Maintain consistent package manager usage throughout the project lifecycle. When a project uses yarn (indicated by yarn.lock), avoid committing npm's package-lock.json file, as having both lock files can cause dependency resolution conflicts and inconsistent builds across different environments. Key practices: - Choose one package manager (npm or yarn) and stick with it - Only commit the lock file corresponding to your chosen package manager - Remove conflicting lock files from version control - Ensure all team members use the same package manager Example violation: ``` # Project structure showing both lock files (problematic) ├── package.json ├── yarn.lock # Using yarn └── package-lock.json # Should not exist when using yarn ``` This practice ensures reproducible builds and prevents confusion about which package manager and dependency versions should be used in different environments.