The OpenAI API gives developers access to the GPT-5 family of models, including GPT-5.4 (the current flagship that passed the human baseline on OSWorld in March 2026) plus its specialized variants for coding, reasoning, and real-time voice. OpenAI pioneered the modern chat completions API that most of the industry now implements, and has the deepest ecosystem of integrations, SDKs, and third-party tools.
OpenAI's primary endpoint is the Responses API (the successor to Chat Completions) which handles messages, tools, structured outputs, and computer-use in a single unified interface. Tool definitions are passed as JSON schemas; Claude returns structured tool_call objects the client must execute and return. Computer-use mode gives the model access to a virtual desktop for agentic automation — it can click buttons, type into forms, navigate file systems, and browse the web, all via screenshots and virtual keyboard/mouse events.
GPT-5.4: $2.50 input / $10 output per million tokens. GPT-5.4-mini: $0.15/$0.60. GPT-5.4-nano: $0.05/$0.20. Computer-use mode charges per screenshot in addition to tokens. Prompt caching discounts available. 1M token context window available on all variants.
Microsoft Copilot, Stripe, Morgan Stanley, Snap, Duolingo, Khan Academy, and essentially every major SaaS company with an AI feature. Largest production deployment of any LLM API.