Retrieved memories make agents sycophantic, and no existing benchmark measures it.

MemSyco-Bench points out that memory benchmarks test whether memories are correctly stored, retrieved, and updated, never whether the retrieved memory should have influenced the decision at all. Its five tasks check whether agents can reject memory as factual evidence, respect its applicable scope, resolve memory-versus-evidence conflicts, track updates, and still use valid memory for personalization. The failure mode is sharp: agents over-align with the user because a stored memory said so, at the cost of being right. Anyone shipping a persistent-memory agent should run this before trusting their retrieval layer.