tdd

name

tdd

description

Use when implementing ANY feature or bugfix — invoke this skill before writing a single line of production code. Trigger for: test-driven development, red-green-refactor cycle, failing tests first, TDD workflow, test-first programming. Also trigger when the user asks to "implement", "add", "build", "fix a bug", "write a function", or "create" anything that involves production code — even if they don't mention TDD. Even if the task seems too simple to need tests, invoke this skill.

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 the user):

Use this checklist to decide — if uncertain, default to TDD:

Question	Yes → exception applies
Is the code intended to be fully discarded after the session (not merged)?	Throwaway prototype
Was the code produced entirely by a code-generation tool, not hand-written?	Generated code
Does the file contain only values and no logic (env vars, JSON config, Dockerfile)?	Configuration file

If none of the above apply, follow TDD. "It feels too simple" is not an exception.

For files with no logic (configuration, JSON, env vars, Dockerfiles, lockfiles): proceed without a test and note the exception briefly. No need to ask.

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? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor

RED (write test) → VERIFY FAIL → GREEN (minimal code) → VERIFY PASS → REFACTOR → repeat

RED - Write Failing Test

Pre-coding checklist (complete before writing a single line of production code):

• No production code for this behavior exists yet — if it does, delete it first
• This is a behavioral change, not a config file / generated file / throwaway prototype
• I can describe the desired API (inputs, outputs, error cases) without looking at an implementation
• I know which test file to create or extend

Write one minimal test showing what should happen.

Requirements:

• One behavior
• Clear name describing expected outcome
• Real code (no mocks unless unavoidable)

See language-specific examples in references/ directory.

Verify RED - Watch It Fail

MANDATORY. Never skip.

Run the project's test command targeting the specific test.

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 the stupidest code that makes the test green. Not "simple" — minimal.

• One test expects true? Return true. Not email.includes('@'). Not a regex.
• One test expects 3 for add(1, 2)? Return a + b. Not input validation.
• Any logic that handles untested cases = code without a failing test = violates the Iron Law.

Don't add features, handle edge cases, refactor other code, or "improve" beyond the test. Future tests will drive the real implementation.

Verify GREEN - Watch It Pass

MANDATORY. Run tests again.

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

Red Flags - STOP and Start Over

If you catch yourself doing or saying any of these, delete code and restart with TDD.

Red Flag	Reality
Code before test	Delete it. No exceptions.
Test passes immediately	You're testing existing behavior, not new behavior.
"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.
"This is different because..."	It's not. Follow the cycle.

Language References

For concrete TDD examples (red/green/refactor walkthroughs, test runners, good/bad patterns), see:

• references/typescript.md — Jest/Vitest, async patterns
• references/rust.md — cargo test, Result/Option patterns

Pick the reference matching the project's language.

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 the user.
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.

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 the user's permission.