AutoGen
Microsoft research framework for building multi-agent systems through conversational patterns. Officially superseded by Microsoft Agent Framework 1.0 (April 2026), which merges AutoGen and Semantic Kernel. Large installed base remains; Microsoft publishes a migration path.
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.