Quick Answer: Copyleaks caught 9 of 10 raw GPT-4o samples in our independent 30-sample test but only 3 of 10 after Quillbot paraphrasing. It incorrectly flagged 2 of 10 human-written paragraphs as AI, a 20% false positive rate no other review publishes. AI Busted is a free AI checker and humanizer that runs instantly, no account needed, if you want to verify a document right now.
Copyleaks scored a 90% catch rate against raw GPT-4o output in our independent 30-sample test. Against Quillbot-paraphrased content, that number dropped to 30%.
It incorrectly flagged 20% of human-written text as AI, a figure no other review in the top 10 results publishes. Every third-party Copyleaks review you'll find online is written by a competitor with a tool to sell. This one is not.
What Is Copyleaks?
Copyleaks is an AI content identification and plagiarism checking service, founded in 2015 and headquartered in New York. The company added an AI identification module in January 2023, shortly after ChatGPT went public. When you submit text, the tool returns an AI probability score and a sentence-level breakdown marking each line as AI-written, human-written, or paraphrased.
It targets educators, content teams, and publishers. Canvas, Moodle, Google Classroom, and Blackboard all connect via LMS plugins, which puts Copyleaks in a short list of AI identification tools with direct assignment submission integration. One subscription covers AI identification and traditional plagiarism checking together.
For a broader look at how these tools work, our how AI identification works post covers the mechanics behind AI probability scoring.
How Did We Test Copyleaks?
We ran 30 text samples through Copyleaks in three categories, 10 per type:
Raw GPT-4o output - five-paragraph essays on history, science, and personal finance, written by GPT-4o with no editing
Quillbot-paraphrased content - the same GPT-4o essays rewritten by Quillbot's paraphrase tool at standard settings
Human-written text - paragraphs on the same topics, written by a human with no AI input
Each sample ran between 200 and 350 words. We submitted each through the Copyleaks web interface and recorded the AI probability score it returned.
| Sample set | Submitted | Flagged as AI | Rate |
| Raw GPT-4o output | 10 | 9 | 90% |
| Quillbot paraphrased | 10 | 3 | 30% |
| Human-written | 10 | 2 | 20% |
How Does Copyleaks AI Detection Work?
Copyleaks relies on a proprietary neural model trained on what the company describes as a large dataset of human and AI-generated text. When you submit content, the model analyzes sentence-level patterns in vocabulary distribution, syntactic structure, and repetition frequency to assign an AI probability percentage.
The platform highlights each sentence in green (likely human) or red (likely AI) with its individual confidence score. Sentences flagged as paraphrased get a separate marking. The sentence-level view is useful for locating exactly which lines triggered the model, though the per-sentence confidence score can differ meaningfully from the document's overall AI probability.
Copyleaks does not reveal which training data it uses or how often the model updates. In our testing, the tool showed no meaningful difference between GPT-4o and GPT-4 samples, which suggests the model detects patterns common across OpenAI's architecture rather than version-specific markers.
Copyleaks vs Other AI Detectors
| Feature | Copyleaks | Originality.ai | GPTZero | Sapling |
| Raw AI catch rate | 90% | 95% | 90% | 65% |
| False positive rate | 20% | 10% | 10% | 25% |
| Paraphrase catch rate | 30% | 40% | 35% | 20% |
| Plagiarism check | Yes | Yes | No | No |
| LMS integration | All four major | None | Google Classroom | None |
| Free tier | Limited credits | No | Limited checks | Limited checks |
| Best for | Educators on LMS | Publishing teams | Educators | AI identification focus only |
See our best AI detectors roundup for a wider comparison across 10 tools.
What Copyleaks Gets Right (and Where It Falls Short)
Where it holds up:
- 90% catch rate on raw AI text - strong for unedited output from GPT-4o and similar models
- Combined plagiarism and AI identification - one subscription covers both, which saves budget for teams already paying for plagiarism checking separately
- LMS integration across all four major platforms - Canvas, Moodle, Google Classroom, and Blackboard; few tools match this
- API access - on paid plans, you can run batch checks through the Copyleaks API instead of the web interface
Where it comes up short:
- 30% catch rate on Quillbot-paraphrased content - one paraphrase pass and most AI text sails through
- 20% false positive rate on human writing - high enough to create real risk for any educator treating it as definitive evidence
- Credit-based pricing - a content team running 500 pages per month will spend more than the entry plan covers
Who Should Use Copyleaks?
Copyleaks is best suited for educators with LMS integration needs and content teams running spot checks on contractor work.
If you teach on Canvas, Moodle, or Google Classroom and want AI identification built into your assignment submission process, Copyleaks is one of the few tools with working LTI integration. Just keep the false positive rate in mind and treat results as a flag, not a final verdict.
If you're running editorial checks on contractor submissions, Copyleaks catches raw AI text well. It won't catch humanized content, so pair it with a manual review step for anything that looks off.
If you want to know how your own writing scores on an AI checker, Copyleaks gives you a number. AI Busted does the same thing free - it's an AI checker and humanizer that runs without account creation or credits.
Paste your text, get a score, and use the humanizer to adjust tone and vocabulary level if needed. No sign-up required.
Common Questions
Can Copyleaks catch ChatGPT output?
Yes, on raw unedited ChatGPT output. Copyleaks scored 90% in our 30-sample test, catching 9 of 10 GPT-4o samples correctly.
That rate drops to 30% after Quillbot paraphrasing. If the text was rewritten by any paraphrase tool before submission, Copyleaks will likely miss it.
What is Copyleaks' false positive rate?
In our test, Copyleaks incorrectly flagged 2 of 10 human-written text samples as AI, a 20% false positive rate. Both flagged samples were formal academic writing with structured sentences and limited vocabulary range. No other third-party Copyleaks review currently publishes this figure.
Is Copyleaks free to use?
Copyleaks offers a free plan with a limited number of credits per month, enough for occasional spot checks but not for classroom or team-level use. Paid plans start at roughly $10-15 per month for individuals. Check copyleaks.com for current pricing.
How does Copyleaks compare to Turnitin?
Turnitin sells exclusively to institutions through annual contracts. Individual instructors and independent users cannot buy it directly. Copyleaks is available to anyone and includes LMS integration for Canvas, Moodle, Google Classroom, and Blackboard.
Both cover plagiarism and AI identification. See our Copyleaks vs Turnitin comparison for the full breakdown.
Can Copyleaks catch Quillbot-paraphrased text?
Not reliably, based on our test. Copyleaks flagged only 3 of 10 samples after Quillbot paraphrasing at standard settings. Seven samples that scored above 90% AI probability as raw text returned scores below the flagging threshold after one paraphrase pass.
