When querying any API method which returns answers (
/answers/ids, etc.) the result does not include the tag information for the related question.
The API does not support filtering answers by tag, but querying for a user's answers is a relatively inexpensive operation for all but the most prolific users. These can subsequently be filtered/sorted without need for additional API calls if the tag data were included for each answer result.
I feel this would add great value to the data without needing to pull all the questions.