Your context window is not your agent's memory

There's a quiet assumption baked into a lot of agent code: that a bigger context window means a better memory. Vendors ship 200K, then 1M, then 2M token windows, and the implied promise is "just put everything in and the model will remember." After building agents that run for weeks, I've come to think this conflates two things that are not the same — and treating them as the same is exactly why long-running agents get dumber over time. The context window is working memory. Real memory is what survives when the window is gone. Mixing them up is like confusing your desk with your filing cabinet. Two different clocks Working memory (the context window) lives for one session, maybe one turn. It's fast, expensive, and volatile. It's where reasoning happens right now . Durable memory lives across sessions. It's slow, cheap, and persistent. It's what the agent knows when it wakes up tomorrow with an empty window. These have different lifespans, different costs, and different access patterns. The moment you try to make one do the other's job, things break: Use the window as memory → everything you "remember" has to be re-loaded every turn, you pay for it every turn, and the instant the session ends it's gone. Use durable storage as working memory → you're reading and writing files mid-reasoning for things that only matter for the next 30 seconds. A good agent keeps them separate on purpose. Why "just use a bigger window" fails Say you have a 1M token window and you stuff the entire history in. Three problems show up, none of which a bigger number fixes: Cost scales with every turn, not every session. That 1M tokens isn't paid once — it's re-sent on each step of a multi-turn task. A 20-step task can mean 20× the bill, mostly re-reading the same stale history. Attention dilutes. "Lost in the middle" is real: models attend most reliably to the start and end of a long context. Bury the one fact that matters under 900K tokens of transcript and recall quality drops, even though