A diff-based LLM-powered grammar check

Since posting about how I use llm to check my grammar, Dr. Drang took inspiration from my setup and wrote about how he implemented a system that shows differences side-by-side, instead of highlighting them as I do. On top of that, his system includes a short rationale for the changes.

I have always liked the idea of a diff-based check, but because I write in Mailmate, helix, and the browser, I couldn’t come up with a system that worked everywhere.

That was until I remembered about Meld, a diffing app.

Before starting, it’s important to have Meld installed and available at the command line (to do so, install it via brew install meld and follow these instructions).

Once that is done (and you have llm installed—see my previous post), I use this macro that I invoke after copying the text:

The short Keyboard Maestro macro to use an LLM to check for my grammar

The script does all the heavy lifting and is self-explanatory:

#!/opt/homebrew/bin/fish

pbpaste > /tmp/original.md
pbpaste | llm -m gemini-3-flash-preview -s "
Given the following text, improve grammar and punctuations. Note that there might be markdown syntax that needs to be preserved

Output the text and add commentary only at the end, after a clear break line (use dashes)" > /tmp/corrected.md
cat /tmp/corrected.md | pbcopy
meld /tmp/original.md /tmp/corrected.md

Once invoked, Meld opens, allowing me to review the changes and copy the text back:

The Meld main window showing this post and the uncorrected version of it — The interface to see the corrections

If you write long lines of text without hard wraps, don’t forget to check the Enable text wrapping box in the preferences pane!