Vercel monorepo: install only a single project dependencies with Yarn

May 4, 2024

If you have a Vercel app as part of a monorepo, it may bother you that by default every build installs the whole monorepo dependencies.

If the Vercel app is the main project in the monorepo, it should go unnoticed, but if you have a bunch of other packages whose dependencies differ a lot from that of the Vercel app, then it will be very obvious that you’re spending a lot of time installing useless stuff on every build.

If you’re using a modern version of Yarn, aka not “Yarn classic”, aka Yarn “Berry” and later, which would be Yarn 4 today, then you can use yarn workspaces focus to do just that.

In Yarn 3, you need to install the workspace-tools plugin via yarn plugin import workspace-tools for this to work. In Yarn 4, the command is supported out of the box.

yarn workspaces focus, when run from inside a specific workspace directory, will install that workspace dependencies, as well as the dependencies of all the workspaces it depends on.

Note: if you’re using nodeLinker: node-modules, the node_modules layout may differ a bit, especially the fact that packages from your other workspaces are no longer installed at the root of the monorepo.

This will make it obvious if you’re implicitly depending on packages that are part of your monorepo but not depended on by your specific workspace.

In order to configure that on Vercel, you can configure the following in vercel.json:

  "installCommand": "yarn workspaces focus"

Note: you’ll probably want to mirror that configuration inside your app settings in the Vercel dashboard in Settings > Build & Development Settings > Install Command, otherwise after you deploy, Vercel will warn you that “the configuration of the current production deployment differ from your current project settings”.

Making it work with Vercel cache

There’s one more problem. Now you download only the dependencies of your project, which is much better, but on subsequent builds, Yarn keeps re-downloading everything again. It’s like there’s no cache!

That’s because Vercel and Yarn 3 and greater don’t play well together. Yarn needs its .yarn/cache and Vercel doesn’t cache it between builds.

More details in this other post. 😉

TLDR: use the following command.

  "installCommand": " yarn workspaces focus"

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