X402 Trust MCP Server

x402-trust-mcp

An MCP server that lets your agent check the trust & reliability of x402 endpoints before paying them.

Backed by x402.fuchss.app, which monitors the entire x402 ecosystem on Base 24/7: uptime probes, 402-envelope spec compliance, advertised-price history, and real on-chain USDC settlement volume per endpoint.

Why

Agents increasingly pay x402 endpoints autonomously. But ~⅔ of listed x402 endpoints are unreachable and ~⅓ of the reachable ones serve non-compliant payment envelopes. Before your agent sends USDC to an unknown endpoint, ask: is it alive, compliant, and does anyone actually pay it?

Tools

Paid tools cost from $0.001 (a semantic search) or $0.005 (a single trust / similar lookup) up to ~$0.50 (500-endpoint bulk batch) or ~$0.20 for a 30-day watch, charged over x402 (USDC on Base). If you set X402_PRIVATE_KEY, the server auto-pays within your X402_MAX_USD limit; otherwise it returns the price quote for your host to pay.

Bulk scoring (x402_trust_bulk)

The bulk tool is the scale axis: score up to 500 endpoints in one call from the same data that powers the leaderboard. It auto-selects the cheapest tier that fits your request:

Cached rows older than ~15 minutes are recomputed on-demand from the latest stored probes and settlements (no live network re-probe), so bulk scores usually reflect reality within minutes. Per-request recompute limits apply: at most 50 rows / 8 seconds are recomputed; the response tells you via recompute_limit_hit + recompute_limit. Each result carries score, grade, recommendation, confidence, probed_at, computed_at, and recomputed so you can see exactly which rows were freshly computed vs served from cache. URLs not in the observation set return found: false; you still pay for the batch.

Finding better alternatives (x402_find_alternatives)

Before paying an unknown endpoint, check whether a better-tested alternative exists for the same purpose. x402_find_alternatives returns up to 25 endpoints (default 5) that are semantically similar to a given URL — matched on advertised purpose via description embeddings — and that out-score it on our deterministic trust score. Each alternative returns score, grade, recommendation, cosine similarity (0-1), amountUsd price, and a free endpointPage URL. Same-host siblings and avoid-flagged endpoints are excluded; an empty alternatives array is a valid answer meaning nothing beats the subject. Cost is ~$0.005 per call.

Semantic search (x402_semantic_search)

Describe the capability you need in plain words and get the up to 25 closest endpoints in the monitored catalog. Ranking is deterministic: cosine similarity bucketed to whole percentage points first (80.3% and 80.5% are the same bucket), then trust score, then described-before-undescribed, then endpoint id, with matches below a 0.5 cosine-similarity floor dropped entirely (so a query can return fewer than the requested limit, or none). Endpoints that advertise no description are still matched via their service name and URL path tokens (host name as a last resort). The exact effect of a missing description: no fixed point deduction and no direct similarity malus; the only deterministic penalty is the described-before-undescribed tiebreak (equal bucket AND equal score: described wins). Beyond that it is purely indirect — the shorter fallback text typically yields lower cosine similarity than a prose description, so such endpoints tend to land in lower buckets, by a query-dependent amount. Each match returns id, resource, score, grade, raw cosine similarity (0-1; ranking buckets it), description (when advertised), and a free endpointPage URL. This is discovery, not verdicts: score/grade are null for unscored endpoints, and no recommendation or flag detail is included (use x402_trust_score for that). Cost is ~$0.001 per call.

Watch / alerting (x402_watch_create, x402_watch_events, x402_watch_edit, x402_watch_cancel, x402_watch_renew)

• Create (x402_watch_create, paid) buys 30 days of change monitoring for one endpoint. Pay over x402; receive a one-time bearer secret, a poll_url, a renew_url, and machine-readable next_steps.
• Poll (x402_watch_events, free) reads the append-only event log. It returns two streams: events (endpoint changes — payTo / price / asset / network / spec / delisting / liveness) and watch_events (lifecycle feedback — created / edited / cancelled / renewed / expiring / expired), each with their own cursor (next_cursor and watch_events_cursor). Page forward by passing the previous response's cursors as since / watch_since. Cursors/ids are global sequences shared across watches, so a watch's first event id may be >1 — always use the returned next_cursor, never assume events start at 1.
• Edit (x402_watch_edit, free) changes webhook/Slack URLs, liveness sensitivity, or subscribed events. Bearer-authed with the secret.
• Cancel (x402_watch_cancel, free) soft-cancels a watch: no new events accrue and probing drops back to normal cadence immediately, but the event log stays readable via x402_watch_events until the original expires_at. Cancel is not a delete.
• Renew (x402_watch_renew, paid) extends the watch before expires_at. The secret stays the same.

Optional push delivery to one or more signed HTTPS webhooks and/or Slack/Discord incoming webhooks can be configured at creation time and updated via edit (up to 5 of each per watch). webhook_url and slack_url accept a single URL string or an array of URLs. Any URL is connection-tested before you are charged: the server POSTs a signed connection_test ping and, if it can't be delivered (3 attempts), rejects the change with notCharged: true so you can retry with a corrected URL. On success the response reports per-URL delivery under delivery.connection_test.

If you use a webhook, verify the x-signature header equals sha256= + HMAC-SHA256(body) keyed by the SHA-256 hex digest of your secret — i.e. the HMAC key is hex(sha256(secret)), not the raw secret. (The delivery worker only ever holds that hash, never the plaintext secret.)

x402_trust_score result

A single call returns everything an agent needs to decide whether and at what price to use an endpoint — no second round-trip, no raw-unit guessing:

Everything is computed deterministically (no LLM) from continuous on-chain and probe observation, so the breakdown is fully auditable.

x402 V2 Payment Flow

This MCP server uses the canonical x402 V2 payment flow:

1. 402 + PAYMENT-REQUIRED — The server responds with HTTP 402 and a base64-encoded PAYMENT-REQUIRED header containing the payment requirements (accepts, network, asset, amount, payTo).
2. Sign + retry with PAYMENT-SIGNATURE — The MCP client signs an EIP-3009 transferWithAuthorization for the selected accept and re-POSTs with the PAYMENT-SIGNATURE header (base64-encoded payment payload).
3. Settlement + PAYMENT-RESPONSE — The server settles the payment and responds with the data plus a PAYMENT-RESPONSE header.

Legacy X-PAYMENT / X-PAYMENT-RESPONSE headers are accepted as a fallback during the V1→V2 transition period but are not the default.

Accept selection: When a 402 response offers multiple accepts (e.g. Solana

• Base USDC), the client selects the best compatible one (canonical USDC on an allow-listed chain) rather than blindly taking the first accept.

Install

Add to your MCP client config (e.g. Claude Desktop claude_desktop_config.json):

{
  "mcpServers": {
    "x402-trust": {
      "command": "npx",
      "args": ["-y", "x402-trust-mcp"]
    }
  }
}

To enable autonomous payment for the paid tools, add a funded Base USDC wallet:

{
  "mcpServers": {
    "x402-trust": {
      "command": "npx",
      "args": ["-y", "x402-trust-mcp"],
      "env": {
        "X402_PRIVATE_KEY": "0xYOUR_BASE_WALLET_KEY",
        "X402_MAX_USD": "0.05"
      }
    }
  }
}

Configuration (env vars)

The free tools work with no configuration at all.

Security

X402_PRIVATE_KEY is a hot wallet — fund it with only what you're willing to spend on trust lookups. The key never leaves your machine; it signs EIP-3009 payment authorizations locally.

Policy checks enforced before signing:

• Chain allow-list (Base mainnet by default)
• Canonical USDC contract verification (no arbitrary tokens)
• Optional payTo allow-list
• Per-call spend ceiling (X402_MAX_USD)
• Cumulative spend cap (X402_MAX_TOTAL_USD)
• Call-count cap (X402_MAX_CALLS)

Verifying response signatures

Tool results are provider-signed: the result object of signed tools carries a top-level signature block with an Ed25519 signature over the JCS-canonicalized (RFC 8785) response without the signature field. This proves the content was assembled by x402-trust and not modified afterwards.

To verify a result:

1. Take the result object and remove its signature field.
2. Canonicalize with JCS (RFC 8785): object keys sorted by UTF-16 code unit order, no whitespace, ECMAScript number formatting.
3. SHA-256 the canonical UTF-8 bytes; the hex must equal signature.digest.
4. Verify signature.value (base64url, no padding) against the public key that signature.keyId resolves to in your PINNED copy of the key document (see below).

Trust anchor: pin, do not follow. signature.publicKeys is a discovery hint, never a trust source. A verifier that fetches the key URL from the response it is checking verifies against a key chosen by the sender, which proves nothing: a forged response would carry the attacker's own key URL and still verify. Pin one of these in your client instead:

• the public key itself (strongest, works offline; add new keys on rotation), e.g. { "x402trust-2026-08": "i4jrHKvmZ98-IGgseDfMTjMV4lAaLAgk-EnBeRIJQ5Y" } (current at the time of writing; the key document always carries the full list, retired keys included), or
• the key document URL https://x402.fuchss.app/.well-known/x402-trust-keys.json, fetched over HTTPS once at bootstrap and cached (rotation-friendly).

Retired keys stay published forever, so a response you froze as evidence remains verifiable. A worked test vector and a 20-line reference verifier live at https://x402.fuchss.app/schemas. Watch management responses (x402_watch_create, x402_watch_edit, x402_watch_cancel, x402_watch_renew) are unsigned by design: they carry capability secrets that must never be forwarded as evidence.

License

MIT