A sub-agent is a specialized Claude instance that runs in an isolated context window with its own allowed tools, memory, and instructions. While your main Claude session handles the conversation, a sub-agent spins up to handle a specific task — investigation, code review, security audit, testing — in parallel with your work. Sub-agents excel at tasks that require: Reading many files without cluttering main context Specialized expertise in one domain Fresh perspective (the agent isn’t biased by earlier conversation) Parallel execution (multiple agents working simultaneously) Sub-Agents vs Skills vs Hooks Before diving into sub-agents, understand the distinctions: