Subagents are agents started by a main agent to handle a focused sub-task, each in its own fresh context window. Each one runs its own agent loop and returns a summary, not its full transcript.
Trade-offs
- Benefit: clean context. A subagent can read hundreds of files and return a few paragraphs, which keeps the main agent’s window clear. This is a context engineering technique as much as a way to run work in parallel.
- Benefit: parallelism. Independent sub-tasks such as search, implementation and review can run at the same time.
- Cost: lossy handoffs. A subagent knows only what its brief says, and the main agent sees only the summary.
- Cost: tokens. Each subagent pays for its own context.
- Rule of thumb: delegate when sub-tasks are truly independent. Otherwise the overhead outweighs the gain.
Arrangements
How subagents are wired together is an architectural choice. It can be a fixed order (sequential, parallel or loop), routing by a coordinator model, multi-level hierarchical decomposition, or an all-to-all swarm. Google’s guide advises starting with a single agent and moving to multi-agent only when one agent must juggle several distinct responsibilities. See agent design patterns.
Related
Verification and evals, where independent reviewer subagents are a common use.