bullet-tracer（子弹追踪器）

Bullet Tracer Skill

From The Pragmatic Programmer: "Use Tracer Bullets to Find the Target" — build a minimal end-to-end vertical slice that touches all layers first, then expand.

Compatibility Note

Not all tickets/features are fully compatible with the tracer bullet approach (e.g., pure refactors, config changes, single-layer fixes). Be smart: apply tracer bullet concepts to compatible parts, use general best practices for the rest.

Philosophy

• Tracer bullet!= prototype: You keep the code, it becomes the foundation
• Vertical slice: Touch ALL layers (UI → API → logic → DB → tests) in one thin path
• Fast feedback: Validate architecture and integration points early
• Hardcoded is OK: Initial tracer can use minimal/fake data

Workflow

Phase 1: Tracer Bullet (MUST DO FIRST)

1. Identify all layers the feature touches (e.g., frontend, API, service, DB, config)
2. Implement the thinnest possible path through ALL layers:

- One happy path only - Hardcoded values acceptable - Skip edge cases, validation, error handling - Must be runnable/testable end-to-end

1. Write one integration test that exercises the full path
2. Verify it works — run the test, manually test if needed

Phase 2: Expand

Only after tracer works, expand systematically:

1. Replace hardcoded values with real implementations
2. Add validation and error handling
3. Add edge cases and additional paths
4. Add unit tests for individual components
5. Refactor for production quality

Example

Feature: "User can save favorite items"

Tracer (Phase 1):

- Frontend: Add button that calls POST /api/favorites with hardcoded itemId
- API: Endpoint that inserts into favorites table
- DB: Create favorites table with user_id, item_id
- Test: Integration test that clicks button, verifies DB row exists

Expand (Phase 2):

• Real item selection UI
• Validation (item exists, not already favorited)
• Error handling and user feedback
• Unfavorite functionality
• List favorites page
• Unit tests for each layer

Rules

1. NEVER skip the tracer phase — always build e2e first
2. NEVER expand before tracer works — resist the urge to "do it properly"
3. Keep tracer minimal — if you're adding "nice to haves", stop
4. Tracer must touch ALL layers — if it doesn't, it's not a tracer
5. Tracer must be testable — if you can't verify it works, add a test

Anti-patterns

❌ Building the perfect DB schema before any code ❌ Implementing full validation before happy path works ❌ Writing all unit tests before integration test ❌ Building frontend in isolation from backend ❌ "I'll connect it later"

Checklist

Before moving to Phase 2, verify:

• Code touches every layer the feature needs
• Can run/test the feature end-to-end
• At least one integration test passes