supadetect vs GPTZero
GPTZero is one of the most widely used AI detectors. supadetect matches its plan sizes and price, and differs in two ways: we are built for Spanish as much as English, and we show a Band whose thresholds are measured per language rather than one percentage.
| supadetect | GPTZero | |
|---|---|---|
| Who can use it | Anyone, with or without an account. | Open to anyone; owned by Superhuman (formerly Grammarly) since June 2026. (Superhuman, checked 2026-11-10) |
| Free use | 5,000 words a month without an account (1,500 per Scan); 10,000 with a free account. | Without an account, up to 10,000 characters per scan; about 10,000 words a month with a free account. (GPTZero, checked 2026-11-10) |
| Paid price | Pro €18.99 a month or €119.88 a year, for 300,000 words a month. | Premium shown at €18.99 a month in euros. (GPTZero, checked 2026-11-10) |
| Spanish | Separate thresholds for Spanish and English, measured on texts in each language; Spanish interface. | Detects 27 languages; reports 95.7% accuracy and a 5.6% false-positive rate on Spanish. (GPTZero blog, checked 2026-11-10) |
| What it publishes about accuracy | Thresholds and error rates per language with confidence intervals, from a test set never used for tuning (below). | States 99.3% accuracy and a 0.24% false-positive rate overall. (GPTZero, checked 2026-11-10) |
Facts about other products come from their own pages or from the reports linked, on the date shown. Prices and features change; check the source before deciding.
The differences that matter
- Spanish. GPTZero reports its own Spanish false-positive rate (see the table). Ours, measured on Spanish texts written before ChatGPT existed, is below.
- How results read. We answer Likely human, Unclear or Likely AI, and we say Unclear when the text could go either way, instead of a single number.
- Your text. We don't store it, and we never train on it.
Our measured numbers
Rendered from the measurement file of Engine 2026.11-oculus-v0. We do not measure other vendors' detectors, so there are no competitor accuracy figures on this page.
| English human texts called Likely AI | 1.2 % (0.9–1.7) |
| English AI texts called Likely AI | 89.2 % (87.1–91.0) |
| Spanish human texts called Likely AI | 1.4 % (1.0–2.0) |
| Spanish AI texts called Likely AI | 99.5 % (98.8–99.8) |
What independent studies found about detectors in general
- Seven detectors flagged 61% of 91 human TOEFL essays written by non-native English speakers as AI-written (Liang et al., 2023). (Patterns / arXiv, checked 2026-11-11)
- A test of 14 detection tools found them "neither accurate nor reliable", and easy to fool with paraphrasing (Weber-Wulff et al., 2023). (International Journal for Educational Integrity, checked 2026-11-11)
- An audit by the University of Chicago's Becker Friedman Institute found large differences between commercial detectors, with only one meeting a 0.5% false-positive cap (Jabarian & Imas). (Becker Friedman Institute, checked 2026-11-10)