Anyone got an idea on how to inform the model that an external process is complete, without showng it to the user in UseChat

Say for instance, i get an object response from the model , and i use that response to call an external api , now, i want to send a message to the model via the chat but invisible to the user with the response of the external api call, so it can inform the user.

I tried append with a system role, but it errors out, functionality: 'system messages are only supported at the beginning of the conversation',"