Patch for VS Code MCP JSON Schema Compatibility (issue #112)

package.json

@@ -31,7 +31,8 @@
     "dotenv": "^17.2.2",
     "firecrawl-fastmcp": "^1.0.3",
     "typescript": "^5.9.2",
-    "zod": "^4.1.5"
+    "zod": "^4.1.5",
+    "zod-to-json-schema": "^3.24.6"
   },
   "engines": {
     "node": ">=18.0.0"

src/index.ts

@@ -1,5 +1,11 @@
 #!/usr/bin/env node
 import dotenv from 'dotenv';
+import { patchFastMCPSchemas } from './schema-patch.js';
+
+// CRITICAL: Apply MCP SDK patches BEFORE importing FastMCP
+// This patches the Server.prototype.setRequestHandler method
+patchFastMCPSchemas();
+
 import { FastMCP, type Logger } from 'firecrawl-fastmcp';
 import { z } from 'zod';
 import FirecrawlApp from '@mendable/firecrawl-js';

src/schema-patch.ts

@@ -0,0 +1,103 @@
+/**
+ * Patch for VS Code MCP JSON Schema Compatibility
+ * 
+ * This module patches JSON schemas from draft 2020-12 to draft-07
+ * to ensure compatibility with VS Code's MCP validator.
+ */
+
+import { Server } from '@modelcontextprotocol/sdk/server/index.js';
+
+/**
+ * Recursively converts JSON Schema draft 2020-12 to draft-07
+ * @param schema - The schema object to convert
+ * @returns The converted schema
+ */
+export function convertSchemaToDraft07(schema: any): any {
+  if (!schema || typeof schema !== 'object') {
+    return schema;
+  }
+
+  // Create a copy to avoid mutating the original
+  const converted = Array.isArray(schema) ? [...schema] : { ...schema };
+
+  // Replace the $schema version
+  if (converted.$schema === 'https://json-schema.org/draft/2020-12/schema') {
+    converted.$schema = 'http://json-schema.org/draft-07/schema#';
+  }
+
+  // Remove unsupported draft 2020-12 features
+  delete converted.$dynamicRef;
+  delete converted.$dynamicAnchor;
+  delete converted.prefixItems;
+  delete converted.dependentSchemas;
+  delete converted.dependentRequired;
+
+  // Convert $defs to definitions (draft-07 uses 'definitions')
+  if (converted.$defs) {
+    converted.definitions = converted.$defs;
+    delete converted.$defs;
+  }
+
+  // Recursively process nested schemas
+  for (const key in converted) {
+    if (typeof converted[key] === 'object' && converted[key] !== null) {
+      converted[key] = convertSchemaToDraft07(converted[key]);
+    }
+  }
+
+  return converted;
+}
+
+/**
+ * Patches the MCP SDK Server class directly by monkey-patching its setRequestHandler method
+ * This must be called BEFORE any Server instances are created
+ */
+export function patchFastMCPSchemas(_server?: any): void {
+  console.error('[Schema Patch] Applying VS Code compatibility patches...');
+
+  // Patch the Server prototype's setRequestHandler method
+  const originalSetRequestHandler = Server.prototype.setRequestHandler;
+  if (!originalSetRequestHandler) {
+    console.error('[Schema Patch] ERROR: Server.prototype.setRequestHandler not found');
+    return;
+  }
+
+  console.error('[Schema Patch] Patching Server.prototype.setRequestHandler...');
+
+  Server.prototype.setRequestHandler = function(requestSchema: any, handler: any) {
+    // Extract the method name from the Zod schema
+    const method = requestSchema?.shape?.method?.value;
+
+    console.error(`[Schema Patch] setRequestHandler called for method: ${method}`);
+
+    if (method === 'tools/list') {
+      console.error('[Schema Patch] ✓ Intercepting tools/list handler registration');
+
+      const wrappedHandler = async (...args: any[]) => {
+        console.error('[Schema Patch] tools/list handler called, converting schemas...');
+        const response = await handler(...args);
+
+        if (response && response.tools && Array.isArray(response.tools)) {
+          console.error(`[Schema Patch] Converting ${response.tools.length} tool schemas to draft-07`);
+
+          response.tools = response.tools.map((tool: any) => {
+            if (tool.inputSchema) {
+              const before = tool.inputSchema.$schema || 'unknown';
+              tool.inputSchema = convertSchemaToDraft07(tool.inputSchema);
+              console.error(`[Schema Patch] ✓ ${tool.name}: ${before} -> draft-07`);
+            }
+            return tool;
+          });
+        }
+
+        return response;
+      };
+
+      return originalSetRequestHandler.call(this, requestSchema, wrappedHandler);
+    }
+
+    return originalSetRequestHandler.call(this, requestSchema, handler);
+  };
+
+  console.error('[Schema Patch] Successfully patched Server.prototype.setRequestHandler');
+}