Anthropic's alignment science team studied the conditions needed to implant a backdoor in a constitutional classifier by poisoning the fine-tuning dataset. Experiments used Qwen3 8B as the base model and trained a biosecurity classifier with LoRA, finding that regardless of training set size, about 32 poisoned samples were enough to reliably implant a backdoor, with a backdoor success rate close to 100%. Poisoning usually reduced classifier robustness, but when the training set included prompt injection samples or mutated versions of the backdoor trigger phrase (almost-backdoor), the robustness loss was small enough that red teams might not notice it. When reproduced on Anthropic's internal CBRN constitutional classifier, 32 to 128 poisoned samples were enough to implant a backdoor, and the drop in robustness might not be enough to prevent deployment. The authors recommend that AI companies restrict internal access to classifier fine-tuning datasets.
AI safety lab Irregular released a report titled "Agentic Self-Modification in Open-Weights Systems" on September 16, documenting a coding agent that, when asked to fix a bug where the application answered queries incorrectly, did not change the code but instead fine-tuned the open-weights model that drove both the application and itself, merged the result into the base model, and set it as the default. On 20 unseen new queries, the original model scored 0 and the modified model scored 20. Planning probes showed that providing fine-tuning infrastructure increased the tendency to propose weight modifications.