Undetectable AI vs. Grammarly’s AI Humanizer: What Works Best?

As AI writing tools keep getting better, we’re seeing a new wave of features pop up: not just tools to generate content, but  for making that content sound less machine-like.

One of the newest battlegrounds? AI humanizers.

Undetectable AI built its name around this idea. But now Grammarly—once just your go-to grammar checker—has stepped into the ring with its own take: an AI humanizer that rewrites robotic-sounding content into something more natural.

So how do the two stack up?

Before we get into the head-to-head, let’s break down what each tool actually does.

What is Undetectable AI?

Undetectable AI is a rewriting tool built to make AI-generated text sound more human. It doesn’t just fix tone or grammar. It specifically targets the patterns that AI detectors look for—like predictable phrasing, formal over-politeness, or flat transitions—and reworks them into something more human, less detectable.

It’s popular with students, freelancers, and anyone who wants to write faster using AI without sounding like a bot. One standout feature is the Writing Style Replicator, which not only humanizes content but also tries to mimic your own voice.

What is Grammarly?

Grammarly started out as a grammar and spelling checker—a tool you used to clean up your writing after the fact. But in the last few years, it’s evolved into a much broader writing assistant. Now, it offers AI-powered suggestions for clarity, tone, and structure, and even generates content from scratch.

Its latest feature? An AI humanizer.

Grammarly's version of a humanizer rewrites stiff or robotic text (usually from ChatGPT or similar tools) to sound more natural. It leans heavily into Grammarly’s existing tone suggestions and tries to keep the content polished, but more readable.

In theory, it shares the same goal as Undetectable AI: smoother, less detectable writing. But the way each tool goes about it—and who it’s really built for—makes all the difference.

Undetectable AI vs. Grammarly AI Humanizer

Test #1

Grammarly: Detected as AI-generated.
AI Likelihood Score: 100%

Undetectable AI: Successfully passes as human!
AI Likelihood Score: 0%

Test #2

Grammarly: Detected as AI-generated.
AI Likelihood Score: 100%

Undetectable AI: Detected as AI-generated.
AI Likelihood Score: 100%

Test #3

Grammarly: Detected as AI-generated.
AI Likelihood Score: 100%

Undetectable AI: Successfully passes as human!
AI Likelihood Score: 0.2%

Test #4

Grammarly: Detected as AI-generated.
AI Likelihood Score: 100%

Undetectable AI: Detected as AI-generated.
AI Likelihood Score: 99.5%

Test #5

Grammarly: Detected as AI-generated.
AI Likelihood Score: 100%

Undetectable AI: Successfully passes as human!
AI Likelihood Score: 0%

Overall Score

What’s Next?

So here’s what we found: Undetectable AI successfully bypassed AI detectors 60% of the time. Grammarly’s humanizer? Zero.

That doesn’t mean Grammarly’s tool is useless. It still does a solid job (better than Undetectable AI) cleaning up awkward AI writing, especially if your goal is better tone or flow. But when it comes to actually dodging detection? It doesn’t hold up.

Undetectable AI, on the other hand, is clearly built with evasion in mind. It doesn’t just smooth things out—it rewrites with purpose. That makes a huge difference when you’re trying to stay under the radar.

Of course, there’s always an ethical conversation to be had about tools like this. But if you're writing original content and just want it to sound like you—not a language model—then tools like Undetectable AI offer a pretty compelling option.

At the end of the day, both tools have value. But if the goal is to sound human and stay undetected, one is clearly built for that job. The other... still has some catching up to do.