Implement MCP server x402 paywall

Author: dimitrov-d

package-lock.json

@@ -48,6 +48,8 @@
 	},
 	"homepage": "https://github.com/GWALN/cli",
 	"dependencies": {
+		"@faremeter/info": "^0.12.0",
+		"@faremeter/middleware": "^0.12.0",
 		"@modelcontextprotocol/sdk": "^1.22.0",
 		"ajv": "^8.17.1",
 		"ajv-formats": "^2.1.1",

package.json

@@ -0,0 +1,82 @@
+/**
+ * @file src/lib/x402.ts
+ * @description X402 payment middleware for MCP server using Corbits and Faremeter.
+ */
+
+import { evm } from '@faremeter/info';
+import { express as faremeter } from '@faremeter/middleware';
+import { type NextFunction, type Request, type Response } from 'express';
+import { readConfig } from '../shared/config';
+
+export interface X402Config {
+  host: string;
+  port: number;
+  amount?: number;
+  asset?: 'USDC';
+  network?: 'base';
+  facilitatorURL?: string;
+  description?: string;
+}
+
+export const BYPASS_PAYMENT_METHODS = [
+  'initialize',
+  'initialized',
+  'notifications/initialized',
+  'tools/list',
+  'prompts/list',
+  'resources/list',
+] as const;
+
+export const PAYWALLED_TOOLS = ['query', 'publish'] as const;
+
+/**
+ * Create MCP x402 payment middleware that only requires payment for specific tools.
+ * @returns Express middleware function
+ */
+export const initializePaymentMiddleware = async (
+  config: X402Config,
+): Promise<(req: Request, res: Response, next: NextFunction) => void> => {
+  const gwalnConfig = readConfig();
+  const walletAddress = gwalnConfig.dkgPublicKey;
+
+  if (!walletAddress) {
+    throw new Error(
+      'No wallet address configured. Set dkgPublicKey in .gwalnrc.json or run "gwaln init".',
+    );
+  }
+
+  const baseUrl = `http://${config.host}:${config.port}/mcp`;
+
+  const paywalledMiddleware = await faremeter.createMiddleware({
+    facilitatorURL: config.facilitatorURL ?? 'https://facilitator.corbits.dev',
+    accepts: [
+      {
+        ...evm.x402Exact({
+          network: config.network ?? 'base',
+          asset: config.asset ?? 'USDC',
+          amount: config.amount ?? 10000, // $0.01 per request
+          payTo: walletAddress,
+        }),
+        resource: baseUrl,
+        description: config.description ?? 'GWALN MCP tools',
+      },
+    ],
+  });
+
+  return (req: Request, res: Response, next: NextFunction): void => {
+    if (req.body?.method && BYPASS_PAYMENT_METHODS.includes(req.body.method)) {
+      next();
+      return;
+    }
+
+    if (req.body?.method === 'tools/call') {
+      const toolName = req.body?.params?.name;
+      if (toolName && PAYWALLED_TOOLS.includes(toolName)) {
+        paywalledMiddleware!(req, res, next);
+        return;
+      }
+    }
+
+    next();
+  };
+};