A custom assistant is a saved AI setup that already knows your job, your files, and your rules, so you stop re-explaining yourself every chat. Every major AI app has one, and you can build it without any code.
The three ingredients
Every custom assistant, on any platform, is built from the same three parts.
- Instructions. A standing note the AI reads before every chat: who it is, who it helps, what to do, and what never to do. This is the most important part.
- Knowledge. Files you upload once so it can refer to them every time: a price list, a style guide, a manual, your notes.
- Tools (optional). Abilities like web search, reading your Google Drive, or making images. Some platforms let advanced users connect other apps.
If you remember one thing: a custom assistant is only as good as its instructions. A great file library with vague instructions still gives vague answers.
Where to build one
Use whichever AI you already use. All three are good, and the names differ more than the ideas.
| ChatGPT: Custom GPTs | Claude: Projects | Gemini: Gems | |
|---|---|---|---|
| What it is | A standalone assistant you can share | A workspace with files, instructions, and its own chat history | A saved "expert" with instructions and files |
| Free to build? | No, needs a paid plan (anyone can use a shared one) | Yes, up to 5 projects | Yes |
| Remembers past chats? | No, only its instructions and files | Keeps its own chat history | No, only its instructions and files |
| Best at | Sharing with anyone by link | Lots of long documents | Staying in sync with Google Drive files |
ChatGPT also has "Projects," which work like Claude's: folders for your own ongoing work. Use a Project for yourself; build a GPT when you want to hand it to others.
One catch: you cannot move an assistant from one platform to another. Keep a copy of your instructions in a plain document so you can rebuild it anywhere.
Plan details as of July 2026; they change often. Source: LumiChats comparison (https://lumichats.com/blog/claude-projects-vs-chatgpt-projects-vs-gemini-gems-2026 (opens in a new tab))
Writing good instructions
Good instructions read like a briefing for a smart new helper on their first day. Copy this template and fill in each line.
# Who you are
You are [role], helping [who] with [what].
# What you know
Use the attached files: [file names and what each one is].
If the answer is not in the files, say so. Do not guess.
# How to respond
- Tone: [friendly / formal / short and direct]
- Length: [e.g., under 150 words unless asked for more]
- Format: [bullets / a table / plain paragraphs]
# Always
- [e.g., ask one clarifying question if the request is unclear]
# Never
- [e.g., give medical or legal advice; make up prices]
Test it with five real questions, including one it should refuse or say "I don't know" to. Fix the instructions after each test. Most good assistants take three or four rounds.
Common mistakes
| Mistake | Fix |
|---|---|
| Trying to do everything in one assistant | Make one assistant per job. A "recipe helper" and a "budget helper" beat one "life helper." |
| Uploading old or messy files | Upload only current, clean files. Outdated files give outdated answers. |
| No "I don't know" rule | Tell it to say so when the answer is not in its files. Otherwise it fills gaps with guesses. |
| Sharing private files by accident | Anyone you share a custom assistant with may be able to get at its files. Never put private data in a shared one. |
| Never updating it | Review the instructions every month or two. Small fixes add up. |
Part 5 walks through a full build, start to finish.
Found this helpful? Raya.ai is built on the ideas in this series. Learn more at raya.ai (opens in a new tab), or download the app on iOS (opens in a new tab) or Android (opens in a new tab).