Chrome quietly shipped a small local model called Gemini Nano, accessible via the Prompt API. A developer just used it to solve a problem every prompt engineer has but rarely fixes: your good prompting habits don’t travel.
The build is called Prompt Polish. It’s a userscript that holds a reusable instruction library (tone rules, format constraints, the stuff you’d normally retype or skip entirely) and rewrites your rough prompt against it right in the input box before you hit send. Works on ChatGPT, Claude, and Gemini. One library, every model.
The twist is where the rewriting happens: on your machine. Chrome’s built-in model handles it, no API key, no external call, nothing leaves your device until you decide to send. It even works offline once the model downloads.
🛠️ How to set it up
- Install the free extension at customaise.com
- Open your instruction library and write down your rules: tone, output format, domain constraints
- Type a rough prompt in ChatGPT, Claude, or Gemini
- Hit Polish before you hit Send
- Review the rewrite, edit if needed, then send ✅
Pro tips
- The most valuable rules to add are the ones you apply mentally every time but forget to type. “Be direct.” “Use bullet points.” “Assume a non-technical reader.” Those habits are exactly what gets skipped on quick prompts, which is also when they matter most.
- The same local model powers two sibling scripts the developer built: Explain This (select any text, get a plain-language popover) and Quiz Me (select an article, get flashcards). Same pattern: 10-second tasks that aren’t worth a full tab round trip.
- A userscript is one file, no build step, no backend. If the defaults don’t match your workflow, fork and adjust in under an hour.
💡 The deeper unlock here is the pattern itself. Chrome now hands a local AI model to any userscript you write. Your specific prompt habits don’t need an app. They need a file.
🔗 Grab Prompt Polish at customaise.com and spend five minutes writing down the rules that already live in your head every time you write a prompt. That list is your library.
I built an on-device prompt polisher so my saved instructions apply before I hit send, on ChatGPT, Claude, and Gemini
by u/schequm in PromptEngineering