GPT-5.1 detector
What the GPT-5.1 detector does
The GPT-5.1 detector reads a passage and reports the writing signals in it: the vocabulary, the sentence shapes, the structural habits that keep turning up in machine-drafted prose. It is a server tool. Your text is posted to our server, one model reads it, and the answer comes back as sentences you can read rather than a number.
You get 5 free runs in any rolling 24 hours. There is no account, no sign-up and no file upload. The box takes 1 to 12,000 characters, and anything outside that range is refused before a run is counted.
What it will not do is tell you a passage came from GPT-5.1 rather than GPT-5, or rather than a person writing carefully. The instruction this check runs under says to describe observable signals and their limits and never to claim which model wrote a text. That is the honest shape of the tool, and the rest of this page explains it.
How it works
How to run a GPT-5.1 check in three steps
Paste, run, read. There is nothing to configure first.
- Step 1
- Paste the passage. Anything from one character to 12,000 is accepted, though a paragraph or two gives the check something real to work with.
- Step 2
- Press the run button. The text goes to our server once and is read by one hosted model at a low temperature setting, so repeat runs on the same text land close to each other.
- Step 3
- Read the findings and the reasons attached to them, then judge them against what you already know about where the text came from.
The signals
What AI writing signals look like in practice
Wikipedia's editors keep a public guideline on signs of AI writing, built from years of cleaning up submitted drafts. It is the most specific list anyone publishes, and it is worth knowing before you read any detector result, because most of it you can check with your own eyes.
- Vocabulary
- Words that appear far more often in generated prose than in ordinary writing: delve, underscore, foster, crucial, intricate, pivotal, tapestry, and sentences that open with additionally.
- Dodging the word is
- Serves as, stands as, represents, functions as and boasts, used where a plain is would do the job.
- Negative parallelism
- Not just X, but also Y. Not X, but Y. Once you have noticed the shape you see it everywhere.
- Sourceless attribution
- Industry reports, experts argue, observers have cited, with no report, expert or observer ever named.
- Groups of three
- Three-item lists appearing where the subject does not actually come in three parts.
- Formatting habits
- Heavy bold inside body text, curly quotes where straight ones belong, emoji used as section separators, and a high em dash count.
The honest answer
Can a GPT-5.1 detector tell 5.1 from GPT-5?
No, and this page is not going to pretend otherwise. Identifying a version would require that version to leave a signature in ordinary prose that its neighbours do not leave, and nobody has published evidence of one. Guides written for people who have to act on a result say the same thing: no detector can prove model origin with certainty.
Our check refuses it at the instruction level rather than as a matter of taste. The system prompt behind this family of routes tells the model never to claim certainty about authorship or about which AI wrote a text, and to describe observable signals and limitations instead.
GPT-5.1 detector is what people type, so the page is written for that phrase. What you actually get is a signal report on your sample.
Getting it wrong
When a GPT-5.1 detector result is wrong
The most reliable way to get a misleading result is to paste writing that is formulaic for human reasons. One detector's own documentation names the categories plainly: academic definitions, legal copy and structured templates. That prose is repetitive and low in variety because the genre demands it, not because a model produced it.
Writing by non-native English speakers runs into the same trap. A smaller working vocabulary and steadier sentence construction look a lot like the pattern detectors read as machine output.
So a finding is a reason to look closer, never a reason to accuse. Nothing here is proof of authorship, and no consequence should rest on a single run of any checker.
Sample size
Why a short sample gives a thin result
The server accepts a single character. That is a technical floor, not a useful one. Signals like uniform sentence length or repeated transitions only exist across several sentences, so a one-line sample gives the check almost nothing to describe.
Detector pages that publish a recommendation tend to ask for at least 80 words and prefer 300 or more. That is a sensible way to think about your sample here too: a few full paragraphs read better than a headline.
The ceiling is 12,000 characters, roughly two thousand words. A longer document goes through in sections, and a representative section usually tells you what the whole would.
Limits
The limits of this free GPT-5.1 checker
Five runs per rolling 24 hours, counted from when you used them rather than reset at midnight. The count is kept against a hashed form of your connection, because there is no account to attach it to.
One run is one model call and one written answer. There is no percentage score, no per-sentence highlighting, no PDF upload, no batch mode and no API. If you need any of those, this is not the tool.
We have no affiliation with OpenAI, and nothing here predicts what any other checker will say about the same text.
In use
Who runs a GPT-5.1 detector, and when
Teachers with a submission that reads oddly. Editors deciding whether to hand a commissioned piece back. Hiring managers reading a cover letter that sounds like every other cover letter. In all three cases the check is the start of a conversation, not the end of one.
The other half of the traffic is writers checking their own drafts, which is the use we would defend most readily. If your writing has drifted into the vocabulary and the three-item lists described above, it is better to find out before a reader does.