isValidForWrite

Whether this level is valid as the regular (non-serial) consistency of a write operation.

SERIAL/LOCAL_SERIAL are the only levels here Cassandra/Scylla reject as a write's regular consistency — the coordinator errors out server-side ("You must use conditional updates for serializable writes"). Those two levels are only meaningful as the separate serial consistency parameter of a conditional (IF) statement — see the serialConsistency parameter on saveIfNotExists()/update() — not as a table's regular write consistency.

See resolveWriteConsistency in kandra-runtime's StatementBuilder and kandra-ktor's Kandra plugin, mirroring isValidForRead's two enforcement points.