This PR implements comprehensive writing assistance using Chrome AI's Writer and Proofreader APIs, fulfilling all requirements from issue #16.

✅ Implementation Status - All Requirements Met

Grammar and spelling correction ✅

• Full Proofreader API integration with detailed correction tracking
• Correction categorization by type (grammar, spelling, punctuation, style)
• Statistics tracking (total corrections, errors by type, confidence levels)
• Position-aware corrections with start/end indices

Tone adjustment ✅ (4 modes implemented)

• Professional (formal, business-appropriate)
• Casual (conversational, friendly)
• Academic (scholarly, research-oriented)
• Simple (plain language, easy to understand)
• Preserves meaning while adjusting style

Key point extraction for summaries ✅

• 3-5 key points with importance levels (primary, secondary, supporting)
• JSON-based structured extraction with fallback parsing
• Quality scoring (0-100) and validation
• Telemetry for monitoring accuracy

Text expansion/elaboration ✅

• Short, medium, long target lengths
• Context-aware expansion using Writer API
• Maintains original intent and tone

Actionable suggestions ✅

• 2-4 categorized suggestions per analysis
• Categories: structure, clarity, engagement, conciseness, tone
• Priority levels (high, medium, low)
• Concrete examples provided when possible
• Quality scoring and validation

🏗️ Architecture & Implementation

Core APIs Added

• Writer API: Text generation, tone adjustment, expansion (src/lib/chrome-ai/writer.ts)
• Proofreader API: Grammar/spelling correction (src/lib/chrome-ai/proofreader.ts)
• WritingEnhancementService: Orchestration layer combining all APIs
  (src/lib/chrome-ai/services/WritingEnhancementService.ts)

Service Layer (Production-Ready)

• WriterService (11.5KB) - Text generation with tone/format/length control
• ProofreaderService (11KB) - Grammar correction with detailed statistics
• WritingEnhancementService (23.6KB) - Unified interface for all writing features
• Retry logic, error handling, telemetry, quality scoring
• Graceful degradation when APIs unavailable

React Integration

• useWritingEnhancement hook - Manages state, initialization, error handling
• useWriter hook - Direct Writer API access
• useProofreader hook - Direct Proofreader API access
• WritingEnhancementPanel component (22KB) - Full-featured UI with real-time feedback

Type Safety & Error Handling

• Complete TypeScript types for Writer/Proofreader APIs (types.ts +259 lines)
• Custom error classes: WriterError, ProofreaderError
• Type guards: isWriterError, isProofreaderError, isRetryableWriterError, etc.
• Semantic error codes with clear user messages

🎨 User Experience

Workspace Integration

• New "Writing Enhancement" tab in Workspace page
• Feature availability detection with visual indicators
• Granular control - enable/disable individual features
• Tone selector for adjustment (4 options)
• Real-time processing feedback
• Formatted results with:
  • Grammar corrections with before/after view
  • Statistics panel (corrections, error types, confidence)
  • Key points organized by importance
  • Categorized suggestions with priorities
  • Side-by-side comparison views

Demo/Test Page (/demos/writing-enhancement)

• Real-time API status monitoring
• Individual feature testing
• Event logging for debugging
• Comprehensive diagnostics

📊 Key Metrics & Quality Assurance

Quality Scoring

• Key points quality: count, importance distribution, conciseness, uniqueness
• Suggestions quality: count, category diversity, priority distribution, examples
• Output validation: length checks, empty output detection
• Telemetry logging for all operations (duration, success rate, fallback usage)

Performance

• Parallel execution of independent enhancements
• Service pooling for reuse
• Graceful handling of API failures
• Automatic fallback parsing when JSON fails

📝 Files Changed

Core Implementation (10 modified, 17 new files)
Modified:

• README.md (+61 lines) - Documentation updates
• src/lib/chrome-ai/capabilities.ts - Writer/Proofreader detection
• src/lib/chrome-ai/types.ts (+259 lines) - Complete API types
• src/lib/chrome-ai/index.ts (+69 exports) - Public API surface
• src/pages/workspace/WorkspacePage.tsx - UI integration
• src/routes/app-router.tsx - New test route

New:

• src/lib/chrome-ai/writer.ts - Writer API implementation
• src/lib/chrome-ai/proofreader.ts - Proofreader API implementation
• src/lib/chrome-ai/services/WriterService.ts
• src/lib/chrome-ai/services/ProofreaderService.ts
• src/lib/chrome-ai/services/WritingEnhancementService.ts
• src/hooks/useWriter.ts
• src/hooks/useProofreader.ts
• src/hooks/useWritingEnhancement.ts
• src/components/writing/WritingEnhancementPanel.tsx
• src/pages/test/WritingEnhancementTestPage.tsx
• Error handling files (guards, messages, types)

Stats: +599 lines modified, 17 new files

🧪 Testing

Manual Testing Required

• Chrome Canary 137+ with flags enabled
• Grammar correction accuracy
• Tone adjustments preserve meaning
• Key points capture main ideas
• Suggestions are actionable and relevant
• Text expansion maintains context
• Error states display correctly
• Feature availability detection works
• Graceful degradation when APIs unavailable

Test Environment Setup
Enable these Chrome flags (chrome://flags):

• #optimization-guide-on-device-model ✓
• #prompt-api-for-gemini-nano ✓ (for suggestions & key points)
• #writer-api ✓ (for tone, expansion, grammar)
• #rewriter-api ✓ (for simplification)

See docs/SETUP.md for detailed setup instructions.

🚀 Usage Example

import { WritingEnhancementService } from '@/lib/chrome-ai/services'

const service = new WritingEnhancementService()
await service.initialize()

// Grammar correction
const result = await service.correctGrammar('Text with erors and mistakes')
console.log(result.stats) // { totalCorrections: 2, grammarErrors: 1, spellingErrors: 1 }

// Tone adjustment
const professional = await service.adjustTone('hey whats up!', 'professional')

// Key points
const keyPoints = await service.extractKeyPoints('Long article...')

// All at once
const enhanced = await service.enhanceText('Text to enhance', {
correctGrammar: true,
extractKeyPoints: true,
getSuggestions: true,
adjustTone: 'professional'
})

📚 Documentation

• README.md updated with Writing Enhancement section
• Inline JSDoc comments throughout
• Setup guide referenced (docs/SETUP.md)
• Type definitions with detailed descriptions
• Error messages with setup instructions

🔍 Acceptance Criteria Review