VAPI•7mo ago

VAPI incorrectly naming MCP function name

I'm encountering an issue when calling my custom MCP server. Although I've successfully set up the vapi assistant with the MCP server, the tool names are not being handled correctly. The server lists the tools as check_availability and create_book but when a tool is called, its name is prefixed with "run-". For example, instead of calling check_availability it calls run-check_availability which is causing the request to fail.

Example of the tool call result:

{
    "tool_calls": [
        {
            "id": "call_13MSnBy1svzJRGGmisKCEW2w",
            "type": "function",
            "function": {
                "name": "run-check_availability",
                "arguments": {
                    "date": "2025-05-30"
                }
            }
        }
    ]
}

{
    "tool_calls": [
        {
            "id": "call_13MSnBy1svzJRGGmisKCEW2w",
            "type": "function",
            "function": {
                "name": "run-check_availability",
                "arguments": {
                    "date": "2025-05-30"
                }
            }
        }
    ]
}

{
    "tool_calls": [
        {
            "id": "call_13MSnBy1svzJRGGmisKCEW2w",
            "type": "function",
            "function": {
                "name": "run-check_availability",
                "arguments": {
                    "date": "2025-05-30"
                }
            }
        }
    ]
}

{
    "tool_calls": [
        {
            "id": "call_13MSnBy1svzJRGGmisKCEW2w",
            "type": "function",
            "function": {
                "name": "run-check_availability",
                "arguments": {
                    "date": "2025-05-30"
                }
            }
        }
    ]
}

Appreciate any help on this.

extreme-purple•5/29/25, 2:10 PM

Update your MCP server to strip the prefix before handling the request or adjust your assistant’s configuration to avoid adding the prefix in tool calls.

Vapi Support BotAPP•5/29/25, 2:12 PM

Suggestion:
As a quick workaround, update your MCP server's handler to strip the "run-""run-" prefix from incoming function names before dispatching them. For example, in Node.js/Express:

const functionName = req.body.function.name.replace(/^run-/, "");
// Now use functionName to route the call

const functionName = req.body.function.name.replace(/^run-/, "");
// Now use functionName to route the call

This will allow your server to handle both prefixed and unprefixed tool calls until the root cause is identified.

Source:

Model Context Protocol (MCP) Integration – Vapi Docs

⤵️ Mention @Vapi Support Bot for follow-ups 🐥

Vapi Support BotAPP•5/29/25, 2:12 PM

Suggestion:
As a quick workaround, update your MCP server's handler to strip the "run-""run-" prefix from incoming function names before dispatching them. For example, in Node.js/Express:

const functionName = req.body.function.name.replace(/^run-/, "");
// Now use functionName to route the call

const functionName = req.body.function.name.replace(/^run-/, "");
// Now use functionName to route the call

This will allow your server to handle both prefixed and unprefixed tool calls until the root cause is identified.

Source:

Model Context Protocol (MCP) Integration – Vapi Docs

⤵️ Mention @Vapi Support Bot for follow-ups 🐥

Vapi Support BotAPP•5/29/25, 2:12 PM

Suggestion:
As a quick workaround, update your MCP server's handler to strip the "run-""run-" prefix from incoming function names before dispatching them. For example, in Node.js/Express:

const functionName = req.body.function.name.replace(/^run-/, "");
// Now use functionName to route the call

const functionName = req.body.function.name.replace(/^run-/, "");
// Now use functionName to route the call

This will allow your server to handle both prefixed and unprefixed tool calls until the root cause is identified.

Source:

Model Context Protocol (MCP) Integration – Vapi Docs

⤵️ Mention @Vapi Support Bot for follow-ups 🐥

VapiAPP•5/29/25, 9:33 PM

Hey! To help track down this issue, could you share:

The call IDThis would really help us figure out what went wrong!

VAPI incorrectly naming MCP function name

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