The OpenAI API gives developers programmatic access to GPT-5.5, GPT-5.4, and the full range of OpenAI models through two primary interfaces: the Responses API (stateful, agentic, recommended for new projects) and the Chat Completions API (stateless, wide compatibility). With GPT-5.5 launching in April 2026 as the new flagship and open-weight gpt-oss models released for local deployment, the ecosystem now spans everything from sub-cent commodity inference to frontier research-grade reasoning. The core paradigm shift is agents and tools over raw completions: built-in web search, computer use, MCP server integration, and background async execution are first-class API features, not just ChatGPT-only capabilities.
What This Cheat Sheet Covers
This topic spans 22 focused tables and 214 indexed concepts. Below is a complete table-by-table outline of this topic, spanning foundational concepts through advanced details.
Table 1: API Endpoints
The OpenAI REST API is versioned under https://api.openai.com/v1. The Responses API (/v1/responses) is the recommended path for new agentic applications; the Chat Completions endpoint remains fully supported for compatibility. All endpoints require an Authorization: Bearer <API_KEY> header.
| Endpoint | Example | Description |
|---|---|---|
POST /v1/responses | • Recommended stateful agentic endpoint • supports built-in tools (web search, computer use, MCP), previous_response_id chaining, background mode, and store. | |
POST /v1/chat/completions | • Stateless multi-turn conversation endpoint • still fully supported • use for legacy integrations or when stateless behavior is required | |
POST /v1/embeddings | Converts text to vector representations for semantic search, clustering, and retrieval. | |
POST /v1/images/generations | • Generates images using gpt-image-2 (flagship) or gpt-image-1.5• DALL-E 2/3 shut down May 12 2026. | |
POST /v1/images/edits | • Inpaints or edits images using a mask • only gpt-image-2 supported | |
POST /v1/audio/speech | • Text-to-speech using gpt-4o-mini-tts or other TTS models• returns audio stream | |
POST /v1/audio/transcriptions | • Speech-to-text using whisper-1 or gpt-4o-transcribe• accepts audio file upload | |
POST /v1/audio/translations | Translates audio to English text using whisper-1. |