How Does AI Text Detection Work?
AI text detectors exploit a fundamental difference between how humans and language models write. Human writers are unpredictable — we vary sentence structure, make unusual word choices, and deviate from the most statistically likely phrasing in ways that feel natural. Large language models such as ChatGPT, Gemini, and Claude are trained to predict the most probable next token, which means their outputs tend to cluster around high-probability word sequences. This predictability is measured as perplexity: a low-perplexity score indicates the text closely follows expected statistical patterns, a strong signal of AI authorship.
A second key signal is burstiness — the variation in sentence length and complexity across a document. Human writing tends to burst between short punchy sentences and long intricate ones. AI writing is more metronomic: sentences follow similar rhythms and lengths, producing a lower burstiness score. AIGeneratedIt's detection pipeline combines perplexity analysis with burstiness scoring and a fine-tuned RoBERTa classifier trained on hundreds of thousands of labeled examples from GPT-3.5, GPT-4, GPT-4o, Gemini, Claude, Llama, Mistral, and Cohere outputs.
The final layer is model attribution: after classifying text as AI-generated, our system estimates the probability distribution across source models. This helps users understand not just whether content is AI-generated, but which tool likely produced it — useful for academic integrity, journalism verification, and enterprise compliance workflows.
Frequently Asked Questions
Can AIGeneratedIt detect text written by ChatGPT-4o?
Yes. Our detector is trained on outputs from GPT-4o, GPT-4, GPT-3.5, Google Gemini 1.5 and 2.0, Anthropic Claude 3.5 and 4, Meta Llama 3, Mistral, Cohere, and Falcon. The RoBERTa classifier is updated monthly as new models are released.
How does per-sentence analysis work?
Rather than scoring the entire document as one block, AIGeneratedIt segments text into individual sentences and scores each independently. This is essential for mixed-content documents where a human writer has partially edited AI-generated text. The result is a color-coded heatmap that highlights exactly which sentences are most likely AI-generated.
Can AI humanizers or paraphrasing tools defeat the detector?
AI humanizing and paraphrasing tools reduce the effectiveness of single-model detectors. AIGeneratedIt's five-model ensemble — RoBERTa, Binoculars, perplexity scorer, Sapling API, and burstiness detector — uses orthogonal signals that are difficult to fool simultaneously. While no detector is perfect, our ensemble is significantly more robust than single-model approaches.
What is the minimum text length for accurate detection?
A minimum of 150 characters (approximately 25–30 words) is required for results to be meaningful. For the highest confidence, we recommend submitting at least 300 characters. Short snippets return a low-confidence flag and should not be used for consequential decisions.
Is AIGeneratedIt free to use?
Yes. The AI text detector is completely free with no account required. Submit text and receive instant results. Pro plans unlock higher character limits, bulk document analysis, API access, and team collaboration features.
What AI Writing Models Does This Detect?
AIGeneratedIt's text detection engine is trained on labeled outputs from all major large language models. Below is the current list of supported AI writing systems:
- OpenAI ChatGPT — GPT-3.5, GPT-4, GPT-4o, GPT-4o mini, GPT-5
- Google Gemini — Gemini 1.0, Gemini 1.5 Pro, Gemini 2.0 Flash, Gemini 2.0 Ultra
- Anthropic Claude — Claude 2, Claude 3 Haiku, Claude 3.5 Sonnet, Claude 4
- Meta Llama — Llama 2, Llama 3, Llama 3.1, Llama 3.3
- Mistral AI — Mistral 7B, Mixtral 8x7B, Mistral Large
- Cohere — Command R, Command R+
- TII Falcon — Falcon 7B, Falcon 40B, Falcon 180B
- Other models — Perplexity AI, Grok, DeepSeek, Qwen, and 40+ additional LLMs