Undetectable AI earned its reputation early. Founded in January 2023 by Christian Perry (CEO), Bars Juhasz (CTO, PhD student from Loughborough University), and Devan Leos, it went from zero to 20 million users by late 2025. For a while, it was the default recommendation in most corners of the internet. But the market has matured fast, and sticking with Undetectable AI in 2026 feels a lot like paying for brand recognition rather than performance.

For a deeper dive on the tool itself, see our full Undetectable AI review. The pricing sits at $19/month for 10,000 words. That puts it in the same range as UndetectedGPT ($19.99/month), but the 88% bypass rate still puts it behind newer competitors that deliver more consistent results at a comparable price. And the free tier? 250 words over 3 days. That's barely enough to test whether the tool works before you're asked to commit.

The 88% bypass rate isn't terrible on paper. It'll get past most detectors most of the time. But "most of the time" is the problem. When you're submitting academic work to [Turnitin](/blog/bypass-turnitin-ai-detection) or publishing content screened by [Originality.ai](/blog/bypass-originality-ai-detection), you need a tool that works nearly every time — not one that rolls the dice on 12% of your submissions. In one independent test, GPTZero still flagged Undetectable AI's humanized output as "likely AI." In another, the tool averaged a 20% AI detection score across essays — which sounds low until you realize that's dangerously close to Turnitin's flagging threshold.

The output quality can also be hit-or-miss on longer texts. Paragraphs sometimes come back with awkward phrasing or subtle meaning shifts that force you to manually edit the result. If you're going to spend time cleaning up the output, the tool isn't fully doing its job.

Let's dig into the specific problems users are reporting, because these aren't edge cases — they're patterns.

The Trustpilot picture is mixed. Undetectable AI holds a 3.4 out of 5 on Trustpilot across roughly 764 reviews. That's not terrible, but the negative reviews reveal recurring themes that matter. The most common complaint? Billing issues. Some reviewers report being charged unexpectedly during or after free trials, including charges appearing months after cancellation. One reviewer claimed they were "charged me 7 months after canceling my free trial." Others report being charged the full annual amount ($60) immediately rather than monthly — despite the marketing suggesting otherwise.

The bypass rate isn't consistent across detectors. Undetectable AI performs differently depending on which detector you're facing. In our testing, it brought Turnitin to about 18% AI — which sounds good until you realize many institutions flag anything above 15-20%. One independent head-to-head test found Undetectable AI averaging 20% AI detection across five essays, with one essay scoring 100% AI — a complete failure on that particular text. GPTZero's own review team tested Undetectable AI and concluded their humanized text was still "likely AI," noting concerns about press mentions on the site that didn't include links to the original coverage.

The 250-word free trial is too small to evaluate anything. You can't meaningfully test an AI humanizer with 250 words. That's about one paragraph. AI detectors behave differently on short vs. long text — the Perkins et al. (2024) study found that text length significantly affects detection accuracy. A paragraph that passes might become a flagged essay once you process the whole thing. And with the free trial lasting only 3 days, you're pressured to decide quickly based on inadequate data.

The "20 million users" claim has been questioned. Undetectable AI's CEO announced 20 million users in September 2025. As of November 2025, the homepage counter showed approximately 19.2 million. The difference may reflect timing, methodology, or rounding, but it's worth noting alongside the unlinked press mentions that GPTZero's team flagged.

We put five tools through identical testing conditions: same 1,000-word GPT-5 essay, same set of major AI detectors (Turnitin, GPTZero, Originality.ai, Copyleaks, ZeroGPT). Every tool was scored on bypass rate, readability, and price. And one alternative beats it on the metrics that matter most: bypass rate, readability, and consistency across detectors.

The numbers speak for themselves, but let's break down why UndetectedGPT is the clear winner here.

A 96% bypass rate doesn't just edge past Undetectable AI's 88%: it puts the tool in a completely different reliability class. That 8-point gap means the difference between occasionally getting flagged and almost never getting flagged. In our testing, UndetectedGPT consistently scored under 5% AI on Turnitin and under 4% on Originality.ai — the two detectors that matter most for academic and professional use. Compare that to Undetectable AI's 18% on Turnitin (borderline) and its variable performance that occasionally hits 100% AI on certain texts.

The readability factor is equally important. Undetectable AI's output can sound stilted, especially on academic or technical content. Words get swapped for synonyms that technically work but feel unnatural — the kind of writing that a professor might not flag as AI but would definitely find odd. UndetectedGPT's 9.2/10 readability score reflects output that actually reads like a human drafted it. Sentence lengths vary naturally. Transitions between ideas feel organic. Your arguments don't get diluted or rearranged.

The Perkins et al. (2024) study found that the most effective approach to bypassing detection targets perplexity (word choice predictability) and burstiness (sentence length variation) simultaneously. That's exactly what UndetectedGPT does — it restructures the statistical patterns that detectors measure, not just the surface-level words. This is why it achieves 96% while Undetectable AI's more superficial approach plateaus at 88%.

At $19.99/month, UndetectedGPT costs more than Undetectable AI's base price, but the results justify the difference. You're getting the highest bypass rate on the market (96% vs 88%) plus a 9.2/10 readability score that no competitor matches. You also get multiple humanization modes so you can adjust intensity depending on whether you're bypassing Turnitin for a research paper or polishing a blog post. The free tier lets you test before you buy, and unlike Undetectable AI's restrictive 250-word, 3-day trial, there's no auto-billing surprise waiting for you.

Let's get into the detector-by-detector breakdown, because the aggregate bypass rate only tells part of the story.

Against Turnitin, Undetectable AI brought our test essay down to 18% AI. Decent, but dangerously close to the 15-20% thresholds many institutions use. One bad run and you're flagged. UndetectedGPT brought the same essay down to 4% AI — well below any threshold in use. Against Originality.ai, Undetectable AI scored 15% while UndetectedGPT scored 3%. Against GPTZero, Undetectable AI's results varied between runs, while UndetectedGPT stayed consistently under 5%. On every single detector we tested, UndetectedGPT produced a lower AI score. Not by a little. By a lot.

The independent data backs this up. A head-to-head test by Gold Penguin found Undetectable AI averaging 20% AI detection across five essays — acceptable overall but with one essay scoring 100% AI. That kind of variance is a dealbreaker for anyone who can't afford a single failure. The Weber-Wulff et al. (2023) study found that all 14 AI detection tools scored below 80% accuracy, which means detectors miss things. But they miss fewer things when your text only needs to clear a 4% bar versus an 18% bar.

On pricing, the current comparison: Undetectable AI charges $19/month for 10,000 words. UndetectedGPT starts at $19.99/month — essentially the same price. But for that extra dollar you get: a 96% bypass rate versus 88%, a readability score of 9.2 versus roughly 8, and consistent sub-5% scores on Turnitin where Undetectable AI hovers around 18%. Both offer free tiers, but Undetectable AI's 250-word, 3-day trial is noticeably more restrictive and comes with billing complaints that UndetectedGPT doesn't have.

When you factor in that UndetectedGPT delivers the highest bypass rate on the market, the best readability, and a free tier to test before paying, the case for switching is straightforward. You're paying for results, not a brand name.

The right pick depends on what you actually need.

If reliability is non-negotiable — meaning you cannot afford to have a submission flagged — UndetectedGPT is the obvious choice. A 96% bypass rate with top-tier readability makes it the safest bet for academic papers, professional content, and anything screened by strict detectors. If you're a student submitting through Turnitin or a marketer publishing content that gets checked by clients, this is the tool that won't let you down. The Liang et al. (2023) Stanford study found that AI detectors flag 61.3% of non-native English essays as AI-generated — if you're an ESL writer, you need every percentage point of bypass rate you can get. See our guide to AI detector false positives for more on this issue.

If you're on a tight budget, Humbot at $12/month or WriteHuman at $18/month offer decent performance for casual use. Just know that a 72-78% bypass rate means roughly 1 in 4 submissions could still get flagged. For low-stakes content where a false flag isn't career-ending, these work. For anything academic? Not worth the risk.

[StealthGPT](/blog/stealthgpt-review) at $32/month (billed weekly) is more expensive than UndetectedGPT, and it delivers a lower bypass rate (82% vs 96%) and independent testing shows wildly inconsistent results. Originality.ai flagged its output at 100% AI in multiple reviews. The Trustpilot rating (4.0/5) is decent, but users report the tool has declined after recent updates.

[GPTinf](/blog/gptinf-review) at $9.99/month (Lite plan) is the weakest performer on this list. It achieves roughly 65% bypass in our testing, and independent tests found even worse results — Originality.ai's team found GPTinf output at 100% AI confidence across all detectors. We were also unable to find a publicly identified team or company behind the tool, which may be a consideration for users who value transparency.

Whatever you choose, always test the output yourself before submitting. Paste the humanized text into a free detector like GPTZero or ZeroGPT and verify the score. Even the best tools occasionally produce a paragraph that needs a quick manual tweak.