Pay Invoice

Instructions

Implementation Reference

• src/tools/nwc/pay_invoice.ts:1-63 (handler)

  Main handler function `registerPayInvoiceTool` that registers and implements the 'pay_invoice' tool. It wraps `client.payInvoice()`, converts sats to millisats for NWC, converts millisats back to sats in response, and returns structured content with preimage and fees_paid_in_sats.

  import { nwc } from "@getalby/sdk";
  import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
  import { z } from "zod";
  
  export function registerPayInvoiceTool(
    server: McpServer,
    client: nwc.NWCClient
  ) {
    server.registerTool(
      "pay_invoice",
      {
        title: "Pay Invoice",
        description: "Pay a lightning invoice",
        inputSchema: {
          invoice: z.string().describe("The lightning invoice to pay"),
          amount_in_sats: z
            .number()
            .describe(
              "Optional amount in sats, only provide if paying a zero-amount invoice"
            )
            .nullish(),
          metadata: z
            .object({})
            .passthrough()
            .describe("Optional metadata to include with the payment")
            .nullish(),
        },
        outputSchema: {
          preimage: z.string().describe("Payment preimage"),
          fees_paid_in_sats: z.number().nullish().describe("Fees paid in sats"), // TODO: remove nullish once Primal supports it
        },
      },
      async (params) => {
        const { fees_paid, preimage, ...result } = await client.payInvoice({
          invoice: params.invoice,
          amount: params.amount_in_sats
            ? params.amount_in_sats * 1000
            : undefined, // Convert sats to millisats for NWC
          metadata: params.metadata || undefined,
        });
  
        // Convert millisats back to sats in the response
        const convertedResult = {
          ...result,
          preimage: preimage || "", // TODO: once Primal supports preimage, remove this
          fees_paid_in_sats:
            typeof fees_paid === "number"
              ? Math.ceil(fees_paid / 1000) // Round up fees when converting millisats to sats
              : undefined,
        };
  
        return {
          content: [
            {
              type: "text",
              text: JSON.stringify(convertedResult, null, 2),
            },
          ],
          structuredContent: convertedResult,
        };
      }
    );
  }

• src/tools/nwc/pay_invoice.ts:9-32 (registration)

  Registration of the 'pay_invoice' tool on the MCP server, including input schema (invoice, amount_in_sats, metadata) and output schema (preimage, fees_paid_in_sats).

  server.registerTool(
    "pay_invoice",
    {
      title: "Pay Invoice",
      description: "Pay a lightning invoice",
      inputSchema: {
        invoice: z.string().describe("The lightning invoice to pay"),
        amount_in_sats: z
          .number()
          .describe(
            "Optional amount in sats, only provide if paying a zero-amount invoice"
          )
          .nullish(),
        metadata: z
          .object({})
          .passthrough()
          .describe("Optional metadata to include with the payment")
          .nullish(),
      },
      outputSchema: {
        preimage: z.string().describe("Payment preimage"),
        fees_paid_in_sats: z.number().nullish().describe("Fees paid in sats"), // TODO: remove nullish once Primal supports it
      },
    },

• src/mcp_server.ts:7-7 (registration)

  Import of `registerPayInvoiceTool` from the pay_invoice module.

  import { registerPayInvoiceTool } from "./tools/nwc/pay_invoice.js";

• src/mcp_server.ts:26-26 (registration)

  Registration call: `registerPayInvoiceTool(server, client)` inside `createMCPServer`.

  registerPayInvoiceTool(server, client);