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.
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.
"Other tests" means the project's suite, not just your file. A
green run of the test you wrote is not a green suite. Before you call
the change done, run the project's test command (bare pytest,
npm test, cargo test — whatever the repo uses) even when your task
named only one test file. A scope statement in your task bounds the
deliverable, not your verification. Any failure that run shows —
including one you didn't cause — goes in your report by name; a red
test you watched scroll past and didn't mention is a report falsified
by omission.
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
When writing or changing any test, read writing-good-tests.md for the rules that keep tests honest:
Name the production change that would make the test fail — before writing it
Assert on real behavior, never on mock behavior
Keep test-only code in test utilities, out of production classes
Understand a dependency's side effects before mocking it
Common Rationalizations
Excuse
Reality
"Too simple to test"
Simple code breaks. Test takes 30 seconds.
"I'll test after"
Tests written after pass immediately — which proves nothing. They may test the wrong thing, test the implementation instead of the behavior, or miss the edge case you forgot. You never watched it fail, so you never proved it can catch the bug. Test-first forces that failure.
"Tests after achieve same goals (spirit not ritual)"
Tests-after answer "what does this do?"; tests-first answer "what should this do?" Tests written after are biased by the code you already wrote — you verify the cases you remembered, not the ones you'd have discovered. Coverage without proof the tests work.
"Already manually tested"
Manual testing is ad-hoc: no record of what you covered, no way to re-run it when the code changes, easy to forget cases under pressure. "Worked when I tried it" ≠ comprehensive. Automated tests run the same way every time.
"Deleting X hours is wasteful"
Sunk cost fallacy — that time is already spent either way. The real choice: rewrite with TDD (high confidence) vs. keep it and bolt tests on after (low confidence, likely bugs). Keeping code you can't trust is the waste.
"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 IS the pragmatic path: catches bugs before commit, prevents regressions, lets you refactor without fear. "Pragmatic" shortcuts mean debugging in production — slower, not faster.
"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.
