What
Print mode is Claude Code's one-shot, non-interactive mode. It can keep a spare process started in advance and waiting, called "parked", until it is claimed for a real session. When a process is parked and there is no resume or continue, it now loads with no messages and does not run session-start hooks. Session-start hooks are commands you configure to run when a session begins. A new takeClaimSessionStartHooks step runs them after the process is claimed.
Why
Your session-start hooks run for the actual session, not while a spare process is sitting idle.