Context Engineering Rule of Thumb: Keep Under 25% of Context Window Regardless of Model Size — Martin Fowler/Ajabbi Workshop Documents the Practical Ceiling

Martin Fowler·medium signal

Martin Fowler's article and Fausto Coppi's Amsterdam workshop on context engineering for coding agents converge on the same finding: keep context usage under 25% of the window, whether 200K or 1M tokens. Beyond that threshold, agent performance degrades as the model struggles to prioritize relevant information. The practical limit for a high-signal CLAUDE.md is 80-120 lines. Coding conventions belong in path-scoped rules, not CLAUDE.md. Engineering work shifts from writing code to managing context, memory, tools, and retrieval.