SvelteKit 3 migration, tested: what sv migrate misses

SvelteKit 3.0 shipped on October 1, 2026, with about thirty breaking changes and an official migration command. I took a SvelteKit 2.40 app of the kind most people still run (Vite 7, TypeScript 5.9, a svelte.config.js, and a few of the APIs that changed), confirmed it built and type-checked with 0 errors, then upgraded it the two ways people will try.

What changed in the packages

SvelteKit 3.0.1 refuses to install next to older tooling. Its peer requirements:

It also needs Node 22 or newer.

Way 1: just bump the package

npm install -D @sveltejs/kit@latest

npm error code ERESOLVE
npm error Found: @sveltejs/vite-plugin-svelte@6.2.4
npm error Could not resolve dependency:
npm error peer @sveltejs/vite-plugin-svelte@"^7.0.0" from @sveltejs/kit@3.0.1

Nothing is installed. You’d have to bump Vite, Svelte, TypeScript, the Vite plugin and the adapter together, which is what the migration tool does for you.

Way 2: the official migration

npx sv migrate sveltekit-3

Then I ran the build.

The build failed three times

Each fix revealed the next error.

1. The CSRF option. The tool moved it into vite.config.ts unchanged:

config_option_removed_check_origin
`config.csrf.checkOrigin` has been removed in favour of `csrf.trustedOrigins`
error during build:
[Error: Failed to load SvelteKit options from Vite config]

2. The preload option. Same story, and this one isn’t mentioned in MIGRATION_TASKS.md at all:

config_option_removed_preload_strategy
`config.output.preloadStrategy` has been removed. `modulepreload` will always be used
error during build:
[Error: Failed to load SvelteKit options from Vite config]

3. The Node polyfills. Left in hooks.server.ts:

"./node/polyfills" is not exported under the conditions ["module", "node", "production", "svelte", "import"]
from package …\node_modules\@sveltejs\kit

The bug the build didn’t catch: a link that 404s

My Kit 2 page had a normal base-path link:

<a href="{base}/about">About</a>

On Kit 2.40 it rendered href="/about". The migration rewrote it to:

<a href="{resolve('')}/about">About</a>

On Kit 3.0.1 that rendered as href=".//about", which a browser resolves to http://localhost/ + //about, a 404. From a nested page like /blog/post it becomes /blog//about. It passed vite build and svelte-check with no warning. The tool did list +page.svelte under “Files to review” in MIGRATION_TASKS.md, but only as a generic $app/paths check, after already writing the broken version.

<a href={resolve('/about')}>About</a>

A behaviour change: fail() now returns its status code

My form action returns fail(400, { missing: true }) when a field is empty. Same action, same empty field:

The page and the returned form data look the same, so users won’t notice. Things that read the HTTP status will: end-to-end tests asserting 200, uptime or error-rate monitoring, and logs that treat 4xx as errors.

TypeScript 7 with SvelteKit 3

Kit 3’s typescript: ^6.0.0 excludes TypeScript 7. The same side-by-side setup from my TypeScript 7 frameworks test works here too:

npm install -D "@typescript/native@npm:typescript@7.0.2" "typescript@npm:@typescript/typescript6@^6.0.2"

Checklist after sv migrate sveltekit-3

1. Run vite build before anything else, and fix config errors one by one (checkOrigin, preloadStrategy and any other removed option).
2. Delete @sveltejs/kit/node/polyfills imports.
3. Search for resolve('') and move the path inside resolve(…).
4. Work through MIGRATION_TASKS.md and any @migration-task comments.
5. Check tests and monitoring that expect 200 from failed form actions.
6. Run the app and click the links. A green build proved nothing about item 3.

Limits of this test

One small app, built to touch the changes most likely to bite: config options, $app/paths, hooks and form actions. It didn’t cover remote functions, shallow routing, parameter matchers, custom adapters or $app/stores, which the migration also rewrites. The release notes list about thirty breaking changes. Read the official migration guide for the rest.

How this was tested

A SvelteKit 2.40.0 app (adapter-auto 6.1.1, Vite 7.3.7, vite-plugin-svelte 6.2.4, TypeScript 5.9.3) that built and passed svelte-check with 0 errors, upgraded with npm install and then with npx sv@1.1.1 migrate sveltekit-3 --tasks all. After each change I ran vite build and svelte-check, then served both versions with vite preview and checked the rendered links and the form action’s HTTP status with curl. Every message above is copied from those runs.