Compressing system-side control context has a sharp, non-linear reliability cliff
"Control Under Compression" (arXiv 2608.01056, 2 Aug 2026) looks at agent control contexts rather than conversation history. A control context is the persistent system-side instruction set. It specifies tools, arguments, policies, execution protocols and recovery. The benchmark ran 15,525 environment-verified runs. Retaining 75% of the context held success near the full-context baseline, at 92.7% and 92.4% against 93.8%. Between 50% and 35% the methods diverged sharply. At 35% they scored 47.0%, 39.0% and 19.9%, depending on the compression method. Failures appeared mainly as tool-execution and action-parsing errors. Reliability varied enough between control contexts that the authors argue no compressor ranking holds universally, and that each context needs its own qualification. So trimming tool and policy prompts is a runtime-reliability decision, and it needs executable testing per context. The safe-looking zone ends abruptly.