LLM Safety Circuits Found in Just 50 Neurons by Unit 42

Forensic Summary

Palo Alto Unit 42 researchers have developed a technique called perturbation probing that identifies the precise feed-forward neurons responsible for LLM safety refusal behaviour, finding that as few as 50 neurons out of 350,208 control safety guardrails in Qwen3-4B. Disabling those neurons altered responses on 80% of tested harmful prompts, demonstrating that RLHF-aligned safety is structurally fragile rather than distributed. The research also introduces an FFN/Skip ratio metric that predicts model safety fragility across 13 models with 81% explanatory power, giving defenders a rapid quantitative tool for comparing alignment robustness.

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Read the full technical deep-dive on Grid the Grey: https://gridthegrey.com/posts/llm-safety-circuits-found-in-just-50-neurons-by-unit-42/