Lexis+ AI vs Westlaw AI

Lexis+ AI from LexisNexis and the AI features in Thomson Reuters' Westlaw add generative AI to the two dominant US legal research databases. LexisNexis launched Lexis+ AI commercially in late 2023 and later added a personalized assistant called Protégé. Thomson Reuters introduced AI-Assisted Research in Westlaw Precision in late 2023. The same year it acquired Casetext and its CoCounsel assistant, which it then folded into its product line. Both rely on retrieval-augmented generation (RAG). Instead of answering from what a language model memorized, the system searches the vendor's own collection of cases, statutes, regulations and secondary sources. It passes the passages it finds to the model and asks it to write an answer that cites them. Each vendor also draws on its own editorial assets: Westlaw: its headnotes, the West Key Number System and KeyCite, plus practice content in Practical Law; and Lexis: its headnotes and Shepard's, plus practice content in Lexis Practical Guidance. Citations link back to the source documents, and citator signals help show whether an authority is still good law. Grounding reduces made-up answers but does not eliminate them. A 2024 study by Stanford researchers found that both Lexis+ AI and Westlaw's AI-Assisted Research gave incorrect or misgrounded answers on a meaningful share of test questions. Westlaw's tool erred more often in that test, and the vendors disputed parts of the method. The subtler failure is misgrounding: citing a real case for a point it does not support. The practical conclusion is to choose between them based on your own testing, not marketing. Build a set of questions from your practice area whose answers you already know. Include: jurisdiction-specific questions; recent changes in the law; questions built on a false premise; and issues where the controlling authority is obscure. Then compare accuracy, whether each tool finds the controlling authority, how easily you can check its citations, and how it handles uncertainty.