Antaeus is a semantic policy infrastructure engine that converts human-written policy documents into versioned artifacts with named test cases, explicit outcomes, and traceable evidence. It separates policy meaning from provider configuration, returning structured allow, review, deny, or failure decisions through a provider-neutral contract. The open-source core runs locally without credentials using a deterministic fixture evaluator, while an experimental OpenAI adapter handles real semantic evaluation with a user-supplied key. A hosted beta offers managed policies and API key access for teams wanting a cloud option.