Simple explanation
Welcome to Day 19. On Day 12 you ran two Claude Code sessions simultaneously. Today you go one level further. Claude Code runs multiple workstreams by itself — you give it one big task and it splits the work, manages the parts, and brings the results back to you. What are sub-agents? A sub-agent is a separate Claude Code process that your main session creates and manages. You give the main session a complex task. It decides how to split it. It creates sub-agents to work on each part in parallel. It collects all the results and combines them. You stay in one session. The work gets done as if you had a whole team. A plain English example Without sub-agents: You ask Claude Code to research five competing websites, summarise each one, and write a comparison report. Claude Code visits them one at a time. You wait for all five before seeing any results. With sub-agents: You ask Claude Code to research five competing websites using sub-agents. It creates five sub-agents — one per site — running simultaneously. All five come back at the same time. Claude Code combines them into one report. You wait a fraction of the time. How to trigger sub-agents Simply include the words "use sub-agents" in your prompt: "Use sub-agents to check each page of my site for SEO issues and compile a single priority list of fixes." "Use sub-agents to research three AI tool competitors and bring back a summary of what each one offers." "Use sub-agents to write draft content for my About page, Resources page, and FAQ page simultaneously — then show me all three for review."
What to do
When sub-agents are most useful: Research tasks — gathering information from multiple sources at the same time. Multi-page builds — building several pages simultaneously when they do not depend on each other. Content generation — writing multiple pieces of content in parallel and reviewing them together. Site-wide checks — running the same check across every page simultaneously instead of one at a time. The one thing to watch Sub-agents use more of your Claude Pro usage allowance than single sessions. Use them for tasks where the parallel speed genuinely saves significant time. Do not use them for simple single-file changes where a normal prompt is faster. Today's practical task 1. Open Claude Code. 2. Type this prompt: "Use sub-agents to review three pages on my site simultaneously — the homepage, the latest lessons page, and the 21-day challenge page. For each page check: does the page load correctly, are there any obvious design inconsistencies, and is there a clear call to action? Bring back a single combined report." 3. While it runs — do not open another session. Just observe how it manages the workstreams. 4. Read the combined report when it comes back. 5. Pick one thing from each page to fix. 6. Fix them one at a time using the 4-part formula. Today's result: You have used sub-agents for the first time. A task that would have taken three separate sessions came back in one.
Copy-paste prompt
Use sub-agents to review the following pages simultaneously: [list your pages]. For each page, check: page loads without errors, design is consistent with the rest of the site, there is one clear call to action, and there are no placeholder texts or broken links. Bring all findings back as a single combined report — one section per page, ordered by priority.
Course note
Key takeaway
Add "use sub-agents" to any prompt that involves multiple parallel tasks. Claude Code splits the work, runs it simultaneously, and brings back combined results. One prompt, multiple workstreams, one report. Use it for research, multi-page builds, and site-wide checks.