When I first examined whether it's possible to fight back against AI-generated plagiarism and how that approach might work, it was January 2023 -- just a few months into the world's exploding awareness of generative AI.
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This is a completely updated version of that original January 2023 article. When I first tested GPT detectors, the best result was 66% correct from one of three available checkers. My most recent set of tests, in February 2025, used up to 10 checkers -- and three of them had perfect scores. This time, just a couple of months later, five did.
Before I go on, though, let's discuss plagiarism and how it relates to our problem. Merriam-Webster defines "plagiarize" as "to steal and pass off (the ideas or words of another) as one's own; use (another's production) without crediting the source."
This definition fits AI-created content well. While someone using an AI tool like Notion AI or ChatGPT isn't stealing content, if that person doesn't credit the words as coming from an AI and claims them as their own, it still meets the dictionary definition of plagiarism.
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To test the AI detectors, I'm using five blocks of text. Two were written by me and three were written by ChatGPT. To test a content detector, I feed each block to the detector separately and record the result. If the detector is correct, I consider the test passed; if it's wrong, I consider it failed.
When a detector provides a percentage, I treat anything above 70% as a strong probability -- whether in favor of human-written or AI-written content -- and consider that the detector's answer. If you want to test a content detector yourself using the same text blocks, you can pull them from this document.
To evaluate AI detectors, I reran my five-test series across 10 detectors. In other words, I cut and pasted 50 individual tests (I had a lot of coffee).
For this update, I added Copyleaks and Monica. I dropped Writefull from my tests because it discontinued its GPT detector. Content Guardian requested inclusion, but I didn't hear back in time for testing accounts.
This table shows overall results. As you can see, five detectors correctly identified human and AI text in all tests.
I tried to ascertain whether there was a tangible pattern of improvement over time, so I constructed a chart comparing the five-test set over time. So far, I've run this series six times, but there's no strong trend. I did increase the number of detectors tested and swapped out a few, but the only consistent result is that Test 5 was reliably identified as human across detectors and dates.
I'll continue to test over time, and hopefully I'll see reliability trend consistently upward.
While there have been some perfect scores, I don't recommend relying solely on these tools to validate human-written content. As shown, writing from non-native speakers often gets rated as generated by an AI.
Even though my hand-crafted content has mostly been rated human-written this round, one detector (GPTZero) declared itself too uncertain to judge, and another (Copyleaks) declared it AI-written. The results are wildly inconsistent across systems.
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Bottom line: I would advocate caution before relying on the results of any -- or all -- of these tools.
Now, let's look at each individual testing tool, listed alphabetically.
This tool was originally produced by an AI content generation firm, Content at Scale. It later migrated to BrandWell.ai, a new name for an AI-centric marketing services company.
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Unfortunately, its accuracy was low. The tool was unable to tell if the AI-generated content in Test 2 was human or AI, as shown in this screenshot:
I find it amusing that Copyleaks declares itself "the most accurate AI detector with over 99% accuracy" when more than half of tested detectors performed better. But marketing folks will be marketing folks -- superlatives are as hard for them to resist as barking at a squirrel (and the FedEx truck, and all the neighbor kids) is for my dog.
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The company's primary offering is a plagiarism checker sold to educational institutions, publishers, and enterprises seeking to ensure content originality and uphold academic integrity.
This tool was built using a machine-learning hub managed by New York-based AI company Hugging Face. While the company has received $40 million in funding to develop its natural language library, the GPT-2 detector appears to be a user-created tool using the Hugging Face Transformers library.
GPTZero has clearly been growing. When I first tested it, the site was bare-bones -- it wasn't even clear whether GPTZero was a company or just someone's passion project. Now, the company has a full team with a mission of "protecting what's human." It offers AI validation tools and a plagiarism checker.
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Unfortunately, performance seems to have declined. In my last two runs, GPTZero correctly identified my text as human-generated. This time, it declared that same text as AI-generated.
Grammarly is well known for helping writers produce grammatically correct content -- that's not what I'm testing here. Grammarly can check for plagiarism and AI content. In the grammar checker, there's a Plagiarism and AI Text Check button in the lower-right corner:
I'm not measuring plagiarism checker accuracy here, but even though Grammarly's AI-check accuracy was poor, the site correctly identified the test text as previously published.
Monica is a new entrant. This service offers an all-in-one AI assistant with a wide range of services. Users can choose from various large language models.
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The company calls Monica the "Best AI Detector Online," but it looks like it runs content through other detectors including ZeroGPT, GPTZero, and Copyleaks. Weirdly, both GPTZero and Copyleaks didn't perform well in my tests, but Monica -- and ZeroGPT -- did.
We're giving it 100% because it earned that rating, but I'll see how it stands up in future tests.
Originality.ai is a commercial service that bills itself as an AI and plagiarism checker. The company sells usage credits: I used 30 credits for this article. They sell 2,000 credits for $12.95 per month. I pumped 1,400 words through the system and used just 1.5% of my monthly allocation.
The last few times I tested QuillBot, results were wildly inconsistent -- multiple passes of the same text yielded wildly different scores. This time, however, it was rock solid and 100% correct. So I'm giving it the win. I'll check back in a few months to see if it holds onto this performance.
Undetectable.ai's big claim is that it can "humanize" AI-generated text so detectors won't flag it. I haven't tested that feature -- it bothers me as a professional author and educator, because it seems like cheating.
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However, the company also has an AI detector, which was very much on point.
The AI detector passed all five tests. Notice the indicators showing flags for other detectors. The company said, "We developed multiple detector algorithms modeled after those major detectors to provide a federated and consensus-based approach. They do not directly feed into the listed models; rather, the models are each trained based on results they've generated. When it says those models flagged it, it's based on the algorithm we created and updated for those models."
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I do have a question about the OpenAI flag, since OpenAI's content detector was discontinued in 2023 due to low accuracy. Even so, Undetectable.ai detected all five tests, earning a perfect 100%.
Writer.com is a service that generates AI writing for corporate teams. Its AI Content Detector tool can scan for generated content. Unfortunately, its accuracy was low. It identified every text block as human-written, even though three of the six tests were written by ChatGPT.
ZeroGPT has matured since I last evaluated it. Then, no company name was listed, and the site was peppered with Google ads and lacked clear monetization. The service worked fairly well but seemed sketchy.
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That sketchy feeling is gone. ZeroGPT now presents as a typical SaaS service, complete with pricing, company name, and contact information. Its accuracy increased as well: last time it was 80%; this time it scored 5 out of 5.
What about you? Have you tried AI content detectors like Copyleaks, Monica, or ZeroGPT? How accurate have they been in your experience? Have you used these tools to protect academic or editorial integrity? Have you encountered situations where human-written work was mistakenly flagged as AI? Are there detectors you trust more than others for evaluating originality? Let us know in the comments below.
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