We developed a MCP server that runs on http://localhost:8080/mcp and its able to connect and provide data to MCP Inspector but for N8N MCP client not. Both applications are running locally but it seems there is bug ( even the endpoint is included in the correct way, the “endpointUrl”: show spaces in the endpoint “http: //127.0.0.1: 8080/mcp” using streamable http option since the n8n detailed log shows: “12:49:55.163 debug Workflow execution finished with error { “error”: { “level”: “warning”, “tags”: { }, “description”: “Could not connect to your MCP server”, “timestamp”: 1755877795160, “context”: { “itemIndex”: 0 }, “functionality”: “configuration-node”, “name”: “NodeOperationError”, “node”: { “parameters”: { “notice”: “”, “endpointUrl”: “http: //127.0.0.1: 8080/mcp”, “serverTransport”: “httpStreamable”, “authentication”: “none”, “include”: “all”, “options”: { } }, “type”: “@n8n/n8n-nodes-langchain.mcpClientTool”, “typeVersion”: 1.1, “position”: [352,224], “id”: “1f9a200c-f127-4839-a47c-7e2886911375”, “name”: “MCP Client”, “rewireOutputLogTo”: “ai_tool” }, “messages”: , “message”: “Error in sub-node MCP Client”, “stack”: “NodeOperationError: Error in sub-node MCP Client\n at ExecuteContext.getInputConnectionData (C: \Users\Adriano\AppData\Roaming\npm\node_modules\n8n\node_modules\n8n-core\src\execution-engine\node-execution-context\utils\get-input-connection-data.ts: 302: 11)\n at processTicksAndRejections (node: internal/process/task_queues: 105: 5)\n at ExecuteContext.getInputConnectionData (C: \Users\Adriano\AppData\Roaming\npm\node_modules\n8n\node_modules\n8n-core\src\execution-engine\node-execution-context\execute-context.ts: 203: 10)\n at ExecuteContext.execute (C: \Users\Adriano\AppData\Roaming\npm\node_modules\n8n\node_modules\@n8n\n8n-nodes-langchain\nodes\ToolExecutor\ToolExecutor.node.ts: 57: 22)\n at WorkflowExecute.runNode (C: \Users\Adriano\AppData\Roaming\npm\node_modules\n8n\node_modules\n8n-core\src\execution-engine\workflow-execute.ts: 1212: 9)\n at C: \Users\Adriano\AppData\Roaming\npm\node_modules\n8n\node_modules\n8n-core\src\execution-engine\workflow-execute.ts: 1582: 27\n at C: \Users\Adriano\AppData\Roaming\npm\node_modules\n8n\node_modules\n8n-core\src\execution-engine\workflow-execute.ts: 2158: 11” }, “workflowId”: “7juyxalWNVlF7d5M”, “file”: “logger-proxy.js”, “function”: “exports.debug” }”. I am using the latest n8n version.
Hey @INVX hope all is good, welcome to the community.
Please
- share details about your setup (including related env vars)
- share your workflow
- share screenshots or the error, of the inspector
- which applications you are referring to when you say “both applications”
Thank you Jabbson.
The vNode MCP server, MCP Inspector and n8n are running in a Windows 11 machine.
N8N_COMMUNITY_PACKAGES_ALLOW_TOOL_USAGE=True in the env vars
MCP Inspector works a expected. The issue just happen trying to use the n8n MCP client.
MCP server was built in Javascript as script into an Industrial software gateway called vNode. It was tested with success with MCP Inspector:
- when you say that n8n is running in a windows 11 machine, do you mean in Docker or with npm/npx?
- please show the output for when you click on Open node in the error.
- please show the MCP Client node setup
I installed initially in Docker but due the issue I decided to try installing with npm.
Below the n8n log and the MCP Client configuration:
Notice that even I fill up in the correct way the endpoint in the log it showed with spaces (my white tag in the log).
Below the MCP server log and the MCP Inspector working as expected:
I’ve noticed in n8n you use /mcp in the path and in the inspector you do not. Could you try to not use /mcp in n8n as well and see if that helps?
Good news! I really discovered that is a bug in the MCP Client node.
After 2 days and nights 
of many searches in Google Gemini, ChatGPT and many changes in my MCP server code, I remember of Grok and just with some interactions it found a reference about the bug in the n8n MCP native client. I installed community “nerding-io/n8n-nodes-mcp” as suggested by Grok and worked in the first time! Thank you for the support Jabbson!
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