Definition
The top-level agent or workflow engine that decides which sub-agent or tool to invoke next.
In practice
Could be an LLM ('ask Claude which agent to route to') or rule-based ('if ticket starts with 'refund' send to billing-agent'). Rule-based is more predictable; LLM-based is more flexible. SMB stacks usually start rule-based and add LLM routing only where the rules get too messy to maintain.