mongo-migration

MongoDB schema migration safety reviewer and migration script generator. ALWAYS use when writing, reviewing, or planning MongoDB schema changes — field additions/removals, index builds, schema validator changes, document type migrations, shard key modifications, or any bulk update touching production collections. Covers index build lock behavior (foreground vs rolling builds), additive schema evolution, _id-range batched updates, write concern tuning during migration, reshardCollection (5.0+), collMod validator changes, and rollback planning. Use even for "just add a field" — MongoDB's schema-less nature makes silent type inconsistencies and missing-field bugs harder to detect than RDBMS constraint violations.