Undetectable AI vs StealthGPT

Undetectable AI's trial is 250 words in total, roughly one long paragraph, and it does not renew. StealthGPT has no free words at all; its $1 trial buys three days of the Pro plan and then bills $29.99 a month unless cancelled, read 9 November 2026. If the question is "can I see the output before paying anything", only Undetectable AI answers yes, and only for 250 words. Larger free tiers exist elsewhere: the free humanizer ranking lists eight tools with free access, up to 3,000 words a request.

The answer flips with the billing period, so both are shown.

Figures read on the vendors' pricing pages on 9 November 2026. On monthly billing StealthGPT is cheaper per word; on yearly billing Undetectable AI is cheaper per word but requires a year paid at once. Price is reported here and never scored, so neither tool earns a "best value" label on this site.

Undetectable AI states no per-request cap on its pricing page, so a 1,500-word essay or a 3,000-word article goes in whole, read 9 November 2026. StealthGPT caps Pro at 1,000 words and Max at 2,000, so the same essay is two pastes on Pro and a 3,000-word article is three. The monthly pools run the other way: 10,000 words on Undetectable AI's entry plan is about six 1,500-word essays, and StealthGPT Pro's 50,000 is about 33.

Neither vendor names rewrite modes on the pricing page, so there is no "light" or "strong" setting to compare. The published method runs each tool on its default setting and, where the interface offers one, its strongest documented mode, and records the names in the raw files on the run date.

No peer-reviewed study has tested either product by name against a named detector in a way this site could cite. What the research does say concerns the category. Krishna and colleagues reported that a paraphraser dropped DetectGPT's detection rate from 70.3 percent to 4.6 percent at a 1 percent false positive rate (Krishna et al. 2023, arXiv 2303.13408, read 9 November 2026). Weber-Wulff and colleagues tested 14 detectors and concluded they were neither accurate nor reliable, with paraphrasing and other obfuscation worsening performance (Weber-Wulff et al. 2023, International Journal for Educational Integrity, read 9 November 2026).

Those papers measured detector readings, not whether the paraphrase kept the writer's claims. That gap is why the method on this site weights meaning kept at 35 points of 100 and detector pass rate at 30, and why neither tool's name is repeated here as a promise. No detector score should be the only evidence in a misconduct case.

Both vendors sell more than one tier, and the shape of the ladder matters if your volume grows. StealthGPT's page lists three writing plans, Pro, Max and Enterprise, with monthly pools of 50,000, 200,000 and 1,000,000 or more words and request caps of 1,000, 2,000 and 10,000 or more, read 9 November 2026. Undetectable AI's page is built around a word slider with a yearly toggle on by default, so the headline shows $5.00 while the charge is $60.00, and the 10,000-word tier is the lowest point on that slider, read 9 November 2026.

A trial that converts is the detail most often missed. StealthGPT's $1 buys access, not a decision; cancelling inside the three days is on the buyer. Undetectable AI's trial ends on its own, and the next step is a deliberate purchase of a year or a month.

The method on this site runs both tools on the same 24 AI-written inputs and 6 human controls, on the default setting and on the strongest documented mode where one exists, and checks every output on GPTZero, Originality.ai, Copyleaks, Pangram and ZeroGPT. Two editors rate each output blind for meaning kept and count errors introduced per 100 words. When a run is published, this page gains the same fields for both tools: meaning kept, errors added, detector readings per detector, and the plan each was run on with its price on the run date. Until then the comparison rests on the plan facts above, which are the facts a buyer can check today.