OpenAI API
API access to GPT-5, GPT-5.5, o3/o4 reasoning models, and the Responses API; plus embeddings, image, audio, and Realtime endpoints. The most widely deployed LLM API in production.
Letta
Letta (formerly MemGPT) is an open-source framework for building stateful AI agents with persistent, editable memory. Agents remember conversations across sessions, search their own memory, and update their knowledge over time — solving context window limitations.