assistantId and custom properties within the assistant parameter in your payload, it's important to understand how Vapi interprets this. If you provide an assistantId, it uses the configuration of the assistant associated with that ID from the dashboard. However, when you also include custom properties within the assistant object (like firstMessage in your case), you're essentially creating a transient, on-the-fly assistant configuration for that call only. This means the call will not inherit other settings from the assistant specified by assistantId but will use the properties you've explicitly included in the assistant object in your payload.firstMessage while retaining the other configurations of your assistant from the dashboard, you would typically need to ensure that all required properties of the assistant are included in the assistant parameter of your payload. This may involve replicating the configurations you've set up in the dashboard within your API request payload.firstMessage of an assistant while keeping the rest of the assistant's configurations intact from the dashboard settings without specifying the entire configuration in the request (1) .firstMessage needs to be dynamic, you might consider programmatically retrieving the assistant's configuration using an API call to List Assistants (2) , modifying the firstMessage in your application logic, and then passing the entire configuration in your call creation API request. This approach, though not dirfirstMessage.List Assistants