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I have an assistant, I am calling it using `assistant-request` feature, so it doesn't have an **assistant ID**, I want to connect the user to the call and during the call I would ask some questions and on the basis of answers if it is our customer then I want to update the system prompt, as well as some assistant configurations like tools, models and voice etc. How can I do it? I have assigned a server URL to a phone number, so when a person calls I got the assistant request.

Just to clarify, you would like to adjust the system prompt, assistant tools, model, and voice after the call has initialized?

Try using the Live Call Control feature. Your server can listen to the transcript and force the pipeline to swap the assistant on the fly. You can pass another transient assistant with a different prompt, voice, etc.
https://docs.vapi.ai/calls/call-features#6-handoff-call

This is the transfer of a call to different assistant. Is there any way I can update the prompt or tools, I don't want the xutosmer to know that the call is transfered. And what about the context of the call? Will it also be tranferred to the new assistant?

context engineering plan allows you to change which messages are going to be passed to the next assistant

you would transfer the call to a different assistant, modified from the original assistant with a new prompt, tools, model, voice, etc.

check out the docs https://docs.vapi.ai/tools/handoff