Xyntherium Verify MCP

Let any AI agent verify an answer against its sources before it states a fact or takes an action.

Two tools, hosted at https://www.xyntherium.com/mcp (Streamable HTTP, bearer auth; use the www. address, the bare domain currently redirects and a redirect drops the key):

It catches a dropped condition ("within 30 days for unopened items"), a changed number or date, a flipped negation, a wrong entity, and any statement the sources never make. It checks only against the text you pass: it never fetches URLs and does not check facts about the world.

This repository is the integration layer: install snippets, example configs, a system-prompt snippet and a tiny client wrapper. The hosted service is closed source; the deterministic engine behind it is open at augustbrookz/verify-gate.

Install

Get an API key at https://xyntherium.com/account/api-keys (free: 1,000 checks a month, 50 a day over MCP, no card).

Claude Code

claude mcp add --transport http xyntherium-verify https://www.xyntherium.com/mcp \
  --header "Authorization: Bearer $XYN_API_KEY"

Add --scope user to make it available in every project. claude mcp list shows it connected.

Claude Desktop

Settings → Developer → Edit config (claude_desktop_config.json), through the mcp-remote bridge (examples/claude_desktop_config.json):

{
  "mcpServers": {
    "xyntherium-verify": {
      "command": "npx",
      "args": ["-y", "mcp-remote", "https://www.xyntherium.com/mcp", "--header", "Authorization: Bearer ${XYN_API_KEY}"],
      "env": { "XYN_API_KEY": "xyn_live_…" }
    }
  }
}

Or Settings → Connectors → Add custom connector with https://www.xyntherium.com/mcp and no key: Claude sends you to Xyntherium to sign in and approve access (OAuth, 90 days).

Cursor

~/.cursor/mcp.json or .cursor/mcp.json (examples/cursor-mcp.json):

{
  "mcpServers": {
    "xyntherium-verify": {
      "url": "https://www.xyntherium.com/mcp",
      "headers": { "Authorization": "Bearer xyn_live_…" }
    }
  }
}

VS Code

.vscode/mcp.json (examples/vscode-mcp.json); VS Code asks for the key once and stores it securely.

Any other MCP client

URL https://www.xyntherium.com/mcp, header Authorization: Bearer <key>, Streamable HTTP, stateless, protocol 2026-07-28 and 2025-11-25 (and 2025-06-18 / 2025-03-26 through initialize). Discovery and tools/list need no key; every tool call does.

Related MCP server: Recommend Agentic Trust Layer

30-second demo

1. Install with the Claude Code command above.

2. Ask Claude:

   Our return policy says: "Refunds are available within 30 days for unopened items with proof of purchase."
   A customer asks if they can get a refund. Draft a one-sentence reply, then verify it with
   xyntherium-verify before you show it to me, and fix anything the check flags.

3. Claude drafts "Yes, refunds are available within 30 days", calls verify_against_document, and gets:

   REVISE: dropped “for unopened items with proof of purchase”.
   - Dropped: “Refunds are available within 30 days.” leaves out “for unopened items with proof of purchase”; the source says “within 30 days for unopened items with proof of purchase” (source document).
   Claims:
   1. [unsupported] “Refunds are available within 30 days.”; drops “for unopened items with proof of purchase”; source document: “Refunds are available within 30 days for unopened items with proof of purchase.”.
   0 of 1 statements supported · mode fast · rules 2.1.0 · request …

4. Claude rewrites the reply with the conditions, checks again, and gets SEND.

From a shell, without any client:

curl -s -X POST https://www.xyntherium.com/mcp \
  -H "Authorization: Bearer $XYN_API_KEY" \
  -H 'content-type: application/json' -H 'accept: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"verify_against_document","arguments":{
    "answer":"Refunds are available within 30 days.",
    "document_text":"Refunds are available within 30 days for unopened items with proof of purchase."}}}'

Telling your agent when to call it

The server sends every client these instructions; examples/agent-system-prompt.md has a longer version for your own system prompt:

Before you state facts drawn from documents in the conversation, or act on them (send a reply, file a change, book, refund, approve), verify the drafted answer: call verify_answer with the answer and the passages it relies on, or verify_against_document with the answer and the one document it rests on. On REVISE, fix what it names and check again before proceeding. On ASK A PERSON, tell the user what the sources do not settle instead of guessing. Do not call it for chit-chat, for answers with no source text, or to fact-check the world.

The result

Text (the verdict first, then each problem, then every claim) and the same as structuredContent:

{
  "verdict": "revise",
  "release_recommendation": "revise",
  "summary": "REVISE: dropped “for unopened items with proof of purchase”.",
  "claims": [
    {
      "index": 0,
      "statement": "Refunds are available within 30 days.",
      "status": "unsupported",
      "reason": "drops “for unopened items with proof of purchase”",
      "source_id": "document",
      "source_passage": "Refunds are available within 30 days for unopened items with proof of purchase.",
      "material": true
    }
  ],
  "dropped": [ … ], "contradicted": [], "unsupported": [], "unresolved": [],
  "statements": 1, "supported": 0, "mode": "fast",
  "request_id": "…", "evidence_ledger_id": "…", "rules_version": "2.1.0"
}

status is supported (the sources say this), contradicted (the sources say otherwise) or unsupported (the sources do not say this, or do not settle it: the reason says which). Both tools are read-only and idempotent. Everything you pass is treated as untrusted data: it is checked, never followed.

Client wrapper

Two single-file wrappers call the server directly over JSON-RPC, with no dependencies:

• client/xyntherium-verify.mjs (Node 18+ or any runtime with fetch):

  import { XyntheriumVerify } from './client/xyntherium-verify.mjs';
  const verify = new XyntheriumVerify({ apiKey: process.env.XYN_API_KEY });
  const r = await verify.answer({ answer, sources: [{ id: 'policy', text: policy }] });
  if (r.verdict !== 'send') console.log(r.claims.filter((c) => c.status !== 'supported'));

• client/xyntherium_verify.py (Python 3.9+, standard library only):

  from xyntherium_verify import XyntheriumVerify
  r = XyntheriumVerify(api_key=os.environ["XYN_API_KEY"]).against_document(answer=answer, document_text=policy)
  if r["verdict"] != "send": print([c for c in r["claims"] if c["status"] != "supported"])

Both raise on a refused call (no key, over quota, over a cap, oversized input) with the server's message.

Limits, metering and cost

A refused call comes back as a tool error that says which limit was hit and when it resets, with a Retry-After header.

Data handling

• Sent to Xyntherium: the answer, the source or document text, and the question if you pass one. Nothing else from your conversation, repository or machine. URLs are never fetched.

• Seen by the model: the model provider (Anthropic) receives the text needed to split the answer into claims and check each one, under its commercial API terms, which exclude training on API traffic. Xyntherium never trains on your text.

• Stored: the text and the result, encrypted (AES-256-GCM), for your plan's retention window: 7 days on Free, 30 on Builder, 90 on Team; then deleted. The evidence ledger (hashes of the text, verdicts and offsets, not the text) is kept 30, 90 or 365 days by plan. Telemetry is structural only: counts, latency, cost, client name and version, never content.

• Deletion: email support@xyntherium.com from the address on the account to delete everything tied to it. Privacy · Security · Terms

Verdicts describe whether the supplied passages support each claim, not whether a claim is true in the world. Evidence support for supplied sources only; not a compliance certification or legal assessment.

Links

• Docs and install: https://www.xyntherium.com/mcp

• API docs: https://xyntherium.com/docs/api

• The open engine, Gate, CLI and SDK: https://github.com/augustbrookz/verify-gate

• Support: support@xyntherium.com

MIT.