vitejs/vite · error · Error

Unknown lang: ${lang}

Error message

Unknown lang: ${lang}

What it means

An exhaustiveness guard inside the lightningcss-based CSS dependency resolver. When resolving `@import`/`@use`, the code switches on `lang` (css/styl/sass/scss/less) and the `default` branch throws. The `satisfies never` annotation makes this a compile-time check too; a runtime hit means a lang value slipped through that the switch does not cover.

Source

Thrown at packages/vite/src/node/plugins/css.ts:3378

              const lang = CSS_LANGS_RE.exec(from)?.[1] as CssLang | undefined
              let resolver: ResolveIdFn
              switch (lang) {
                case 'css':
                case 'sss':
                case 'styl':
                case 'stylus':
                case undefined:
                  resolver = atImportResolvers.css
                  break
                case 'sass':
                case 'scss':
                  resolver = atImportResolvers.sass
                  break
                case 'less':
                  resolver = atImportResolvers.less
                  break
                default:
                  throw new Error(`Unknown lang: ${lang satisfies never}`)
              }

              const resolved = await resolver(environment, id, from)
              if (resolved) {
                deps.add(resolved)
                return resolved
              }
              return id
            },
          },
          minify: config.isProduction && !!config.build.cssMinify,
          sourceMap:
            config.command === 'build'
              ? !!config.build.sourcemap
              : config.css.devSourcemap,
          analyzeDependencies: { preserveImports: true },
          cssModules: cssModuleRE.test(id)
            ? (config.css.lightningcss?.cssModules ?? true)

View on GitHub (pinned to 89620f09af)

Solutions

1. Report the `lang` value from the message — it indicates a Vite-internal or plugin-introduced inconsistency.
2. Remove custom plugins that alter CSS file extensions/langs to isolate the cause.
3. Update Vite to the latest version; lang-handling bugs are patched regularly.

try {
  await transformCss();
} catch (e) {
  if (/Unknown lang:/.test(e.message)) {
    console.error('Unexpected CSS lang — disable custom CSS lang plugins and update Vite');
  }
  throw e;
}

When it happens

Trigger: Calling the lightningcss `@import` resolver path with a `lang` value outside `{ undefined, 'styl', 'stylus', 'sass', 'scss', 'less' }`. This is gated by `CSS_LANGS_RE`, so it normally only happens with a bug in lang detection.

Common situations: Internal Vite bug after adding a new CSS dialect, a custom plugin that mutates the id extension/lang before this resolver runs, or a malformed lang string in a request.

Related errors

• Unsupported dependency type: ${dep.type}
• HTML proxy index in "${id}" not found
• base or replace is required
• Preprocessor dependency "${lang}" not found. Did you install
• Unsupported target "${e}"

AI-assisted analysis of vitejs/vite@89620f09af (2026-08-03). Data as JSON: /data/errors/75eb339cc063986a.json. Report an issue: GitHub.