"Collectionless" collection names include $cmd.aggregate, $cmd.listCollections, and others. Some commands (like most writes, including createIndexes) already reject these namespaces, while others (like listIndexes, drop) accept the namespace and treat it as a non-existing collection, returning "OK" responses as if they were actually doing meaningful processing.

It would be convenient if there was a generic way to reject collectionless namespaces for commands that require a real collection. I tried implementing something like this in IDL (see link in comments) but it only looked for "$cmd.aggregate". Maybe that PR could be extended to check for generic "collectionless" namespaces, possibly via CommandHelpers::ensureValidCollectionName() (although that might reject other internal but valid namespaces?). Alternatively, maybe there is another approach that doesn't require IDL changes.