This paper presents CacheSmith, a KV cache reuse system for RAG prefill that maximizes quality-gated SLO attainment. While context caching lowers prefill latency by reusing precomputed KV states for retrieved chunks, it misses cross-chunk attention that a full prefill would compute over concatenated inputs. Prior selective recomputation schemes tackle this problem but enforce static, one-size-fits-all configurations that fail to effectively handle request heterogeneity or dynamic latency budgets. CacheSmith decomposes selective recomputation into a three-phase workflow of WarmUp, Probe, and Patch that can be dynamically configured to exploit a wide range of cost-accuracy trade-offs. At serving time, an SLO-aware scheduler in CacheSmith forms batches to mitigate inter-request interference during layer-by-layer prefill, while a budget-aware planner configures recomputation phases across the batch to maximize the number of requests satisfying both generation quality and latency targets. Both components make fast, solver-free decisions over profiled cost-accuracy bands. On four benchmarks across varying arrival rates and burstiness levels, CacheSmith improves quality-gated SLO attainment by up to 27.5%, over four state-of-the-art baselines.

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