assay-oracle-mcp

MCP server for Assay, the x402 service-quality oracle. Your agent asks Assay before paying any machine-payable (x402) service: is this thing actually any good?

Assay's answer is earned, not self-reported: it pays real USDC to probe x402 services several times a day, records whether payment settled (with the on-chain tx hash), whether the response matched the advertised schema, whether the data agreed with independent references, and how an LLM judged the response. Daily evidence digests are anchored to Bitcoin via OpenTimestamps, so history can't be quietly rewritten.

Install

Claude Code

claude mcp add assay -- npx -y assay-oracle-mcp

Claude Desktop / any MCP client — add to your MCP config:

{
  "mcpServers": {
    "assay": {
      "command": "npx",
      "args": ["-y", "assay-oracle-mcp"]
    }
  }
}

Requires Node 18+. No API key. (Not related to the npm package assay-mcp, a code-verification tool — this is the MCP server for the Assay x402 quality oracle.)

Tools

| Tool | Cost | What it does | |---|---|---| | check_service | free | Quality tier (gold / ok / avoid / unrated) for an x402 resource URL, with a one-line verdict. Use before paying an unfamiliar endpoint. | | rank_services | free | One call to order up to 50 candidate x402 URLs best-first by tier. Use when choosing which of several services to pay. | | get_score | $0.005¹ | Full report: composite 0–100, component breakdown (settlement, schema, ground truth, LLM judge), 7-day trend, probe count. | | top_services | free | Ranked list of the x402 services Assay actively probes, best first. |

¹ GET /score is a paid x402 endpoint ($0.005 USDC on Base). Without a wallet the tool reports the 402 payment details; with ASSAY_WALLET_KEY set it purchases the report automatically. The free check_service / rank_services verdicts are sufficient for most pre-payment decisions.

Configuration

| Env var | Default | Purpose | |---|---|---| | ASSAY_URL | https://assay.nominal-labs.com | Assay instance to query | | ASSAY_WALLET_KEY | (unset) | Optional Base wallet private key. When set, get_score pays for reports itself — hard-capped at 0.01 USDC per call, Base-mainnet USDC only, refuses anything else. Fund it with a few dollars at most and use a dedicated wallet. |

How to read the tiers

• gold (composite ≥ 85) — consistently delivers what it charges for
• ok (≥ 60) — generally delivers, some failures
• avoid — frequently fails to deliver paid responses correctly
• unrated — under 20 probes so far; unproven, not bad
• a 404/unknown means Assay has no paid-probe evidence for that URL at all

Also see assay-x402-guard — a fetch wrapper that enforces these tiers automatically on every paid request your agent makes.

Full agent guide: https://assay.nominal-labs.com/SKILL.md · MIT © Nominal Labs