Yarn Berry "Cache path does not exist" / Hardened Mode - Fix in CI

Yarn Berry CI installs run with --immutable, which forbids changes to the lockfile and (with a committed offline cache) expects every package to already be present. A drifted lockfile or an incomplete cache fails the install.

What this error means

yarn install --immutable fails because the lockfile would change, or because a cache entry it expected is missing. It is deterministic - the committed state simply does not satisfy the immutable contract.

Yarn output

➤ YN0028: The lockfile would have been modified by this install,
which is explicitly forbidden.
➤ YN0000: Failed with errors in ...s
# or, with zero-installs cache committed:
Error: cache path does not exist for some-pkg@npm:1.2.3

Common causes

The lockfile is out of sync under --immutable

A dependency change that was not committed to yarn.lock means the immutable install would have to modify it, which YN0028 forbids.

An incomplete committed offline cache (zero-installs)

With zero-installs, the .yarn/cache is committed. If a package’s cache artifact is missing, the immutable install cannot fetch it and fails.

How to fix it

Regenerate and commit lockfile (and cache)

Run a normal install locally to update yarn.lock (and the committed cache for zero-installs), then commit.

Terminal

yarn install
git add yarn.lock .yarn/cache
git commit -m "chore: sync yarn lockfile and cache"

Confirm the cache strategy

For zero-installs, ensure .yarn/cache is fully committed (not gitignored).
For non-zero-installs, cache .yarn/cache in CI keyed on yarn.lock instead of committing it.
Run yarn install after any dependency change so the lockfile and cache stay consistent.

How to prevent it

Commit yarn.lock with every dependency change.
Keep CI on --immutable and fix drift in the repo.
Match the cache approach (zero-installs vs CI cache) consistently.