# Reindex migrations

`REQUIRES_REINDEX` always names distinct source and shadow targets and a distinct target
`EmbeddingSpace`. Direct deployment swaps and same-index vector mixing fail validation.

Preflight proves the frozen source fingerprint/count, target absence, space and capability health,
credential, storage, scheduler capacity, adapter availability, rollback retention, evaluation suite
and current approval. Backfill priority is `BACKGROUND`; batches, concurrency and in-flight work are
bounded. Each checkpoint records cursor, generation, deterministic item/result fingerprints,
completed/retryable/permanent counts, bounded errors and duration.

A retry replaces the prior attempt's counters rather than double-counting them. Completed matching
batches are idempotent. Old versions/generations and changed item fingerprints fail closed. A batch
is complete only after finite shape/space validation, committed destination write and matching
content hashes. Cancellation and pause preserve the shadow for retention-aware resume or cleanup.