How do I know the open-weight base model I am fine-tuning has not been poisoned?

Backdoors planted in pre-trained model weights persist through full-parameter fine-tuning, adapter training, and RLHF updates because the trigger patterns survive objective-shifting and partial-freezing strategies. These triggers are invisible to standard behavioral safety tests and benchmark evaluation. Detecting them requires white-box weight analysis that the average fine-tuning practitioner never runs, and major model hubs apply no mandatory scanning before a checkpoint is made publicly downloadable. An organization building a production system on a compromised base model has no signal anything is wrong until the trigger fires in deployment.