Svelte 5 Migration Troubleshooting: AI-Optimized Reference

Critical Failure Scenarios

Build Failures (Immediate Blockers)

• Severity: Critical - Application won't start
• Frequency: 80% of migrations encounter dependency issues
• Root Cause: Library compatibility gaps with Svelte 5
• Impact: Complete development stoppage

Primary Failure Pattern: Module not found: Error: Can't resolve 'svelte/internal'

• Detection: npm ls | grep -E "(UNMET|missing)"
• Solution: rm package-lock.json node_modules -rf && npm install
• Fallback: Check individual package Svelte 5 compatibility

Runtime Rune Context Errors

• Error: Cannot read properties of undefined (reading '$state')
• Severity: Critical - App crashes at runtime
• Cause: Runes executed outside component context
• Detection: grep -n "\$state|\$derived|\$effect" src/**/*.js src/**/*.ts
• Fix: Move runes to component scope or use proper stores

TypeScript Component Type Failures

• Error: Type '{ prop: string }' is not assignable to type 'ComponentProps<MyComponent>'
• Severity: High - Blocks TypeScript builds
• Cause: Component prop types changed from class-based to function-based
• Migration Pattern:

  // BROKEN: Svelte 4 pattern
  export interface Props { name: string; }
  
  // FIXED: Svelte 5 pattern
  interface Props { name: string; }
  let { name }: Props = $props();
  

Configuration Requirements

Dependency Management

Essential Version Requirements:

• Node.js: 16+ (production requirement)
• @testing-library/svelte: Latest version required
• Svelte DevTools: Updated browser extension mandatory

Breaking Dependency Patterns:

• Libraries pinned to Svelte 4 while core updated to Svelte 5
• Old svelte/internal imports not migrated
• Store libraries expecting class-based components

Testing Configuration Failures

Test Library Compatibility Issues:

• Error: component.$$set is not a function
• Cause: Testing libraries don't understand Svelte 5 components
• Solutions:
  1. Update: npm install @testing-library/svelte@latest
  2. Switch to browser testing: npm install vitest-browser-svelte
• Migration Impact: Browser testing more reliable with runes

Resource Investment Requirements

Time Allocation (Production Migration)

• Simple Apps (< 20 components): 2-4 hours debugging post-migration
• Medium Apps (20-100 components): 1-2 days manual fixes
• Large Apps (100+ components): 3-5 days incremental migration
• Enterprise Apps: Plan 1-2 weeks including testing and deployment

Expertise Requirements

• Basic Migration: Junior developer with Svelte knowledge
• Complex Patterns: Senior developer familiar with reactivity systems
• TypeScript Issues: Developer experienced with advanced TypeScript patterns
• Production Deployment: DevOps knowledge for build pipeline updates

Hidden Costs

• Learning Curve: Runes paradigm fundamentally different from reactive statements
• Testing Updates: Entire test suite may need rewriting
• Team Training: All developers need runes education
• CI/CD Updates: Build pipelines need Svelte 5 configuration

Critical Warning Patterns

Migration Tool Limitations

Tool Gives Up Scenarios:

• Complex reactive statements with multiple conditions
• Store integration with complex logic
• Event dispatcher patterns with conditional logic
• Lifecycle hooks with async dependencies

Failure Indicators:

• @migration: This reactive statement is too complex
• TypeScript errors after successful migration
• Components work in browser but tests fail
• Performance degradation post-migration

Production-Only Failures

Development vs Production Discrepancies:

• Hydration mismatches in SSR applications
• Bundle size increase due to compatibility imports
• Performance regression from unnecessary compatibility layers
• Environment-specific build failures

Technical Specifications

Breaking Change Thresholds

• UI Performance: Breaks at 1000+ spans making debugging impossible
• Bundle Size: Can increase 20-30% due to compatibility imports
• Test Execution: 2-3x slower with outdated testing libraries
• Memory Usage: Runes more efficient but migration creates temporary overhead

Reactivity System Changes

Old Pattern Conversion:

// BREAKS: Complex reactive statement
$: {
  if (condition) {
    doMultipleThings();
    updateState();
  }
}

// FIXED: Convert to $effect
$effect(() => {
  if (condition) {
    doMultipleThings();
    updateState();
  }
});

Store Migration Patterns

Critical Decision Point: Keep stores vs convert to runes

• Stores: Recommended for shared state between components
• Runes: Component-local state only
• Breaking: Mixed patterns in same component cause conflicts

Common Misconceptions

"Migration Tool Handles Everything"

Reality: Tool handles 60-70% of patterns automatically

• Complex reactive statements require manual conversion
• Store integration patterns often break
• TypeScript component types need manual fixes
• Testing patterns require complete rewrite

"Performance Automatically Improves"

Reality: Initial migration may decrease performance

• Compatibility imports add overhead
• Over-reactive effects from poor conversion
• Bundle size increases temporarily
• Optimization requires post-migration cleanup

"Backward Compatibility is Seamless"

Reality: Subtle behavior changes affect functionality

• Reactivity timing more precise (effects run less frequently)
• Component initialization order changes
• Event handling timing differences
• SSR/hydration behavior modifications

Implementation Decision Framework

When to Migrate Incrementally

• Large codebases (100+ components)
• Active development teams
• Production applications with uptime requirements
• Complex store integrations

When to Migrate All at Once

• Small applications (< 20 components)
• Development/staging environments
• Simple reactive patterns
• Dedicated migration time available

When to Delay Migration

• Critical business deadlines within 2 weeks
• Team lacks Svelte 5 expertise
• Dependencies not Svelte 5 compatible
• Limited testing coverage

Debugging Workflow Priority

Phase 1: Build Stability (30 minutes)

1. Dependency conflict resolution
2. Import error fixes
3. Basic TypeScript errors
4. Configuration updates

Phase 2: Runtime Fixes (2-4 hours)

1. Rune context errors
2. Store integration repairs
3. Component communication patterns
4. Lifecycle hook conversions

Phase 3: Testing Restoration (4-8 hours)

1. Testing library updates
2. Test pattern conversions
3. Component interaction tests
4. E2E test timing fixes

Phase 4: Production Optimization (1-2 days)

1. Performance regression analysis
2. Bundle size optimization
3. SSR/hydration fixes
4. Deployment pipeline updates

Quality Indicators

Migration Success Metrics

• Build Success: No errors, minimal warnings
• Test Coverage: Maintained or improved
• Bundle Size: Within 10% of original
• Performance: Equal or better than Svelte 4
• Team Velocity: Maintained after 1-week adjustment

Red Flags Requiring Rollback

• Build failures persisting after 4 hours debugging
• Test suite failures above 20%
• Performance degradation above 50%
• Critical business functionality broken
• Team productivity severely impacted

Resource Links by Problem Category

Build Issues

• Compatibility Tracker
• Migration Tool Source
• Dependency Issues

TypeScript Problems

• Component Type Migration
• Advanced TypeScript Patterns

Testing Solutions

• Modern Testing Approach
• Testing Library Migration

Performance Optimization

• Bundle Analysis
• Svelte 5 Performance

Community Support

• Real-time Help
• Migration Experiences
• Production Migration Reports

Critical Success Factors

1. Plan Migration Time: Budget 3x initial estimate for complex apps
2. Test Environment First: Never migrate production directly
3. Incremental Approach: Migrate in small batches for large apps
4. Team Preparation: Train developers on runes before migration
5. Rollback Plan: Maintain ability to revert quickly
6. Monitor Performance: Track metrics throughout migration process