2026-04-16 · claudecert.com
Leave headroom: the 70% rule
Plan for ~70% of the window so you have room for outputs and unexpected tool result blocks.
Context windows aren't budgets you spend down to zero. Tool calls return variable-size results. Models occasionally produce more output than you expected. Compaction needs room to write a summary in.
Target ~70% utilization. When you hit it, compact. Track tokens per *section* of your prompt (system, tools, history) so you know which section to compact first.