Add optional real-time grammar feedback powered by Harper

[TyceHerrman]

Problem

Cai currently checks writing only after the user selects content and presses ⌥C. Fix Grammar is an LLM action, so Cai doesn't flag issues while the user is typing.

Proposal

Add an opt-in Live Writing Feedback mode powered by Harper:

• Monitor the focused editable field through macOS Accessibility.
• Lint locally with Harper after a short debounce.
• Underline spelling, punctuation, grammar, and style issues as they appear.
• Show Harper's explanation and suggestions when an underline is selected.
• Allow applying or ignoring a suggestion, disabling its rule, or adding a word to the dictionary.
• Keep Cai's existing ⌥C actions for optional AI rewriting.

Live checking shouldn't call an LLM, silently change text, or inspect background applications.

Privacy and compatibility

• Off by default, with per-app controls and an easy pause switch.
• Monitor only the focused nonsecure text field.
• Keep all linting on-device.
• Start with standard Accessibility text controls.
• If an application doesn't expose reliable text-range bounds, show a feedback list or disable live checking rather than using continuous OCR.

A persistent harper-core or harper-ls engine could supply structured lint spans and suggestions for an Accessibility-positioned overlay.

This would give Cai a useful hybrid workflow: continuous deterministic feedback from Harper plus user-invoked AI rewriting when wanted.

[soyasis]

Thanks @TyceHerrman! Interesting idea, never thought about this angle, but to be very honest it does feel more like a new product at this stage than a Cai feature :)

One thing that could be worth exploring is the harper-cli. Cai can run shell actions on your selection, so you could get deterministic, no-LLM grammar feedback with no new permissions. It would still be behind a ⌥C action, but the always-on mode is not something I'd want to ship...

harper-cli calls itself "experimental", but it should be easy to build this as a Cai extension

Out of curiosity, what do you use Cai for? Grammar being your first ask makes me think something writing-heavy?

[soyasis]

Turns out Harper ships their own macOS app now (beta, free, Apache-2.0, fully on-device) and it does exactly this system-wide: https://writewithharper.com/desktop

Or did I misunderstood your idea?

[TyceHerrman]

Yea I saw the desktop app for Harper recently too (have used it as lsp for a while). That kicked off a quick check for what else is out there. In comparing to other desktop apps out there I saw Cai (haven't tried Cai yet but got the gist of what it does). Cai would actually be a more powerful desktop app to use as my daily driver for all the features you already have, but I still would want the functionality of what Harper desktop does, real time deterministic checking and highlighting, just packaged up in Cai. A Cai extension that does that would work too I think (haven't tried Cai not really explored how your extensions work yet)