There is a model picker that currently allows you to switch between Claude Sonnet 4.0 and Claude Sonnet 3.7

And yes, Kiro is agentic, so it can (and often does) execute a long running multi-turn workflow in response to your interactions, however, the billing model is based on your manual interaction that kicks off the workflow (via chat, spec, or hook), even if that agent workflow takes many turns for Kiro to complete