Karpathy's CLAUDE.md just crossed 220k GitHub stars. Here's why it works.

A plain-text CLAUDE.md file placed in a repo root gives Claude Code persistent session context, solving the cold-start problem where the model silently makes wrong assumptions about stack choices and prior decisions. The file encodes four behavioral rules including ask-don't-assume and surface inconsistencies before writing code. Forrest Chang's implementation hit 220k GitHub stars in roughly three months after Karpathy articulated the underlying failure modes in January.