sv migrate - Saves You From Manual Migration Hell

Before this tool existed, upgrading Svelte was a nightmare of find-and-replace across hundreds of files. You'd spend a weekend manually converting every let count = 0 to let count = $state(0) and inevitably fuck something up.

Enter `sv migrate` - the official CLI tool that handles about 80% of that tedious work automatically. But here's the reality check: it's not magic, and understanding where it succeeds and fails will save you hours of debugging frustration.

What It Migrates (The Easy Stuff)

Svelte 5 runes conversion - Converts your old reactive patterns:

• let count = 0 becomes `let count = $state(0)`
• $: doubled = count * 2 becomes `let doubled = $derived(count * 2)`
• export let name becomes destructured `$props()`
• on:click becomes onclick (finally!)

SvelteKit upgrades - Updates routing and API patterns for SvelteKit 2, though honestly most of this shit should have been backwards compatible in the first place.

Legacy cleanup - Converts old Svelte 3/4 patterns that should have been deprecated years ago.

What It Can't Handle (The Reality Check)

Here's where sv migrate throws in the towel and leaves you @migration comments to fix manually:

Complex reactive patterns - If you have weird `$:` statements doing multiple things, the tool bails out. Custom stores with intricate reactivity? Good luck.

Third-party library integration - Your component library that depends on Svelte 4 internals? The migration tool doesn't give a shit about that. You'll spend hours figuring out compatibility.

Weird edge cases - Custom event dispatchers, slot manipulation, dynamic component patterns - anything that's not vanilla Svelte gets left for you to debug.

The Real Migration Experience

Small projects (under 20 components) usually migrate cleanly in seconds. Medium projects (50+ components) work mostly fine, but expect to spend an hour hunting down @migration comments. Large projects with complex patterns? Clear your calendar for a day and prepare for some serious debugging sessions.

The tool is pretty good at not breaking working code, but "not breaking" doesn't mean "works perfectly." You'll get a lot of legacy compatibility imports that make your bundle bigger while you slowly fix edge cases.

Bottom line: sv migrate saves you from manually updating hundreds of files, but it's not magic. Complex apps will still require significant manual work to fully migrate to idiomatic Svelte 5 patterns.

Now that you understand what the tool can and can't handle, let's walk through the actual migration process. This isn't just running a command and hoping for the best - there's a specific workflow that minimizes pain and maximizes success.

Prerequisites (Don't Skip This Shit)

You need Node.js 16+ (exact version doesn't matter much despite what the docs claim) and an existing Svelte project. Most importantly: commit your fucking code first because this tool occasionally corrupts files when it hits weird syntax patterns.

Before you run anything:

git add . && git commit -m "Before migration - pray for me"

Seriously. I've seen this tool corrupt files when it hits weird syntax patterns. Always have a clean git state.

Basic command:

npx sv migrate

This gives you an interactive picker. Or jump straight to Svelte 5:

npx sv migrate svelte-5

What Actually Happens During Migration

The tool scans your components and converts old patterns to new ones. Here's what works reliably:

State conversion - Basic reactive variables get converted:

• let count = 0 → `let count = $state(0)`
• $: doubled = count * 2 → `let doubled = $derived(count * 2)`
• Simple side effects get wrapped in `$effect()`

Props and events - Standard patterns convert fine:

• export let name → proper `$props()` destructuring
• on:click → onclick (thank god)
• Basic `createEventDispatcher` → callback props

What breaks the tool:

• Weird `$:` statements that do multiple things
• Custom stores with complex reactivity
• Components that manipulate DOM directly
• Anything using undocumented Svelte internals

The @migration Comment Hell

When the tool can't figure out how to convert something, it leaves you @migration comments. These are your homework assignments. Common ones:

<!-- @migration: This reactive statement is too complex to migrate automatically -->
$: {
  if (condition) {
    someComplexLogic();
    updateMultipleThings();
  }
}

You'll need to manually convert these to $effect() or split them up. Fun times.

What Gets Updated (Besides Your Components)

The tool also updates:

• package.json dependencies (bumps Svelte to v5)
• TypeScript configs (usually works fine)
• SvelteKit configs (if you're using SvelteKit)

Sometimes this breaks your build because your deps aren't Svelte 5 compatible yet. That's on you to fix.

VS Code Integration (Actually Useful)

The Svelte VS Code extension has a "Migrate Component" command that works on individual files. This is actually better than the CLI for complex components because you can see exactly what changes before committing.

The Svelte Playground also has a migrate button for testing patterns.

When It All Goes Sideways

If the migration fucks everything up (happens sometimes with complex projects):

git checkout . # nuclear option - back to square one

Or revert specific files:

git checkout HEAD -- src/components/broken-component.svelte

Pro tip: migrate on a branch, test everything, then merge. Don't YOLO it on main.

The theoretical knowledge is helpful, but let's get into the practical reality of migration. This section covers what actually happens when you run the tool on real projects, complete with realistic timelines and the inevitable gotchas you'll encounter.

Before You Start (Learn From My Mistakes)

I've migrated a dozen Svelte projects to v5. Here's what I wish I knew before the first one went sideways:

Pre-migration checklist (actually important):

• git checkout -b svelte-5-migration - obvious but critical
• `git status` should be clean - no uncommitted changes
• Run your tests - they should pass before migration
• Check your component library compatibility - most stuff breaks initially

Dependency hell warning:
Popular libraries like `@tailwindcss/forms`, older UI libraries, and custom Svelte components from NPM will probably break. Check the compatibility discussions but honestly it's hit-or-miss. Budget time for finding alternatives.

The Actual Migration Process (With Real Timelines)

1. Run the tool:

npx sv migrate svelte-5

Reality check on timing:

• Simple projects (10-20 components): 30 seconds, maybe 1 issue to fix
• Medium projects (50 components): 2-3 minutes, 10-15 `@migration` comments to resolve
• Large projects (100+ components): 5-10 minutes to run, then you spend 3-6 hours debugging

2. Find your homework:

grep -r "@migration" src/

This shows you every place the tool gave up. On a typical medium app, expect 10-20 spots to manually fix.

3. Fix dependencies:

npm install

This usually works but sometimes breaks spectacularly. Common issues:

• `npm ERR! peer dep missing` - some library needs Svelte 4
• Build fails because TypeScript configs are fucked
• Tests break because testing libraries don't understand runes yet.

Real Migration Scenarios (From Actual Experience)

Small apps - Usually migrate clean. Maybe one weird $: statement to fix manually. Done in an hour.

Medium apps with standard patterns - Migration runs fine, 10-15 manual fixes needed. Half-day of work if you know what you're doing.

Large apps with custom stores - Prepare for pain. The tool converts basic stuff but custom store patterns often break. Budget 2-3 days.

Apps using component libraries - Check compatibility first. Most UI libraries had Svelte 5 support by early 2025, but if you're using some random NPM package, you're probably fucked.

TypeScript projects - Usually fine, but sometimes you discover type errors that were hidden before. The new Component types are stricter.

When Shit Breaks (Common Failure Modes)

Custom stores with lifecycle dependencies - If your stores depend on component lifecycles or complex $: patterns, manual migration required.

Dynamic component patterns - <svelte:component this={component}> patterns sometimes need manual fixes.

Edge case reactive patterns - Anything with multiple $: statements that depend on each other usually needs manual conversion to $effect().

Build tool incompatibility - Vite configs sometimes need updates, especially if you use custom preprocessors.

Testing After Migration (What Actually Breaks)

Run your tests immediately:

npm test

Component tests using @testing-library/svelte usually keep working because they test behavior, not implementation. But expect some failures from:

• Tests that depend on specific DOM structure (runes change how things render)
• Anything testing component internals or lifecycle behavior
• Tests using component stores or complex reactive patterns

Manual testing checklist (learned the hard way):

• Forms - make sure two-way binding still works
• Complex state flows - reactive computations often break subtly
• Component communication - props/events sometimes get weird
• Check the browser console - lots of deprecation warnings initially
• Animations/transitions - these sometimes break with runes

Performance reality check:
Svelte 5 is supposed to be faster, and usually is. But if you had weird optimization patterns before, performance might change. Don't assume it's automatically better.

When You Need to Rollback (It Happens)

Nuclear option:

git checkout . && git clean -fd

Back to square one. Use this when the migration completely fucked everything.

Selective revert:

git checkout HEAD -- src/components/broken-shit.svelte

Revert specific files while keeping successful migrations.

My approach: Migrate, test core functionality, commit what works, revert what doesn't. Repeat until done.

Use the Svelte Playground migrate feature to test individual component patterns before applying manual fixes.

The Bottom Line

sv migrate handles the mechanical stuff well but it's not magic. It'll save you hours of manual find-and-replace, but complex apps still need significant manual work. Don't expect to run the command and be done - expect to run the command and then spend time debugging edge cases.

Treat it as a very good starting point, not a complete solution. The tool gets you 80% there; the remaining 20% is where you earn your salary as a developer.

Key takeaway: Plan for a phased approach. Use the tool to handle the grunt work, then methodically work through the @migration comments. Test frequently, commit often, and don't rush. A careful migration beats a fast one that breaks production.