Bug with yarn and bun when running `rails new`

rubyonrails-core
1

I think I’ve found a bug with the rails new command.

When I run this command, it sets up the project with both yarn and bun, creating two lockfiles.

rails new project_name -j=esbuild -c=tailwind -d=postgresql

Problems

Here are the duplicate lock files after running that command:

$ ls -1
app
bin
bun.lock <-- bun lock
config
config.ru
db
Dockerfile
Gemfile
Gemfile.lock
lib
log
node_modules
package.json
Procfile.dev
public
Rakefile
README.md
script
storage
test
tmp
vendor
yarn.lock <-- yarn lock

Here’s the Procfile.dev with duplicate package managers — yarn and bun:

web: env RUBY_DEBUG_OPEN=true bin/rails server
js: yarn build --watch
css: bun run build:css --watch

I change bun run to yarn like this:

web: env RUBY_DEBUG_OPEN=true bin/rails server
js: yarn build --watch
css: yarn build:css --watch

Then I delete the yarn.lock, bun.lock, and node_modules.

I run yarn again to reinstall the node_modules cleanly.

Then I run bin/rails test, and the bun.lock file reappears.

Then I delete the yarn.lock, bun.lock, and node_modules, and run yarn again.

I replace bunx from the "build:css" script in package.json with npx, resulting in this:

  "scripts": {
    "build": "esbuild app/javascript/*.* --bundle --sourcemap --format=esm --outdir=app/assets/builds --public-path=/assets",
    "build:css": "npx @tailwindcss/cli -i ./app/assets/stylesheets/application.tailwind.css -o ./app/assets/builds/application.css --minify"
  },

Then bin/rails test and the bun.lock file returns.

I don’t think there should be two lockfiles in the project.

System

M1 MacBook with Sequoia 15.3.2.

$ rails -v
Rails 8.0.2

$ bun -v
1.2.8

$ ruby -v
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [arm64-darwin24]

$ yarn -v
1.22.22

(I’d rather be using pnpm, but I haven’t figured out how to do that yet.)

2

This problem also happens when running rake by itself. A bun.lock file appears.

3

Now I’m getting this other problem.

Edit: rails new was silently failing when yarn wasn’t installed after a Node version update with nvm.

I’m still getting the double lockfile once yarn is back on the path.

4

I’m still trying to figure this out, and I think the bug is in cssbundling-rails, so I’m going to file a bug there.

5

I tried to reproduce this but got another issue on main, and sent a PR:

I think now the issue will be what you reported, rails new finishes but yarn install never ran.

Once the app is there, can you run yarn install to fix it? Or there are some other steps we might need?