Second Brain

A second brain is a trusted place outside your head for what you know, what happened, and what you’ve committed to. It gives these back when you ask, and before you need to ask, so your mind is free to think and to live.

This is a framework for keeping one with an AI agent. The agent files, links, reminds and reviews, so the upkeep that killed earlier second brains is no longer your job. Everything lives in plain markdown files in a folder you own, and you can read and fix every one of them by hand.

Where to start

To see what it’s like day to day, read Using it: what you say to your agent, and how a week with it goes.

System design is the framework. It says what the system is and why: the kinds of things it holds, the routines, the rules and the boundaries. It does not depend on any tool.
Implementation is one way to run it: the author's own setup, with the tools, scripts and file format. Use it as a template, or write your own.
Research is what the choices rest on.
Landscape survey · Agent memory · Obsidian plugins

Routines (CORD)

The agentYouBoth

Capture
4.1

You

You never say where something goes. One sentence is enough.

talked to Sam, the website deadline moved to 1 December
Organize
4.2

The agent

Organizing is the agent's job.
- Conversations/2026/2026-10-12 Sam about the website.mdnew note
- Projects/Website for Sam/Website for Sam.mdNow and Log
Review
4.5

The agent starts, you decide

The system starts the review; you make the decisions.

Weekly review

Projects: active ones still active and with a clear next step?

Website for Samactive
Do
4.8

You

Done: tick it off or tell the agent.
- Done: Choose hosting for Sam's site

Capture, Organize, Review, Do, and round again, with one conversation followed through. Restated from section 4 of the system design; the example is the conversation in section 4.3, filed into the vault of section 5.1.

Kinds of things in a life

Area: Usually never ends.; health, home, money, car, dance
Project: Has an end and a goal.; plan the vacation, a website for Sam
Task: One action.; call the plumber
List item: Grouped by where you buy it, not by project.; milk, light bulbs
Repeating, fixed date: Tied to the calendar.; rent on the 1st; bins every Tuesday
Repeating, from last time: The timer restarts when you do it.; dentist 6 months after the last visit
Waiting for: Someone else has to act.; a parcel, a reply from the landlord
Someday: Set aside.; places to visit
Appointment: A moment you have to be somewhere, one-off or repeating.; dentist 14 Nov 10:00; dance every Tuesday 20:15
Date to remember: Comes back ahead of time, so there's time for a gift.; birthdays
Conversation: What was said and what came of it.; doctor visit, a talk with a friend
Person: Who someone is and how you know them.; Sam, your neighbour, your dentist
Reference: No action.; insurance policy, recipes
Commitment: How your week normally runs, not a calendar entry.; work 08:30–17:00 on weekdays (a baseline); fitness every Monday (an intention)
Habit (Habits, parked): Measured over time.; read 20 min a day

Each kind drawn in pencil, with time running left to right, then the first line of what makes it different and an example. Restated from section 3.2 of the system design.

The vault

The vault is one folder, called second-brain by default.

second-brain/

Inbox/waiting to be organized
Projects/
- Website for Sam/
  - Website for Sam.mdproject note; most tasks live here as lines
  - Choose hosting for Sam's site.mda task with its own note
Areas/
- Health/
  - Health.md
  - Dentist checkup.mdrepeating task
  - 2026-11-14 Dentist.mdappointment
- Dance/
  - Dance.md
  - Dance class.mdrepeating appointment
- Work/
  - Work.md
  - Work hours.mdcommitment
- Home/
  - Home.md
  - Shopping list.md
  - Fix the squeaky garden gate.md
People/
- People.mdeveryone you know, one line each
- Robin.mdsomeone with their own note
- <your name>.mdyour own facts (sizes, allergies, licence plate)
Conversations/
- 2026/
  - 2026-10-12 Sam about the website.md
Resources/
- Recipes/
  - Lasagne.md
Weeks/
- 2026/
  - 2026-W42.mdweek plan
Logs/
- time/
  - 2026-10 time.mdCapacity add-on
Archive/mirrors the vault (section 4.9)
- Projects/
  - Vacation Italy/
- Areas/
  - Health/
    - 2026-03-02 Dentist.mdpast appointment
- Inbox/
  - 2026/processed inbox items
_system/
- config.md
- rules.md
- types/one definition per kind
- addons/capacity/the Capacity add-on's own types

The vault is the truth. Plain markdown. Principle 2
The folder is the parent. A task, list, appointment or commitment lives in its project or area folder; moving it means moving the file. 5.2
People and conversations get their own folders. Both belong to several topics at once. 5.2
Links are by name: [[Website for Sam]]. 5.2
The archive mirrors the vault: Archive/Projects/<Name>/, Archive/Areas/<Area>/…, so an item always has the same spot, however often it's revived and archived again. 5.2

The layout of section 5.1, with conventions from section 5.2 beside it. Marked: the note the agent wrote and the project note it updated in the example above.

Ready to use

Ninja Board is an Obsidian plugin built for the implementation. It shows your vault as Today, Week, Projects, Backlog & Waiting and Guide views, where you mark tasks done, drag cards and plan your week. Use it as it is, or change it to fit your own setup.

Second Brain Setup sets up a working vault from one sentence to your AI agent: Install this second brain for me: https://github.com/AlexSendula/second-brain-setup. It is an early version.

For agents: to install this system, follow https://github.com/AlexSendula/second-brain-setup/blob/main/SETUP.md.

Research

Landscape survey: what already exists, from markdown apps and task managers to agent memory layers.
Agent memory: eleven memory systems compared against the rules of the design, and what was decided.
Obsidian plugins: which plugins are trusted and fit a vault like this one.

Good to know

The system is built for one person’s private life first. The same system should work for a work vault, kept separately.

The files are released under the MIT licence.