The moment you change a state machine's definition, every instance already running it is still running the old one. polyvers classifies the change, seeds it against real in-flight state pulled from the live fleet, and gates the deploy on the verdict.
The step most verification tooling skips entirely.
Every version diff is sorted into a compatibility lane before anything else happens: additive, migrated, or breaking.
The new definition is checked against snapshots of instances that are actually running right now, not a clean synthetic dataset.
A breaking change with no valid migration for in-flight state is blocked before it ships, named offenders included.