The Google Gemini CLI is an open-source terminal assistant designed for high levels of extensibility and customization. Users can expand its capabilities through a pluggable extension system, custom slash commands, and integration with Model Context Protocol (MCP) servers for external data access. Advanced control is provided by a lifecycle hook system and a declarative policy engine that manages tool execution safety. The assistant's behavior and persona can be further refined using hierarchical context files (GEMINI.md) or by completely overriding the system prompt. For professional environments, the CLI supports enterprise-grade security, including sandboxed tool execution, centralized configuration, and administrative allowlists. Additionally, the tool offers a headless mode for automation and IDE integrations to streamline developer workflows directly within the editor.