How to check an AI answer for hallucinations
- Paste the answer from ChatGPT, Gemini, Claude, Copilot or any other assistant, including its links and reference list. You can also click Upload file to load a .txt, .pdf or .docx document.
- Click Check for hallucinations. The tool finds every source in the text and checks them three at a time; each one turns green, red or amber as its result comes in.
- Read the summary at the top: how many sources were verified, how many were not found and how many could not be checked.
- Work through Claims to fact-check, the list of sentences most likely to contain invented facts.
What is sent where
Your text stays in your browser. Only the items the tool finds are looked up: links and arXiv IDs through our link checker, DOIs and reference lines in Crossref (the public database of scholarly publications), and ISBNs in Open Library. The sentences around them, and the rest of your text, are not sent anywhere.
What gets checked
| Found in the text | How it is checked |
|---|---|
DOIs, such as 10.1038/nature14539 | Looked up in Crossref, which returns the real title, journal and year. If Crossref does not have it, the tool checks whether doi.org can resolve it; a DOI that is not registered anywhere is marked as not found. |
| Links (URLs) | Our server requests the page. HTTP 404 or 410 means the page does not exist. HTTP 401, 403, 429 or a 5xx error means the site refused or failed to answer, so the link is marked as could not be checked. |
arXiv IDs, such as arXiv:1706.03762 | The matching arXiv page is requested to see whether the paper exists. |
| ISBNs | The check digit is validated first: an ISBN with a wrong check digit cannot belong to any book. Valid numbers are then looked up in Open Library. |
Reference-list lines, such as Author, A. (2019). Title. Journal | Searched in Crossref. It counts as a match when about 70% of the words in the found title appear in your reference line. |
Reading the results
Each source gets one of three outcomes:
- Verified: the source exists. For DOIs and references the real title is shown, so you can compare it with the citation.
- Not found: the page returned 404 or 410, the DOI is not registered, the arXiv ID does not exist, or the ISBN is invalid. Invented DOIs and links to pages that never existed are classic hallucinations, and any red item deserves suspicion.
- Could not be checked: the site blocked our request, the reference was not found in Crossref, or a valid ISBN is missing from Open Library. This is not evidence of a hallucination. Search for the source yourself.
Claims to fact-check
Language models most often hallucinate in specific details, so the tool lists sentences that contain them: statistics and percentages, amounts of money, dates, phrases such as "according to", superlatives like "the first", "the largest", "always" and "never", and quotes attributed to a person. It cannot tell whether they are true; it shows you where to look. The tool also flags knowledge-cutoff disclaimers such as "as of my last training update", a reminder that the answer may be out of date.
The most important limitation
This tool confirms that a source exists, not that it supports the claim. A common hallucination cites a real paper for a number that appears nowhere in it. A green tick means "this paper is real", nothing more. Before you rely on a claim, open the source and find the passage that supports it.
Crossref covers most journal articles but misses many books, reports, theses and conference papers. A reference marked as could not be checked may be perfectly real. If you cannot find it in a library catalogue, Google Scholar or the publisher's website either, treat it as invented.
Tips
- Ask the chatbot to give a DOI or link for every claim. Specific identifiers can be checked; vague mentions of "a 2023 study" cannot.
- For a link marked as could not be checked, open it in your browser. The Redirect Checker shows where a link really leads.
- To check the references in a PDF paper or report, convert it with PDF to Text and paste the reference list.
Frequently asked questions
What is an AI hallucination?
A hallucination is a statement that a language model presents as fact but that is false or unsupported: an invented study, a fake quote, a wrong date or a link to a page that never existed.
Does it check whether the facts are true?
No. It checks whether cited sources exist and lists the sentences you should verify. Judging whether a claim is true still needs a person reading the source.
Is my text sent to a server?
Only the links, DOIs, arXiv IDs, ISBNs and reference lines it finds are looked up, via our link checker, Crossref and Open Library. The rest of your text stays in your browser.
Why does a real website show "could not be checked"?
Many sites block automated requests or limit how often they answer, which returns codes like 403 or 429. That says nothing about whether the page exists, so open it yourself.