TIDY is always executed for all entities when a request fails. The system would not know in advance if there were any build materials for the entities. Thus, the TIDY phase is invoked. This is considered a good "just in case" mechanism to clean up any build materials left over or to determine if they should be kept for subsequent re-release which happens with interdependency.
TIDY is also valid for HELD entities when a request completes successfully. Assuming an interdependent request with entities A and B on first execution, the build for A could succeed but then the build for B fail. In that instance, the build materials for A would be retained. If the user then held entity A before resubmitting the request, the tidy phase needs to be executed in order to get rid of the retained build materials from the first submission.
This Frequently Asked Question applies to all supported releases of ESP Alchemist.