An agent with a 1M-token context still meets you as a total stranger tomorrow — the context window is RAM, wiped the moment the session ends. Real memory is an engineering system you build around the model: what to save, when to update it, and what to forget. This is the taxonomy, the storage trade-offs, and the write policy that separates a gimmick from a colleague.
Prompt engineering was about wording one message. Context engineering is about managing the entire context window as a scarce budget — what goes in, in what order, and what gets evicted. For a backend engineer, it's working-set management applied to an LLM.