x402 over MCP · Roundlot docs

Payment happens per tool call inside MCP, using the same _meta convention as @x402/mcp.

MCP over streamable HTTP is JSON-RPC on one endpoint, so an HTTP 402 can't price individual tools. Roundlot prices each tool call instead. initialize, tools/list and catalog never ask for payment.

The round trip

Call a paid tool. The result has isError: true and carries the x402 PaymentRequired object in structuredContent and JSON-encoded in content[0].text.
Sign the payment exactly as for REST.
Call the same tool again with the payload in _meta["x402/payment"].
The result arrives with the settlement receipt in _meta["x402/payment-response"].

{ "method": "tools/call",
  "params": { "name": "quote",
    "arguments": { "symbol": "NVDA", "side": "buy", "size": "100" },
    "_meta": { "x402/payment": { "x402Version": 2, "payload": { "...": "signed authorization" } } } } }

Compatibility

This is the wire format of Coinbase's @x402/mcp, so its createx402MCPClient can pay Roundlot. The server side is implemented on OKX's @okxweb3/app-x402-core rather than with @x402/mcp itself: that library is built on a different x402 core and calls resource-server methods OKX's core doesn't have.

Clients that can't sign, such as Claude Desktop, see a second text block with the price and the REST alternative.