breaking: move `Link` header preloading to a config option by teemingc · Pull Request #15939 · sveltejs/kit

teemingc · 2026-06-01T14:02:28Z

This PR moves Link header preloading from the default to an option instead to avoid blowing up common self-hosted reverse proxies. The default is now to use the HTTP link element.

~~Should this be strictly considered a breaking change and done only in SvelteKit 3?~~ Yes

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🦋 Changeset detected

Latest commit: 4960061

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svelte-docs-bot · 2026-06-01T14:04:04Z

Preview: https://svelte-dev-git-preview-kit-15939-svelte.vercel.app/

teemingc · 2026-06-01T14:13:12Z


 	/** @param {string} tag */
 	add_http_equiv(tag) {
-		if (!this.#prerendering) return;


These function are gated behind a if (prerendering) check already so it was confusing to move the function call outside that guard and find that it still wasn't doing anything because of these additional guards.

benmccann · 2026-06-01T14:15:14Z

Should this go into v3 so people aren't surprised by the behavior change?

teemingc · 2026-06-01T14:18:34Z

Yeah, I think so

Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>

…into link-header-preload

teemingc · 2026-06-01T18:45:04Z

Some naive performance tests:

link element: https://pagespeed.web.dev/analysis/https-kit-link-element-vercel-app/xw4vgzhh1t?form_factor=mobile
- "render blocking requests" diagnosis due to styles not being preloaded through the Link header?
- "network dependency tree" from chaining critical requests
Link header: https://pagespeed.web.dev/analysis/https-kit-link-header-vercel-app/cvjawoounw?form_factor=mobile
- 0.1 seconds more for first contentful paint. Probably an anomaly

Testing with slower throttling in the browser tools shows that there really isn't much of a difference

This PR was opened by the [Changesets release](https://github.com/changesets/action) GitHub action. When you're ready to do a release, you can merge this and the packages will be published to npm automatically. If you're not ready to do a release yet, that's fine, whenever you add more changesets to version-3, this PR will be updated. ⚠️

⚠️

⚠️ `version-3` is currently in **pre mode** so this branch has prereleases rather than normal releases. If you want to exit prereleases, run `changeset pre exit` on `version-3`. ⚠️

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⚠️ # Releases ## @sveltejs/adapter-auto@8.0.0-next.0 ### Major Changes - breaking: require SvelteKit 3 ([#15506](#15506)) ### Patch Changes - Updated dependencies [[`fa335bd`](fa335bd), [`3031d89`](3031d89), [`cb9d416`](cb9d416), [`caf3a18`](caf3a18), [`4777827`](4777827), [`a2792e2`](a2792e2), [`ba36148`](ba36148), [`48e8710`](48e8710), [`e2f3075`](e2f3075), [`047d6a0`](047d6a0), [`87603d1`](87603d1), [`096962c`](096962c), [`d545970`](d545970), [`e2f3075`](e2f3075), [`d06affc`](d06affc), [`8af47eb`](8af47eb), [`e2f3075`](e2f3075), [`5c4d130`](5c4d130), [`3f11f35`](3f11f35), [`caf3a18`](caf3a18), [`8823037`](8823037), [`1d76212`](1d76212), [`0dc0548`](0dc0548), [`00d81fa`](00d81fa)]: - @sveltejs/kit@3.0.0-next.0 ## @sveltejs/adapter-cloudflare@8.0.0-next.0 ### Major Changes - breaking: upgrade `@cloudflare/workers-types` to 4.20260219.0 ([#15347](#15347)) - breaking: upgrade minimum `wrangler` version to ^4.67.0 ([#15347](#15347)) - breaking: remove `platform.context` in favour of `platform.ctx` ([#15347](#15347)) - breaking: require SvelteKit 3 ([#15506](#15506)) ### Patch Changes - chore: check the `WORKERS_CI` environment variable to determine if we're building for Cloudflare Workers ([#13733](#13733)) - Updated dependencies [[`fa335bd`](fa335bd), [`3031d89`](3031d89), [`cb9d416`](cb9d416), [`caf3a18`](caf3a18), [`4777827`](4777827), [`a2792e2`](a2792e2), [`ba36148`](ba36148), [`48e8710`](48e8710), [`e2f3075`](e2f3075), [`047d6a0`](047d6a0), [`87603d1`](87603d1), [`096962c`](096962c), [`d545970`](d545970), [`e2f3075`](e2f3075), [`d06affc`](d06affc), [`8af47eb`](8af47eb), [`e2f3075`](e2f3075), [`5c4d130`](5c4d130), [`3f11f35`](3f11f35), [`caf3a18`](caf3a18), [`8823037`](8823037), [`1d76212`](1d76212), [`0dc0548`](0dc0548), [`00d81fa`](00d81fa)]: - @sveltejs/kit@3.0.0-next.0 ## @sveltejs/adapter-netlify@7.0.0-next.0 ### Major Changes - chore: use `rolldown` for edge function bundling ([#15432](#15432)) - breaking: write output that conforms to the stable [Netlify Frameworks API](https://docs.netlify.com/build/frameworks/frameworks-api/). ([#15294](#15294)) Deploying and previewing with Netlify CLI now requires [v17.31.0](https://github.com/netlify/cli/releases/tag/v17.31.0) or later. Run `npm i -g netlify-cli@latest` to upgrade. - breaking: require SvelteKit 3 ([#15506](#15506)) - breaking: edge function build target is now `es2022` ([#15432](#15432)) ### Patch Changes - Updated dependencies [[`fa335bd`](fa335bd), [`3031d89`](3031d89), [`cb9d416`](cb9d416), [`caf3a18`](caf3a18), [`4777827`](4777827), [`a2792e2`](a2792e2), [`ba36148`](ba36148), [`48e8710`](48e8710), [`e2f3075`](e2f3075), [`047d6a0`](047d6a0), [`87603d1`](87603d1), [`096962c`](096962c), [`d545970`](d545970), [`e2f3075`](e2f3075), [`d06affc`](d06affc), [`8af47eb`](8af47eb), [`e2f3075`](e2f3075), [`5c4d130`](5c4d130), [`3f11f35`](3f11f35), [`caf3a18`](caf3a18), [`8823037`](8823037), [`1d76212`](1d76212), [`0dc0548`](0dc0548), [`00d81fa`](00d81fa)]: - @sveltejs/kit@3.0.0-next.0 ## @sveltejs/adapter-node@6.0.0-next.0 ### Major Changes - chore: migrate from rollup to rolldown ([#15297](#15297)) - breaking: require SvelteKit 3 ([#15506](#15506)) ### Patch Changes - Updated dependencies [[`fa335bd`](fa335bd), [`3031d89`](3031d89), [`cb9d416`](cb9d416), [`caf3a18`](caf3a18), [`4777827`](4777827), [`a2792e2`](a2792e2), [`ba36148`](ba36148), [`48e8710`](48e8710), [`e2f3075`](e2f3075), [`047d6a0`](047d6a0), [`87603d1`](87603d1), [`096962c`](096962c), [`d545970`](d545970), [`e2f3075`](e2f3075), [`d06affc`](d06affc), [`8af47eb`](8af47eb), [`e2f3075`](e2f3075), [`5c4d130`](5c4d130), [`3f11f35`](3f11f35), [`caf3a18`](caf3a18), [`8823037`](8823037), [`1d76212`](1d76212), [`0dc0548`](0dc0548), [`00d81fa`](00d81fa)]: - @sveltejs/kit@3.0.0-next.0 ## @sveltejs/adapter-static@4.0.0-next.0 ### Major Changes - breaking: require SvelteKit 3 ([#15506](#15506)) ### Patch Changes - Updated dependencies [[`fa335bd`](fa335bd), [`3031d89`](3031d89), [`cb9d416`](cb9d416), [`caf3a18`](caf3a18), [`4777827`](4777827), [`a2792e2`](a2792e2), [`ba36148`](ba36148), [`48e8710`](48e8710), [`e2f3075`](e2f3075), [`047d6a0`](047d6a0), [`87603d1`](87603d1), [`096962c`](096962c), [`d545970`](d545970), [`e2f3075`](e2f3075), [`d06affc`](d06affc), [`8af47eb`](8af47eb), [`e2f3075`](e2f3075), [`5c4d130`](5c4d130), [`3f11f35`](3f11f35), [`caf3a18`](caf3a18), [`8823037`](8823037), [`1d76212`](1d76212), [`0dc0548`](0dc0548), [`00d81fa`](00d81fa)]: - @sveltejs/kit@3.0.0-next.0 ## @sveltejs/adapter-vercel@7.0.0-next.0 ### Major Changes - chore: use `rolldown` for edge function bundling ([#15432](#15432)) - breaking: edge function build target is now `es2022` ([#15432](#15432)) ### Patch Changes - Updated dependencies [[`fa335bd`](fa335bd), [`3031d89`](3031d89), [`cb9d416`](cb9d416), [`caf3a18`](caf3a18), [`4777827`](4777827), [`a2792e2`](a2792e2), [`ba36148`](ba36148), [`48e8710`](48e8710), [`e2f3075`](e2f3075), [`047d6a0`](047d6a0), [`87603d1`](87603d1), [`096962c`](096962c), [`d545970`](d545970), [`e2f3075`](e2f3075), [`d06affc`](d06affc), [`8af47eb`](8af47eb), [`e2f3075`](e2f3075), [`5c4d130`](5c4d130), [`3f11f35`](3f11f35), [`caf3a18`](caf3a18), [`8823037`](8823037), [`1d76212`](1d76212), [`0dc0548`](0dc0548), [`00d81fa`](00d81fa)]: - @sveltejs/kit@3.0.0-next.0 ## @sveltejs/enhanced-img@1.0.0-next.0 ### Major Changes - breaking: require Node 22 or newer ([#12548](#12548)) ### Minor Changes - breaking: require Vite 8 and `vite-plugin-svelte` 7 ([#15542](#15542)) ## @sveltejs/kit@3.0.0-next.0 ### Major Changes - breaking: TypeScript 6 is now the minimum required version ([#15930](#15930)) - breaking: upgrade to cookie v1. Cookie names must now contain only ASCII characters ([#13386](#13386)) - breaking: require Node 22 or newer ([#12548](#12548)) - breaking: remove the `preloadStrategy` option. `modulepreload` will always be used ([#15256](#15256)) - breaking: default the cookie `path` option to `'/'` ([#15398](#15398)) - breaking: remove `@sveltejs/kit/node/polyfills` ([#15430](#15430)) - breaking: add `config.kit.output.linkHeaderPreload` to preload using the `Link` header ([#15939](#15939)) - breaking: require `@sveltejs/vite-plugin-svelte` v7 ([#15371](#15371)) - breaking: remove `createEntries` from the `Builder` object passed to adapter functions ([#15509](#15509)) - breaking: remove the deprecated CSRF `checkOrigin` option in favor of `trustedOrigins` ([#15437](#15437)) - breaking: the `delta` property now only exists for `popstate` navigation events ([#15522](#15522)) - breaking: remove deprecated `pragma` header in version polling for improved CORS support ([#15428](#15428)) - breaking: require Svelte 5.48.0 or newer ([#15371](#15371)) - chore: change `error`, `isHttpError`, `redirect`, and `isRedirect` to refer to public type instead of internal class ([#15250](#15250)) - breaking: require Vite 8. Provides new functionality even for existing Vite 8 users such as faster builds with Vite hook filters and more powerful SvelteKit adapters with the Vite environment API ([#15371](#15371)) - breaking: remove `data-sveltekit-*` option `'off'` in favour of `false` ([#15907](#15907)) ### Minor Changes - feat: resolve paths using the Vite config `root` option instead of `process.cwd()` to better support monorepo configurations such as Vitest workspaces ([#15469](#15469)) - chore: deprecate `Response` helpers in favor of platform-provided alternatives ([#15448](#15448)) - feat: explicit env vars ([#15934](#15934)) ### Patch Changes - fix: remove check for svelte.config.js before running `sync` ([#15946](#15946)) - fix: generate a placeholder tsconfig.json to squelch sync-time warnings ([#15948](#15948)) - chore: remove dependency on kleur ([#12548](#12548)) - chore: remove dependency on `set-cookie-parser` ([#15384](#15384)) - fix: allow use of `$app/env/public` in service workers ([#15950](#15950)) ## @sveltejs/package@3.0.0-next.0 ### Major Changes - breaking: require Node 22 or newer ([#12548](#12548)) ### Patch Changes - chore: remove dependency on kleur ([#12548](#12548)) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

add option and test

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teemingc mentioned this pull request Jun 1, 2026

fix: do not preload route JS chunks in Link header when SSR is disabled #15933

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teemingc added 2 commits June 1, 2026 22:06

fix unit test

7b134ea

fix link in docs

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teemingc commented Jun 1, 2026

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teemingc marked this pull request as ready for review June 1, 2026 14:14

changeset

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teemingc marked this pull request as draft June 1, 2026 14:19

teemingc added 2 commits June 1, 2026 22:19

switch changeset to major

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Merge branch 'version-3' into link-header-preload

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teemingc changed the base branch from main to version-3 June 1, 2026 14:23

teemingc added the breaking change label Jun 1, 2026

teemingc marked this pull request as ready for review June 1, 2026 14:23

teemingc changed the title ~~fix: move Link header preloading to a config option~~ breaking: move Link header preloading to a config option Jun 1, 2026

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Comment thread packages/kit/src/runtime/server/page/render.js Outdated

Comment thread packages/kit/test/apps/basics/test/server.test.js Outdated

teemingc and others added 6 commits June 1, 2026 22:39

Update packages/kit/src/runtime/server/page/render.js

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Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>

Update packages/kit/test/apps/basics/test/server.test.js

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Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>

fix test

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Merge branch 'link-header-preload' of https://github.com/sveltejs/kit …

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…into link-header-preload

silly

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fix test app

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Rich-Harris approved these changes Jun 2, 2026

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teemingc merged commit 48e8710 into version-3 Jun 2, 2026
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teemingc deleted the link-header-preload branch June 2, 2026 17:19

github-actions Bot mentioned this pull request Jun 2, 2026

Version Packages (next) #15311

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breaking: move `Link` header preloading to a config option#15939

breaking: move `Link` header preloading to a config option#15939
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