Can Claude AI fix your grammar mistakes? No, it can’t, but there’s an AI that will.

Reading Time: 11 minutes
(Image by Maria Korolov via Midjourney.)

Yesterday, I put the new Claude AI from Anthropic through its paces. And it was the best structural and content editor I’ve ever seen. It can take a 75,000-word book file upload, read it in seconds, and find plot holes, loose threads, and unsatisfying character arcs… its release is literally a life-changing event for authors.

But can it do a grammar check?

I didn’t bother trying it out yesterday, other than just giving it a few ungrammatical sentences to see if it could catch problems. It could, though with one sentence I had to ask it twice. I gave it a 90 to 95 percent accuracy score and moved on. After all, there are plenty of good, free grammar checkers already out there. Microsoft Word has an AI-powered grammar checker built right in. So does Google Docs. If those aren’t enough, Grammarly has a very good free plan, and ProWritingAid … well, it HAS a free plan. I wouldn’t call it good, but it’s there.

Those other grammar checkers are designed to be extremely thorough and convenient, which makes them great for the job.

Generative AIs like ChatGPT and Claude are designed to be creative. That’s why the word “generative” is right there in the name. They weren’t originally designed to be accurate and complete, though people are using them as if they are. There’s a lot of pressure on their developers to improve that aspect, but it might take a while for the chatbots to get there.

But maybe 90 to 95 percent accuracy is good enough?

I decided to test Claude out this morning. First, I needed an error-ridden piece of text to try out. I didn’t have one lying around, and Google didn’t come up with anything, just individual sentences, not full stories. So I had another chatbot write me a bad story for Claude to practice on.

The other chatbot was ChatGPT.

Here was my prompt:

Write me a 500-word sci-fi story that sounds like it was written by a non-native English speaker. Make it full of grammar problems. make it cut off abruptly at the end — don’t write an ending or conclusion. Make it weird and unsatisfying.

Here’s what it gave me:


In year of 3030, big ship floating in space, over planet that’s red like blood. Ship shiny, full of humans and machines. Captain of ship is Zar, eyes like ice and heart of fire. Zar is brave, he seen many star and black hole.

One day, Zar looking at strange green light in blackness of space. It’s light like no seen before, not star, not nebula, not comet. He take decision to go near. He tells engine man, “Put power full, we go light.”

Engine man was name Kri. Kri said, “But Zar, we not knowing what is light. It can be danger.” But Zar only laugh. “Kri, danger is what making life interesting, yes?” Kri shaking head, but he obey captain. Ship goes to light.

When they are getting near, green light is growing big. It’s very big now, like whole universe is green. Suddenly, Kri is shouting, “Zar, machine telling me, it is not light. It’s green hole!” Zar is feeling fear. Green hole, it is story told by old ones, saying if ship go in, never coming out.

But too late now. Ship is already sucked in green hole. Everything is shaking, alarms are ringing, people are screaming. Zar is holding wheel hard, trying to get out, but green hole is stronger. Ship is going into green hole, until everything is green, and then everything is black.

Zar is waking up. He is in ship, but everything quiet. No alarm. No screaming. Only green light outside window. Zar is getting up, going to window. Outside is big green planet. It is beautiful, with tall mountains and deep ocean. But Zar is feeling strange. He is feeling alone.

Zar is walking to engine room. It is empty. No Kri. No anyone. Zar is feeling fear again. Where everyone is going? He is running to every room, looking for people. But all empty. Only Zar in big ship.

Then, Zar is hearing voice. It’s soft like whisper, saying, “Welcome, Zar. We been waiting.” Zar is looking around, but no seeing anyone. “Who you?” he is asking. But voice only laughing. It is cold laugh, like ice.

Voice saying, “We are the green. We are the universe. You are in our world now.” Zar is feeling angry. “Where my people?” he asking. Voice is laughing again. “They here, Zar. They with us now.”

Zar is not understanding. He is feeling alone and afraid. But then, he is looking at window again. On green planet, he is seeing something. Small figures, walking on surface. It is humans. It is his people.

Zar is feeling joy. His people are safe. But then, he is feeling something else. Something wrong. People are not moving right. They are moving like…

Suddenly, ship is shaking. Zar is falling down, hitting head. Everything is going black. And then, green light is coming, growing big, covering everything, until it is only…

Okay, this doesn’t feel like a story that a non-native English speaker would write. I’ve edited a lot of those, and this isn’t it. Instead, it feels like a story written by someone PRETENDING to be a non-native English speaker. Frankly, it feels a little racist to me.

Anyhow, the story is short enough that I could just copy-and-paste it into Claude, but I’m going to create a text file instead, and upload it, so that I could simulate a book edit.

If you want to follow along with me, here’s the file: Story of Zar by ChatGPT.

Okay, now I’m going to head over to Claude, upload the story, and have it do a complete edit.

Here’s the prompt:

Please do a thorough copyedit of this story. For each problem you find, show me the sentence with the error, explain the problem, and suggest two or three fixes for it.

After pasting in the prompt, I hit the little paperclip icon to upload the file.

Next to the paperclip, there’s a purple arrow — that’s the button that tells Claude to get started.

It gave me a very, very long response. I would not at all recommend taking this approach for a whole book. The chatbot interface isn’t designed for grammar checking. Again, the built-in grammar checkers or add-ons like Grammarly work inside your document and are much easier to use.

But here’s the start of Claude’s responses:


Here are some suggested edits for the story:

In year of 3030, big ship floating in space, over planet that’s red like blood. Ship shiny, full of humans and machines. Captain of ship is Zar, eyes like ice and heart of fire. Zar is brave, he seen many star and black hole.

    • The use of “In year of 3030” is awkward. Better: “In the year 3030, a big ship floated in space, over a planet that was red like blood.”

    • “Ship shiny” should be more descriptive. Better: “The ship gleamed brightly” or “The ship’s metallic hull shone brightly.”

    • “He seen many star and black hole” is grammatically incorrect. Better: “He had seen many stars and black holes.”

And on and on. Claude went through every single paragraph in the story and seems to have found all the errors.

I recommend that if you do use Claude to do this, that you ask it to work one paragraph at a time instead of doing the whole book all at once. Then, since presumably you’re doing this for educational reasons, ask follow-up questions about each error. For example, you can ask Claude to explain why your sentence is grammatically incorrect.

I asked Claude to explain the problems in the first sentence in more detail, as if it was talking to a five-year-old.

This is what it said:


Okay, I’ll try to explain the correction in simpler terms:

The original sentence was:

“He seen many star and black hole.”

This sounds wrong because:

“Seen” should be “saw” when talking about something that happened before. You use “saw” for things in the past.

There are many stars, not just one star. So it should be “stars”.

There are many black holes, not just one. So it should be “black holes”.

You need to add the word “and” between stars and black holes to connect them.

So the sentence should be:

“He saw many stars and black holes.”

I changed it to:

“He had seen many stars and black holes.”

This means the same thing, but “had seen” sounds a little better than just “saw” in this sentence. It makes it more clear this was something he saw a long time ago.

Does this help explain why I changed that sentence, using simpler words and examples? Please let me know if you need me to clarify or expand on any part of the explanation!

As you can see, Claude made a mistake. It said that I needed the word “and” between “stars” and “black holes.” But the original story had the word “and.” It hallucinated a problem that wasn’t there.

But, overall, I’d say that Claude is pretty good at spotting and explaining grammar issues. It’s not perfect, and it’s not a replacement for an actual grammar checker, but it’s very useful if you’re just learning grammar.

Then, I checked Claude out on punctuation.

There’s a particular error that writers often make, and grammar checkers tend not to catch. Can you spot it in the following passage?

 “Can’t be more than a half hour!” His voice cracked the steady silence, “Footing’s not too bad up here!”

“Don’t you have some work you could be doing?” Mary scratched the white dust on her cheeks, “You could work on expanding your understanding.”

“The Boogie Woogies were good lords when they ruled here,” The old man said. He knew the Boogie Woogies had not sent spies because they didn’t want to exasperate a conflict.

Did you see all the problems? If you didn’t, check out this Rabbit With a Red Pen guide to punctuating dialogue.

These sentences — with identifying details changed — are based on ones that I actually saw recently in a work by a new author.

I asked Claude to evaluate the sentences for grammar and punctuation. Claude did not do well.

Here’s what it produced for the first paragraph:


“Can’t be more than a half hour!” His voice cracked the steady silence, “Footing’s not too bad up here!”

    • The first quoted sentence is grammatically correct.

    • The second quoted sentence should not be capitalized, since it is a continuation of the same quote. It should be: “Footing’s not too bad up here!”

    • There should be a comma after “silence” when attaching the dialogue tag, so it should be: “His voice cracked the steady silence, “Footing’s not too bad up here!””

Claude gets this wrong. The phrase “His voice cracked the steady silence” shouldn’t be connected to the second quote. The first quote already cracked the silence, so this piece of text should either be connected to the first quote, or a separate sentence all on its own.

  • Option 1: “Can’t be more than a half hour!” his voice cracked the steady silence. “Footing’s not too bad up here!”
  • Option 2: “Can’t be more than a half hour!” His voice cracked the steady silence. “Footing’s not too bad up here!”

Either way, you need a period after “silence,” not a comma.


“Don’t you have some work you could be doing?” Mary scratched the white dust on her cheeks, “You could work on expanding your understanding.”

    • The first quoted sentence is grammatically correct.

    • There should be a period after “cheeks” since it is an action tag, not a dialogue tag.

    • The second quoted sentence should start with a capital letter since it is a new sentence.

It should be: “Don’t you have some work you could be doing?” Mary scratched the white dust on her cheeks. “You could work on expanding your understanding.”

This one Claude gets right, except that the second quoted sentence already starts with a capital letter. Again, Claude hallucinated an error that wasn’t there.


“The Boogie Woogies were good lords when they ruled here,” The old man said.

    • There should not be a comma after “here”. The comma makes it a comma splice.

    • It should be: “The Boogie Woogies were good lords when they ruled here,” the old man said.

Claude got this half right. “The old man said” shouldn’t start with a capital letter, and it fixed it in the corrected version. But the problem it actually identified — the comma — was, in fact, in the correct place, and Claude did, in fact, punctuate the sentence correctly in its rewrite.


He knew the Boogie Woogies had not sent spies because they didn’t want to exasperate a conflict.

    • “Exasperate” should be “exacerbate”.

    • Otherwise this sentence is grammatically correct.

Claude caught the wrong word choice.

So, with dialogue punctuation, Claude is only half-accurate, and has a tendency to make stuff up.

Now for the real test. I took my novella Krim Times, which is about 17,000 words long, and added 11 dialogue punctuation errors throughout the text.

My prompt:

Please review this book for punctuation errors, particularly the use of quotes, commas and periods in dialogues.

The first response wasn’t particularly useful.


Here are my notes on the punctuation in the dialogue of the book:

    • Quotation marks are used inconsistently, sometimes placed outside periods and commas, sometimes placed inside. They should be placed systematically, either inside or outside punctuation.

    • Commas are sometimes missing after dialogue tags like “said Trask”. There should be a comma after the dialogue tag before the next quote.

    • Periods at the end of dialogue are sometimes missing when the quote is followed by a dialogue tag. The period should come before the closing quotation mark.

    • There are some run-on sentences in the dialogues that could be broken up into separate sentences for clarity.

… and so on. These are all pretty generic suggestions. I need details.

I asked Claude to list all sentences where the periods are missing after dialogue tags, and to give me the original sentence and its correction.


Here are the sentences with missing periods after dialogue tags and my corrections:


“It’s gone for good,” said Trask “We’ll never find out what happened.”


“It’s gone for good,” said Trask. “We’ll never find out what happened.”


“You mean, the originals?” asked the scribe “We sent them back to the newspaper. We just keep the ads that we’re going to run again in the next issue.”


“You mean, the originals?” asked the scribe. “We sent them back to the newspaper. We just keep the ads that we’re going to run again in the next issue.”


“I found his name,” Trask finally said “Here it is. William Lockton, at some party.”


“I found his name,” Trask finally said. “Here it is. William Lockton, at some party.”

In the first correction, that sentence is, in fact in the text — but the period is, in fact, there in my version. Claude made up an error that doesn’t exist.

In the second correction, Claude, again, hallucinated a missing period.

In the third correction, Claude not only hallucinated a missing period but replaced the word “he” with “Trask.”

I tried again, multiple times, using different prompts. In the end, Claude was only able to find two of the dialogue punctuation errors I added in, and made many suggestions that were incorrect or hallucinated.

So, if you’re just learning grammar, you can use Claude as a writing tutor.

But if you need a tool that can catch and fix all your spelling, grammar, and punctuation mistakes, Claude isn’t the way to go.

How do other AI-powered grammar checkers compare?

Google Docs, Microsoft Word, Grammarly, and ProWritingAid all use AI to detect spelling, grammar, and punctuation problems. But they don’t use generative AI. They’re using an older variant of AI, called machine learning, which is actually far more accurate for specific tasks and does not make stuff up. Machine learning is “narrow” AI — it can do one thing, but it does that thing accurately and quickly.

Google Docs, Microsoft Word, and Grammarly are all super convenient and free. The first two have the grammar checkers built right in, and Grammarly has a plugin that makes it act as if it was built in. I highly recommend any of these three for a first pass through your document because of the ease and convenience of the tools.

I ran my three test paragraphs through Google Docs, and the only problem it caught was the use of the word “exasperate” instead of “exacerbate”. It also highlighted “Woogies” as a misspelled word, but that’s to be expected.

Microsoft Word missed all the problems, including “exasperate”. It only caught the “Woogies”.

Grammarly did the same as Microsoft Word.

I was very disappointed with all three tools.

But ProWritingAid caught all four problems — the three punctuation errors and the word “exasperate” as well. So, if you’re having problems punctuating your dialogues, that’s the tool for you.

The free plan has a 500-word limit, however, and the premium plan is $10 per month. Plus, you have to sign up for a whole year at a time, which is $120. If you want to try it out on a month-by-month basis, the price is $30 per month. However, both of the paid options have a 14-day money-back guarantee.

Overall, I would recommend Claude only as a writing tutor, to help you structure your writing better.

If you want a quick grammar, spelling and punctuation scan, just use the checker that comes with your favorite word processor, or get the free Grammarly add-on.

If you’re getting ready to publish your book and need it to be perfect — and you know you have issues with dialogue punctuation in particular — then use ProWritingAid.

MetaStellar editor and publisher Maria Korolov is a science fiction novelist, writing stories set in a future virtual world. And, during the day, she is an award-winning freelance technology journalist who covers artificial intelligence, cybersecurity and enterprise virtual reality. See her Amazon author page here and follow her on Twitter, Facebook, or LinkedIn, and check out her latest videos on the Maria Korolov YouTube channel. Email her at [email protected]. She is also the editor and publisher of Hypergrid Business, one of the top global sites covering virtual reality.

Leave a Comment

Your email address will not be published. Required fields are marked *