fix: run `reroute` in a middleware function before split functions by teemingc · Pull Request #16590 · sveltejs/kit

teemingc · 2026-07-30T20:00:11Z

See #12296 for context

This PR adds a middleware that runs reroute beforehand so that, when there are multiple functions, the correct one is invoked instead of not matching any function matchers.

The Vercel middleware uses a simplified version of the rewrite helper from @vercel/functions to avoid pulling in the whole bundle.

rewrite docs - https://vercel.com/docs/functions/edge-middleware/middleware-api#rewrites
rewrite implementation - https://github.com/vercel/vercel/blob/cefda60a603d60cc35e4697c36e751cca411e6bb/packages/functions/src/middleware.ts#L101
edge middleware example - https://github.com/vercel/examples/tree/main/build-output-api/edge-middleware

The Netlify middleware resolves the URL then passes it to the next handler.

There's currently an issue with Vercel rewrites discarding the SvelteKit form action in the URL because the query parameter doesn't have a value vercel/vercel#12902

TODOS

implement Vercel middleware
add split test app on Vercel

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pkg-svelte-dev · 2026-07-30T20:00:15Z

Install the latest version of @sveltejs/kit from 29e21be:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/29e21be8e7c7f23875442bb75ba1bb8050b5bd3c

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16590

changeset-bot · 2026-07-30T20:00:16Z

🦋 Changeset detected

Latest commit: 29e21be

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

This PR includes changesets to release 3 packages

Name	Type
@sveltejs/adapter-netlify	Major
@sveltejs/adapter-vercel	Major
@sveltejs/kit	Minor

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svelte-docs-bot · 2026-07-30T20:01:54Z

Preview: https://svelte-dev-git-preview-kit-16590-svelte.vercel.app/

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

Co-authored-by: Tee Ming <chewteeming01@gmail.com>

teemingc · 2026-08-04T18:02:35Z

+	let new_request;
+
+	// `reroute` hooks receive pathnames, so they must not run for route-ID resolution requests
+	if (remote_id || is_route_id_resolution_request) {


Don't really like that we have to duplicate the "should run reroute" check logic between here and respond.js but I'm not really sure what's the best way to dedupe it right now

teemingc · 2026-08-04T18:04:46Z

-/**
- * @param {URL} url
- */
-export function get_remote_id(url) {


Had to move this to a less packed module. Right now Rolldown can't treeshake anything that dynamically imports a module. This causes things like await import('otel') or await import('node:async_hooks') in any module to completely break treeshaking. We don't want otel bundled into the reroute middleware.

Rich-Harris · 2026-08-05T16:54:23Z

+		new_request = new Request(request);
+	} else {
+		const { url: normalized_url, denormalize } = normalizeUrl(url);
+		const resolved_path = await reroute({ url: normalized_url, fetch });


Unfortunately I don't think this is right — normally fetch would be the internal fetch implementation that handles root-relative paths and applies handleFetch (and forwards cookies etc)

Ugh, you're right and that complicates things... I wonder if we take a different approach and try to handle this in the catch-all serverless function instead.

…16665) alternative to #16590 fixes #11879 This PR makes the assumption that a catch-all serverless function is deployed with no routes in its manifest (the alternative was an explicit flag). It then exports a helper which should be called in such a serverless function after `server.respond` so that it can invoke the correct handler instead of simply returning a 404 since it has no route information in its SSR manifest. The advantage of this approach compared to deploying a middleware that only runs reroute is that it's simpler implementation-wise and it allows the user's `handleFetch`, `init` hooks, etc. to run correctly without any additional effort from us. The disadvantages are that: - `reroute` will run twice: once in the catch-all function then again in the correct function. We _could_ choose to skip the second `reroute` based on some `header` value or context (if any) that can be passed between serverless functions. Otherwise, I chose to omit this so that the `reroute` hook can't be skipped from outside influence (someone setting the header themselves without running `reroute` first) - Slightly bigger serverless function bundle than a middleware which just runs `reroute`. Not that big of an issue I think... --- ### Please don't delete this checklist! Before submitting the PR, please make sure you do the following: - [x] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs - [x] This message body should clearly illustrate what problems it solves. - [x] Ideally, include a test that fails without this PR but passes with it. ### Tests - [ ] Run the tests with `pnpm test` and lint the project with `pnpm lint` and `pnpm check` ### Changesets - [x] If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running `pnpm changeset` and following the prompts. Changesets that add features should be `minor` and those that fix bugs should be `patch`. Please prefix changeset messages with `feat:`, `fix:`, or `chore:`. ### Edits - [x] Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed. --------- Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com> Co-authored-by: Rich Harris <richard.a.harris@gmail.com> Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

teemingc added 3 commits July 31, 2026 03:58

add reroute adapter primitives in kit

0745c8f

add fix and test for netlify

7b61281

add kit changeset

da5a2bc

teemingc mentioned this pull request Jul 30, 2026

fix: generate edge middleware to run reroute #12296

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teemingc added needs-platform-tests This PR needs to run platform tests in order to merge. pkg:adapter-vercel Pertaining to the Vercel adapter pkg:adapter-netlify labels Jul 30, 2026

Merge branch 'version-3' into fix-split-reroute

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teemingc added 3 commits July 31, 2026 04:02

add adapter changeset

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vercel fix?

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reserved func name

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teemingc marked this pull request as ready for review July 30, 2026 20:45

teemingc commented Jul 30, 2026

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teemingc marked this pull request as draft July 30, 2026 20:51

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teemingc added 5 commits July 31, 2026 05:05

dry

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Update respond.js

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default to false

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avoid changing url

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Merge branch 'version-3' into fix-split-reroute

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teemingc marked this pull request as ready for review July 31, 2026 11:05

teemingc added 2 commits July 31, 2026 22:11

add split test app

44e9b3a

make basic test specific to split app

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teemingc mentioned this pull request Jul 31, 2026

chore: add Vercel split: true test app #16601

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teemingc added 3 commits July 31, 2026 22:20

revert

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Update package.json

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Merge branch 'chore-vercel-split-test' into fix-split-reroute

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Update packages/adapter-vercel/index.js

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

teemingc commented Aug 3, 2026

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Apply suggestions from code review

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Co-authored-by: Tee Ming <chewteeming01@gmail.com>

Rich-Harris reviewed Aug 3, 2026

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teemingc added 4 commits August 4, 2026 23:59

Merge branch 'version-3' into fix-split-reroute

fd3d200

generate reroute module for middlewares

de53193

fix alias

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Delete 15-@sveltejs-kit-adapter.md

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vercel Bot reviewed Aug 4, 2026

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Comment thread packages/kit/src/core/adapt/builder.js

avoid bundling otel into reroute module

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teemingc commented Aug 4, 2026

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Merge branch 'version-3' into fix-split-reroute

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teemingc added 4 commits August 5, 2026 02:08

Apply suggestion from @teemingc

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Update index.js

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avoid unnecessary prebundling

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replace global constants used in paths file

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teemingc marked this pull request as draft August 4, 2026 18:38

teemingc added 2 commits August 5, 2026 22:27

Merge branch 'version-3' into fix-split-reroute

12cc7b7

ensure resolved_path is updated when url.pathname is modified

b375a12

teemingc marked this pull request as ready for review August 5, 2026 14:38

Update rolldown.config.js

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teemingc requested a review from Rich-Harris August 5, 2026 15:00

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teemingc marked this pull request as draft August 5, 2026 18:02

teemingc mentioned this pull request Aug 6, 2026

feat: add applyReroute helper for catch-all serverless functions #16665

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teemingc closed this Aug 6, 2026

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fix: run `reroute` in a middleware function before split functions - #16590

fix: run `reroute` in a middleware function before split functions#16590
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