A market definition that lives in one head drifts between Monday and Thursday. Written as one sentence with an exclusion, it becomes a gate later work reads.
Written by instinct, or not written at all. You sell to whoever answers, so the list is everyone and the message is nobody. For a forty-person firm the sentence lives in one head, and it changes between Monday and Thursday.
By hand today
“If you narrow your market to a more uniform group, like ‘large hospitals’, or ‘small law firms’ - approaches that apply to one group will not work for another”
DenisM, on Hacker News
That is the stage, described by a founder in one line. Narrowing is not a branding exercise. It is the decision that makes every later piece of work either possible or wasted, because an approach built for one group does not carry to another.
Above a certain size the sentence becomes somebody’s job. Job adverts for product marketing roles ask the hire to own positioning end to end. Below that size the founder owns it, usually in their head, where it drifts. Ask three people in a small company who the customer is and you get three answers, all of them reasonable, none of them the same.
The drift stays invisible while the sentence stays unwritten. It shows up later, in a week of work that went to the wrong reader.
What it costs you each week
Six of ten articles were pulled in one audit, because they had been written for a feed and not for a buyer. The sentence was missing, so the work shipped and then came back.
Source: ArkOne content audit, 9 August 2026
No honest hour count exists for the deciding itself, so none is given here. The cost lands downstream, in work already paid for.
ArkOne read its own published articles in an audit dated 9 August 2026. Ten of them had been written from what the feed was discussing rather than from who the company sells to. Six of the ten were archived. The research, the drafting, the images and the review had all been spent before anyone asked whether the reader was the buyer.
A small company pays the same cost in a smaller currency: a landing page rewritten twice, a month of posts nobody replies to, an advert aimed at a title that never signs anything.
The build
Open Claude Code. It reads your won deals, your lost deals and your last ten posts, then drafts the sentence. The sentence names one buyer and one kind of company you will not sell to. After that it becomes a gate.
The folder needs to hold what you already have: an export of won deals, an export of lost ones, and the last ten things you published. Nothing needs to be tidy. Then say what you want.
Read won-deals.csv, lost-deals.csv and the last ten posts in posts/.
Draft one sentence naming the buyer we sell to: the title that signs,
the size of company they run, and one thing that must be true of that
company. Then name one kind of company we will not sell to, and why.
List the three deals that support the sentence and the two that argue
against it. Write the sentence, the exclusion and that evidence to
buyer.md.
What comes back is a short file: one sentence, one exclusion, and two lists of real deals. Read the list that argues against the sentence first. That is where the sentence is either wrong or usefully narrower than you expected.
The file is the point. A sentence in a document is a note; a sentence in a file that later work reads is a gate. ArkOne keeps its own version in docs/content-pillars.md, inside the same repository as its website. That file names one buyer, the titles that count as that buyer, and the claims that may never be made. Anything written for publication is checked against it before the writing starts. Six of ten archived articles is what the file was built out of.
You build your own. The path is the pattern to copy, not a product to install: one file, kept where the work happens, holding the sentence, the exclusion and the date it was ruled. Ask Claude Code to check the next draft against buyer.md and to say plainly which line it fails. A draft that fits nothing in the file does not get written.
What it feeds
The ICP a machine reads. The next chapter turns the sentence into a stored object: titles, a size band, and one gate that rules a company out. That is only possible because the sentence is already written down, in one place, with an exclusion beside it.
The number that proves this stage worked is the count of drafts, adverts or lists the gate refused, set against everything produced. A gate that has never refused anything is not a gate. On ArkOne’s own audit the ratio was six refused out of ten.
What stays with a person
The company you refuse. A tool can rank and sort. It cannot decide which paying customer you will stop chasing.
The draft comes back with a buyer and an exclusion. The exclusion is the part that costs something, because it usually names work you are currently taking. No tool can make that call, since it depends on what you can afford to turn away this quarter.
A person rules once. The ruling goes into the same file the next run reads, with the date beside it. When the answer changes, it changes in one place, and everything downstream changes with it.
The tools you already pay for
Nothing new to buy.
CRM holds the buyer as one row, not as a paragraph in a slide deck.
Your writing is the evidence: what you published, and what people replied to.
Notes are wherever your team already writes things down. The file matters more than the tool holding it.
The sentence is written, and the company you will not sell to is named beside it. Then the ICP can be built.
Ask: Could you say in one sentence who you sell to, and who you will not?
This is chapter 1 of twelve; the contents list every stage, and chapter 2 turns the sentence into something a machine reads. The book, Building a real GTM engine with Claude Code and Codex, end to end, runs the whole engine in order, one chapter per stage. A free starter edition for firms under fifty people is coming through the 100k programme.