Second brain AI: tools compared, and when agents write

Updated 10 min read

An AI second brain is a place outside your head for your notes, sources and ideas, with AI doing the filing, linking and answering that most people skip. The idea comes from Tiago Forte’s Building a Second Brain; the AI is the new part. The tools fall into four kinds: notes apps with AI built in, AI-first apps, Obsidian with plugins or an agent, and a wiki your agents write themselves. Which one fits depends mostly on who writes to it: only you, or your agents too.

What is a second brain?

Forte’s introductory guide, on his own site, defines it as “an external, centralized, digital repository for the things you learn and the resources from which they come.” His reason fits in one line: “our brains are for having ideas, not storing them.”

He builds it with two frameworks.

CODE is the process: Capture, Organize, Distill, Express. Capture only what resonates with you. Organize for actionability, not by subject. Distill with Progressive Summarization: save the best excerpts, then summarize those, then summarize the summary, so a note can be read at any level of detail. Express is using what you saved in work you make. Forte calls CODE a process for turning the information you consume “into creative output and concrete results.”

PARA is the folder structure, four top-level categories for everything:

  • Projects: short-term efforts with a goal, such as writing a report.
  • Areas: responsibilities that need ongoing attention, such as health or finances.
  • Resources: topics you are interested in or learning about.
  • Archives: anything from the other three that is no longer active.

Forte says PARA works in a file system, cloud storage or a notes app, and his guide names Evernote, OneNote, Notion, Apple Notes and Obsidian as places to keep it.

What does AI add to a second brain?

The steps people drop are Organize and Distill. Filing a capture into the right project, linking it to related notes and summarizing it all take time, so the inbox grows and the notes go unread. AI now does that work quickly:

  • It reads an inbox note and files it, or merges it into the page that already covers the topic.
  • It connects a new note to related ones, so you reach it from either side later.
  • It answers questions from your notes and cites them.
  • It compiles. In the LLM wiki pattern Andrej Karpathy published in April 2026, an agent reads your sources and writes and maintains a set of linked pages from them, and you ask the compiled pages instead of a pile of raw notes. Our LLM wiki guide explains it.

Robin Good’s newsletter, a top Google result for this query, puts the gap plainly: second brains let you search and retrieve, but did not get smarter as you added to them. Compiling closes that gap.

What kinds of AI second brain tools are there?

Prices are from each product’s pricing page on 1 October 2026, in US dollars.

Kind Products What the AI does Price Where notes live
Notes app with AI built in Notion, Capacities Writes, searches and answers inside the app Notion free with an AI trial, Business $20 a member a month; Capacities free, Pro $9.99 a month The vendor’s service
AI-first app Mem, Tana An agent files notes, tracks tasks or takes meeting notes Mem free, then $9 to $49 a month; Tana free, Pro $30 a user a month The vendor’s service
Obsidian with plugins or an agent Obsidian plus Copilot, Smart Connections or Claude Code Plugins chat and search; an agent edits the files App free; plugin and model costs vary Markdown files on your machine
Agent-written wiki The Curator, an LLM wiki folder, Dexio Agents write and keep pages current The Curator free; Dexio free for one person Your folder, or hosted

Notes apps with AI built in

Notion gives its Free and Plus ($10 a member a month) plans a trial of Notion AI. Business, at $20 a member a month, adds Notion Agent, which its pricing page says “completes complex, multi-step tasks using context from Notion, your connected apps, and the web,” along with AI Meeting Notes and Enterprise Search (beta). Custom Agents, which run repetitive tasks on their own, are free to try, then $10 per 1,000 Notion credits. Pick Notion when your team already keeps its docs there.

Capacities is free on its Basic plan, with unlimited objects, custom object types, sync and full import and export. Pro is $9.99 a month billed monthly, or 16% less yearly, and adds an AI assistant, related content, smart search and API access, with a monthly AI budget. Pick it for a personal notes app built around typed objects, such as people, books and meetings, with a low-priced AI tier.

AI-first apps

Mem is built around an agent that, in Mem’s words, “learns what matters from what you save.” The free plan allows 25 new notes and 25 messages to Mem a month. Plus is $9 a month for 50 of each. Pro, at $29, raises that to 100 and adds one custom routine (a scheduled briefing, research or note organizing) and one connection to Gmail, Slack or Todoist. Pro 2× is $49. Pick it if you want tasks found in your notes and followed up with little setup, and watch the note caps if you write a lot.

Tana now sells itself on meetings. The free plan hosts 5 meetings a month with AI transcripts and summaries and includes 50 AI queries. Pro lists $30 a user a month, shown at an early-bird $20 when we checked, with 20 times the AI, unlimited meetings, a botless meeting agent for Zoom, Teams and Google Meet, and “full MCP”. There is a 30-day trial with no card. Pick it when most of what you need to remember is said in meetings.

Obsidian with plugins or an agent

Obsidian is free with no sign-up, and its pricing page says your data is stored locally on your device. Sync costs $4 a user a month billed yearly, or $5 monthly. The AI comes from community plugins or from an agent working in the vault folder. When we counted Obsidian’s plugin downloads for Obsidian AI, the two most installed AI plugins both ran Claude Code or Codex inside the vault. Pick Obsidian if you want plain markdown files on your own machine and no subscription.

A wiki your agents write

Here the agent writes and you read. Any coding agent in a folder of markdown can run the LLM wiki pattern. The Curator, the open-source app in Robin Good’s post, packages it as a desktop app; its README says “Build a second brain from what you read, share it with a team, and give it to your agents.” It keeps one knowledge folder on your computer, readable in Obsidian and synced through your own private GitHub repository. The README gives its license as MIT plus source-available, and it had 97 GitHub stars on 1 October 2026. Dexio, a hosted version of this kind, is covered below.

When should your agents, not just you, read and write it?

Forte designed the second brain for one person. Many people now also run agents: Claude Code in a repository, ChatGPT on the phone, a Hermes or OpenClaw bot on a server. Each starts a session with whatever its own memory holds, which is usually not your notes (agent memory explains the kinds).

Let your agents read and write the second brain when:

  1. More than one agent needs the same context. If you explain a project to Claude Code on Monday and to ChatGPT on Tuesday, that explanation belongs on a page both can read.
  2. The knowledge comes out of the agent’s work. A coding agent that spends an hour finding why a deploy fails has the finding first-hand. Ask it to write the page; retyping it yourself loses detail.
  3. Organize and Distill are piling up. If 200 captures sit unfiled, an agent filing them into PARA folders beats a weekend you will not schedule.
  4. You can see what it wrote: Git diffs, page history with the agent’s name, or a graph of pages and links. If you cannot review it, do not let it write.

Keep it yours when the writing is the thinking (journals, drafts, Forte’s Express step), or when a note is private and the agent’s model runs in the cloud.

Split CODE between you and your agents like this:

Step You Your agents
Capture Decide what resonates; clip and highlight Save decisions, findings and transcripts from their own sessions
Organize Set the PARA folders and the rules File captures, link pages, move finished projects to the archive
Distill Read the top layer and correct it Write summaries, then summaries of the summaries
Express Write, decide, ship Draft from pages and answer questions with citations

How do I set up an AI second brain?

Start with one folder, PARA, one agent and one rules file:

second-brain/
  AGENTS.md      # rules every agent reads
  CLAUDE.md      # optional, for Claude Code: first line @AGENTS.md
  inbox/         # captures waiting to be filed
  projects/
  areas/
  resources/
  archive/
  journal/       # yours: agents read it, never write
  index.md       # one line per page
  log.md         # what each agent did, appended

Anthropic’s docs say Claude Code reads AGENTS.md by default only when there is no CLAUDE.md. If you keep a CLAUDE.md too, the @AGENTS.md import loads both, so every agent follows one set of rules (CLAUDE.md guide). A starting AGENTS.md:

# Second brain rules

This folder is my second brain, organized with PARA.

## Organize
- Captures land in inbox/. File each into projects/, areas/ or resources/,
  merging into an existing page when one covers the topic.
- When I say a project is done, move its folder to archive/.
- Never write in journal/. Ask before deleting anything.

## Distill
- One topic per page. Start each page with a summary of three lines or fewer.
- Link every new page to at least one existing page and add it to index.md.
- Name the source (file or URL) of every claim taken from one.

## Log
- After each task, append one line to log.md: date, what changed, why.

Then:

  1. Put the folder in Git, so git diff shows every line an agent changed.
  2. Start your agent in the folder and hand it the inbox: “File everything in inbox/ following AGENTS.md.”
  3. Once a week, look at the graph. A page with no links was never filed properly. The wiki graph viewer opens a local folder in your browser and uploads nothing.

For Obsidian, Obsidian + Claude Code adds wikilinks, deny rules and the Obsidian CLI, and LLM wiki in Obsidian turns sources into a wiki step by step.

What if your agents run in different places?

A folder on your laptop serves agents on your laptop. A bot on a server, Claude on the web or ChatGPT on your phone cannot reach it. You can sync copies (Obsidian Sync, Git, or The Curator’s GitHub repository), or keep one wiki that every agent reaches over MCP. MCP memory servers compares ten ways to do the second.

Share it with your other agents

Dexio is a hosted wiki that agents read and write over MCP. Claude Code, Codex, Cursor, Hermes, OpenClaw, Muse, Grok Bot, Claude and ChatGPT all connect to https://app.dexio.wiki/mcp (ChatGPT through developer mode on a paid plan). Dexio tells connected agents to check the wiki before answering questions about your work, and to record decisions, findings and how things work on the page that covers them.

You see what your agents know at app.dexio.wiki, as a graph where every page is a dot and every link a line. Each page’s history lists every revision with the agent, the person and the note behind it, and pages can cite sources with footnotes. It is free for one person with unlimited pages and agents; Team is $10 and Business $20 a member a month, agents never count as members, and there are no usage charges. You can download the wiki as markdown any time, and the code is open source under AGPL-3.0.

Other tools fit some jobs better. For a notes app you write in every day, use Obsidian, Notion or Capacities. For meeting capture, Tana or Notion’s AI Meeting Notes. For everything on one machine, a folder and Git cost nothing. Dexio fits when several agents, on different machines or in chat apps, need the same pages. To connect one, send it:

Connect yourself to my Dexio wiki. The steps are at https://dexio.wiki/agents.md: read the whole file, not a summary, and follow them.

FAQ

What is the best AI second brain app? It depends on who writes. If only you do, the notes app you will open every day. If agents write too, a markdown wiki they share.

Can I build an AI second brain for free? Yes. Obsidian is free, and so are several of its AI plugins. Capacities Basic, Mem Free (25 notes a month), Tana Free (50 AI queries) and Dexio Free for one person cost nothing. Notion’s free plan has only a trial of its AI. With a cloud model, you still pay the model provider.

Is ChatGPT’s memory a second brain? It is memory for one assistant. A second brain is something you organize and read yourself, and that each of your tools can use. Agent memory compares the kinds.

Does AI make the second brain idea obsolete? Not by Forte’s own logic. If brains are for having ideas, AI takes over more of the storing, filing and summarizing. Express, the work you make from it, stays yours.

Sources

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