adverse-sapphire•4d ago

Assistant Not Triggering Custom Tool WebhooksMessage

Hello VAPI Support, I'm experiencing an issue where my custom tool works when tested directly, but doesn't trigger the webhook when called by my assistant during conversations. DETAILS: - Tool ID: 23a97c05-eb36-4551-b152-2907f3a9ef45 - Tool Name: check_availability - Assistant ID: ed1cee38-797d-4e48-9af5-... - Webhook URL: https://imel.app.n8n.cloud/webhook/vapi-receptionist WHAT WORKS: ✅ Testing the tool directly in VAPI Dashboard → webhook receives request ✅ My webhook endpoint (n8n) works perfectly WHAT DOESN'T WORK: ❌ When assistant calls the tool during chat → webhook never receives request ❌ VAPI logs show "No result returned" error ❌ n8n shows no execution when assistant calls tool I've tried: - Removing and re-adding tools to assistant - Recreating the tool - Verifying all configurations - Checking async settings (it's OFF) - Testing with different prompts The assistant calls the tool (I can see it in VAPI chat logs), but the webhook is never triggered. This only happens when the assistant calls it, not when I test the tool directly. Can you help diagnose why the assistant isn't actually sending webhook requests to my endpoint? Thank you!

2 Replies

Vapi Support Bot•4d ago

Actionable Fix:
The "No result returned" error usually means your webhook isn't returning the exact JSON structure Vapi expects. Make sure your webhook always responds with HTTP 200 and this format (replace call_123 with the actual toolCallId from the request):

{
  "results": [
    {
      "toolCallId": "call_123",
      "result": "Your single-line string result"
    }
  ]
}

{
  "results": [
    {
      "toolCallId": "call_123",
      "result": "Your single-line string result"
    }
  ]
}

Even for errors, use the same structure with an "error" field instead of "result".
Double-check your webhook's response to match this exactly—this should resolve the issue.
See Vapi troubleshooting guide for details. Source: - Custom Tools Troubleshooting – Vapi Documentation

⤵️ Mention @Vapi Support Bot for follow-ups 🐥

flat-fuchsia•2d ago

I can help troubleshoot this. Since direct tool tests work, the issue likely lies in how the assistant invokes the tool or handles its output. Could you confirm if the assistant is set to wait for the tool’s response and if output mapping is configured? @seda

Assistant Not Triggering Custom Tool WebhooksMessage

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