Svelte Release Notes

Major Changes

• breaking: reject query parameters beginning with x-sveltekit- (#17125)

Patch Changes

• fix: support bigint params in server-side route resolution (#17130)

• fix: generate types when dev server starts (#17034)

• fix: validate prerender concurrency (#17145)

• fix: manipulate stack trace for errors that happen while generating prerender inputs (#17113)

• fix: keep memory flat when precompressing many files (#16993)

• fix: render remote form actions while prerendering (#17139)

• fix: ignore Vitest browser loader HTML transforms (#17092)

• fix: prevent failed link preloads from causing unhandled promise rejections in production (#17181)

• fix: enforce request body size limits when Content-Type is absent (#17127)

• fix: ignore nested outDir files outside generated (#17150)

• fix: follow HTTP redirects from authentication proxies when enhancing form submissions (#17106) …

Major Changes

• breaking: record the list of static assets at build time, files added to the output afterwards are not served (#16908)

• breaking: validate static assets with content-hash ETags and stop sending Last-Modified (#16908)

• breaking: serve static assets only to GET and HEAD requests (#16908)

Minor Changes

• feat: serve static assets from tables recorded at adapt time, instead of using sirv (#16908)

Patch Changes

• chore: bundle the adapter's server source directly instead of prebuilding it with rolldown (#17109)

• chore: hand adapt-time values to the server as a module instead of patching built chunks (#17103)

• chore: avoid bundling the server twice (#17068) …

Major Changes

• breaking: write static files based on the publish adapter option rather than reading the netlify.toml file (#17078)

Minor Changes

• feat: support split and edge options combined (#17045)

Patch Changes

• chore: include edge function configuration in the function module (#17058)
• Updated dependencies [abf47df, df3f003, 3c044cd, 509dd14, 5d8722c, 462111a, 9f3342f, 224e791, 8614b01, 65f93c8, [147f09a](https://github.com/sveltejs/kit/commit/147f09ab7397727f17b5f35ff753…

Major Changes

• breaking: build the server with Vite instead of a second Bun.build pass, keeping production dependencies external; buildOptions now only applies to executables (#17120)

Patch Changes

• Updated dependencies [abf47df, df3f003, 3c044cd, 509dd14, 5d8722c, 462111a, 9f3342f, 224e791, 8614b01, 65f93c8, 147f09a, 09b8de8, [68c6ef9](https://github.com/sveltejs/kit/commit/68c6ef97fad9baf6c883e203b1ceb686e2ca7…

Major Changes

• major: error when a client-requested single-flight mutation isn't respected by the server, allow the server to explicitly ignore refreshes (#16892)

• breaking: remove Server constructor and SSRManifest from public types (#16876)

• breaking: replace the builder.generateManifest with builder.generateServerInstance and builder.manifest (#16875)

Minor Changes

• feat: allow adapters to receive the Svelte config as a function argument when adding Vite plugins (#16986)

• feat: allow adapters to override getRequest and setResponse during vite dev and vite preview (#16753)

Patch Changes

• fix: respect paths.relative during development for client files (#17053)

• fix: clarify circular imports from src/env (#17014)

• breaking: populate env vars before instrumentation.server.js is evaluated and update the adapter instrumentation API (#16303)

• fix: record a history traversal before resolving its route (#16959) …

Major Changes

• breaking: populate env vars before instrumentation.server.js is evaluated and update the adapter instrumentation API (#16303)

• breaking: replace the builder.generateManifest with builder.generateServerInstance and builder.manifest (#16875)

Patch Changes

• fix: pass the requested pathname, including any trailing slash, to ISR functions (#16903)

• fix: trace process.cwd()-relative files from the project directory and never glob from the filesystem root (#16972)

• Updated dependencies [9a4b343, ee2f55b, 65673a4, 1b79845, b12bb78, 03f1687, 49f0808, e7c74bf, [0876d63](https://github.com/svelt…

Major Changes

• breaking: populate env vars before instrumentation.server.js is evaluated and update the adapter instrumentation API (#16303)

• breaking: replace the builder.generateManifest with builder.generateServerInstance and builder.manifest (#16875)

Patch Changes

• Updated dependencies [9a4b343, ee2f55b, 65673a4, 1b79845, b12bb78, 03f1687, 49f0808, e7c74bf, 0876d63, 9a2b9ba, c9f54b5, [22b8152](https://github.com/sveltejs/kit/commit/22b8152f…

Major Changes

• breaking: populate env vars before instrumentation.server.js is evaluated and update the adapter instrumentation API (#16303)

• breaking: replace the builder.generateManifest with builder.generateServerInstance and builder.manifest (#16875)

Patch Changes

• fix: route requests with omitted optional parameters to split serverless functions (#17020)

• fix: copy _redirects when deploying edge function (#17047)

• chore: replace @iarna/toml with smol-toml (#16948)

• chore: identify serverless functions with a UUID instead of Netlify's function token (#17044)

• fix: ensure serverless function files have unique names (#17049) …

This month, Svelte 5.57 shipped with new SvelteMap methods and a few quality-of-life additions while SvelteKit 3 got closer to the finish line with its Release Candidate.

The sv CLI also got a new ai-tools add-on that replaces the old mcp one, and sv@next now ships a task-based sveltekit-3 migration for existing apps.

Let’s dive a bit deeper!

What’s new in Svelte

• SvelteMap now has getOrInsert and getOrInsertComputed methods for the common “read or initialize” pattern (5.57.0, Docs, #18728)
• createContext now returns a third has function so you can check whether a context has been set without triggering the get error (5.57.0, Docs, #18472)
• <select> now supports the defaultValue attribute, so the select reverts to that value on form reset (5.57.0, #18591)
• svelte/server now exports the RenderOutput, SyncRenderOutput, Csp and Sha256Source types for typing server render output and CSP sources (5.57.0, #18648)

For the full list of patches and bug fixes, see the Svelte CHANGELOG. …

SvelteKit 3 is now in the Release Candidate phase. If all goes well — meaning that people like you try it out and find that it works as expected — we will follow it up with a stable release in the near future, with no further breaking changes.

But there are some breaking changes since SvelteKit 2. We’re taking advantage of this release to prune some of the weeds in the codebase and lay the groundwork for SvelteKit’s continued evolution, more on which below.

To migrate an existing app, you can use the next version of sv migrate:

npx sv@next migrate sveltekit-3 --tasks all --confirm

This will automatically migrate as much of your code as possible. For everything else, it will generate a TODO list for you (or your clanker of choice) to work through. Wherever possible, SvelteKit will print useful diagnostic warnings and errors if you try to run code that hasn’t yet been updated.

To create a new app, run sv create:

npx sv@next create my-new-app

What’s changed?

For the full list of changes, consult the migration guide over on next.svelte.dev which is where the SvelteKit 3 documentation will live until the stable release. Most changes are fairly minor, but a few are worth calling out: …

The biggest news this month is the first @next releases of SvelteKit 3. Thirteen preview versions shipped in July: previewing new $app/manifest and $app/service-worker modules, improved API availability and type checking in service workers, tracing out of the experimental namespace, shallow routing baked into goto and a lot more. It’s a prerelease, but it’s worth trying out to see what’s coming to SvelteKit!

Alongside the preview releases, the stable line kept moving with submitted on remote forms and a new home for defineEnvVars. The language tools also picked up zero-config +error.svelte props so error pages get their page and error types with no extra setup.

And, in case you missed it, Svelte Summit Ljubljana 2026 is happening November 19-20, with a workshop day on November 18, the day before the summit. Save the date!

What’s new in SvelteKit

• Remote forms now expose a submitted property so you can react to the moment a form is submitted without waiting for the response (2.69.0, Docs, #14811)
• defineEnvVars has moved from @sveltejs/kit to @sveltejs/kit/env so environment helpers live in a dedicated subpath (2.70.0, Docs, #16378) …