ToneShift – Rewrite Text with Chrome’s Built-in AI

Inspiration

While reading online articles, I realized how often we need to adjust tone from formal for reports to casual for messages yet rewriting manually wastes time. AI writing tools exist, but they require cloud access, risking privacy and increasing latency. ToneShift was born to solve this a fully client-side Chrome extension that lets users rewrite text instantly in different tones using Gemini Nano’s on-device AI.

What it does

ToneShift is a Chrome Extension powered by Chrome’s built-in Gemini Nano (Prompt API) that rewrites any selected text on a webpage into your preferred tone "Formal, Friendly, or Casual " with a single click. It runs entirely on-device, ensuring privacy, instant response, and offline functionality. It rewrite text in real-time into three tones (Formal, Friendly, Casual) Works directly on any webpage (Wikipedia, email drafts, blog editors, etc.) 100% on-device AI (no cloud calls, no data leaks) Uses Chrome’s built-in Gemini Nano (Prompt API) One-click “Copy to Clipboard” for easy reuse

How we built it

ToneShift was developed using JavaScript, HTML, and CSS under Chrome’s Manifest V3. It integrates the Prompt API with Gemini Nano to perform tone rewriting directly on the client — no network calls, no data leaving the browser. The extension uses Chrome’s context menus to detect selected text, applies AI transformation, and displays results through a responsive popup interface.

Challenges we ran into

Handling deprecated clipboard APIs and ensuring secure copy actions. Ensuring seamless DOM manipulation for rewriting selected text. Understanding how Gemini Nano’s on-device AI behaves compared to traditional cloud-based models. Designing a minimal yet intuitive user workflow suitable for non-technical users.

What we learned

How to integrate Chrome’s built-in AI APIs (Prompt API) for client-side inference. Creating a context-aware UI within Chrome extensions. Managing asynchronous AI operations without external servers. Optimizing user experience for offline, private AI interactions.

What's next for ToneShift – Rewrite Text with Chrome’s Built-in AI

ToneShift is evolving the way we edit and refine text directly in Chrome. With Chrome’s built-in AI, users can now rewrite text effortlessly, adapting tone, style, and clarity in real time. Seamless Integration: Works natively in Chrome no extensions needed These are next steps that would be taken.

Built With

• chrome
• css
• extensions
• html
• javascript
• promptapi