Why 1-million-token context windows won't save your 50-turn agentic workflows, and the concrete engineering patterns, mathematical models, and benchmarks to master context compaction. The Long-Context Illusion in Production In the early days of building LLM applications, the context window was a tight bottleneck. Managing a 4,096-token limit for GPT-3.5 required aggressive prompt slicing, brittle truncation heuristics, and constant vector-store lookups. When foundation model