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Add a fact-checking MCP server to Claude or Cursor

20 October 2026

Add a fact-checking MCP server to Claude or Cursor

Short answer: Screwtape.ai runs a remote MCP server over streamable HTTP at https://screwtape.ai/mcp. It has two tools: judge, which checks one output against the evidence and criteria you pass and returns pass, fail or retry, and consensus, which scores agreement across 2 to 16 answers. Add the URL with an Authorization: Bearer header to Cursor's mcp.json or Claude Code's .mcp.json, or as a custom connector in Claude Desktop and claude.ai. It's listed in the official MCP Registry as ai.screwtape/screwtape.

Screwtape.ai is an agent validation API. It is not the arXiv paper that is also called Screwtape.

What "fact-checking" means here: Screwtape checks an answer against evidence you supply, using LLM judgment when a model key is configured or deterministic claim-versus-evidence token grounding when it isn't. It doesn't search the web for evidence. Pair it with a search or retrieval tool if you need one.

Cursor

~/.cursor/mcp.json (global) or .cursor/mcp.json (project). Cursor resolves ${env:NAME} inside headers (Cursor MCP docs):

{
  "mcpServers": {
    "screwtape": {
      "url": "https://screwtape.ai/mcp",
      "headers": {
        "Authorization": "Bearer ${env:SCREWTAPE_API_KEY}"
      }
    }
  }
}

Claude Code

Project .mcp.json. Claude Code needs "type": "http" for URL servers and expands ${VAR} in headers (Claude Code MCP docs):

{
  "mcpServers": {
    "screwtape": {
      "type": "http",
      "url": "https://screwtape.ai/mcp",
      "headers": {
        "Authorization": "Bearer ${SCREWTAPE_API_KEY}"
      }
    }
  }
}

Or from the CLI. Note that your shell expands the variable, so the literal key ends up stored in your Claude config:

claude mcp add --transport http screwtape https://screwtape.ai/mcp \
  --header "Authorization: Bearer $SCREWTAPE_API_KEY"

Claude Desktop and claude.ai (custom connector)

Remote servers are added as custom connectors, not in claude_desktop_config.json. Per Anthropic's help center:

  1. Go to Customize > Connectors, click Add, hover over Custom and select Web.
  2. Name it screwtape and enter https://screwtape.ai/mcp.
  3. Under Request headers, add Authorization: Bearer <your key>.

Notes from the same docs: on Team and Enterprise plans an Owner adds the connector, Free plans are limited to one custom connector, and the connection is made from Anthropic's cloud, not from your machine. That's fine here because the server is public.

Test the server with curl

tools/list doesn't need a key:

curl -s https://screwtape.ai/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'

On 2026-10-20 this returned the two tools. judge requires task and agent_output, with optional evidence, criteria and retain. consensus requires task and answers, with 2 to 16 answers. tools/call uses the same Bearer key, quota and x402 payment as the REST API. Without either, it returns a JSON-RPC error with code -32002 ("Payment required"). GET /mcp returns 405 because the server doesn't open an SSE stream. Supported protocol versions are 2025-03-26 and 2025-06-18, per the server card.

Example prompt once connected

Use the screwtape judge tool. Task: "State the crew size and launch site." Output: "The crew size was 5. The mission launched from Vandenberg." Evidence: "The mission launched on 2024-03-01 from Cape Canaveral." and "The crew size was 4." Report the verdict and reasons.

FAQ

Is there an MCP server for fact-checking agent output?

Yes. Screwtape.ai's MCP server at https://screwtape.ai/mcp exposes a judge tool that checks an output against the evidence and criteria you pass and returns pass, fail or retry with a score and reasons. A consensus tool scores agreement across several agents. It does not search the web for evidence.

Does the Screwtape MCP server work with Claude Desktop?

Yes, as a remote custom connector added under Customize > Connectors. Enter https://screwtape.ai/mcp and add an Authorization: Bearer request header with your key.

Which transport does it use?

Streamable HTTP. The server card lists protocol versions 2025-03-26 and 2025-06-18. It answers POST requests. GET returns 405 because it does not open an SSE stream.

Is it in the official MCP Registry?

Yes. On 2026-10-20 the official MCP Registry listed it as ai.screwtape/screwtape with status active.

Do I need a key to try it?

Listing the tools needs no key. Calling them needs a free key (100 calls per UTC day) from https://screwtape.ai/signup/, or an x402 payment of $0.005 USDC on Base per call.

Sources