[elliott-with-the-longest-name-on-github]

Vercel has no server at runtime for prerendered pages, so adapter-vercel handles trailingSlash for them in the routing table. Currently it adds two routes per prerendered page: one that serves the page and one that redirects the wrong trailing slash to the right one. Vercel only allows 2048 routes per deployment, so a blog with a thousand posts fails to deploy, even though every post comes from the same /blog/[slug] route.

Basically, the adapter can't do better than per-page routes without knowing which trailing slash each route wants. It can't tell whether /blog/ should redirect everything, or whether /blog/about-me has its own route with a different setting. This PR gives adapters that information. When every route agrees, the adapter replaces all of the per-page routes with one global rule. That's the case for most apps, since 'never' is the default.

What changed

• @sveltejs/kit: builder.routes now includes each route's trailingSlash. It's resolved the same way as at runtime: the page's option, then the endpoint's, then 'never'.
• @sveltejs/adapter-vercel / @sveltejs/adapter-static (the copy it uses when building on Vercel):
  • Every route uses 'never': a single 308 from ^/(.+)/$ to /$1, placed before the filesystem phase.
  • Every route uses 'always': this is harder, because Vercel ignores the trailing slash when it looks up a static file, and it also serves any foo/index.html at foo. So /foo would find the page before any redirect after the filesystem phase could run.
    • Prerendered pages are renamed to foo/__index.html, which Vercel doesn't alias to foo.
    • A miss route rewrites /foo/ to /foo/__index.html. Vercel reverts miss rewrites that don't find a file, so other requests are unaffected.
    • Anything left that doesn't end in a slash gets redirected after the filesystem phase. By that point static assets have already been served, so they aren't affected. /_app/*, __data.json and __route.js requests are excluded.
  • With paths.base, there's also a redirect from the base to base/, since Kit always gives the root a trailing slash in that case.
  • Anything else (mixed options, 'ignore', or an older Kit) keeps the existing per-page routes. fix: combine prerendered trailing slash redirects into as few Vercel routes as possible #17523 makes that fallback much smaller.
• Static ISR routes are now inserted before the first handle marker rather than before handle: filesystem, since a miss section can now come first.

Why only when every route agrees

I considered emitting a regex per route, ordered by Kit's route priority, with exceptions for conflicting routes. That falls apart quickly:

• reroute means a route's pattern doesn't tell you which URLs it serves. You can end up with the edge removing a slash and Kit adding it back, forever.
• Param matchers aren't regexes, so they can't be expressed in the routing table.

If every route agrees, none of that matters, because whichever route ends up handling a URL wants the same thing.

A few behaviour changes worth knowing about:

• Server-rendered requests with the wrong slash are now redirected at the edge instead of in the function. The result is the same, and it skips a function call.
• URLs that don't match any route now get a 308 before their 404.
• In 'always' apps, a +server.js endpoint that doesn't export trailingSlash defaults to 'never'. That means the app isn't uniform, so it uses the fallback.

Tests

• split (which has no trailingSlash options, and has a reroute hook) now has prerendered pages, plus build and platform tests for the 'never' rule.
• A new trailing-slash app uses 'always' everywhere. It covers prerendered and server-rendered pages, endpoints, static assets with and without extensions, the query string and client-side navigation (which exercises __data.json).
• basic mixes options and keeps covering the fallback.
• I ran all three apps' platform tests against real deployments.

CI setup needed before merging: the new app needs a Vercel project with its root directory set to packages/adapter-vercel/test/apps/trailing-slash, and its ID stored as the VERCEL_PROJECT_ID_TRAILING_SLASH secret. The workflows already reference it.

With paths.base set, the 'always' rename needs #17525, which fixes the overrides keys for apps that use a base path. Once that's merged, I'll rebase this and add the base to the rename as well.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated
kit-ts-always	Error		Oct 22, 2026 6:18pm UTC
kit-ts-split	Error		Oct 22, 2026 6:18pm UTC

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 2d0cb08:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/2d0cb08c7a222220c842921e0aaf003f7f412a72

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17522

[changeset-bot]

🦋 Changeset detected

Latest commit: 2d0cb08

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@sveltejs/kit	Minor
@sveltejs/adapter-vercel	Patch
@sveltejs/adapter-static	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-17522-svelte.vercel.app/