| name | test-driven-development |
| description | Use for any feature, bug fix, refactor, or behavior change — enforces RED-GREEN-REFACTOR. Write the failing test first, watch it fail, write minimal code to pass, refactor. Carve out only throwaway prototypes, generated code, configuration files. Adapted from Hermes Agent / obra/superpowers. |
| lastReviewed | "2026-06-07T00:00:00.000Z" |
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 (use spike instead)
- 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'
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.
Run the specific test through the workspace terminal:
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
Bad:
def add(a, b):
result = a + b
logging.info(f"Adding {a} + {b} = {result}")
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.
pytest tests/test_feature.py::test_specific_behavior -v
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.
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:
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. |
Integration With Other Skills
With agent-delegation
When dispatching a worker subagent (per agent-delegation), enforce TDD in the goal:
"Implement [feature] using strict TDD. Follow test-driven-development skill: write failing test FIRST, run to verify failure, write minimal code to pass, run to verify pass, refactor if needed, commit. Project test command: pytest tests/ -q."
With systematic-debugging
Bug found? Write failing test reproducing it. Follow TDD cycle. The test proves the fix and prevents regression. See systematic-debugging — never fix bugs without a test.
With plan
plan calls for TDD per task. Every implementation task in a plan should start with RED.
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.
Related
- systematic-debugging — bug-found path that produces the failing test
- plan — every plan task should embed RED-GREEN-REFACTOR
- spike — TDD exception lane for throwaway feasibility experiments
- code-review — post-write companion; TDD is pre-write, code-review is the review gate
- agent-delegation — enforce TDD on delegated work via the goal prompt
Would Revise If
- Event-based: ≥3 heir feedback files flagging the Iron Law as obstruction within 90 days; OR zero RED-GREEN-REFACTOR cycle citations in heir commits by 2026-09-07 (skill is decorative — heirs ignore it). First trigger sinks to Mall (still installable on demand) rather than removing entirely.
- Date-based: 2026-09-07 (90 days from adoption). If the "When to Use" carve-outs (throwaway prototypes, generated code, configuration files) consistently mis-classify heir work and produce friction, broaden the carve-outs explicitly.
- Counter-evidence: if a heir reports that TDD enforcement caused a real regression (test-first led to a worse design that test-after would have caught), document the case and revise the Iron Law's absoluteness.
Attribution
Adapted from Hermes Agent (Nous Research, MIT), which itself adapted the discipline from obra/superpowers (MIT). Both upstream sources MIT-licensed.