Incremental TDD

Incremental TDD

Overview

Write ONE test. Make it pass. Repeat.

TDD is not "write all tests, then implement." It's an incremental cycle where each test teaches you what to build next.

Core principle: Writing tests in batch defeats the design discovery benefit of TDD.

The Iron Law

ONE FAILING TEST AT A TIME

Write a single failing test → Implement minimal code to pass → Refactor if needed → Write NEXT failing test.

No exceptions:

• Not "write a few failing tests"
• Not "cover main scenarios first"
• Not "write comprehensive test suite then implement"
• ONE failing test, make it pass, then next failing test

Clarification: Having multiple GREEN (passing) tests is fine. The rule is about FAILING tests. Only write ONE failing test at a time.

The Cycle

1. Write ONE simple test (simplest case)
2. Run ALL tests - watch it fail (RED)
3. Write minimal code to pass (GREEN)
4. Run ALL tests - verify all pass (still GREEN)
5. Refactor if needed (REFACTOR)
   5a. Check code quality (duplication, complexity)
   5b. Review ADR compliance (dispatch reviewer subagent with adr-review skill)
   5c. Fix issues found
6. Run ALL tests - verify still passing
7. Commit
8. GOTO 1 for next test

IMPORTANT: Always run ALL tests, not just the one you're working on.

This catches regressions immediately. If you only run the current test, you won't notice if your changes broke existing functionality.

REFACTOR phase includes ADR review: After tests pass, dispatch a reviewer subagent with the adr-review skill to check compliance with all project ADRs. This catches issues that tests don't verify (naming, structure, documentation standards).

Two Stages of RED

Compile error? Add empty stub. Run again. Then implement.

Compile error ≠ implement. Stub first, run again, see behavior failure, THEN implement.

Why One Test at a Time Matters

The REFACTOR step is where design emerges.

When you write ONE test at a time:

1. RED: Write one failing test
2. GREEN: Minimal code to pass
3. REFACTOR: Evaluate the emerging design, improve it
4. Now you understand the design → inform next test

Critical insight: After each test passes, you can evaluate the design and discover what's next. Writing 3-5 tests at once commits you to a design before seeing what you need.

❌ Batch: Write 5 tests → implement everything ✓ Incremental: Write ONE test → implement → evaluate → next test

ADR Review in REFACTOR Phase

After tests pass (GREEN), check ADR compliance BEFORE committing.

Use a reviewer subagent with adr-review skill:

**Dispatch reviewer subagent:**

You are reviewing code for ADR compliance in the REFACTOR phase.

**Your task:** Use the adr-review skill to review [file/​function]

**Instructions:**
1. Use Skill tool to load: adr-review
2. Follow the skill to review code against all ADRs
3. Report violations with specific line numbers and fixes
4. STOP and report back

Be strict - flag all violations per the skill.

After review:

• Fix violations found
• Re-run ALL tests (ensure still GREEN)
• Only then commit

Why separate subagent?

• Separates concerns: behavior (tests) vs structure (ADRs)
• Reviewer can't rationalize "close enough"
• Clear authority: reviewer reports, you decide

When to skip ADR review:

• No ADRs exist in project yet
• Trivial change (typo fix, comment update)
• Emergency hotfix (review after deployment)

Quick Reference

Common Rationalizations (All Wrong)

Seeing yourself in this table? Write ONE test. Stop rationalizing.

Red Flags - You're Doing Batch TDD

STOP if you:

• Write more than one FAILING test() / it() block before implementing
• Think "let me cover the main scenarios" and write them all
• Plan out all test cases AND write them all at once
• Write multiple failing tests numbered test1, test2, test3
• Jump straight from compile error to full implementation (skipping stage 2 RED)
• Think "I'll just implement it all at once since I know what it needs"

All of these mean: Keep only ONE failing test. Comment out or delete the rest. Make it pass. Then uncomment/​write the next one.

For two-stage RED: Add stub first, see behavior failure, THEN implement.

Integration with RED-GREEN-REFACTOR

This skill enforces the incremental nature of RED-GREEN-REFACTOR:

1. RED: Write ONE failing test
2. GREEN: Minimal code to pass THAT test
3. REFACTOR: Clean up if needed
4. Repeat: Write NEXT test

You cannot skip step 2 (implementing) by writing multiple tests in step 1.

The Bottom Line

TDD means ONE test at a time.

Not "tests first, then code." Not "batch of tests, then batch of code."

One test → Implementation → Next test.

That's the cycle. Follow it.