# ObsDrop

Coding agents mostly append to their history, so prefix-cache reuse reaches 98–99%, yet text-level context compaction rewrites earlier tokens and forces the whole suffix to be recomputed. ObsDrop moves compaction from the prompt into the KV cache: it keeps the logical token prefix unchanged while releasing the KV of stale tool observations and excluding those positions from attention, implemented in SGLang and Sparse-vLLM. At 16 concurrent agents it reaches 2.45–2.85× throughput with FlashInfer and a 5.37× peak with Triton relative to no compaction, with task quality close to no compaction.

Coding agents mostly append to their history, so prefix-cache reuse reaches 98–99%, yet text-level context compaction rewrites earlier tokens and forces the whole suffix to be recomputed. ObsDrop moves compaction from the prompt into the KV cache: it keeps the logical token prefix unchanged while releasing the KV of stale tool observations and excluding those positions from attention, implemented in SGLang and Sparse-vLLM. At 16 concurrent agents it reaches 2.45–2.85× throughput with FlashInfer and a 5.37× peak with Triton relative to no compaction, with task quality close to no compaction.