Lexis+ AI, CoCounsel and Westlaw AI: Is There an API?

Short answer: It depends on the product. LexisNexis lists a Protégé API, which gives programs access to the AI assistant behind Lexis+ with Protégé (the product that used to be called Lexis+ AI), and you ask for access through a form. For Thomson Reuters's CoCounsel and for Westlaw's AI features, we found no public developer API on any page we could read. What Thomson Reuters offers instead is an add-on for Claude that it calls a pilot, open to CoCounsel Legal subscribers, plus contract-only APIs for some Westlaw data.

TL;DR

• Lexis+ AI is now Lexis+ with Protégé. LexisNexis made the change in February 2026. Its Lexis APIs page lists a Protégé API with "Ask" and "Summarize" functions.
• Access to the Protégé API is by request. Approved teams get credentials for the LexisNexis developer portal, where the documentation and a test area sit.
• CoCounsel has no public developer API that we could find. Thomson Reuters offers a connector for Claude, labeled a pilot, for people who already subscribe to CoCounsel Legal.
• Westlaw's AI features live inside the Westlaw app. Thomson Reuters's separate APIs for Westlaw data are sold by contract. See our guide to the Westlaw API.
• "API" can mean three different things here. It can mean an API for the AI assistant, an API for the data under it, or a connector that lets another AI tool call the assistant. Ask which one you are being offered.

Why the names are confusing

Three things changed names or owners in the last few years, and search results mix the old and new names.

CoCounsel began at Casetext. Casetext launched CoCounsel on March 1, 2023, according to a Thomson Reuters investor presentation. Thomson Reuters signed a definitive agreement to acquire Casetext on June 26, 2023, and announced it had closed the deal on August 17, 2023, for $650 million in cash. Its release lists CoCounsel as one of Casetext's key products. The CoCounsel name has since grown. Thomson Reuters now uses it for tax and audit products as well as the legal one, and it launched CoCounsel Legal, with a research tool called Deep Research, on August 5, 2025.

Lexis+ AI was renamed. LexisNexis's product page says: "As of February 2026, Lexis+ AI has been renamed Lexis+ with Protégé." Protégé is the name of the AI assistant inside it. If you read an older article about "Lexis+ AI," it is the same product line.

Westlaw has tiers. Thomson Reuters's Westlaw page currently lists Westlaw Edge, Westlaw Edge with AI-Assisted Research, and Westlaw Advantage, which adds Deep Research. CoCounsel Legal is sold next to them.

A short history of the assistants

The assistants arrived fast, and the date order helps you read the claims around them.

Casetext was founded in 2013, according to Thomson Reuters's acquisition release. That same release calls CoCounsel "an AI legal assistant powered by GPT-4." GPT-4 is a large language model from OpenAI, the kind of software that writes answers in plain sentences. LexisNexis followed on October 25, 2023, when it announced general availability of Lexis+ AI for US customers. On January 27, 2025 it announced the US general availability of Protégé, a personalized assistant that uses AI agents (programs that carry out several steps toward a goal on their own).

In between came a change that has nothing to do with law. On November 25, 2024, Anthropic published the Model Context Protocol as an open standard. Its announcement described the problem it wanted to fix: "Every new data source requires its own custom implementation, making truly connected systems difficult to scale." Before a standard like that, a company that wanted its content inside someone else's AI tool had to build a custom connection each time. A standard connector makes that cheaper, which is why connectors have become a common way to reach an assistant.

A connector is still not the same as an API for building your own product. The first lets a person use the assistant from inside another tool. The second lets your software call a documented service. The difference shapes what you are allowed to build.

Product by product

Lexis+ AI (Lexis+ with Protégé)

LexisNexis's Lexis APIs page has a section titled "LexisNexis Protégé API," marked "NEW!" and described as "Programmatic access to generative AI features and tasks." It says "secure endpoints expose Protégé's core 'Ask' and 'Summarize' functionality, returning authoritative answers and citations drawn from LexisNexis Primary Law, Secondary Materials, Practical Guidance, and more." (Endpoints are the web addresses a program calls.)

On access, the page says: "You can request access through the form on this page. Once approved, your team will receive credentials and onboarding details for the LexisNexis Developer Portal, where you can explore documentation and test endpoints in a sandbox environment." On cost, it says "API access and pricing depend on the specific products and data volumes your organization needs," and it publishes no price.

So an API exists, and it is real, but it is gated. We read the public parts of the LexisNexis developer portal and found no reference pages for the Protégé API. That fits the page's own description: documentation arrives after approval. For the wider LexisNexis catalog, see our guide to the LexisNexis API.

CoCounsel

We found no CoCounsel developer API, no public reference pages and no sign-up for keys on any Thomson Reuters page we could read. That comes with a limit. Thomson Reuters's developer portal is a JavaScript application whose catalog our tools could not read, so a person with a browser should check it. In 2024 Thomson Reuters wrote that as its products become more AI-centric, "APIs will play a crucial role in delivering advanced 'AI Skills' directly into customer workflows." That is a stated direction. It does not say an API for CoCounsel exists today.

What Thomson Reuters does publish is a CoCounsel Legal MCP connector for Claude. (MCP, the Model Context Protocol, is a standard way for an AI tool to call an outside service.) Its help page says:

"Thomson Reuters offers this connector as a pilot and may modify, suspend, or withdraw it at any time, in whole or in part, without notice."

You need "an active Thomson Reuters account with a CoCounsel Legal package subscription." The page says the connector "deliberately exposes a single tool: Ask CoCounsel," plus support tools to start a session, upload files and fetch results. So you can ask CoCounsel for work from inside Claude. Building your own product on CoCounsel would need a documented interface, and we found none.

Thomson Reuters's August 20, 2026 release on the next generation of CoCounsel Legal mentions an "expanded CoCounsel Legal MCP with Claude in August" and a "forthcoming MCP connection" to Amazon Quick. Both are connectors for AI tools. Neither is described as a developer API.

The connector page also sets a rule you should know about: "These terms prohibit the use of Thomson Reuters Content with another Professional AI Tool, either directly or through an interface such as an MCP." If you are building an AI product, that matters.

Westlaw's AI products

Westlaw's AI features, including AI-Assisted Research and Deep Research, are described on Thomson Reuters's product pages as parts of the Westlaw app. We found no page that describes an API for them. The Westlaw Advantage page describes features and sales contacts, and describes no API.

Thomson Reuters does sell APIs for some Westlaw data, such as Westlaw US Litigation Analytics, through its developer portal. The company's own announcement says keys come from "sales for a subscription." That is a data route, and it is covered in our guide to the Westlaw API.

The table

What to check before you choose

Start with what you want to build, because the three meanings of "API" lead to different products.

If you want an AI assistant inside your own tools, ask the vendor:

1. Is there an API for the assistant itself, and is the documentation open or only after approval?
2. May my product show the answers to my customers, or only to my own staff?
3. Do the terms allow the content to be used with other AI tools?
4. How are answers cited, and can I check each citation against the source?
5. What are the price, the limits and the support terms?

If you want the law itself to ground your own AI, you need the source text, found by citation, and a clear licence. Ask the questions in our ten questions to ask a legal data vendor. A primary law data API, such as Vaquill AI's for US statutes, regulations, court rules and agency guidance, returns the text and a link to the official source. Our guide to US law data maps the options.

If you only want to know what an assistant does with your data, our guide on whether Westlaw and Lexis train AI on your data covers it. For reviews of the products themselves, see our reviews of Lexis+ AI and CoCounsel.

FAQ

Does Lexis+ AI have an API?

LexisNexis lists a Protégé API for the AI assistant behind Lexis+ with Protégé, the new name for Lexis+ AI. It offers "Ask" and "Summarize" functions. You request access through a form, and approved teams get portal credentials.

Does CoCounsel have a public API?

We found none on any Thomson Reuters page we could read. Thomson Reuters publishes a CoCounsel Legal connector for Claude, which it calls a pilot, for people with a CoCounsel Legal subscription. The developer portal's catalog needs a browser, so check it directly.

Who owns CoCounsel?

Thomson Reuters. CoCounsel began as a Casetext product. Thomson Reuters signed an agreement to buy Casetext on June 26, 2023 and closed the $650 million cash deal on August 17, 2023.

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

Yes. LexisNexis says Lexis+ AI was renamed Lexis+ with Protégé as of February 2026. Protégé is the AI assistant inside it.

Does Westlaw's AI have an API?

We found no API for Westlaw's AI features, such as AI-Assisted Research or Deep Research. Thomson Reuters offers separate APIs for some Westlaw data by contract, and its CoCounsel connector for Claude can reach Westlaw-grounded research for subscribers.

Where do I ask for access?

For LexisNexis, use the request form on its Lexis APIs page. For Thomson Reuters, its developer announcement says to connect with its sales team, and CoCounsel connector questions go to its CoCounsel support team.

Can I use Thomson Reuters content in another AI tool?

Its connector terms say no for another professional AI tool, including through an MCP. Read your own contract for anything broader.

For readers who need the law itself behind their AI, see the US primary law API.

Priyansh Khodiyar

Co-Founder & CTO

Priyansh leads engineering and AI at Vaquill AI: the pipelines that pull statutes, regulations and court rules from every US jurisdiction's official publisher, and the REST API, MCP server and open dataset that serve them.