Summary of necessary driver changes   $map and $filter get a new option, arrayIndexAs . Similar to as , this is used to specify the variable name that the index of the current element is assigned to. $reduce gets three new options: as , arrayIndexAs , valueAs New variables are introduced: $$THIS and $VALUE are introduced as aliases for the existing $$this and $$value variables and added to improve consistency with other variables that are uppercase (e.g. $$KEEP , $$REMOVE ). The new $$IDX variable is introduced to contain the index value if indexValueAs is not used. Drivers with aggregation pipeline builders need to add support for the new options, and for the new variables if they use an enum to specify valid system variables. Context for other referenced/linked tickets  Syntax document: https://docs.google.com/document/d/1V0RwknDCgEEo6ueZLnZyRU_yt3gs9lNm7d37dzI8uN0/edit?tab=t.0