Show HN: Grammit – Local-only AI grammar checker (Chrome extension)

Hey HN, I wanted a grammar checker that didn’t send my writing to someone's servers, so we built Grammit, a Chrome extension that runs grammar checks locally using an LLM. Your text never leaves your computer during checking.

Here’s a 2-minute overview: https://www.loom.com/share/baf501ee6cf14a919a7384128246ed67

Because it uses an LLM, it catches more than spelling and grammar. For example, it can correct some wrong statements like “The first US president was Benjamin Franklin.”

Grammit also includes an in-page writing assistant that can rephrase or draft new text. It also uses the local LLM.

We used many new web features to build this, such as:

- Chrome’s new Prompt API to talk to the local model.

- Anchor Positioning API to place the UI with minimal impact on the DOM.

- CSS Custom Highlights API for inline error marking.

- The new CSS sign() function to create CSS-driven layout with discontinuities.

Part of the fun of being early adopters of bleeding edge tech is we’re discovering new Chrome bugs (e.g., https://issues.chromium.org/issues/428354426, https://issues.chromium.org/issues/428039224).

I’d love your feedback on:

- Where the UX feels rough

- What do you think of the corrections and suggestions

Happy to answer questions about the tech or the Prompt API. Thanks for trying it out!

Chrome Web Store extension link: https://chromewebstore.google.com/detail/grammit-the-ai-gram...