From marsai-default
RED-GREEN-REFACTOR implementation methodology - write failing test first, minimal implementation to pass, then refactor. Ensures tests verify behavior.
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Write the test first. Watch it fail. Write minimal code to pass.
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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.
Always:
Exceptions (ask your human partner):
Thinking "skip TDD just this once"? Stop. That's rationalization.
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
Write code before the test? You have ONLY two options:
If you wrote code before test:
rm [files] # or git reset --hard
Not tomorrow. Not after asking. NOW.
Delete means DELETE:
rm -rf the_file.py ✓git reset --hard ✓These are NOT deleting (violations):
git stash - That's hiding, not deletingmv file.py file.py.bak - That's keepingDelete means gone forever. No recovery possible.
Then start over with TDD.
You CANNOT:
If you think you have a legitimate reason to skip deletion:
The rule has ZERO exceptions. ZERO.
Untested code WILL cause production incidents. These are CERTAINTIES:
❌ "This time is different" → Every developer who thought this experienced these consequences.
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:
ZERO exceptions. Anniversary, deadline, manager pressure, sunk cost - NONE of these override deletion.
You CANNOT create alternatives:
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.
Cycle: RED (write failing test) → verify fails → GREEN (minimal code) → verify passes → REFACTOR (clean up) → verify still green → repeat
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:
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.
| 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 passes? You're testing existing behavior. Fix test.
Test errors? Fix error, re-run until it fails correctly.
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.
MANDATORY.
npm test path/to/test.test.ts
Confirm:
Test fails? Fix code, not test.
Other tests fail? Fix now.
After green only:
Keep tests green. Don't add behavior.
Next failing test for next feature.
| 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 |
| 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. |
| 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. |
All of these mean: Delete code. Start over with TDD.
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.
Before marking work complete:
Can't check all boxes? You skipped TDD. Start over.
| 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. |
Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.
Never fix bugs without a test.
This skill uses these universal patterns:
skills/shared-patterns/state-tracking.mdskills/shared-patterns/failure-recovery.mdskills/shared-patterns/exit-criteria.mdskills/shared-patterns/todowrite-integration.mdApply ALL patterns when using this skill.
| 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.
Production code → test exists and failed first
Otherwise → not TDD
No exceptions without your human partner's permission.
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 |
The following requirements CANNOT be waived:
| 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 |
| 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." |
| 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 |