TDD: enforce RED-GREEN-REFACTOR, tests before code.

Skill metadata


Source	Bundled (installed by default)
Path	`skills/software-development/test-driven-development`
Version	`1.1.0`
Author	Mibyan (adapted from obra/superpowers)
License	MIT
Platforms	linux, macos, windows
Tags	`testing`, `tdd`, `development`, `quality`, `red-green-refactor`
Related skills	`systematic-debugging`, `subagent-driven-development`

Reference: full SKILL.md

The following is the complete skill definition that Mibyan loads when this skill is triggered. This is what the agent sees as instructions when the skill is active.

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 first):

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? 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 Cycle

RED — Write Failing Test

Write one minimal test showing what should happen. Good test:

def test_retries_failed_operations_3_times():
    attempts = 0
    def operation():
        nonlocal attempts
        attempts += 1
        if attempts < 3:
            raise Exception('fail')
        return 'success'

    result = retry_operation(operation)

    assert result == 'success'
    assert attempts == 3

Clear name, tests real behavior, one thing. Bad test:

def test_retry_works():
    mock = MagicMock()
    mock.side_effect = [Exception(), Exception(), 'success']
    result = retry_operation(mock)
    assert result == 'success'  # What about retry count? Timing?

Vague name, tests mock not real code. Requirements:

One behavior per test
Clear descriptive name (“and” in name? Split it)
Real code, not mocks (unless truly unavoidable)
Name describes behavior, not implementation

Verify RED — Watch It Fail

MANDATORY. Never skip.

# Use terminal tool to run the specific test
pytest tests/test_feature.py::test_specific_behavior -v

Confirm:

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

Test passes immediately? You’re testing existing behavior. Fix the test. Test errors? Fix the error, re-run until it fails correctly.

GREEN — Minimal Code

Write the simplest code to pass the test. Nothing more. Good:

def add(a, b):
    return a + b  # Nothing extra

Bad:

def add(a, b):
    result = a + b
    logging.info(f"Adding {a} + {b} = {result}")  # Extra!
    return result

Don’t add features, refactor other code, or “improve” beyond the test. Cheating is OK in GREEN:

Hardcode return values
Copy-paste
Duplicate code
Skip edge cases

We’ll fix it in REFACTOR.

Verify GREEN — Watch It Pass

MANDATORY.

# Run the specific test
pytest tests/test_feature.py::test_specific_behavior -v

# Then run ALL tests to check for regressions
pytest tests/ -q

Confirm:

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

Test fails? Fix the code, not the test. Other tests fail? Fix regressions now.

REFACTOR — Clean Up

After green only:

Remove duplication
Improve names
Extract helpers
Simplify expressions

Keep tests green throughout. Don’t add behavior. If tests fail during refactor: Undo immediately. Take smaller steps.

Repeat

Next failing test for next behavior. One cycle at a time.

Avoid Horizontal Slices

Do not write all tests first and then all implementation. That is horizontal slicing: RED becomes “write a pile of imagined tests” and GREEN becomes “make the pile pass.” It produces brittle tests because the tests are designed before the implementation has taught you what behavior and interface actually matter. Use vertical tracer bullets instead:

WRONG:
  RED:   test1, test2, test3, test4
  GREEN: impl1, impl2, impl3, impl4

RIGHT:
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3

A tracer bullet is one end-to-end behavior slice. It proves the path works, teaches you about the interface, and keeps each next test grounded in what you just learned.

Why Order Matters

“I’ll write tests after to verify it works” Tests written after code pass immediately. Passing immediately proves nothing:

Might test the wrong thing
Might test implementation, not behavior
Might miss edge cases you forgot
You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something. “I already manually tested all the edge cases” Manual testing is ad-hoc. You think you tested everything but:

No record of what you tested
Can’t re-run when code changes
Easy to forget cases under pressure
“It worked when I tried it” ≠ comprehensive

Automated tests are systematic. They run the same way every time. “Deleting X hours of work is wasteful” Sunk cost fallacy. The time is already gone. Your choice now:

Delete and rewrite with TDD (high confidence)
Keep it and add tests after (low confidence, likely bugs)

The “waste” is keeping code you can’t trust. “TDD is dogmatic, being pragmatic means adapting” TDD IS pragmatic:

Finds bugs before commit (faster than debugging after)
Prevents regressions (tests catch breaks immediately)
Documents behavior (tests show how to use code)
Enables refactoring (change freely, tests catch breaks)

“Pragmatic” shortcuts = debugging in production = slower. “Tests after achieve the same goals — it’s spirit not ritual” No. Tests-after answer “What does this do?” Tests-first answer “What should this do?” Tests-after are biased by your implementation. You test what you built, not what’s required. Tests-first force edge case discovery before implementing.

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 the 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 the code you touch.

Red Flags — STOP and Start Over

If you catch yourself doing any of these, delete the code and restart with TDD:

Code before test
Test after implementation
Test passes immediately on first run
Can’t explain why test failed
Tests added “later”
Rationalizing “just this once”
“I already manually tested it”
“Tests after achieve the same purpose”
“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.

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 the wished-for API. Write the assertion first. Ask the user.
Test too complicated	Design too complicated. Simplify the interface.
Must mock everything	Code too coupled. Use dependency injection.
Test setup huge	Extract helpers. Still complex? Simplify the design.

Mibyan Integration

Running Tests

Use the terminal tool to run tests at each step:

# RED — verify failure
terminal("pytest tests/test_feature.py::test_name -v")

# GREEN — verify pass
terminal("pytest tests/test_feature.py::test_name -v")

# Full suite — verify no regressions
terminal("pytest tests/ -q")

With delegate_task

When dispatching subagents for implementation, enforce TDD in the goal:

delegate_task(
    goal="Implement [feature] using strict TDD",
    context="""
    Follow test-driven-development skill:
    1. Write failing test FIRST
    2. Run test to verify it fails
    3. Write minimal code to pass
    4. Run test to verify it passes
    5. Refactor if needed
    6. Commit

    Project test command: pytest tests/ -q
    Project structure: [describe relevant files]
    """,
    toolsets=['terminal', 'file']
)

With systematic-debugging

Bug found? Write failing test reproducing it. Follow TDD cycle. The test proves the fix and prevents regression. Never fix bugs without a test.

Testing Anti-Patterns

Testing mock behavior instead of real behavior — mocks should verify interactions, not replace the system under test
Testing implementation details — test behavior/results, not internal method calls
Happy path only — always test edge cases, errors, and boundaries
Brittle tests — tests should verify behavior, not structure; refactoring shouldn’t break them

Final Rule

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

No exceptions without the user’s explicit permission.

تطبيق ديسكتوب

البداية

استخدام مبيان

الميزات

منصات المراسلة

التكاملات

الأدلة والدروس

دليل المطوّر

المرجع

Test Driven Development — TDD: enforce RED-GREEN-REFACTOR, tests before code

Skill metadata

Reference: full SKILL.md

Test-Driven Development (TDD)

Overview

When to Use

The Iron Law

Red-Green-Refactor Cycle

RED — Write Failing Test

Verify RED — Watch It Fail

GREEN — Minimal Code

Verify GREEN — Watch It Pass

REFACTOR — Clean Up

Repeat

Avoid Horizontal Slices

Why Order Matters

Common Rationalizations

Red Flags — STOP and Start Over

Verification Checklist

When Stuck

Mibyan Integration

Running Tests

With delegate_task

With systematic-debugging

Testing Anti-Patterns

Final Rule

​Skill metadata

​Reference: full SKILL.md

​Test-Driven Development (TDD)

​Overview

​When to Use

​The Iron Law

​Red-Green-Refactor Cycle

​RED — Write Failing Test

​Verify RED — Watch It Fail

​GREEN — Minimal Code

​Verify GREEN — Watch It Pass

​REFACTOR — Clean Up

​Repeat

​Avoid Horizontal Slices

​Why Order Matters

​Common Rationalizations

​Red Flags — STOP and Start Over

​Verification Checklist

​When Stuck

​Mibyan Integration

​Running Tests

​With delegate_task

​With systematic-debugging

​Testing Anti-Patterns

​Final Rule

Skill metadata

Reference: full SKILL.md

Test-Driven Development (TDD)

Overview

When to Use

The Iron Law

Red-Green-Refactor Cycle

RED — Write Failing Test

Verify RED — Watch It Fail

GREEN — Minimal Code

Verify GREEN — Watch It Pass

REFACTOR — Clean Up

Repeat

Avoid Horizontal Slices

Why Order Matters

Common Rationalizations

Red Flags — STOP and Start Over

Verification Checklist

When Stuck

Mibyan Integration

Running Tests

With delegate_task

With systematic-debugging

Testing Anti-Patterns

Final Rule