Hallucination
Also: hallucinations, hallucinate
When the model states something that sounds right but is made up, including cases, quotes, and facts that do not exist.
“In Harper v. Lindqvist (2019 ONCA 412), the Court held that notice must be in writing.”
No such case exists. The name, the year and the citation all look right.“Clause 14.2 of the lease (page 6) says notice must be in writing.”
You can open page 6 and look.The model does not know which of these it has done. It produces the most plausible-sounding text, and a fake case is very plausible-sounding text. The only defence is to open the source.
The fuller definition
Hallucination is a common term for generated content that is false, fabricated, or unsupported by the relevant evidence, despite being presented plausibly.
Go deeper
Delivery may be possible in the first week of April, subject to inspection.
Delivery was promised on 1 April.
The output changes certainty, invents a specific day, and loses a condition. A link to the real source makes checking possible; it does not make that summary accurate.
A concrete example
An assistant invents a case citation or converts a conditional proposal into an unconditional commitment.
The distinction that matters
A confident tone, low temperature, or accompanying citation does not establish accuracy.
Related terms
Grounding
Basing an answer on a named source or set of facts. A link to a source does not prove the answer follows from it.
Reliability & securityEvaluation (eval)
Testing an AI tool on a set of tasks to see how well it actually does.
Documents & retrievalCitation
A pointer to the source that supports a statement, so someone can check it.
Where it is taught
Test an AI workflow on work you can assessSource and further reading
Anthropic · Evaluating AI agentsThe definition and example are written for this library. The link gives background, official documentation or legal authority as labelled. Legal scope depends on the jurisdiction and current text; product features depend on release and settings.