OpenAI provides direct access to GPT and related models through the official OpenAI API.
Endpoints
GPT-4.1
Chat Completions sends conversational prompts and returns model-generated replies. This is the main endpoint used by most AI apps.
GPT-5.6 Sol, GPT-5.6 Terra, GPT-5.6 Luna, GPT-5.3 Codex
Responses is OpenAI's newer API for multi-turn conversations, tool use, and structured outputs in a single request.