Motivated by context management in LLM agents, we study a repeated rent-or-buy model with schedule-independent history growth and a fixed reset price $K$, a discrete form of dynamic TCP acknowledgment: every call pays as rent the history accumulated since the last reset, and a reset clears it. The classical threshold rule is $2$-competitive, and no deterministic rule guarantees a smaller ratio. We show that, measured additively, its loss is governed by the changes in the growth: if the growth is constant on $P$ runs, the threshold rule, knowing neither the runs nor the horizon, pays at most $(2P-1)K$ more than the optimal schedule chosen with hindsight, and the coefficient $2P-1$ is sharp. If the growth is only within a factor $1+\varepsilon$ of $P$ runs, the bound holds up to a factor $1+\varepsilon$ and an extra $\varepsilon K$. The proof is a potential argument built from the relative value functions of the single-rate problems. On two runs no deterministic rule guarantees a loss below $2K$. A rule that also resets when the growth decreases guarantees $2K$, while the threshold rule can lose arbitrarily close to $3K$.

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