Create your own AI assistant: three routes

The short version

• Three features, one principle: a project in ChatGPT, a project in Claude, a skill in Gemini. Each stores instructions and files so you don't have to enter them again every time.
• Custom GPTs and Gems are no longer an option: according to OpenAI, personal accounts can't create new GPTs, and retirement has been announced. Google is moving Gems on personal accounts to skills from November 17, 2026.
• The work is in the briefing, not the setup. Setup takes five minutes, the briefing an afternoon.
• All three can be done for free, with limits on files and the number of projects.

The briefing: write it once, use it everywhere

A briefing is the fixed instruction your assistant knows in every chat: who you are, what it does for you, what the result looks like and what it must never do. It has five parts. Three of them, role, task and format, are also named by Google in its tips for Gems, the predecessor of skills; the fifth matters more to me than all the others:

• Role: who the assistant writes for and in what context. Tone and vocabulary depend on it.
• Task: what it does and what it doesn't. An assistant that writes drafts for you to review is a different thing from one that decides.
• Style: how your texts sound. It comes from your real emails, not from adjectives, otherwise every assistant sounds the same.
• Format: length and structure of the result. Otherwise you rework every draft.
• Taboos: what it must never do. Without them it fills gaps with inventions. One example: “If information on price, date or deadline is missing, write [CLARIFY] in its place.”

The style comes from your emails

You derive the style from five to ten real emails, with a fixed analysis instruction that separates recurring habits from one-off details and carries over no names, dates or commitments. In the test from my video on emails in your own style, three sample emails from three different situations were already enough. The result:

Still, check a result like this: just because the AI describes a pattern doesn't mean it applies to every email.

Vague or specific

Whether a briefing holds up almost always comes down to precision:

Then add two or three templates as files: a proposal you were happy to send, a typical reply to an enquiry, a polite no. The assistant needs no more than that at the start.

Three routes, the same assistant

What the three features can do according to the providers' help pages, as of October 25, 2026:

Sources: OpenAI on projects, Anthropic on projects, Google on skills, Google on notebooks. Anthropic is rolling out a new version of projects in stages, starting with Claude Code; the table describes the current version in chat. Gemini also has notebooks: a folder with sources, custom instructions and the chats on one topic, closest to projects in ChatGPT and Claude. In the EEA you connect Gemini Notebook to Gemini once yourself. For the briefing, a skill is enough; for many documents on one topic, a notebook fits better. For your work this means: the briefing is the same everywhere, only the place you enter it differs. If you switch providers, you simply take briefing and templates with you.

Why not a custom GPT or a Gem any more

Many guides still recommend a custom GPT for your own assistant. That is out of date. According to OpenAI, personal ChatGPT accounts, that is Free, Go, Plus and Pro, can no longer create or publish new GPTs; existing ones remain usable until retirement. OpenAI has announced it will retire custom GPTs, for Enterprise workspaces on December 11, 2026, with other plans expected to follow. As the replacement OpenAI names plugins and a planned migration flow.

Gemini is moving the same way. According to Google, Gems on personal accounts become read-only on November 17, 2026 and are removed on December 17, 2026. Google converts them to skills automatically; some file types don't carry over, such as audio, video and legacy Office formats. Workspace accounts follow in 2027.

For you as an individual that means: a project in ChatGPT and Claude, a skill in Gemini. Both take the same instructions and files, both work without a subscription, and there is nothing to migrate later.

How to set it up

1. 1

   Collect examples

   Five to ten emails or proposals you were happy to send. They are the basis for the style.

2. 2

   Have the style derived

   Paste the examples into a chat with a fixed analysis instruction. Check the result and cut whatever doesn't fit.

3. 3

   Store briefing and templates

   The briefing as project or skill instructions, two or three templates as files. Five minutes.

4. 4

   Test with three real tasks

   An enquiry, a proposal, a polite no. Whatever misses becomes a rule in the briefing.

What the assistant can't do

• It only knows what is in there. Anything missing from the briefing or files, it may invent. Hence the [CLARIFY] taboo.
• It checks nothing. You check names, numbers and dates yourself, every time.
• It connects no programs. If the reply should be created automatically from your contact form, you need a flow, for example with n8n.
• It sounds only as good as the briefing. How to spot AI-written text and avoid it is in the guide AI writing in your own voice.

Where it slips after two weeks

From client work I know four places where a set-up assistant starts to slip again:

• The style drifts. Corrections happen in the chat and never reach the briefing. The next email sounds like before.
• One assistant is meant to do five tasks. Proposals, refusals, posts and minutes in one briefing pull in different directions. One assistant per task works better.
• Nobody checks names and numbers. The draft sounds right, so it goes out. Checking belongs in the routine.
• In a team, everyone uses it differently. Without agreeing who changes the briefing and what the assistant is for, everyone soon has their own version.

In the AI Workflow Sprint we build this for two or three of your tasks, test it in your daily work and sharpen it.

Sources

Retrieved on October 25, 2026.

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