Type Hardening Skill

Purpose

Gradually improve type safety by:

• Replacing magic strings with enums/constants
• Narrowing any/unknown to specific types
• Using shared types instead of inline definitions
• Adding type imports where missing

When to Use

• Code review finds string literals that should be enums
• any types that can be narrowed
• Inline types duplicating shared definitions
• Post-refactoring type cleanup
• Technical debt reduction sprints
• Triggered by /harden-types command or /refactor --strategy=types

Core Principles

1. Check Existing First: Always search for existing types before creating new ones
2. Small Batches: 1-3 related changes at a time
3. Verify Immediately: Type check after every change
4. No Logic Changes: Purely structural/type improvements
5. Commit Often: Atomic commits per successful batch
6. New Types Allowed: Create new types when genuinely needed for clean code
7. No unknown Cheating: Replacing any with unknown is NOT an improvement - narrow to a SPECIFIC type or leave it (with a TODO comment if needed)

Instructions

Step 1: Discover Existing Types

Before ANY changes, locate existing type definitions:

# Find Prisma enums (common source of types)
grep -E "^enum " backend/prisma/schema.prisma 2>/dev/null || true

# Find shared types
find . -path ./node_modules -prune -o -name "*.types.ts" -print 2>/dev/null
find . -path ./node_modules -prune -o -name "types.ts" -print 2>/dev/null

# Find constants files
find . -path ./node_modules -prune -o -name "*.constants.ts" -print 2>/dev/null

# Common locations to check:
# - shared/types/
# - src/types/
# - backend/src/types/
# - backend/src/generated/prisma/ (Prisma enums)

Record findings before proceeding.

Step 2: Analyze Target File

Read the target file and identify:

### Type Hardening Opportunities

1. **String Literals** → Should use enum/constant
   - Line 45: `'admin'` → `UserRole.admin`
   - Line 89: `'pending'` → `JobStatus.PENDING`

2. **Any Types** → Should be narrowed to SPECIFIC types
   - Line 23: `any` → `UserWithRole` ✅
   - Line 67: `any` → `ApiResponse<T>` ✅
   - ❌ WRONG: `any` → `unknown` (this is cheating, not fixing)

3. **Inline Types** → Should use shared
   - Line 102: `{ id: string; name: string }` → `UserBasic`

4. **Missing Imports** → Need to add
   - `UserRole` from `@prisma/client` or generated types

Step 3: Prioritize Changes

Work in this order for safest progression:

1. Prisma Enums First (most reliable)

   // ❌ BEFORE
   if (user.role === 'admin')
   
   // ✅ AFTER
   import { UserRole } from '../generated/prisma/index.js';
   if (user.role === UserRole.admin)
   

2. Shared Types Second (well-established)

   // ❌ BEFORE
   type NotificationType = 'info' | 'success' | 'warning' | 'error';
   
   // ✅ AFTER
   import type { NotificationType } from '@shared/types/notification';
   

3. Constants Third (project-specific)

   // ❌ BEFORE
   const provider = 'openai';
   
   // ✅ AFTER
   import { AI_PROVIDERS } from '../types/ai.constants';
   const provider = AI_PROVIDERS.OPENAI;
   

4. New Types Last (when genuinely needed)

   // Only create new types when:
   // - No existing type serves the purpose
   // - The type will be used in multiple places
   // - It improves code clarity significantly
   

Step 4: Apply Changes (Batch of 1-3)

# Edit the file with precise changes
# Use Edit tool for exact string replacement

Example transformation:

// Before (lines 44-48)
async function promoteUser(userId: string): Promise<void> {
  const user = await userRepo.findById(userId);
  if (user.role === 'user') {
    await userRepo.update(userId, { role: 'admin' });
  }
}

// After
import { UserRole } from '../generated/prisma/index.js';

async function promoteUser(userId: string): Promise<void> {
  const user = await userRepo.findById(userId);
  if (user.role === UserRole.user) {
    await userRepo.update(userId, { role: UserRole.admin });
  }
}

Step 5: Verify Immediately

# Backend files
cd backend && npx tsc --noEmit

# Frontend files
npx tsc --noEmit

# If using ESLint
npx eslint <modified-file>

If verification fails:

1. ❌ DO NOT proceed to next batch
2. Analyze the error
3. Fix or revert the change
4. Re-verify before continuing

Step 6: Commit Atomic Change

git add <modified-file>
git commit -m "refactor: harden types in <filename>

- Replace string literals with <EnumName>
- Narrow any to <TypeName>
- No behavior change

Co-Authored-By: Claude <noreply@anthropic.com>"

Step 7: Iterate or Complete

Continue if:

• More hardening opportunities exist
• User requested multiple files
• Within iteration limit

Stop if:

• File is clean (no more opportunities)
• Verification failed (fix first)
• User requested stop
• Iteration limit reached

Output Format

Progress Report

## Type Hardening Report: `backend/src/services/auth.service.ts`

### Changes Applied
| Line | Before | After | Status |
|------|--------|-------|--------|
| 45 | `'admin'` | `UserRole.admin` | ✅ |
| 89 | `'user'` | `UserRole.user` | ✅ |
| 123 | `any` | `AuthPayload` | ✅ |

### Verification
- TypeScript: ✅ 0 errors
- Lint: ✅ passed

### Remaining Opportunities
- Line 156: `'pending'` could use `JobStatus.PENDING`
- Line 201: `unknown` could narrow to `TokenPayload`

### Next Action
Continue? (y/n/skip to file X)

Integration Points

With Refactor Agent

Referenced as Pattern 6 in /refactor:

### Pattern 6: Type Hardening
**When**: String literals, `any` types, inline types matching shared
**How**: Use `type-hardening` skill
**Reference**: `.claude/skills/quality/type-hardening/SKILL.md`

With Quality Gate

Can be part of quality checks:

### Optional: Type Strictness Check
Use `type-hardening` skill in analysis-only mode to identify
opportunities without making changes.

With Gemini Delegation

Safe for delegation with guardrails:

## Gemini Handoff: Type Hardening
Scope: [specific files]
Task: Replace string literals with existing enums
Verification: tsc --noEmit must pass

Common Type Sources

Source	Location	Example
Prisma Enums	backend/src/generated/prisma/	UserRole, JobStatus
Shared Types	shared/types/	NotificationType, ApiResponse
Domain Types	src/types/	UserWithRole, SceneData
Constants	*/constants.ts	AI_PROVIDERS, CACHE_TTL

Error Recovery

Import Resolution Failed

# Check if type is exported
grep "export.*TypeName" shared/types/

# Check tsconfig paths
cat tsconfig.json | grep -A5 "paths"

Type Mismatch After Change

# Revert the change
git checkout -- <file>

# Analyze why it failed
# - Was the enum value different? (admin vs ADMIN)
# - Was the type path wrong?
# - Was there a missing export?

Best Practices

1. Never guess - Always verify the type exists before using it
2. Match casing - UserRole.admin not UserRole.ADMIN (check actual enum)
3. Preserve semantics - 'admin' becomes UserRole.admin, not Role.ADMIN
4. Import correctly - Use import type for type-only imports
5. Test critical paths - Extra verification for auth/security code