OpenAI API

The OpenAI API gives you programmatic access to every model OpenAI ships: GPT-5.5 and GPT-5.4 for general intelligence, GPT-4.1 for cost-efficient production work, o3 and o4-mini for reasoning, plus gpt-image-1 for images, Whisper for speech, and the embeddings models for semantic search. It is the most-used commercial AI API by a wide margin, and most production LLM stacks start here. The chat completions and responses endpoints are the workhorses. You send a list of messages with roles (system, user, assistant), optionally with tools or structured output schemas, and stream the response back token by token. Function calling lets the model invoke your code, structured outputs guarantee a JSON shape, and the Responses API stitches multi-turn agent loops together with built-in tool use. Model choice drives most of the cost. GPT-5.4 at $2.50/$15 per million tokens is the recommended production default in 2026: 1M-token context window, beats GPT-4.1 on every benchmark, and supports cached input at $0.25/M for repeated prefixes. GPT-4.1 Nano at $0.10/$0.40 is the right pick for classification and extraction at scale. GPT-5.5 at $5/$30 is the flagship for complex agent and long-context work; GPT-5.5 Pro at $30/$180 unlocks the highest-stakes reasoning. For hard reasoning, o3 at $2/$8 now matches GPT-4.1 on price and replaced the older o1 at $15/$60. o4-mini at $1.10/$4.40 covers reasoning at a fraction of the cost. Image generation runs through gpt-image-1, which superseded DALL-E 3. Whisper handles transcription; gpt-4o-transcribe is the higher-accuracy option. Embeddings power semantic search and RAG. Fine-tuning is supported on GPT-4o and earlier generations; the GPT-5 family is not yet fine-tunable. The Batch API discounts every model by 50% in exchange for a 24-hour SLA, the single biggest cost lever for non-real-time workloads.