Describe the bug

In Vite's experimental full-bundle dev mode (experimental.bundledDev: true), importing a .css file in a project that also uses PostCSS causes a hard build-time crash. The dev server starts, but as soon as the CSS module is pulled in through Rolldown's lazy bundling, the build fails and never recovers. The browser is stuck on the bundledDev "Bundling in progress" screen with the error overlay, and no page reload recovers — the bundle itself fails to build.

What I expect: the CSS module is processed by PostCSS as CSS and the page renders.
What actually happens: vite:css runs PostCSS on a JavaScript proxy module and PostCSS throws while trying to parse JS as CSS.

✘ Build error: Build failed with 1 error:

[plugin vite:css] /path/to/src/global.css?rolldown-lazy=1:15:41
CssSyntaxError: [postcss] /path/to/src/global.css?rolldown-lazy=1:15:42: Unclosed string
    at Input.error (.../node_modules/postcss/lib/input.js:135:16)
    at unclosed (.../node_modules/postcss/lib/tokenize.js:46:17)
    at Object.nextToken (.../node_modules/postcss/lib/tokenize.js:161:15)
    at Parser.other (.../node_modules/postcss/lib/parser.js:428:30)
    at Parser.parse (.../node_modules/postcss/lib/parser.js:479:16)
    at parse (.../node_modules/postcss/lib/parse.js:11:12)
    at new LazyResult (.../node_modules/postcss/lib/lazy-result.js:165:16)
    at Processor.process (.../node_modules/postcss/lib/processor.js:53:14)
    at runPostCSS (.../node_modules/vite/dist/node/chunks/node.js:21166:66)
    at async compilePostCSS (.../node_modules/vite/dist/node/chunks/node.js:21150:6)

Root cause

In bundledDev mode, Rolldown's lazy bundling wraps each lazily-loaded module in a JavaScript proxy. For a CSS module it keeps the original id and appends a query, …/src/global.css?rolldown-lazy=1, but the content of that module is JavaScript, roughly:

const lazyExports = (async () => {
  delete __rolldown_runtime__.modules["src/global.css?rolldown-lazy=1"];
  await import(/* @vite-ignore */ `/@vite/lazy?id=${encodeURIComponent(...)}&clientId=...`);
  return __rolldown_runtime__.loadExports("src/global.css");
})();
export { lazyExports as 'rolldown:exports' };

Vite's vite:css plugin decides whether to transform a module by matching the file extension:

const CSS_LANGS_RE = /\.(css|less|sass|scss|styl|stylus|pcss|postcss|sss)(?:$|\?)/

and only skips ids matching commonjsProxyRE and SPECIAL_QUERY_RE (/[?&](?:worker|sharedworker|raw|url)\b/). The ?rolldown-lazy query is not excluded, so vite:css runs PostCSS (runPostCSS / compilePostCSS) on the JavaScript proxy module. PostCSS tries to parse JS as CSS and throws Unclosed string (the quoted export name in export { lazyExports as '…' }).

Suggested fix

vite:css's transform should skip ids that carry the ?rolldown-lazy query — those are Rolldown JS proxies, not CSS (e.g. add it to the special-query exclusion or short-circuit when the query is present).

I have not submitted a PR for this.

Reproduction

https://github.com/happycollision/vite8-bundleddev-postcss-repro

The repository is the smallest project that reproduces it: a vanilla Vite app with experimental.bundledDev: true, a postcss.config.js registering autoprefixer, an index.html, an entry src/main.js that imports a .css file, and a valid src/global.css.

Steps to reproduce

git clone https://github.com/happycollision/vite8-bundleddev-postcss-repro
cd vite8-bundleddev-postcss-repro
npm install
npm run dev
# open http://localhost:5173/ in a browser

The dev server terminal prints the CssSyntaxError: Unclosed string build error shown above, and the browser is stuck on the "Bundling in progress" overlay. This happens with the dev server (npm run dev); vite build is not affected.

Details that matter for reproduction:

• PostCSS must actually run. The crash is inside runPostCSS/compilePostCSS, so a real PostCSS plugin (autoprefixer) is configured. The sample CSS uses user-select, which autoprefixer rewrites — confirming PostCSS is in the pipeline, not no-op'd.
• The CSS is loaded through lazy bundling. In the repro src/main.js uses await import('./global.css'), which forces Rolldown to emit the ?rolldown-lazy=1 proxy. (A statically-imported CSS module that lands in the initial entry bundle is inlined eagerly and does not hit this path.)
• The .css itself is perfectly valid — this is not a CSS authoring error.

Regression range (bisected)

Verified in the repro: switching to [email protected] renders the app and processes the CSS with no error; switching back to [email protected] crashes with the trace above. Introduced in 8.0.13 by "bundled-dev: add lazy bundling support" (#21406).

Related issues

This is distinct from the two existing bundledDev reports:

• vite 8.0.13 regression in dev mode with bundledDev:true and react lazy loading with resolve.alias #22454 (closed — "fixed in 8.0.14"): an alias × dynamic-import edge case. Different trigger, and that fix does not address this — our case is still broken in 8.0.16.
• Static asset imports fail to load on first page load in dev mode with bundledDev: true #22596 (open): a first-load asset issue, i.e. a recoverable runtime/first-load failure.

This issue is different in kind: it is a hard build-time PostCSS crash (vite:css running PostCSS on a ?rolldown-lazy=1 JS proxy), not a recoverable first-load runtime failure. The bundle never builds, so a reload does not fix it.

System Info

  System:
    OS: macOS 26.5.1
    CPU: (16) arm64 Apple M4 Max
    Memory: 1.22 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.14.1
    npm: 11.11.0
    pnpm: 10.33.0
  Browsers:
    Chrome: 149.0.7827.54
    Firefox: 151.0.2
    Safari: 26.5
  npmPackages:
    vite: 8.0.16 => 8.0.16
    rolldown: 1.0.3 (bundled via [email protected])

Used Package Manager

npm

Logs

✘ Build error: Build failed with 1 error:

[plugin vite:css] /path/to/src/global.css?rolldown-lazy=1:15:41
CssSyntaxError: [postcss] /path/to/src/global.css?rolldown-lazy=1:15:42: Unclosed string
    at Input.error (.../node_modules/postcss/lib/input.js:135:16)
    at unclosed (.../node_modules/postcss/lib/tokenize.js:46:17)
    at Object.nextToken (.../node_modules/postcss/lib/tokenize.js:161:15)
    at Parser.other (.../node_modules/postcss/lib/parser.js:428:30)
    at Parser.parse (.../node_modules/postcss/lib/parser.js:479:16)
    at parse (.../node_modules/postcss/lib/parse.js:11:12)
    at new LazyResult (.../node_modules/postcss/lib/lazy-result.js:165:16)
    at Processor.process (.../node_modules/postcss/lib/processor.js:53:14)
    at runPostCSS (.../node_modules/vite/dist/node/chunks/node.js:21166:66)
    at async compilePostCSS (.../node_modules/vite/dist/node/chunks/node.js:21150:6)

Validations

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