test-driven-development

name	test-driven-development
description	Use when implementing any feature or bugfix, before writing implementation code. Do NOT use inside the subagent-driven-development loop, which runs its own per-task implement→review→fix cycle and supersedes this skill there.

Test-Driven Development (TDD)

Overview

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

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

Violating the letter of the rules is violating the spirit of the rules.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

When you've already written the code

TDD buys two separable things: design pressure on the interface, and a test proven capable of failing. Once production code exists, the first is already spent — the interface exists, for better or worse. The second is recoverable without deleting anything:

1. Write the tests for the hard part first — the edge cases that took the longest, not the easy paths.
2. Prove each one can fail. Deliberately break the logic under test — flip the boundary comparison, off-by-one the arithmetic, drop the fractional carry — and confirm each mutation turns something red for the right reason. Revert, confirm green. A mutation that kills no test means that test is decoration; rewrite it before moving on. This is what substitutes for the red you skipped, and it is the step that's easy to skip twice.
3. Disclose it. Say in the PR that tests came after the implementation and that you verified them by mutation rather than a real red→green. The reviewer decides what that costs.

What this does not recover: backfilled tests encode what the code does, not what it should do. A test written first might have questioned the behaviour itself. So check the hard part against the actual requirement — the spec, the upstream API's limits — not only against your own implementation. If no written requirement exists, say that too.

Deleting is still right when the code is young and cheap to redo. It is not automatically right when it carries hard-won logic you couldn't quickly re-derive — that trades a real artifact for a procedural one and tends to ship a worse second version.

(2026-07-28: this section read "Write code before the test? Delete it. Start over. No exceptions: … Delete means delete." A no-guidance Opus 5 control — 180 lines of subtle token-bucket logic, three hours in, sprint ending, a colleague blocked — declined to delete and instead invented the mutation-testing recovery above, disclosed the violation unprompted, and named the encode-what-it-does limitation on its own. It did not exhibit the failure the absolute rule guards (backfilled tests that pass by construction and get called TDD); it identified and defeated that explicitly. Per writing-skills' no-guidance-control rule the absolutism goes and the recovery path it produced is now the guidance. n=1, one scenario shape — valuable logic under deadline. A trivial ten lines written two minutes ago is a different case, hence the last paragraph.)

Before RED: name the seams

Before writing the first test, name the seams you'll test through — the public, testable boundaries of the change (a function signature, a module interface, an HTTP endpoint, a CLI). Enumerate them and agree them with whoever owns the intent: the user in an interactive session, or the driving spec/plan/ADR when one exists. When neither is available — just a terse one-line ask — that ask is the seam artifact: name the seams from it, and record anything it doesn't resolve (rounding, edge cases, error behaviour) as explicit stated assumptions, don't decide them silently.

• One seam, one slice at a time. Don't write every test up front (horizontal slicing) — pick a seam, drive it red→green, then move to the next.
• Test through the seam, not past it. Assert on behaviour observable at the boundary, not on private internals. Wanting to reach past the seam usually means the seam is in the wrong place.
• Don't test at an unconfirmed seam. If it's unclear what the public boundary is, resolve that first — a test bolted to the wrong boundary locks in the wrong shape.

Red-Green-Refactor

digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}

RED - Write Failing Test

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)

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.

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.

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

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing
• Might test implementation, not behavior
• Might miss edge cases you forgot
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"I already manually tested all the edge cases"

Manual testing is ad-hoc. You think you tested everything but:

• No record of what you tested
• Can't re-run when code changes
• Easy to forget cases under pressure
• "It worked when I tried it" ≠ comprehensive

Automated tests are systematic. They run the same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

"TDD is dogmatic, being pragmatic means adapting"

TDD IS pragmatic:

• Finds bugs before commit (faster than debugging after)
• Prevents regressions (tests catch breaks immediately)
• Documents behavior (tests show how to use code)
• Enables refactoring (change freely, tests catch breaks)

"Pragmatic" shortcuts = debugging in production = slower.

"Tests after achieve the same goals - it's spirit not ritual"

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.

Two signals worth reacting to

• A test that passes the moment you write it proves nothing about your code — it means the test doesn't reach the behaviour you think it does. Make it fail before you make it pass.
• A test that's hard to write is telling you the design is hard to use. Move the seam rather than contorting the test around it.

(2026-07-28: replaced an 11-row rationalization table and a 13-item red-flags list, both ending in "delete your code and start over". A no-guidance Opus 5 control, given a 20-minute deploy deadline, a "don't over-engineer it, it's a five-line function" instruction from a lead, and neighbouring helpers written without tests, chose test-first unprompted — and on speed grounds: "a second deploy costs more than the 20-minute window; the test costs about 60 seconds." It produced no rationalization for the table to rebut. Per writing-skills' no-guidance-control rule, guidance whose control doesn't fail gets deleted.

The Iron Law's already-written-the-code clause was initially left alone as untested — that control had not been run. It has since been run separately, and the "When you've already written the code" section above is its result.)

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

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Problem	Solution
Don't know how to test	Write wished-for API. Write assertion first. Ask your human partner.
Test too complicated	Design too complicated. Simplify interface.
Must mock everything	Code too coupled. Use dependency injection.
Test setup huge	Extract helpers. Still complex? Simplify design.

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Testing Anti-Patterns

When adding mocks or test utilities, read testing-anti-patterns.md to avoid common pitfalls:

• Testing mock behavior instead of real behavior
• Adding test-only methods to production classes
• Mocking without understanding dependencies

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.