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tdd

Skill4 starsChanged 5 months agoMITView on GitHub

Discipline skill — TDD red-green-refactor cycle. Use when implementing any feature or bugfix, before writing implementation code.

194 other projects use a version of this, 5 of them edited.

Adopted by

Most recently taken up by

Used with

Projects with it also use capture-learning-tools, design-frontend-dev and 10 more.

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Files found in the same public projects as a version of this one, more often than they turn up anywhere else. Each project counts once, large collections count for less, and this file's own repository is left out.
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The file

Show its rules and text

We split this skill into the rules it gives an agent

18 rules. 0 are common, 18 are distinctive, and 0 are very specific to this project.

  • 0

    Common

    Most of the 186 edited versions we read keep these.

  • 18

    Distinctive

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Write the test first. Watch it fail.

    line 9
  2. Distinctive:

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

    line 11
  3. Distinctive:

    Write code before the test?

    line 34
  4. Distinctive:

    Don't keep it as "reference"

    line 36
  5. Distinctive:

    Don't "adapt" it while writing tests

    line 37
  6. Distinctive:

    Don't look at it

    line 38
  7. Distinctive:

    Write one minimal test showing what should happen.

    line 45
  8. Distinctive:

    Test fails (not errors)

    line 55
  9. Distinctive:

    Test passes? You're testing existing behavior.

    line 59
  10. Distinctive:

    Write the simplest code to pass the test.

    line 63
  11. Distinctive:

    Don't add features, refactor other code, or "improve" beyond the test.

    line 65
  12. Distinctive:

    Test fails? Fix code, not test.

    line 73
  13. Distinctive:

    Keep tests green. Don't add behavior.

    line 83
  14. Distinctive:

    Write failing test reproducing the bug

    line 124
  15. Distinctive:

    Verify RED (test fails as expected)

    line 125
  16. Distinctive:

    Verify GREEN (test passes, all tests pass)

    line 127
  17. Distinctive:

    Never fix bugs without a test.

    line 130
  18. Distinctive:

    Before marking work complete:

    line 143

“Common” is measured: kept by at least 6 in 10 of the 186 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

Test-Driven Development (TDD)

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.

Usage

/tdd

Invoke this skill before implementing any feature or bugfix. It establishes the discipline for your implementation session.

When to use

  • New features
  • Bug fixes
  • Refactoring
  • Behavior changes

Exceptions (ask the user): throwaway prototypes, generated code, configuration files.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

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

Red-Green-Refactor

RED - Write Failing Test

Write one minimal test showing what should happen.

Requirements:

  • One behavior per test
  • Clear name that describes behavior
  • Real code (no mocks unless unavoidable)

Run the test. Confirm:

  • Test fails (not errors)
  • Failure message is expected
  • Fails because feature missing (not typos)

Test passes? You're testing existing behavior. Fix the test.

GREEN - Minimal Code

Write the simplest code to pass the test.

Don't add features, refactor other code, or "improve" beyond the test.

Run the test. Confirm:

  • Test passes
  • Other tests still pass
  • Output pristine (no errors, warnings)

Test fails? Fix code, not test.

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 behavior.

Good Tests

QualityGoodBad
MinimalOne thing. "and" in name? Split it.test('validates email and domain and whitespace')
ClearName describes behaviortest('test1')
Shows intentDemonstrates desired APIObscures what code should do

Common Rationalizations

ExcuseReality
"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 is not systematic. No record, can't re-run.
"Deleting X hours is wasteful"Sunk cost fallacy. Keeping unverified code is technical debt.
"Need to explore first"Fine. Throw away exploration, start with TDD.
"Test hard = design unclear"Listen to the test. Hard to test = hard to use.
"TDD will slow me down"TDD is faster than debugging.

Red Flags - STOP and Start Over

  • Code before test
  • Test after implementation
  • Test passes immediately
  • Can't explain why test failed
  • Rationalizing "just this once"
  • "I already manually tested it"
  • "Keep as reference"

All of these mean: Delete code. Start over with TDD.

Bug Fix Flow

  1. Write failing test reproducing the bug
  2. Verify RED (test fails as expected)
  3. Implement minimal fix
  4. Verify GREEN (test passes, all tests pass)
  5. Refactor if needed

Never fix bugs without a test.

When Stuck

ProblemSolution
Don't know how to testWrite wished-for API. Write assertion first.
Test too complicatedDesign too complicated. Simplify interface.
Must mock everythingCode too coupled. Use dependency injection.
Test setup hugeExtract helpers. Still complex? Simplify design.

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
  • 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.

Related Skills

  • /debug — systematic debugging when tests reveal unexpected failures
  • /verify — verification before claiming work is complete

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