The tassert is triggered when a stage which sets HostTypeRequirement::kAllShardHosts is followed by a stage with which requires a shard but which does not set mergeShardId, such as $unionWith (kTargetedShards) or $group with allowDiskUse=true which disqualifies the mongos.

We also need to have no collation on the command and the collection to be untracked.

The following fail:

[\{$planCacheStats: {allHosts: true}}, $unionWith: \{...}]

[\{$planCacheStats: {allHosts: true}}, $group: \{...}], {allowDiskUse: true}

Other stages which could be used for kAllShardHosts are collStats, $currentOp.

I've attached a reproduction script.