A Brain API is an interface that exposes “AI brain” capabilities—like memory, retrieval, prompt programs, and tool use—so your app can call them like standard API functions. Instead of wiring an LLM directly into each feature, a Brain API acts as a shared intelligence layer: it can remember user context, retrieve knowledge from approved sources, and produce structured outputs your product can reliably consume. This approach helps you build AI features that compound over time, rather than resetting every session.