Issue Custom async function tool returns "Unknown tool: undefined" during live calls instead of calling the configured webhook. Setup - BYO phone number (Zadarma SIP) - Tool type:

function

,

async: true

- Tool has

server.url

configured - Tool is in assistant's

model.toolIds

array - GPT-4o model What I See in Call Transcript

AI calls: load_user_memory
     VAPI returns: "Unknown tool: undefined"

- ✅ Same webhook works when tested directly (returns correct data) - ✅ Tool exists and is properly configured - ✅ Other 3 tools on same assistant work fine ## Question Why does VAPI return "Unknown tool: undefined" instead of calling the webhook? Is there something special needed for async tools on BYO numbers? Has anyone else experienced this with async tools?

Hey! The "Unknown tool: undefined" usually happens when the assistant can’t resolve the toolId during live BYO calls, even if it works in testing. I can help debug your async tool setup and ensure the webhook triggers correctly. Are you seeing this only on voice calls or also on text interactions? Knowing this helps pinpoint if it’s a BYO-specific or general async tool issue. @bsbrown_78

Hi @bsbrown_78 please watch out for @User . He says he knows the answer to your problem and he can help. But he will ask for money upfront. I made the mistake of doing so and never heard back from him. I already reported him in the # report-scam channel. But I wanted you to be aware of him. Cheers!