fix(mcp): clarify missing remote server URL

packages/components/nodes/tools/MCP/core.test.ts

@@ -5,7 +5,8 @@ import {
     validateEnvironmentVariables,
     validateMCPServerConfig,
     sanitizeMCPToolDescription,
-    sanitizeMCPToolName
+    sanitizeMCPToolName,
+    MCPToolkit
 } from './core'
 
 describe('MCP Security Validations', () => {
@@ -753,3 +754,13 @@ describe('sanitizeMCPToolName', () => {
         warnSpy.mockRestore()
     })
 })
+
+describe('MCPToolkit', () => {
+    it('explains how to configure a missing remote URL', async () => {
+        const toolkit = new MCPToolkit({ command: 'npx', args: ['some-server'] }, 'sse')
+
+        await expect(toolkit.createClient()).rejects.toThrow(
+            'URL is required for remote MCP transport. For stdio servers, set CUSTOM_MCP_PROTOCOL=stdio and provide command/args.'
+        )
+    })
+})

packages/components/nodes/tools/MCP/core.ts

@@ -118,7 +118,9 @@ export class MCPToolkit extends BaseToolkit {
             await client.connect(transport)
         } else {
             if (this.serverParams.url === undefined) {
-                throw new Error('URL is required for SSE transport')
+                throw new Error(
+                    'URL is required for remote MCP transport. For stdio servers, set CUSTOM_MCP_PROTOCOL=stdio and provide command/args.'
+                )
             }
 
             const baseUrl = new URL(this.serverParams.url)