You hand one model a goal with nine moving parts — pull the numbers, check them against last quarter, draft the summary, file the ticket — and it does four, forgets two, and invents the rest. Nothing was broken. You gave one context window a job that needed a team.
An agent is a model that can take actions, and delegation is the first pattern you reach for when one agent stops coping. A planner reads the goal, splits it into subtasks, and routes each one. Workers execute, each holding only the slice of context its own subtask needs and only the tools that subtask requires. The planner never runs the work itself.