gate: medium/high complexity? → route to brainstorming or writing-plans first. Cycle: RED (write test → watch it fail) → GREEN (minimal code → watch it pass) → REFACTOR (clean up → keep green) Regression: shared module → related tests. contract change → producer + consumer. core logic → old + new tests. → Done when: all tests pass, every new function has a test that failed first, TDD preflight gate passed.

Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

If you didn't watch the test fail, you don't know if it tests the right thing.

When to Use

New features, bug fixes, refactoring, behavior/logic changes, interface/data contract changes, cross-module or shared module changes, core logic refactors.

Exceptions (ask your human partner): throwaway prototypes, generated code, config files, pure docs cleanup, read-only diagnosis, comment-only changes.

Preflight Gate

TDD is the implementation discipline for an approved behavior or atomic task. It is not a substitute for task routing, product clarification, or planning.

Before writing tests or production code, stop and route to brainstorming or writing-plans if the current request has any medium- or high-complexity signal:

• multiple files, modules, pages, screens, services, or owners
• user-visible flows such as navigation, onboarding, checkout, lifecycle, or recovery paths
• state transitions, routing rules, API or data contracts, compatibility boundaries, migrations, permissions, or persistence
• more than one acceptance path or manual/visual verification requirement
• unclear product behavior, competing constraints, or long-running execution

For these tasks, require a baseline read-set, plan, and atomic tasks before TDD. High-complexity or ambiguous tasks also need a spec/design review before planning. Only proceed directly with TDD for low-complexity work whose intent, owner, compatibility boundary, and verification path are already clear.

When a medium- or high-complexity task needs project records, use the Aegis Project Workspace lazily. Prefer python scripts/aegis-workspace.py init --root <target-project-root> when the helper is available in the active method-pack checkout. If the task needs a process trail under work/, prefer python scripts/aegis-workspace.py new-work --root <target-project-root> ... so the intent, checkpoint, drift, and evidence paths are indexed and structurally checkable:

docs/aegis/
  README.md
  INDEX.md
  BASELINE-GOVERNANCE.md
  adr/
  baseline/
  specs/
  plans/
  work/YYYY-MM-DD-<task-slug>/
    10-intent.md
    20-checkpoint.md
    90-evidence.md
    99-reflection.md

Do not promote reusable project facts, decisions, specs, or plans into those directories unless the workflow needs them and no existing project authority already owns them.

Red-Green-Refactor

RED - Write Failing Test

State: input | output | boundary | acceptance criteria. Check existing test coverage first. Write one minimal test showing what should happen.

```typescript test('retries failed operations 3 times', async () => { let attempts = 0; const operation = () => { attempts++; if (attempts < 3) throw new Error('fail'); return 'success'; };

const result = await retryOperation(operation);

expect(result).toBe('success'); expect(attempts).toBe(3); });

Clear name, tests real behavior, one thing
</Good>

<Bad>
```typescript
test('retry works', async () => {
  const mock = jest.fn()
    .mockRejectedValueOnce(new Error())
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retryOperation(mock);
  expect(mock).toHaveBeenCalledTimes(3);
});

Vague name, tests mock not code

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)
• If a new feature changes user-observable behavior, prefer one minimal end-to-end or integration test for the main path before narrower unit tests
• For user-visible work, cover the main journey and the highest-risk experience or operational floor before treating unit tests as sufficient
• Add unit tests for core rules, boundary conditions, and error branches

Verify RED - Watch It Fail

MANDATORY. Never skip.

npm test path/to/test.test.ts

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos)

Test passes? You're testing existing behavior. Fix test.

Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test.

```typescript async function retryOperation(fn: () => Promise): Promise { for (let i = 0; i < 3; i++) { try { return await fn(); } catch (e) { if (i === 2) throw e; } } throw new Error('unreachable'); } ``` Just enough to pass ```typescript async function retryOperation( fn: () => Promise, options?: { maxRetries?: number; backoff?: 'linear' | 'exponential'; onRetry?: (attempt: number) => void; } ): Promise { // YAGNI } ``` Over-engineered

Don't add features, refactor other code, or "improve" beyond the test.

Fix the real owner of the behavior. Do not add a new fallback, adapter, or branch unless the debugging or design workflow identifies why it is necessary and what old path retires.

Verify GREEN - Watch It Pass

MANDATORY.

npm test path/to/test.test.ts

Confirm:

• Test passes
• Other tests still pass
• Output pristine (no errors, warnings)

Test fails? Fix code, not test.

Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Repeat

Next failing test for next feature.

Regression Scope

At minimum, run the target test you just changed or added. Broaden regression based on impact:

• Shared module change -> related module tests
• Interface or data contract change -> producer and consumer tests
• Cross-module behavior change -> integration or end-to-end path
• Core logic refactor -> old behavior regression tests plus new behavior tests

If the current environment cannot run automated tests, state the blocker and provide reproducible manual verification steps.

Good Tests

Quality	Good	Bad
Minimal	One thing. "and" in name? Split it.	test('validates email and domain and whitespace')
Clear	Name describes behavior	test('test1')
Shows intent	Demonstrates desired API	Obscures what code should do

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic"
• "This is different because..."

All of these mean: Delete code. Start over with TDD.

Example: Bug Fix

Bug: Empty email accepted

RED

test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});

Verify RED

$ npm test
FAIL: expected 'Email required', got undefined

GREEN

function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...
}

Verify GREEN

$ npm test
PASS

REFACTOR Extract validation for multiple fields if needed.

Verification Checklist

• Defined input, output, boundaries, compatibility, acceptance criteria
• Every new function/method has a test that failed first
• All tests pass, output pristine
• Regression: shared/contract/core changes ran related tests
• If automation blocked → blocker + manual steps documented

Can't check all boxes? Start over.

Exploration and Emergency Exceptions

Exploratory spikes are allowed only as throwaway learning. When the spike ends, convert confirmed behavior into tests before formal implementation.

Emergency hotfixes may prioritize the smallest safe repair when delay is more dangerous than incomplete TDD. Record the reason, keep the change narrow, and add the missing regression test in the same slice or the next nearest slice.

When Stuck

Don't know how to test → write wished-for API first. Test too complicated → simplify design. Must mock everything → reduce coupling.

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Never fix bugs without a test.