To allocate transactions to blocks, cryptocurrencies use auction-esque transaction fee mechanisms (TFMs). A conjecture of Roughgarden (2024) asks whether there is a TFM that is incentive compatible for both users and the miner and is also resistant to a collusion that allows them to jointly increase overall welfare. We resolve this question in the negative for deterministic TFMs, through a comprehensive characterization of designs robust to this form of collusion. For randomized TFMs, we show limits on the efficiency they can achieve.

Y. Gafni, A. Yaish, Transaction fee mechanisms robust to welfare-increasing collusion, Games and Economic Behavior 157 (2026) 351-375.