afraid-scarlet
afraid-scarlet•2mo ago

MCP Integration Bug: Tool Function Names Not Properly Exposed

The Issue Vapi's MCP integration creates a non-existent wrapper function instead of exposing actual MCP server tools, causing "Unknown tool" errors in production calls. Expected vs Actual Behavior MCP Server defines: 
@mcp.tool
def get_appointment_timeslots(zip_code: str, address: str, city: str): pass

@mcp.tool  
def create_appointment(correlation_id: str, first_name: str, ...): pass
@mcp.tool
def get_appointment_timeslots(zip_code: str, address: str, city: str): pass

@mcp.tool  
def create_appointment(correlation_id: str, first_name: str, ...): pass
Should expose: get_appointment_timeslots and create_appointment
Vapi exposes: book_appointments (doesn't exist in MCP server) Production Failure AI attempted book_appointments 3 times, all failed: 
// Request
{"function": {"name": "book_appointments", "arguments": "{\"action\": \"get_appointment_timeslots\", ...}"}}

// Response  
{"result": "[{\"type\": \"text\", \"text\": \"Unknown tool: book_appointments\"}]"}
// Request
{"function": {"name": "book_appointments", "arguments": "{\"action\": \"get_appointment_timeslots\", ...}"}}

// Response  
{"result": "[{\"type\": \"text\", \"text\": \"Unknown tool: book_appointments\"}]"}
Customer couldn't book appointment due to this bug. Root Cause Vapi incorrectly parses MCP tools/list JSON-RPC responses. Instead of exposing individual tools per the MCP specification, it creates a wrapper that doesn't exist. Vapi's tool config shows the mismatch: 
{
  "function": {"name": "book_appointments"},  // ❌ Fake wrapper
  "metadata": {
    "tools": [
      {"function": {"name": "ngrokfree-get_appointment_timeslots"}},  // ✅ Real tools
      {"function": {"name": "ngrokfree-create_appointment"}}
    ]
  }
}
{
  "function": {"name": "book_appointments"},  // ❌ Fake wrapper
  "metadata": {
    "tools": [
      {"function": {"name": "ngrokfree-get_appointment_timeslots"}},  // ✅ Real tools
      {"function": {"name": "ngrokfree-create_appointment"}}
    ]
  }
}
Comparison Claude Desktop (works correctly): - front-desk-appointment:get_appointment_timeslots ✅ - front-desk-appointment:create_appointment ✅ Impact on MCP's Purpose This bug defeats the entire point of MCP. The protocol exists to expose groups of related tools through a single server, but Vapi's implementation forces developers to create individual tools one-by-one instead of leveraging MCP's server-based tool discovery. This makes MCP integration pointless on Vapi.
2 Replies
afraid-scarlet
afraid-scarletOP•2mo ago
https://github.com/VapiAI/mcp-server/issues/7
GitHub
MCP Integration Bug: Tool Function Names Not Properly Exposed · Is...
The Issue Vapi's MCP integration creates a non-existent wrapper function instead of exposing actual MCP server tools, causing "Unknown tool" errors in production calls. Expected vs Ac...
Sahil
Sahil•4w ago
Hey, Can you send me the latest call_id? Along with that, can you provide me the duration in the recording where you had this issue?

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