# Best AI detector for long texts of 300 words or more (tested)

Source: https://toolbenchmarks.com/best-ai-detector-for/long-texts/  
Tested: 30 October 2026. Publisher: Tool Benchmarks. Data licence: CC BY 4.0.

> Desklib is the best AI detector for texts of 300 words or more in our test. It flagged none of 28 human texts and caught 54.4% of AI texts, a balanced accuracy of 77.2%. Fast-DetectGPT scored 77.4% by catching more AI text, 69.0%, but it flagged 4 of 28 human texts. Our longest texts were a few hundred words, so multi-page essays were not tested.

| Detector | Balanced accuracy | Catches AI text | Flags human text |
|---|---|---|---|
| Fast-DetectGPT (Qwen2.5-1.5B) | 77.4% | 69.0% | 14.3% |
| Desklib AI text detector | 77.2% | 54.4% | 0.0% |
| MAGE | 71.7% | 57.6% | 14.3% |
| Fakespot AI text detector | 59.7% | 76.6% | 57.1% |
| E5-small LoRA detector | 58.4% | 81.0% | 64.3% |
| OpenAI RoBERTa detector | 52.0% | 7.6% | 3.6% |
| TMR (Target Mining RoBERTa) | 50.0% | 100.0% | 100.0% |

## More words help some detectors and hurt others

A longer text gives a detector more to measure, and [MAGE](/ai-detectors/mage/) used it. Its balanced accuracy rose from 51.1% on texts under 200 words to 71.7% at 300 words or more, and it caught 57.6% of long AI texts against 20.6% of short ones. Fast-DetectGPT barely moved between length buckets, scoring 77.9% on short texts and 77.4% on long ones, while Desklib rose from 71.8% to 77.2%.

Two detectors went the other way. Fakespot flagged 34.2% of short human texts and 57.1% of long ones, and E5-small LoRA 39.5% and 64.3%. Both also caught more long AI text, 76.6% and 81.0%, but their balanced accuracy on long texts stayed low, at 59.7% and 58.4%. Our data does not show why. One possibility is that both models treat the structure of a longer piece, several paragraphs on one topic, as a sign of AI. TMR flagged all 28 long human texts and all 158 long AI texts, which fits its habit of flagging any multi-paragraph text.

## Desklib or Fast-DetectGPT for long documents

On balanced accuracy the two are tied for practical purposes, 77.4% for Fast-DetectGPT and 77.2% for [Desklib](/ai-detectors/desklib/). They differ in which mistake they make. Fast-DetectGPT caught 69.0% of long AI texts and flagged 4 of 28 human ones (14.3%, with a 95% interval from 5.7% to 31.5%). Desklib caught 54.4% and flagged none. Fast-DetectGPT's threshold was also chosen on our calibration split, a step the other detectors did not get, so its score here is a little flattering.

If a flag could cost someone a grade or a job, Desklib's profile is the safer one. Its clean record rests on 28 texts, though, and the interval for its false positive rate runs up to 12.1%. It also missed many AI texts, so a pass is no proof of human writing.

## What "long" means here

The bucket starts at 300 words and ends at the length of our longest texts, a few hundred words. The 28 human texts in it come from encyclopedia articles, news reports, Q&A answers and fiction. None are academic abstracts, which are shorter, so the abstract problem described on the [academic writing page](/best-ai-detector-for/academic-writing/) does not show up in these numbers. The [leaderboard](/ai-detectors/) has the results for all lengths combined.

## Questions people ask

### Are AI detectors more accurate on longer texts?

Some are. MAGE went from 51.1% balanced accuracy under 200 words to 71.7% at 300 words or more. Fast-DetectGPT changed little, and Desklib rose from 71.8% to 77.2%. Fakespot and E5-small LoRA flagged more human writing as length grew.

### How long should a text be for an AI detector to work?

Longer is not always better. Desklib's best length bucket was 200 to 299 words, where it scored 77.6% balanced accuracy and caught 63.5% of AI texts, against 54.4% at 300 words or more.

### Can I use these results for a student essay?

Only loosely. Our long human texts are encyclopedia articles, news reports, forum answers and fiction passages of a few hundred words, not student essays. On them Desklib flagged 0 of 28 human texts, with a 95% interval up to 12.1%.