How to create a personal AI writing style
By Jack Stovell · published 2026-09-20
Most guides tell you to write a persona: "You are a witty, plain-spoken product marketer who values clarity." The assistant reads it and produces the same witty, plain-spoken draft it produces for everyone who wrote that sentence. A personal style is not a persona. It is a set of measurements only your writing produces, stated so precisely that a model cannot drift from them without noticing.
Here is how to build one, from the one-page version to the full profile.
Build the one-page version now. Paste 120 words or more of your own writing below and the prompt builder measures it and writes the system prompt in numbers: sentence length and variation, contraction rate, punctuation habits, formality, humour, how you open and how you argue.
Step 1. Choose the writing that is you
Three or more pieces, ideally 3,000 words between them, written without help: a newsletter, a long email, a post you were proud of. Not the pieces an editor smoothed. The measurement is only as personal as the sample.
Step 2. Measure, do not describe
Sentence length and its variation. Contractions per 1,000 words. Commas per sentence. Asides in brackets. Semicolons, exclamation marks, ellipses. The pronoun mix. Then the judged attributes: formality on a 0 to 10 scale, humour register, confidence against hedging, opening style, argument structure, rhythm. A full profile measures 45 of these; the prompt builder does the ones that matter most for a prompt.
Step 3. State the rules the model breaks by default
No em dashes. No generic transitions. No "not X but Y". A short sentence in every paragraph. The measured numbers are the ceiling for elevation, not the floor. These go into every prompt because every model reaches for the same tells when nothing forbids them.
Step 4. Put it where the assistant reads it
ChatGPT: Settings, then Custom instructions. Claude: a Project's instructions or a Style. Gemini: Saved info. Any assistant: the system prompt. The connector version skips the pasting: ScriptGrain plugs into ChatGPT and Claude directly and hands them the full profile, then scores what comes back.
Step 5. Score, edit, repeat
A style you never check is a style you slowly lose. Score each draft against the measurements, fix the two features that moved, and keep your edits: a profile that learns from them gets closer every month.
Questions
How much of my writing do I need?
120 words is enough for the one-page prompt; the numbers steady from about 300. A full profile works best on three or more pieces totalling 3,000 words, because it also keeps the samples for the model to anchor on.
Is a persona paragraph useless?
Not useless: it sets the intent. It just cannot carry the habits that make a voice yours, because you cannot see most of them. Put the persona in one line and the measurements underneath it.
Does the prompt builder store my writing?
No. The text is measured once (one call for the judged attributes), the numbers come back, and the text is discarded. The fingerprint that comes with it keeps scores only.