I need to validate that an address is in the right format, how do I do this? I am creating an api that can check and then respoond but how does it loop back to asking address if it is wrong and then checking again?

Use a custom validation tool in vapi to check the address format, then in your assistant’s flow, prompt the user for their address, call the validation tool, and if invalid, ask them to re-enter it repeatedly until a valid address is provided before proceeding..

What type of tool would I use

By calling your API, and loop back to re-prompt the user if the address is invalid..