When would you use a multi-agent system instead of a single agent?

Use multiple agents when a task benefits from separation of concerns: distinct roles (planner, coder, reviewer, researcher), parallelizable subtasks, or isolating context so each agent has a focused, smaller prompt. A common pattern is an orchestrator/supervisor that decomposes work and delegates to specialist sub-agents, each with its own tools and instructions. Benefits: modularity, parallelism, and cleaner context per agent. Costs: coordination overhead, higher token and latency spend, and harder debugging, errors can cascade between agents, and shared state gets tricky. Multi-agent shines for broad, parallelizable research or workflows with clearly separable roles. For most tasks a single well-equipped agent is simpler and cheaper; reach for multi-agent only when a single context genuinely can't hold the task or roles truly conflict.