@cryptoapis-io/mcp-x402-accept

Put an x402 paywall in front of an MCP server you already run — charge AI agents per tool call without changing a line of that server's code.

agent  →  mcp-x402-accept (your process)  →  your existing MCP server

The proxy is an MCP server to the agent and an MCP client to yours. It mirrors every tool your server exposes; the ones you price are paywalled, the rest pass through free.

When to use this (and when not to)

Your situation	Use
You can edit your MCP server (Node/TS)	x402-merchant-sdk/mcp — three lines around your handler, nothing in the data path
You cannot — third-party, closed-source, or not Node	this proxy

If you can edit your server, do that instead. It is simpler and strictly fewer moving parts. This package exists for the case where that is not an option.

Why self-hosted only

You run this process. We never do, and we do not offer a hosted version.

That is a deliberate product decision, not a default you can flip. A proxy sits between an agent and your server, so a hosted one would mean CryptoAPIs holding your upstream server's credentials, sitting in the cleartext path of your tool arguments and results, and owing you uptime on your own revenue. For a merchant whose product is the data flowing through those calls, that is not a reasonable thing to ask. Self-hosting removes the question rather than answering it.

What that guarantees, structurally:

• Tool arguments and results are opaque to the proxy. It reads the tool name (is this priced?) and _meta["x402/payment"] (is it paid for?) — nothing else. They are never parsed, logged, buffered, or cached.
• Your upstream credentials are your own env vars, in your own process.
• The only outbound calls are to the CryptoAPIs facilitator, to /verify and /settle the payment itself.

Install

npm install -g @cryptoapis-io/mcp-x402-accept

Configure

{
  "upstream": {
    "transport": "stdio",
    "command": "node",
    "args": ["/path/to/your-mcp-server.js"],
    "env": { "YOUR_UPSTREAM_TOKEN": "${YOUR_UPSTREAM_TOKEN}" }
  },

  "apiKey": "${CRYPTOAPIS_API_KEY}",
  "payTo":  "0xYourReceivingAddress",

  // Only these are charged for. Everything else your server exposes stays FREE.
  "pricedTools": {
    "premium_data": {
      "network": "eip155:8453",
      "asset":   "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "amount":  "10000"
    }
  }
}

${VAR} reads from the environment, so the config file itself holds no secrets and is safe to commit.

amount is in atomic units — USDC has 6 decimals, so "10000" is $0.01. Getting this wrong by 10^6 is the easiest mistake here.

An HTTP upstream instead of stdio:

"upstream": {
  "transport": "http",
  "url": "https://your-server.example.com/mcp",
  "headers": { "Authorization": "Bearer ${YOUR_UPSTREAM_TOKEN}" }
}

To accept several assets or networks for one tool, pass an array — the agent picks one. Offer only what GET https://ai.cryptoapis.io/x402/merchant/supported lists (public, no key); anything else is advertised and then rejected at /verify.

Run

cryptoapis-mcp-x402-accept --config ./x402-accept.config.json

Then point agents at the proxy instead of your server. In Claude Code:

claude mcp add my-paid-tools -- cryptoapis-mcp-x402-accept --config /abs/path/to/config.json

Everything is validated at boot — the config, the upstream connection, and whether the tools you priced actually exist upstream. A typo'd tool name is a hard failure, not a silent revenue hole: the alternative is believing a tool is paywalled while it quietly serves for free.

What an agent sees

Step	Result
Unpaid call to a priced tool	isError: true carrying PaymentRequired in both structuredContent and content[0].text
Agent pays	Signs locally, retries with the payload in _meta["x402/payment"] — raw JSON, no base64
Paid call	Your tool's real result, with the receipt in _meta["x402/payment-response"]
Call to an unpriced tool	Forwarded, free, unchanged

Your server is never called until payment has settled. The proxy charges and then forwards — it does not forward and then bill. An unpaid call cannot reach your server at all, so it cannot be made to do the work for free.

Priced tools advertise [paid: x402] in their description so an agent knows the cost before calling. An agent that discovers a price only by being refused has already wasted a round-trip.

Paying it

Agents pay with @cryptoapis-io/mcp-x402-pay or, in code, @cryptoapis-io/x402-buyer-sdk/mcp.

Notes

• Requires a CryptoAPIs API key with the X402_FACILITATOR feature.
• "settle": false verifies without settling — useful while wiring up, but advisory only: the buyer proved they could pay, not that they did. Never ship it.
• Test end-to-end on Base Sepolia (eip155:84532) before charging real money.
• Implements the x402 v2 MCP transport (spec), sharing one implementation with the merchant SDK — a spec fix lands in both.