Dep scan fails with UNRESOLVED_IMPORT in virtual-module: when a .svelte file in node_modules is imported by a deep path (html-types branch return no longer falls through to bare-import resolve) #23669

Describe the bug

In Vite 8, a bare import that ends in .svelte and resolves into node_modules (for example import Banner from 'svelte-lib/Banner.svelte') is not recorded as an optimizable dependency. The scanner crawls it as source instead. When that file has a relative import (import Icon from './Icon.svelte'), the import is resolved against the virtual-module: importer and fails. The whole scan is then skipped:

(!) Failed to run dependency scan. Skipping dependency pre-bundling. Error:   Failed to scan for dependencies from entries:
  <root>/index.html

  Build failed with 1 error:

[UNRESOLVED_IMPORT] Could not resolve './Icon.svelte' in virtual-module:<root>/node_modules/svelte-lib/Banner.svelte?id=0

The visible consequence is that dependencies are only discovered at first page load. The optimizer re-runs mid-load (optimized dependencies changed. reloading), and requests for the stale /node_modules/.vite/deps/* URLs get 504. Depending on timing, the first load can fail with "Importing a module script failed" and leave a blank page until a manual reload.

Importing the same component through the package's barrel (import { Banner } from 'svelte-lib') scans cleanly, and svelte-lib is pre-bundled at startup.

Cause. In vite:dep-scan:resolve, the html-types branch intends to hand optimizable node_modules files to the bare-import branch below it:

if (htmlTypesRE.test(id)) {
  const resolved = await resolve(id, importer)
  if (!resolved) return
  // It is possible for the scanner to scan html types in node_modules.
  // If we can optimize this html type, skip it so it's handled by the
  // bare import resolve, and recorded as optimization dep.
  if (
    isInNodeModules(resolved) &&
    isOptimizable(resolved, optimizeDepsOptions)
  )
    return
  if (shouldExternalizeDep(resolved, id)) {
    return externalUnlessEntry({ path: id })
  }
  return resolved
}

With esbuild these were separate onResolve callbacks, so returning undefined passed control to the next callback (the bare-import onResolve), which recorded the dep and externalized it. In the rolldown version both branches live in the same resolveId function. The return exits that function, so the bare-import branch (// bare imports: record and externalize, L624 onward) never runs for this id. Rolldown's own resolution then picks the file up, vite:dep-scan:load:html extracts its <script> into virtual-module:…?id=0, and relative imports inside it fail to resolve.

The trigger is any framework whose plugin adds its extension to optimizeDeps.extensions (@sveltejs/vite-plugin-svelte 7 always adds .svelte, so isOptimizable is true), combined with a deep .svelte import from a package installed in node_modules.

Experiment that fixes it. Make the optimizable case fall through instead of returning. Only lines 618-621 change, and they are wrapped:

if (!(isInNodeModules(resolved) && isOptimizable(resolved, optimizeDepsOptions))) {
  if (shouldExternalizeDep(resolved, id)) {
    return externalUnlessEntry({ path: id })
  }
  return resolved
}
// otherwise fall through to the bare-import branch below

I applied the equivalent edit to node_modules/vite/dist/node/chunks/node.js in the reproduction below (vite 8.3.3). With it, the scan succeeds and _metadata.json lists svelte-lib/Banner.svelte as an optimized dependency.

Reproduction

Minimal project, described inline. There are seven files plus a packed library tarball.

package.json

{
  "name": "vite-svelte-deep-import-scan-repro",
  "private": true,
  "type": "module",
  "scripts": { "dev": "vite --force" },
  "dependencies": { "svelte-lib": "file:./svelte-lib-1.0.0.tgz" },
  "devDependencies": {
    "@sveltejs/vite-plugin-svelte": "^7.3.0",
    "svelte": "^5.56.9",
    "vite": "8.3.3"
  }
}

vite.config.js

import { defineConfig } from 'vite'
import { svelte } from '@sveltejs/vite-plugin-svelte'

export default defineConfig({ plugins: [svelte()] })

index.html

<!doctype html>
<html>
  <body>
    <div id="app"></div>
    <script type="module" src="/src/main.js"></script>
  </body>
</html>

src/main.js

import { mount } from 'svelte'
import App from './App.svelte'

mount(App, { target: document.getElementById('app') })

src/App.svelte

<script>
  // Deep import of a .svelte file from a dependency: the dependency scan fails.
  import Banner from 'svelte-lib/Banner.svelte'
  // Barrel import of the same component: the dependency scan succeeds.
  // import { Banner } from 'svelte-lib'
</script>

<Banner />

The library lives in lib/ and is packed with npm pack into svelte-lib-1.0.0.tgz. A tarball is used so the package is a real copy under node_modules. A file:./lib folder dependency is symlinked, resolves outside node_modules, and does not take this code path.

lib/package.json

{
  "name": "svelte-lib",
  "version": "1.0.0",
  "type": "module",
  "exports": {
    ".": "./index.js",
    "./*": "./*"
  },
  "peerDependencies": { "svelte": "^5.0.0" }
}

lib/index.js

export { default as Banner } from './Banner.svelte'
export { default as Icon } from './Icon.svelte'

lib/Banner.svelte

<script>
  import Icon from './Icon.svelte'
</script>

<p><Icon /> hello</p>

lib/Icon.svelte

<span>*</span>

Steps to reproduce

1. Create the files above.
2. (cd lib && npm pack --pack-destination ..)
3. npm install
4. npx vite --force. The output shows Failed to run dependency scan with [UNRESOLVED_IMPORT] Could not resolve './Icon.svelte' in virtual-module:…/node_modules/svelte-lib/Banner.svelte?id=0.
5. In src/App.svelte, switch to the barrel import (import { Banner } from 'svelte-lib') and run npx vite --force again. The scan succeeds and node_modules/.vite/deps/_metadata.json lists svelte-lib under optimized.

This minimal project reproduces it on vite 8.3.3. The same error appears with vite 8.2.1 in a larger project, and the code is unchanged on main at the commit linked above.

System Info

  System:
    OS: macOS 27.0.1
    CPU: (16) arm64 Apple M4 Max
    Memory: 2.01 GB / 48.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 26.9.0 - /opt/homebrew/bin/node
    npm: 11.19.1 - /opt/homebrew/bin/npm
    pnpm: 12.4.2 - /opt/homebrew/bin/pnpm
    Deno: 2.9.7 - /opt/homebrew/bin/deno
  Browsers:
    Brave Browser: 143.1.85.111
    Chrome: 154.0.8037.95
    Safari: 27.0.1
  npmPackages:
    @sveltejs/vite-plugin-svelte: ^7.3.0 => 7.3.1
    svelte: ^5.56.9 => 5.57.2
    vite: 8.3.3 => 8.3.3

rolldown is not a direct dependency, so envinfo does not list it. npm ls rolldown reports rolldown@1.2.12.

Used Package Manager

npm

Logs

[vite-plugin-svelte] no Svelte config found at <root> - using default configuration.
[vite] (client) Forced re-optimization of dependencies
[vite] (ssr) Forced re-optimization of dependencies
(!) Failed to run dependency scan. Skipping dependency pre-bundling. Error:   Failed to scan for dependencies from entries:
  <root>/index.html

  Build failed with 1 error:

[UNRESOLVED_IMPORT] Could not resolve './Icon.svelte' in virtual-module:<root>/node_modules/svelte-lib/Banner.svelte?id=0
   ╭─[ virtual-module:<root>/node_modules/svelte-lib/Banner.svelte?id=0:2:20 ]
   │
 2 │   import Icon from './Icon.svelte'
   │                    ───────┬───────
   │                           ╰───────── Module not found.
   │
   │ Help: 'virtual-module:<root>/node_modules/svelte-lib/Banner.svelte?id=0' is imported by the following path:
   │         - virtual-module:<root>/node_modules/svelte-lib/Banner.svelte?id=0
   │         - node_modules/svelte-lib/Banner.svelte
   │         - virtual-module:<root>/src/App.svelte?id=0
   │         - src/App.svelte
   │         - src/main.js
   │         - index.html
───╯

    at aggregateBindingErrorsIntoJsError (<root>/node_modules/rolldown/dist/shared/error-Bj1xBdEY.mjs:49:18)
    at unwrapBindingResult (<root>/node_modules/rolldown/dist/shared/error-Bj1xBdEY.mjs:19:128)
    at scan (<root>/node_modules/rolldown/dist/experimental-index.mjs:241:3)
    at async Object.build (<root>/node_modules/vite/dist/node/chunks/node.js:28068:3)
    at async scan (<root>/node_modules/vite/dist/node/chunks/node.js:28007:4)
    at async <root>/node_modules/vite/dist/node/chunks/node.js:35783:15

  VITE v8.3.3  ready in 605 ms

[vite] (client) Forced re-optimization of dependencies
[vite] (ssr) Forced re-optimization of dependencies

  VITE v8.3.3  ready in 310 ms

[vite] (client) Forced re-optimization of dependencies
[vite] (ssr) Forced re-optimization of dependencies

  VITE v8.3.3  ready in 296 ms

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• The provided reproduction is a minimal reproducible example of the bug.