Less 5 removes `FileManager` make it incompatible

[yhx-12243]

Describe the bug

As

vite/packages/vite/src/node/plugins/css.ts

Line 2910 in dcf88bd

ViteLessManager ??= class ViteManager extends FileManager {

shows, the less css adapter requires the less API FileManager.

However, less 5 removes this API (less/less.js@937d1e4), result in the following error:

[plugin vite:css] src/index.less:undefined:undefined
Error: [less] Class extends value undefined is not a constructor or null at process (node_modules/vite/dist/node/chunks/node.js:23770:45)
  at async compileCSSPreprocessors (node_modules/vite/dist/node/chunks/node.js:23041:27)
  at async compileCSS (node_modules/vite/dist/node/chunks/node.js:23069:30)
  at async TransformPluginContextImpl.handler (node_modules/vite/dist/node/chunks/node.js:22556:47)
  at async plugin (node_modules/rolldown/dist/shared/bindingify-input-options-C-Zsy1EG.mjs:1537:16)
  at async plugin.<computed> (node_modules/rolldown/dist/shared/bindingify-input-options-C-Zsy1EG.mjs:2125:12)

Reproduction

https://stackblitz.com/edit/vitejs-vite-392jf7f8

Steps to reproduce

Run npm install followed by npm run build

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 22.22.3 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    vite: ^8.2.0 => 8.2.1

Used Package Manager

npm

Logs

Click to expand!

✗ Build failed in 5.15s
error during build:
Build failed with 1 error:

Error: [less] Class extends value undefined is not a constructor or null
    at napi_create_error (file:///tmp/rolldown-1.2.4/node_modules/.pnpm/@emnapi+core@2.0.0-alpha.4/node_modules/@emnapi/core/dist/emnapi-core.js:4610:23)
    at napi_create_error (file:///tmp/rolldown-1.2.4/node_modules/.pnpm/@emnapi+core@2.0.0-alpha.4/node_modules/@emnapi/core/dist/emnapi-core.js:4610:23)
    at aggregateBindingErrorsIntoJsError (file:///home/projects/vitejs-vite-392jf7f8/node_modules/rolldown/dist/shared/error-CVc7IgvG.mjs:67:18)
    at Module.unwrapBindingResult (file:///home/projects/vitejs-vite-392jf7f8/node_modules/rolldown/dist/shared/error-CVc7IgvG.mjs:37:128)
    at #build (file:///home/projects/vitejs-vite-392jf7f8/node_modules/rolldown/dist/shared/rolldown-DiYVDns9.mjs:147:54)
    at async buildEnvironment (file:///home/projects/vitejs-vite-392jf7f8/node_modules/vite/dist/node/chunks/node.js:33814:66)
    at async Object.build (file:///home/projects/vitejs-vite-392jf7f8/node_modules/vite/dist/node/chunks/node.js:34234:19)
    at async Object.buildApp (file:///home/projects/vitejs-vite-392jf7f8/node_modules/vite/dist/node/chunks/node.js:34231:153)
    at async CAC.eval (file:///home/projects/vitejs-vite-392jf7f8/node_modules/vite/dist/node/cli.js:791:3) {
  errors: [Getter/Setter]
}

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[solomonaustin]

Some findings that may save triage time — the removal is broader than FileManager.

Less 5 is a rewrite: lib/index.js describes itself as "a Less-compatible API backed by the Jess compiler", and the whole plugin/file-manager architecture is gone. Checked against the published less@5.0.0-alpha.1:

resolved version : [ 5, 0, 0 ]
FileManager      : undefined
PluginLoader     : undefined
own keys         : version, render, renderFile, logger, lesscHelper, Compiler

Vite needs two things here — less.FileManager to subclass (css.ts#L2910) and pluginManager.addFileManager() to register the resolver (css.ts#L2949). Neither exists in Less 5, so this isn't a matter of adapting to a renamed export: there's no interception point left for Vite's @import resolution, and supporting Less 5 would need an upstream API first.

Two things that may affect how you want to prioritise it:

• 5.0.0-alpha.1 is published under the alpha dist-tag only — latest is still 4.9.0 — so only explicit opt-in is affected.
• packages/vite/package.json already declares less: ^4.0.0 in peerDependencies, so Less 5 is outside the supported range.

Given that, I opened #23271 to replace the opaque Class extends value undefined is not a constructor or null with a message that names the incompatibility and points at less@^4. Verified both ways against a local build (less@5 → new error, less@4.9.0 → builds clean).

Happy to close that in favour of a different direction if you'd rather handle it another way, e.g. a version check at preprocessor load time.

[sapphi-red]

less v5 is still alpha and is not ready for Vite. Let's wait for them.
https://github.com/less/less.js/blob/v5.0.0-alpha.1/CHANGELOG.md#:~:text=The%20broader%20Less,4%20fixture%20corpus.