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Research on multi-agent pathfinding (MAPF) has recently shifted towards problem variants that are closer to actual applications. Such variants often include the assignment of multiple goals to agents. To solve them, researchers have extended the Conflict Based Search (CBS) algorithm to multiple goals. This extension might look straightforward at first sight but it is tricky and this has already led to the development of algorithms that despite claiming to be optimal, return suboptimal solutions for some MAPF instances. In this paper, we provide a detailed analysis of the issue to raise awareness among the search community so that this mistake will not be perpetuated. Furthermore, a first evaluation against an optimal implementation is conducted which shows why this issue might have been difficult to spot. In only one of the randomly generated instances, the suboptimal behavior emerged.
Mouratidis et al. (Sat,) studied this question.