Subagents

A subagent is a separate Claude process that the main session can spawn to do a piece of work. It runs with its own context window, its own tool list, optionally its own model, and returns a single summary message back to the parent. Claude Code ships bundled subagents for common delegation patterns, including general-purpose, Explore, and Plan; custom ones live in .claude/agents/<name>.md as markdown with YAML frontmatter.

The whole point is context isolation. A subagent can grep through fifty files and read twenty of them, and the main thread only sees the conclusion. That’s what makes long sessions stay coherent — noise lives in the child, signal comes back to the parent.

This chapter starts with the mindset (context firewall, not productivity theatre), then shows when the built-ins already do the job, then how to write a custom subagent that gets routed to, scoped tightly, and briefed well. The last two Items cover orchestration — parallel versus background — and verification, because an agent’s summary describes what it intended to do, not what it actually did.