Write the company brain

You'll finish with: A finished CLAUDE.md, under 200 lines, specific to you.

This is the hour that makes everything else work. In Claude Code it's a file called CLAUDE.md that loads automatically every session. In a chat project it's the custom instructions box. In Cowork it's the instructions field.

Shorter is better

Counterintuitive, but Anthropic's own docs are clear: longer files eat context and Claude follows them less consistently. Aim under 200 lines. Write only what you'd otherwise re-explain.

Be specific enough to check

If two people could disagree about whether Claude followed the instruction, it's too vague.

Name the pain properly

The clients section is where most files go soft:

The second produces completely different writing.

Same question, with and without the file

No context
"Write a follow-up to a customer"
Generic. Could be any business on earth. You rewrite the lot.
With your file
"Write a follow-up to a customer"
Names the job, your terms, your voice. You tweak a line and send it.

Where it goes

Depends which one you opened in the last chapter. It's the same box in each — just a different name.

Chat projectYour project→Set instructions
Cowork projectYour project→Settings→Instructions
Claude CodeYour business folder→a file named CLAUDE.md

In Claude Code the folder ends up looking like this:

smith-plumbing/
├── CLAUDE.md
├── clients/
└── business/
Plain English

A file ending in .md is a plain text file. Nothing special opens it — Notepad on Windows, TextEdit on a Mac, or the editor built into Claude. The .md just tells programs the text uses a few simple marks for headings and lists.

Two traps when you make one. Windows Notepad quietly adds .txt unless you set Save as type to All Files first. Mac TextEdit needs Format → Make Plain Text. If your file ends up called CLAUDE.md.txt, that's what happened — rename it and it will work.

Facing a blank page? Don't. The setup builder asks you these same questions one at a time and writes the file for you — then you edit a draft instead of starting from nothing. That is a much easier job, and it takes about twenty minutes. Come back here when you have it.

Do this now
  1. Open the instructions box for your surface using the path above. In Claude Code, run /init and it drafts one from your folder — otherwise create the file yourself.
  2. Paste the skeleton below and fill in every bracket.
  3. Delete any section you can't fill in honestly. Empty sections are worse than missing ones.
  4. Count the lines. Over 200, cut.
How to know it worked

Open a brand new chat in that project and ask: "What do you know about my business?" It should tell you back, specifically, without you pasting anything.

If it answers vaguely or asks who you are, the file didn't load. In Claude Code, run /context and check CLAUDE.md is listed under Memory files.

# [Company] — Operating Context

## Who we are
[What the business does, who it serves, how it makes money.]

## Who I am
[Name, role, what you do daily, what you don't do.]

## Our clients
[Type 1] — the pain: [concrete, not abstract]
[Type 2] — the pain: [concrete, not abstract]

## What we sell
[Offer] — [what it's for, roughly what it costs]
We do NOT sell: [list it]

## How we talk
- [Voice rule]
- Banned words: [yours]
- Sounds like us: "[a real sentence]"

## Systems we run on
Invoicing: [tool] · CRM: [tool] · Email: [tool]

## Standing rules
- Never send anything to a client without showing me first
- Never quote a price I haven't confirmed
- Flag uncertainty instead of guessing

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