Fresh Memory, Stale Plans: Derivation Currency for Distributed LLM-Agent Memory

A large language model (LLM) agent that inherits a plan through shared memory can hold the latest requirement yet act on a plan derived from an older one: fresh memory, stale plan. Freshness checks miss this failure because they compare local copies with current state (observation currency) rather than the inputs the plan was derived from (derivation currency). PlanFence makes derivation currency checkable after a handoff. Stored plans carry exact links to their recorded inputs; before a protected action, PlanFence follows those links to an action-specific dependency frontier, asks each input's owner for its current head, refreshes what changed, and allows one replan before blocking. Application code supplies the links and declares the scope; no shared memory service is required. Holding the native S-Bus validator fixed, supplying inherited input versions raises detected handoff conflicts from 0/30 to 30/30: retained evidence is the missing ingredient. In 30 live five-agent workflows with a revision inserted after planning, a freshness-only executor acts on the stale plan every time, whereas PlanFence, like a centralized-lineage baseline that requires a shared store, completes all 30 correctly. In matched replay under emulated LTE traces, PlanFence's stall stays within 143-237 ms per action across a 64$\times$ range of update rates while per-update synchronization grows from 52 to 1,196 ms; synchronization is cheaper only at the lowest tested rates. Scoping queries to the declared dependencies holds traffic at 8.1 KiB per action, a tenth of all-key validation at 128 keys; at full scope the two tie.