Write Marketing Copy That Sounds Like Your Business

You ask an assistant for a promotion email. What comes back is grammatical, on topic, and could belong to any business in your category. Then you spend twenty minutes rewriting it into something a customer would recognise as you, which is roughly the time it would have taken to write from scratch.

The fix is not a longer prompt each time. It is a container that holds your voice permanently, so every draft starts from your material instead of the model's defaults. All three major assistants have one. They are named differently, they hold different amounts, and only two of the three are properly documented.

Three containers for a house voice, and how much each one holds

ChatGPT calls it Custom Instructions. OpenAI's help article gives two different paths depending on device: "In your Settings, select Personalization" on web and desktop, and "In your Settings, select Customize ChatGPT" on iOS and Android. The size cap is the number to know before you start writing, because it decides how much of your voice actually fits. The article states that free and Go users get up to 1,500 characters, while Plus, Pro, Enterprise, Business and Education users get up to 5,000.

Fifteen hundred characters is one solid paragraph. Five thousand is about a page. Plan the text you paste around whichever ceiling your plan gives you, because the assistant will not warn you that your best guidance got truncated.

Gemini calls it a Gem. Google's help page walks the path: go to gemini.google.com, then "On the left, click Explore Gems" and select New Gem, then "Name your Gem and write instructions for it to follow." There is a Knowledge section with an Add files button, so a Gem carries documents as well as instructions. One line on that page is worth reading twice before you close the tab: "Using the preview window does not automatically save your Gem." Preview all you like, then click Save.

Claude splits the job across two features. Projects are the container. Anthropic's help centre describes them plainly: "Projects allow you to create self-contained workspaces with their own chat histories and knowledge bases." Free accounts can create a maximum of five projects, and the same page notes that the enhanced project knowledge which retrieves from your uploaded files is limited to paid plans (Pro, Max, Team or Enterprise). For a business voice that means the free tier will read your uploads, but the paid tiers handle a bigger pile of them better.

Claude's second feature is Styles, and this is where the documentation gives out. Anthropic's personalization article says only that you should "Use styles when you want to customize how Claude formats and delivers its responses" and, separately, "Use profile instructions for account-wide settings that affect all your interactions with Claude." It does not publish the click path, the number of writing samples a custom style takes, or any size limit. The old announcement URL at anthropic.com/news/styles now redirects to claude.com/blog/styles, which returns a 404. So the feature is real and referenced in the help centre, and the steps for building one are not documented anywhere Anthropic currently publishes. Find the control in the chat composer and expect to work it out by clicking.

Give it your own writing rather than a list of adjectives

The common mistake is filling that container with adjectives. "Friendly but professional, warm, approachable, expert." Every business in the world describes itself that way, and a model trained on the whole web already knows what generic friendly-professional copy reads like. Adjectives get you closer to the average, which is the opposite of the goal.

Put artefacts in instead. Four things, in this order.

1. Three to five pieces of your actual copy. The email a customer replied to. The service page you wrote yourself before you got busy. A voicemail script. Real sentences carry your sentence length, your comfort with jargon, and how much you hedge. No adjective list encodes that.
2. A banned-words list, written from your own drafts. Read your last ten pieces of published copy and note the words you never use. If you have never once written "solutions" or "best-in-class", say so explicitly. A negative list is easier for a model to follow than a positive one because it needs no judgement.
3. The facts that must never change. Trading hours, service area, what you do not do, the exact spelling and capitalisation of your business name, the guarantee wording. In Gemini this belongs in the Knowledge section rather than the instructions box, because it is reference material. In Claude it belongs in the project knowledge base.
4. One worked before and after. Paste a piece of generic copy, then paste your rewrite of it, and label them. That single pair does more than a page of description, because it shows the transformation rather than naming it.

On the 1,500-character plans, items 1 and 2 will not both fit in Custom Instructions. Put the banned-words list and one short sample in the instructions, and keep the longer samples in a document you paste at the start of a chat.

Where AI marketing copy turns into an FTC problem

An assistant asked to write a landing page will happily produce a customer testimonial, a satisfaction percentage and a five-star quote, because those are what landing pages contain. None of it is real, and in the United States publishing it is now specifically illegal rather than merely bad practice.

The Federal Trade Commission's final rule on consumer reviews and testimonials, announced in a press release dated August 2024, bans several practices by name. The first category on the FTC's own list covers "reviews and testimonials that misrepresent that they are by someone who does not exist, such as AI-generated fake reviews". The rule also covers reviews by company insiders without a clear and conspicuous disclosure of the connection, buying or selling "fake indicators of social media influence, such as followers or views generated by a bot", and using threats or intimidation "to prevent or remove a negative consumer review". The FTC states the rule became effective 60 days after publication in the Federal Register, and that it allows the agency to seek civil penalties against knowing violators.

The practical rule for a small business is short. Every number, quote and named customer in AI-drafted copy has to be one you can point to in your own records before it ships. If you cannot produce the invoice, the email or the signed permission, cut the sentence.

The same discipline applies to claims that are not testimonials. "Fastest in the county" and "trusted by hundreds of local families" are the kind of phrases a model produces to fill a gap in a template. If the assistant wrote it and you cannot substantiate it, it is your problem once it is on your website, not the assistant's.

Jobs to keep out of the assistant entirely

Voice storage works for the repeated, low-stakes end of marketing. Service descriptions, promotion emails, social posts about something that genuinely happened, replies to reviews, rewrites of copy you already approved. That is most of the volume in a small business, and it is the part that eats evenings.

Three categories are worth keeping in your own hands. Anything that states a price or a term of business, because a hallucinated number in a promotion is a commitment you may have to honour. Anything about a specific named customer, which is a permission question before it is a copy question. And anything you would describe as your best piece of writing, because a container full of your average work will pull a draft toward your average, and averaging is exactly what these tools do well.

One maintenance note that is easy to skip. The container ages. If you change your pricing, drop a service or move premises and the Gem's Knowledge section still holds last year's document, the assistant will keep producing confident copy about a business you no longer run. Put a recurring reminder against whichever container you chose and open it every quarter, which is the same rhythm the rest of this series uses for the stack as a whole.