Lexis AI vs. Westlaw AI: Features and Test Results

Lexis AI vs. Westlaw AI comes down to two product families. LexisNexis sells Lexis+ with Protégé, the product formerly called Lexis+ AI. Thomson Reuters sells AI inside Westlaw, now led by Westlaw Advantage, alongside its CoCounsel assistant. Both promise answers grounded in their own editorial content. The only rigorous independent test of both found real differences, but it is now more than two years old.

This comparison uses each vendor’s own product pages, checked October 24, 2026, and the Stanford study that tested both. For what the tools cost, see our guide to what legal AI tools cost.

What Each Product Includes

LexisNexis says on its Lexis+ with Protégé page that Lexis+ AI was renamed Lexis+ with Protégé in February 2026. The page describes drafting of transactional agreements and litigation documents, research grounded in LexisNexis content and the customer’s own documents, and a Verify feature that checks citations in AI responses and drafts and flags unsupported statements. It also lists reusable AI agent skills, including a library of skills from LexisNexis and Claude, review tables for extracting terms across large document sets, shared Workrooms, document Vaults, and connections to iManage, SharePoint and NetDocuments. LexisNexis says it never uses customer data to train AI models.

Thomson Reuters splits its offer in two. Its Westlaw Advantage page describes Deep Research, an agentic feature that plans and performs multistep research and delivers a report with sources. The same page says Westlaw Advantage differs from Westlaw Edge with AI-Assisted Research by running multistep processes without being prompted at each step. Its CoCounsel Legal page describes an assistant that reasons from Westlaw content, Practical Law guidance spanning more than 100 jurisdictions and the firm’s own documents. It lists connections to iManage, SharePoint, HighQ, Smokeball and Box, use inside Word, Outlook and Teams, and a CoCounsel Legal MCP for use within Claude.

On paper, the two now look alike: agentic research, drafting, document review and links into the firm’s document systems. The differences are in the content underneath and in how each performs.

What the Stanford Test Found

The Stanford study, published in 2025 in the Journal of Empirical Legal Studies, ran more than 200 preregistered queries through Lexis+ AI and Westlaw AI-Assisted Research, among other tools. Lexis+ AI was the highest performer, answering 65% of queries accurately. Westlaw AI-Assisted Research was accurate 42% of the time and hallucinated nearly twice as often as the other legal tools tested. Across the legal tools, hallucination rates ran from 17% to 33%.

The dates matter. The researchers ran the Lexis+ AI queries between March 22 and April 22, 2024, and the Westlaw queries between May 23 and 27, 2024. The paper notes that LexisNexis released a second-generation tool after testing and that the results do not speak to it. Both vendors have since shipped new products, including Protégé and Westlaw Advantage, that this study did not test. Treat the results as evidence that grounded legal AI still makes errors, not as a current ranking.

Pricing and Data Terms

Neither vendor publishes a price for its AI tier. The LexisNexis law firm store describes Lexis+ with Protégé as all of Lexis+ plus generative AI and directs buyers to call. Thomson Reuters sells CoCounsel Legal through an online plans page with one-, two- and three-year terms, and says longer terms typically save money, but the figures sit inside a configurator. Ask both for quotes covering the same users, content and term.

On data, the LexisNexis page states the no-training commitment directly. Ask Thomson Reuters for its equivalent terms in writing, along with retention periods for prompts and uploaded documents. Our guide to ABA Formal Opinion 512 covers why those terms matter.

Common Questions

Is Lexis+ AI the same as Lexis+ with Protégé?

Yes. LexisNexis says Lexis+ AI was renamed Lexis+ with Protégé in February 2026.

Is Lexis AI or Westlaw AI more accurate?

In the Stanford study, Lexis+ AI was accurate on 65% of queries and Westlaw AI-Assisted Research on 42%. Both products have changed since that 2024 testing.

What is the difference between Westlaw Advantage and Westlaw Edge?

Thomson Reuters says Westlaw Advantage adds agentic AI that runs multistep research processes, beyond Westlaw Edge with AI-Assisted Research.

Is CoCounsel part of Westlaw?

They are sold together and separately. Thomson Reuters lists Westlaw Advantage with CoCounsel Essentials as one option and CoCounsel Legal as its full assistant.

The Verdict

Partly Real. Both products are real and increasingly similar on features, and the one independent test favored Lexis. But that test predates the current versions, so firms should run their own side-by-side trial on their own research questions before choosing.