Developed and delivered a Filesystem API for sidecar containers within the modal-labs/modal-client repository, enabling secure read, write, and file management operations inside sandboxed environments. The work involved integrating new API endpoints with the existing container execution flow to support robust file manipulation in sidecar workflows, aligning with the STO-80 backlog for traceable feature delivery. Using Python, the developer focused on backend and API development, ensuring that the new functionality operated seamlessly within the established architecture. Emphasis was placed on testing and secure sandbox integration, resulting in a single, well-scoped feature that enhances file operations for containerized applications.