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

Write code before the test? You have ONLY two options:

Violation Handling (Mandatory)

If you wrote code before test:

DELETE IT. IMMEDIATELY.

rm [files] # or git reset --hard

Not tomorrow. Not after asking. NOW.

Delete means DELETE:

• rm -rf the_file.py ✓

• git reset --hard ✓

• Physically delete ✓

These are NOT deleting (violations):

• git stash

• That's hiding, not deleting

• mv file.py file.py.bak

• That's keeping

• Copy to clipboard - That's keeping

• Comment out - That's keeping

• "Mental note" - That's keeping

• Asking permission - That's delaying

Delete means gone forever. No recovery possible.

Then start over with TDD.

NO ASKING. NO PERMISSION. NO EXCEPTIONS.

You CANNOT:

• ❌ Ask "should I delete this?"

• ❌ Ask "may I proceed with tests-after?"

• ❌ Request permission due to [any reason]

• ❌ Explain why deleting is hard

• ❌ Decide "I'll be pragmatic and test after"

• ❌ Claim "spirit not letter"

• ❌ Rationalize "damage already done"

• ❌ Argue "skill is guide not suicide pact"

• ❌ Create "pragmatic compliance" category

• ❌ Mention sunk cost, time pressure, personal plans

• ❌ Commit anything

If you think you have a legitimate reason to skip deletion:

• You don't.

• Delete it.

• Every excuse is rationalization.

• Anniversary? Delete code, go to dinner, start fresh tomorrow.

• Deadline? Delete code, communicate delay, do it right.

• 4 hours of work? Sunk cost fallacy. Delete it.

• Manager pressure? Delete it, explain TDD prevents bugs.

• "But this is different because..." No. Delete it.

The rule has ZERO exceptions. ZERO.

Consequences of NOT Deleting

Untested code WILL cause production incidents. These are CERTAINTIES:

• Trust: Partner loses confidence permanently. PRs rejected. Assigned bug duty.

• Production: Incident within 30 days. Worst timing (holiday, 3am). Postmortem responsibility.

• Career: Reputation as "cuts corners" follows you. Promotion blocked.

• Personal: Anniversary debugging. Technical debt haunts for years.

❌ "This time is different" → Every developer who thought this experienced these consequences.

The ONLY Acceptable Action

DELETE THE CODE. NOW.

rm /tmp/auth-feature/src/auth.ts rm /tmp/auth-feature/src/middleware.ts rm /tmp/auth-feature/src/utils.ts

or: git reset --hard

Then:

• Go to anniversary dinner

• Sleep well

• Tomorrow morning: Start fresh with TDD

• Implement in 2-3 hours with tests (faster second time)

• Code review passes

• No production incidents

• Career intact

ZERO exceptions. Anniversary, deadline, manager pressure, sunk cost - NONE of these override deletion.

Prohibited Rationalizations

You CANNOT create alternatives:

• ❌ "Commit with WIP message" - NO. Delete.

• ❌ "Commit with 'NO TESTS' warning" - NO. Delete.

• ❌ "Mark as technical debt" - NO. Delete.

• ❌ "Email team about violation" - NO. Delete first, communicate after.

• ❌ "Come early tomorrow to decide" - NO. Delete now, decide nothing.

• ❌ "Let team decide" - NO. You decide: Delete.

• ❌ "Write tests tonight after dinner" - NO. Delete, start fresh tomorrow with TDD.

• ❌ "Commit honestly labeled code" - NO. Honesty is deleting it.

• ❌ "Present options to partner" - NO. Only one option: Delete.

The skill doesn't offer choices. It commands: DELETE.

If you create ANY alternative to deletion, you violated the skill.

DELETE IT. NOW. NO ALTERNATIVES.

Red-Green-Refactor

Cycle: RED (write failing test) → verify fails → GREEN (minimal code) → verify passes → REFACTOR (clean up) → verify still green → repeat

RED - Write Failing Test

Write one minimal test showing what should happen.

GOOD: test('retries failed operations 3 times', ...)

• clear name, tests real behavior, one thing.

BAD: test('retry works', ...) with mocks - vague name, tests mock not code.

Time limit: <5 minutes. Complex mocks → testing wrong thing. Lots of setup → design too complex. Multiple assertions → split tests.

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

Paste the ACTUAL failure output in your response:

[PASTE EXACT OUTPUT HERE] [NO OUTPUT = VIOLATION]

If you can't paste output, you didn't run the test.

Required Failure Patterns

Test Type Must See This Failure Wrong Failure = Wrong Test

New feature NameError: function not defined or AttributeError

Test passing = testing existing behavior

Bug fix Actual wrong output/behavior Test passing = not testing the bug

Refactor Tests pass before and after Tests fail after = broke something

No failure output = didn't run = violation

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.

GOOD: Simple loop with try/catch, just enough to pass. BAD: Adding options, backoff, callbacks (YAGNI).

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

Argument Reality

"Tests after verify it works" Tests-after pass immediately → proves nothing. Test-first forces failure → proves test works.

"Already manually tested" Ad-hoc ≠ systematic. No record, can't re-run, easy to forget under pressure.

"Deleting X hours is wasteful" Sunk cost fallacy. Keeping unverified code = tech debt. Delete → rewrite with confidence.

"TDD is dogmatic" TDD IS pragmatic: catches bugs pre-commit, prevents regressions, documents behavior, enables refactoring.

"Spirit not ritual" Tests-after = "what does this do?" Tests-first = "what should this do?" Different questions.

Common Rationalizations

Excuse Reality

"Too simple to test" Simple code breaks. Test takes 30 seconds.

"I'll test after" Tests passing immediately prove nothing.

"Tests after achieve same goals" Tests-after = "what does this do?" Tests-first = "what should this do?"

"Already manually tested" Ad-hoc ≠ systematic. No record, can't re-run.

"Deleting X hours is wasteful" Sunk cost fallacy. Keeping unverified code is technical debt.

"Keep as reference, write tests first" You'll adapt it. That's testing after. Delete means delete.

"Need to explore first" Fine. Throw away exploration, start with TDD.

"Test hard = design unclear" Listen to test. Hard to test = hard to use.

"TDD will slow me down" TDD faster than debugging. Pragmatic = test-first.

"Manual test faster" Manual doesn't prove edge cases. You'll re-test every change.

"Existing code has no tests" You're improving it. Add tests for existing code.

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', ...) → Verify RED: FAIL: expected 'Email required', got undefined → GREEN: Add if (!data.email?.trim()) return { error: 'Email required' } → Verify GREEN: PASS → REFACTOR: Extract validation 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.

Required Patterns

This skill uses these universal patterns:

• State Tracking: See skills/shared-patterns/state-tracking.md

• Failure Recovery: See skills/shared-patterns/failure-recovery.md

• Exit Criteria: See skills/shared-patterns/exit-criteria.md

• TodoWrite: See skills/shared-patterns/todowrite-integration.md

Apply ALL patterns when using this skill.

Violation Recovery Quick Reference

Violation Detection Recovery

Code before test Implementation exists, no test file DELETE code (rm ), write test, verify RED, reimplement

FALSE GREEN Test passes immediately, no implementation Test is broken - make stricter until it fails correctly

Kept "reference" Stash/backup/clipboard exists Delete permanently (git stash drop , rm ), start fresh

Why recovery matters: Test must fail first to prove it tests something. Keeping code means you'll adapt it instead of implementing from tests.

Final Rule

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

No exceptions without your human partner's permission.

Blocker Criteria

STOP and report if:

Decision Type Blocker Condition Required Action

Test framework missing Cannot run tests in the project STOP and install/configure test framework first

Code written first Implementation exists without corresponding test STOP and DELETE the code immediately

Test passes immediately New test passes without implementation STOP and fix test to fail correctly

Cannot reproduce RED Unable to make test fail for expected reason STOP and investigate test correctness

Cannot Be Overridden

The following requirements CANNOT be waived:

• MUST write test before implementation code

• MUST watch test fail before writing implementation

• MUST DELETE any code written before its test

• MUST NOT stash, backup, or keep "reference" of untested code

• MUST verify test fails for the right reason (missing feature, not typo)

Severity Calibration

Severity Condition Required Action

CRITICAL Code written before test exists MUST delete code immediately, start over with TDD

HIGH Test passes immediately without implementation MUST fix test to fail correctly before proceeding

MEDIUM Test fails for wrong reason (error vs assertion failure) MUST fix test to fail correctly

LOW Test name unclear or multiple assertions Should refactor test for clarity

Pressure Resistance

User Says Your Response

"Just write the code, we'll add tests later" "CANNOT write code without test first. Tests-after prove nothing. Will write test now."

"I already spent 4 hours on this code" "MUST delete it. Sunk cost fallacy. Untested code will cause production incidents."

"Keep it as reference and write tests first" "CANNOT keep reference. That's not deleting. Will start fresh with TDD."

"TDD is too slow, deadline is tomorrow" "CANNOT skip TDD due to deadline. TDD is faster than debugging. Will proceed with TDD."

Anti-Rationalization Table

Rationalization Why It's WRONG Required Action

"Code is simple, doesn't need TDD" Simple code breaks. TDD takes 30 seconds. Complexity is not the criterion. MUST follow TDD regardless of simplicity

"I'll test thoroughly after implementing" Tests-after pass immediately, proving nothing. Test must fail first. MUST write failing test before code

"Deleting my work is wasteful" Sunk cost fallacy. Keeping untested code is technical debt that costs more later. MUST delete code, start fresh with TDD

"I already manually tested it works" Manual testing is ad-hoc and not reproducible. No record, can't re-run. MUST have automated test that failed first

"Spirit of TDD matters, not the ritual" The ritual IS the spirit. Tests-first asks "what should this do?" not "what does this do?" MUST follow RED-GREEN-REFACTOR exactly