WitCert: Sound Runtime Risk Observability and Gating for KV-Cache Quantization

WitCert introduces a runtime meter for KV-cache quantization that provides per-layer, per-head, and per-step upper bounds on the total variation between exact and compressed attention. Its deterministic certificate is designed to remain sound for cache-preserving black-box quantizers and adaptive queries, while a tighter probabilistic certificate targets a controlled subtractively dithered INT8 quantizer under an explicit request-level failure budget. An environment-guarded SGLang integration supports live measurement and risk-based gating. The authors report that gating raises raw-cast FP8 performance from 22.8 to 79.7 on hard RULER tasks, and that certified INT8 serves 1.88 times more KV tokens at equal memory.