GuardDog is a command-line interface tool designed to identify malicious packages in PyPI and npm repositories. The tool operates by running a set of heuristics on package source code through Semgrep rules and analyzing package metadata to detect potentially harmful components. The tool supports scanning both local and remote packages from PyPI and npm ecosystems. Users can scan the most recent version of a package or specify particular versions for analysis. GuardDog allows selective rule application, enabling users to run specific heuristics or exclude certain rules from the scanning process. Installation options include pip installation for Python environments or Docker container deployment. The Docker image provides cross-platform compatibility, with Windows systems requiring Docker as the only supported installation method. GuardDog can process various package formats including compressed archives and local directories containing packages. The tool provides flexibility in scanning approaches, allowing users to target individual packages or batch process multiple packages stored in local directories.