Lenz Fact-Check

Lenz Fact-Check

The Lenz MCP server: check the factual claims in a draft or an answer against independent sources, from any MCP client.

Website · Get an API key · Developer docs

One-click install (OAuth — no API key needed).

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Lenz is a fact-checking platform. It takes a factual claim, runs it through a multi-model pipeline (reading the claim → research → debate → panel review → conclusion) against independent sources, and returns a verdict with a confidence level. This is the official remote MCP server — a hosted Model Context Protocol endpoint that exposes Lenz's fact-checking as tools your AI assistant can call.

It verifies claims against the open web, independent of whatever context your model was given — so it complements retrieval/groundedness checkers rather than replacing them.

• Hosted, no install: point your client at https://lenz.io/mcp. Nothing to run locally.
• Auth: OAuth (no key — for clients that support it) or a free Lenz API key (Authorization: Bearer lenz_…).
• Transport: Streamable HTTP, stateless. Supports the 2025-11-25 and 2026-07-28 protocol revisions.

Tools

Language. Lenz answers in the language of the text it is given, so pass the claim in the user's own words and language, not translated. The language argument is for a user who explicitly asks for the answer in another language (one of en, es, de, fr, it, pt, nl, sv, da, no, fi, bg); leave it unset otherwise. Verdicts and written results follow the language, and a quick check's reviewer note follows the text the reviewer saw. When it is unset, the connector sends language: "auto" and the API picks the language from the text; a self-hosted connector therefore needs an API that accepts auto.

Prompts. Clients that surface MCP prompts (Claude shows them under +) get two: Check this text with Lenz and Check your last answer with Lenz. Each takes the text to check and starts a quick check. Lenz cannot see the conversation, so Check your last answer needs the answer pasted in.

Verdicts are directional, not absolute: confidence is returned as bucketed

language with a caveat, not a calibrated probability. A quick verdict is a first read;

when a deep check disagrees with it, the deep check is the one to rely on, because it

shows its sources and its reasoning.

The result card

In Claude and the ChatGPT app, a check comes back as a card in the conversation as well as text. From the card you can run the deep check on a claim (Check against sources, a standard-depth verify_claim), open a source, or pick which claims in a draft to check in depth. When a deep check started from the card finishes, the assistant is told the result: silently in Claude, and by a short message in the chat in ChatGPT. Every other client gets the same results as text; nothing depends on the card.

Quickstart

Two ways to connect, depending on your client:

• OAuth — for clients that support it (e.g. Claude connectors). No key to paste; you sign in to Lenz and authorize the connection.
• API key — works with any MCP client via an Authorization header.

Connect with OAuth (no API key)

Claude (web, desktop and mobile): Lenz is an official Claude connector. Open the Lenz connector page and click Connect to Claude, or in the app: Settings → Extensions → Browse extensions, search for Lenz and click +. Either way it is a one-time OAuth sign-in with nothing to configure, and one account covers all three.

For any other client that supports OAuth for MCP, add the server with just its URL and no headers:

{

"mcpServers": { "lenz": { "type": "http", "url": "https://lenz.io/mcp" } } }

The first time you use it, your client walks you through a one-time sign-in: you authenticate on Lenz's own screen and authorize the connection — no key is stored in your config. Your assistant then fact-checks on your behalf against your Lenz account's credits. You can revoke the connection at any time; see the privacy policy.

Connect with an API key

1. Get a free API key at lenz.io/api-credentials (format lenz_…).

2. Add the server to your client (examples below). Authenticate with Authorization: Bearer .

Claude Code

As a plugin (the connector plus the lenz-fact-check skill; sign in with /mcp on first use):

claude plugin marketplace add lenzhq/lenz-mcp

claude plugin install lenz@lenz

Or the connector alone, with an API key:

claude mcp add --scope user --transport http lenz https://lenz.io/mcp \

--header "Authorization: Bearer ${LENZ_API_KEY}"

--scope user makes Lenz available in every project; without it, Claude Code adds the server only to the directory you run the command in.

Claude Desktop / any client that reads .mcp.json

{

"mcpServers": { "lenz": { "type": "http", "url": "https://lenz.io/mcp", "headers": { "Authorization": "Bearer ${LENZ_API_KEY}" } } } }

Cursor

One click: use the Add Lenz to Cursor button above — it adds the server and signs you in via OAuth (no key to paste).

Manual: Settings → MCP → Add new MCP server → type HTTP, URL https://lenz.io/mcp, and add a header Authorization: Bearer . (Or drop the JSON above into .cursor/mcp.json.)

VS Code

One click: use the Install in VS Code button above (OAuth, no key). Manual, with a key:

code --add-mcp '{"name":"lenz","