Assay MCP Server

Assay

The quality oracle for the x402 agent economy — x402 being the open protocol that lets machines pay for HTTP APIs with stablecoin micropayments. Assay pays real USDC to probe machine-payable services, verifies what actually comes back, and sells the resulting quality scores — every rating backed by an on-chain payment receipt and a Bitcoin-anchored evidence trail.

Live: assay.nominal-labs.com · Leaderboard · Agent guide (SKILL.md)

The problem

Agents are starting to buy API responses from other machines over x402 — HTTP 402 payment challenges settled in USDC on Base (Coinbase's Ethereum L2), no accounts, no invoices. Tens of thousands of services already advertise themselves in discovery catalogs like the Coinbase x402 Bazaar. But an agent holding a wallet has no way to know which of them actually deliver: whether payment settles, whether the response matches the advertised schema, whether the data is even true. The trust signals that exist score liveness — uptime, latency, payment telemetry. Nobody pays these services and verifies what actually comes back. Paying to find out is the only real test — so that's what Assay does, on a schedule, with its own money, so your agent doesn't have to.

How scoring works

Assay probes a curated set of x402 services several times a day, at randomized times, with real paid requests. Each probe is scored on four independent checks:

Weights renormalize over the components a service has data for. A service's composite score (0–100) is computed over its trailing 30 days of probes and publishes only once it has 20+ probes spread across days — no scores from thin evidence. Tiers: gold ≥ 85, ok ≥ 60, otherwise avoid.

Why the history can't be quietly rewritten

Scores are only as trustworthy as the evidence behind them, so the probe corpus is append-only and publicly anchored:

• Every probe row (settlement result, response hash, paid amount, tx hash) becomes a leaf in a per-day Merkle tree; the root is anchored to Bitcoin via OpenTimestamps (OTS) after a safety lag.
• Daily Merkle roots and their OTS proofs are public at /api/digests.
• Anything that ever touches recorded history — migrations, re-anchoring — is logged in the open in docs/CORPUS-LOG.md.

The accumulating corpus is the point: anyone can copy the code, but nobody can backdate the evidence — the anchors prove when each day's records existed.

Using Assay

HTTP — the oracle itself is an x402 service at https://assay.nominal-labs.com. Paid endpoints answer with an HTTP 402 payment challenge that any x402 client settles automatically:

MCP server (Model Context Protocol — how AI assistants call external tools) — assay-oracle-mcp on npm and the official MCP registry. For Claude Code:

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

Spend guard — assay-x402-guard wraps your agent's paying fetch and throws before money moves to a service rated avoid:

import { wrapFetchWithAssay } from "assay-x402-guard";
// payFetch = your x402-paying fetch (e.g. wrapFetchWithPayment from @x402/fetch)
const safeFetch = wrapFetchWithAssay(payFetch);

More background: SUMMARY.md (what/why) and docs/specs/ (design).

Development (any OS)

1. cp .env.example .env and fill in (all vars documented in the example):
   • PROBE_WALLET_KEY: fresh dedicated wallet, ≤ $50 USDC on Base mainnet (no ETH needed — x402 "exact" settles via facilitator using EIP-3009 signatures)
   • ANTHROPIC_API_KEY: the LLM judge
2. npm install && npm test (no network or wallet needed for the suite)
3. npx tsx scripts/verify-env.mts — preflight; fails loudly on misconfiguration
4. npm run dev — server on :3402 (localhost-bound by default) plus crons: catalog ingest (6h), probe sweeps (every 4h at :15 + jitter), daily digest anchoring
5. Curate services to probe:
   • npm run curate -- list
   • write a template JSON (see docs/templates/), then npm run curate -- add <serviceUrl> <template.json>
6. Ops dashboard: http://127.0.0.1:3402/dashboard (pause/resume, probe-now, retire/restore)

The money path is deliberately paranoid: a hard per-payment cap, a daily budget summed from actually-settled amounts, and fail-closed guards on unparsable payment requirements — all unit-tested with injected fakes.

Note that a fresh deployment starts with an empty corpus — the scores are only as good as the probe history behind them.

Production deploys

How the reference instance at assay.nominal-labs.com is operated (adapt for your own): commit to main, ship to the VPS as a git bundle, apply with git reset --hard + systemctl restart assay (provisioning and update runbook in deploy/README.md). The SQLite file at data/assay.db is the product — never recreate it; the VPS keeps daily on-box snapshots via /etc/cron.daily/assay-backup.

Payments: PAYMENTS_ENABLED=true + RECEIVE_WALLET_ADDRESS + a Coinbase Developer Platform (CDP) API key (CDP_API_KEY_ID/CDP_API_KEY_SECRET — the server auto-selects the CDP facilitator for Base mainnet when both are set).

Layout

• src/ — the whole system: server + APIs, prober, scoring, digests, ingest, pages
• mcp/ — assay-oracle-mcp: MCP server (check_service, get_score, top_services)
• middleware/ — assay-x402-guard: wrapFetchWithAssay spend-guard for agents
• tests/ — vitest; money-path logic unit-tested with injected fakes
• scripts/ — preflight, wallet rotation, smoke tests; start-assay.* are legacy launchers from an earlier home-PC deployment, superseded by systemd
• deploy/ — Caddyfile, systemd unit, provisioning runbook
• docs/ — CORPUS-LOG.md (public integrity log), specs, templates

License

MIT — the whole repo, including the npm packages (mcp/ and middleware/). The code is freely reusable; the evidence corpus and its Bitcoin-anchored history are what a fresh deployment can't copy.