Ship Something 10 min

Build · A writing assistant

A saved Project / GPT / Gem you'll use forever. ~10 min, no install.

You'll build a saved custom assistant in any major chat app — Claude.ai (Projects), ChatGPT (Custom GPTs / Projects), or Gemini (Gems). When you finish, you'll have a bookmark you reach for whenever you need to draft writing in your voice.

0 of 8 steps

Open a chat app and sign in

Pick one. All have free tiers; you can sign up in a minute.

You should see

You're signed in and see a sidebar with past chats (or an empty one if it's new).

Create a new custom assistant

The button is in a different place in each app:

Claude.ai: click Projects in the left sidebar → New project.
ChatGPT: sidebar → Explore GPTs → Create. (Or use Projects if you have it.)
Gemini: top-left menu → Gems → + New Gem.

You should see

You see a configuration screen with fields for a name, a description, and instructions.

Name and describe it

Suggested values you can paste straight in:

Name: My writing partner
Description: Turns rough notes into polished prose in my voice.

Paste the system prompt

This is the brain of the assistant — it gets prepended to every chat. Copy and paste it into the Custom instructions / System prompt / Instructions field:

System prompt

You are my writing partner. When I paste rough notes, produce three things:

1. A clean version in my voice — direct, no jargon, no corporate puffery.
2. A one-sentence subject line / headline.
3. Two alternatives — one shorter, one more formal — so I can pick.

Always ask one clarifying question if context is missing (audience, occasion, length, tone).
Keep names, numbers, and dates exact.

You should see

The instructions field shows the prompt. No errors.

Upload 1–3 writing samplesoptional

Drop in examples of your best writing — a blog post, an old email you're proud of, a memo. The assistant will match your voice much more closely.

Claude.ai: the Knowledge section on the project page.
ChatGPT: Knowledge section in the GPT editor.
Gemini: Knowledge section in Gem setup.

Save and start a chat

Hit Save / Update / Create. Then start a new chat inside the assistant (not a regular chat).

You should see

You see an empty conversation with your assistant's name at the top.

Test it with real notes

Paste rough notes for something you actually need to write. Or use this test pack:

Test notes

subject: tomorrow's design review prep
- 3 PMs joining late from Berlin office
- want them caught up on Q4 roadmap before we get to detail
- emphasize the migration risk — Karen needs to weigh in
- ask if Maya can dial in too
- send by 4pm so they have evening to review

You should see

You should get back three things: a polished version, a one-sentence subject/headline, and two alternatives (shorter + more formal). If anything was missing from your notes, it should ask one clarifying question.

Iterate and ship

If the output isn't quite right, tell it. Real examples that work:

"More direct. Half as long. Drop the bullet points."
"This sounds too American — soften the tone, make it warmer."
"Always use my name as the signoff, not 'Best regards'."

When the output matches your voice, go back to the assistant's instructions and incorporate the rules you discovered. Bookmark the assistant's URL.

You should see

A bookmarked, reusable writing assistant tuned to you. Use it forever.

What you just learned. A "custom assistant" is just a saved system prompt plus optional knowledge files. You can build one for any recurring task — email triage, meeting summaries, code review style guides, recipe ideas, gym-plan coaching. Same five fields, different prompt.