I've come to trust their grammar corrections implicitly. So much that I've setup a "Fix Grammar" command with Obsidian Copilot that I'll just let it run on a whole chapter (YOLO mode):

    Proofread the following text for grammar, punctuation, and typos while
    strictly preserving the author's voice, pacing, and intentional stylistic
    choices (such as sentence fragments or character dialogue quirks).

    Rules:
    - Correct objective spelling errors, misused words, and unintended punctuation mistakes.
    - Smooth out unintentional syntactic snags without homogenizing unique phrasing.
    - Retain all Markdown formatting.
    - Return ONLY the corrected text with no intro, summary, or explanations.

    Text to edit:
    {selection}

The complexity in this command is there for phi4:14b and qwen3:14b which I run locally via ollama. If the text doesn't have any Obsidian callouts or similar, fancy stuff I just use either of those and they do a fantastic job in seconds. For Big AI (e.g. Gemini, GPT-whatevs, Claude) you can literally just tell it, "fix the grammar." No need for the lengthy command.

NOTE: I am decent with English grammar so most of what needs fixing is typos I didn't spot or misplaced commas and periods inside/outside of quotes (I always screw that up without thinking—even though I know the rules! LOL). Occasionally, Big AI (Gemini, specifically) have disagreements about whether a comma is necessary in a particular spot but it's always of no real consequence.

> About a year ago, all three big AI vendors could end up with strange "corrections" that would throw you off, but not anymore.

Without a history of diffs created by this method, I don't believe you.

> I am decent with English grammar so most of what needs fixing is typos I didn't spot or misplaced commas and periods inside/outside of quotes (I always screw that up without thinking—even though I know the rules! LOL).

The rules there vary by style guide and are not objective.

LLMs seem like massive overkill for something the red and green squiggles can already accomplish.