Free AI Detectors Compared: Privacy & Accuracy
Searches like "GPTZero alternative" and "free AI detector no sign up" show that people want a tool that is free, quick and does not keep their files. This guide explains how the main kinds of AI detector differ so you can pick the right one for the job. We built AIScan Lens, so we are open about what it does and does not do.
Three kinds of AI detector
1. Text classifiers
Well-known text checkers such as GPTZero, ZeroGPT, Originality.ai and Copyleaks usually score writing with a trained model plus statistics such as perplexity (how predictable the wording is) and burstiness (how much sentence length varies). They return a probability. Many require an account or limit free use, and your text is sent to a server.
2. Watermark detectors
Google's SynthID Detector looks for an invisible watermark that Google and its partners embed. It is precise for those tools and silent about everything else.
3. Provenance and metadata readers
These read labels that already sit inside the file: C2PA Content Credentials, IPTC AI tags and generation parameters. AIScan Lens belongs here for images and video.
How they compare
| Question | Text classifiers | SynthID | AIScan Lens |
|---|---|---|---|
| Checks text | Yes | No | Yes (indicative estimate) |
| Checks images | Some | Yes (its own watermark) | Yes (metadata evidence) |
| Works on Midjourney or FLUX | Varies | No | When metadata survives |
| Needs an account | Often | Check the site | No |
| File leaves your device | Usually | Check the site | No, analysed in your browser |
| Gives a number with no evidence | Usually | No | No, says inconclusive |
Features of third-party tools change often, so check each tool's current terms before relying on this table.
Why privacy matters
Students paste essays, journalists upload unpublished photos and companies check drafts. Server-based tools need that content to travel to someone else's computer. AIScan Lens runs its analysis in your browser, so the file is never uploaded. Read the full methodology and our privacy policy.
Why accuracy claims need care
No detector is perfect. Text detectors can flag human writing, especially from non-native English speakers and very formal prose, as AI. See Can AI text detectors be trusted?. A fair tool shows its evidence and admits uncertainty. Be sceptical of anything that promises 99 percent accuracy without explaining how it was measured.
Which should you use?
- Checking an image or video from an unknown source: start with a metadata check, then SynthID for Google and partner content.
- Reviewing writing: use a text tool only as one signal, never as proof, and talk to the author.
- Sensitive material: choose a tool that does not upload your file.