Undetectable AI vs. Grammarly’s AI Humanizer: What’s Better with ChatGPT?

AI-generated writing has come a long way — and now the tools that make it sound less like AI are catching up too.

We’re not just talking about grammar checkers or tone polishers. We’re talking about full-on “humanizers” — tools that take AI-written content and make it feel like a person actually sat down and wrote it.

Undetectable AI has been focused on this space from the beginning. Grammarly? Not so much. But with its new AI humanizer, Grammarly is stepping into that lane. So we figured it’s worth asking: which one actually does a better job at making AI writing feel human when paired with ChatGPT?

Let’s break it down.

What is Undetectable AI?

Undetectable AI isn’t built to make your writing sound prettier — it’s here to make your AI-generated content sound undetectable.

That means it doesn’t just rewrite for tone or grammar. It targets the exact markers that trigger AI detection tools: overused phrases, overly structured transitions, and that telltale “machine voice” most large language models can’t shake. 

It’s mostly used by students, writers, and marketers who want the speed of AI without getting flagged for it. And it does that job well. Especially with features like the Writing Style Replicator, which lets you train the system to sound like you, not just a generic human voice.

So if your goal is to pass AI detection — or at least fly under the radar — this tool was literally designed for that.

What is Grammarly?

Grammarly used to just catch your typos. But today, it’s trying to be your full-on writing partner. And to be fair, it’s come a long way.

Today’s Grammarly can suggest rewrites, change tone, and even generate content from scratch. The new AI humanizer is its answer to the growing need for less robotic-sounding content — the kind of output that doesn’t immediately scream “ChatGPT wrote this.”

But here’s the thing: Grammarly isn’t really trying to hide the AI-ness of your writing. It’s more about smoothing it out. The humanizer leans into friendly, easy-to-read polish — less for evasion, more for engagement.

That makes it great for emails, blogs, and social posts. But if your goal is to avoid detection? Grammarly’s not the tool for that.

Undetectable AI vs. Grammarly: Head-to-Head with ChatGPT

Test #1

Undetectable AI: Successfully camouflaged as human-written.
AI Likelihood Score: 0%

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

Test #2

Undetectable AI: Detected as AI-generated.
AI Likelihood Score: 64.4%

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

Test #3

Undetectable AI: Successfully camouflaged as human-written.
AI Likelihood Score: 0%

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

Test #4

Undetectable AI: Successfully camouflaged as human-written.
AI Likelihood Score: 0.7%

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

Test #5

Undetectable AI: Successfully camouflaged as human-written.
AI Likelihood Score: 0%

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

Overall Score

The Bottom Line

If you’re just trying to improve tone and polish, Grammarly’s AI humanizer is fine. It makes things sound better, clearer, friendlier — which is exactly what most users want.

But if your goal is to rewrite AI content so that it doesn’t feel like AI at all, Grammarly just doesn’t go far enough, for now.

Undetectable AI is a tool with a very specific purpose — and that focus shows. It’s not just a humanizer. It’s a strategist. It understands what makes content sound synthetic and actively works against it.

Bottom line: Grammarly makes AI content sound better. Undetectable AI makes it sound real.

Two different tools. Two different goals. It just depends on what you’re aiming for.