Steam achievements
Batch unlocks, owned DLC
Steam achievement sets: batched unlocks fail on unowned DLC
Valve's Achievement Sets beta lets Steam games group achievements by base game, DLC and other sets. Steam refuses unlocks for DLC the account doesn't own or for archived sets, and one rejected item can fail an entire batch write. A C# developer maintaining an achievement manager explains the schema changes, the parsing code and the ownership-check fix.