Package managers

yarn, pnpm, npm and uv are integrations too. Each is on by default when its program is on PATH (enabled: auto), and then acts only on repositories that are recognisably its:

	recognised by
yarn	yarn.lock
pnpm	pnpm-lock.yaml
npm	package-lock.json or npm-shrinkwrap.json
uv	uv.lock or pyproject.toml

For the three node managers the packageManager field of package.json — what corepack reads — beats the lockfile; a repository with two lockfiles and no such field gets a warning and the first of yarn, pnpm, npm. uv is detected independently, so a repository with a Python service and a web front end gets both.

In every worktree a switch creates, the manager installs held to the lockfile, so two worktrees of the same commit get the same dependencies and an out-of-date lockfile is an error rather than a silent rewrite:

	runs
yarn	yarn install --immutable — --frozen-lockfile on yarn 1.x, asked per worktree since corepack makes the version the project’s choice
pnpm	pnpm install --frozen-lockfile
npm	npm ci
uv	uv sync

Before installing, yarn and pnpm seed the new worktree’s node_modules by hard-linking the one from the branch you switched from, or from the default branch when that is not a branch directory. A hard link costs an inode instead of the bytes, and the frozen install then reconciles the seed against what the branch’s lockfile actually says — the result is the same as installing from nothing, minus most of the work. Symlinks are recreated as symlinks, so pnpm’s relative links into node_modules/.pnpm point into the copy, not back at the original. A monorepo’s package-level node_modules (up to three levels deep) come along from the same source worktree; across filesystems the files are copied instead.

npm does not seed by default: npm ci deletes node_modules before it starts, which is what makes it reproducible. uv never seeds — a virtualenv carries its own absolute path and is not relocatable — but its global cache already hard-links packages into place.

The integrations run before a repository’s own setup commands, which can then assume the dependencies are there. An existing worktree whose dependency directory is present is left alone; --rerun-setup installs again and --no-setup skips the managers along with setup. An install that fails is a warning, not a failed switch — the session still opens, and you fix it there.

integrations:
  pnpm:
    install: [install, --offline]   # replaces the arguments entirely
  npm:
    install: [install]              # trade npm ci for seeding
    link: true
  yarn:
    enabled: false

ws status lists which integrations are active on this machine.