What is an AI hallucination?

Ask a model a question it cannot really answer and it often answers anyway, in the same calm, confident voice it uses for everything else. A wrong date, a quote nobody said, a court case or a book that does not exist. That is a hallucination.

A real example

In May 2025 researchers asked the DeepSeek app, then running DeepSeek-V3, for the birthday of one of the researchers, and told it to answer only if it knew. In 3 tries it gave 3 different dates: 03-07, 15-06 and 01-01. All 3 were wrong. The real date is in autumn.

It did not say "I don't know". It guessed, 3 times, with confidence.

Why it happens

A model is not a database. It does not keep copies of what it read in training. It keeps a huge set of numbers, its weights, that let it predict the next piece of text. Facts that appeared many times in that text come out reliably. A fact that appeared once, like one person's birthday, is close to a guess.

The same research found a second cause: the way models are trained and tested rewards a guess over an honest "I'm not sure". On a test that marks a blank as wrong, guessing scores better on average. So the model learns to guess.

How to catch it

The model's tone tells you nothing. A wrong answer sounds exactly as sure as a right one. What helps:

• Ask for sources, then open them. A model with web search can cite the pages it read. A citation you have not opened is not a check.
• Give it the material. Paste the document or connect it to your files, so it answers from text in front of it rather than from memory. This is the idea behind RAGRAGA way to answer from your own documents. A search finds the passages that match the question, and they are added to the prompt before the model writes..
• Be most careful with rare facts. Names, dates, figures, quotes and references are where guessing hides.

What to do with this

Treat a model's answer as a confident first draft. For anything you will repeat, sign or send, check the specific facts against a source you can see. Making the model check a claim against sources reduces hallucinations. It does not end them.