---
name: kaizen
description: Guide for continuous improvement, error proofing, and standardization. Use this skill when the user wants to improve code quality, refactor, or discuss process improvements.
---

# Kaizen: Continuous Improvement

## 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- Update outdated comments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

<Good>
```typescript
// Iteration 1: Make it work
const calculateTotal = (items: Item[]) => {
  let total = 0;
  for (let i = 0; i < items.length; i++) {
    total += items[i].price * items[i].quantity;
  }
  return total;
};

// 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
</Bad>

#### 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>
```typescript
// Error: string status can be any value
type OrderBad = {
  status: string; // Can be "pending", "PENDING", "pnding", anything!
  total: number;
};

// 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
</Good>

#### Validation Error Proofing

<Good>
```typescript
// Error: Validation after use
const processPayment = (amount: number) => {
  const fee = amount * 0.03; // Used before validation!
  if (amount <= 0) throw new Error('Invalid amount');
  // ...
};

// 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
</Good>

#### Configuration Error Proofing

<Good>
```typescript
// Error: Optional config with unsafe defaults
type ConfigBad = {
  apiKey?: string;
  timeout?: number;
};

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
</Good>

<Bad>
```typescript
// Existing pattern uses classes
class UserAPIClient { /* ... */ }

// New code introduces different pattern without discussion
const getOrder = async (id: string): Promise<Order> => {
// 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> = { ok: true; value: T } | { ok: false; error: E };

// All services follow this pattern
const fetchUser = async (id: string): Promise<Result<User, Error>> => {
  try {
    const user = await db.users.findById(id);
    if (!user) {
      return { ok: false, error: new Error('User not found') };
    }
    return { ok: true, value: user };
  } catch (err) {
    return { ok: false, error: err as Error };
  }
};

// Callers use consistent pattern
const result = await fetchUser('123');
if (!result.ok) {
  logger.error('Failed to fetch user', result.error);
  return;
}
const user = result.value; // Type-safe!
````

Standard pattern across codebase
</Good>

#### Documentation Standards

<Good>
```typescript
/**
 * Retries an async operation with exponential backoff.
 *
 * Why: Network requests fail temporarily; retrying improves reliability
 * When to use: External API calls, database operations
 * When not to use: User input validation, internal function calls
 *
 * @example
 * const result = await retry(
 *   () => fetch('https://api.example.com/data'),
 *   { maxAttempts: 3, baseDelay: 1000 }
 * );
 */
const retry = async <T>(
  operation: () => Promise<T>,
  options: RetryOptions
): Promise<T> => {
  // Implementation...
};
```
Documents why, when, and how
</Good>

#### In Practice

**Before adding new patterns:**

- Search codebase for similar problems solved
- Check CLAUDE.md for project conventions
- Discuss with team if breaking from pattern
- Update docs when introducing new pattern

**When writing code:**

- Match existing file structure
- Use same naming conventions
- Follow same error handling approach
- Import from same locations

**When reviewing:**

- Check consistency with existing code
- Point to examples in codebase
- Suggest aligning with standards
- Update CLAUDE.md if new standard emerges

### 4. Just-In-Time (JIT)

Build what's needed now. No more, no less. Avoid premature optimization and over-engineering.

#### Principles

**YAGNI (You Aren't Gonna Need It):**

- Implement only current requirements
- No "just in case" features
- No "we might need this later" code
- Delete speculation

**Simplest thing that works:**

- Start with straightforward solution
- Add complexity only when needed
- Refactor when requirements change
- Don't anticipate future needs

**Optimize when measured:**

- No premature optimization
- Profile before optimizing
- Measure impact of changes
- Accept "good enough" performance

#### YAGNI in Action

<Good>
```typescript
// Current requirement: Log errors to console
const logError = (error: Error) => {
  console.error(error.message);
};
```
Simple, meets current need
</Good>

<Bad>
```typescript
// Over-engineered for "future needs"
interface LogTransport {
  write(level: LogLevel, message: string, meta?: LogMetadata): Promise<void>;
}

class ConsoleTransport implements LogTransport { /_... _/ }
class FileTransport implements LogTransport { /_ ... _/ }
class RemoteTransport implements LogTransport { /_ ..._/ }

class Logger {
private transports: LogTransport[] = [];
private queue: LogEntry[] = [];
private rateLimiter: RateLimiter;
private formatter: LogFormatter;

// 200 lines of code for "maybe we'll need it"
}

const logError = (error: Error) => {
Logger.getInstance().log('error', error.message);
};

````
Building for imaginary future requirements
</Bad>

**When to add complexity:**
- Current requirement demands it
- Pain points identified through use
- Measured performance issues
- Multiple use cases emerged

<Good>
```typescript
// Start simple
const formatCurrency = (amount: number): string => {
  return `$${amount.toFixed(2)}`;
};

// Requirement evolves: support multiple currencies
const formatCurrency = (amount: number, currency: string): string => {
  const symbols = { USD: '$', EUR: '€', GBP: '£' };
  return `${symbols[currency]}${amount.toFixed(2)}`;
};

// Requirement evolves: support localization
const formatCurrency = (amount: number, locale: string): string => {
  return new Intl.NumberFormat(locale, {\n    style: 'currency',
    currency: locale === 'en-US' ? 'USD' : 'EUR',
  }).format(amount);
};
````

Complexity added only when needed
</Good>

#### Premature Abstraction

<Bad>
```typescript
// One use case, but building generic framework
abstract class BaseCRUDService<T> {
  abstract getAll(): Promise<T[]>;
  abstract getById(id: string): Promise<T>;
  abstract create(data: Partial<T>): Promise<T>;
  abstract update(id: string, data: Partial<T>): Promise<T>;
  abstract delete(id: string): Promise<void>;
}

class GenericRepository<T> { /_300 lines _/ }
class QueryBuilder<T> { /_ 200 lines_/ }
// ... building entire ORM for single table

````
Massive abstraction for uncertain future
</Bad>

<Good>
```typescript
// Simple functions for current needs
const getUsers = async (): Promise<User[]> => {
  return db.query('SELECT * FROM users');
};

const getUserById = async (id: string): Promise<User | null> => {
  return db.query('SELECT * FROM users WHERE id = $1', [id]);
};

// When pattern emerges across multiple entities, then abstract
````

Abstract only when pattern proven across 3+ cases
</Good>

#### Performance Optimization

<Good>
```typescript
// Current: Simple approach
const filterActiveUsers = (users: User[]): User[] => {
  return users.filter(user => user.isActive);
};

// Benchmark shows: 50ms for 1000 users (acceptable)
// ✓ Ship it, no optimization needed

// Later: After profiling shows this is bottleneck
// Then optimize with indexed lookup or caching

````
Optimize based on measurement, not assumptions
</Good>

<Bad>
```typescript
// Premature optimization
const filterActiveUsers = (users: User[]): User[] => {
  // "This might be slow, so let's cache and index"
  const cache = new WeakMap();
  const indexed = buildBTreeIndex(users, 'isActive');
  // 100 lines of optimization code
  // Adds complexity, harder to maintain
  // No evidence it was needed
};\
````

Complex solution for unmeasured problem
</Bad>

#### In Practice

**When implementing:**

- Solve the immediate problem
- Use straightforward approach
- Resist "what if" thinking
- Delete speculative code

**When optimizing:**

- Profile first, optimize second
- Measure before and after
- Document why optimization needed
- Keep simple version in tests

**When abstracting:**

- Wait for 3+ similar cases (Rule of Three)
- Make abstraction as simple as possible
- Prefer duplication over wrong abstraction
- Refactor when pattern clear

## Integration with Commands

The Kaizen skill guides how you work. The commands provide structured analysis:

- **`/why`**: Root cause analysis (5 Whys)
- **`/cause-and-effect`**: Multi-factor analysis (Fishbone)
- **`/plan-do-check-act`**: Iterative improvement cycles
- **`/analyse-problem`**: Comprehensive documentation (A3)
- **`/analyse`**: Smart method selection (Gemba/VSM/Muda)

Use commands for structured problem-solving. Apply skill for day-to-day development.

## Red Flags

**Violating Continuous Improvement:**

- "I'll refactor it later" (never happens)
- Leaving code worse than you found it
- Big bang rewrites instead of incremental

**Violating Poka-Yoke:**

- "Users should just be careful"
- Validation after use instead of before
- Optional config with no validation

**Violating Standardized Work:**

- "I prefer to do it my way"
- Not checking existing patterns
- Ignoring project conventions

**Violating Just-In-Time:**

- "We might need this someday"
- Building frameworks before using them
- Optimizing without measuring

## Remember

**Kaizen is about:**

- Small improvements continuously
- Preventing errors by design
- Following proven patterns
- Building only what's needed

**Not about:**

- Perfection on first try
- Massive refactoring projects
- Clever abstractions
- Premature optimization

**Mindset:** Good enough today, better tomorrow. Repeat.
