CSS side-effect imports become unreferenced when multiple entry points import the same CSS file

[jimmy-sc]

Describe the bug

When two or more entry points independently import the same CSS file as a side effect (import './style.css'), the CSS asset is emitted but no output chunk references it. The CSS is completely disconnected from the module graph — no chunk has it in viteMetadata.importedCss, and no JS file in the build output contains any reference to it.

Reproduction

https://github.com/jimmy-sc/vite-css-orphan-repro

git clone https://github.com/jimmy-sc/vite-css-orphan-repro
cd vite-css-orphan-repro
pnpm install
npx vite build

The setup:

• src/search.js — imports search.css, exports SearchView
• src/settings-search.js — independently imports search.css, exports its own view
• src/mail-search.js (+ calendar, contacts, tasks, files) — imports SearchView from search.js
• preserveEntrySignatures: 'strict' with entryFileNames: '[name].js'

Steps to reproduce

Run npx vite build. The debug plugin in vite.config.js logs chunk metadata:

search.js:              css=[], imports=[assets/search-xxx.js]
settings-search.js:     css=[], imports=[assets/search-xxx.js]
assets/search-xxx.js:   css=[assets/search-yyy.css], codeLen=0, imports=[]

CSS asset "assets/search-yyy.css": referenced by NOTHING (bug!)

The orphaned chunk assets/search-xxx.js has the CSS reference but has empty code and is not written to disk. grep -r "search-yyy" src/dist/ returns nothing — zero references to the CSS file anywhere in the output.

Expected behavior

Each entry chunk that imports search.css should have the CSS file in its viteMetadata.importedCss, or the CSS should be on a chunk that is reachable through the entry's import chain and written to disk.

Root cause

This appears to be caused by Rolldown's side-effect deduplication (rolldown#8804). When multiple entries import the same CSS file, Rolldown deduplicates the CSS import into a single empty chunk. That chunk exists in the generateBundle bundle object but is not written to disk, and no written chunk imports it.

Real-world impact

In our application (~600 entry points), this affects 48 modules. CSS loads in the main app because a bundle plugin walks the full import graph and collects CSS from all chunks. But when modules are loaded individually (e.g. in child browser windows), the CSS is missing because the entry chunk's import chain never reaches the CSS reference.

System Info

System:
  OS: macOS 26.4
  CPU: (14) arm64 Apple M4 Pro
  Memory: 134.08 MB / 48.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 25.9.0 - /opt/homebrew/bin/node
  npm: 11.12.1 - /opt/homebrew/bin/npm
  pnpm: 10.33.0 - /opt/homebrew/bin/pnpm
Browsers:
  Chrome: 146.0.7680.178
  Safari: 26.4

Broken: vite 8.0.3+ (rolldown 1.0.0-rc.12+) through vite 8.0.8 (rolldown 1.0.0-rc.15)
Works: vite 8.0.0 (rolldown 1.0.0-rc.9)

Used Package Manager

pnpm

Logs

No response

[sapphi-red]

Due to how the CSS is processed, the generateBundle hook in the plugins without enforce: 'post' will see that kind of state. If you print out chunk.code, you can actually see that assets/search-xxx.js actually exists at that point and is imported by other chunks. The removal is done by this code.

Adding enforce: 'post' to the plugin would make the plugin see the state after the removal is done. You can also use the manifest if that's easier.

[jimmy-sc]

Thank you, that was exactly it! Adding enforce: 'post' to our plugin makes it run after vite:css-post has cleaned up the empty CSS chunks and transferred importedCss to the parent chunks. Our plugin was simply running too early and seeing the intermediate state.

The side-by-side comparison makes it very clear:

Without enforce: 'post' — orphaned empty chunk holds the CSS, entry chunks have importedCss=[]:

search.js:              css=[], imports=[assets/search-xxx.js]
assets/search-xxx.js:   css=[assets/search-yyy.css], codeLen=0

With enforce: 'post' — Vite has cleaned up, CSS is on the entry chunks:

search.js:              css=[assets/search-yyy.css], imports=[]

I've updated the reproduction repo with both a before/after debug plugin so you can see the difference: https://github.com/jimmy-sc/vite-css-orphan-repro

Should this issue be closed, or would it be worth documenting somewhere that plugins reading viteMetadata.importedCss in generateBundle need enforce: 'post' to see the final state?

[sapphi-red]

Let's document it once there's another person finding this behavior confusing as this is a niche use case.

[doberkofler]

Let's document it once there's another person finding this behavior confusing as this is a niche use case.

I guess I am the second person ;-)