3 Ways To Build An AI Second Brain | Learn AI With Mariah

People make a second brain sound complicated. It isn’t.

A second brain is one place where all your important information lives. Point AI at it and you stop re-explaining your job and your goals in every chat, because the AI already knows where everything is.

Here are three ways to build one, ranked easiest first. Claude Projects takes 15 minutes and zero tech skill. Obsidian takes an hour and suits note takers. A folder of files takes 30 minutes and is the most powerful. You can switch later, because all three are just files.

Read This Before You Build Anything

Second brains fail because people build the container and never fill it. Put five real files in on day one. Skip the folder system and the tags for now. Five files with actual information about you and your work.

Start here if you’ve never done this. Projects work on every Claude plan, including Free (Free gets up to five). You drop your files in, write a short set of instructions, and every chat inside it already knows who you are. (Claude Help Center)

Create the project

In Claude, click Projects in the sidebar (or go to claude.ai/projects), then + New project. Name it after what it’s for, like My Business or Job Search, one per big area of your life.

Upload the right things

Click the + in the project knowledge panel and upload, in this order:

A one-page doc about you. Role, goals this year, who you work with, what you want help with. The prompt below writes it with you.
Your recurring documents. Brand guide, pricing sheet, client list. Anything you’ve pasted into a chat more than twice.
Three examples of your best work, so it learns your standard instead of guessing.

Don’t upload your whole hard drive. On paid plans, once the knowledge gets close to Claude’s memory limit, it switches to searching your files instead of reading them all.

Write the project instructions

Click Set project instructions. Almost everyone skips this, and it’s what makes the project feel smart. Write who you are, what you want done here, how to talk to you, and which file to check first, like always check the brand guide before writing anything customer facing.

Keep it current

At the end of a good chat, use the save prompt below and upload the note back in. Once a month, delete what’s out of date.

The Catch Worth Knowing

In a big project it searches for what looks relevant, so a broad question can miss a detail sitting right there. Ask narrow questions and name the file you care about. Everything also lives inside Claude, so moving to another AI means re-uploading by hand.

The one the internet is obsessed with. Overhyped, but genuinely good. Your notes live as plain markdown files on your own computer, and it’s free for any use, work included. (Obsidian license)

Install it and make a vault

Download it from obsidian.md. On first open it asks you to create a vault, which is just a folder on your computer. If you use iCloud, Dropbox or Google Drive, put the vault in that synced folder so it backs up. Make one vault, not three.

Use four folders

inbox for quick captures. notes for finished thinking, which is what you want the AI reading. projects for one file per active project. reference for things you didn’t write, like saved articles and transcripts. Skip tags for two weeks and let the structure follow what you actually have.

Connect AI to it

Your vault is a normal folder, so the easiest move is pointing Claude Code or Codex at it (setup is in No. 1 below). Or drag the notes you care about into a Claude Project.

The Warning That Matters Most

Obsidian has thousands of community plugins, and that’s where people lose. People install twenty and quit by month two with a beautiful empty vault. Install one or two at most in your first month. Zero is fine.

This is the setup I run my businesses on. It’s a folder on my desktop with hundreds of plain files that an AI agent reads, adds to and pulls from in seconds. Nothing sits between your information and the AI, and you own every file.

Make the folder

Make a folder called brain with three folders inside: about-me, projects and reference. Add more later if you need them.

Write the one file that matters most

In about-me, save a file called context.md. Every AI you point at this folder should read it first. Paste this prompt into Claude or ChatGPT and it interviews you and writes the file.

Write My context.md

You are helping me write context.md, the one file every AI I use will read first so I never have to re-explain myself. Interview me. Ask ONE question at a time and wait for my answer before the next. Cover these six areas: 1. Who I am: my name, what I do, what my job or business actually is. 2. What I'm working toward: goals for this year, with numbers if I have them. 3. How I work: my schedule, my tools, what I do myself and what I hand off. 4. How I want AI to talk to me: tone, length, when to push back. 5. What I keep having to explain: background I'm tired of typing into every chat. 6. What's in my second brain: the folders or files I have (or plan to have), one line each. If an answer is vague, ask one follow-up to get something specific (a number, a name, an example). Don't ask more than 12 questions total. When you're done, write the file in markdown: - A heading for each of the six areas - Short, plain sentences, written in first person as me - No filler, no adjectives I didn't use, nothing you guessed. If I skipped something, write "TBD" under it. - Keep it under 500 words End by listing the 3 facts in the file most likely to go out of date, so I know what to update first.

Name files like a human

Every file is a .md file, which is just a text file. Name it so you know what’s inside without opening it: brand-voice.md, 2026-goals.md, client-acme-notes.md. Lowercase with dashes, one topic per file.

Point Claude Code or Codex at it

Claude Code needs a paid Claude plan (Pro or higher). In the Claude desktop app, open Claude Code and choose your brain folder. Codex is a mode in the ChatGPT desktop app, and Free plans get it with tighter limits. Open it and pick the same folder.

Then ask something you’d normally have to explain first: read context.md, then tell me what I should focus on this week. End good sessions with the save prompt, and the brain grows while you use it.

Save This Session

We're wrapping up this session. Save what matters to my second brain. 1. Write a short markdown note with: - A title line: today's date plus the topic - Decisions we made (one bullet each, with the reason) - Anything I told you about myself, my work or my preferences that isn't already in my files - Open questions and next steps, with who owns each one 2. Keep it under 250 words. Use my words where you can. Don't include anything we tried and rejected unless the reason is worth remembering. 3. Suggest a lowercase-with-dashes file name and which folder it belongs in (about-me, projects or reference). 4. If anything we discussed contradicts a file I already have, name the file and the line so I can fix it. If you can write files in this folder, save the note there. If you can't, give it to me in one block I can copy.

3 Ways To BuildAn AI Second Brain

3 Ways To Build
An AI Second Brain