Artificial intelligence (AI) agents built on large language models increasingly work in teams, where one agent divides tasks or resources among others that the software marks with a team, a role or a similar label. A model that simply up-weights any recipient sharing a string with its own context would favor same-label peers, so a shared-label tilt alone says little about groups. Borrowing the minimal-group paradigm of social psychology, we had six language models divide 100 points among 19 peers that differed only in carrying one of two invented words, in groups of unequal size (32,400 decisions). Varying group size separates two effects. In five of the six models, at the most unequal split, a recipient with the rarer label received 1.3 to 3.3 times the points of one with the common label even when the decider was not told its own label: a rarity effect that involves no match to the decider. In four of them it accounts for 35% to 55% of the extra favoritism that agents in the smaller group show their own group, and for more in the fifth. Telling the decider its label adds an own-group effect, positive in every model and in five of the six indistinguishable from label matching. With the labels hidden, neither appears. Because the labels mean nothing, bias tests built on lists of named social groups are not designed to detect either effect.

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