Why LLM Agents Fail: Four Mechanisms of Cognitive Decay and the Reasoning Harness Layer

LLM agents fail in four predictable, mechanism-level ways. Attention decay, reasoning decay, sycophantic collapse, hallucination drift. The current stack (prompting, fine-tuning, RAG, agent loops) cannot close them because each layer operates inside the same decaying chain. The fix is an external layer we call a reasoning harness. If you have built an agent that runs more than ten steps, you have watched it drift. Plans fragment. The system prompt you wrote at the top of the context stops bindin