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**The maintained set is an intent, not a snapshot.** An unnamed set is stored as the intent to maintain every index, which lance re-resolves whenever it builds a MemTable, so an index created after the spec installs is maintained too. A named set is checked here, where the caller can be told which names exist; an index of a kind the MemWAL cannot mirror fails a named set and is skipped by an unnamed one. **A newly created index reaches a writer that is already open.** lancedb caches a `ShardWriter` per shard and reuses it, so `create_index` installs the current set on the cached writer. A failure there is reported rather than propagated: the index is committed durably by then, and the writer this process holds may be fenced or poisoned, which is no reason to fail creating an index. **A table carries one spec.** `set_lsm_write_spec` fails while one is installed and names `unset_lsm_write_spec` in the error, because the generations already written were homed under that spec. Accepting a repeat that changed only the maintained set would hand a caller who also passed a different sharding a success that changed nothing they asked for. **The read path asks what is actually carried.** An unnamed set leaves the stored list empty, so the list alone cannot decide whether an index is maintained. The read asks the spec *and* every resident MemTable whether it carries the index, so a MemTable built before the index joined the set refuses the read rather than answering without its rows. Both spec routes report an unnamed set as `null` rather than `[]`: maintain whatever the table gains, and maintain nothing, are different settings. ## Known gap The guard above covers resident MemTables. Once a writer flushes and closes none is resident, so a retained generation written before the index existed still gets an indexed read that omits its rows. This window is **wider than on main**: a set resolved at install time excluded an index created later, so the read was refused; an intent includes it, and the refusal only holds while a MemTable is resident. Closing it needs the generation's own coverage, which this layer cannot see — its snapshot carries a generation and a path — so it belongs in lance's scanner or flush path. ## Testing `cargo nextest run -p lancedb --lib --features remote` — **1479 passed, 3 skipped**. `cargo fmt --all --check`, and `cargo clippy --features remote --lib --tests --examples -- -D warnings`, both clean. Covered: the round trip of an unnamed set, a set over an installed spec refused, a spec replaced through unset, a read refused while a resident MemTable predates the index, and an index created mid-write reaching the open writer. Two Python binding tests move with the semantics: an unnamed set now reads back as `None` rather than the list it resolves to, and the refusal is matched on `already set`. Neither was run locally — no Python build here — so CI is the check on them.