A subagent is a fresh Claude instance that does a self-contained job in its own context window and hands back only a summary. This episode covers using them the Act I way: one-off delegation to keep your main session's context clean. **Why isolation matters.** The context window fills fast and Claude's performance degrades as it does. From Anthropic's [context-window walkthrough](https://code.claude.com/docs/en/context-window): a research subagent read ~6,100 tokens of files and returned a 420-token result. The reading never touched your main thread. The [best-practices guide](https://code.claude.com/docs/en/best-practices) calls subagents one of the most powerful tools available because context is your fundamental constraint. **The Agent tool.** As of Claude Code 2.1.63 the Task tool was renamed to Agent (old `Task(...)` references still work as aliases). Subagents cannot spawn other subagents, which is why the orchestrator pattern needs agent teams later. **Built-in subagents.** Explore (read-only, Haiku, codebase search), Plan (read-only, used in plan mode), general-purpose (all tools, inherits your model), plus helpers statusline-setup and claude-code-guide. Explore and Plan skip CLAUDE.md and git status to stay fast. **Custom subagents.** Markdown files with YAML frontmatter in `.claude/agents/` (project, checked into git) or `~/.claude/agents/` (user). The fields that matter: `name`, `description` (drives automatic delegation), `tools` (allowlist; omit to inherit all), and `model` (`sonnet`/`opus`/`haiku`/full ID/`inherit`). Manage them with the `/agents` command. See [Create custom subagents](https://code.claude.com/docs/en/sub-agents). **Proactive delegation.** Put "use proactively" in the `description` field so Claude reaches for the agent on its own. **Worked examples.** A codebase-search delegation that keeps file reads out of your window, and the official read-only `code-reviewer` (restricted to Read, Grep, Glob, Bash) that judges your diff with fresh eyes. **Subagent vs skill vs slash command.** A skill runs in your main context; a subagent runs isolated and returns a summary; a slash command is a typed entry point. Offload noisy, self-contained work to a subagent. **The pitfall.** Subagents start blank, with no conversation history. Vague delegation produces confident, wrong answers. Scope tightly and restate load-bearing constraints. Plus the costs: tokens still get spent, cold-start latency, and no mid-task steering (a subagent can't even ask you a clarifying question).