What are subagents, and when do they help?
Subagents, and why a fresh notepad helps.
You ask Claude Code: "Compare these three vendors' API docs and tell me which one fits our use case."
Each set of docs is 40 pages. If one agent reads all 120 pages, its desk is buried before it even starts comparing (see "Why long chats make AI worse").
So instead, it sends out helpers.
Subagent: a second copy of the AI, started by the main one to do one piece of the job on its own clean desk.
It's like a manager sending an intern to the library with a note: "Read these docs, come back with one page on pricing and limits."
The intern reads all 40 pages. The manager only ever sees the one page.
The main desk stays clear. It holds three short summaries, not 120 pages.
Mistakes stay contained. If one intern goes down a rabbit hole, it doesn't poison the main conversation (see "Why AI gets stuck on its first idea").
They can work at the same time. Three helpers can read in parallel.
The intern only knows what's in the note. The subagent starts with a blank desk. Anything the main agent forgets to put in the brief, the subagent never hears.
Detail gets lost on the way back. The summary is all that returns. If the key fact was on page 31 and didn't make the summary, it's gone.
It uses more of your allowance. Every subagent reads its own pile. One Claude Code user who analysed their own sessions found subagents accounted for about a quarter of their usage.
Bad note: "Look into vendor B."
Good note: "Read vendor B's API docs. We need: monthly price for 1 million calls, rate limits, and whether they support single sign-on. Answer in under 200 words. Quote the page for each number."
Same as briefing a person (see "AI can't read your mind").
Made-up example.
Good fit: big reading jobs, separate pieces of work that don't depend on each other, searching through a lot of files.
Poor fit: small tasks, or work where each step depends on the last. The handover costs more than it saves.
For big research or reading tasks, ask for subagents. "Use a separate subagent for each vendor and bring me back a summary of each."
Ask for sources in the summaries. "Quote the page or file for each fact." That way you can check what didn't come back.
Skip them for small jobs. One desk is fine when the pile is small.