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.
Anthropic API
API access to Claude model family including Claude 3.5 Sonnet, Haiku, and Opus. Excels at coding, analysis, long-context tasks, and safety-critical applications.