Randomly Bit-Flipping LLM Weights Kills Every Model at About 23 Flips, and Protecting One Bit Raises That to 490,000

spock.is, via r/LocalLLaMA·medium signalXBlueskyLinkedInCopy link

A 2026-08-20 writeup that hit 600 upvotes on r/LocalLLaMA simulated cosmic-ray strikes on Qwen2.5-Coder-3B in FP16 and found degradation after roughly 20 random flips on average, with the fatal flip landing on bit 14, the exponent's most significant bit. Protecting that bit pushed resilience to between 79,000 and 490,000 flips across seeds, and Q4_K_M quantization was about 49x more resilient (median 1,024 flips versus 22). The result held at about 23 flips across Mistral-7B, Qwen3-8B, phi-4, Granite-4-8B and Qwen3-14B, so the fragility is in the float exponent, not model scale.