Your LLM application's context window changes over time — new documents added, old ones updated, user state evolves. Design a system for managing this. Cover: how to version prompts and context so you can reproduce past outputs, how to handle backfilling when embeddings or retrieval logic changes, how to track which version of context produced which output for debugging, how to manage context drift when a long conversation accumulates stale information, and how to test that context changes don't regress output quality.