OpenAI details training activation classifiers for safety and alignment

OpenAI describes training activation classifiers, or probes, that read a model's internal residual stream activations to flag misaligned or policy-violating assistant completions, and says it has deployed them on internal and external traffic of its most capable models starting from GPT-5.6. The probes use an Attention probe architecture with MLP plus QV-attention and an exponential moving average, are trained on up to 1M messages labeled by reasoning-model judges, and run in streaming fashion on the inference engine so a completion can be blocked mid-turn. OpenAI reports excellent probe transfer across many post-training stages but more limited transfer across pre-training checkpoints, and says probes were overall more robust to jailbreaks than content-based classifiers in its red-teaming.