NPM Workspaces Optional Peer Dep Resolution Bug

README.md

NPM Workspace Optional Peer Dependency Resolution Bug

With the above workspace setup/package configurations, the change in npm/cli#8431 specifically breaks installation of the platform-specific packages for lightningcss.

The lightningcss-darwin-arm64 et. al dependencies are all being flagged as "optional peer" dependencies, despite technically only being "optional" through the lightningcss dep. (verified via npm why lightningcss-darwin-arm64). What's interesting, though, is that lightningcss itself is correctly being flagged as a "dev" dependency (not optional, not peer).

The only fix I've been able to find for this is either installing lightningcss in the root package.json, or installing lightningcss as a dependency of services/b, even though it's unused.

apps_a_package.json

{
  "name": "@test-repro/a",
  "version": "0.0.0",
  "private": true,
  "devDependencies": {
    "lightningcss": "^1.30.1",
    "vite": "^7.0.6"
  }
}

npm_why_output.txt

[email protected] dev optional peer
node_modules/lightningcss-darwin-arm64
  optional lightningcss-darwin-arm64@"1.30.1" from [email protected]
  node_modules/lightningcss
    peerOptional lightningcss@"^1.21.0" from [email protected]
    node_modules/vite
      dev vite@"^7.0.6" from @test-repro/[email protected]
      apps/a
        @test-repro/[email protected]
        node_modules/@test-repro/a
          workspace apps/a from the root project
      dev vite@"^7.0.6" from @test-repro/[email protected]
      services/b
        @test-repro/[email protected]
        node_modules/@test-repro/b
          workspace services/b from the root project
    dev lightningcss@"^1.30.1" from @test-repro/[email protected]
    apps/a
      @test-repro/[email protected]
      node_modules/@test-repro/a
        workspace apps/a from the root project

package.json

{
  "name": "npm-bug-repro",
  "version": "0.0.0",
  "private": true,
  "workspaces": [
    "apps/*",
    "services/*"
  ]
}

services_b_package.json

{
  "name": "@test-repro/b",
  "version": "0.0.0",
  "private": true,
  "devDependencies": {
    "vite": "^7.0.6"
  }
}