Metric37 vs Undetectable AI vs QuillBot Humanizer

If you use AI to write, you have probably tried at least one "humanizer" tool. The market is crowded, the promises are identical, and most of them quietly produce the same mediocre output. This comparison looks at three of the most popular options — Metric37, Undetectable AI, and QuillBot's humanizer — and breaks down where they actually differ.

How Each Tool Works

The core difference is in the pipeline. QuillBot and Undetectable AI both rely on single-pass synonym swapping or paraphrasing. They take your text, substitute words, rearrange some clauses, and hand it back. Metric37 takes a fundamentally different approach: it uses a multi-LLM rewriting pipeline followed by an evaluation gate.

In practice, that means Metric37 sends your text through a large language model (with a fallback chain across providers), then runs the output through a second model that scores it for naturalness. If the rewrite does not pass the quality threshold, the system retries automatically. The result is text that reads like a human wrote it from scratch, not text that reads like a thesaurus attacked it.

Feature Comparison

Output Quality

This is where the tools diverge most. Synonym-swapping tools produce text that technically uses different words but retains the same cadence, structure, and predictability of the original AI output. They solve the "detector evasion" problem at the surface level without fixing the underlying issue: the writing still sounds like a language model.

Metric37's multi-pass approach restructures paragraphs, varies sentence length, introduces natural imperfections, and adjusts rhythm. The eval gate catches rewrites that still read like AI and forces a retry. That second layer is critical — without it, even LLM-based rewriting can produce output that drifts back toward default AI patterns.

QuillBot is best understood as a paraphrasing tool that happens to reduce some AI signals. It was not designed specifically for humanization. Undetectable AI is purpose-built for detector evasion, but its single-pass approach means it trades accuracy for speed.

Pricing

Raw pricing looks similar, but the comparison is misleading without considering output quality. If a synonym swapper produces text that still gets flagged — or worse, text that reads awkwardly and needs manual editing — the time cost erases any price advantage. The question is whether you are paying for word throughput or for writing that actually works.

Version History and Workflow

One underrated difference: Metric37 keeps a full version history for every document. You can re-humanize with a different tone, compare versions with a word-level diff, or restore an earlier version. This turns humanization from a one-shot operation into an iterative workflow.

Neither Undetectable AI nor QuillBot offer version tracking. Once you close the tab, the previous output is gone. For anyone producing content at scale — marketing teams, agencies, freelancers — that lack of history creates unnecessary friction.

Detection Evasion

All three tools reduce AI detection scores, but through different mechanisms. Undetectable AI optimizes specifically for passing detectors like GPTZero and Originality.ai, sometimes at the cost of readability. QuillBot reduces detection incidentally through paraphrasing. Metric37 reduces detection as a side effect of producing genuinely better writing — text with natural variation, imperfections, and voice.

The distinction matters because detectors keep improving. Tools that game specific detectors today may stop working tomorrow. Tools that produce authentically human-sounding prose are more durable.

Bottom Line

If your goal is the cheapest possible detector evasion and you do not care about readability, Undetectable AI or QuillBot will get you part of the way. If your goal is text that sounds like a skilled human wrote it — text you would actually publish under your name — Metric37 is the tool built for that job. The multi-LLM pipeline, eval gate, and version history exist because humanization is not a synonym problem. It is a writing problem.