A Grammar Checker Your AI Assistant Can Actually Call

Most grammar checkers have a peculiar side effect: they make your writing more robotic. They flatten contractions, formalize casual phrasing, and "correct" the small stylistic choices that made the text sound like you. You fix the comma splices and lose the voice.

SupWriter's grammar checker works in two steps to avoid that, and the SupWriter MCP server lets your AI assistant call it directly — so you can clean up a draft inside Claude, ChatGPT, or your editor without copy-pasting into yet another tool.

The two-step problem

Here's the trap. A pure grammar fixer optimizes for correctness, and "correct" English tends toward the bland, even, textbook register that AI detectors and human readers both find a little lifeless. So SupWriter does it in two passes: first it fixes the grammar, spelling, and punctuation; then it runs a humanization pass so the corrected text still reads naturally instead of like a style guide.

The grammar_check tool does both in a single call. You don't manage the steps — you just ask for clean text, and you get clean text that still sounds human.

The three modes

You pass a mode argument depending on how heavy a hand you want:

• basic — grammar, spelling, and punctuation only. Available on every plan. Use it when the draft is solid and you just want the errors gone.
• thorough — everything in basic, plus sentence structure, subject-verb agreement, and tense consistency. Pro and Ultra plans.
• style — grammar, spelling, and punctuation, plus word choice, conciseness, and flow. Pro and Ultra plans. This is the one to reach for on something you're about to publish.

You can also pass a language so it applies the right rules for non-English text.

In practice you'd say something like: "Clean up the grammar in this paragraph, style mode," and the assistant calls grammar_check with mode: "style" and returns polished text — no tab switching, no losing your place.

How it's billed

Grammar checks draw on your SupWriter plan's word credits, the same pool the rest of the tools use. Because the result goes through a humanization pass, you're charged on the cleaned output: one credit per output word. If a premium mode isn't available on your plan, the tool quietly downgrades to basic rather than erroring, so a request never just dies.

What it won't do

A grammar checker is a polish pass. It won't verify your facts, it won't restructure a confused argument into a clear one, and it won't catch a claim that's simply wrong. Treat it as the last 5% — the layer that removes friction for the reader — not as an editor that fixes weak thinking.

If your goal is to rephrase rather than correct, that's a different tool: see our note on humanizing vs paraphrasing, and the paraphraser MCP which is built for rewriting while preserving meaning.

Connecting it

The grammar checker ships in the same connector as the rest of SupWriter's tools. Add https://supwriter.com/api/mcp in your client's connector settings, sign in, and approve. Full walkthroughs for Claude, ChatGPT, and dev tools are a click away.

Once connected, "fix the grammar in the selection" becomes a single sentence to your assistant — and the result still sounds like you wrote it.