Does QuillBot get flagged by AI detectors?
Paraphrasing changes the words. Detectors mostly read the rhythm, which paraphrasing leaves alone or makes worse.
The assumption behind every paraphrasing tool is that a detector reads word choice, so changing the words changes the verdict. That is half right, and the half it gets wrong is the half that carries most of the signal.
What a paraphraser changes
Vocabulary and local phrasing. It swaps words for near-synonyms and reorders clauses inside a sentence.
What it generally does not change is the shape of the text: how long each sentence is relative to the others, how the paragraphs are structured, and how evenly the whole thing runs.
That shape is the strongest signal most detectors read. Human academic writing in our corpus varies its sentence length with a standard deviation of 8.7 words. Generated text sits at 6.4. A paraphraser working sentence by sentence preserves the sentence boundaries it was given, so the number barely moves.
Where it can make things worse
Uniform output. A paraphraser applies the same transformation everywhere, which can flatten the natural unevenness of writing that was already yours. Text that went in varied comes out smoother.
Odd word choices. Reaching for a less common synonym produces phrasing a person would not have chosen. That reads strangely to a reader, and it does not help with a classifier that is reading structure rather than vocabulary.
A second uniform layer. Human writing passed through a paraphraser has one person's irregularity replaced by one tool's consistency.
What actually moves a score
Structure, and it takes deliberate work rather than a button.
Sentence length variation. Split a long sentence, merge two short ones, put a four-word sentence next to a thirty-word one. This is the single highest-leverage change available.
Connective openings. "Moreover", "Furthermore", "Additionally" open 2.4% of human sentences in our corpus against 5.8% of generated ones. Most can be deleted with nothing lost.
Specific detail. Name the method, give the number, use the real example. Generalisation is the habit that reads as generated, and paraphrasing tends to increase it rather than reduce it.
Full list in what makes text read as machine written.
The other thing worth knowing
Running your own writing through a paraphraser can raise the score on text you wrote yourself, for the reasons above. People reach for these tools when they are worried about a flag, and the tool can move them in the wrong direction.
If that has happened, the fix is the same list. See do paraphrasing tools beat AI detection.
Checking either version
Run the original and the paraphrased version and compare per paragraph. That tells you what the tool actually did to your text rather than what it claims to do.
Ours is free and unlimited with no account, has no word cap, scores every paragraph separately, and never stores your text. Our wrong-flag rate on genuine human writing is 0.4%, published, against Turnitin's 4%.