Lenz MCP Server

Fact-check claims against independent sources — from any MCP client.

Website · Get an API key · Developer docs

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

Lenz is a fact-checking platform. It takes a factual claim, runs it through a multi-model pipeline (framing → 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.

Tools

Verdicts are directional, not absolute — confidence is returned as bucketed language with a caveat, not a raw score. Surface it, and the link back to Lenz, to the user.

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 <your-key>.

Claude Code

claude mcp add --transport http lenz https://lenz.io/mcp \
  --header "Authorization: Bearer ${LENZ_API_KEY}"

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 <your-key>. (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","type":"http","url":"https://lenz.io/mcp","headers":{"Authorization":"Bearer ${LENZ_API_KEY}"}}'

ChatGPT

At this time, ChatGPT connects to Lenz as a custom app through OpenAI's Developer mode — over OAuth, so there is no API key to paste. It is set up on chatgpt.com; the ChatGPT desktop and mobile apps cannot create one.

On a personal account (Plus or Pro). Settings → Plugins → turn on Developer mode at the bottom → Plugins → Browse plugins → + next to Search → name it Lenz, URL https://lenz.io/mcp, Authentication OAuth → tick I understand and want to continue → Create → Sign in with Lenz → Try in Chat.

In a Business, Enterprise or Edu workspace. An owner or admin publishes it once: Workspace settings → Apps → + Create, confirm Enable developer mode, fill in the same form with OAuth, then Drafts → Publish, set who can use it, and Publish again. Each member then goes to Settings → Plugins → Lenz → Connect and signs in.

A custom app is off by default in every new chat: + and tick Lenz. Naming Lenz in the question is what makes ChatGPT reach for it rather than answer from memory.

MCP Inspector (try the tools by hand)

npx @modelcontextprotocol/inspector
# Transport: Streamable HTTP · URL: https://lenz.io/mcp
# Header: Authorization: Bearer <your-key>

Example

You: Is it true that honey never spoils?

The assistant calls assess_claim("Honey never spoils") and gets back: Mostly True — high confidence. Properly sealed honey can keep effectively indefinitely thanks to its low moisture and acidity; the caveat is contamination or added water. For a sourced deep-dive it can escalate with verify_claim.

Credits

Every tool call draws on one pool of credits on your Lenz account. There is no separate budget per tool: spending on assess_claim reduces what is left for verify_claim, and vice versa. check_usage returns the balance (credits_remaining) alongside costs, the live price list — read the weight from there rather than assuming one.

verify_claim is the expensive path by an order of magnitude; assess_claim and ask_followup are the cheap ones. verify_claim also takes an optional depth: "low" runs a shallower research pass — fewer sources, faster, the same models — for half the credits of the default "standard". That price sits under cost_options.verify.depth.low on check_usage. You are charged for the depth you request; the depth on the completed result is the depth the verdict was produced at, so a "low" request answered from an existing deeper check reads "standard" — the echo describes the evidence, the charge follows the request.

check_usage also reports assess_remaining and verify_remaining, which are the SAME balance projected into each tool's own unit — "how many of these could I still make" — not separate allowances.

Free keys include a monthly allowance that resets each period; grants and top-ups add non-expiring bonus credits on top, spent only once the allowance is gone. See plans at lenz.io/plans.

When you run out, tools return status: "quota_exhausted" with a message and a manage_url pointing at the plans page. It is not retryable — the balance is spent until you top up or the monthly allowance resets. An agent should say so plainly rather than retrying or quietly skipping the check.

status: "rate_limited" is a different thing: a rate limit (today, the per-account daily extract cap, which costs no credits at all). That one does clear on its own, and the result carries retry_after_seconds telling you when.

Skills

Prefer a guided workflow to calling the tools yourself? The lenz-fact-check skill turns "is this true?" into a structured pass — it extracts the checkable claims, runs assess_claim on each, escalates high-stakes ones to a deep verify_claim, and reports verdicts with confidence and sources (with the directional-not-absolute caveat built in). Point your agent at skills/lenz-fact-check/SKILL.md.

Links

• Website: lenz.io
• Get a key: lenz.io/api-credentials
• Developer docs: lenz.io/developers
• SDKs: Python · Node

Support

Questions or issues? Open an issue or get in touch.

License

Apache-2.0 © lenzhq. The Lenz name and logos (assets/) are trademarks of lenzhq and are not granted by the license — see NOTICE.

Maintainer

@Pavel12431432