TDD

Value: Feedback -- short cycles with verifiable evidence keep AI-generated code honest and the human in control. Tests express intent; evidence confirms progress.

Purpose

Teaches a five-step TDD cycle (RED, DOMAIN, GREEN, DOMAIN, COMMIT) that adapts to whatever harness runs it. Detects available delegation primitives and routes to guided mode (human drives each phase) or automated mode (system orchestrates phases). Prevents primitive obsession, skipped reviews, and untested complexity regardless of mode.

Practices

The Five-Step Cycle

Every feature is built by repeating: RED -> DOMAIN -> GREEN -> DOMAIN -> COMMIT.

1. RED -- Write one failing test with one assertion. Only edit test files. Write the code you wish you had -- reference types and functions that do not exist yet. Run the test. Paste the failure output. Stop. Done when: tests run and FAIL (compilation error OR assertion failure).

2. DOMAIN (after RED) -- Review the test for primitive obsession and invalid-state risks. Create type definitions with stub bodies (todo!(), raise NotImplementedError, etc.). Do not implement logic. Stop. Done when: tests COMPILE but still FAIL (assertion/panic, not compilation error).

3. GREEN -- Write the minimal code to make the test pass. Only edit production files. Run the test. Paste passing output. Stop. Done when: tests PASS with minimal implementation.

4. DOMAIN (after GREEN) -- Review the implementation for domain violations: anemic models, leaked validation, primitive obsession that slipped through. If violations found, raise a concern and propose a revision. Done when: types are clean and tests still pass.

5. COMMIT -- Run the full test suite. Stage all changes and create a git commit referencing the GWT scenario. This is a hard gate: no new RED phase may begin until this commit exists. Done when: git commit created with all tests passing.

After step 5, either start the next RED phase or tidy the code (structural changes only, separate commit).

A compilation failure IS a test failure. Do not pre-create types to avoid compilation errors. Types flow FROM tests, never precede them.

Domain review has veto power over primitive obsession and invalid-state representability. Vetoes escalate to the human after two rounds.

User-Facing Modes

Guided mode (/tdd red, /tdd domain, /tdd green, /tdd commit): Each phase loads references/{phase}.md with detailed instructions for that step. For experienced engineers who want explicit phase control. Works on any harness -- no delegation primitives required. The human decides when to advance phases.

Automated mode (/tdd or /tdd auto): The system detects harness capabilities, selects an execution strategy, and orchestrates the full cycle. The user sees working code, not sausage-making. For verbose output showing phase transitions and evidence, use /tdd auto --verbose.

Capability Detection (Automated Mode)

When automated mode activates, detect available primitives in this order:

1. Agent teams available? Check for TeamCreate tool. If present, use the agent teams strategy with persistent pair sessions.
2. Subagents available? Check for Task tool (subagent spawning). If present, use the serial subagents strategy with focused per-phase agents.
3. Fallback. Use the chaining strategy -- role-switch internally between phases within a single context.

Select the most capable strategy available. Do not attempt a higher strategy when its primitives are missing.

Execution Strategy: Chaining (Fallback)

Used when no delegation primitives are available. The agent plays each role sequentially:

1. Load references/red.md. Execute the RED phase.
2. Load references/domain.md. Execute DOMAIN review of the test.
3. Load references/green.md. Execute the GREEN phase.
4. Load references/domain.md. Execute DOMAIN review of the implementation.
5. Load references/commit.md. Execute the COMMIT phase.
6. Repeat.

Role boundaries are advisory in this mode. The agent must self-enforce phase boundaries: only edit file types permitted by the current phase (see references/phase-boundaries.md).

Execution Strategy: Serial Subagents

Used when the Task tool is available for spawning focused subagents. Each phase runs in an isolated subagent with constrained scope.

• Spawn each phase agent using the prompt template in references/{phase}-prompt.md.
• The orchestrator follows references/orchestrator.md for coordination rules.
• Structural handoff schema (references/handoff-schema.md): every phase agent must return evidence fields (test output, file paths changed, domain concerns). Missing evidence fields = handoff blocked. The orchestrator does not proceed to the next phase until the schema is satisfied.
• Context isolation provides structural enforcement: each subagent receives only the files relevant to its phase.

Execution Strategy: Agent Teams

Used when TeamCreate is available for persistent agent sessions. Maximum enforcement through role specialization and persistent pair context.

• Follow references/ping-pong-pairing.md for pair session lifecycle, role selection, structured handoffs, and drill-down ownership.
• Both engineers persist for the entire TDD cycle of a vertical slice. Handoffs happen via lightweight structured messages, not agent recreation.
• Track pairing history in .team/pairing-history.json. Do not repeat either of the last 2 pairings.
• The orchestrator monitors and intervenes only for external clarification routing or blocking disagreements.

Phase Boundary Rules

Each phase edits only its own file types. This prevents drift. See references/phase-boundaries.md for the complete file-type matrix.

If blocked by a boundary, stop and return to the orchestrator (automated) or report to the user (guided). Never circumvent boundaries.

Walking Skeleton First

The first vertical slice must be a walking skeleton: the thinnest end-to-end path proving all architectural layers connect. It may use hardcoded values or stubs. Build it before any other slice. It de-risks the architecture and gives subsequent slices a proven wiring path to extend.

Outside-In TDD

Start from an acceptance test at the application boundary -- the point where external input enters the system. Drill inward through unit tests. The outer acceptance test stays RED while inner unit tests go through their own red-green-domain-commit cycles. The slice is complete only when the outer acceptance test passes.

A test that calls internal functions directly is a unit test, not an acceptance test -- even if it asserts on user-visible behavior.

Harness-Specific Guidance

If running on Claude Code, also read references/claude-code.md for harness-specific rules including hook-based enforcement. For maximum mechanical enforcement, ask the bootstrap skill to install optional hooks from references/hooks/claude-code-hooks.json.

Enforcement Note

Enforcement is proportional to capability:

• Guided mode: Advisory. The skill text instructs correct behavior but cannot prevent violations. The human enforces by controlling phase transitions.
• Automated mode (chaining): Advisory with self-enforcement. The agent follows phase boundaries by convention.
• Automated mode (serial subagents): Structural enforcement via context isolation and handoff schemas. Subagents receive only phase-relevant files. Missing evidence blocks handoffs.
• Automated mode (agent teams): Maximum enforcement through role specialization. Neither engineer can skip review because the other is watching. Persistent context means accumulated understanding, not just rules.
• Optional hooks (Claude Code): Mechanical enforcement. Pre-tool-use hooks block unauthorized file edits per phase. See references/claude-code.md.

No mode guarantees perfect discipline. If you observe violations -- production code edited during RED, domain review skipped, commits missing -- point it out.

Verification

After completing a cycle, verify:

• Every failing test was written BEFORE its implementation
• Domain review occurred after EVERY RED and GREEN phase
• Phase boundary rules were respected (file-type restrictions)
• Evidence (test output) was provided at each handoff
• Commit exists for every completed RED-GREEN cycle
• Walking skeleton completed first (first vertical slice)

HARD GATE -- COMMIT (must pass before any new RED phase):

• All tests pass
• Git commit created with message referencing the current GWT scenario
• No new RED phase started before this commit was made

Dependencies

This skill works standalone. For enhanced workflows, it integrates with:

• domain-modeling: Strengthens the domain review phases with parse-don't-validate, semantic types, and invalid-state prevention principles.
• code-review: Three-stage review (spec compliance, code quality, domain integrity) after TDD cycles complete.
• mutation-testing: Validates test quality by checking that tests detect injected mutations in production code.
• ensemble-team: Provides real-world expert personas for pair selection and mob review.

Missing a dependency? Install with:

npx skills add jwilger/agent-skills --skill domain-modeling