Overhead scales with the number of agents
Every subagent carries its own system prompt and tool schemas. Six subagents at 1,200 tokens each is 7,200 tokens of budget spent before any of them reads anything.
The planner subtracts that first, because allocating percentages of a total that includes overhead produces budgets nobody can actually work within.
When to collapse the topology
If overhead exceeds roughly thirty percent of the budget, the design is paying more for coordination than for work. Fewer agents with broader scope is usually both cheaper and more reliable.
Multi-agent structures earn their cost when sub-tasks need genuinely different tools or different models. Splitting purely to give each agent a tidier prompt rarely pays.