msd-core/gsd-core/references/tdd.md

TDD is about design quality, not coverage metrics. The red-green-refactor cycle forces you to think about behavior before implementation, producing cleaner interfaces and more testable code.

Principle: If you can describe the behavior as expect(fn(input)).toBe(output) before writing fn, TDD improves the result.

Key insight: TDD work is fundamentally heavier than standard tasks—it requires 2-3 execution cycles (RED → GREEN → REFACTOR), each with file reads, test runs, and potential debugging. TDD features get dedicated plans to ensure full context is available throughout the cycle.

<when_to_use_tdd>

When TDD Improves Quality

TDD candidates (create a TDD plan):

• Business logic with defined inputs/outputs
• API endpoints with request/response contracts
• Data transformations, parsing, formatting
• Validation rules and constraints
• Algorithms with testable behavior
• State machines and workflows
• Utility functions with clear specifications

Skip TDD (use standard plan with type="auto" tasks):

• UI layout, styling, visual components
• Configuration changes
• Glue code connecting existing components
• One-off scripts and migrations
• Simple CRUD with no business logic
• Exploratory prototyping

Heuristic: Can you write expect(fn(input)).toBe(output) before writing fn? → Yes: Create a TDD plan → No: Use standard plan, add tests after if needed </when_to_use_tdd>

<tdd_plan_structure>

TDD Plan Structure

Each TDD plan implements one feature through the full RED-GREEN-REFACTOR cycle.

---
phase: XX-name
plan: NN
type: tdd
---

<objective>
[What feature and why]
Purpose: [Design benefit of TDD for this feature]
Output: [Working, tested feature]
</objective>

<context>
@.planning/PROJECT.md
@.planning/ROADMAP.md
@relevant/source/files.ts
</context>

<feature>
  <name>[Feature name]</name>
  <files>[source file, test file]</files>
  <behavior>
    [Expected behavior in testable terms]
    Cases: input → expected output
  </behavior>
  <implementation>[How to implement once tests pass]</implementation>
</feature>

<verification>
[Test command that proves feature works]
</verification>

<success_criteria>
- Failing test written and committed
- Implementation passes test
- Refactor complete (if needed)
- All 2-3 commits present
</success_criteria>

<output>
After completion, create SUMMARY.md with:
- RED: What test was written, why it failed
- GREEN: What implementation made it pass
- REFACTOR: What cleanup was done (if any)
- Commits: List of commits produced
</output>

One feature per TDD plan. If features are trivial enough to batch, they're trivial enough to skip TDD—use a standard plan and add tests after. </tdd_plan_structure>

<execution_flow>

Red-Green-Refactor Cycle

RED - Write failing test:

1. Create test file following project conventions
2. Write test describing expected behavior (from <behavior> element)
3. Run test - it MUST fail intentionally (#3770): the TARGET test you named must be the test that fails, on an assertion for the planned behavior. A nonzero exit alone is NOT RED — syntax errors, zero-test discovery, fixture crashes, parser errors, and unrelated assertions are INVALID_RED and must not authorize GREEN.
4. Persist the RED evidence record (command, exit code, failing test, expected result, actual result) and verify it: gsd_run check tdd-red-evidence <record.json>. Only verdict RED_EVIDENCE_OK satisfies the RED gate; INVALID_RED blocks GREEN until the RED phase is fixed.
5. If test passes: feature exists or test is wrong. Investigate.
6. Commit: test({phase}-{plan}): add failing test for [feature]

GREEN - Implement to pass:

1. Write minimal code to make test pass
2. No cleverness, no optimization - just make it work
3. Run test - it MUST pass
4. Commit: feat({phase}-{plan}): implement [feature]

REFACTOR (if needed):

1. Clean up implementation if obvious improvements exist
2. Run tests - MUST still pass
3. Only commit if changes made: refactor({phase}-{plan}): clean up [feature]

Result: Each TDD plan produces 2-3 atomic commits. </execution_flow>

<test_quality>

Good Tests vs Bad Tests

Test behavior, not implementation:

• Good: "returns formatted date string"
• Bad: "calls formatDate helper with correct params"
• Tests should survive refactors

One concept per test:

• Good: Separate tests for valid input, empty input, malformed input
• Bad: Single test checking all edge cases with multiple assertions

Descriptive names:

• Good: "should reject empty email", "returns null for invalid ID"
• Bad: "test1", "handles error", "works correctly"

No implementation details:

• Good: Test public API, observable behavior
• Bad: Mock internals, test private methods, assert on internal state </test_quality>

<framework_setup>

Test Framework Setup (If None Exists)

When executing a TDD plan but no test framework is configured, set it up as part of the RED phase:

1. Detect project type:

# JavaScript/TypeScript
if [ -f package.json ]; then echo "node"; fi

# Python
if [ -f requirements.txt ] || [ -f pyproject.toml ]; then echo "python"; fi

# Go
if [ -f go.mod ]; then echo "go"; fi

# Rust
if [ -f Cargo.toml ]; then echo "rust"; fi

2. Install minimal framework:

Project	Framework	Install
Node.js	Jest	npm install -D jest @types/jest ts-jest
Node.js (Vite)	Vitest	npm install -D vitest
Python	pytest	pip install pytest
Go	testing	Built-in
Rust	cargo test	Built-in

3. Create config if needed:

• Jest: jest.config.js with ts-jest preset
• Vitest: vitest.config.ts with test globals
• pytest: pytest.ini or pyproject.toml section

4. Verify setup:

# Run empty test suite - should pass with 0 tests
npm test  # Node
pytest    # Python
go test ./...  # Go
cargo test    # Rust

5. Create first test file: Follow project conventions for test location:

• *.test.ts / *.spec.ts next to source
• __tests__/ directory
• tests/ directory at root

Framework setup is a one-time cost included in the first TDD plan's RED phase. </framework_setup>

<error_handling>

Error Handling

Test doesn't fail in RED phase:

• Feature may already exist - investigate
• Test may be wrong (not testing what you think)
• Fix before proceeding

Test doesn't pass in GREEN phase:

• Debug implementation
• Don't skip to refactor
• Keep iterating until green

Tests fail in REFACTOR phase:

• Undo refactor
• Commit was premature
• Refactor in smaller steps

Unrelated tests break:

• Stop and investigate
• May indicate coupling issue
• Fix before proceeding </error_handling>

<commit_pattern>

Commit Pattern for TDD Plans

TDD plans produce 2-3 atomic commits (one per phase):

test(08-02): add failing test for email validation

- Tests valid email formats accepted
- Tests invalid formats rejected
- Tests empty input handling

feat(08-02): implement email validation

- Regex pattern matches RFC 5322
- Returns boolean for validity
- Handles edge cases (empty, null)

refactor(08-02): extract regex to constant (optional)

- Moved pattern to EMAIL_REGEX constant
- No behavior changes
- Tests still pass

Comparison with standard plans:

• Standard plans: 1 commit per task, 2-4 commits per plan
• TDD plans: 2-3 commits for single feature

Both follow same format: {type}({phase}-{plan}): {description}

Benefits:

• Each commit independently revertable
• Git bisect works at commit level
• Clear history showing TDD discipline
• Consistent with overall commit strategy </commit_pattern>

<gate_enforcement>

Gate Enforcement Rules

When workflow.tdd_mode is enabled in config, the RED/GREEN/REFACTOR gate sequence is enforced for all type: tdd plans.

Gate Definitions

Gate	Required	Commit Pattern	Validation
RED	Yes	test({phase}-{plan}): ...	Test exists AND fails before implementation — intentionally: check tdd-red-evidence returns RED_EVIDENCE_OK (target test failed on an assertion for the behavior; anything else is INVALID_RED)
GREEN	Yes	feat({phase}-{plan}): ...	Test passes after implementation
REFACTOR	No	refactor({phase}-{plan}): ...	Tests still pass after cleanup

Fail-Fast Rules

1. Unexpected GREEN in RED phase: If the test passes before any implementation code is written, STOP. The feature may already exist or the test is wrong. Investigate before proceeding.
2. INVALID_RED in RED phase (#3770): A nonzero exit is not RED by itself. Zero-test discovery, fixture/load crashes, nonzero exits with no failing test, unrelated failing tests, and unexpected greens all classify as INVALID_RED (gsd_run check tdd-red-evidence). STOP and fix the RED phase — do NOT proceed to GREEN.
3. Missing RED commit: If no test(...) commit precedes the feat(...) commit, the TDD discipline was violated. Flag in SUMMARY.md.
4. REFACTOR breaks tests: Undo the refactor immediately. Commit was premature — refactor in smaller steps.

Executor Gate Validation

After completing a type: tdd plan, the executor validates the git log:

# The commit protocol promises no zero-padding for ${PHASE}/${PLAN} — strip both and
# match the commit-scope position anchored (#4003). #4619: PHASE may be decimal/
# N-segment; zero-strip only the leading integer segment, escape the rest.
PHASE_INT=${PHASE%%.*}; PHASE_FRAC=${PHASE#"$PHASE_INT"}
PHASE_N="$((10#$PHASE_INT))${PHASE_FRAC//./\\.}"
PLAN_N=$((10#${PLAN}))
# Check for RED gate commit
git log --oneline -E --grep="^test\((0*${PHASE_N})-(0*${PLAN_N})\):" | head -1
# Check for GREEN gate commit  
git log --oneline -E --grep="^feat\((0*${PHASE_N})-(0*${PLAN_N})\):" | head -1
# Check for optional REFACTOR gate commit
git log --oneline -E --grep="^refactor\((0*${PHASE_N})-(0*${PLAN_N})\):" | head -1

If RED or GREEN gate commits are missing, add a ## TDD Gate Compliance section to SUMMARY.md with the violation details. </gate_enforcement>

<end_of_phase_review>

End-of-Phase TDD Review Checkpoint

When workflow.tdd_mode is enabled, the execute-phase orchestrator inserts a collaborative review checkpoint after all waves complete but before phase verification.

Review Checkpoint Format

### TDD REVIEW — Phase {X}

TDD Plans: {count} | Gate violations: {count}

| Plan | RED | GREEN | REFACTOR | Status |
|------|-----|-------|----------|--------|
| {id} |  ✓  |   ✓   |    ✓     | Pass   |
| {id} |  ✓  |   ✗   |    —     | FAIL   |

{If violations exist:}
⚠ Gate violations are advisory — review before advancing.

What the Review Checks

1. Gate sequence: Each TDD plan has RED → GREEN commits in order
2. Test quality: RED phase tests fail for the right reason (not import errors or syntax)
3. Minimal GREEN: Implementation is minimal — no premature optimization in GREEN phase
4. Refactor discipline: If REFACTOR commit exists, tests still pass

This checkpoint is advisory — it does not block phase completion but surfaces TDD discipline issues for human review. </end_of_phase_review>

<context_budget>

Context Budget

TDD plans target ~40% context usage (lower than standard plans' ~50%).

Why lower:

• RED phase: write test, run test, potentially debug why it didn't fail
• GREEN phase: implement, run test, potentially iterate on failures
• REFACTOR phase: modify code, run tests, verify no regressions

Each phase involves reading files, running commands, analyzing output. The back-and-forth is inherently heavier than linear task execution.

Single feature focus ensures full quality throughout the cycle. </context_budget>