Claude already had subagents. This is a new mode for the main agent to be in (bespoke context oriented to delegation), combined with a team-oriented task system and a mailbox system for subagents to communicate with each other. All integrated into the harness in a way that plugins can't achieve.
Wow there goes a lot of harnesses out the window. The main limitation of subagents was they couldn’t communicate back and forth with the main agent. How do we invoke swarm mode in Claude Code?