Why do AI deep-research reports cite URLs that don’t exist?

A citation is a promise: this page exists, and it says what I said it says. Most deep-research reports break that promise.

A University of Pennsylvania study, funded through DARPA’s SciFy program, measured citation URL validity across commercial LLMs and deep-research agents (Rao, Wong, and Callison-Burch, arXiv:2604.03173). On their DRBench set, 3 to 13 percent of cited URLs have no record in the Wayback Machine — they likely never existed. Five to 18 percent do not resolve at all.

Deep-research agents cite more, and they hallucinate more. Pooled hallucination is 10.7 percent against 4.8 percent for search-augmented models. Gemini Deep Research sits at the top of that table: 13.3 percent hallucinated, 18.5 percent non-resolving. More citations is not more proof.

The failure is architectural. A one-shot report can emit a URL from retrieval, from memory, or from a blend of the two. Some models fabricate every dead URL they emit. Others retrieve a real page that has since gone dark. The memo does not tell you which.

If you run a newsroom, a trade association, or a research desk, that is the product you are being sold: a fluent brief with a bibliography you still have to check by hand.

The rule is the same in business and in print: source it, or don’t say it. A fluent answer is not proof.

Aculeus makes Starglass, a collaborative research agent. Deep Research does not start until you approve an editable Case Plan. The Living Case keeps cited source links, the conversation, and the open questions. Nothing auto-starts or publishes automatically. A reviewed Living Case can be shared as a public, read-only record. Judge the method, not a showcase.

If a URL does not exist, it is not a source. Treat it that way.

Source. Delip Rao, Eric Wong, and Chris Callison-Burch, “Detecting and Correcting Reference Hallucinations in Commercial LLMs and Deep Research Agents,” arXiv:2604.03173, 2026. https://arxiv.org/abs/2604.03173