September 12, 2026 · 8 min read

What Is Context Rot

A model with a million token window still does not read it evenly. It handles the front and the back with care and grows careless in the middle, and the more you load in, the wider that careless zone gets. That is context rot, the measurable drop in quality as the input grows, and it starts long before the window is full. In a July 2025 report, Chroma tested 18 current models, among them GPT 4.1, Claude 4, Gemini 2.5, and Qwen3, and found every one got worse as the input got longer, even on finding a fact or copying text. Two years earlier a Stanford team named the lost in the middle effect, where accuracy fell more than 30 percent when the answer sat in the middle rather than the edges. What context rot is, why a bigger window is not a bigger memory, and the smaller cleaner window that fixes it.