diff --git a/.github/workflows/claude-code-review.yml b/.github/workflows/claude-code-review.yml
index 205b0fe..7bb0b4f 100644
--- a/.github/workflows/claude-code-review.yml
+++ b/.github/workflows/claude-code-review.yml
@@ -37,6 +37,8 @@ jobs:
         with:
           claude_code_oauth_token: ${{ secrets.CLAUDE_CODE_OAUTH_TOKEN }}
           prompt: |
+            **모든 리뷰와 코멘트는 한글로 작성하세요. (코드는 영어 유지)**
+
             REPO: ${{ github.repository }}
             PR NUMBER: ${{ github.event.pull_request.number }}
 
diff --git a/.github/workflows/claude.yml b/.github/workflows/claude.yml
index 412cef9..d5a984f 100644
--- a/.github/workflows/claude.yml
+++ b/.github/workflows/claude.yml
@@ -41,7 +41,7 @@ jobs:
             actions: read
 
           # Optional: Give a custom prompt to Claude. If this is not specified, Claude will perform the instructions specified in the comment that tagged it.
-          # prompt: 'Update the pull request description to include a summary of changes.'
+          prompt: '모든 응답은 한글로 작성하세요. (코드와 커밋 메시지는 영어 유지)'
 
           # Optional: Add claude_args to customize behavior and configuration
           # See https://github.com/anthropics/claude-code-action/blob/main/docs/usage.md
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..bf64b0d
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,189 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Language
+
+**All user-facing communication must be in Korean (한글).** Code, commit messages, and internal comments remain in English.
+
+## Project Overview
+
+QuickUUID is a Next.js 15 application for generating UUIDs with extensive customization options. Built with React 19, TypeScript, and Tailwind CSS v4, it emphasizes performance, accessibility, and clean architecture through contract-based design patterns.
+
+## Development Commands
+
+### Essential Commands
+```bash
+npm run dev          # Start development server with Turbopack
+npm run build        # Production build with Turbopack
+npm run typecheck    # TypeScript compilation check (no output)
+npm run lint         # ESLint check
+npm run format       # Format code with Prettier
+```
+
+### Testing
+```bash
+npm test             # Run Jest unit/integration tests
+npm run test:watch   # Jest in watch mode
+npm run test:e2e     # Run Playwright E2E tests (starts dev server automatically)
+npm run test:e2e-ui  # Playwright with UI for debugging
+npm run perf:test    # Run performance benchmarks
+```
+
+**Important Testing Notes:**
+- E2E tests run on `http://localhost:3001` (configured in playwright.config.ts)
+- E2E tests automatically start the dev server if not running
+- Contract tests validate API contracts defined in `src/types/contracts.ts`
+
+## Architecture Overview
+
+### Contract-Based Design Pattern
+The codebase follows a strict contract-based architecture where all utilities implement well-defined interfaces from `src/types/contracts.ts`:
+
+- **UUIDGeneratorAPI**: UUID generation with crypto API support
+- **FormatUtilsAPI**: UUID formatting and validation
+- **StorageAPI**: LocalStorage persistence with error handling
+- **ClipboardAPI**: Clipboard operations with fallback support
+
+All implementations in `src/lib/` must adhere to these contracts. Contract tests in `tests/contract/` validate compliance.
+
+### Core Architecture Layers
+
+**1. Page Layer** (`src/app/page.tsx`)
+- Main application entry point
+- Integrates all components with error boundaries
+- Handles keyboard navigation and focus management
+- Uses code splitting with React.lazy for performance
+
+**2. Hook Layer** (`src/hooks/`)
+- `use-uuid-generator.ts`: Master hook integrating all utilities
+- Custom hooks for clipboard, debounce, focus management, keyboard nav, storage, theme
+- Hooks orchestrate utilities from the lib layer and manage React state
+
+**3. Utility Layer** (`src/lib/`)
+Each utility is a factory function returning an object implementing its contract:
+- `uuid-generator.ts`: Core UUID generation with batch optimization (>100 UUIDs)
+- `format-utils.ts`: UUID formatting with multiple options
+- `storage-utils.ts`: LocalStorage with error handling and schema versioning
+- `clipboard-utils.ts`: Clipboard API with fallback to document.execCommand
+- `performance-utils.ts`: Performance monitoring and health tracking
+- `validation-utils.ts`: Input validation and sanitization
+- `logger.ts`: Centralized logging (use instead of console)
+
+**4. Component Layer** (`src/components/`)
+- `control-panel.tsx`: Configuration UI
+- `result-display.tsx`: UUID output with virtualized list
+- `virtualized-uuid-list.tsx`: High-performance rendering for large UUID lists
+- UI components from `src/components/ui/` (Radix UI + shadcn/ui)
+
+### Type System (`src/types/`)
+- `index.ts`: Core type definitions (Config, Result, State)
+- `contracts.ts`: API interfaces that utilities must implement
+
+### Path Aliases
+Use `@/` for all imports: `@/hooks`, `@/lib`, `@/components`, `@/types`
+
+## Key Technical Patterns
+
+### Performance Optimization
+- **Batch Generation**: >100 UUIDs use optimized `generateUUIDBatch()` method
+- **Code Splitting**: Components lazy-loaded with Suspense
+- **Virtualization**: Large UUID lists use virtualized rendering
+- **Debouncing**: Config changes debounced to prevent excessive regeneration
+- **Performance Monitoring**: `PerformanceManager` tracks generation health
+
+### Error Handling
+- All async operations wrapped in try-catch
+- Error boundaries at page level
+- Fallback mechanisms for clipboard and crypto APIs
+- Validation at boundaries (user input, storage, API calls)
+
+### Accessibility
+- Focus management with `use-focus-management.ts`
+- Keyboard navigation with `use-keyboard-navigation.ts`
+- Screen reader announcements with `use-screen-reader.ts`
+- Skip links and proper ARIA attributes
+- Focus visible styles in `src/styles/focus.css`
+
+### State Management
+- No external state library (React hooks only)
+- `use-uuid-generator.ts` is the single source of truth
+- LocalStorage for config persistence with schema versioning
+- Debounced config updates to prevent thrashing
+
+## Testing Strategy
+
+### Test Organization
+```
+tests/
+├── contract/      # API contract validation tests
+├── integration/   # Cross-utility integration tests
+├── regression/    # Bug regression tests
+└── e2e/          # Playwright end-to-end tests
+```
+
+### Contract Testing
+Every utility in `src/lib/` must have a corresponding contract test in `tests/contract/` that validates it implements its API interface correctly. This ensures architectural consistency.
+
+### Running Single Tests
+```bash
+# Jest (unit/integration)
+npm test -- uuid-generator          # Run specific test file
+npm test -- --testNamePattern="v4"  # Run specific test case
+
+# Playwright (E2E)
+npm run test:e2e -- uuid-generation.spec.ts  # Specific spec
+npm run test:e2e -- --project=chromium       # Single browser
+```
+
+## Code Quality Standards
+
+### Logging
+**Never use `console.*` directly**. Use the logger from `src/lib/logger.ts`:
+```typescript
+import { logger, storageLogger, clipboardLogger } from '@/lib/logger';
+logger.info('message', { data });  // Structured logging
+```
+
+ESLint enforces this rule (warns on console usage except in logger.ts and tools/).
+
+### TypeScript
+- Strict mode enabled
+- Run `npm run typecheck` before commits
+- No `any` types (ESLint warns)
+- Use contract interfaces from `src/types/contracts.ts`
+
+### Import Patterns
+- Use `@/` path alias exclusively
+- Import types from `@/types` or `@/types/contracts`
+- Use named exports (no default exports for utilities)
+
+## Next.js Specifics
+
+### App Router
+- Using Next.js 15 App Router (`src/app/`)
+- Client components marked with `"use client"`
+- Server components by default
+
+### Turbopack
+Development and production builds use Turbopack (`--turbopack` flag). This is significantly faster than Webpack.
+
+### Build Output
+Console removal happens automatically in production builds (next.config.ts).
+
+## Common Gotchas
+
+1. **Performance Tests**: Run `npm run perf:test` to validate UUID generation stays under 50ms for 1000 UUIDs
+2. **E2E Port**: Playwright expects dev server on port 3001, not 3000
+3. **Batch Threshold**: UUID generation switches to batch mode at 100+ UUIDs
+4. **Storage Schema**: LocalStorage has versioned schema in `StoredPreferences` type
+5. **Contract Compliance**: When modifying utilities, update contract tests to validate interface adherence
+
+## Development Workflow
+
+1. Make changes to utilities/components
+2. Run `npm run typecheck` to catch TypeScript errors
+3. Run `npm test` for unit/integration tests
+4. Run `npm run test:e2e` for full E2E validation
+5. Run `npm run perf:test` if touching generation code
+6. Format with `npm run format` before commit
diff --git a/src/lib/test-helper.ts b/src/lib/test-helper.ts
new file mode 100644
index 0000000..4bb21e2
--- /dev/null
+++ b/src/lib/test-helper.ts
@@ -0,0 +1,16 @@
+/**
+ * Test helper utilities for GitHub Actions validation
+ */
+
+export function formatTimestamp(date: Date = new Date()): string {
+  return date.toISOString();
+}
+
+export function generateTestId(): string {
+  return `test-${Date.now()}`;
+}
+
+export const TEST_CONFIG = {
+  maxRetries: 3,
+  timeout: 5000,
+} as const;