I have tested the new mcp client extensively and have been getting this error most of the time: {"error": "MCP error -32603: Tool not found"} Sometimes it works and sometimes it doesn't.

Based on the information I found, the "MCP error -32603: Tool not found" error occurs when Vapi can't locate a required tool. This intermittent issue could be caused by incorrect

toolCallId

usage. Try ensuring your response format follows this exact structure:

json
{
"results": [{
  "toolCallId":"1. message.toolCalls[].id",
  "result":"Your tool response here"
}]
}

Make sure the

toolCallId

is dynamically replaced with the actual ID from the incoming request. Source: - [Discord conversation about toolCallId issues](https://discord.com/channels/1211482211119796234/1325000684231196775)

Hi, My MCP server is N8N. The problem is that the tool is not even triggered for the error to occur.

Formatting issues, such as returning a stringified object instead of a JSON object, or incorrect webhook configurations, can prevent the tool from being recognized or invoked. Ensure your n8n webhook returns the MCP tool response in the correct object format, for example: { "results": \[ { "toolCallId": "your-tool-call-id", "result": "your result string" } \] } and not as a stringified JSON.

Thanks for the feedback. It worked using webhook and within webook querying mcp. However, I was unable to use the mcp client directly from VAPI. Since my mcp server is n8n, the n8n node does not allow structuring a return json response. So, sometimes it works and sometimes it doesn't. When it works, it returns the json in the conversation (I'm testing via chat in VAPI). When it doesn't work, I get the error {"error": "MCP error -32603: Tool not found"}

There might be an issue with your MCP tool configuration. The error you're getting indicates that the tool isn't always registered or available to your assistant. Double-check that your MCP server URL is correct and securely configured in the Vapi Dashboard. Also, ensure your tool configuration is complete and that your MCP server consistently provides the list of available tools in the expected format.

There may be some instability of N8N, since the MCP server was recently released. Today, the MCP Server node does not allow much configuration. It simply connects to the desired service and provides a MCP query url.

Hey Gsoares, your right its on their side but still I will keep your thread under investigation and let you know if I find anything related to it.