What the system sees is built, not accumulated
The pattern three independent teams arrived at. The window holds only what the current step needs. Everything else waits outside it, in storage, and is brought in by reference.
The window is not the memory. It is the desk. The filing cabinet is somewhere else, and the useful question about any system is who owns the cabinet and how it decides what to take out of it.
Sources: Packer et al., MemGPT, arXiv:2310.08560, UC Berkeley (October 2023), since commercialised as Letta. Hangfei Lin, Google Developers Blog (December 2025). Yichao Ji, "Context Engineering for AI Agents: Lessons from Building Manus" (July 2025).