The collStats command includes information about in-progress index builds in its results. It compares the in-memory state from the IndexCatalog with the on-disk state using the DurableCatalog interface to see if an index is available for queries or it is still building.

The current collStats implementation does not determine if an index is visible in the current snapshot before querying the on-disk state. A similar issue affecting $indexStats was discussed and subsequently addressed in SERVER-51133.