If you're deploying Firebase functions from a monorepo, you might have encountered a particularly frustrating bug: a shared package fix lands, type-checks, and passes CI, but the deployed function still runs the old, stale code. The problem isn't in your code or your tests; it's a subtle interaction with how Firebase functions bundle dependencies, specifically when those dependencies are local packages within your monorepo.

The Illusion of Freshness: Why Your Code Appears Current

When you're working in a monorepo, it's natural to expect that once you update a shared utility package and your CI pipeline passes, those changes will be reflected everywhere that package is used. For many parts of a modern application, this assumption holds true. Your local development environment picks up the changes, your tests run against the new code, and your type-checker confirms everything is in order.

However, Firebase Cloud Functions introduce a wrinkle. Unlike other deployment targets that might build directly from your source or a pre-compiled artifact, Firebase functions, particularly when dealing with local file: dependencies in package.json, have a specific bundling mechanism. They package each local dependency as a .tgz archive. This archive is then included in the final function bundle that gets uploaded to Google Cloud.