vitejs/vite · error · Error

Name in package.json is required if option "build.lib.cssFil

Error message

Name in package.json is required if option "build.lib.cssFileName" is not provided.

What it means

Thrown by `resolveLibCssFilename` when building a library (`build.lib`) and none of `lib.cssFileName`, `lib.fileName`, or a `name` field in the nearest `package.json` is available to derive the output CSS filename. Vite needs a name to produce `<name>.css` when you have not given an explicit one.

Source

Thrown at packages/vite/src/node/plugins/css.ts:3668

  return targets
}

export function resolveLibCssFilename(
  libOptions: LibraryOptions,
  root: string,
  packageCache?: PackageCache,
): string {
  if (typeof libOptions.cssFileName === 'string') {
    return `${libOptions.cssFileName}.css`
  } else if (typeof libOptions.fileName === 'string') {
    return `${libOptions.fileName}.css`
  }

  const packageJson = findNearestMainPackageData(root, packageCache)?.data
  const name = packageJson ? getPkgName(packageJson.name) : undefined

  if (!name)
    throw new Error(
      'Name in package.json is required if option "build.lib.cssFileName" is not provided.',
    )

  return `${name}.css`
}

View on GitHub (pinned to 89620f09af)

Solutions

Add a `name` field to the nearest `package.json` (e.g. `"name": "my-lib"`).
Set `build.lib.cssFileName` explicitly to bypass name inference.
Set `build.lib.fileName` (it is also used to derive the CSS filename).

Example fix

// before — package.json has no name
{ "version": "1.0.0" }
// vite.config.ts
export default defineConfig({ build: { lib: { entry: 'src/index.ts' } } });
// after — package.json
{ "name": "my-lib", "version": "1.0.0" }

Defensive patterns

Strategy: validation

Validate before calling

import { readFileSync, existsSync } from 'node:fs';
import path from 'node:path';

function validateLibCssName(root, lib) {
  if (lib?.cssFileName || lib?.fileName) return;
  const pkgPath = path.join(root, 'package.json');
  if (existsSync(pkgPath)) {
    const pkg = JSON.parse(readFileSync(pkgPath,'utf8'));
    if (!pkg.name) throw new Error('package.json needs a "name" for library CSS output');
  } else {
    throw new Error('No package.json found — set build.lib.cssFileName');
  }
}
// validateLibCssName(config.root, config.build.lib);

Prevention

Always set `name` in `package.json` for library packages.
Set `build.lib.cssFileName` explicitly to avoid reliance on name inference.
Run library builds in CI to catch missing-name errors early.

When it happens

Trigger: Running a library build (`build.lib` truthy) where the package produces CSS, `build.lib.cssFileName` and `build.lib.fileName` are unset, and the nearest `package.json` has no `name` field (or there is no package.json at all).

Common situations: A new package scaffolded without a `name` in `package.json`; a build run from a directory whose nearest `package.json` is a stub; a monorepo package whose `name` was removed during refactoring.

Related errors

AI-assisted analysis of vitejs/vite@89620f09af (2026-08-03). Data as JSON: /data/errors/06373b79e91bab40.json. Report an issue: GitHub.