Plan vs Agent
You pick one of two submodes when you start the session, and the session stays in that submode for its lifetime.
The descriptions come straight from the slash-command registry.
/agent is “Switch to Agent mode (applies changes directly)”. /plan is “Switch to Plan mode (creates plan before making changes)”. Use Plan when you want a written game plan before any code lands. Use Agent for direct edits.
How chat mode differs from Plan & Build
Both modes use the same chat surface, the same slash commands, the same model picker, and (when configured) the same worktree. The differences:
If you started in chat mode and realize the task is bigger than a fix, open a fresh session from the new-session page and pick Plan & Build. There is no in-place upgrade from a
CHAT session to a SPEC session.