How to Warm Dependency Caches in CI

A cache only helps if it exists. Warming it from main means PRs never pay the cold-install tax.

GitHub Actions caches are scoped: a PR can read caches from its base branch but not from sibling branches. Build the cache on main so every PR inherits it.

Build the cache on the default branch

Run a scheduled or push-triggered job on main that installs dependencies and saves the cache. PRs then restore it.

Use restore-keys for partial hits

A fallback key lets a slightly stale cache restore and update incrementally instead of starting cold.

.github/workflows/ci.yml

- uses: actions/cache@v4
  with:
    path: ~/.npm
    key: npm-${{ hashFiles('package-lock.json') }}
    restore-keys: |
      npm-

Skip the cold start entirely

Warm-pool managed runners (like Latchkey) keep dependency layers hot across jobs, so even the first PR run starts warm instead of provisioning and downloading from scratch.

Key takeaways

PRs can only read base-branch caches, so warm main.
restore-keys give partial hits instead of cold starts.
Warm-pool runners keep caches hot across jobs.

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