Every few weeks another framework ships a bigger context window and someone declares agent memory solved. It isn't. I've watched three separate production agents degrade in the exact same way — not because they forgot something important, but because they remembered too much and couldn't tell what mattered anymore. The fix wasn't more storage. It was an eviction policy.
The thesis
Here's the hot take: retention is not the hard problem in agent memory. Forgetting is. Most teams building "memory" into their agents are really building append-only logs with a vector index bolted on for search. That's not memory, it's a diary nobody edits. And a diary that never gets edited eventually buries the one relevant entry under ten thousand irrelevant ones — at which point your retrieval step is doing archaeology, not reasoning.
The symptom is specific and recognizable if you've run an agent past a few hundred sessions: retrieval quality degrades, but the failure looks like a prompting problem, not a memory problem. The agent contradicts itself. It re-asks questions it already has answers to. It surfaces a fact that was true three iterations ago and has since been superseded, and states it with total confidence because nothing marked it stale. You go tune the retrieval prompt, add reranking, bump top_k. None of it fixes the actual defect, which is that the memory store has no concept of a fact going bad.
