Deja records a non-deterministic call (today: an Anthropic LLM call) the first time a test makes it and replays the recorded response on every run after that, so tests that exercise real model behavior stay fast, offline, and deterministic. Ships RSpec helpers (use_llm_cache, expect_llm_called, forbid_calls) and a meet_requirements matcher that judges free-text requirements with the model and caches the verdict.