Do LLM Outages Affect the Stock Market?

Is growing investor/trader use of large language models (LLM) extinguishing known stock return anomalies? In their March 2026 paper entitled "Do LLMs Make Markets More Efficient?", Runjing Lu, Yongxin Xu and Luka Vulicevic examine how use of LLMs is affecting reactions of individual stocks to recent newsworthy events with and without outages of LLMs from three major providers (ChatGPT, Claude and Gemini). Together, these three account for nearly 80% of LLM usage. They classify outages as (1) any, (2) single-provider severe or (3) multi-provider, as documented by each provider. They focus on outages that coincide with news releases and persist beyond the NYSE close. They use RavenPack Event Sentiment Scores for articles from the Dow Jones Newswire that have ticker-specific relevance scores above 75. They control for time-varying stock/firm characteristics, past returns, new type and calendar effects. They measure daily abnormal stock returns relative to those of a characteristic-matched benchmark portfolio. Using daily outage, stock/firm and news/sentiment data during March 2023 through November 2025, they find that: