Anthropics has released Sandbox Runtime (srt), a lightweight OS-level sandboxing tool that enforces filesystem and network restrictions on processes without requiring containers. The tool leverages native sandboxing primitives—Apple's sandbox-exec with Seatbelt profiles on macOS, bubblewrap with network namespace isolation on Linux, and Windows Filtering Platform (WFP) with restricted user tokens on Windows. Network traffic is mediated through HTTP and SOCKS5 proxies running on the host, enforcing domain allowlists and denylists. Filesystem access follows a deny-then-allow model for reads (with allowRead taking precedence over denyRead) and an allow-only model for writes (where denyWrite takes precedence). By default, all network and write access is denied. Configuration is managed via a JSON settings file (~/.srt-settings.json) or programmatically through the TypeScript library API. The tool supports dynamic runtime configuration updates via a control file descriptor, per-command network allowlists with cryptographically random IDs (minimum 128 bits), and violation tracking by command attribution keys. It is designed for securing AI agents, MCP servers, and arbitrary CLI processes, with a research preview status and evolving APIs.