Drift is the default, and it's slow
Ask a model for a post today and one next week and you get two posts. Both fine. Slightly different sentence lengths, a different idea of how formal you are, a heading style that wasn't there before.
No single output is wrong, which is exactly why this is hard to catch. Nobody reviewing Thursday's caption thinks "this is off-brand." The drift only shows up when you scroll your own feed a month later and it reads like three different businesses took turns.
Prompts don't prevent this. A prompt is a description of what you want, written fresh each time, and each rewrite is a small vote for a slightly different brand. Consistency has to live somewhere that isn't retyped.
Write the rules down, and be specific enough to be wrong
Most brand guidelines fail because they are unfalsifiable. "Friendly but professional" cannot be checked. Two people will read it differently and both will be defensible, which means it settles no arguments.
Rules worth having are the ones a stranger could apply and you could mark as broken. Sentence case for headings, never title case. Contractions, always. Prices as figures, never as "affordable." Second person, present tense. Say "book," not "schedule your appointment today."
The test is simple: could someone who has never met you follow it, and could you point at a line and say that one breaks the rule? If not, it isn't a rule yet, it's a mood.
The banned list does more work than the style guide
Telling a model what to write shapes the output a little. Telling it what never to write shapes it a lot, because the tells are finite and repetitive.
Every business has words that would never come out of its founder's mouth. Leverage. Elevate. Unlock. Seamless. Delve. "It's not just a haircut, it's an experience." A list of thirty words and two sentence constructions you refuse to publish will do more for consistency than three pages describing your values.
Ours is written down for the same reason yours should be. It gets updated whenever something ships that made us wince.
Our team works from a brand system
Our team keeps the same rules for copy, layout, voice, colours and asset formats from one project to the next. New work starts with those rules already loaded, rather than being described again in a prompt.
That covers the visual side too, and the visual side is where drift is most obvious to a stranger. One typeface pairing. One set of colours with fixed roles — this one is the accent, and it is the accent everywhere. Fixed crops per platform, so the carousel and the reel and the landing page hero are recognisably the same hand.
Our people edit every draft against those rules before it reaches you. The model is fast and it is consistent-ish; a person is what makes it consistent.
What stays consistent, and how you'd check
Pages, carousels, video captions and outreach email use the same voice and the same visual rules. A prospect who sees your Instagram, then your landing page, then a cold email should not have to work out whether it's the same business.
There's a cheap audit for this. Put twelve recent assets on one screen — posts, an ad, an email, a page — and look at them together rather than one at a time. Drift is invisible in sequence and obvious in a grid. If three of the twelve feel like guests, you've found your gap.
Do it once a quarter. It takes ten minutes and it catches things no individual approval ever will.
Consistency isn't the same as never changing
Worth saying plainly, because "on brand" gets used to shut down anything new.
A brand system should be revised on purpose and rarely. What you want to eliminate is accidental variation — the kind that happens because nobody wrote the rule down. Deliberate change is a different act: you decide, you update the rules, and everything made after that follows the new version.
The difference between the two is whether you could name the day it changed. If you can, that's a decision. If you can't, that's drift.
Work with Sundayable
Sundayable's done-for-you content work is Organic Growth at $1,000 CAD a month. Ads, sales email and automation are separate lines, sold on their own. Book a free consultation and we'll show you work made against your rules before you commit to anything.