@Shubham Bajaj Would it be possible to have our server send a response to an assistant-request so the call gets automatically transferred to the fallback number? Sending a malformed json does not transfer the call and plays a VAPI message to the caller. callid: a683fd52-5d38-4329-80a8-a0f5e9859e25 callid: 0ce0300d-59be-478d-b424-9a0e6b911fd2 Sending a 400 or 500 message returns a message saying that the server responded with error ###. callid: 93aeecc7-cced-4623-862f-af418ecbd04c callid: 3721e8c3-6b53-4976-b5da-4ed1dd2f4534 This is not the desired outcome.

To automatically transfer a call to a fallback number when an error occurs (such as a pipeline error), you can use an assistant hook like this in your assistant configuration:

json
{
  "hooks": [{
    "on": "call.ending",
    "filters": [{
      "type": "oneOf",
      "key": "call.endedReason",
      "oneOf": ["pipeline-error"]
    }],
    "do": [{
      "type": "transfer",
      "destination": {
        "type": "number",
        "number": "+1234567890"
      }
    }]
  }]
}

Replace

+1234567890

with your fallback number. This will transfer the call if a pipeline error occurs, which is the closest supported approach to your use case according to the documentation. Note: Directly returning a transfer instruction in the assistant-request response for transient assistants is not documented as supported. [See more in the documentation.](https://docs.vapi.ai/assistants/assistant-hooks#example-transfer-on-pipeline-error) Source: - [Assistant hooks](https://docs.vapi.ai/assistants/assistant-hooks#example-transfer-on-pipeline-error)

how can i apply this to transient assistants? The idea would be to have the call transferred if the backend fails to produce an assistant json or even fails to respond.

Yes, you can intercept vapi assistant-request or transfer-destination-request webhook and respond with a connect action (or dynamic transfer JSON) pointing to your fallback number so the call auto-forwards..