Crystal is a free, MIT-licensed desktop app that lets you spin up multiple Claude Code and Codex sessions working on different tasks simultaneously, each in its own git worktree. It runs locally and is great for batch work and parallel exploration, but sessions are independently dispatched and reviewed by hand. (See Crystal's repository for its current status.)
Fleet is designed for continuous, autonomous operation. Agents persist, have roles, communicate through an event bus, and react to external events like GitHub label changes. The system is less about spinning up sessions interactively and more about maintaining a running team that handles your development workflow without a person at the keyboard.