AI Coding Assistants Know the React Key Rule But Routinely Ignore It in Practice

AI coding assistants can correctly explain why using array indexes as React keys is problematic, yet frequently generate exactly that pattern in real codebases. The issue is not a knowledge gap — models have encountered this React best practice thousands of times during training. The problem arises because generating code token-by-token under multiple simultaneous decisions differs from directly recalling a known rule. Index keys tend to appear when data lacks an obvious unique identifier, when list rendering is a small part of a larger request, or in early-stage placeholder components that often ship unchanged. This highlights a broader gap between an AI model possessing information and reliably applying it during active code generation.