Pattern: Prefix-Cache Stability Is Becoming a First-Class Coding-Agent Design Axis

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DeepSeek-Reasonix (27.9k stars, a single static Go binary) is explicitly "engineered around prefix-cache stability — leave it running": it injects a small stable environment summary at startup, prunes stale tool output *before* compaction so the cache prefix survives, and runs optional executor/planner two-model setups in separate cache-stable sessions. vLLM's K3 work solves the same problem from the serving side with copy-on-write partial prefix matching. The pattern worth stealing: treat your agent's context prefix as an append-only structure and never rewrite its head, because every early edit invalidates the entire downstream cache.