Dev server: non-deterministic route node indices due to unsorted readdirSync

[notnotjake]

Describe the bug

When running the dev server with bun (bun --bun vite dev) hydration errors can occur. In practice these are intermittent but can be reproduced by anything that calls sync.create() (see reproduction steps below).

As best as I understand it, create_manifest_data assigns numeric node indices based on the order fs.readdirSync() returns directory entries. This ordering is not guaranteed by POSIX and differs between runtimes: node returns entries alphabetically, while bun returns them in filesystem/creation order.

This means the same project produces different client/app.js manifests depending on which runtime calls sync.create(). When the dev server's in-memory SSR manifest uses one ordering but the on-disk client manifest uses another, the SSR-emitted node_ids point to the wrong components, causing hydration failures and wrong page content.

This can happen if while you are running the dev server, you run a different command with node or your IDE or AI agent run something with node that triggers the sync.

Root cause (?)

packages/kit/src/core/sync/create_manifest_data/index.js line 217:

const files = fs.readdirSync(dir).map((name) => ({

Node indices are assigned from this traversal order (line 416) before sort_routes() is called (line 464).

Reproduction

1. Create a SvelteKit project with multiple route groups - e.g. (app), (marketing)
2. Start the dev server with bun --bun vite dev
3. In a separate terminal, run sync.create() under Node:

   node --input-type=module -e '
   import { load_config } from "./node_modules/@sveltejs/kit/src/core/config/index.js";
   import * as sync from "./node_modules/@sveltejs/kit/src/core/sync/sync.js";
   const config = await load_config();
   sync.all(config, "development");
   '

4. Navigate to any route — wrong page content loads, console shows HierarchyRequestError: The operation would yield an incorrect node tree

Logs

System Info

System:
    OS: macOS 26.3
    CPU: (10) arm64 Apple M1 Max
    Memory: 179.69 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 25.6.0 - /opt/homebrew/bin/node
    npm: 11.8.0 - /opt/homebrew/bin/npm
    bun: 1.3.9 - /Users/jake/.bun/bin/bun
  Browsers:
    Chrome: 144.0.7559.134
    Safari: 26.3
  npmPackages:
    @sveltejs/kit: ^2.50.2 => 2.50.2
    @sveltejs/vite-plugin-svelte: ^6.2.4 => 6.2.4
    svelte: ^5.50.1 => 5.50.1
    vite: ^7.3.1 => 7.3.1

Severity

annoyance

Additional Information

No response

[notnotjake]

I created a fork and made the following change which fixed the issue:

main...notnotjake:sveltekit:fix/sort-route-manifest-traversal

packages/kit/src/core/sync/create_manifest_data/index.js - line 217

- const files = fs.readdirSync(dir).map((name) => ({
+ const files = fs.readdirSync(dir).sort().map((name) => ({

It doesn't seem to have impacted anything else but I don't understand the code well enough to say whether this is a clean fix or if it has side effects.