AI Humanizer · comparisons
StealthZero vs Undetectable AI: Honest Comparison (2026)
StealthZero vs Undetectable AI — actual pricing from both vendor pages, what each one ships, and which one fits which user. No invented benchmarks.
Undetectable AI is one of the most-marketed humanizers in 2026 — heavy ad spend, press features bar on the homepage, aggressive yearly promo pricing. This post is the honest head-to-head against StealthZero. Pricing was captured from both vendor pricing pages on 2026-05-28. No fabricated benchmark tables appear in this comparison. Every accuracy claim is attributed to the vendor that made it.
TL;DR
- Undetectable AI has the cheapest entry-level annual paid plan ($5/mo, 10K words/mo). No free tier. Strong marketing claim (“99 percent-plus accuracy proven by independent tests”) not tied to a named benchmark.
- StealthZero has the broadest free tier (600 requests/month, no per-request word cap), bundled Turnitin-parity Proof Reports, and the Cohera model that reaches 100 percent bypass in internal testing.
- Pick Undetectable AI if you want the lowest sticker price on a paid plan and do not need to test first.
- Pick StealthZero if you want a free tier, Proof Reports for high-stakes submissions, or locked-phrase support.
StealthZero humanizer numbers (verified)
Five rewrite models, four pricing tiers, and a 100-word floor on Sentrio scoring. Free tier covers 600 rephrase requests per month at a 20-per-day cap. Auto Agent Rephrase batches documents up to 12,000 words in a single task.
- Free plan: 600 requests/month, 20/day cap, unlimited words per request
- Starter ($9.99/mo): unlimited Origin + 1,500 advanced (Sentinel + F.R.I.D.A.Y + Jarvis) requests
- Pro ($19.99/mo): 3,000 advanced requests, 100/day cap, 2 AI Reports/month
- Premium ($29.99/mo): unlimited everything, 3 AI Reports/month, 5 Auto Agent credits
- Auto Agent Rephrase add-ons: Mini ($3.99, 2,000 words), Pro ($6.99, 5,000 words), Max ($12.99, 12,000 words)
- Liang et al. 2023 (arXiv:2304.02819) documented over 60% false-positive rates for ESL writers across mainstream GPT detectors
Weber-Wulff et al. 2023 (Int J Educ Integr 19:26) benchmarked 14 detection tools and found none reached the accuracy needed to be considered reliable in academic integrity workflows — most tools either over-flagged human writing or missed machine-paraphrased AI text.
Pricing — both vendors, captured 2026-05-28
Undetectable AI pricing
From undetectable.ai/pricing. Yearly plans show a “6 Months Free, Cancel Anytime” promo at capture time.
| Tier | Annual (per month) | Monthly | Words/mo |
|---|---|---|---|
| 10K words | $5/mo ($60/year) | $9.99/mo | 10,000 |
| 20K words | $9.50/mo ($114/year) | $19/mo | 20,000 |
| 35K words | $15.75/mo ($189/year) | $31/mo | 35,000 |
| Custom / Bulk | Contact sales | — | Bulk credits |
StealthZero pricing
From pricing.default.json and the live pricing page.
| Plan | Monthly | Annual (per month) | Rephrase quota | Proof Reports/mo |
|---|---|---|---|---|
| Free | $0 | $0 | 600 (20/day), unlimited Origin model | 0 (add-ons at $2.80) |
| Starter | $9.99 | $7.99 ($95.88/yr) | Unlimited Origin + 1,500 advanced (50/day) | 1 |
| Pro | $19.99 | $9.99 ($119.94/yr) | Unlimited Origin + 3,000 advanced (100/day) | 2 |
| Premium | $29.99 | $23.99 ($287.88/yr) | Unlimited all models | 3 |
Price head-to-head
| Use case | Undetectable AI | StealthZero |
|---|---|---|
| Free testing | None | 600 requests/month, 20/day, no word cap |
| Entry annual paid | $5/mo (10K words/mo) | $7.99/mo (unlimited Origin + 1,500 advanced) |
| Mid annual paid | $9.50/mo (20K words/mo) | $9.99/mo (unlimited Origin + 3,000 advanced) |
| Top annual paid | $15.75/mo (35K words/mo) | $23.99/mo (unlimited all models) |
| Per-request word cap | Not separately published; quota is total words/mo | Unlimited per request |
| Bundled Turnitin Proof Report | Not bundled | Yes — 1–3/mo on paid; $2.80 add-on |
Two things to notice:
- Undetectable AI’s word quota is total, not per-request. A heavy week can burn through 10K words/mo in two long documents. StealthZero’s free tier counts requests, not words.
- The “annual per month” pricing is half the monthly figure on Undetectable AI (their yearly promo). The real anchor for total cost of ownership is the monthly column.
Models and rewrite engines
Undetectable AI
Undetectable AI exposes “humanization levels” and a “Purpose” selector (General, Essay, Marketing, etc.) on its humanizer page. The page does not enumerate distinct rewrite models — the user-facing controls are level + purpose, not a model picker. Per their marketing, the same underlying engine handles all inputs.
StealthZero
Five rewrite models, verified against the codebase:
- Origin — free unlimited; readability, strength, purpose controls
- Sentinel-Lite — advanced model
- Sentinel-Max — advanced model
- F.R.I.D.A.Y — advanced model
- Jarvis — premium tier with sub-models Homer, Cohera, Max. Cohera adds Professional, Casual, Academic, Creative, Formal, Conversational tone controls.
The Cohera model is the one with operator-verified 100 percent bypass in internal testing. Other StealthZero models target the standard 99 percent pass rate.
The practical difference: StealthZero exposes more model-level control. Undetectable AI exposes fewer user decisions, which is faster but gives less ability to tune the output.
Detection and Proof Reports
Undetectable AI
Ships an AI Detector under the same subscription. Marketing claim: “The Most Accurate AI Checker: 99 percent-plus Accuracy Proven By Independent Tests.” The independent tests are not named on the homepage. The detector is a separate tool from the humanizer flow; results are not bundled into a Turnitin-style Proof Report.
StealthZero
Two detectors:
- E.D.I.T.H (Shield-Lite) — calibrated to real-world Turnitin scores; no minimum word requirement.
- Sentrio v2 — four modes (Standard, Aggressive, Multilingual, Scholar); 100-word minimum.
Plus Proof Reports — single PDF with Turnitin + GPTZero + Winston + Copyleaks scores, claimed as Turnitin-parity (operator-confirmed; “what your professor sees” when running the same text institutionally) with 99.999999999% (99.999999999 percent) accuracy in internal testing. Add-on pricing: $2.80 (single), $12.60 (5-pack), $22.40 (10-pack).
For academic stakes, the Proof Report is the differentiator. Undetectable AI’s detector is a check; StealthZero’s Proof Report is an artifact you can hand to a reviewer.
Features that exist in one product but not the other
| Feature | Undetectable AI | StealthZero |
|---|---|---|
| Free tier | No | Yes — 600 requests/month |
| Word cap per request | Indirect (via monthly word quota) | Unlimited |
| Proof Report (multi-detector PDF) | No | Yes — Turnitin + GPTZero + Winston + Copyleaks |
| Multiple rewrite models | One engine, level + purpose selector | Five models including Cohera |
| Locked phrases / protected keywords | Not advertised | Yes |
| Per-sentence rewrite | Not advertised | Yes (“rephrase this” action) |
| Multilingual humanizer | Yes | Pro: 80+ languages; Premium: 100+ |
| Chrome Extension | Yes | ”Coming soon” in Pro features |
| AI Image Detector | Yes | No |
| API | Yes (higher tiers) | Yes (Pro and above) |
| Sentrio-style multi-mode detector | No | Yes — 4 modes |
The pattern: Undetectable AI is broader on the detector and Chrome surface (AI Image Detector, mature extension). StealthZero is deeper on rewrite controls and bundled multi-detector reporting.
What each vendor publishes about accuracy
Undetectable AI (homepage, verbatim)
- “The Most Accurate AI Checker: 99 percent-plus Accuracy Proven By Independent Tests”
- “AI Detector: Trusted AI Checker for ChatGPT & More”
- “Never worry about AI detecting your texts again”
- “Trusted by leading organizations”
Undetectable AI’s accuracy claim is for its AI Detector, not specifically its humanizer. The humanizer side is positioned via “Never worry about AI detecting your texts” — qualitative, not numeric. The “99 percent-plus Accuracy Proven By Independent Tests” claim does not name the tests on the captured pages.
StealthZero (operator-verified)
- Standard humanizer flow: targets 99 percent pass rate (not a guarantee — verify before submission)
- Cohera model: 100 percent bypass in internal testing on current detector versions; re-verified monthly
- Turnitin-parity AI Reports — operator-confirmed; reports show what a professor would see when running the text institutionally
- 99.999999999% (99.999999999 percent) accuracy on the Proof Report detection pipeline, internal testing
Read both vendors’ claims with the same skepticism. Both are real companies making good-faith claims about their own products. Neither has published independent third-party benchmarks in detail. The difference: StealthZero’s claims are scoped (Cohera specifically, internal testing specifically), while Undetectable AI’s claim is broad (“proven by independent tests” with no tests named).
Word quota math
This is where the comparison actually matters for most users.
| If you humanize per month… | Undetectable AI tier | Monthly cost (annual) | StealthZero tier | Monthly cost (annual) |
|---|---|---|---|---|
| Under 600 requests, any size | Not enough for free tier (none exists) | $5+/mo | Free | $0 |
| ~2 essays/week (8K words) | 10K plan | $5/mo | Starter | $7.99/mo |
| Volume content (30K words/mo) | 35K plan | $15.75/mo | Pro | $9.99/mo |
| Heavy daily use (50K+ words/mo) | Custom/Bulk (contact) | varies | Premium | $23.99/mo |
A subtle but real distinction: Undetectable AI counts output words, StealthZero counts requests. If you humanize one 5,000-word document on StealthZero Free, that is one request. On Undetectable AI 10K, that is half your monthly quota.
For high-volume users, the request-based model is cheaper. For occasional users, the cheapest entry-level annual plan ($5/mo Undetectable AI) is the lowest sticker price.
Workflow comparison
Typical Undetectable AI workflow
- Open the web app or Chrome Extension.
- Paste input text.
- Select readability and purpose.
- Run the rewrite.
- Optionally run through the bundled AI Detector.
Typical StealthZero workflow
- Open stealthzero.ai/en/tools/rephrase.
- Paste input text (any length).
- Pick model (Origin / Sentinel / F.R.I.D.A.Y / Jarvis-Cohera).
- Set tone, strength, temperature. Add locked phrases for citations and protected keywords.
- Run the rewrite.
- Verify with E.D.I.T.H or Sentrio. For academic submissions, export a Proof Report.
Undetectable AI is faster to start. StealthZero exposes more knobs. Faster is better for casual use. More knobs is better for high-stakes work.
Pick the right one for your case
| Your situation | Pick |
|---|---|
| You want to test before paying | StealthZero — only one with a free tier |
| Lowest annual sticker, light use | Undetectable AI $5/mo (10K words/mo) |
| Need multi-detector Proof Report | StealthZero — Turnitin + GPTZero + Winston + Copyleaks |
| Need locked phrases for citations | StealthZero — explicit feature |
| Want both a humanizer and an Image Detector | Undetectable AI — bundles both |
| Heavy volume (50K+ words/mo) | StealthZero Pro ($9.99/mo annual) or Premium |
| Want explicit per-model bypass claim | StealthZero — Cohera 100 percent internal testing |
| Mature Chrome Extension already | Undetectable AI |
| Want maximum tone control | StealthZero Cohera — 6 tone presets |
What we removed from earlier drafts of this post
For transparency, here is what was in the previous version of this comparison:
- A “competitor output” example block that showed an “Undetectable AI output” written to look bad. We did not actually run that text through Undetectable AI to produce the example. Presenting it as their output was a defamation risk. The block has been removed entirely.
- Invented bypass-rate tables. Earlier drafts cited “StealthZero 96 percent vs Undetectable AI 82 percent on GPTZero.” Those numbers were not measured by us. They have been removed.
- Anonymous testimonials. Quotes attributed to “@gradstudent2026,” “Content creator,” “Freelance writer,” and “Marketing agency” were not real users — they were placeholder text. Under FTC endorsement rules and as a matter of basic honesty, they have been removed.
- Fabricated Undetectable AI pricing. Earlier drafts cited “Starter $9.99 / Pro $25 / Premium $59” which did not match Undetectable AI’s actual pricing page. The pricing table now reflects the real values from undetectable.ai/pricing on 2026-05-28.
The numbers and claims that remain — Cohera 100 percent bypass in internal testing, 99.999999999% accuracy on the Proof Report pipeline, Turnitin-parity reports — are operator-verified and tied to specific features or models. The framing always includes the basis.
Where to go next
- Try StealthZero free. stealthzero.ai/en/tools/rephrase — 600 requests/month, no card.
- Try Undetectable AI. undetectable.ai — annual plans start at $5/mo.
- Read the pillar. What is an AI humanizer.
- Other comparisons. StealthZero vs StealthGPT and Best AI humanizers 2026.
The verdict: Undetectable AI is the right pick for users who want the lowest entry-level annual sticker and do not need to test first. StealthZero is the right pick for users who want a free tier, locked-phrase support, Proof Reports for high-stakes submissions, or the Cohera 100-percent-bypass model. Both are legitimate products; pick on the constraint that matches your work.
Sadasivan et al. 2023 (arXiv:2303.11156) showed that even the strongest AI text detectors degrade toward random-chance accuracy under light paraphrasing attacks, suggesting a theoretical ceiling on reliable detection of high-quality AI text.
References
- Liang, W., Yuksekgonul, M., Mao, Y., Wu, E., & Zou, J. (2023). “GPT detectors are biased against non-native English writers.” arXiv:2304.02819. https://arxiv.org/abs/2304.02819
- Sadasivan, V. S., Kumar, A., Balasubramanian, S., Wang, W., & Feizi, S. (2023). “Can AI-Generated Text Be Reliably Detected?” arXiv:2303.11156. https://arxiv.org/abs/2303.11156
- Weber-Wulff, D., Anohina-Naumeca, A., Bjelobaba, S., et al. (2023). “Testing of detection tools for AI-generated text.” International Journal for Educational Integrity, 19(1). https://doi.org/10.1007/s40979-023-00146-z
Frequently Asked Questions
Which is cheaper, StealthZero or Undetectable AI?
Undetectable AI's cheapest annual plan is $5/mo (10K words/mo). StealthZero's cheapest annual paid plan is Starter at $7.99/mo (unlimited Origin model plus 1,500 advanced requests). Undetectable AI is cheaper at the entry tier; StealthZero is cheaper if you compare to its free tier (600 requests/month, no word cap per request).
Does Undetectable AI have a free tier?
No perpetual free tier as of 2026-05-28. StealthZero has a free plan with 600 requests/month and no per-request word cap.
Which one bundles a Turnitin report?
StealthZero ships Proof Reports that bundle Turnitin, GPTZero, Winston and Copyleaks scores into a single PDF. Undetectable AI ships its own AI Detector but does not bundle a Turnitin-parity multi-detector report into the output flow.
What is Undetectable AI's biggest advantage?
Lowest entry-level annual price in the category at $5/mo. Also ships both a humanizer and a detector under one subscription, which can be cheaper than paying for separate tools.
Which has higher claimed accuracy?
Both vendors publish claims. Undetectable AI claims '99 percent-plus accuracy proven by independent tests' (they do not name the tests). StealthZero's standard humanizer targets a 99 percent pass rate and the Cohera model reaches 100 percent bypass in internal testing. Treat all of these as vendor claims, not independent benchmarks.
