---

name: kaizen description: Use when Code implementation and refactoring, architecturing or designing systems, process and workflow improvements, error handling and validation. Provide tehniquest to avoid over-engineering and apply iterative improvements.

Kaizen: Continuous Improvement

Apply continuous improvement mindset - suggest small iterative improvements, error-proof designs, follow established patterns, avoid over-engineering; automatically applied to guide quality and simplicity

Overview

Small improvements, continuously. Error-proof by design. Follow what works. Build only what's needed.

Core principle: Many small improvements beat one big change. Prevent errors at design time, not with fixes.

When to Use

Always applied for:

• Code implementation and refactoring
• Architecture and design decisions
• Process and workflow improvements
• Error handling and validation

Philosophy: Quality through incremental progress and prevention, not perfection through massive effort.

The Four Pillars

1. Continuous Improvement (Kaizen)

Small, frequent improvements compound into major gains.

Principles

Incremental over revolutionary:

• Make smallest viable change that improves quality
• One improvement at a time
• Verify each change before next
• Build momentum through small wins

Always leave code better:

• Fix small issues as you encounter them
• Refactor while you work (within scope)
• Update outdated comments
• Remove dead code when you see it

Iterative refinement:

• First version: make it work
• Second pass: make it clear
• Third pass: make it efficient
• Don't try all three at once

// Iteration 2: Make it clear (refactor) const calculateTotal = (items: Item[]): number => { return items.reduce((total, item) => { return total + (item.price * item.quantity); }, 0); };

// Iteration 3: Make it robust (add validation) const calculateTotal = (items: Item[]): number => { if (!items?.length) return 0;

return items.reduce((total, item) => { if (item.price < 0 || item.quantity < 0) { throw new Error('Price and quantity must be non-negative'); } return total + (item.price * item.quantity); }, 0); };

Each step is complete, tested, and working
</Good>

<Bad>
```typescript
// Trying to do everything at once
const calculateTotal = (items: Item[]): number => {
  // Validate, optimize, add features, handle edge cases all together
  if (!items?.length) return 0;
  const validItems = items.filter(item => {
    if (item.price < 0) throw new Error('Negative price');
    if (item.quantity < 0) throw new Error('Negative quantity');
    return item.quantity > 0; // Also filtering zero quantities
  });
  // Plus caching, plus logging, plus currency conversion...
  return validItems.reduce(...); // Too many concerns at once
};

Overwhelming, error-prone, hard to verify

In Practice

When implementing features:

1. Start with simplest version that works
2. Add one improvement (error handling, validation, etc.)
3. Test and verify
4. Repeat if time permits
5. Don't try to make it perfect immediately

When refactoring:

• Fix one smell at a time
• Commit after each improvement
• Keep tests passing throughout
• Stop when "good enough" (diminishing returns)

When reviewing code:

• Suggest incremental improvements (not rewrites)
• Prioritize: critical → important → nice-to-have
• Focus on highest-impact changes first
• Accept "better than before" even if not perfect

2. Poka-Yoke (Error Proofing)

Design systems that prevent errors at compile/design time, not runtime.

Principles

Make errors impossible:

• Type system catches mistakes
• Compiler enforces contracts
• Invalid states unrepresentable
• Errors caught early (left of production)

Design for safety:

• Fail fast and loudly
• Provide helpful error messages
• Make correct path obvious
• Make incorrect path difficult

Defense in layers:

1. Type system (compile time)
2. Validation (runtime, early)
3. Guards (preconditions)
4. Error boundaries (graceful degradation)

Type System Error Proofing

// Good: Only valid states possible type OrderStatus = 'pending' | 'processing' | 'shipped' | 'delivered'; type Order = { status: OrderStatus; total: number; };

// Better: States with associated data type Order = | { status: 'pending'; createdAt: Date } | { status: 'processing'; startedAt: Date; estimatedCompletion: Date } | { status: 'shipped'; trackingNumber: string; shippedAt: Date } | { status: 'delivered'; deliveredAt: Date; signature: string };

// Now impossible to have shipped without trackingNumber

Type system prevents entire classes of errors
</Good>

<Good>
```typescript
// Make invalid states unrepresentable
type NonEmptyArray<T> = [T, ...T[]];

const firstItem = <T>(items: NonEmptyArray<T>): T => {
  return items[0]; // Always safe, never undefined!
};

// Caller must prove array is non-empty
const items: number[] = [1, 2, 3];
if (items.length > 0) {
  firstItem(items as NonEmptyArray<number>); // Safe
}

Function signature guarantees safety

Validation Error Proofing

// Good: Validate immediately const processPayment = (amount: number) => { if (amount <= 0) { throw new Error('Payment amount must be positive'); } if (amount > 10000) { throw new Error('Payment exceeds maximum allowed'); }

const fee = amount * 0.03; // ... now safe to use };

// Better: Validation at boundary with branded type type PositiveNumber = number & { readonly __brand: 'PositiveNumber' };

const validatePositive = (n: number): PositiveNumber => { if (n <= 0) throw new Error('Must be positive'); return n as PositiveNumber; };

const processPayment = (amount: PositiveNumber) => { // amount is guaranteed positive, no need to check const fee = amount * 0.03; };

// Validate at system boundary const handlePaymentRequest = (req: Request) => { const amount = validatePositive(req.body.amount); // Validate once processPayment(amount); // Use everywhere safely };

Validate once at boundary, safe everywhere else
</Good>

#### Guards and Preconditions

<Good>
```typescript
// Early returns prevent deeply nested code
const processUser = (user: User | null) => {
  if (!user) {
    logger.error('User not found');
    return;
  }
  
  if (!user.email) {
    logger.error('User email missing');
    return;
  }
  
  if (!user.isActive) {
    logger.info('User inactive, skipping');
    return;
  }
  
  // Main logic here, guaranteed user is valid and active
  sendEmail(user.email, 'Welcome!');
};

Guards make assumptions explicit and enforced

Configuration Error Proofing

const client = new APIClient({ timeout: 5000 }); // apiKey missing!

// Good: Required config, fails early type Config = { apiKey: string; timeout: number; };

const loadConfig = (): Config => { const apiKey = process.env.API_KEY; if (!apiKey) { throw new Error('API_KEY environment variable required'); }

return { apiKey, timeout: 5000, }; };

// App fails at startup if config invalid, not during request const config = loadConfig(); const client = new APIClient(config);

Fail at startup, not in production
</Good>

#### In Practice

**When designing APIs:**
- Use types to constrain inputs
- Make invalid states unrepresentable
- Return Result<T, E> instead of throwing
- Document preconditions in types

**When handling errors:**
- Validate at system boundaries
- Use guards for preconditions
- Fail fast with clear messages
- Log context for debugging

**When configuring:**
- Required over optional with defaults
- Validate all config at startup
- Fail deployment if config invalid
- Don't allow partial configurations

### 3. Standardized Work

Follow established patterns. Document what works. Make good practices easy to follow.

#### Principles

**Consistency over cleverness:**
- Follow existing codebase patterns
- Don't reinvent solved problems
- New pattern only if significantly better
- Team agreement on new patterns

**Documentation lives with code:**
- README for setup and architecture
- CLAUDE.md for AI coding conventions
- Comments for "why", not "what"
- Examples for complex patterns

**Automate standards:**
- Linters enforce style
- Type checks enforce contracts
- Tests verify behavior
- CI/CD enforces quality gates

#### Following Patterns

<Good>
```typescript
// Existing codebase pattern for API clients
class UserAPIClient {
  async getUser(id: string): Promise<User> {
    return this.fetch(`/users/${id}`);
  }
}

// New code follows the same pattern
class OrderAPIClient {
  async getOrder(id: string): Promise<Order> {
    return this.fetch(`/orders/${id}`);
  }
}

Consistency makes codebase predictable

// New code introduces different pattern without discussion const getOrder = async (id: string): Promise => { // Breaking consistency "because I prefer functions" };

Inconsistency creates confusion
</Bad>

#### Error Handling Patterns

<Good>
```typescript
// Project standard: Result type for recoverable errors
type Result<T, E> = {